Part 32 - - Offline - Biogas Technology in the Third World

Part 32 - - Offline - Biogas Technology in the Third World
A project of Volunteers in Asia
Biogas Technoloav
by: Andrew Barnett,
$Q the Third
Lcr Pyle and S.K, Subramanian
Development Research Centre
60 Queen Street
P.O. Box 8500
Ottawa, Canada KlG 389
Paper copies are $10.00;
developing countries.
to serious
groups in
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A Multidisciplinary
Andrew Barnett
Leo Pyle
S. K. Subramanian
The International Development ResearchCentre is a public corporation
created by the Parliament of Canada in 1970to support research designed
to adapt scienceand technology to the needsof developing countries. The
Centre’s activity is concentrated in five sectors: agriculture, food and
nutrition sciences;health sciences;information sciences;publications; and
social sciences.IDRC is financed solely by the Government of Canada; its
policies, however, are set by an international Board of Governors. The
Centre’s headquartersare in Ottawa, Canada. Regional offices are located
in Africa, Asia, Latin America, and the Middle East.
,G1978 International Development ResearchCentre
Postal Address: Box 8500, Ottawa, Canada K IG 3H9
Head Office: 60 Queen Street, Ottawa
Barnett, A.
Pyle, L.
Subramanian, S.K.
Biogas technology in the Third World : a multidisciplinary review.
Ottawa, Ont.. IDRC. 1978. 132p. : ill.
/IDRC publication,/. Compilation on bio/gas/ generation in
/developing country/s and the /technical aspect/sof /biomass/ and organic
/ waste/ fermentation - discussesother /energy source/savailableto /rural
community/s, feasibility of small-scaledigestorsfor the production of /fuel/
and /fertilizer/, /social implication/s and /economic aspect/s of biogas
technology; includes /case study/s from /Asia/, /bibliography/c notes,
/statistical data/.
U DC: 662.76
Microfiche edition available
ISBN: O-88936-162-2
IDRC- 103e
A Multidisciplinary
Andrew Barnett
Leo Pyle
S. K. Subramanian
About the authors 5
Anaerobic digestion: the technical options
Leo Pyle 9
Alternative systems based on anaerobic
digestion: digester designs 3 I
Alternative treatment systems centred on biogas 43
Technical parameters affecting digester performance 49
State-ot-the-art review 50
Research and development priorities:
some suggestions 65
Biogas technology:
a social and economic assessment
Andrew Barnett
The general approach 68
Valuation of common inputs and outputs 78
Case studies 84
The social and economic determinants of the dema: .; 3r biogas 92
An approach to research priorities 95
Biogas systems in Asia: a survey
S.K. Subramanian
India 97
The Republic of Korea IO1
The Philippines 102
Thailand 104
Indonesia IO5
Japan 105
Other countries I 36
Interest of the international agencies in Asia 108
Technological aspects of the region’s experience 108
Social and economic issues I 14
Problems of evaluation 119
Summary 120
1. Continuous digestion: typical gas yields 123
2. Continuous/batch
digester rates: some models and results 124
Cellulose digestion 125
3. Suggestions for studies in core technology 126
About the Authors
Andrew Barnett received his MA in Development Economics from the
University of Sussex and then specialized in the problems of cost-benefit
analysis, first with the Ministry of Overseas Development, London, and then as
a free-lance consultant. At the time of writing 0,;s report he was at the Institute
of Development Studies, University of Sussex, Brighton. He is currently
Advisor to the Science and Technology Policy Programme of the International
Development Research Centre at the Science Policy Research Unit, University
of Sussex.
Leo Pyle obtained his PhD from Cambridge University in Chemical
Engineering. He has taught at Imperial College, London for the la!;? 10 years
during which time he was Visiting Professor both at Queen’s 1iniversity,
Kingston and the University of Chile. He has carried out research on anaerobic
fermentation kinetics and is currently Chairman of the Methane and Fertilizer
Working Group of the Intermediate Technology Development Group, London.
SK. Subramanian gained his PhD from Birmingham University in
Chemical Engineering and Petroleum Technology and worked in the chemical
industry in India for a number of years. Following a period as Assistant Director
of the National Chemical Laboratory, Poona, he moved to the Department of
Science and Technology of the Government of India and was Secretary to the
Indian National Council of Science and Technology. Later, he became Director
of. Technical Management at the Management Development Institute, New
Delhi. More recently he has joined the Asian Productivity Organization, Tokyo.
A microapproach to the social and
economic appraisal of rural technologies,
which stresses both the need to examine
technologies in their social context and the
need to compare biogas investments with
alternative uses of the resources available in
specific rural locations, is presented in the
second chapter. This approach is discussed
in relation to a number of the better
attempts that have been made to evaluate
The third chapter complements the other
two by presenting practical field experience. It i; based on an extensive survey of a
large number of biogas plants and their
in India,
Thailand, Indonesia, the Philippines, South
Korea, and Japan.
The technology associated with the
production of methane at the village level is
in a much greater state of change than is
popularly assumed. However, much of the
data on both the new systems and the more
traditional ones are generally unreliable and
inconsistent. It would therefore appear
advisable, before any major commitments
are made to this form of energy and fertilizer production, that a much more
systematic approach be made to research,
development, and evaluation.
Many of the technical and economic
evaluations that have been carried out so far
have been applied to only a limited set of the
known techniques, and comparisons have
been made between biogas and other
systems at the ‘high’ end of the technology
spectrum. Given the current bias in the
distribution of the world’s research and
effort it is hardly surprising
‘Biogas technology is based on the phenomenon that when organic matter containing that in these comparisons the undercellulose is fermented in the absence of air developed [email protected] techniques sometimes
(ailaerobically) a t*ombustible gas (methane) is appear to be inferior. With the fluid state of
biogas technology and the unusual interest
Biogas technology’ represents one of a
number of village-scale technologies that are
currently enjoying a certain vogue among
governments and aid agencies and that offer
the technical possibility of more decentralized approaches to development.
However, the t=tchnical and economic
evaluation of these technologies has ofren
been rudimemary. Therefore, there is a real
danger that attempts are being made at
wide-scale introduction of these techniques
in the rural areas of’the Third World before
it is known whether they are in any sense
appropriate to the problems of rural
In response to the interest in biogas and
other rural energy systems shown by a
number of Asian researchers, the International Development Research Centre
commissioned this state-of-t he-art review
so that it might form a basis of further discussions concerning the direction of future
biogas research.
This book, which is divided into three
chapters, represents a multidisciplinary
approach to the problem and attempts to
review existing work rather ,than to
champion particular solutions. The first
thapter establishes in broad terms the
energy options facing rural communities in
the Third World and considers in detail just
what is known about the technical aspects of
biogas production. This is done by assembling details of known small-scale
digester designs and by reviewing the literatur.: to establish the technical parameters
determining digester performance. ,
combine to produce both the appropriate
hardware for various situations and the
infrastructure that’ is necessary to ensure
that the hardware is widely used.
Our objective, then, is to stressthe need to
examine a wider range of technical and
economic alternatives for meeting the
energy and fertilizer needs of rural peoples.
It is our hope that this survey contributes to
this process by showing what has already
been done, by pointing out pitfalls, and by
indicating the major gaps that still remain.
currently being shown in it, it would seemto
be relatively easy to design and build biogas
plants that could be operated in rural situations to meet certain social objectives anu
yet still compete with ‘higher’ technologies
even in conventional terms of profit and
capital required per unit of output.
The viability of a particular biogas plant
design depends on the particular environment in which it operates. Therefore, the
research problem becomes one of providing
a structure in which technologists,
ecozomists, and users of the technology can
Anaerobic Digestion: The Technical Options
Leo Pyle
the context of a given technical and social
system. This section attempts to be no more
than one input to such an evaluation.
The major considerations that are likely
to weigh in considering the contribution of
biogas to a rural or village environment
include its contribution to supplies of fuel,
energy, and fertilizer, and to waste treatment, public health control, and sanitation,
as well as its use of local resources (material
and human).
There will also be a number of wider considerations such as the technology’s contribution to indigenous technologies and social
and economic development.
Thus we can consider various alternatives, for example: alternative methods of
supplying local energy/fuel needs (including
methods of production, distribution, and
use); alternative methods of using local
materials (e.g. cowdung, crop residues, and
wastes); alternative methods of supplying
fertilizer needs; alternatives in public health
control; alternative systems with anaerobic
as the “core,” including
different end uses; and finally, alternative
arrangements/designs of the digester for
biogas production. Although there will be
some overlap among these different categories, this listing affords a convenient basis
for discussion.
Whatever the context of the pcl:jsible application oi biogas technology+,choices will
always have to be made between’alternative
courses of action. The choice may be to
install a methane generator or to do nothing
at all, or lhere may well be complicated
issues of choice between a number of alternative developments. This preliminary
section sketches some of the technical alternatives that could be involved and suggests
sources for further study.
An effort is made both to put in perspective the state-of-the-art of biogas technology
and also to help define the range of system
objectives and boundaries relevant to this
discussion. Existing digester designs (‘core’
technology) are therefore described and
their features compared and contrasted; this
description being later extended to include
the ‘peripheral’ technologies involved in
different biogas systems.
The more important aspects and features
of the science and technology of biogas both core and peripheral - are then discussed in order to delineate the more significant ways in which digester behavior can be
affected, changed, and measured, and to
outline the areas where there are clear gaps
in knowledge or where research and development work is progressing, or might be
pursued. The problems of evaluating biogas
performance, and of deciding where research and development might be justified
are also discussed.
It must be emphasized that this discussion
is not meant to be prescriptive. An evaluation of the merits of biogas technology, or of
the value of investment in research and
development, can only be made within a
clearly defined set of objectives and within
Energy and Fuel
The main sources of energy that could be
provided to rural areas are listed in Table 1.
This gives a qualitative picture of the substitutability of the different energy sources
for household, agricultural,
or (small)
Table 1. Main energysourcesthat could possibly
Coke, coal
Straw, vegetable
Solar energy
be provided
~Includes. for example.
pump sets.
4 ncludes steam.
NOTE: (X) represents methods that are likely
Agriculture/ Industry
Pow&l Transport energy*
to rural
to he very expensive,
industrial use, but gives no indication of the
costs or likely appropriateness of the energy
It is also relevant to consider the major
existing or potential production methods
for the fuels listed, because this consideration can weigh very heavily when
considering the choice of technology, and
because of wider implications, such as
ecological suitability. The primary energy
sources can be classified as broadly nonrenewable or renewable: nonrenewable
sources include coal, coke, oil, natural gas,
and nuclear; renewable include solar, wood,
dung, vegetable matter, water, and wind (see
Table 2).
Although some of the fuels listed are
classified as renewable, this may only be true
under controlled conditions. For example,
the use of wood as a major source of energy
in some developing countries has led to
deforestation on an extremely serious scale.
No attempt will be made to describe
current patterns of energy use in developing
countries: it is sufficient to note the extremely high dependence in rural, areas on
noncommercial fuels. Many of the methods
of use are carried out with very low ef10
he of limited
or need further
ficiency or with serious health consequences
(e.g. burning of dung indoors creates a
source of eye complaints). The questions of
energy utilization in the rural areas of poor
countries are described and analyzed by
Makhijani and Poole (1975) and Makhijani
The appropriateness of a particular
energy source for a given situation depends
UPC: a number of factors, such as the availability of prim&-y resources (e.g. coal, water,
etc.) and the economies of scale in production. For example, a possible benefit of
using crop residues or solar energy is that
these might be feasible on quite small scales
and thus be appropriate to rural areas at an
early stage of mechanization and development. The practicality of processes centred
on biogas production will be the subject of
later discussion. First, however, it is instructive to consider alternative methods of gas
Alternative Methods of Gas
The combustible fraction of biogas is
methane; however, there are other possible
Table 2. Some renewable and nonrenewable
sourcesof common energyand fuel types.
End sourceof
Nonrenewable Renewable
enerev/ fuel
Coal, oil, gas- Hydroelectricity
fired power
Natural gas, Celluloses,
oil. coal
wastes, etc.
combustible gases such as (mixtures of)
hydrogen, carbon monoxide, and higher
r-paraffins (such as butane and propane).
In industrialized countries methane is
much used as a fuel or chemical feedstock. It
is a major constituent of natural gas, and can
also be manufactured by gasification or
reforming from fossil and nonfossil fuels to
yield substitute natural gas (SNG). These
processes are highly sophisticated technically and are carried out under elevated
temperatures and pressures. It seems highly
unlikely that they will be serious contenders
as technologies appropriate to rural application either in terms of levels of technology
employed, capital intensity, or scale of
operation. However, for large-scale operations gasification must be considered
seriously. Ifeadi and Brown (1975) consider
the process promising above 100tonnes/day
(equivalent to the manure from 17000 dairy
cows !)
An alternative route, which may in the
future be more appropriate, is pyrolysis. In
this process the biomass (wood, coal,
vegetable, etc.) is heated in an air-lean environment to 400-1000 “C. The products
depend on the feed material and the
operating conditions, but usually comprise
three phases: a solid char, which can be subsequently used as fuel; an oily fraction; and a
combustible gas (which may have considerable CO and Hz content) that can be further
processed to produce a gas with a high
methane content.
The promise of the process, for relatively
small-scale applications, is reviewed by Pyle
( 1977). For example, Tatom et al. ( 1975)
report a small, transportable pyrolysis unit
able to treat wood, peanut shells, sugar
waste, trash, etc. The fuel products from the
unit are char (with a heating value (H.V.) of
30000 kJ/kg), oil, and gas. Preliminary
experience shows that the process is
promising for low moisture content wastes
where the energy needed to evaporate the
moisture is small. However, a good deal of
work remains to be done to develop appropriate designs. Makhijani and Poole (1975,
p. 100) have commented favourably on the
Chiz;+. experience with pyrolysis.
1 Y.r same range of feed materials can also
be fermented anaerobically (at 30-60 “C) to
produce biogas. The three methods sketched
in Fig. 1 can be seenas potential alternatives
within the framework of Fig. 2. And, in fact,
whether the wastes are animal or vegetable
products, they can be considered to have
their basic energy supply in the capture (by
photosynthesis) of solar enei gy (see Table
Figure 3 illustrates this point and shows
the potential for gas cleaning and C07,
water, and nutrient recycling.
Materials Suitability
McCann and Saddler ( 1976) considered
the economics gf pyrolysing wheat straw
using the Garrett process and found that it
yielded between 0.32 and 0.40 tonnes of oil
(at a lower calorific value than fuel oil) per
tonne raw material. All the gas produced
and most of the solid char was recycled to
provide heat for the process. Despite this,
the preliminary economic analysis of the
process (under Australian ‘conditions) looks
extremely promising. However, if a wet
manure is used to feed the pyrolysis unit, it is
necessary to evaporate about 80-85% of the
weight before treatment. On the basis of 1
tonne wet matter, containing 200 kg dry
matter (calorific value 14500 kJ/kg), one
would need to evaporate up to 800 kg of
water (i.e. 4 kg water/ kg dry matter) at an
1 :
Fig. 1. Three principal methods for anaerobic conversion of feed materials to biogas.
Fig- 2. The three .methods qf conversion shouw in Fig. I can be seen as potent;ial alternatives within a
framework of energy supply.
Table 3. Examples of fuel-gas production
Pine bark
Rice straw
Cellulosic refuse
Wood, paper
Water hyacinth*
Unicellular algae4
Hydrolysis + digestion
Hydrolysis + digestion
Hydrolysis + digestion
methods from biogas (from Klass 1976).
= 3700-16
500 kJ /m’;
900 OC
ambient pressure
200-700 “C
ambient pressure
540 OC, 70 bar
30 OC, 30 davs
48 OC, IO-28 days
48 OC, 28 days
33-48 OC, 20-50 days
35-55 OC, 30 days
low value gas, char, oil
gas, char,
high value gas, char
intermediate value gas
intermediate value gas
intermediate value gas
intermediate value gas
intermediate value gas
= 16 500-30 000 kJ / m3; high
30 000 kJ / m3.
2Eichhornia crassipes.
3Macrocystis pyrlfera (giant kelp).
4Scendismus spp.. Chlorella spp.
Fig. 3. Wastes, whether animal or vegetable products, have their basic energy supplev in the capture (by
of solar energy.
energy cost of approximately 2500 kJ/,kg of crop residues. It is, however, necessary to
water or 10000 kJ/ kg dry matter. The feed- view the question within a wider context in
stock energy (To) used for evaporation is order to establish an optimum strategy for
shown, as a function of moisture content, in local communities. For example, one has to
Fig. 4. Thus for materials of high moisture consider current patterns of usageto ensure
content there is a strong argument in favour that people are not worse off - in material
of anaerobic digestion becauseno evapora- or resource terms - as well as establish
tion is necessary.
alternative methods of use. For example,
sugar cane may be used as a fuel, or alterAlternative Methods of Waste and natively, the cane can be used in paper
manufacture, which may be more ‘profBiomass Utilization
itable.’ However, the implications of a
The preceding discussion has touched on switch in end use raise very complex issues.
production of fuel gas from animal and Our purpose is not to oversimplify the issues
vegetable wastes, and conceivably, from involved but rather to point out some of the
Alternatives Based on Animal and
Crop Wastes
Biogas production is often suggested in
situations where animal wastes are used as a
major source of household energy. The
potential advantages include: (1) the replacement of an inefficient (but traditional)
fuel with a more efficient and flexible one;
(2) the recoupment of the fertilizer value of
the waste, which is lost if the dung is burned;
and (3) the benefits to public health
(especially in reducing eye diseases) if the
cleaner, less smokey, gas is used. However,
the question remains: Does this represent
the best use of the waste?
Some of the alternatives for biochemical
and chemical processing of raw materials
containing cellulose are outlined in Fig. 5.
Fig. 4. effect qffeed moisture content on amount
of energy available .for biogas production.
more obvious choices, becausethe question
of feasibility can only be raised within
particular situations and sets of objectives.
The need for organic matter in agriculture
is well recognized. In the context of fertilizer
shortages and increasing prices, the need to
use organic wastes has taken on a new
dimension, and their contribution certainly
cannot be underrated. For example, it has
Fig. 5. Simplifipd product chart based on cellulose.
been relrurtsd that SSTbof China’s cultivated
land is treated with organic manures (night
soil, compost, green manures, etc.). One
major advantage of this type of manuring is
its contribution to recycling and conservation of plant nutrients (see Gotaas 1956).
There is strong evidence of the need for
naturally derived nutrients and humus in
situations where heavy reliance is placed on
‘chemical* fertilizers (see Dhua 1975).
When dung is used not as manure but as a
fuel, a link in the nutrient cycle is broken.
This could be avoided if the manure was
used as an input for anaerobic digesters. It
can be argued that this would be preferable
to composting (where the nutrient qualities
are conserved, but the fuel value of the
wastes is lost). However, one must also consider the zero capital-cost of unmechanized
composting, and the more complex issues
related to public health (e.g. pathogen
Although one would like to calculate the
‘trade-off between the net benefits of
anaerobic digestion and the fertilizer value
of composted materials, it is not possible at
this stage to obtain quantitative measures of
the fertilizer/nutrient value of the slurry
effluent from bioga; plants, nor to compare
the value with composted materials.
It is also noteworthy that cornposting,
being an aerobic process, is exothermic.
Under normal conditions this heat is wasted
(it is very low grade heat) except insofar as it
is responsible for the destruction of
pathogens. However, it may be worthwhile
in some circumstances to use the heat from
composting material as an energy input to a
biogas plant. This can be done by surrounding the biogas plant with compost.
From the point of view of efficiency of
converting feed (grass, grain, etc.) into
human food, animals leave a good deal to be
desired: the major proportion of the feed is
converted not into meat protein but into
protein in the manure. Reuse of the
digestible portion of the feces could lead to
significant reductions in the overall cost
(and therefore increased efficiency) of
raising cattle (Pimental 1975), swine, and
poultry. There are a number of ways in
which this can be done (Perrigo and
Demmitt 1975).
Use of Untreated Material
There have been a number of studies of
the use of untreated or slightly treated
manures as a ration supplement, and
Perrigo and Demmitt report generally
favourable results when the use of bovine
manures was limited to about 10% of the
COW’S diet. The practice is currently forbidden in the USA and parts of Western
Europe: it is, however, practiced in the U.K.,
where dried poul!;.~ tllanure is currently
being used succ~.ssf~~!ly.b~~~ausethe costs
involved are small, v~-e!ul ti(trention should
be paid to the poa:ihiLtier of‘ this method.
Manuric Modified Silage
Anthony has successfully developed a
process where manure can be incorporated
into silage. The manure is mixed with
-Bermuda grass (in ratios up to 1.3:1)and left
to ferment. The ‘wastelage’ contains lOY0
crude protein and 60% digestible nutrients
(dry basis), and has produced positive results in feeding trials.
Fermentation-Based Processes
Two broad types of processes using
microorganisms to increase the nutritive
content of manures are discussed by Perrigo
and Demmit: (I 975). The first involves the
use of bacterial cultures to ferment the
manure to silage. The second involves culturing the microorganisms on the manure
substrate, harvesting them, and finally processing them to increase digestibility. For
example, Ward and Seckler ( 1975)discuss a
process where a high-protein fraction from
cattle manure is fed directly to poultry. The
proposed method involves anaerobic fermentation of the waste followed by
fractionation to three products A, B, and C.
These three fractions are characterized in
Table 4.
Ward and Seckler claim that the highprotein fraction from one dairy cow can
support 30 hens. In fact, because poultry
manure is high in uric acid, when it is dried it
pressure and/ or the use of acids or bases
(e.g. caustic soda) can be used to improve
the digestibility of cattle manures (Stidham
Potential use
et al. 1973; Klopfenstein and Koer 1973).
High fibre
Feed for cattle,
Robb and Evans (1976) report the use of
(equiv. to corn
sodium hydroxide in the recovery of nutrisilage)
tive materials from cereal straw. Hydrolysis
High protein
Feed for ruminants
has also been used to replace the slow
or nonruminants
breakdown of ligno-cellulosic
Soil amendment
High ash
plant tissues to sugars, for subsequent
fermentation to single-cell protein (Worgan
Curiously, there are few reports of such
is itself a possible feed supplement. Thus,
this method offers a potential- reduction in methods as part of biogas production, where
the acreage needed for producing animal the rate of hydrolysis of the feed is largely
feed. Although presertly unproven, this responsible for the slowness of the process.
method should be considered as one alter- This point is considered in more detail later.
native among many. With the recent interest
The methods discussed above for waste
shown in processes of this type, CGn- recycling have as a major objective the imsiderable improvement may be expected in provement of the efficiency (viewed in input:
the near future.
output terms) of animals as food producers.
An alternative method of converting The same arguments hold if cattle, for
wastes into protein is their use as a substrate example, are used for work. In other words,
in algal culture. In principle various wastes the object is to increase the ratio of human
can be fed to ponds in which unicellular or food:animal food or work:food, and the
filamentous green or blue-green algae are feasibility has to be determined by weighing
cultured, Yields of approximately 80000 the cost of treatment and recycling against
kg/ha/yr (dry matter) have been achieved the improvement in efficiency.
As has been seen, it may be advantageous
under laboratory conditions (Boersma et al.
1975, Pantastico 1976). The efficiency cf to use the animal waste (whether treated or
these waste ponds depends on numerous not) as a food additive for another
factors: the physical conditions (incident population.
light (most algae cannot grow oxidatively on
There are clearly a number of implicasewage in the dark), pH, temperature, tions of such schemes, and these are
mixing); the quantity and biochemical discussed below in the context of integrated
oxygen demand (BOD) loading of the feed; food, energy, and waste-treatment cycles.
the pond dimensions; nutritional conditions Before discussing integrated systems, howsuch as the presence of micro- and macro- ever, we will consider other possible uses of
nutrients; and the species present in the wastes or biomass.
population, their frequency of harvesting,
Other Fermentation Processes
etc. (Shaw 1973).
Considerably wider possibilities than
Physicochemical Processes
those outlined above exist. Three such proOne ofthe major limitations on the easeof cessesare considered here, and more inforutilization of cellulose-based materials is mation is presented in the recent symposium
their rate of hydrolysis to produce sugars, by UNITAR (1976).
which can be more directly utilized. A
Protein production from carbonumber of physicochemical pretreatments
hydrate wastes
have been proposed to prepare manure and
agricultural wastes for refeeding, recycling,
Carbohydrates are the largest renewable
or processing. For example, heating under source of carbon compounds available for
Table 4. Characteristics of the three fractions
-- deriwd from fermentation-based processes. -
conversion into protein for human or
animal food. A variety of materials can be
used as substrates for the production of
edible protein in the form of yeasts (e.g. C.
fungi (e.g. F. seruitectun~), and
bacteria (e.g. Edwrid~ia
co/i). However,
there is still work to be done on processing
methods and on the palatability and
acceptability of such products before these
processes are likely to be used on a wide
scale for bulk food preparation.
Worgan ( 1973) gives a good review of
some of the methods of producing edible
protein. Their relative biological efficiency
may be judged from Table 5, which is based
Table 5. Relative biological efficiency of some
methods of producing edible protein based on
using either sucroseor oat husksas the carbohydrate source (from Tables 7 and 8, Worgan
(g) to yield
100 g protein
C. utilis
F. semirectum
E. coli
Beef cattle
doubling time
on using both sucroseand oat husk as carbohydrate sources.
Relative biological efficiency is also
reflected in land productivity calculations:
for sugar beet the yield of protein can be
2800 kg.ha yr: the yield of beef protein is,
by contrast, about 42 kg,iha/yr. Of course,
cattle are more than just beef producers:
their wastes can be utilized and they are
essential sources of power for much of the
world’s population.
Production of glucose, alcohols,
Starches and celluloses can be used to
produce sugars. Starch, for example, can be
hydrolyzed by acids or enzymes and then
fermented to ethyl alcohol. Celluloses too
can be broken down in a similar way. Processesfor the enzymatic hydrolysis of cellulose to glucose have recently been developed, but the estimated cost is, at present.
high (McCann and Saddler (1976) quote ;i
figure of $US 0.22-0.55,/kg glucose) and
ethyl alcohol made by the route is probabh,
uneconomical (Fig. 5 illustrates the
The hydrolysis of starch is much easier
than the hydrolysis of cellulose, and
McCann and Saddler ( 1976) claim that
alcohoi production from starchy substrates
(e.g. cassava)is cheaper than from cellulose.
The costs of production of various fuels estimated by McCann and Saddler are given in
the Table 6. The potential of cassava for
fuel, food, and industrial chemical (via
alcohol) production is at present a virtually
unexplored region.
An excellent discussion of ethanol production by fermentation is given in the
review paper by Trevelyan (1975), where
both the process and end-use alternatives
are discussed in detail.
The different end uses of cellulose can be
assessed by comparing the value (market
price) of the various products. Table 7
(Dunlap 1975)gives such estimates, but does
not take into account the difficulties or costs
associated with processing nor the va!ue of
by-products (for example, in biogas production the fertilizer value of the slurry is
extremely significant).
Alternative Sources of Biomass
Biogas production is usually considered
as a method of treating animal and vegetable
wastes. However, it is advisable to consider
wider possibilities; for example, the feasibility of growing renewable crops for energy
Potential raw material can be divided into
two classes: (1) land grown; and (2) water
grown (fresh or sea). The choice of the most
appropriate crop will depend on many technical and social factors. Included in the
technical factors are: attainable growth rates
under the climatic conditions in question;
nutrient and water demands; ease of
Table 6. Costs (calculated for Australian conditions) of some photobiological
Saleable by-products
As fuel
Raw material
Cassava tops
& tubers
Enzyme hydrolysis /
Batch fermentation
Cereal straw
Cereal straw
Acid hydrolysis/
Enzyme hydrolysis/
Bacterial fermentation
Bacterial fermentation
Flash pyrolysis
(Garrett process)
Flash pyrolysis
(Garrett process)
Fibre (animal
feed), fuse1
Biomass slurry
Biomass slurry
‘Comparative costs of other energy sources at time of study ($I 109J):Kuwait
gallon, taxed) 4.45; diesel fuel (350 per gallon, untaxed) 2.0; No. 6 fuel oil ($75
*Includes energy cost of harvesting, transport, process fuels, electricity, and
fuels (from McCann and Saddler).
$I 109J
energy input energy input
NUEP Overall
20. I
crude oil (US$!O per bbl)
per tonne) 1.7; and natural
1.25; syncrude from coal 1.2-1.9; gasoline
gas (Cooper Basin) I. IS.
(70~ per
Napier grass
Sugar cane
Annual crops
Rain forest
Temperate (Europe)
Annual crops
g/ m*lday
harvesting; water content of harvested crop;
amount of harvested feed; and reactivity or
A summary of known average or
achievable yields, with some approximate
cost estimations (under U.S. conditions), is
given in Tables 7- 11, and a good discussion
of the suitability of the highest yielding of
these potential sources is given in Alich and
Inman (1975).
(70 of total radiation)
animal by-products (blood, bone meal, etc.),
(treated) hirman wastes, and siurry from
biogas plants, combined with chemical
or by chemical fertilizers alone
(urea, ammonium bicarbonate, ammonium
phosphates, potassium sulfates).
There is a worldwide trend toward the use
of high analysis and complex (chemical)
fertilizers. Since the early seventies,
Alternative Sources of Plant
Table 8. Photosynthetic
yields (mg CO*
fixed /cmz/ h) at normal and enhanced CO2 gas
levels (from U.K./ISES 1976).
Nutrients vital for plant growth and development are based on carbon, oxygen,
hydrogen, macronutrients
phosphorus, and potassium), and, finally,
secondary and micronutrients. In the caseof
micronutrients, toxic effects from oversupply can cause serious problems.
The first four elements occur in air and
water; phosphorus, potassium, and nitrogen
‘fixed’ by organisms are found in the soil and
are subject to exhaustion as they are removed by plants. Their availability depends
on environmental factors such as temperature, moisture, and acidity. Soils may be
replenished by applying nutrients in the
form of organic or ‘nktural fertilizers such
Maize, sorghum,
sugar cane’
Soybean, sugar beet
Oats, wheat, barley
Tomato, cucumber,
Tree species,grapes,
are the only C.,-species
in the table.
Table 9. Some high short-term
dry weight yields of crops and their short-term
efficiencies (from U.K. / ISES 1976).
Sugar cane
Sudan grass
Palm oil
Napier grass
Bullrush millett
Sugar cane
Rye grass
Sugar beet
Wheat (spring)
Bermuda grass
Sugar beet
US, California
US, California
US, Georgia
S. Australia
US, Texas
US, California
US, California
US, California
(whole year)
El Salvador
Australia, NT
g/m’/day can be converted to
Table 10. Productivity
by multiplying
and energy conversion in agricultural
by 3.65.
crops on an annual basis.
US, Illinois
Canada, Ottawa
US, Kentucky
US, Washington
US, Washington
5 (grain)
5 (grain)
12 (grain)
30 (total)
7 (grain)
7 (grainj
Table IO. Productivity and energyconversionin agricultural crops on an annual basis(concluded).
US, California
US, California
Australia, NSW
US, California
US, California
7 (grain)
Napier grass
Sugar cane
Oil palm
Sugar beet
Rice +
El Salvador
Puerto Rico
Hawaii (2 crops)
Australia, NT
7 (grain)
23 (grain)
Table 11. Biomass yields and estimated costs
(from Saddleret al. 1976;Klass 1975). _
Elephant grass
Sugar cane
($/ 10 BTU)
Corn silage
Sugar cane
Land/water based
however, escalating prices and supply
shortages due to the increasing costs of raw
material and energy have affected both
production and freight costs. The international price of ammonia, for example, increased nine-fold between 1972 and the end
of 1974, and led to declining demand,
especially in developing countries. At the
same time, experience has shown that
fertilizer supply is perhaps the single most
important technical factor in agricultural
growth. The need for increases in productivity per hectare is increasingly important
and calls for an acceleration in the supply of
fertilizers. The extent to which chemical
fertilizers will supply this need will depend
on factors that vary from country to
country: distribution and credit facilities;
availabllity of appropriate supplies; supply
of complementary inputs; know-how and
skills to run production facilities near
On average 7%85% of the major nutrients
capacity; and technical backup to the
40-50% of the organic matter in the feed
are present in the manure. Urine contains
40-70% of the fertilizer value of the manure.
Organic Fertilizers
Taiganides and Hazen calculated the
Recent practice has tended toward mono- potential annual value of the manure per
cultural production based on chemical 1000 lb (450 kg) live weight to be $7 1 for
fertilizers. There is plenty of evidence, how- poultry, $42 for hog, and $26 for cow
ever, that biologically supplied or recycled manure (in 1966 US dollars), valued at the
organic nutrients (cornposting, biological cost of equal amounts of commercial NPK.
nitrogen fixation. nutrient recycling) reprePantastico (1976) outlines the elements
sent an alternative to reliance on chemical that are relevant to an assessmentof the role
of organic nutrients, and gives data on the
It is impossible to generalize on the composition of manures and composts of
potential contribution from wastes because different types (compost, stable manure,
their composition varies enormously. night soil, raw straw, plant ash, and green
Taiganides and Hazen (1966) provide manure). It has been estimated that wastes
typical NPK contents, and Ames (1976) from animals, plants, and humans could
gives some average figures for a range of supply developing countries with six to eight
organic manures (Table 12). An approxi- times more nutrients than they derive from
mdte idea of the potential contribution of chemical fertilizers. These figures can be
animal manures can be seenfrom Tables 13 compared with estimates of the NPK conand 14.
centrations of fertilizer and manure in the
Table 12. Average chemical composition
(%) of some organic manures (from Ames 1976).
Bu1k.r organic manures
Farmyard manure
Compost (urban)
Compost (rural)
Green manures (various averages)
Edible oil cakes
Cotton seed (decorticated)
Cotton seed(undecorticated)
Manure of animal origin
Dried blood
Fish manure
Bird guano
Bone meal (raw)
Bone meal (steamed)
Activated sludge (dry)
Settled sludge (dry)
Night soil
Human urine
Cattle dung and urine mixed
Horse dung and urine mixed
Sheep dung and urine mixed
0 5-o 7
0.4-O. 5
that about 16% of the N present in the
digested sludge is present as dissolved ammonia, which evaporates on standing (see
also Idnani and Varadarajan 1974). The
proportion of nitrogen as ammonia varies
with the feed: for rice straw the loss is only 810% (Acharya 1958).
Table 13. Average daily manure production and
composition of -hens, -swine, and cattle (from
Tables 2 and 3. Taie;anides and Hazen 1966).
t 1.8-2.3 kg) (45 kg) (450 kg)
Wet manure
Total solids
(o/r wet basis)
Volatile solids
(?Q dry basis)
(% dry basis)
(o/c dry basis)
(or0dry basis)
Where the fertilizer (i.e. nitrogen) content
of the slurry is important, it is essential to
minimize volatilization losses by using
proper storage and application methods.
Tanks or lagoons are perhaps the most
satisfactory storage method; whereas, loss
during application can be minimized if the
sludge is injected below, rather than spread
on the soil surface.
Table 14. Major fertilizing elements per 450 kg live animal weight (from Tables 2 and 3, Taiganides and
Hazen 1966).
Wet manure
Total mineral matter
Organic matter
kg/ day
W yr
kg/ day
I 4600
0. I ‘7
USA (Table IS), which show that manures Table 15. Estimates of the quantity and value of
generally have a relatively low nutritive nitrogen fixed by some of the principal legumes.
value (but of course the organic matter itself
serves a vital function).
Quantity of N
Other possible sources of fertilizer
fixed in a
Value as
(Briones and Briones 1976) are industrial
growing season fertilizer of N
fixed (% per
wastes (e.g. mud press from sugarcane
(kg per
mills), bean meals, and garbage.
Lucerne (alfalfa)
Other temperate
pasture legumes
Tropical pasture
Biogas Plants as a Source of
Several authors have commented on the
fertilizer quality of digester slurry (for example, Acharya 1958). Unfortunately data
on the fertilizer value of the slurry from
biogas plants are inadequate. Theoretically,
little of the NPK fed to the digester should
be lost during the process because the only
loss of N is as gaseous ammonia and this is
small during digestion. Acharya points out
‘Ranges of values in recent findings - due to the
complexirics of the factors involved these ranges are
wide, with extreme values in both directions relatively
*The nitrogen fixed was valued conservatively at a
1975 ‘farm gate’ cost of SO.30/kgN.
and cereals gives a substantial increase in
cereal yields. On the other hand, some
legumes make very high phosphorus
demands and economic responses are
achieved only by fertilizing with superphosphate.
The enormous range of options in the
provision of chemical fertilizers is too wide
for discussion here, but two further comments are perhaps in order. First, the technologies for nitrogenous fertilizer production are marked by considerable apparent
economics of scale, as a consequenceof their
high capital intensity. A methodology for
comparing such widely differing alternatives as large-scale capital-intensive
ammonia production and smaller, more
labour-intensive, technologies (e.g. biogas)
is discussed in the next chapter. It should be
Biological Nitrogen Fixation
kept in mind that problems have also been
due to either high transport
Despite the ever-increasing use of
chemically fixed nitrogenous fertilizers it costs or to operation at much less than full
has been estimated that biological fixation capacity. Second, the existence of an ‘anticontributes about four times as much nitro- chemical’ lobby must be acknowledged. It is,
gen to the soil. One example is the forma- however, not our intention to take one side
tion of nodules on the roots of various or another in this debate: the relative lack of
legumes (pulses, beans, nuts, peas, some attention to chemical fertilizers must not be
clovers) by bacteria. Bacteria associated taken as implying any prejudice on the
with the roots of nonlegumes (rice, sugar writer’s part.
cane, etc.) are also important, especially in
the tropics. In addition, other bacteria and
Public Health, Waste Treatment,
blue-green algae (Pantastico 1976) can
and Pollution Control
contribute to the process. The possible
contribution of some legumes to the supply
A major source of concern throughout the
of N fertilizer is given in Table 16.
world is the safe treatment and disposal of
Not all the nitrogen fixed finds its way wastes. Here we are primarily concerned
into the soil (which points to the urgent need with the disposal of organic wastes, many of
for research work), but rotation of legumes which are likely to be degradable. These
Other plant nutrients may be conserved
and made more availabie if plant residues
are recycled through a digester. The result of
using slurries from anaerobic digesters on
the land is much the same as using any other
kind of compost: the humus material plays a
vital role in improving soil properties and
Generally, ‘SO-70% of the degradable
organics fed to the digester are decomposed,
and only a small proportion (lo-20%) of the
carbon is converted to cellular matter. Thus,
problems arising from using the sludge on
land are much smaller than those from using
aerobically treated wastes because the
smaller quantity of bacterial matter
minimizes both smells and insect development.
Table 16. Average oxygen demand data for farm-animal wastes (from Taiganides and Hazen 1966).
.Animal size
(kg! day)
(kg/ day)
(kg/kg VS)
(kg/kg W
(kg/ day)
I .04
wastes are derived from a wide range of
sources: animal manures and wastes; farm
wastes including residues; night soil; contaminated waste waters; industrial wastes,
including wastes from agroindustry; and
general solid wastes, garbage. Wastes range
in nature from solids to liquids, but our
main interest is centred on paste, semiliquid,
or slurry-type wastes.
Among the public health considerations
that dictate the need for waste disposal
facilities are: the transmission of disease
vectors, pathogens, etc. (and especially
fecal-borne diseases), and associated contamination problems; the problems caused
by disposing untreated wastes on land or
water (odours, insect colonies etc.); the
potential of using valuable materials, if they
can be recovered or recycled; and regulations (if any) covering the quality of discharges.
The range of choices available will
broadly be: to do nothing; to treat at the
source (e.g. composting latrine); or to treat
centrally (e.g. community waste treatment
Industrial waste disposal and control
cannot be dealt with here although many of
the treatment policies will be applicable.
Useful background to the use of anaerobic
fermentation in controlling
effluents can be found in Fair et al. ( 1966)
and Mosey ( 1974).
would be exerted on a water body if the
waste were discharged into an (aerobic)
water course. IK is usually measured at 20 OC
over 5 days (hence BODS). The chemical
oxygen demand (COD) can also be used as
an index, but the test does not differentiate
between biologically degradable and inert
matter. Under some conditions, BOD and
COD of a waste will correlate; more often,
the correlation is not good. If the tests correlate, the COD measure is superior because
it is both faster and more accurate. In
assessing animal and human wastes these
measures are best expressed in terms of
BOD (or COD) (as kilograms oxygen
demand) per kilogram of volatile solid (VS)
in the waste. Tables 17 and 18 give average
BOD data for farm animal wastesand mean
figures for BOD and COD for animals and
humans. The COD/-kg live weight is about
the same for hens, pigs, and cattle; the BOD
of cattle waste is considerably lower,
probably becauseof the larger proport ion of
cellulose (which is attacked slowly), The
population equivalent (PE) is the number of
humans to produce the same daily quantity
of waste measured as BOD. In calculating
the load on a treatment system, the liquid
quantity discharged is also very important.
The wide variability (reflecting feeding
patterns, health) in the data cannot be overemphasized: the figures quoted are only a
Pollutant Strengths
Treatment Methods
There are a number of indexes of the
pollutant strength of an organic waste. One
is the biochemical oxygen demand (BOD),
which measures the oxygen demand that
The earliest and simplest form of handling
wastes, the cesspool, developed into the
septic tank, in which the detectable oxygen
level is zero and conditions are anaerobic.
Table 17. Mean BOD and COD of farm-animal wastes (Taiganides and Hazen 1966).
(kg/ day)
(BOD basis)
0.09 I
Live weig!rt
Man (excrement)
Man (total)
The optimum growth conditions for the
phases in a facultative pond differ. The
minimum temperature for effective operaMaximum
tion is about 15 OC, and in most climates
loading rate Detention
4 m will be the maximum depth for an
(kg BOD,’
unheated pond. Odours are caused if the
Pond type
acid concentration becomes too
22.4- 168
One potential advantage of anaerobic
over aerobic treatment systems is that they
‘i.e. per hectare pond surface.
can be loaded more heavily. The figures in
Table 19allow a rough comparison of ponds
loaded with waste water and suspended
Ponds must be carefully designed; for
Perhaps the cheapest and simplest type of
device available is a pond (Fig. 6). The zones example, in shallow ponds algae can form a
that are illustrated can coexist in a single blanket giving excessive fatty acid
pond or exist as separate pond types. The generation even when the (obligate) bacteria
organic matter is converted into: a gaseous needed for methane generation cannot
product; algae, which can be harvested, dis- function because of the oxygen present.
charged, or allowed to settle; and organic Aerobic ponds should be less than 0.3 m
volatile acids, methane, carbon dioxide, and deep; facultative or anaerobic ponds need to
stabilized sludge. The reactions in the be at least 2 m deep. The range of possibilianaerobic part are thus essentially the same ties for relatively simple treatment systems is
shown in Table 20.
as those in an anaerobic digester.
Table 18. Pond characteristics (from Dugan and
Oswald 1968).
- - -s-w
Fig. 6. A pond is perhaps the cheapest and simplest type qf treatment device. This profile indicates the
zones that may exist within a single pond.
.I able 19. Enclosed animal-waste treatment systems (from Loehr 1971).
+ Water 3
Holding tank +
+ Water -
Pond +
Land disposal
Cheap, simple
Land disposal
+ Water -
Anaerobic - Aerobic unit
( high load
In-house oxidation
or Land disposal
Holding unit
Needs space; better odour
control than (1)
Land disposal
Puilt into animal house;
low handling problems;
no need for excess water;
semi-solid wastes to dispose of
Solids to land
Can handle highly concentrated
wastes; secondary (aerobic)
treatment necessary
Allows treatment adapted
to waste
Separation at source<
Liquids to treatment
Drying. incineration
Expensive if mechanized;
drying gives solid fuel or
fertilizer; nutriem losses
Possible handling problems
Pescod (197 1) notes that there are
relatively few waste-water treatment
systems; waste solids (night soil) are often
collected, but the operation of septic tanks,
cesspools, etc. leaves much to be desired,
and the resulting sludges must be treated
carefully. Pescod discusses the anaerobic
digestion of partially stabilized wastes from
septic tanks. In the tropics this is generally
not necessary, but his results indicate the
technical feasibility of anaerobic digesters
fed with night soil or sludge. Treatment is
possible at loading rates of up to 4.5 g
VS/ day/ litre digester volume - some three
times the normal loading rate for primary
sludge treatment. Pescod advocates the use
of lagoons for drying stabilized sludge cake,
but also cautions of the dangers of open
lagoons in the tropics citing the growth of
mosquito larvae (Cuie4x pipiens) on the
surface of the supernatant liquor. It is impossible to disagree with his conclusion that
there is a dearth of published information
relating to developing countries and that a
good deal of research and development is
necessaryto develop rational design criteria.
Jewel1 ( 1975) presents a wide-ranging
discussion of methods and possibilities in
animal-waste treatment with a primary
emphasis on intensive agriculture.
The use of manure and composted farm
and animal wastes is an important alternative technology that has the advantage of requiring little capital investment. Properly
handled, manure has great value both as a
source of nutrients and organic matter
(Gotaas ,19X1,the journal Compost Science,
Klausner et al. 1971, Ames 1976).
One of the major problems in both urban
and rural communities is the safe disposal of
human wastes. Integrated sewerageschemes
are costly. Septic tanks have certain advantages, but they require quite large
quantities of water and are unsuitable for
areas with high groundwater levels, poor
percolation properties, etc. The ‘pit’ toilet is
a sanitary device that is effective if properly
located (away from houses, rural areas),
constructed, and maintained: it can,
however, be a source of odours and groundwater contamination. Recently, there has
been 2 good deal of interest in simple
‘composting’ toilets that do not need any
water and from which the waste products
can safely be used as fertilizer. Typical
examples of such latrines are the Farallones
Table 20. Summary of operating conditions of a range of different digester designs (1 ft3
Digester size
Name of
No. of
Vol. I
Vol. II
Total Vol
No. (Qty)
54 0’
54 ft’
;MlU I sed.
loo ft’
5 (45 kg/d)
I80 f1’
about 30
7/g (60 kg/d)
100 ft’
loo ft’
7ooo It’
30 +
130 cows or 800 pigs
8 (80kg/d)
6 approx
(60 kg! d)
no conlrol
IO00 lb/d
no control
230 kgjd
5 people
"0 con1rol
no data
(3ooO kgld)
5 6m3
no data
465 It’
465 fl’
no da:a
14. I water
16.4 temp.
16.1 t
132 X
230 ft’
mixed feed -
(6-8 tonid)
no data
100 ft’
I IO f1’
300 ItI
approx 9m’
1fmoo 8’
3otMO ft’
(or equivalent)
IO-15 hogs
plus waler
(17 kg/d)
IO-15 hogs
plus water
(I7 kg/d)
IO-15 hogs
plus water
( I7 kg/day)
see various papers by Singh
no data
= WL%rnJ, to convert lb/ftJ/day
65 Ib!d
52.5 Ibid
230 kg/d
to kg/mj/day
multiply by 16.02).
% solids
0.97 lb
I30 R’
7 kg/ml/d
?lWJI hg
= 15wo I da>
3-4 days
no data
I day
45 kg/d
6.75 kg/d
100 f1’
no data
60 days
( 1976)
60 kg/d
IO kg!d
180 ft’
no data
30 days
40 kg/d
5 kg/d
1:I (?)
100 ft’
no data
no data,
30 days
6 kg/ms/d
7000 ItI
Rain f 1963)
18%mmax 200-400
85 kg/d
100 kgid
4rnJi d
20% +
no data
3 kg/m’:d
at 35°C)
no data
30 days +
no data
42 days
20 days
no data
no data
no data
2OOmr / d
30 days
Lapp f 1974)
no data
kg/ ml/d
20 days
Lapp t 1974)
309 ft’
no data
no data
no data
no data
50-70 day\
no data
8.0 kgid
16.0 kg/d
50 k& d
no data
no data
Mann (1962)
RG (21 5
1.6 (volumes
I .3 per day)
0.42 (volumes
0.52 per dayj
300 II’
wooO fl’
17300 It,
5 Ib!ft’!d
I/2 to 1
per day
4.25 kgid
about 60% reduction
6 days
See Paper
C’hung PO
4.25 kg/d
about 60% reduction
it days
See Paper
chung PO
t 1974)
4.25 kg/d
about 6090 reduction
16 days
See Paper
Chung Pa
f iY74l
30 kg:d
Total X
840 kgdd
30% T.S.
25-30 days
25-30 days
14Ooo fl’
30-60 days
composting privy (van der Ryan 1976), the
Toa-Pa tine (de Jounge 1976),or the similar
Clivus system, which is essentially a
container (able to hold some 4-months
waste) with provision for controlled airflow
through the aerobic composting heap. Care
is needed to ensure that the high temperatures for destruction of the harmful microorganisms (i.e. parasite eggs, protozoa, and
viruses) can be attained uniformally. In cold
climates, this may well be a serious drawback. The latrines cited are relatively
expensive (the Toa-Throne costs up to
$1000, the Farallones latrine is quoted at
around $lOO), but cheaper versions are
available. The returns to such an investment
are partly indirect (improved health, etc.)
and partly direct (provision of a fertilizer
Details of conventional waste disposal
methods are to be found in the standard
texts on sanitary engineering; most are
designed to handle relatively dilute wastes
aerobically. More substrates can be handled
aerobically than anaerobically, but the
contact between oxygen and the substrate.
has so far limited aerobic systems to dilute,
largely stabilized, materials with solids
present as fine suspended particles. In
systems where solid wastes are degraded
aerobically, the main problem is to maintain
the solids in a relatively dry, loose matrix.
The large energy releases associated with
aerobic processesenable a high proportion
of the substrate carbon and nitrogen to be
converted to microbial cells, themselves
subject to further microbial attack, so that
disposal of the wet sludge can sometimes
lead to further pollution.
There are two major potential advantages
of anaerobic digestion systems. First, the
possibility of stabilizing waste material for
subsequent safe disposal, and collecting the
gas produced during digestion. Second, it
should be possible to treat more concentrated slurries than with aerobic systems.
In practice, as noted above, the process is
not foolproof. Even under controlled operation the slurry may contain considerable
fatty acids, ammonia, and other nitrogenous
compounds. Further treatment may thus be
necessary before the liquid reaches acceptable levels for discharge. Anaerobic
systems can certainly treat more concentrated wastes. COD levels of 4000 mgllitre
or BOD’s of approximately 100000 ppm are
possible (Hobson et al. 1974); and solids
concentrations between 2 and 10,yOare
normal, as ccmpared with < 1% in the case
of aerobic treatment processes. Anaerobic
processesare generally considerably slower,
which may lead to cost problems. Anaerobic
systems can also handle a range of organic
solids (Klein 1972).
Pathogen Destruction
The choice of technology also relates to
the possibility of disease transmission. Here
we are primarily concerned with fecal-borne
The diseasesfor which causative or vector
organisms are associated with fecal wastes
fall into four broad groups, according to the
vector: viruses, e.g. poliomyelitis, hepatitis,
gastroenteritis; protozoa, e.g. amebic
dysentery; bacteria, e.g. typhoid, paratyphoid, dysentery, cholera, TB, enteritis,
salmonellosis; helminths, e.g. roundworm,
pinworm, sheep liver fluke, bilharziasis.
The hazards associated with treatment
processes that handle human excreta thus
depend on the incidence of the various
organisms, their survival rates, and their
subsequent viability in secondary treatment,
storage, and discharge to the land.
Most organisms are destroyed during
aerobic composting (Gotaas 1956) if
temperatures exceed 60 “C for longer than
0.5-l hour. Most resistant are the eggs of
Ascaris Zumbricoides (roundworm). These
can survive 14 days at 35 OC. Low temperature processes must be evaluated carefully
(there are a number of WHO documents
that cover this field).
Anaerobic digestion (above 30-35 “C) is
as good as any other practical treatment
process for human excreta. However, there
is little documentation dealing with outbreaks of disease in relation to the use of
night soil and other wastes on the land.
Detailed results relating to anaerobic
digesters are discussed more fully later.
Alternative Systems Based on
Anaerobic Digestion: Digester
the most usual measures are the BOD,
COD, VS, or total organic carbon. BOD
levels of up to 10” ppm may be treated by
anaerobic fermentors. In practice, the
concern is more likely to be with solidTwo broad objectives that are often bearing inputs such as manures. Slurries
associated with anaerobic digestion are: the with solid contents of about 10% are likely
treatment of wastes prior to disposal; and to be quite paste-like, and designs to handle
the generation of methane. These are by no concentrated slurries efficiently would minimeans the only objectives: nor are they of mize water requirements and the required
comparable importance. The design of digester volume.
The most frequently used measure of the
digesters reflects the need to meet these
objectives. In this section, the main design biodegradable proportion of the feed is the
variants are outlined and a preliminary at- VS content, which is a close approximation
tempt is made to compare the performance to the potential substrate; with a mixed
of different designs despite the lack of data. substrate any overall measure is likely to be
over-simplified. The conditions in the
Feed Materials and Measures of
digester itself will depend on both the concentration and mean retention time of the
feed. In the case of a continuous digester of
Laboratory studies have shown that a volume V, feed rate Q(mJ/day), and feed
is concentration C(kgVSj m3) loading rate can
wide range of organic matter
biodegradable by anaerobic fermentation. be defined as:
However, one single design is unlikely to
cope adequately witl+ all possible substrates LR=$g kgVS/mJ digester volume/day
because of the different physical conditions
and properties and different rates of
=- C
fermentation that are involved. Designs
have been reported that deal with: soluble
where @= hydraulic retention time.
industrial wastes; sewage and human
wastes; animal manures (for which many
This is often used as a basis for comparing
designs are available): and vegetable and
different digesters. If one digester operates
general farm wastes.
The more soluble and easily degradable at a higher loading rate than another then
the substrate, the more easyis the design and either it can process a greater quantity of
operation. In particular, pig wastes are con- substrate in the same retention time or
sidered to be relatively easy to handle handle the samequantity in a smaller time or
(Hobson et al. 1974);whereas,a good deal of in a smaller volume.
trouble has been reported with straw
bagasse or other low density vegetable
Batch Versus Continuous
matter. Such materials very easily form an
A summary of the operating conditions of
impenetrable scum on the surface of the
digester. However, there is considerable ad- a range of designs is given in Table 20, which
vantage in fermenting vegetable matter serves as the basic reference for this section.
Digesters can be broadly divided into
because the potential methane production
rate per unit mass is higher than that from either batch or continuous flow. In a batch
cow manures that have, in effect, already operation, the raw materials (substrate) are
been through an anaerobic digester (i.e. the charged into the digestion vessel and the
fermentation process can be considered in
Concentrations may be measured in three stages.
In stage I, the bacterial population begins
various ways, more or less appropriate to
establish itself, and following a lag
digestion. In the case of contaminated water
many days) gas evolution begins.
The gas is likely to be unusable (or even
dangerous) as a fuel, with a high
concentration of hydrogen sulfide. In stage
II (2-4 weeks) the gas production rate increases, passes through a maximum and
then begins to decrease. In stage III the gas
production rate falls off gradually. The total
time for virtually complete digestion is
about 60-90 days.
In a continuous operation, the substrate is
fed to the digester continuously, so that,
once the operation is established the rates of
gas production, and input and output are
steady with time (see Fig. 7).
Fig. 7. Schematic process qf a continuous
operation. Once established, the rates qf gas
production and input and output are stead!’ with
Maya farm in the Philippines (Obias 1975)
where 32 batch digesters, based on swine
manures, are in operation. An early
European design (Lessage and Abiet 19S2)
was a batch process, and Mann (1962) has
recorded a number of other simple designs.
The degree of mixing varies considerably
in both batch and continuous processes.The
simplest situation is zero mixing. Some
simple batch fermentors are of this type, but
they are quite inefficient (e.g. the early
versions of the ‘Ducellier’ type were simply
loaded with prerotted manure and allowed
to digest over a long period). Other batch
digesters are mixed by a centrally located
stirrer (for relatively dilute wastes), or in the
Schmidt and Eggersgluss-type design, by
using a pump to circulate the liquid
Mixing reduces stratification and thereby
improves contact between the organisms
and the substrate; it has also been suggested
that mixing increases the rate of
decomposition by releasing small trapped
gas bubbles from the microbial cell matrix
(Finney and Evans 1975) but there is no
direct evidence for this. In the absence of
mixing the material stratifies as sketched in
Fig. 8.
The majority of digester designs are intended for continuous operation. It is often
claimed that continuous digestion is more
efficient (i.e. has higher gas production rates SCUM-{\\\\\\\\\
per unit digester volume) than batch
operations. There is, however, little direct ACTIVE/l
evidence of this, and given the relatively high
reported failure rate of simple digesters this
assertion may be wl’ong.
In fact, one advantage of a batch
operation is that daily attention is not as
crucial as with continuous operation where Fig. 8. In the absence qf mixing, the digester
the maintenance of steady operating condicontents tend to strat[[v.
tions is vitally important. It is possible to
obtain an approximately constant rate of
. gas production by having a small number of
Stirring also breaks the scum layer, which
batch digesters connected to one manifold
if undisturbed can lead to inefficient
serving a central gas storage facility. One of digestion and can even provide a seal on the
the more successful installations is at the digester. If buoyant vegetable matter (e.g.
be digested mixing is vital (cf.
Trends in Technology, Sept. 1974, p. 3).
The simplest types of unstirred
continuous devices can be extremely
inefficient, especially if no attempts are
made to reduce liquid bypassing. With long
retention times (>30 days) and low loading
rates (<1.6 kg VS/mJ digester/day) the
digester, which is little more than a septic
tank, will perform more or less adequately.
Many designs, and more recent designs in
developing countries, are of this type (see
Table 20 for operating data). Operating
efficiencies (i.e. rates of gas production/ unit
volumes of digester) are low (<<I volume/
Despite their simplicity, care must still be
taken if the units are to operate successfully.
Some typical sources of trouble that are
common to many designs (but which can be
by careful
design and
construction) are: blocking of inlet/ outlet
omit all bends,
pipes (remedy:
constrictions); leakages to surrounding land
(remedy: careful construction, favoured by
designs); and gasometer
toppling/jamming (remedy: design guides
correctly) (see e.g. Sathianathan 1976).
If the vessel is stagnant then undigested
substrate (especially leaves and large
particles) will collect, leading to an
interruption of the process. Some designs
are particularly prone to this; for instance,
rectangular designs and possibly the design
proposed by Chan, and Richard (1975). This
exemphfies the possible advantage of batch
processing at simple levels of technology, for
batch digesters can handle a wider range of
substrates (e.g. chopped vegetables) than the
continuous digester (cf. Mann 1962, p. 240).
The gas output from a batch digester can be
made approximately constant either by
operating single units in rotation or by
connecting a series of digesters, each
operated out of phase, to a central
Rectangular digesters of the ‘Chan’ type
probably have a rather nonideal internal
fluid flow pattern. Some designs explicitly
aim to eliminate mixing and Fry’s ( 1974)
design is perhaps the best known example.
Here the attempt is to attain “plug flow”
with (inevitably) a degree of stratification.
These designs are fairly simple to engineer
and manufacture, and what little data there
are suggest that they are not too inefficient.
Fry’s design is primarily for animal
(especially swine) manures but could possibly be adapted for vegetable matter
(especially if the digester is at a slight angle
to the horizontal). Fry (Neur A lchemr) iVeu)sletter no. 3, 1973) discusses how the design
copes with scum accumulation by using a
small drag device. Some gas storage space is
provided above the digesting materials but
an additional gasometer is necessary.
Other simple designs with little or no
agitation appear to operate continuously
problems from accumulating
sludges. The KVIC designs from .India
(Sathianathan 1976) are examples of successful design. Some modifications of the
septic tank, although not efficient as gas
producers, allow regular and continuous
removal of settled sludge, which has not
happened in the septic tank. Figure 9 shows
Fig. 9. A mod$ed ltnhofj’tank
and a second
type qf sludge digester.
Such well-stirred digesters are known 2s
Imhoff tank and another type of
‘high-rate’ digesters and have been successsludge digester.
The second of these designs incorporates fully used with urban sewage (Meynell
a simple sedimentation/ separation volume, 1976), and various animal wastes (Hobson
(this is a feature of many sewagewaste treat- et al. 1974). Their efficiency is seen in their
ment systems and of the tu.o-stage designs ability to handle large input flows (loading
proposed by Ram Bux Singh 1971 and rates up to 10 times those in conventional
digesters can be achieved; typical improveSathianathan 1976).
It is not clear whether the larger KVIC ment is two or three times). (Typical loading
biogas plants - with a dividing wall in the rates: conventional: O-6-1.6kg VS/&/day;
digester (see Fig. 10) -- operate as two-stage high rate: 2.4-6.4 kg VS/ mJ/day.) A qualitytive comparison of the digester types outlined above is given in Table 2 1 (from
Meynell 1976, p. 47).
Quantitative Comparisons of
Digester Efficiencies
Unfortunately there is little reliable data
a serious comparison of digester effiSLURRY OUT
ciencies. Table 20 represents a modest
attempt to collect typical data, but it must be
emphasized that much of the data on
‘simple’ digesters operating in developing
countries is speculative and perhaps hopeful rather than realistic. In many cases it is
impossible to assess how closely plants
approach their design or claimed perforFig. 10. Simplifird diagram of the K VIC digester.
mance. Some of the data are of extremely
doubtful quality (cf. Malynicz’ discussion of
a paper by Chan, Univ. Papua New Guinea
devices; there is some mixing, and it is reThe wide range of operating conditions
ported that performance depends on the and efficiencies is shown by the data in Table
20. In general, retention times of 30 or more
digester depth or the length/depth (L/D)
ratio. Presumably the ‘correct’ choice of days are typical of ‘conventional’ poorly
L/D ration ensures reasonable *mixing (e.g. mixed digesters; 10-20 day retention times
are possible for high rate digesters; and
by gas bubbles).
That mixing improves the rate of Hobson et al. (1975) have shown that stable
digestion has been shown in full-scale and digestion can be achieved with continuously
laboratory-scale studies. Current practice in loaded digesters (on pig waste) at 2-3 days
developed-country applications is toward retention time, although performance
continuous stirring (as opposed to 20 begins to fall off below about 10days. Overminutes or so per day in the Ram Bux Singh all reductions in total solids of approxdesigns). Mixing may be by stirrers or imately 400/o,volatile fatty acides and BOD
agitators (hand/cattle powered for inter- of about 90%, and COD of 40% are typical.
mittent stirring, continuously powered for These figures can be compared with the data
constant stirring), jet pumps (Ram Bux of Chung PO et al. (1975) in an exemplary
Singh 1971), or by pumping the digester study of a simple digester (also using pig
contents in a recycle. This can be coupled waste) (Table 22).
with external heat exchangers (See Meynell
Some data are plotted in Fig. 1IA. But,
there is currently insufficient data to com34
Table 21. Comparison
of different types of digester systems.
Volumes. retention
60 or
Agriculvolumes more
up to 25%
or seasonal, solid
fibrous or
to digest
Gas Degree of
duction required
Irregular Little
and dis- once
contin- started
Messy and
tc, start
or regular
flows. less
fibre content.
5- 15%
and scum
can be
Less than 30-60
5% solids
Not very
More sophisticated Automatic
Unheated None
High Rate Agricultural
More so- Can be
(low organic
High rate
‘Liquid retention time.
60 @
wide range
Fig. I I. A
of operating
conditions exists. Here, reported gas production
plotted against retention time.
rates tire
Cell Holdup
pare the performance of different digesters
and different levels of sophistication.
The wide range in operating conditions is
however shown by Table 20 and Fig. I I B.
Retention times range from as low as 2-3
days (with soluble wastes) to 60-70 days in
the case of a Fry digester, Loading rates
from 0.5 (sewage works) through 2.9 gobar
gas) to as high as I I kg VSmJ/day are
reported. The efficiency of gas production in
digester volumes produced per day varies
from around 0.50 (Imhoff. Gobar Gas, Fry)
to as high as 4 (Rowett Research, Hobson et
The process schemes illustrated in Fig. 7
rely on the establishment in situ of a viable
Bacterial growth rates under anaerobic
conditions are slow and thus, in the case of
continuous operation, it is possible that the
throughput of material could be fast enough
to remove bacteria as quickly as they are
able to form (i.e. ‘wash out’). Even with long
retention times the population densities of
the bacteria may be so low as to limit the
fermentation rate. Various schemes have
been devised to attempt to overcome this by
using two stages so that the two main
bacteriological processes can operate under
more favourable conditions (e.g. Ghosh and
Pohland 1974). So far these ideas have not
been demonstrated with full-scale plants
using animal or vegetable waste feeds.
Another scheme is to settle out some of the
microbial particles (which are attached to
the solid substrates) after the digester, and
then to recycle either a fraction of the sludge
and / or the microorganisms; the net effect is
to increase the microorganism concentration in the digester. This type of ‘contact’
process is illustrated in Fig. 12.
It has been found necessary in some cases
(e.g. meat-packing wastes) to use vacuum
Table 22. Results obtained with a simple digester
fed’ uith pig waste (Table 3. Chung PO et al.
Retention time (day j
I gas kg TS destroyed
1 gas kg VS destroyed
i gas kg COD destroyed
I gas kg BOD dt;troyed
TS reduction (?r)
VS reduction (?c)
COD reduction (%)
BOD reduction (q)
‘Loading 0.768g I’S litre day.
@ j I., ,
:“. .I”
,m ”
degassing to remove dissolved and entrapped gases (McCabe and Eckenfelder
1953, vol II (Steffen A.I.)). Torpey and
Melbinger (1967) found that the required
digester volume was reduced by two-thirds,
indicating that with recycling the potential
for capital savings is substantial. There is
considerable room for study of this type of
development in relation to farm wastes. An
alternative for increasing the microbial
holdup or concentration is the ‘anaerobic
filter’ in which the wastespassthrough a bed
of concentrated solids, perhaps immobilized
on inert particles. This has only been
demonstrated on low strength soluble
wastes, but extension to higher solids concentrations should be possible.
Temperature and its Control
The digester temperature is a key variable
in determining the rate of fermentation. In
rough terms, fermentation starts at about
IO OC, and increases rapidly with increasing
up to the normally
recommended operating temperatures of
30-35 “C. At higher temperatures (50-60 OC)
thermophilic bacteria take over and the
rates are substantially higher (e.g. McCarty
(1964) quotes relative rates of 1.9:1at 55 and
35 “C).
Anaerobic digesters are extremely
sensitive to fluctuations in temperature
(Trevelyan 1975). Catastrophic failures can
result from temperature changes of only :L
few centigrade degrees caused by the addition of cold-water feed, a rapid drop in external temperature, etc. Thermal-stability
control is therefore very important.
The process only generates a little heat,
which is insufficient to keep the temperature at 35 “C if the input and surrounding
temperatures are more than 5 “C less. There
is thus a need for good insulation and (in
colder climates) for some method of heating.
Many simple digesters (Chan, KVIC
(gobar), Fisher, etc.) are, like the septic
tank, poorly insulated and unheated. In
winter or at i Ight, temperatures inevitably
fall and so does production. Some more
sophisticated designs have heating and insulation (cf. Ram Bux Singh 1971). Although some designs rely on capturing
incident solar heat, this is a small input if the
heating surface is a (painted) gasometer. The
heat demand of the process is shown schematically (but not to scale) in Fig. 13, and
possible heat economies, or utilization
methods, are shown in Fig. 14.
Heat lossesfrom a below-ground digester
are not zero even in hot climates. There are
no theoretical
problems in design
Fig. 13. Heat demand C$ digestion process (not to scale).
Fig. 14. t%ssibIe hear economies, or utilization methods, that may be emplo?ved in rhe digestion
calculations: heat transfer coefficients.
thermal properties, etc. are all known sufi
ficiently accurately. It mi;y also be possible
to save on gas use by using solar heaters
either to preheat the feed or to run a simple
hot-water circulation loop as a thermosyphon.
Lesage and Abiet ( 1952) provide a
comprehensive discussion of insulation and
simple heating systems. One should minimize the digester surface area (cylindrical
section being the best basis). Lesage and
Abiet also describe an ingenious heating
system in which the digester is surrounded
by a thick compost layer (the heat liberated
during aerobic composting is high and although ‘low grade’ is well-suited to economizing on thermal energy utilization).
High-rate digesters are almost invariably
heated. In practice, internal coils or an external heat exchanger, through which the
digester contents are pumped, are used. The
latter method is efficient and avoids the
problems of scale formation and local overheating near the internal tubes.
It is generally accepted that operation
under thermophilic conditions is rarely
worthwhile when handling rural waste
inputs. But laboratory results (Cooney and
Wise E975)suggest that the question may be
open, arid Japaneseexperience using soluble
industrial wastes is better under thermophilic rather than mesophilic conditions.
Clearly there is a trade-off between capital
savings and increased efficiency at higher
temperatures, and the costs (in use of gas to
heat the digester) of maintaining the
temperature. ‘The location of the optimum
will depend on local factor prices and should
be examined carefully before automatically
assuming that operation at 35 “C is ‘best.’
Many problems in operating digesters
derive from operation with low surrounding
temperatures. A simple inventory of the heat
loads on the digester (Fig. 13) will give a
useful guide to the most likely sources of
energy conservation.
Batch reactors are also subject to temperature fluctuations but not to regular disturbance from cold feed. The use of simple but
uncontrolled external heating by an aerobic
composting pile may be especially beneficial.
Loading Rates
The loading rates of digesters vary widely
(Table 20). The advantages of operating at
high concentration (loading rates) are to:
minimize digester volumes at the same overall residence time; cut down heat load on
system; and reduce water requirements and
water disposal problems.
A major problem in dealing with very
concentrated wastes (greater than 10%
solids) is handling the very stiff slurry. It
may, however, be possible to operate at
higher input concentrations. Some of the
early batch systems reviewed by Tietjens
(1975) used concentrated loadings and
Wong-Chong (1975) discusses operation at
20% dry solids content. This could give a
volume reduction of up to 50% over a conventiona! digester and reduce the problems
of disposal/ treatment of the supernatant
liquor. Problems were encountered due to
ammonium buildup (toxicity) with high
protein content wastes; clearly a good deal
of work still remains to be done.
There is a difference between the simple
gobar gas deep-well Indian designs and the
rectangular section designs of Chan and
Richard. The Indian plants operate at high
solids loading and use much less water than
the rectangtilar designs. There is also no
separation between the slurry and supernatant liquid. The units seemto be quite well
mixed and have operated for many years
without any sludge buildup. The rectangular
designs (claimed to be designed for detention times of about 1-2days, which seemimpossibly low for proper operation) consume
large quantities of water and are very
troublesome in that there is a need to remove
accumulated sludge at regular intervals.
Unfortunately there are not enough data
available for a comparison of the economics
of high rate/high loading systems with
simple digesters.
ability. Other authors (e.g. Finlay 1976)
mention the possible advantages of adding
small quantities of urea; others recommend
the use of urine. Careful laboratory studies
(c.f. Idnani’s work) show that the effects 3f
mixed substrates are not simple.
The ability of batch reactors to handle
vegetable wastes has been mentioned. However, there is little guidance on which to base
a precise evaluation of the possibilities of
Laboratory and pilot-scale studies have
shown that grass, (Boshoff 1965; Hadjitofi
1976), coffee bean wastes, etc. may be
fermented, yet it is reported (Anon. 1976)
that digesters are not able to handle
bagasse, coir, and insoluble cellulosic
materials. In many situations pretreatment
of the digester feed (chopping, soaking, etc.)
may be necessary, but there is little data
available to enable the degree (or cost) of
these operations to be specified. Ram Bux
Singh ( 197I) mentions the problems of scum
formation and the flotation of buoyant
vegetable matter to the surface (taking with
them attached microorganisms).
0 bviously, the operating conditions
(crudely measured by the carbon/nitrogen
ratio) must be maintained in the desired
range for fermentation to proceed. Assuming that this is so, Sathianathan quotes
production rates of biogas per kilogram dry
matter for a range of substrates (and gives
data for the digestion of mixtures of night
soil/manures, Table 23), These figures show
the relative disadvantage of cow dung
against other substrates.
Feed Composition
Most continuous digesters are designed to
handle animal wastes. Various studies have
been made (Acharya, ldnani and coworkers) to investigate the effects of adding
small quantities of other organic matter in
an attempt to increase the digester efficiency. Sathianathan (1975, p. 38) for
example, implies that it is useful to add
nitrogen (in leguminous plants) to accelerate the fermentation of cow dung,
which may be limited by nitrogen avail40
Table 23. Production rates of bi.ogas per kilogram
of dry matter (from Sathianathan 1976).
Pig manure
Cow manure
Chicken manure
Straw, grass
Green vegetables/wastes
Production rate
(mJ/kg DM)
vertically, so that the pressure inside the
gasholder remains constant (the pressure is
determined by the weight of the holder and
its cross-sectional area). The advantage is
that the gas supply pressure remains
constant (at a few inches water) and gas
supply to the consumer is steady and controllable.
Possible disadvantages are: price (the
gasholder is the most expensive part of a
gobar gas unit (ICAR 1976); corrosion resulting from the acidic conditions and the
H2S inside the digester and corrosion on the
outside lip of the holder (regular painting
and maintenance is need& to remedy this);
and problems with gas offtake - many
systems employ a flexible gas offtake via the
top of the gasholder, but the pipes can crack
and give serio-usoperational problems. One
remed), is to take the gas via a fixed pipe
taking care that liquid does not get into the
pipe. The corrosion problems of a separate
gasholder are less severe than with an
integral holder.
Alternatives have been suggested for the
metal gasholder. One possibility is to use a
wooden/ bamboo framework covered with
plastic. It may also be possible to use ferrocement. Another possibility is the ‘neoprene
bag’ digester under trial in Taiwan (Chung
POet al. 1974).? Le digester is made of 0.55mm hypalon laminated with neoprene and
reinforced with nylon sheet. The digester
and gasholder can be combined in one bag,
so the potential cost of the digester becomes
extremely low.
Digesters can be made from relatively
Engineering Design, Construction
cheap local materials (stone, mortar,
cement), but must be constructed extremely
The broad design categories have been carefully to avoid leakage. One ingenious
discussed. One factor that could dra- possibility is a digester design of Chinese
matically change the economics of biogas origin presently being promoted in Pakistan
generation would be a sharp reduction in the (Appropriate
Technology Development
capital cost of the digester (see Barrett 1978; Division, Govt. of Pakistan 1976). The
1975; Prasad et al. 1974; principle of this unit, which can handle
Sat hianathan 1975).
animal manures and some vegetable wastes
There are two possible digester types: (especially as ‘accelerators’), is shown in Fig.
is entirely
those with an integral gasometer and those 15. The construction
that feed a separate gasholder. In both cases brick/ cement and incorporates no moving
the gasholder itself is usually an inverted parts. It is possible to maintain an approximetal cylinder or ‘box’ that is free to move mately constant gas pressure because inHe also quotes figures that show a decrease in total gas production and gas
production per kilogram of VS added as the
concentration of solids in the feed increases.
It is not clear, however, if these data refer to
constant detention times. Chung PO et al
(1974) show an increase in efficiency as the
retention time increases at constai;:
substrate feed rate (i.e. decreasing inlf:t
concentration): the economic optimum as a
function of concentration, loading, residence time, and substrate material is clearly
well worth further study (seeAppendix 1for
a detailed list of operating data).
Many authors suggest the use of ‘starters’
or ‘seeds’ by adding discharged slurry from
one digester to promote another (e.g.
Sathianathan 1975, p. 41; Alicbusan 1976).
This scheme is similar to the cell-recycle
schemes discussed earlier. For batch and
continuous fermentors improved rates of
gas yield have been reported, which is hardly
surprising because an acclimatized population and partly digested slurry is being recycled. Care must be taken to avoid the
buildup of toxins at high recycle rates.
In view of the interest in using effluents as
a source of nutrients for algae, water
hyacinths, etc. (Prasad et al. 1974), there are
few studies of digester behaviour when the
feedstock is comprised of algae or water
hyacinth. There is an urgent need for further
detailed study of such processes (see for
example Chemical and Engineering News,
22 March 1976, p. 23; Wolverton and
McDonald 1976).
Fig. 15. This ingenious Jigester design of Chinese origin can handle animal manures and some
vegetable )t-astes. It is constructed entirel~~ of brick and cement and incorporates no moving parts.
creasing gas volume in the storage chamber
expels some of the liquid content of the
digester. If the cross-sectional area is large
the change in liquid height and thus gas
pressure is small. The only possible
drawback lies in exposure of the fermentor
contents to the air, but diffusion of oxygen
into the digester slurry is usually negligib!e.
The schematic diagram (Fig. 16) shows
another variation: here water moves in
response to changes in gas volume,
Neither cost nor operating data are as yet
available for this digester. However, a range
of designs covering a scaleof operation from
a single household to a whole community
exist; thus, developments along these lines
seem very promising.
Fig. 16. General arrangement of an enclosed
biogas plant in which the pressure of the biogas
inside the gas storage tank is kept constant by
automatic adjustment of the liquid pressure (A
intake chamber; B fermentation
tank; C gas
storage tank; D outlet chamber; Eliquidpressure
tank: F gas pipe).
All the desigr,: shown operate under a
positive pressure: usually of a few inches
water gauge; though in the case of the
Chinese process the pressure in the gas can
be several feet of water. One consequence is
that the concentration of COZ dissolved il:
the fermentor (and thus pH) will be
decreased. Whether this could depress the
pH so far as to inhibit the methanogenic
bacteria remains to be seen. Other authors
have suggestedthat operation under a slight
vacuum is beneficial (Sat hianathan 1975),
presumably because the dissolved CO? is
lower and the pH higher. This is similar to
the idea of Graef and Andrews (1974) to
scrub CO1 from the gas phase. Operation
under vacuum is a hazardous business
because of the possibility of air leaking into
the digester, and it is not recommended.
There are also (undocumented) reports that
production is inhibited in deep-well
digesters. This could be due to the increase
in pressure at the bottom of a 5-m digester
(the pressure would be about 1.5 atmospheres).
Sathianathan reports that maximum gas
production is obtained (presumably in an
unstirred digester) with diameter to depth
ratios between 0.66 and 1.0. However,
practical digesters often have ratios in the
order of 0.25 (implications for heat loss fro,m
the digester must also be taken into
Effects of sca!e have not been properly
studied. There are undoubtedly strong
economies of scale as far as capital costs are
concerned. There are also certain technical
disadvantages in very small (3-4 m3) plants.
First, heat losses are high, which makes
them uneconomical, and also makes it
difficult to achieve stable temperatures.
Small continuous plants also tend to be
more unstable in operation because slight
errors in feeding are magnified. It is also
difficult to justify the expense of improvements in design (e.g. mixing, gas recirculation, etc., see Malina and Miholits), which
could lead LO substantially improved
efficiencies. However, there may well be
arguments for extremely small (oil-drum
scale) units, to provide small gas outputs
(enough to boil a gallon or two of water per
day), since these can be constructed at close
to zero cost, ,md may well be appropriate for
individual families.
Alternative Treatment
Systems Centred on Biogas
The alternatives within which anaerobic
digestion could be the core are summarized
in Fig. 17. The alternative treatments and
end uses of the gas product from the digestion are summarized in Fig. 18.
It is convenient to separate the variations
associated with handling, treating, and
utilizing the liquid and solid wastes(Fig. 17)
from variations associated with gas
The choice among alternatives depends
on a number of system parameters, among
which the crucial elements are the: quantities and types of waste available; forms of
local social organization; objectives and
defined locally; regulations
governing discharges, etc.; scale of
operation; and opportunity costs of the
inputs - fertilizer, fuel and power, land,
water, labour, capital, etc.
Waste Treatment, Nutrient
An extended background discussion to
this topic is given earlier. The alternatives
sketched in Fig. 17 reflect, in approximate
order of increasing technical complexity and
integration, the ways in which the technical
component may vary.
Alternatives 1 and 2
This is the case in which there is no
processing. Attempts at improving these
options must centre on: controlling nutrient
losses by good farm practice, improving
nutrient value to the land, and using in areas
where contamination is negligible; and
where evaporation and nutrient lossesseem
impossible to control, increasing burning
appliailce efficiency, and controlling
hazard/ contamination by e.g. oven design.
Fig. 17. Summar.r qf alternative treatment g-stems within which anaerobic digestion could be the core.
C02, H2S
Fig. 18. Alternative
treatments and end uses qf the gas.from the digestion process.
Alternative 3
to increase streams A and B in Fig. 20 at the
expense of stream D. There still remains the
question of the efficiency of utilization of the
nutrient stream B, and this depends on the
end use. Stream B could be used as a feed to
a cereal crop.
The potential fuel value of the wastes is
I--* This alter=nativecan be regarded as an
attempt to respond to the drawbacks of alternative 1 by improving the nutrient
quality of the feed to the land (decreasing
stream A in Fig. 19), and using the liquid
waste more efficiently. The public health
aspects are also improved.
Alternative 5
This improves the efficiency of the cyc!e
and meets pollution control standards by
making more efficient use of the liquid and
solid streams; losses between treatment and
cereal production are reduced.
Alternative 4
In its simplest form (waste __t digester
--c slurry and gas) this is the ‘core’ technology that is the main subject of this report.
Here the explicit attempt is to utilize both
the potential nutrient and fuel functions of
the waste (Fig. 20). There should be some
trade-off between the fuel value A and
nutrient value (stream B) of the products but
this has not been seriously studied.
More refined core technologies (e.g. highrate digesters) offer the benefits of higher
efficiencies of conversion to gasand digested
solid. So, too, does the use of pretreated
waste as an input (i.e. inputs that have been
macerated, partially composted, or decomposed). The net effect (at ca:zutab:e cost) is
Alternative 6
This attempts to make a more rational use
of the waste stream. Here the only motivation for including a digester in the cycle is to
obtain energy (as fuel) from the system, for
sewage/ animal wastes can themselves be
used as direct feeds (variant 6b Fig. 17)
(Shaw 1973; McCarry 1971; McGarry et al.
1972). In this system there are sufficient
degrees of freedom to allow sets of objectives to be met more closely than in a
simple once-through, one-unit process
(alternatives 1 or 2). As Eusebio ( 1976)
Fig. 19. Energy flows through alternatives I, 2, and 3 qf Fig. 17.
points out the basic ideas are: to meet
pollution control requirements on the
effluent; and to use the nutrients (especially
organic N) more efficiently. Considering
Fig. 20, for example, the two major lossesor
inefficiencies in the nutrient cycle are: loss
due to seepage, evaporation, etc.; and inefficient take-up by the crop.
Alternative 6 attempts to minimize
seepage loss by using the nitrogen in the
liquid waste stream before it is returned to
the land or discharged to a watercourse. In
practice, and especially in the tropics, the
use of algal ponds is one method of using to
-bestadvantage the local conditions, because
high rates of algal growth are possible with
photosynthesizing algae. The algae can then
be used as a feed supplement for cattle or
fish or fed to the digester. The nitrogencontaining water from the algal pond can
also be used as a source of nutrients for fish
or ducks (Chan 1973).
There is~a danger of thinking that zerocost solutions are possible or, alternatively,
that integrated rural systems offer something for nothing. They do not. Every step in
the processing stream has a degree of inefficiency; as one r, ves along the process
stream so, inevitably, the marginal costs of
recovering nutrients, for example, become
more expensive. These processes must be
evaluated carefully and realistically.
Fig. 20. Alternative 4 (Fig. 17) attempts to utilize both the potential nutrient andfueifunctions
Alternative Gas Handling/End
of the
Uses the gas (i.e. its calorific value and flame
Again, there is a scale of increasing
complexity, sophistication, and cost (Fig.
18). The gas must be combustible (which
implies rejecting gas produced early in the
batch cycle or within a short time after startup of a continuous tank). Under no condition should the gas be burnt until the
process has settled down. The gas stream
leaving the digester contains methane,
carbon dioxide, negligible traces of other
gases(HIS, H2), and is saturated with water.
This gas cools along the pipeline and water
condensesout in the line (just as it condenses
on the inside of the gas holder). It is extremely important, then, that the pipes be at
a slight angle to the horizontal and that
provision be made for draining off the
The gas, even as a 50:50 methane/carbon
dioxide mixture is combustible. Why then
bother to purify the gas of CO*? (The case
for removing H,S is strong because it is so
noxious.) The arguments in favour of
removing CG2 for household use are that
this will improve the burning properties of
temperature). However, reliable burners for
lean methane gases are available and there
seems relatively little incentive for purification.
If the scale of production is sufficient (gas
requirements can be calculated on the basis
of a calorific value of the gas of 18-26J/ cm3
see Tables 24 and 25) then various alternative end uses of the gas are possible.
Table 24. Comparison of the calorific value of
biogas and other fuel gases (Meyneii i975).
Calorific value
Coal gas
Natural gas
38.9-8 1.4
NOTE: Variation depends upon degree of saturation
and percentage com.posi!ion of component gases.
of contamination and sludge formation; no
need to add tetra-ethyl lead; and more
homogeneous mixture conditions in the
Volume of equivalent
The same authors report the effects of
CO,:CH, mixtures on engine performance.
16 m3
Natural gas
The effects of increasing methane content on
24.3 litres
Liquid butane
the specific power output are given in Fig.
19.7 litres
2 1. Whether it is worth removing CO?
17.4 litres
Diesel oil
depends on the trade-off between purification costs and improved performance.
If the engine is stationary (pump set, etc.)
The gas can be used to power internal or used for local travel/ power (e.g. a tractor)
combustion engines, pump sets, etc. (a good there is little incentive to compress the gas
description of running efficiencies is to be unless storage is a problem. If it is intended
found in Sathianathan 1976, p. 72f). The to use the methane as a fuel for a car then it
carbon dioxide acts as a diluent and affects certainly is imperative to minimize storage
the performance. Neyeloff and Gunkel space and handling difficulties by compres(1975) point out the possible advantages of sing the fuel. Methane, unfortunately, does
gaseous fuels: anti-knock qualities; absence not liquefy easily (critical T and P: -82.5 OC,
Table 25. Volume of other fuels with a calorific
value equivalent of 2X m’ of biogas (at 22.2 J :crn’
= 622 Mega J) (Meynell 1975).
RATIO 15: 1
Fig. 21. lY<ftw qf \*aricus CO,:CH, mixtures on engine performance as measured h 19specific. poM-er
output (Nql-ehf, and Gunkel 197.5).
46.0 bar) so that intermediate compression
(perhaps using a simple single-stage
compressor) to about 140 kg/cm2 would be
possible. Meynell (1975) showed that a
cylinder 1.6 m x 0.27 m diameter would
hold about 54 litres, weigh - 60 kg, and
contain the equivalent of about 16 litres of
petrol, i.e. approximately three times that
needed for petrol storage. This assumesthat
the carbon dioxide has been scrubbed from
the gas. If the objective of the biogas is to
provide fuel for transport rather than to
substitute for other fuels, one should instead
consider alcohol production. Trevelyan
( 1975) gives a good description of the possibilities of alcohol as a fuel for combustion
engines. Makhijani and Poole (1975) further
discuss energy/fuel alternatives,
It is unrealistic to consider alcohol for
household fuel, and there may be arguments
in favour of using compressed bottled gas to
serve a community from a central facility.
The costs to evaluate these alternatives can
be calculated on the basis of existing
The possible reuse of carbon dioxide
merits serious consideration. Carbon
dioxide can be regenerated easily from lime
water and could be used as dry ice for local
health service, refrigerators, etc., or possibly
to promote algal growth. These possibilities
have not yet been evaluated seriously.
Technical Parameters
Affecting Digester
In this section the main variables and
measures of digester behaviour are
enumerated, and some attempt is made to
summarize the state of knowledge on the
significance of these variables. Later a
number of the more important areas related
to the assessment,design, and operation of
digesters are reviewed.
in the section dealing with alternative
digester designs. We can separate the major
influencks into three broad groups:
parameters characterizing the mode of
operation; more specific design parameters;
and inputs and possible disturbances. In
view of earlier discussions, many of the
variables are listed with little or no commentary.
Mode of Operation
flow, or
can either be a batch (mixed,
mixed, or unmixed) or continuous
partially mixed, unmixed, plug
anaerobic contact) operation.
Design Parameters (Associated
with Fabrication etc.)
Materials of construction; configuration,
length/ diameter ratio; number of stages;
and heating arrangements are involved.
Processes (especially continuous ones)
depend on various inputs and are subject to
due to
fluctuations in environmental conditions
(temperature, rainfall, etc.) or in feed
materials (composition, quantity, operator
errors, etc.). It is extremely difficult to
monitor or control some of these inputs, and
it is important to know the relative importance of the main inputs and the sensitivity
of the process. It is also necessary to devise
methods to monitor process performance,
detect incipient malfunctions, and to correct
malfunctions (see Table 26).
The acceptable ranges of the majority of
these variables have been discussedin earlier
sections. Others are discussed in more detail
State Variables
The most s.ignificant measurable variables
that reflecr (and influence) digester
behaviour are given in Table 27, along with
Major Influences on Digester
indication of the measurement technique
Many of these parameters and variables
In practice, only a few of the state
have already been introduced and discussed variables are measurabie on a day-to-day
Table 26. Inputs and disturbances of digestion
BOD, COD feed
Feed composition
Feed: physical state,
Feed concentration
Retention time
Loading rate
Bacterial. or seed
Feed temperature
Toxic materials
Nutrient content
C/N ratio
Heat input
Heat losses
Ambient temperature/conditions
‘Sec0ndar.v’ disrurhance.+
Gas composition
Digester temperature
No (but
the fermentation process it is highly interacting in the sense that few if any input or
control variables affect only one measured
or state variable. At different feed compositions, the acclimatized bacterial population
will presumably be different, leading to
different alkalinity and pH conditions and
ultimately to different gas compositions and
production rates. Similarly, a change in feed
composition will trigger changes in all these
variables. The more significant state
variables or ‘indicators’ are discussed below,
State-of-the-Art Review
The important areas relevant to digester
design, operation, and use that are outlined
in this section are: technical feasibility data;
the microbiology/ bacteriology of anaerobic
fermentation; the kinetics of digestion; engineering design aspects; operation and
control of digesters; gas handling and use;
instrumentation for operation and control;
and problems related to ‘peripheral’ technologies - oxidation ponds, etc.
NOTE: A rather subjective opinion has been taken as
to what
is ‘easily’
or ‘cheaply’
categories are thus e.vrreme!r subjective
and inexact.
‘By adding !ime. urea etc. to control pH or stimulate
the operation.
is known of the dynamic
effects of (small)
changes in these variables.
which are, of course. interrelated.
‘Dry solids could probably
be measured
but monitoring
of loading rate is impractical
in rural application.
4By using ‘starters’ or recycles - again. there is little
to draw on.
5At some cost.
hBy adding known quantities
of specified nutrients.
‘Effect as yet unexplored.
“These rrre [email protected] ‘state’ (i.e. dependent
which themselves can affect further behaviour.
basis. Clearly, the operator will recognize
the symptoms of a failing or malfunctioning
digester -- usually, in the first instance,
through a fall in gas production. Some
variables are lessimportant or sensitive than
others. Given the complex microbiology of
Feasibility Data
If a given substrate or substrate mixture is
biodegradable to methane under anaerobic
conditions, there are a series of supplementary questions to be asked: Under what
conditions does the process work (best)?
What is the likely methane yield per unit
weight of substrate? What are the water,
energy, and nutrient requirements? What is
the slurry production rate (and its characteristics)? What will be the measured
operating conditions under normal operation? What size of equipment is needed? It
would be advantageous to be able to answer
all these questions to within a defined degree
of accuracy when assessingthe feasibility of
a proposed project.
There is enough information on digester
operation and sufficiently well-proven basic
information to be able to answer most of
these questions.
of Substrate
There is no completely adequate theory of
biodegradability. Nonetheless there is a con50
Table 27. The most significant measurable variables that reflect (and influence) digester behaviour.
Easily (cheaply)
acceptable range
Toxic materials
Gas production rate
Gas composition
Nutrient levels
IO-60 OC
6.4-7.5 (-4.0)
<-DO mV redox pot.
2000-35000 mg/ litre
0 WJ
depends on species
Cell content
Bacterial population
>50% CH4
depends on nutrient
(Variable + depends
on water regulation)
- (higher the
No (?)
Thermometer etc.
pH meter: litmus
Specific technique
Orsat analysis
Laborious analysis
KOTE: A rather sub.iective opirfon has been taken as to what is ‘easily’ or ‘cheaply’
are thus ta.t-trcww/,l~subJecti\ e and inexact.
The variables noted with a question mark are ones that can be measured relatively
feasible within the context of village technology.
siderable amount of laboratory and
practical information on the anaerobic decomposition of organic materials. Lignins
are degraded slowly if at all; insoluble compounds degrade more slowly than do soluble
ones. In practice, animal and human wastes
can be degraded and many vegetable and
crop residues and wastes from agricultural
processing can be fermented. Good sources
of information
are .Meynell (1976),
Sathianathan (1976), Buswell and Boruff
( 1932). and the series of papers from ICAR.
Mosey ( 1974) covers the processing of
industrial and urban wastes.
The categories
but may not yet be
measure of chemical composition is the
carbon/nitrogen ratio, which gives some
guidance as to the range of feedstuffs that
can be handled. Generally C/ N ratios in the
range lo-30 are recommended, but this
figure is not absolute. Typical C/N ratios of
some feeds are given in Table 28.
Table 28. Typical carbon/ nitrogen ratiosof some
(% dry weight)
The major indicators or determinants are
the concentration and composition of the
feed, the temperature, and the pH. There is
little variation in these parameters among
different feedstuffs, and concentrations of
up to 10% (dry weight) can usually be
handled. It may be necessary to adjust the
composition of the feed to ensure that the
process is not limited by the lack of a particular nutrient. The simplest overall
Night soil
Chicken manure
Horse manure
Hay, grass
Hay, alfalfa
Oat straw
Wheat straw
Temperatures in the range 15-60 OC can
be used, but the temperature is usually
chosen to be 30-35 “C (see earlier comments).
The equilibrium pH, which should be
approximately 7-8 (i.e. slightly alkaline),
’ will be established in a self-regulating way
when the process functions correctly (see
also “Equipment size”).
conditions, raw materials, etc. This variability is shown in the published data and
has occasionally led to outrageous claims
for potential yields. It would be useful to be
able to set bounds on performance to assist
in feasibility studies, check claimed
behaviour, etc.
Likely Yields
Buswell and Mueller (1952) produced a
simplified overall picture of the anaerobic
of a typical substrate
(CnHaOb) to carbon dioxide and methane.
The overall stoichiometry is oversimple (for
example, it neglects cell formation), but it
represents the limit of what could happen.
Their equation is
Yields depend on the detention or batch
time. Typical detention times for continuous
processes in conventional digesters are approximately 30 days, when operation
efficiencies (expressed as percentage
destruction over the digester) are about
50-70%. Under these conditions some
typical gas yields are presented in Table 29.
CnHaOb+ W20)
Table 29. Typical reported yields from anaerobic
Gas Yield
CO, + (ft;-$)
The composition of the gas depends on
the substrate, and, in principle, is
Gas yield
predictable. The total gas yield (CO, + CHJ
can also be calculated, a priori, because 1 kg
matter fed) (O/cmethane)
of carbon in the substra.te will yield l/ 12
kmole gas product. Thus, per kilogram of
Cow dung
carbon decomposed, the yield of gas should
Chicken manure
be (22.4/ 12)m3 gas (measured at STP) or
Pig manure
1.867 m3 gas.
Farm wastes
On this basis, Table 30 has been derived,
Elephant grass
using average carbon contents of the
Chicken manure;
‘paper pulp
materials. The values can be interpreted in
Chicken manure/
two ways: first, as the maximum gas yields
grass clippings
possible per unit mass dry matter fed to the
Sewage sludge
digester; second, as the maximum gas yields
possible per unit quantity of dry matter
Appendix 1 gives a comprehensive listing of destroyed. It is assumed that none of the
experimental data, and Table 20 operating substrate leaves as cellular matter or intermediate volatile acids. It is also assumed
Knowing the quantity of feed available that all the carbon in the feed is susceptible
and its approximate composition gas yields to anaerobic digestion. Actual yields (Table
can be calculated. It is interesting to seehow 29) are, of course, somewhat lower than the
far the data can be based on fundamental calculated values. The tabulated values,
however, give a rough guide to the effect of
substrate on yield.
a typical continuous
Limits on Performance
digester, with feed containing around 2%
The efficiency of a biogas plant depends volatile solids (of which C = 50%), the
on many factors - the design, operating carbon content in the feed will be approxi52
Table 30. Maximum
gas yields (from various sources).
Young grass clippings
Grass clippings
Manure (large)
Oat straw
Wheat straw
Horse manure
Cow manure
Pig manure
Sheep manure
Chicken manure
Steer manure
NOTE: To convert ft3/ lb to m3/ kg multiply
(ft3/ lb DM)
by 0.62.
mately 1% which is approximately 1
kmole/mJ feed. With a loading rate of X kg
VS/ m3/day the maximum gas yield would
thus be 1.867 (X/2) digester volumes/day.
X digester
or, very approximately,
volumes/day. Loading rates vary from 0.8
to 3.5 kg VS,/mj/day, so the maximum gas
yields that can conceivably be produced
range from 0.8 to 3.5 digester volumes/day.
Thus, there is considerable economic
incentive to seek designs that operate at high
loading rates.
It is possible that some of the CO* and
CH4 may be lost in the slurry. For a 50:50
gas mixture at 1 bar at 35 OC, there will be
0.2 litre dissolved CO* and 0.01 iilre CH,
(measured at 0 OC, 1 bar) per litre slurry.
Thus, in a plant operating at a loading rate X
kg/mJ/day and producing X digester
volumes gas/day, the effluent slurry will
carry with it some (0.0 I / 8) digester volumes
of CH, per day, which is negligible in
comparison with the total gas production
Gas Yields and COD Removal
Very often the quality of an effluent is
measured in terms of its COD value. Using
the Buswell and.i.!Mueller stoichiometry, if
the substrate were oxidized ccmpletely it require (n + a/4 - b) kmole oxygen per
kmole subst.tate,
C,H,O, + (n i-i
-‘z b,
nC02 + ; Hz0
Moreover, one kmole substrate digested
anaerobically should yield (n/2 + a/8 - b/4)
kmole methane, so that I m3 methane
produced is equivalent to
digestion process. Again, a simplified
approach yields useful information about
the overall feasibility of the process.
2.85 kg COD C,H,O, (+ H,O) +
n a b
In other words, it is possible to calculate the
(T-8 +z) co, +++i
-4 4 CH,
equivalence between COD removal and
methane generation directly from first If the standard heat of combustion of the
principles. However, because some carbon substrate is (---AH,,) then, because the
must go to producing cells and volatile acids standard heat of combustion of methane is
this is likely to underestimate the COD -88 345 kJ/ kmol, the standard heat of reacreduction.
tion of the fermentation is
For example, Chung PO et al. (1974)
found that the processefficiency varied from
1.2 to 1.4 m3 gas per kg COD (pig swine (-A H,,) + (n/2 + a/8 - b/4) 88 345
digester). which, assuming 50% methane, is
kJ/ kmole substrate
equivalent to 1.7 kg COD/m3 methane.
Water, Energy, Nutrient
Water requirements depend on the concentration of the input stream and the
hydraulic retention time. Typical Indian
designs operate with a water:animal manure
ratio of about I : 1, which is equivalent to a
dry matter concentration in the feed of
about 9%. Other designs (e.g. Richard 1975;
Chan 1973) operate with dilute feeds and
very short residence times (a few days) so
that water consumption rates are extremely
high. If water is relatively expensive or
scarce attempts should be made to
economize or reuse it.
The energy required to maintain stable
operation at a desired temperature can be
calculated with some confidence from basic
principles. The principle sources of heat loss
from the system have been discussed before.
Each term can be calculated directly
good estimates of local
environmental and subsoil conditions
(especially temperature)
are known
(examples will be found in Jewel1 1975).
Other thermodynamic properties are discussed below.
Calorific Values of Substrate and
Product, and Heat of Reaction
(n/2 + a/8 - b/4)
+ 88 345 kJ/ kmole
(neglecting heats of solution).
In general, the process is mildly
exothermic. The higher the proportion of
substrate diverted to carbon dioxide, the
higher the heat release but this is of little
avail in practical circumstances.
On the basis of these figures, one can
calculate heating and insulation requirements. Such calculations can be carried out
from first principles.
In practice, it is usually assumedthat with
organic wastes of natural origin there is no
need to provide additional nutrients (if the
C/N ratio is approximately correct). This
assumption will not be true in the case of
many industrial wastes.
Slurry Production Rate
This will be very nearly equal to the volumetric flow rate of input matter. The slurry
contains all the nitrogen from the input, and
some 50% of the input organic matter.
Measured Operating Conditions
If designed and operated properly, the
temperature should be close to the desired
value. CJasflow rate (and composition) have
McCarty ( 1964) discusses the thermodynamic implications of the anaerobic
above. The PH should settle
down to a value around- 7.2. Trevelyan
(1975) shows that it is possible to calculate
the pH, using basic information on the
buffering capacity of the process. For
example, if protein were fermented at a
concentration of 1 g atom Cllitre, the final
pH would be above 8, and would lead to
problems of ammonia toxicity. The possibilities of toxicity can also be calculated
from first principles (Mosey 1974).
In other words, assuming complete decomposition of the substrate to methane,
carbon dioxide. and water, useful limits can
be put on the digester performance.
Equipment Size
This cannot be calculated from first
principles in any rigorous way for the
substrates of interest becausethis requires a
knowledge of the process kinetics. In
practice, it can be taken that a simple unit
will have a gas production rate of about
0.5- 1 c.igestervolumes per day. Knowing the
gas yield per unit quantity of substrate it is
possible to put an approximate size on the
equipment, but there is a good deal of room
for improvement in this respect.
There are a number of excellent up-todate reviews of the microbiology of
anaerobic fermentation (e.g. Hobson et al.
1974; Trevelyan 1975). Rather than present
a detailed review here, the main features of
the microbiology as presently understood
are discussed, with special reference to the
implications for digester behaviour, future
developments, etc.
Anaerobic microbial metabolisms may
take place whenever the supply of oxygen is
stopped or is so limited that aerobic processesquickly remove the oxygen. Thus, it
takes place below the surface in still waters,
ponds, or lagoons. Generally it is characterized by extremely small energy changes
per unit substrate decomposed (McCarty
1971). Moreover, the overall process can be
approximately divided into three sequential
stages, of which the first two are so in-
timately linked that they are often considered together. Thus, the relatively small
energy yield from the overall conversion is
divided into even smaller packets and distributed among the different bacteria involved. As a result, the production of solids
( i.e. microbial cells) is small, which is
particularly advantageous for waste stabilization and disposal (Pfeffer 1966). In
aerobic processes,on the other hand, energy
changes are large and the quantities of
microbial solid to be disposed of are often
embarrassingly large.
In practice, most organic wastesconsist of
a range of materials (carbohydrates, proteins, lipids, fats, and salts), and the general
scheme (adapted from Hobson et al. 1974)
of their fermentation is shown in Fig. 22.
Ideas based on the behaviour of pure
bacterial cultures in the presence of single
pure substrates are likely to have rather
limited application because a balanced
microbial flora, dependent on the feed, seed,
etc., is essential to the process, One of the
factors determining the composition of the
mixed culture flora (some constituents of
which are essential while others are present
fortuitously) is the energy available from the
biochemical reactions that are spread
among the bacteria. A mixed culture acts
synergistically, that is it can do more than is
estimated by ‘summing’ the effects of pure
cultures acting on single substrates. For
example, the presence of additional substrates or bacterial strains can modify the
process yield (by suppressing or accelerating
the degradation of a particular substrate)
(Hobson et al. 1974, p. 147). An essential
feature of the behaviour of mixed cultures,
which is so far incompletely understood or
studied, lies in the interactions and interdependence among different bacterial
There are also difficulties in transferring
results from one set of anaerobic conditions
to another. For example, the multicomponent stomach of ruminant animals is
among the oldest established anaerobic processes. Optimum conditions in the rumen
are such as to minimize methane production
and in practice the rumen has developed to
SO; NOti
H2. C02. ETHANOL (?)
CH4. COz
Fig. 22. Most organic wastes consist qfa range of materials that can he,fermented.follow~in~ this general
scheme (adaptedjrom Hohson et al. 1974).
that end (nonetheless, Trevelyan quotes
typical methane yields from a COW of
100-500litresl day, or 5- 10% of the calorific
value of the diet!). On the “L‘IbI
,.+I.-- hand,
optimum design of a digester seeks maximum methane production a.ndthe minimum
production of acids and microbial cells.
The first stage of digestion is the hydrolysis (by extracellular enzymes) of complex
organic substances to soluble monomeric or
dimeric compounds (e.g. cellulose, glucose).
A wide range of cellulolytic and other
bacteria have been identified and related to
this stage; their population depends on the
feed composition. The cellulolytic bacteria
are often classified in two groups: the
mesophilic bacteria, which have an
optimum temperature range of about 35-40
“C, and the thermophilic, with an optimum
about 55-60 *C. Another important feature
is that the synergistic (or cooperative) action
of these bacteria can lead to a faster removal
of cellulose than by pure cultures. The
optimum pH range for the bacteria is in the
range pH 5-7.
There is good evidence (for example,
I-Iobson et ai. 1974; Chan 197i; and recent
work in our own laboratories, Hadjitofi
1976) that cellulose hydrolysis is often the
slowest (rate limiting) step in anaerobic
digestion. Hobson also reviews the likely
processes in the breakdown of proteins and
lipids. The simpler compounds resulting
from this first stage of digestion serve two
functions: they contribute to the overall reduction and stabilization of the waste, and
they are vital sources of energy and cell components fo,r the bacteria.
A good deal is known of the main requirements for bacterial growth and function:
that is energy (via organic compounds),
nitrogen, and various trace elements and
salts. The popular literature may be rather
misleading in this respect because a good
deal tends to be made of the nitrogen requirements (as expressed in C/N ratio). This
is not as crucial as is sometimes suggested,
and usually only with rather specialized industrial wastes are these major components
seriously out of balance. There has been
little reported work on the nutrient requirements or supplementation necessary for
wastes typical of developing countries.
In stage two the carbohydrates resulting
from the first stage are fermented to one or
mort of: hydrogen; carbon dioxide; formic,
acetic, propionic, butyric, vaieric, lactic, and
other acids; and simple alcohols. This stage
is the principal source of energy for the
bacteria in the digester; however, the microbiology of the processes involving acidforming bacteria is inco,~~pieteiy
understood. The proportion of the different
products from stage two depends on the
flora present, the substrate composition,
and the environmental conditions. These
will depend to a considerable extent on the
rate of hydrolysis (i.e. stage one). Whether
ail, or most, of the intermediate products
listed above can be attacked directly by
methanogenic bacteria is still a contentious
point. Some authors consider that the only
substrates for the final stage are carbon
dioxide, hydrogen, formic acid, and acetic
acids. Acetate (a salt or ester of acetic acid) is
often the single most important intermediate (Smith and Mah 1966 state that
73% of the methane originated from acetate
in the digestion of sewagesludge). It may be,
as Treveiyan (1975) notes, that the reactions
of other acids are coupled together, so that
the overall picture is of several methane
producers acting serially.
In the third, or methanogenic stage the
soluble products are converted. The energy
involved in these reactions is small and in
consequence the amount of bacterial ceil
formation is also small; on the other hand,
some of the ammonia in the liquid resulting
from stages one and two is utilized by the
methanogenic bacteria. In fact, the
methanogenic bacteria are completely dependent on the primary stage bacteria for
growth. Besides depending on them for the
provision of nitrogen (as ammonia) and the
limited number of substrates that can be
utilized, an oxidation-reduction potential
( Eh) below -330 mV is needed for growth. In
mixed cultures, the metabolic activities of
the facultative anaerobes in the primary
stages serve to reduce the Eh to the required
level; the methanogenic bacteria themselves
cannot produce these reduced conditions.
Only a very few methanogenic bacteria
have been isolated in pure culture. The first
strain considered to be pure (Methanobacterium omelianski) was subsequently
shown to be a symbiotic association of two
species, one producing acetate and hydrogen from ethanol and the other using the
hydrogen to reduce carbon dioxide to
methane. The five known pure strains ail
reduce carbon dioxide by hydrogen to
CO, + 4H2+CHJ + 2H,O
Four can convert formate to methane, but
probably via hydrogen and carbon dioxide.
These two simple reactions are held to be
responsible for ail the methane production
in ruminants (where the turnover time is
about 1 day). This observation can be reconciled with earlier remarks about the importance of acetate as an intermediate because
the growth rate of the acetate-utilizing
bacteria is much slower than that of the
hydrogen-utilizing bacteria. In a digester
with a residence time of a day or two,
acetate-users are bound to be unimportant;
at retention times of, say, 20 days th.epicture
may change considerably, as the bacterial
population itself changes. Thus one must
beware of over-simple statements that the
methanogenic bacteria have slow growth
rates (which ones?).As Hobson et al. (1974)
point out, the question of growth rates in
natural habitats is extremely complex.
Slow-growing bacteria are often very
sensitive to shock changes in operating
conditions, which may weii lead to digester
failure. Indeed, digesters are generally less
able to cope with rapid changes in temperature, feed composition, acidity, etc., than
with slow changes. Presumably this is
related to the doubling time of the bacteria,
which in the case of some methanogenic
strains is of the order of several days. This
suggests that the system will encounter
partial pressure (pC0,) (i.e. volume fraction
of CO? in gas above the fermentor x total
pressure). Typically, at 35 “C the concentration of dis&lved CO, = 0.592 pC0,
litres/ litre water. Thus, gas composition and
operating pressure affect the pH and, ultimately, digester performance. If the acidity
in the digester begins to build up (i.e. pH
falls), the proportion of CO2 in the gas
increases, leading to a further drop in pH. In
other words, the system has a limited degree
of self-regulation, and it is easy to see how
the system becomes unstable.
It is advisable to maintain a moderate
total alkalinity (as CaCO,) (valves of 200035000 mgi litre are usually suggested);at low
values, a slight increase in volatile acid concentration leads to a large drop in pH. On
the other hand, at high values the ammonium ion dissociates (to NHI, and H+). A
number of authors (McCarty 1964; Mosey
1974) consider that toxicity may be due to
free ammonia, and Mosey has reported the
conditions for toxicity (as a function of pH),
with an upper limit of 3000 mg/litre N as
NH3. If the process is thermophilic and the
substrate contains a high proportion of
protein, the system could be self-toxic;
Trevelyan (1975) illustrates this and shows
the value of simple overall calculations in
predicting such conditions. Other forms of
toxicity (e.g. due to the presence of salts or
heavy metals) have been considered and it
may be concluded that reasonable guidelines exist to help the unwary (Mosey 1974).
One important consideration relates to
attempts to control the pH. If the pH falls, it
is often suggested that lime be added;
however, lime reacts with COZ to produce
calcium carbonate and at alkalinities above
about 1000 mg/litre this produces an insoluble deposit. McCarty ( 1964)argues that
sodium bicarbonate is a far better buffer. As
Mosey (1974) notes, lime has the dual disadvantage of removing an important
substrate for the bacteria (C02) and increasing the likelihood of scale formation in
the digester.
pH = 6.3 + log (HCO, -)/dissolved C02)
In the event that the pH needs to be
The concentration of dissolved carbon decreased, hydrochloric acid can be used
dioxide depends on the temperature and (but not sulfuric or nitric acids).
severedifficulties in adapting to changes on
a much shorter time scalethan this. It is this
sensitivity, usually manifested in falling pH
as acids accumulate that gives rise to a
‘stuck’ or ‘slow’ digester, which is a major
problem in continuous fermentation.
(Another sign of incipient failure is either (or
both) a falling gas production rate and/or
an increasing carbon dioxide concentration.)
The methane bacteria are extremely
sensitive to some.factors. They are obligate
anaerobes and their growth is inhibited by
small amounts of oxygen or an oxidizing
agent. They are slow growing (it has been
generally accepted that growth rates from 4
to 10 days are typical, although Ghosh and
Pohland (1974) argue convincingly that
generation times are oithe order of 5 hours),
and are at a disadvantage because of their
low numbers. They are particularly sensitive
to nH. Methane production is satisfactory
between pH 6.6 and 7.6 (Dague 1968), but
methane formation is inhibited below 6.6
and conditions become toxic below 6.2. The
first group of bacteria will continue functioning until pH 4.5, and Bolchardt (197 1)
found that with great care methane formation continued down to such levels. This can
be considered an exception, however.
The pH of the system depends on the rate
at which intermediates are fermented to
methane and carbon dioxide, i.e. on the
alkalinity and volatile acid concentration. It
;Jrobably makes little sense to talk of an
optimum pH because this is the integral
result of the different contributions from the
various reactions; moreover, the optimum
pH levels for the separate stages of the
processescould be different.
The system can usually ‘absorb’ fluctuations in acid or base concentrations because
of the natural buffering provided by the
ammonia and bicarbonate ions. The
buffering provided by the carbon dioxide/
bicarbonate system is represented by
and industrial development. Nutrient requirements and the ways in which different
The main vectors and causative species compete for limited nutrients are
organisms in fecal-borne diseaseswere sum- little understood; at a practical level there is
marized earlier. Some of the available little information on nutrient requirements
results on the kill rates of pathogens during a.nd the returns on them.
anaerobic fermentation are summarized in
Doubtless, as the role of the different
Table 3 1. With digester temperatures above bacterial strains becomes clearer, it will
35 aC and detention times of greater than 14 become possible to devise methods or to
days most vectors will be destroyed. The create environments to give improvements
eggs of the roundworm Ascaris lumbriin efficiency. This will require a good deal of
coides are a major exception.
microbiological and empirical work of the
In Europe, sludges are occasionally highest order.
pasteurized before discharge. This appears
infeasible in most developing countries.
The Rate of Methane Generation
Under normal operating conditions the
public health control aspects of anaerobic
As discussed earlier, the rate of methane
digesters handling human excreta are com- production (a major determinant of the
parable with any other feasible technique. digester volume) depends on a wide range of
However, the behaviour of digesters parameters. Ideally, one would like to have
operating at very low retention times should simple functional relations between the rate
be studied carefully.
of decomposition of substrate per unit
volume (rs) or the rate of methane generaLikely Developments
tion (rn), and the various influential paraA major characteristic of anaerobic diges- meters. Without such relations rational and
tion in practice is that the process depends rigorous design is hardly possible.
Attempts to develop these relations have
on an acclimatized mixed culture of
bacteria. Very little is known of the popu- taken two broad routes: to use empirical relation dynamics or ecology of these cultures lations as a basis for correlating the rate with
(see Hattingh and Toerien 1969), and still the primary variables; and to base the form
lessof ways in which particular strains might of correlation on a more soundly based
be encouraged or suppressed if this were theoretical model. Although the latter
useful in improving process efficiency. The course has much to commend it, it is fraught
room for improvement in speeding up the with difficulties, given the complexity of the
fermentation process and for improving its process. Consider, for example, the
robustness or stability is enormous and it processes involved in the decomposition of
may well be that unless such improvements cow dung. The feed contains a range of orcan be made biogas fermentation will always ganic materials (carbohydrates, lipids,
be at best a marginal contributor to rural proteins) with varying degrees of degrada-
Kill Rates of Pathogens
Table 3 I. Kill rates of pathogens during anaerobic fermentation.
Organism -
Salmonella spp.
Salmonella ryphosa
Myobacterium tuberculosis
Ascaris lumbricoides
Poliovirus- I
Retention time
Kill rate
bility, solubility, etc. These materials are
hydrolyzed, with extracellular enzymes, to a
range of simpler organic compounds, which
in turn decompose to yet simpler intermediates (volatile acids, hydrogen, etc.).
Finally, the met hanogenic bacteria are
responsible for the last stage of the process
- the production of methane and carbon
dioxide, which are subsequently released.
The problem is thus complicated by: the fact
that a large number of chemical species,
enzymes, and bacteria are involved in ways
which are incompletely understood; the fact
that the process involves a complex set of
interacting and possibly competing reactions or physical operations; and the
possible constraints imposed by limiting
reactants, species, or nutrients.
Theories for the fermentation of pure
substrates suggest that the rate of substrate
utilization should follow the form of the
Monod (or Michaelis-Menten) equation
rs =
(Imax Sx
Kj +s
where: S = (limiting) substrate concentration; K, = (half) constant; x = concentration
of bacterial cells; qmas = maximum substrate utilization rate (per unit cell cone).
I’s =-kS
while the rate constant, k, followed an
Arrhenius relation with temperature
k = k, exp (-E/RT)
For low substrate concentrations
wastes): What is the correct ‘measure’ for s?
(the concentration of dry matter, volatile
There is a good deal of evidence to support the hypothesis that the methanogenic
step is the rate-limiting step in the case of
soluble substrates; therefore, one can use a
Michaelis-Menten form of rate equation to
correlate the data. There is no such
unanimity when it comes to the more practically interesting substrates such as grass.
For example, Chan (197 1) fitted data on the
continuous fermentation of cellulose to the
concluding that the rate limiting step was the
hydrolysis stage. Pfeffer ( 1968) concluded
that the rate limiting step was the methanogenie stageat low detention times (< 10days)
and the hydrolysis stage at higher detention
times. Hadjitofi (1976) has recently shown
that the limiting step is probably the
hydrolysis stage at least down to detention
times of about 10 days, and that the rate of
reaction of the substrate is best correlated by
a simple first-order relationship:
In qualitative form this agrees with
Boshoff s ( 1968) results on insoluble
substrates using batch reactions, but
because so little is know,n of the dynamics of
the process it would be premature to attempt
to draw stronger conclusions. It is difficult
to gain an unambiguous measure of the
bacterial cell (or biomass) concentration
with insoluble substrates, but Hadjitofi
found that his results did not follow the dependence on x expected from the MichaelisMenten equation. The reasons for, and
implications of, this finding are not yet fully
established. Of more direct interest than the
rate of substrate removal is the rate of
methane production, but there is agreement
that the two are related.
(Imax sx
and for high concentrations
rs +4max x
When the rate equation is coupled with
material balances on the substrate and
bacterial matter, overall design equations
relating input and output concentrations of
substrate to retention time, etc. are obtained. In the case of simple reactions, such
modeling procedures are reasonably well established and allow one to interpret results
from batch or continuous experiments in a
consistent manner (Atkinson 1975). This is,
however, not so for the complex situations
under consideration here. A further
rrn= Yr,
question exists in the case of insoluble substrates (e.g. grass, manure, vegetable where the coefficient Y depends on the sub60
can be estimated from first principles with a reasonable degreeof accuracy).
Some typical results are given in Appendix
A large number of studies report kinetic
information in a very simplified form - in
terms of the volume of gas produced per
kilogram VS added or destroyed (see
Appendix 1). (Usually, values range from
0.45 to 0.6 rn3/ kg VS added.) A good deal of
care should be taken in using these results
becausemany studies give rather incomplete
information on the experimental conditions. In addition, there is a good deal of
difference between a figure of, say, 0.4
m3jkg VS added, and 0.4 m3/kg VS
destroyed. The enormous range of operating
efficiencies reported earlier should be a
warning to all who choose to oversimplify
digester performance.
There is a pressing need for further
experimental data and interpretation if one
is ever to reach the situation of being able to
evaluate the optimum set of operating
conditions. Because there are differences in
the implications of the kinetic models, it is
not yet possible to describe accurately the
behaviour of a digester over the range of
retention times from the minimum (what is
it for grass, dung?) upward; nor can one
compare quantitatively different digester
configurations. Laboratory, pilot, and fullscale experimental trials are required.
Full details of the process are not completely understood and it may well be that
the problem is so complex as to defy quantitative analysis (Hobson et al. 1974). The
main hope for developing relatively simple
models for the rate process is that one of the
many stagesinvolved in the reaction set is so
slow as to control the overall rate of
A number of hypotheses have postulated
that the rate limiting step is: the initial
hydrolysis step (Chan I97 I; Hadjitofi 1976);
or the methanogenic step (related to the rate
of growth of methanogenic bacteria)
(Pohland and Ghosh 1971; Andrews 1964;
McCarty 1964; Lawrence and McCarty
1969); or the release of carbon dioxide/
methane from the bacterial cellular matrix
(Finney and Evans 1975).
It might be thought that these considerations are excessively academic but they are
not. If, for example, it can be shown that the
rate limiting step is the methanogenic step
then it could be concluded that the size or
physical state of the feed substrate would be
Part of the problem in resolving competing claims is that much of the work so far
reported has used soluble substrates
(glucose, acetic acid), or relatively easily
degradable feeds (sewage sludge). Even
these studies vary considerably in their conclusions: for example, growth times of
methanogenic bacteria are variously estimated at from a few hours to several days.
The limiting mechanisms, even with soluble
pure substrates, are not unequivocally
established. One should not immediately
apply results from pure substrates to the
fermentation of mixed insoluble substrates
because there may well be synergistic or
antagonistic effects to alter the picture.
Finally, much of the data contained in the
literature is of very dubious value because it
is not clear whether the digester ever reached
a true acclimatized steady state (requiring
2-3 residence times, or more if a ‘seed’ is
used). It is thus important to record and
control all the parameters that can affect
digester performance.
Design and Engineering
Earlier discussion described the wide
range of operating conditions and efficiencies that are achieved in biogas units. One
can conclude that reliable, if conservative,
designs exist and it seems very likely that
many digester failures are due to either: poor
design and construction; or poor operating
Design and construction faults include:
poor quality construction; lack of advice/
repair backup; blocked inlet/ outlet pipes (in
bends); sludge buildup in digester; gasholders that cannot be moved/ maintained
easily; tilting/jamming gasholders; possible
washout if flash storms drain through
digester; scum accumulation; water accumulation in gas lines; and design close to
quently ar j regularly the digester can be fed,
the better (Hobson et al. 1975).Air must be
excluded completely.
An excellent guide to the operation and
maintenance of gobar-gas plants has been
produced by Finlay (1976). Apart from
setting out the procedures for normal operation, he also g+“4 an invaiuabie check list
for abnormal conditions.
(In many
conditions the wisest course of action is to
leave well enough alone.) It should be
standard practice to have documentation/
instructions of this type for all operational
plants. Apart from mechanical failures,
blockages, etc., the main indicators of
digester performance are:
Gas production - if this falls steadily, the
digester is failing. On the other hand, there
are inevitably day-to-day variations in this
Table 32. For accuratecomparisonsto be made, parameter.
pH and volatile acids - if the volatile
at least the following measurementsmust be
acids concentration increases, the process is
-amcin danger of becoming unbalanced. Because
of the buffering capacity, these changes will
state variables not be noted immediately in pH changes;
input and output output
Gas rate pH
Dry matter
therefore, pH is no& a very sensitive indiTotal solids
C02:CH, Temperature
Volatile solids
(mixing -power
bicarbonate alkalinity
provides the basic buffering mechanism and
if this capacity is reduced to the point where
Water content
the alkalinity and volatile acids are equivalent, trouble is imminent (alkalinity can be
A major area for improvement is a reduc- measured by titration).
Smell - normally, the odour of the
tion in capital cost as a consequence of one
is not unpleasant, but if conditions
or more of: improved efficiency (higher
are upset, the odour will become unloading, etc.); changes in configuration/
materials of construction; or increased pleasant; however, this is not a very rapid
nor sensitive test.
thermal efficiency/conservation.
The main methods of controlling/ balancIt should also be noted that the teching
the digester are: (1) maintaining the
nology is susceptible to on-line improvement by regular monitoring of the major bacterial population - It is not easy to
variables and by controlled changes in correct for changes in population as exoperating conditions (i.e. evolutionary emplified by, for example, a buildup of
operation, which is a proven method for volatile acids. The pH can be controlled to
some extent (Mosey 1974) by liming or other
handling complex systems).
additions; alternatively, a small increase in
temperature should promote the methOperation and Control
anogenic bacteria at the expense of the acid
Perhaps the single most important rule in formers; (2) [email protected] feeding (already comoperating a digester is to attempt to mented on); (3) mixing and time-for digesmaintain the operating conditions (via the tion; (4) maintaining uniform and steady
input) as steady as possible. The more fre- temperature; and (5) pH control - If it
washout conditions (too low detention
time). Bad operating practices include: lack
of responsible/ knowledgeable people to
care for plant; nonexistent operating
instructions (especially to deal with faults);
insufficient feed material; irregular feed; and
lack of maintenance.
There is a need to establish ‘good practice’
engineering standards in the design,
construction, and operation of plants. This
could lead to substantial immediate improvements all-around: to date, there has
been surprisingly little diffusion of knowhow. Monitoring and collecting data over
reasonable periods of time would allow
accurate comparisons to be made, and
would involve measuring at least the items
shown in Table 32.
One problem alluded to earlier is the posbecomes necessary to add alkali to the
digester to raise the pH, it is important not sibility of water condensing and blocking
to allow the concentration of any cations to the gas line. This can be avoided by a simple
reach toxic levels (McCarty 1964). Chemi- drain.
cals (lime, sodium hydroxide, ammonia)
should always be added gradually.
Methods for removing H,S and CO2 are
well-established (Meynell 1975; SathiaThe range of end uses for biogas have nathan 1975) and relatively cheap. Again,
been discussed. The requirements for the emphasis should be on good practice,
various household uses can be estimated on and the production of standardized, robust,
the basis of known consumption rates, and a simple devices.
typical set of data is given in Table 33.
The main areas for development studies
Generally there are few problems of a re- into the technology are: the use in engines
search and development nature in using (to evaluate performance characteristics
biogas. The dangers and limitations in over the complete range of interest); cheap
handling are well documented, as is burner distribution systems using local materials;
design (see Sathianathan 1975). The main possible end uses of CO,; and development
comment to make is that great care should of cheap, efficient, versatile burners and
be taken not to sacrifice safety or reliability
ovens. All these projects are ones that deso as to produce cheap burners or stoves. On pend on local needs,priorities, and therefore
the other hand, a major source of loss and definition. Similarly, the relative economics
inefficiency in gas utilization is at the of compression, purification, etc. are best
burner, and the design of cheap and efficient handled locally. There are, to repeat, no
burners and stoves should have a high problems in calculating power requirements
and costs.
Gas Storage and Handling
Table 33. Uses of methane and requirements (from Sathianathan
Gasoline engine]
Equivalent to:
(a) Gasoline
(b) Diesel oil
Quantity (m3)
0.36-0.7 1
5-cm diam. burner/h
IO-cm diam. burner/h
I5-cm diam. burner/h
per person/ day
boiling water/litre
1 mantle lamp/h
2 mantle lamps/h
3 mantle lamps/h
flame operated mJ/h
per m3 refrigerated space
mJ/h per m3 incubator space
per kW/h
per kW/h
1.00-I. 18
per litre
per litre
1.1 l-l.39
per litre
per litre
‘at 25% efticiency.
A wide range of measurements can be
made on an operating digester. The list of
measurements given earlier may be taken to
be the minimum necessary for evaluation
and comparison of digesters.
As far as the operator is concerned, however, the object will clearly be to attempt to
achieve stable operation at minimum cost.
There are two ways of approaching this
problem: (1) Attempt to controi, as far as
possible, all inputs to the process (i.e. water,
waste, temperature of feed, etc.). As
emphasized above, the closer is the operation to constant conditions, the better. This
can be achieved by proper measurement of
the quantities involved, etc.; (2) Monitor the
variables most sensitive to digester performance. In practice, the easiest measurement
to take is the temperature. As we have seen,
pH is not very sensitive, but is clearly a key
parameter. A simple indicator test would be
extremely useful. Gas composition can be
measured easily and fairly accurately (e.g.
organic analysis), and the gas production
rate, although perhaps not easy to measure
directly, can be monitored under constant
demand conditions by a skilled operator.
Relatively large-scale operations allow
one to monitor more variables, to control
the inputs more carefully, and to incorporate more control. As noted earlier, there
are very strong arguments in favour of a
batch operation because the process is less
sensitive than a continuous plant.
Aigae and Oxidation Ponds
As discussed earlier, there is a significant
potential in the growth and utilization of
algae. The use of oxidation ponds (without
algal growth) has been discussed, and there
1slittle more to be added here, other than to
warn that in tropical climates it is quite
possible that algal growth will commence.
This may well be a severe embarassment
with respect to the major objective of
controlling and stabilizing the liquid and
solid wastes.
There have been a number of studies of
algal growth rates on various substrates
(sewage, McGarry 1971; pig manure,
Boersma et al. 1975; digester effluents,
Obias 1976). There is little doubt that processes incorporating algal ponds and subsequent fish ponds are potentially viable.
Their economics seem, at present, unclear
(see Sumicad 1975). The major danger at
this stage is to take ‘best condition’ figures
from a laboratory study and base an assessment of what will happen in a practical
situation on this. It is even worse to take
even more optimistic figures as an indication
of what might happen (see Malynizc 1973).
Shaw ( 1973)gives a good survey oft he range
of problems and factors affecting algal
growth: environmental conditions; pond
design; loading; throughput;
supply; and algal population.
Perhaps the single most important
problem in the technology is the collection
and drying of the algae. Unicellular algae are
not easy to filter and collect. There is ciearly
need here for engineering studies (as a
function of algal species) of the methods of
algal separation (floatation? flocculation?
centrifuging?) and their costs, as well as
microbiological/public health studies of the
algae and their consequences.
A typical chain of design calculations
(excluding financial and economic evaluation) would follow the sequence: specify
gas requirement; estimate substrate requirement for range of possible substrates;
estimate water requirement; estimate slurry
production rate, composition, BOD; choose
digester temperature; estimate digester
volume, dimension, requirement, materials
including adaption to local availability; and
estimate energy losses. It is possible to
calculate all material and energy flows
implicit in this chain of calculations to an
accuracy sufficient for preliminary feasibility studies. This is possible for both batch
and continuous processes. The material
requirements for digester and peripheral
equipment manufacture can also be estimated although only approximate design is
Detailed calculation and optimization are
not feasible at this stage; however, some of
the key areas for further technical work are
discussed in the following section.
Research and Development
Priorities: Some Suggestions
with respect to community-scale operations
(see following chapter by Andrew Barnett).
(3) Given the wide range of competing
These comments are tentative suggestions possibilities, work should, where possible,
that attempt to summarize in a general way be placed firmly in a context that recognizes
the existence of alternative solutions or
the most useful lines for future work.
(1) The clearest gains to be made in the systems, and preferably be related to
core technology are in the area of capital systems studies of the problem.
reduction -. especially by seeking methods
(4) A very large proportion of the work
of I educing digester and gas-holder volumes needed is actually develop-ment and the
and/or by suitable choice of construction establishment of good engineering practice
materials (see Appendix 3).
rather than research.
(5) It is taken for granted that all
(2) It seems that, in general terms, both
technical and socioeconomic factors favour laboratory/piiot-scale studies will involve a
larger rather than smaller units (i.e. degree of modeling - that is, relating becommunity level rather than household), haviour to basicsand/ or dealing with results
and a higher priority would thus be justified statistically.
Biogas Technology:
A Social and Economic Assessment
Andrew Barnett
In this chapter the problems of assessing
the worth of the technicaPoptions described
previously are discussed. The evaluation of
biogas systems presents a number of
problems associated with making choices
between production techniques that are
small in scale and are considered appropriate for use in the rural areas of the Third
World. These problems have been discussed
(Stewart 1973), but very few of the empirical
studies of choices in nonagricultural rural
technologies have provided a firm enough
base on which major policy decisions might
be made (Bhalla 1975; Carr 1976). It is
hoped tt?at by UUUl
the social anuII
economic aspects of biogas technology we
will contribute to the more general debate
about technical choice at the village level.
The appraisal of biogas technology
involves establishing the set of alternatives
with which it is to be compared. Biogas
plants have to be seen as one of a number of
possible uses of (rural) resources, but from
the economist’s point of view, these other
resource uses need not necessarily be connected with energy or fertilizer. If this logic
is followed the value of biogas techniques
becomes a function of the genuine alternatives that there are to biogas. Much of this
chapter is therefore devoted to establishing
just what these alternatives might be and
what objectives they are to meet.
Once the importance of alternative re--^ -A:sources uses is understood, the piac;~lcaiiiy
of biogas plants may be expected to vary
between locations; their successwill depend
on the particular circumstances in which the
investment takes place. For example, where
an investment in a village is isolated from the
rest of the economy by difficult communications or the lack of cash, surpluses and
‘In this paper the terms technique and
shortages can build up around the project
technology are used somewhat loosely. The term
very quickly and these can affect the project
biogastechnologyis used to signify a collection either by starving it of inputs or by reducing
of hardware, knowledge, and supporting systems
the value of its output. In a different way, the
associated with the production of methane gas on
availability of an alternative source of
a scale and at a level of sophistication that is at
energy, such as electricity, will vary subleast theoretically possible in the villages of the
Third World. The term includes both those stantially from location to location and this
will also affect the need for (and therefore
plants that currently exist and those that could
exist with the current state of knowledge. The
the value of) a source of energy such as
biogas technology referred to here excludes those
aspects of the production of methane (other than
It is not only the physical environment
knowledge) associated with the larger-scale and
that can affect the worth of biogas; the
more sophisticated processes that are currently
assessment of biogas technology must also
used in developed countries (usually as part of
undertaken in the context of the social
sewage disposal systems). The term biogas
and economic structure in which it is
technique is used to describe a particular piece of
developed and used. The influence of social
hardware and is used to distinguish biogas plants
of differing scales and designs.
structures is therefore the second major
theme ot thus chapter. Different social
groups want different things and they value
them accordingly. Social structures regulate
how much access individuals have to the
capital necessary to use biogas technology,
and influence the distribution of the effects
that biogas plants produce. The importance
of some of these effects is evident when it is
realized that small-scale biogas plants have
had harmful effects on the distribution of
income in certain circumstances.
The third theme of this chapter is the
selection of research priorities. This selection is complicated by the sensitivity of
biogas to changes in particular village
characteristics. In such circumstances, it
may be more important to establish an
appropriate structure and process for
making choices about particular biogas
techniques and for determining research
priorities, than to establish the research
priorities themselves. When the value of any
particular change in biogas plants is likely to
be so influenced by the iocation ofthe plant,
research priorities themselves will vary
between locations- This means that the
social and economic assessment of this
particular technology may be more a matter
of deciding in conjunction with villagers and
engineers which aspects of the technology
might be developed to meet a particular set
of problems, than evaluating a static set of
known techniques for making gas. The
technology is currently undergoing considerable change, but so far only a small
number of known designs for biogas
production have been built and tested. In the
future it must be expected that a new set of
techniques will evolve that will greatly
reduce the ccsts of biogas; costs will be
reduced both by increasing the efficiency of
the plants and more importantly by
reducing capital costs through the use of
new designs and different construction
In these changing circumstances it should
be stressed that the current enthusiasm for
biogas should not be interpreted as meaning
that the technology has already been shown
empirically to be the best meansof satisfying
many of the needs of rural peoples. Nor can
the conclusion be justified that biogas has
no future without more detailed analysis of
the current situation in rural areas and the
characteristics of the new designs.
An attempt is made to direct the reader
throngh a range of problems and errors that
might be expected in the evaluation of
biogas and these are then illustrated by
previously published attempts. No attempt
is made to provide a ‘cookbook’ of evaluation procedures.
The analysis of rural technologies can be
carried out at various levels of sophistication. There is considerable danger, when
attempting to consider a large number of
possible problems in a somewhat abstract
way, of merely adding to the mystification of
the problem and further alienating those
who will be affected by the choice of a
particular technique. This chapter clearly
presupposes a structure in which ‘we’ try to
make decisions about ‘them.’ This is not the
only way. The problem of the choice of a
particular technique may well appear much
simpler to those that are actually affected by
the choice. The problem of development is
more one of getting the social structure right
rather than one of deciding which particular
gadget is to be preferred.
It is assumed that the chapter by Leo Pyle
has been read to gain some understanding of
the biogas production processes.It is further
assumed that the procedures of social costbenefit analysis are known, or can be
learned from the sources quoted in this
The chapter is divided into five sections: a
general approach setting out the framework
in which biogas technology can be analyzed;
a valuation of common inputs and outputs
of biogas plants; five casestudies of attempts
to carry out social and economic evaluations
of biogas plants; the social and economic
determinants of the demand for biogas; and
an approach to research priorities.
The General Approach
The primary need is for a logical and
consistent framework in which the problem
of the evaluation of investments in biogas
can be analyzed and appropriate questions
can be examined. Such a framework should
serve two main purposes: it should make
explicit the assumptions that! :,lve to be
made in the analysis and it should force the
evaluator to examine the full range of
possible alternatives.
The evaluation of biogas investments can
either be approached as a macroproblem,
setting the investments in the wider context
of the economv’s overall fuel and rural
development policies, or it can be treated as
a microproblem. in which the returns to a
single investment are examined at a specific
location and within a specific set of macroconditions. Clearly macro and microlevels
interact, in that the macrodecisions depend
to an extent on information at the microlevel on the viability of the individual investments. A review of the literature on the
evaluation of biogas systems shows that
these microdata are not yet available; this is
either becauseof the imprecise nature of the
data used in the few analysesthat do exist or
because of the difficulties experienced in
successfully running the existing plants. In
addition tn this. the vishility of an investment at the village level (such as biogas)
depends crucially on the particular characteristics of the location of each investment.
It is therefore important to first establish
which village characteristics have the most
influence on the viability of the biogas investment and from this generalize to the
more macrolevel about the distribution of
these characteristics
country. For these reasons it is suggested
that the problem of the evaluation of biogas
should be treated, initially at least, as a
The most widely used logical framework
for the evaluation of microinvestment
decisions is social cost-benefit analysis
(SCBA). Considerable advances have been
made in recent years in refining the logical
consistency of these appraisal systems and it
is recommended that at least one of these
systems is used for the evaluation of biogas.
The most recent manual has been produced
by the World Bank (Squire and van der Tak
1975). This is a particularly clear, if
somewhat condensed, version of the
approach (seealso UNIDO 1972; Little and
Mirrlees 1974; Irvin 1976).
The approach adopted by all the recent
manuals df SCBA is predominantly economic, but this is only one of a number of
possible dimensions against which the
impact of an investment can be judged.
What is suggested,therefore, is that a ‘softer’
form of SCBA be adopted, using the
framework set out in the various manuals as
a guide, but taking into account a greater
range of possible social and environmental
effects and attempting to give sufficient
weight to those effects that cannot be
precisely measured on a scale such as that
provided by money values. This ‘softness’
does not necessarily imply a weakening of
the overall logic of the analysis but reflects
the reality that many important events are
neither economic nor can they be precisely
The cost-benefit approach attempts to
determine the physical relationship between
inputs and outputs associated with a
particular investment and then places
economic and social values on these events.
I t is in essencea process for weighing the
various characteristics of alternative courses
of action - and as such is the decision
process of everyday life.
The Political Framework
Investment decisions and the assessment
of the costs and benefits that result from the
investment are primarily political decisions
influenced by two factors: the nature of the
group making the decision, and the social
and economic structure of the society. It is a
political process because the decisionmakers are forced to make an explicit choice
about an investment that helps one group of
people rather than another. The position of
the decision-makers in the social structure
will strongly influence their views on how
investment should be used to further
development objectives. These views may
differ drastically from the views of the
people that will be affected by the investment, and more importantly may fail to take
into consideration the way in which the
existing structure wili affect the actual
distribution of costs and benefits.
The social structure influences the
distribution of costs and benefits among
social groups in a number of ways: for
instance, where some of the factors of production are monopolized by a particular
of a new
group, the introduction
technology, however beneficial to the individual owner, may merely raise the
amount of surplus that can be expropriated
by the monopolists. Conversely, investments in new technologies can a.lso be used
to alte:: the existing distribution of power by
helping to break dependent relations.
All too often the choice of techniques and
particularly the advocation of ‘anpropriate’
technologies is abstracted from ie realities
of political and social structure (Cooper
1973). The introduction of small-scale
biogas plants, for instance, may have the
effect of fostering individual actions rather
than cooperative action; it may satisfy the
needs of only those who can afford plants,
thus reducing the pressure for more redistributive solutions; it may increase the
value of inputs such as cow dung and subsequently deprive the poorer sections of
society of its use. Similar problems were well
documented following the introduction of
high-yielding varieties of food grains during
the so-called Green Revolution in India
(Griffin 1972).
It is hoped that the approach to the
evaluation of biogas plants suggested here
will help to make explicit the choices that
have to-be made and the poiitical nature o
these choices.
The Examination of Alternatives
The evaluation of the impact of an investment is, in principle, the comparison of the
situation ‘with the investment’ and the situation ‘with the next best alternative investment.’ This concept of ‘opportunity cost’ is
cruciai to the approach of social cost-benefit
analysis and is described in detail later.
At this stage the important point is to
decide what realistic alternatives there are to
the investment in biogas: what is to be compared with the biogas system?
In a microanalysis such as the one
proposed here, the ‘next best alternative investment’ is likely to be another investment
in the village: such an investment might be in
an irrigation pump, in land drainage, in
paying’ off previous debts, in buying new
land etc. From the standpoint of the whole
economy the ‘next best alternative investment’ might encompass a wider range of
activities including the production of fuels
and fertilizer by other (possibly larger scale)
processes. The set of possible alternatives
can obviously be very large and depends
both on who controls the available investment resources and on the characteristics of
the economy.
Some evaluations of biogas take a much
narrower view than the one implied by
SCBA and consider only a number of different techniques for producing methane by
anaerobic digestion. It is quite clear that
there are many possible designs and scales
for biogas production that have not yet been
built or tested. The appraisal of these
designs is an important task, but it is equally
important to consider how much the
products (methane gas and slurry) are required and whether this is the best possible
use of the resources involved.
An improvement on this narrow approach
is to evaluate the techniques for methane
production in relation to the existing means
of satisfying the needs for fuel and fertilizer
(this is the approach adopted by the Indian
Council for Agricultural Research, Government of India 1976). But this does not
answer the more fundamental question of
whether investment in methane is the best
use of the available resources.
To expand the range of options that might
be compared with biogas a useful approach
is to consider the range of functions that the
biogas plant might achieve. Biogas is advocated on a number of grounds each of
which can be achieved by other more or less
‘good’ techniques. Some of the options are
discussed below.
(1) Biogas can be seen as providing a fuel
and can therefore be evaluated in terms of its
ability to meet some of the villages’ energy
needs in comparison with other sources of
energy. The comparison might include: (a)
firewood, which in many areas is becoming 1975, p. 158) and animal (usually pig)
increasingly time-consuming to collect manure (Solley and Yarrow 1975); here the
becauseof its scarcityJ(Makhijani 1976);(b) alternatives for comparison might again be
electricity, which is not usually used for cornposting processesor more conventional
cooking but can have very low marginal waste disposal through lagoons and septic
costs where there is surplus capacity in tanks.
(5) Investment in biogas might alterexisting generating capacity and the village
is close to existing power transmission lines natively be seen as a means of utilizing
(Prasad et al. 1974); and (c) at a different village resources that are currently going to
level, the comparison can be legitimately ‘waste’ (or at least are being underutilized).
made with alternative practices associated This way of looking at the problems gives
with the use of existing fuels; for instance, certain insights and i,Q-nnother formulation
considerably less wood might be consumed of the more general economic problem of
if the design of stoves were made more the optimum use of all resources.
efficient (Makhijani 1976. p. 27).
It is essential that the researcher specifies
(2) Biogas plants provide fertilizer in the precisely which options are to be considered
form of the spent slurry and might therefore and justifies the exclusion of others. It
be compared with aerobic cornposting would certainly be unfortunate if the
processes or the provision of chemical impetus that now surrounds biogas were
fertilizer. (Disney 1976 compares biogas used to divert attention from the modification and utilization of the mass of other
with urea production in India.)
.(3) Biogas has been advocated as a applications of technology at the village
substitute for other activities that are con- level that would appear to produce at least
sidered harmful or wasteful, such as the as promising returns as biogas.
burning of dung and wood. The dung might
However, the fact that biogas is currently
be better used as fertilizer, and the burning fashionable and is being promoted in a
of wood has resulted in deforestation of number of countries
is sufficient
some areas, which in turn, has led to erosion justification to modify what would appear,
and flooding (Republic of Korea 1975).4The at the moment at least, to be a rather excomparison here might be with cornposting pensive and unreliable set of techniques. But
or the growing of trees and other plants this concern must not overshadow the more
(such :aswater hyacinth) for fuel (Makhijani
general search for the ‘best’ use of village
1975, p. 114-124).
resources; it does not matter how good the
(4) Another function for the biogas plant methods of social and economic evaluation
is the safe disposal of human (Sathianathan are if they are applied to the wrong set of
Once the logical framework has been
and the broad set of alternatives
3Makhijani 1976, p. 26; wood, straw etc. have
been a traditional source of fuel in rural areas in established, the next stage can be broken
many developing countries. Makhijani suggests down into two tasks: the identification and
estimation of the physical quantities
that there is considerable evidence that collecting
wood for fuel now requires an increasing amount
involved as inputs to and outputs from the
of labour time. In rural India, anywhere from 50 project; and the placing of social and
to 200 or more days of work per family are now
economic values on these quantities. This
required to ensure adequate fuel supplies.
division is useful in terms of exposition
because it helps to make explicit the various
assumptions that have to be made, and be4Republic of Korea 1975, p. 2. Forest products
cause many of the errors are made in the
and straw account for 92Yc of Korea’s rural fuel
estimation of the inputs and outputs rather
needs. Soil erosion,
than in the process of evaluation, which is
denudation of hillsides and the reduction of soil
fertility associated with burning straw, are a commonly considered to be most difficult
major problem in Korea.
part of the analysis.
extent that different social groups consider
certain impacts more worthy of estimation
than others, but the problem of specification
As an initial stage the inputs and outputs mentioned here is a question of the kinds of
directly connected with the investment have impacts that are to be considered (and
to be listed and the relationships among measured in such physical terms as kilothem established. The establishment of these grams, number of people affected etc.)
relationships is the work of engineers,” but it
The kinds of impacts that might be
is necessaryto stress here that a large source included in the social and environmental
of uncertainty and error in the analysis of dimensions were discussed at a recent conbiogas techniques arises in the estimation of ference of the United Nations Environment
these relationships under normal working
Programme ( 1976). The social and environconditions. The evidence in the literature is mental dimension? were each broken down
often unclear and a large range of values can into two strands. The social dimension was
be found for most of the essential related to impacts associated either with
input/ output relations (seechapter by Pyle). structural development or cultural comBiogas plants, as currently designed and patibility; the environmental impacts were
used do not appear to be very reliable (see related to either the quality of human life or
Table 20), and the actual quantity of gas ecological balances. Structural development
produced would appear to be considerably discriminates among the impacts of alterless than the ‘design capacity’ sometimes native investments on the basis of the degree
used in the cost-benefit analyses.
to which they promote self-reliance, involve
public participation in decision-making and
implementation, or reduce dependence at
Three Dimensions
individual, village, and national levels.
The impact of the biogas investment and Other impacts associareci with structural
its alternatives can be compared against a development are the reduction in inequalinumber of possible ‘dimensions.’ Usually, ties among groups and individuals in terms
the only dimension that is considered of the distribution of needs such as coninvolves the direct technical inputs and sumption, education, work, and power.
outputs of the biogas plant and the
subsequent economic analysis of these investment alternatives in terms of whether
quantities. It is advocated that two other they build on the endogenous traditions of
dimensions be considered to give a broader the society or whether they run counter to
view of the impacts of the investment. In them.
addition to the technical/economic dimenThe quality of human life aspects of the
sion, consideration might be given to environmental dimension incorporate those
impacts along the social and environmental characteristics of projects that satisfy such
dimensions. The content of these two addi- needs as the need for creativity and the need
tional dimensions must be specified in some for local initiatives. Additionally, it lays
detail and relevant ‘indicators’ of impacts emphasis on the replacing of one type of
defined. The problems of specification are activity (such as boring, hard, dirty,
quite distinct from the problems associated repetitive, degrading work) with another.
with the social and economic valuation of The impacts related to ecological balances
impacts once they have been established in include the extent to which nonrenewable
physical terms. Values are involved to the natural resources are used, the extent of
pollution, etc.
A number of these impacts have been
SMoulik and Srivastava (1975) suggested that
in previuus analyses of the
“about 71$%~ of plant owners experienced
dimension. Impacts
technical problems. A large number of plants in technical/economic
in this way are
the sample were closed due to these problems” (p.
effects, effects on the
Physical Input and Output
distribution of income, and the costs of
pollution. The argument here is that there is
considerable value in not trying to load too
much on the economic analysis, particularly
at the second stage where the economic and
values are ascribed.
incorporation of many separate impacts
into the economic dimension tends to shift
the balance of power over decision-making
in favour of the project appraiser and away
from those who will be affected by the
investment. Clear problems of employment
become reduced to esoteric discussions of
shadow wage rates (Squire and van der Tak
1975, p 29). The separation of impacts into
three ctimensions reduces the number of
assumptions that have to be made by the
project analyst and provides a useful
checklist of impacts that must be examined
for each investment alternative.
A large number of other impacts might
just as well have been included instead of the
brief list suggested in the UNEP paper. A
number of alternative
along the environmental
dimension can be found in the work of the
International Council of Scientific Unions
(1975) (SCOPE), and Marstrand et al.
(1974) (SPRU). An attempt to incorporate
more of the social dimension is to be found
in Szekely ( 1975).
The point is that a wider range of physical
impacts than is usually implied by economic
analysis should be considered in the
evaluation of new production techniques.
The problem of integrating these impacts
into the decision-making process is not
necessarily simple and will be discussed
technical analysis. But other impacts have to
be measured on less powerful scales where the impact can only be said to be
greater or less than another or, at an even
lower level, where the impact can merely be
seen to exist or not to exist. Many choices in
everyday life are made on the basis of data
that cannot be precisely measured, but there
is a tendency in much (economic) project
analysis to pay more attention to precisely
quantified data to the exclusion of all else.
This bias in quantification sets up a
corresponding bias when social and
economic values are placed on these impacts
- higher value is placed on the more
measurable aspects of the problem. The
procedure suggested in the section entitled
“valuation” allows for the inclusion of data
that can only be quantified on less powerful
The emphasis so far has been on trying to
expand the range of problems that might be
examined and the factors that might be
taken into account. At some point, however,
limits have to be placed on the problem to
define its boundaries.
Any investment takes place as part of a
subsystem of events, and these subsystems
connect with other subsystems. In the case
of village technologies, these systems are
often compact and the alteration of one part
of the system considerably affects another.
This compactness poses particular problems
in the appraisal of rural projects if the
project is partially isolated from a ‘market’
that can easily dispose of surplus outputs
that are created and supply needed scarce
inputs. This isolation stems from the costs of
transport and travel and from the relatively
The selection of impact indicators is large proportion of rural activities that are
clearly a crucial stage in the appraisal not monetized. In such situations the usual
pritcess, and it is apparent that of the many assumptions of microproject appraisal that
possible indicators there is considerable the project is ‘marginal’ to the rest of the
variation in the extent to which they can be economy cannot be made, in the sensethat
The highest
of its existence will not have an effect on prices
measurement (cardinal scales) in which it is (particularly ‘world prices’) (Squire and van
possible to say that an impact is so many der Tak 1975, p. 32). While such
times greater or smaller than another, can be interrelatedness is a problem for SCB ‘\, it
ascribed to many of the impacts and these may be of considerable benefit to Jr-al
are often the core of the economic/
peoples as the by-products of one village
become the very cheap inputs of
anot her.
The usefulnessof the various appraisals of
biogas largely hinges on the extent to which
these important effects within the system
have been included. Apart from the impact
on the social and environmental
dimensions, wl- :ch are often not considered,
the analyses often disregard those effects
that take place in part of the system not
immediately surrounding the project. Such
effects have been called ‘second round’ or
‘linkage’ effects. An example of these types
of‘ effects occurs when the output of the
PI eject under consideration increases the
supply of a particular
sufficiently to reduce its price within the
locality. Other activities that in turn use this
commodity as an input will be affected
’ beneficially by this reduction in price. These
benefits, which occur in the ‘second’ round
of transactions within the system, or which
induce additional
‘linked’ investment
further down the chain of the system, can
legitimately be included as benefits to the
project being appraised. Similar effects can
also arise in connection with the project’s
need for inputs. An increase in the demand
for cow dung, which might result from the
introduction of a biogas plant, can have the
second round effect of reducing the
availability of cow dung to other existing
users further ‘up’ the chain. lmprovements
in the efficiency with which wood is burned
may not only have beneficial ecological
effects in terms of deforestation and erosion,
but it can also have harmful second round
effects on those people whose sole source of
income is the collection and sale of firewood
(such a situation might arise with the woudcollecting ‘tribals’ of western Maharashtra
in India).
The question arises as to how many of
these effects it is necessary(or cost-effective)
to examine. In the evaluation of investments
within a single commercial venture, it is
often the case that (good or bad) effects that
do not affect the firm itself, because they do
not take place within the physical confines
of the enterprise, need not be considered.
-Jheseeffects are described as ‘externalities’
because they are external to the enterprise.
In such analyses the limit to the number of
effects that must be examined is clearly
defined and is largely reflected by the items
in the firm’s accounts. In social cost-benefit
analysis no such clear limit (either ot
geography or system) exists and there
canno! be a clear set of ‘external’ effects.
By their nature thore cannot be a firm set
of rules to determine which effects should be
considered in social cost-benefit analysis.
The choice of a cut-off point, beyond which
effects need not be considered, is a matter of
judgement and experience; judgement about
the likely size of possible effects --~ the
smaller the effect the less its exciusion
matters from the microanalysis --- and
experience of the kinds of effects that have
been encountered
with previous
investments. The choice and specification of
the different dimensions along which
projects are to be evaluated have a certain
role in this respect because they draw the
attention of the project analyst to a range of
possible effects. The application of social
cost-benefit analysis in villages would seem
to be less a problem of the rigorous
application of a set of economic techniques,
but more one of understanding the
variability of the context in which the
investment is taking place.
Three points emerge from this discussion
about the appraisal of biogas investments.
First, it is vitally important to examine ihe
appropriate system surrounding the project.
This includes examining the chain along
which inputs will actually arrive at the
project and along which the outputs will
proceed after it. It also involves ensuring
that similar ‘levels’ of system are being
in the examination
alternatives. A common error is of the type
in which the cost of fertilizer or fuel from
biogas is compared with the cost of similar
of larger-scale
techniques. However, no account is taken of
the costs associated with the delivery of the
output of the large-scale unit to the point of
consumption (the village);6 these are the
transmission costs of electricity or the
delivery cost of fertilizer or kerosene. It may
be useful to use flow charts to represent the
systems being compared.
Second, as the choice is always between
alternatives, it may be that the second round
effects will be of a similar form and size
whichever investment is made in a specific
location. To the extent that this is true these
effects can be excluded from the analysis
without affecting the decision as to the best
investment. However, in practice this logical
nicety is more usually the refuge of the selfjustifying analyst!
Third, the importance of these second
round effects will depend on how large the
investment is. Even if the investment is quite
small, the introduction of many such
investments (as with the widespread
introduction of biogas plants) may well add
up to the sort of nonmarginal change that
can alter many of the parameters of even a
highly responsive ‘market’ system and make
microproject appraisal difficult to apply.
This raisesthe question of how reliable a few
micro cost-benefit studies of biogas are
likely to be for deciding on such
nonmarginal changes as the introduction of
many thousands of biogas plants in high
concentrations. The ‘whole’ is obviously
going to be greater than the sum of the
Again, a compromise must be reached
between the ease of analysis and the extent
to which the analysis describes the complex
reality. What can be said is that the
compromise should not favour economic
rather that the social and environmental
dimensions, nor should it favour the
in preference
to the
Reliability of the System
The estimation of the physical input/
output relations associated with biogas
investment must also take into account the
reliability of the system and its robustness. A
number of factors which influence the
production of gas are discussed in the
chapter ’ by Pyle, and include air
temperature, shock loading, poisons, and
variations in inputs. Where possible,
‘expected’ values should be used in which
each possible level of output is weighted by
the probability of its occurrence as described
by Squire and van der Tak (1975, p. 44).
More generally, it is important to consider
whether changes in design or operating
procedure, which increase the amount of gas
produced, alter the reliability of the system.
It is ,,itely that a tradeoff exists between
increases in gas output volumes and the
reliability of the system.
The lack of reliability of the system in
providing adequate supplies of fuel has been
quoted as a major reason for the nonacceptance of methane generators.7 Where
this problem is important, the physical
resources required to provide a backup
source of fuel (and cooking stoves etc.)
should be included as additional costs for
the biogas system.
The various inputs and outputs identified
on the technical, social, and environmental
dimensions have to be valued. Valuation is
made explicitly and implicitly in terms of a
set of objectives, and these objectives are
likely to vary from person to person and
among different social groups. The sorts of
groups whose objectives
might be
considered in the evaluation of village
technologies are: (1) the government (as a
proxy for society?); (2) the owners of the
investment - the farmers; (3) the village as a
whole; and (4) the various elements within
the village: large farmers, other landowners,
landless people, women, etc. The actual
choice will depend on the local
circumstances and the distribution of the
effects of the investment among these
“This is particularly true when the only cost
‘Singh (1976) contends that operational
allocated for a competing product is its unit difficulties are the single most important negative
,production cast. See for instance, Government of
India (ICAR) 1976, p. 28, where no cost of
transporting keroseneor fertilizer is included.
factor militating against the acceptance of biogas
by individual families. See also Moulik and
Srivastava (1975).
Each of these groups might be expected to
consider different things important and to
value them differently. The most widespread
svstem of valuation ‘might be termed
financial anaiysis, but it is clearly only one of
many possible systems. Financial analysis is
usually carried out using the existing set cf
market prices to value impacts, and the
ob-jective chosen is usually related to the
maximization of profits over some specified
time or the maintenance of some minimum
acceptable income level. Such analysis is
normall!. conducted in terms of the effects
on the iri\,cstor alone.
Social cost-benefit analysir. ii:.ovides an
alternative system of values and has usually
been carried out from the point of view of
the costs and benefits to government. The
objectives have been largely economic,
involving some combination of such
considerations as the rate of growth of the
economy (and therecore the amount of
investable surplus arising from the
investment and the level of employment)
and the distribution of consumption among
various groups. Such analysis takes as a
starting point that mar-ket prices do not
necessarily reflect the values of society.
The essential problem of va!uation is to
find some way of combining the values
placed on the \.arious impacts so as to reduce
them to some manageable aggregate. In this
u.ay, the ‘costs’ can be set against the
‘benefits’ by the decision-makers. The use of
money values as a basis on which to make
these valuations is attractive: rhis is the basis
on which many choices in everyday life are
made and it is widely understood. It is
normal to take market prices as a starting
point -for the aggregation of impacts into
costs and benefits, but two fundamental
reservations must be made.
Market prices do not necessarily reflect
the valuations that a particular group might
place on project impacts. The reasons for
this are discussed at great length in the
literature (Squire and van der Tak 1975, p.
IS-18), but it is clear that market prices
reflect the current distribution of income
and production- which may well be
considered to be unsatisfactory. Prices are
also the result of so-called ‘market
imperfections’ such as monopoly elements,
lack of knowledge by buyers and sellers,
transportation costs etc. For these reasons
adjustments have to be made in the analysis
to market-price valuations. These adjusted
prices are often termed ‘shadow prices’ or
‘accounting prices’ and have been defined as
“the value of the contribution to the
country’s basic socio-economic objectives
made by any marginal change in the
availability of commodities or factors of
production” (Squire and van der Tak 1975,
p. 26). Such a definition need not necessarily
relate to the ‘country’s objectives’ but can
relate to any specified group within the
country. The precise procedures for
constructing shadow prices have now
become well established, and they are
described in considerable detail in the
project appraisal manual mentioned in the
first section of this paper.
It should be stressed, however, that all the
systems described in SCBA manuals utilize
a government objective related to the rate of
growth of the economy. This objective is
sometimes modified by consideration of the
need to trade off economic growth for
greater employment or for a more- equal
distribution of consumption. Costs and
benefits are therefore calculated in terms of
their contribution to, or savings of,
consumption (as in the UNIDO Guidelines)
or investment (as in Little and Mirrlees, and
Squire and van der Tak). A number of ways
of modifying shadow prices to reflect other
subobjectives are discussed in the literature;
for instance, the placing of higher value on
consumption that goes to satisfy the basic
human needs of underprivileged groups.
Certain social and environmental effects can
also be e%asily
incorporated in this analysis as
costs or benefits: pollution damage as a cost,
and increased consumption of luxury items
valued at zero, etc. But, as argued above,
there is considerable value in keeping these
subobjectives separate.
A number of impacts are not usefully
valued in ‘money’ terms, and certain
objectives are more easily understood when
not aggregated with economic objectives.
The impacts of alternative investments can
be displayed in relation to a number of
with no attempt made to
aggregate the impacts into a single index of
net benefit. It should be possible to
construct a matrix showing the impacts in
terms of various criteria for all the
investments considered (see Fig. 23).
Each cell of the matrix contains a gradation
goodineutral/ bad ( +. 0, - ) or the
appropriate score on a cardinal scale, such
as rate of return, net present value, etc. If
need be, some of the criteria can be given
infinite weights such that if the project has a
negative impact on a particular criterion,
this outweighs any possible advantages on
all the other criteria. Such an infinite
weighting might be given to certain
pollution standards or to the worsening of
income distribution.
The problem of adopting such a matrix
displaying the possible effects of investment
choices is that difficulties arise in trying to
trade off one criterion against another. For
instance, how is the choice to be made
between one investment that scores high on
one criterion and low on another and an
alternative investment that has exactly the
opposite characteristics? First, it is rarely the
case that decision-makers can specify in
advance the precise (mathematical) weights
that they would give to different criteria
(such weights can be estimated in retrospect
on the basis of previous decisions, but there
is no reason why the decision-makers should
consistently stick to such a valuation,
UNIDO 1972,Chapter 18). Second, the data
can be presented in such a way that they
show how much of the score on one criterion
has to be given up to gain increases in
another; for instance, the reduction in the
investable surplus generated by the project
that would have to be given up to increase
the level of employment.
Again no hard and fast rule can be
adopted. It seems that in many cases the
data are sufficient, when presented in the
matrix form, for decisions to be made -~ or
at least for areas to be identified in which
greater elaboration is required. This is
particularly true when the number of
options and the number of criteria are
relatively small. A matrix of ten investment
alternatives and up to five criteria would be
a considerable improvement on the level of
information on which many decisions are
currently based; yet, it would be sufficiently
small for the comparisons to be made by eye.
A certain degree of aggregation cannot be
avoided and a choice has to be made
between the need to present the largest
amount of information that can be usefully
handled, and the need not to bias the result
by attaching inappropriate weights to the
A number
environmental impacts for instance might
usefully be aggregated into a single scale
ranging from +5 to -5.
Possible criteria that might be used in a
decision matrix might include: the financial
(money) returns of the project to particular
groups (the owner, poor sections etc); the
net present value of the social returns (from
the SCBA) to the government and other
groups; an indicator of the relative effects of
the projects on the distribution of income;
an indicator of the employment generated
per unit of capital invested (or per unit of
some other scarce resource); an indicator of
the damage to the environment, if any; and
an indicator of the contribution to the
village’s self-reliance. The selection of the
criteria will depend on who is making the
decision: for a particular group many of
these criteria will be unnecessary and a
decision will be more clear cut.
Such criteria do not have to be used in the
purely passive function of screening out
investment alternatives; they can also be
used actively to define the kind of
technology that is required. With the bias in
the current distribution of research and
development expenditure in the world, it is
Valuation According to Market
likely that many curren;. village level
technologies are less developed than the
technologies used in urban and richer
societies. Therefore, the evaluation of the
existing range of techniques may well show
the more developed ‘western* techniques to
be superior. But, it would seem possible to
design techniques for use at the village level
that would satisfy a number of social
objectives and also produce high social and
financial returns.
Valuation of Common
Inputs and Outputs
The general idea behind the valuation
suggested here is that of ‘opportunity cost.’
That is, all inputs and outputs of the project
being appraised should be valued in terms of
the loss to the chosen obiective that would
have resulted had they been put to their next
best alternative use rather than in the project
being appraised. This principle is of crucial
importance, particularly where many of the
items in the eva!uation are not traded.
The procedure involves deciding what
real alternatives there are to the use of
particular inputs and outputs. In villages,
the real alternatives (rather than fanciful
ones) are often difficult to establish, but the
use’ actually
considerably affects the viability of each
investment. It is for this reason that the
conclusions about the feasibility of a scheme
in one location are often difficult to
generalize to another.
The principles of opportunity cost
valuation will become more apparent in the
discussion of the following five broad
categories of inputs and outputs associated
with biogas: cellulosic organic material such
as dung inputs and slurry outputs; methane
gas; labour; capital; and other outputs such
as the improvements in public health.
In certain areas cow dung is bought and
sold, and this price might be an indicator of
the return that could be achieved from other
uses of dung. The argument is that dung
would be sold for more money if the market
price underestimated the uses to which it
could be put, and would have no sales value
at all if the market price exceededthe return
that could be expected. However, it is
known that cow dung is only partially
exchanged for money - in India only 2% is
traded according to an ICAR Report
(Government of India 1976, p. 2), and less
than 5% is sold in Pakistan (Government of
Pakistan 1969, p, 67). Therefore, the dung
that is traded may not be representative of
all the dung available. This may be because
those who sell dung may place a low value
on it becausethey have no land on which to
use it as fertilizer and cannot use it all as a
fuel; conversely those wishing to buy the
dung might have no money.
Cellulosic Organic Material
There would seem to be five possible
opportunity costs for the valuation of these
Valuation in Terms of the Use of
Dung as a Fertilizer
In this valuation it is argued that the next
best thing to putting the dung into the
methane generator is to put it directly on the
fields. The opportunity cost of the dung
when used in the generator is the loss of the
fertilizer value of the dung when used on the
fields. Two points need to be emphasized.
First, the dung will also have a fertilizer
value once it has been passed through the
generator (i.e. as slurry), and although there
is no reason to believe that the fertilizer
value of dung will equal that for slurry (some
studies show a superiority for the slurry,
Sathianathan 1975, p. 81-83; also see case
study 1) the fact that similar procedures for
valuation are used means that any errors
that are made on the cost side in the
valuation of the dung will be off-set to some
extent by similar errors on the benefit side in
the valuation of the slurry. Second, the
fertilizer value of the dung might be changed
if it were aerobically composted before
being put on the land. This emphasizes that
it is important to consider the net cost of the
dung to the methane project. If the dung is
put directly into the generator, certain
savings might be made in terms of the labour
and other costs associated with the
cornposting process. The fertilizer value of
the compost is counted as a cost to the
project and the savings resulting from not
having to do the cornposting are counted as
The value of the dung (and the spent
slurry) as fertilizer can be obtained in two
ways. The more usual method is to establish
the content in the dung of the chemicals that
are useful to plant growth (N, P, K) and
value these at the farm-gate cost (i.e.
including transport etc.) of an equivalent
amount of factory produced chemical
fertilizer. A number of problems arise with
this method of valuation. First, it cannot be
assumed that the plant nutrients contained
in the dung can be q,.!antified with certainty,
as considerable variation is indicated in the
literature. Second, the price of the factory
produced chemicals may bear no relation to
the ‘social’ costs of production. This may be
becauseof taxes and subsidies (which can be
easily taken into account) or because the
market cost of production does not reflect
the ‘social’ costs of production. The points
made earlier about the inadequacy of
market prices apply equally here, but the
point that has particular relevance is that the
foreign exchange cost of the product might
not be sufficiently reflected in the price. The
manual of project appraisal that has been
recommended (Squire and van der Tak
1975) approaches this problem by valuing
such products in terms of ‘world’ (or more
precisely import/export)
prices. Other
approaches attach a special shadow price
to the foreign exchange
component of the costs. A third problem
with this approach is the assumption that
the beneficial effects of dung, compost, or
slurry are equivalent to their content of
(chemical) plant nutrients such as nitrogen,
potassium, and phosphorus. This is clearly
not the case. These ‘organic’ fertilizers also
have the added effect on soil structure of
adding humus, which increases moisture
retention in the soil and helps prevent soil
erosion etc. If these differences between
organic and chemical fertilizer
significant, either an adjustment can be
made in the value of the dung or these
benefits can be picked up by the second
method of obtaining the fertilizer value of
The fertilizer value of dung and slurry can
alternatively be estimated in terms of the net
increase in crop output resulting from their
use. This is, in principle, the most correct
formulation of the opportunity cost of the
dung (or slurry), but it may also be the most
difficult to establish. Not only are there the
problems of establishing the physical
increase in crop output due joieiy to the
dung, but theoretically it is also necessaryto
establish the correct shadow price for the
Valuation in Terms of Use as a
The amount of dung used as fuel varies
considerably from region to region and
among the estimates of the various
researchers. The National Council for
Applied Economic Research put the figure
for India at 22%; the highest figure is given
for Bihar State with 60% of the dung
diverted for fuel purposes.8 The heat value
of the dung can be estimated, together with
the amount of usable heat that will be
obtained in the current stoves, and this can
be related to the cost of providing an
alternative fuel (kerosene, electricity if this
were used for cooking, wood, or coal).
Valuation in Terms of a ‘Free
This is a situation in which the input was
previously unused. This would be most
unlikely with dung, but in certain situations
it might exist with crop wastes. In this
situation the opportunity cost might be
thought to be close to zero (with the only
HGovernmentof India (ICAR) (1976, p. lo),
Government of Pakistan (1969, p. 63, suggest
that lessthan 4% of collectedmanureis usedasa
fuel. Berger (1976,p. 11) suggeststhat in both
Nepal and Korea, wood is the major source of
fuel in rural areas.Seealso Mardon ( 1976,p. 16).
cost being the costs of COILscion). But this
raises a fundamental issue within this sort of
analysis: although the crop wastes were not
being used, they probably could have been.
Should the opportunity cost therefore be
zero or the net benefit foregone had the
‘waste’ been used more productively? This is
perhaps a complicated way of saying that if a
commodity had previously been unused it is
worth considering the full range of possible
uses for it before throwing it into the
methane generator!
Valuation in Terms of a ‘Nuisance’
This is a situation in which the dung or
other waste was previously a nuisance and
had to be removed or treated at a cost. This
can arise with intensive animal rearing
where the dung is seen as an effluent
problem rather than as an assetg.In such a
case, the ‘cost’ of using the dung in the
methane generator would, in fact, enter into
the analysis as a benefit (i.e. as a negative
cost - a cost with the opposite sign to all the
other costs).
Methane Gas
The same principles for valuation apply
here as with the dung; indeed it should now
be clear that the difference between a cost
and a benefit, between an input and an
output, is merely the sign (plus or minus).
The gas is valued in relation to the real cost
of alternative energy sources (including the
cost of supply). It is worth noting, that if the
alternative energy source were electricity
(electricity is rarely used for cooking but
clearly is an alternative if the biogas is to be
used for lighting or motive power), there is a
question as to what price should be used for
electricity. Apart from subsidies,‘0 a
characteristic of electricity generation is
huge economies of scale combined with
considerable ‘lumpiness’ in the size of
possible investments. This means that the
marginal cost of supplying electricity from a
‘Such is the case in Fiji and other Pacific
countries where the sanitary disposal of wastes
from intensive commercial piggeries is necessary
(see Solly and Yarrow 1975).
plant that already exists, but is operating at
less than full capacity, might be extremely
small - merely the marginal costs of the
power plant and the additional transmission
equipment. Where villages are close to
existing main-line grids, the cost of
electricity might be very low indeed (though
the price charged by government is likely to
equate to the average cost - illustrating a
difference between the private and social
returns to such a plan). A similar problem
calculated on the basis of a new plant, the
oldest plant, or some sort of average of the
two. It is to be expected that these prices will
vary considerably.
If the alternative to biogas is wood (and
ICAR suggeststhat for India wood satisfies
58.6$Voof rural fuel needs, Government of
India (ICAR) 1976, p. 9; Berger 1976, p. 11;
Mardon 1976, p. 16) the cost in terms of the
opportunity cost of the labour required to
collect the wood might considerably exceed
any other valuation such as the market price
or the thermal equivalent of another fuel. It
has even been suggested that the time taken
to collect wood in some areas has changed in
recent years from a minor chore to the fulltime occupation of individual members of
the family (Makhijani 1976, p. 26). The
other cost associated with the use of wood is
that of deforestation leading to erosion and
flooding (Republic of Korea 1975, p. 2).
Theoretically, it might be possible to
establish the loss of crops resulting from the
erosion, or the cost of making good the
damage done by erosion. But, in most
additional weighting is applied to the cost of
wood to see what effect it has on the
viability of the various schemes (conversely
it might be useful to show what weighting
would have to be put on the cost of wood in
order to make the biogas plant compete with
other processes for the production of
fertilizer and fuel).
and Poole (1975, p. 98). The
actual cost of electricity to a large Indian village is
about 8-10 US cents per KWH. However, the
rural customers are only charged about 2 cents
because of government subsidies.
Certain costs associated with the use of
fuel before the introd.uction of the methane
investment, such as the cooking stove, are
‘sunk costs’ that would have been
undertaken whether the investment in
biogas is made or not. (For a discussion of
sunk costs seeSquire and van der Tak 1975,
p. 21.) Such costs should theoretically not
appear as part of the costs associated with
the alternatives to biogas; the comparison is
between the future fixed and variable costs
of biogas production and the future fixed
and variable costs of the alternative
(but also see previous section “The
Reliability of the System”).
In certain circumstances the value of the
gas might be assumed to be equal to the cost
of production (less the value of fertilizer).
This valuation might be used when the
comparison is with some other form of
If the CO1 produced in conjunction with
the methane is separated in a usable form,
this should also be valued in terms of its net
benefit further down the chain of
investments. If the CO> is used to increase
plant growth in greenhouses, its value is the
value of the increased crops minus the cost
of separating the gas and delivering it to the
The opportunities for using the gas might
well be increased if the gas can be
compressed, stored, and transported. The
technical problems and the costs, both in
terms of expensive cylinders and the cost of
compression, would seem to present
considerable problems, but compression
would expand the range of possible users for
the gas and would allow for its sale.11If the
gas can be sold then the number of people
who might be willing to run and own biogas
plants is increased by those who do not
themselves have sufficient demand for the
gas. If the gascan be transported, this would
make possible the building of larger
community-scale plants.
Problems of Consumer Surplus
The previous two sections have described
a number of possible ways of valuing dung
and gas. The actual valuation chosen will
depend on a view of the real alternative use
of the dung or the real substitute for the gas.
The justification for the biogas plant is not
only that there will be savings in alternative
fuels (wood) :rnd fertilizer (chemical
nitrogen etc.), but that there will be a greater
quantity of fuel and fertilizer available when
the biogas plant starts operating. This
is accounted
greater quantity
automatically in the analysis with the
estimation of the fuel and fertilizer output of
the plant. However, the valuation suggested
in the previous sections may have to be
modified becauseit may not be legitimate to
value all this extra fuel output in terms of the
price (‘cost’) paid for the smaller quantity of
fuel that was used prior to the introduction
of biogas. The people cannot ‘buy’ the new
increased quantity of gas at the old price.
Figure 24 shows the demand for fuel and
two alternative supplies: wood and biogas.
With the introduction of biogas the quantity
of fuel used rises from q1 to q2. It has been
suggested above that the valuation could
either be in terms of the cost of wood p, or
the cost of biogas productionpz. lfp, is used
this sets the gross benefit of the project equal
to p, x q, (or the rectangle p, r q2 o). From
this would be subtracted the previous
situationp, x ql, to determine the net effects.
This clearly overestimates the benefits of the
scheme by the area urs. The comparison
between p1 x qr with p2 x q2 underestimates
the benefits by the area uts.
“Both Prasad et al. (1974, p. 1358) and
Makhijani (1976, p. 17) feel that the storage of
large quantities of methane is prohibitively
expensive under current village conditions.
Fig. 24. Diagrammatic representation qf the
demand .for .fuel and the supply qf wood and
This difference in valuation would only
matter if the demand for fuel were, in fact,
downward sloping as shown in Fig. 24. If in
any real situation, over the relevant range of
prices and quantities, the demand for fuel
was not influenced by price (i.e. the demand
curve in Fig. 24 was horizontal), then the
valuation of gas in terms of the wood price
would be correct. But if the demand curve
does slope downward, the correct procedure
is to measure the gross benefit to the project
in terms of the whole areay, us qZ.This may
be approximated to the average between the
old and the new price and the average of the
new and old quantity:
cc/3+ PrWl x h + 4x4
A corresponding problem arises with the
valuation of the gas in terms of the labour
cost of collecting an equivalent amount of
wood. The labour saved as a result of using
the gas cannot exceed the amount of labour
actually used in the situation without the
biogas plant when the wood was collected. It
would be an error to attribute to the gas the
value saved had a larger amount of wood
(equivalent to the increased volume of fuel
provided by the gas) been collected. This
kind of error is often made when the cost of
wood is calculated per unit weight (kg) and
this unit cost figure is then used to value the
increased amount of fuel provided by the
certain nutrients (in the water soluble
ammonia) will be lost. In practice, it might
be expected that at certain times of the ye:-ir
the slurry would be stored and dried and at
others, put straight on to the fields. The
valuation of labour can therefore be the
deciding factor in the comparison of largeand small-scale techniques (see Disney 1976
and case studies presented later).
Labour is clearly not a homogeneous
category. Some of the tasks associated with
biogas can be done by unemployed unskilled
labour (possibly the labour of children), but
other tasks require considerably more skill.
Each type of labour might be expected to
have a different opportunity cost. The
opportunity cost of women’s labour is likely
to be different from that of men in many
In many developing countries the value of
skilled labour may be underestimated by its
market price. The value of a skilled
technician needed to rectify the problems of
a biogas plant may be much greater to the
farmer than the cash amount that he is
charged. It is important therefore to cost
into the analysis of biogas a realistic
valuation for skilled labour. At the limit,
certain levels of skill just will not be
available at the village level and the cost of
such skills is effectively infinitely high - a
plant design that requires theseskills cannot
work in the village situation. Such skills
might be associated with the maintenances
of gas compression equipment.
The valuation of labour is given extensive
treatment in project appraisal manuals
because the employment of labour is often
considered both a cost and a benefit to the
Capital is extensively treated in the costproject. In relation to biogas plants, the
labour input to family-sized plants for benefit manuals in terms of its opportunity
mixing and charging is often quite small (of cost, and in terms of problems of
the order of 20 minutes per day), and it may ‘discounting’ costs and benefits that occur
over differing time profiles to a common
well be that the use of this labour has no ‘present value’ (Squire and van der Tak
opportunity cost. However, the valuation of 1975, p. 75). In choices associated with
labour used for carrying and spreading the village investment, what constitutes capital,
spent slurry will depend crucially on the its availability, and its alternative uses is
characteristics of the labour situation in the
difficult to establish.
village and on previous practices for
handling fertilizer and dung.12 The tran‘2Berger(1976, p, 4) estimatesthat about an
sportation of the wet slurry will be more hour is necessary for diluting and mixing slurry.
difficult and costly than the transport of the There are no estimates available for the amount
much drier dung. If the slurry is dried, of time involved in disposing of slurry.
If the capital referred to is at least partly
the funds that the government is willing to
invest in the scheme. then its opportunity
cost might be expected to differ from the
opportunity cost of the capital cant rolled by
the people in the village.
Many of the designs for biogas that
currently exist are v’eryexpensive in terms of
the sorts of (capital) items that have to be
purchased from outside the village particularly the cost of the sheet steel that is
often used for the gas storage container.13
New designs are clearly possible that reduce
the number of these purchased items either
by switching to cheaper materials (wood and
ntastics) or by switching to materials that
eiist in the village and have very low
opportunity costs (oil drums, local bricks,
With village investment, the absolute size
of the ‘initial investment’ is an important
factor. It is of little importance that the rate
of return is good, or the net present value
(NPV) is positive, if the necessary level of
initial capital is not available to the villager.
Access to capital is an essential part of the
political process and capital is clearly not
rationed solely on the basis of its cost (the
rate of interest). Access to capital will be a
major determinant in the rate of adoption of
biogas plants and therefore in the pattern of
their ownership.
Where loans are involved, the cash
returns to the project also become
many of the
opportunity-cost benefits of the biogas
investment may be quite large, the cash
value of these returns might be insufficient
to pay back required loans (Moulik and
Srivastava 1975, p. 60-62). When this is the
case, there may be good reason for the
government to subsidize the loan so that the
social returns rather than financial returns
are maximized.14 An analysis based solely
on the cash transactions associated with the
investment is therefore a necessary part of
the evaluation of biogas systems.
13Berger (1976, p. 13) shows the cost of the
cover is over one-third of the total construction
costs. Prasad et al. (1974) give 35% quoting
Other Outputs
A number of other outputs associated
with biogas investments, which pose
particular problems of quantification and
valuation, were dealt with in general terms
earlier. An output such as the reduction in
the transmission of human pathogens,
which could result from the processing of
human waste through a biogas plant, is a
particular case in point. The increase in the
value of human waste when it can be gasified
might help encourage people to dispose of
excreta safely. It is unlikely to be possible to
specify in precise quantitative terms the
reduction in illness that would result from
the safe disposal of excreta because many
other factors influence health. The safe
disposal of excreta may be a necessary
condition for the improvement of health,
but it is not a sufficient condition. Even if the
effect on health could be quantified there
would be other problems in placing values
on such a reduction.
This problem can be approached in two
ways. First, when biogas is being compared
with other ‘high’ levels of technology, such
as the factory production of urea, the
benefits from the safe disposal of human
waste can be introduced as an explicit, but
qualitative, objective in the decision matrix
discussed earlier. It is unlikely that the
factory production of urea will have any
public health benefit in the villages, and it is
then possible to see what value would have
to be placed on the safe disposal of excreta
to make the biogas plant as ‘profitable’ as
the urea plant (it is, of course, possible that
no such weighting is necessary).
Second, when the comparison is among
different biogas designs, all of which have
some potential public health benefits, a
conflict can arise between the need to
dispose of human waste safely and the
objective of producing the maximum
amount of gas per day in a digester of fixed
size. This can occur when the retention time
141nKorea when the governmentterminated
its heavy subsidy t3 the cons.-uction costs of
biogas plants there was nearly a complete
cessationin the number of plants installed.
KVIC (1975, p. 1355).
needed to kill all the pathogens in the
excreta exceeds the retention time that
would give the maximum gas production
per unit of time. In this case, it is only
possible to show the reduction in the daily
production of gas that would result from
extending the retention time long enough to
kill the pathogens. It is then left to the
decision-maker to decide whether the
benefit of improved health is worth the cost
of the gas lost.
Benefits resulting from the possible
improvement in the domestic environment
following the reduction in wood and dung
smoke that results from the introduction of
biogas would have to be accounted for in a
similar way.
The reduction in the time taken for
cooking when using biogas compared with
using dung and wood would first be
balanced against the extra time taken in
managing the biogas plant. Any surplus
labour would then be valued according to
the opportunity costs of that particular type
of labour.
be compared with the value of the liquid part
of the slurry in its alternative use - most
likely the returns from putting it on the fields
and the subsequent increase in crop yields.
The advantages of taking the fish
production option might be savings in
transport costs of the wet slurry to the fields
and the greater utilization by the algae of the
plant nutrients in the slurry.
It is not always necessary to evaluate the
various intermediate outputs (such as the
slurry) in the chain because the purpose of
the analysis is to weigh the primary inputs to
the system against the final outputs. This
valuation can only take place when the
system has reached a stable state. There is a
certain danger of ‘double counting’ in the
analysis of systems when intermediate
products are valued: first, any commodity
must be ascribed the same value regardless
of whether it is an output or an input of the
system; second, the value of an intermediate
output cannot be valued as a net benefit to
the project if it is subsequently used as an
input elsewhere in the system, It would seem
that in one study this mistake was made and
the values of the slurry (as fertilizer), the
Opportunity Costs in Relation
algae (as protein), and the fish (as protein)
to Chains or Systems of
were all counted as net benefits to the same
project (Philippines de la Salla University).
Biogas plants are often advocated as part
Perhaps the largest source of error in the
of a larger system in which the output of one evaluation of these chains of investment is
part of the system becomes an input to that the physical input/output relations of
another. There is evidence to suggest that the system are incorrectly estimated. The
additional investments to provide inputs or proponents of these systems become overto use outputs can raise the return from the enthusiastic and leave out of the analysis
biogas plant (for instance, when the liquid certain inputs that are required to make the
from the slurry of biogas plants is connected cycle work. Examples can be found of
to ponds in which algae are grown and systems that seemto be out of balance (Tyler
subsequently fed to fish etc. - see chapter 1973).
by Pyle). This poses no problem to the
opportunity-cost analysis (all investments
are in principle part of such a system), but in
Case Studies
practice, care has to be taken in sorting the
net effects on the whole system from these
The following case studies have been
additional upstream and downstream selected to illustrate different approaches to
investments. The benefits to the biogas the evaluation of biogas technology and to
investment from putting part of the slurry highlight some of the points made
through ponds to grow algae and then fish, previously. The number of thorough
will be the value of the fish minus the cost of economic evaluations of biogas is very small
constructing and running the various ponds. and those chosen here are among the best,
Investmen? in these projects would have to but the purpose of this section is to point out
possible points of weakness in the case
studies rather than to praise them. The
of these case studies
concentrates on the conceptual problems
involved rather than on an evaluation f the
empirical data. It is important to note that
quite wide variations in many of the crucial
parameters are to be found among the case
studies, even when similar plants are
involved, and this raises some doubts as to
the precise conclusions that can be drawn
from these studies (see for instance Table
Case Study 1
(ICAR 1976. particular[v
pages 253.5, 61-62)
which gas is produced; the proportion of
dung that was burned in the previous
situation; and different ways of valuing the
methane gas.
By way of illustration, the numbers are
presented for a 60 ft3/ day ( 1.7mj/day) plant
using the assumptions that there is a 50% efficiency of gas production, the gas is valued
at the cost of a thermally equivalent amount
of kerosene, and either all the dung was
previously burned (a) or all the dung was
p,eviously used as fertilizer (6).
Inputs per year
Dung + Generator + Labour
Outputs per year
Gas + Slurry fertilizer
This is a very professional report that
Each of these items can be valued acattempts to fill the gap in detailed cording to the data supplied by ICAR on an
information about the viabi!ity of biogas annual basis.
plants in India. A comparison is made of six
The opportunity cost of the dung:
sizes of biogas plant and current fuel and (a) either as fuel Pz x D
fertilizer practices. No consideration is given
= 0.0545 x 730 x 5
to the wider aspects of other village
= Rs198.92/year; or
investments or to alternative means of (b) as fertilizer P3 x M,
satisfying fuel and fertilizer needs. It is
= 0.04 x 2.50 x 730 x 5
stated (on p. 25-26) that the net returns per
= Rs365/ year
plant per year (R) from the investment in (c) the opportunity cost of the generator,
each size of biogas plant, over the returns
including the labour cost of running it:
from the existing practice for fuel and fertiP,(g) x 60 ft3 x (I-% downtime) x 365
lizer, is equal to the gross value of the
= 0.0165 x 60 x 0.9 x 365
methane gas and the slurry (A), minus the
= Rs329
value of the dung, had part of it been burned (d) the opportunity cost of the gas:
directly and the rest used as farm yard
P,(t) x C x K x 365
manure (B), minus the cost of the main= 0.0186 x 0.5 x 60 x 365
tenance of the biogas plant (E). So that R =
= RS203.67
(A - B - E). The net present value is then the (4) the opportunity cost of the slurry as
discounted value of R, minus the investment
cost (I).
f’4 x M,
0.05 x 3.65 x 730 x 5
The gas is valued according to the market
= Rs666.125
price of a thermally equivalent amount of
kerosene; the slurry is valued as a fertilizer,
but no indication is given as to the method of Where:
K= capacity of the cow dung gas plant
valuation; and the dung is valued both in
(ftJ/day) ( 1 ft3 = 0.028 m3)
terms of its fertilizer value (but again no
I= investment in the gas plant (Rs)
method is given) and the market price of its
dung produced per animal per day
equivalent thermal value of kerosene. This
analysis iliustrates how simple ‘sensitivity’
N= number of animals required per plant
analysis can be carried out to show how
IV= wet dung processedper plant per year
sensitive the profitability of the schemesare
to changes in the value of: the efficiency with
= 365 Nd(kg)
D= dry dung available per year from
dung of equivalent quantity as processedin a gas plant (kg)
G= methane gas produced per plant per
year = 365 K (ft?)
M,= farmyard manure obtained per year
from dung of equivalent quantity as
processed in a gas plant (kg)
M,= gobar-gas manure obtained per plant
per year (kg)
c= efficiency of gas production (varying
from 0 to 1)
.f= proportion of cow dung used as fuel
in the existing system (varying from
Oto 1)
lTf= proportion of cow dung used for
making manure in the existing system
A= gross return per year from cow dung
gas plant from value of methane gas
and gobar-gas manure (Rs)
B= gross return from existing practice
from value of dung fuel and farmyard
manure (Rs)
E= recurring maintenance expenditure
per plant per year (Rs)
R= net return per plant per year over
existing usage from the investment in
a gas plant (Rs)
i= rate of interest (per rupee per annum)
n= economic life of the plant in years
P,= price of methane gas in Rs per ft3
(subscript g is generation cost, t is
thermal equivalent cost)
fz= price of dung fuel in Rs per kg
P3= price of farmyard manure in Rs per
Pa= price of gobar-gas manure in Rs per
Inputs per year
Outputs per year
(a) Rs 198.92 + 329 Rs 666.125 + 203.6
Rs 666.125 + 203.6
(6) Rs 365 + 329
Therefore profit, per year = (a) Rs342 and (6)
Rs 176.
The net present value (NPV) is given by
the formula (I-( I +r)-n)/r, and for a project
life of 20 years and at 10% interest it equals
8.5 136 (where r = 0. I and n = 2). Therefore,
NPV = (a) 29 I2 and (h) 1498.This compares
with results in the ICAR report that show a =
3460 and h q 2040. The difference is due to
the exclusion of the labour costs of running
the plant in the ICAR calculation.
A number of points emerge from this
It is worth noting that the above
calculation is equivalent to comparing the
situation with the investment and the
situation without it; this can be easily
’ A. The situation ‘with’ is
Dung + Generator + Labour -+
Gas Heat + Fertilizer
B. The situation ‘without’ is
Dung + Stove + Labour ---+ Dung Heat
If B is subtracted from A, and the stove is
considered a ‘sunk cost’ because it is an
expense incurred before the decision to go
ahead with biogas, the net effect is:
Generator + Gas Heat - Dung Heat +
which is the same as:
Dung (Heat) + Generator - Gas(Heat) +
Slurry Fertilizer
The value of the slurry in this calculation
is far greater than the value of the gas (Rs666
compared with Rs204/year). But it is unnecessary to anaerobically digest dung
unless gas is required. It would certainly be
worth comparing the situation of composting-plus-kerosene with the biogas situation; the gross returns of the compost might
be lower, but it might involve considerably
less investment than biogas.
The slurry was considered tc be much
more valuable than the dung by ICAR. This
is possible, but again the opportunity cost of
the dung should perhaps be the net value of
composting the dung rather than its value
unprocessed in ?ny way.
Certain costs were not apparently
included in the analysis: water; cooking
equipment for use with gas; costs of
the ga.s (though this is
mentioned); end the transport of kerosene
(and fertilizer?).
No explanation is given for the valuation
of dung and slurry, nor is this considered
sufficiently important to merit inclusion in
the sensitivity analysis. Inputs (such as crop
residues) other than dung are also not
considered in the calculations, though they
are mentioned.
In the valuation of gas in terms of
kerosene, there is no discussion of the use of
market prices. For instance, a shadow
foreign exchange rate might have been used
to more truly reflect the cost to the economy
of kerosene. The gas might also have been
valued in terms of the thermal equivalent of
wood; wood is mentioned as an expensive
fuel in the report.
The sensitivity of the calculations to
changes in the life of the project and the
interest rate were calculated. Reductions in
interest rate and increases in life expectancy
have the effect of improving the net present
The problems of income distribution were
considered to the extent that it is stated that:
“it would be evident from the above
description that the present plant designs are
beneficial solely to the cattle-owning rural
The private cost-benefit analysis is
certainly an attempt to bring out the
imp0rtan.t point that the returns to the
actual investor are somewhat different from
the social returns. It is not clear, however,
why it is assumed that the farmer would
have paid cash for chemical fertilizers had he
not had the biogas plant; nor is it clear why
the farmer would not also have paid cash for
kerosene. The results would also have been
very different if the fertilizer value of the
dung had been raised by a cornposting
Case Study 2
(Dimey 1976. parricular!r
pages 9-15)
The paper starts with the view that “the
superficial attractiveness of intermediate
technology may. . . be overstated by some of
its more enthusiastic supporters.” To support this view a comparison is made of the
production of nitrogen fertilizer by existing
designs of biogas nlants (in India) and by the
more conventional process that produces
chemical nitrogen in the form of urea.
Calculations are carried out to show the cost
per tonne of nitrogen produced by each
technology. If the capital costs associated
with the production of an equivalent
amount of nitrogen are correct, current
biogas plants use between 2.5 and 8 times
more capital per unit of nitrogen output
than conventional
intermediate techniques are not necessarily
less capital intensive (per unit of output)
than the ‘high’ technology, but as Disney
also mentions, biogas plants not only
produce nitrogen but also gas. It is argued in
his paper that with the historical process of
technical change in which costs have tended
to be reduced largely at the capital intensive
(K/L) end of the spectrum of available
techniques, the superiority of western
technology is to be expected.
The study is well argued and illustrates
another form of the evaluation of biogas this time in comparison with another
provider of fertilizer. The gas from the
biogas plant is treated as a ‘negative cost’
(i.e. as a benefit) to the production of
nitrogen. The costs of transporting chemical
nitrogen to the user, which are essential in a
comparison of this kind, are included in the
A partial sensitivity analysis was carried
out for changes in both the shadow price for
foreign exchange and the cost of
transporting the urea for the conventional
plant. For the biogas plant, sensitivity to
changes in the capital costs, the proportion
of nitrogen in the slurry, and the
cost of the dung were
Gas is valued in terms of a thermally
equivalent amount of electricity (at
RsO.O12/ft$ 1 ft3 = 0.028mJ); the cost of
dung is allowed to vary widely between
Rs 100 and 20 per tonne and is basedto some
extent on coal as an alternative fuel; and the
slurry is valued in terms of its nitrogen content ( 1.6%).
The major problems with this analysis are
that labour is inadequately valued for the
biogas plants, and the selection of variables
to be altered in the sensitivity analysis might
be considered biased. (Some of these
problems have been corrected in the revised
version published in “Development and
Labour is represented as 86Ycof the total
costs of the small biogas plant’s costs and
37$Qof the larger plant’s costs (assuming the
‘high cost of dung). The valuation of labour
is therefore crucial in determining the
competitiveness of the biogas system. A
figure of Rs 12; day is used even though the
Indian Council for Agricultural Research
used Rs4iday. Disney does adjust his
labour costs to some extent before making
his final calculations by reducing the
amount of manpower required to run the
plants. but the iabour bill is still a substantial
input. In certain village situations it might
be expected that labour would have a very
low (even zero) opportunity cost and some
people consider that one of the greatest
advantages of the biogas system is that it can
utilize this resource. If a zero opportunity
cost of labour is used to test the sensitivity of
the analysis to changes in the cost of labour,
all the biogas plants produce nitrogen more
cheaply than the smallest conventional plant
(with the exception of the smallest biogas
plant when the dung price is ‘high’, seeTable
The sensitivity of the paper’s conclusions
might also have been tested against changes
in other variables.
The ratio of slurry to gas (27 kg/ 150ft3) is
similar to the figures used by Prasad et al.
1974, but less than the figures used by the
lndian Council for Agricultural Research
(which would be equivalent to 87 kg at the
150 ft3 level) and by Sathianat han ( 1975)
who usesa range of between 27 and 99 kg at
150 ft3. These increases would considerably
influence the balance of costs and reduce the
cost per unit of nitrogen produced.
The value of gas (RsO.012) is less than the
figure used by ICAR (Rs0.0186). A value
such as ICAR’s produces a gas value per
tonne of nitrogen of Rs6500 rather than the
Rs4200 used by Disney. If this lower figure is
used the cost per tonne of nitrogen is further
reduced by (6500-4200) Rs2300, making the
larger biogas plants cheaper than all the
conventional urea techniques even using the
‘high’ cost of dung. It is not clear why the gas
should have been valued in terms of
electricity rather than wood, kerosene, or
even coal as coal was used to establish the
opportunity cost of dung.
The volume of gas actually produced is
assumed equal to the plant’s design capacity.
This is unlikely and therefore the cost per
tonne of nitrogen would be higher than is
suggested here.
Two further points are of interest.
No interest is charged for the use of the
capital in any of the calculations in this
paper. It is usual to calculate annual capital
costs as an ‘annuity’ - what equal annual
amounts have a present value equal to the
cost of the capital: for example
Annual Payment = K G ( 1 -(l
+r) -)z
where K = capital = 3360, r = lO$$oper annum, and n = 10.
The value of the bracket can be found in
discounting tables (e.g. Lawson and Windle
1974): 336OS6.1446 = 546 annually. This is
more correct than taking the total interest
payable over the 10 years (3360 x 0.1 x 10 =
3360) and adding this to the capital costs
(3360 + 3360 = 6720) and spreading this
equally over ten years (6720 f 10 = 672
An essential assumption of Disney’s
analysis is that slurry has no value other
than as nitrogen. This is not in fact the case
because the slurry also contains humus and
Table 34. Costs (Rs) per tonne of producing nitrogen using biogas plants, when the cost of dung is
assumedto be either ‘high’ or ‘low.’ compared with urea plants.
cost of
biogas plant
biogas plant
urea plant
urea plant
I 828.70
trace elements. Therefore, the value of these
extra elements should be subtracted from
the cost of producing nitrogen in the biogas
Case Study 3
(Prusud et al. iY74, particrtlar[~-
pages 13.53. 1355, 1364)
This is perhaps the most widely quoted
and influential article written on biogas in
recent years. It is a masterly marshaling of a
huge amount of data into a coherent and
easily read argument. It raises a number of
issuesand considers many of the alternatives
with which biogas can be compared. The
comparisons between biogas and rural
electrification on the one hand, and biogas
and urea plants on the other, represent only
a small proportion of the paper and are
“intended mainly to stimulate detailed costbenefit analysis of alternatives.” However,
for the purposes of this casestudy only these
two comparisons are considered.
Comparison with Electricity
A quite wide range of inputs and outputs
are identified for a 5000 ftJ/day ( 140
mJ/day) plant and some of them are
quantified. The largest omission from the
list is the cost (the opportunity cost) of the
dung. The assumption implicit in an analysis
presented in this form is that the dung has no
other use (a zero opportunity cost) and that
only the cost associated with its use is the
cost of collection. The inclusion of an
opportunity cost for the dung would make
quite a difference to the apparent viablility of
the biogas plants. If the figures used by the
Indian Council for Agricultural Research
are used (and these seem to be on the high
side) the cost of the necessary dung would
have been in the region of Rs 134 12/year (if
the dung had previously been burned) and
Rs24609/year (had the dung been used as a
fertilizer). This is compared with a benefit
stream of Rs38300/ year and a running cost
(excluding capital) of Rs 12206. If the ICAR
figures are used then a corresponding
adjustment would have to be made to
increase the value of the slurry.
The point that is being made here is not
whether any one particular value is correct
but rather that it is unlikely that the fertilizer
and the gas benefits could be obtained
without some (opportunity) costs elsewhere
associated biith the use of the dung, etc.
The estimation of the annual capital costs
of the plant is omitted from early calculations, but is included in more detailed
calculation at Rs4350 per year. This is an
annuity equivalent to a 30-year plant life and
an interest rate of [email protected]/year. If these costs
are included, the total annual costs of the
5000-ft3 plant rise by 36% from Rs 12206 to
If the figures used for rural electrification
are correct, it appears that there is a massive
subsidy being paid to electricity generation.
This could either be a direct subsidy from
the government or it could be an indirect
the government
underwriting the losses of the electricity
company that these figures suggest. A third
possibility is that the subsidy is a transfer
payment from the urban population if the
profits arising from electricity salesin urban
areas are sufficient to allow the electricity
company to sell electricity to the rural
population at the figures quoted.
The comparison of biogas with electricity
also illustrates a possible confusion between
‘prices’ and ‘values.’ For instance, no value is
attached to the free provision of gas for
cooking to all the village houses, but at the
same time, prices are attached to the slurry
that is produced by the biogas plant, but
which may well not actually be bought for
cash by the villagers. It is essential to
separate the analysis into two distinct parts:
one in which both the costs and the benefits
are valued according to their ‘social
opportunity cost’ and another in which the
costs and benefits are valued according to
the actual cash transactions that occur.
Comparison with a Urea Plant
This is a considerably more rudimentary
analysis than the comparison
electricity. It attempts merely to show the
rough orders of magnitude associated with
each system. The comparison is made
largely on the basis of the ratios of capital to
output {or capital to turnover) and capital to
labour. This formulation makes it difficult
to compare the two processesas the costs per
tonne of nitrogen delivered to the farmer
cannot be calculated because costs are not
given for the inputs to each process: dung,
naphtha, labour, and the transport of the
urea. Again, there is a possible implication
that the inputs, particularly of dung and
other compostable material, are considered
to have a zero cost.
It is interesting to note that it is assumed
here that a p!ant that produces about 5000
ftj of gas per day produces about 8.8 tonnes
of nitrogen per year. This is considerably
higher than the figure used by Disney (5.8
tonnes/year from a 5600-ft3 plant) and is
higher than the figures that appear
elsewhere in the paper: 2.7 tonnes in the urea
calculation and 4.4. tonnes in the electricity
calculation. The large amount of nitrogen
contained in the slurry is partly due to the
fact that it is composted with “refuse etc.” If
this is the casethen it might be expected that
some value for the refuse should be added to
the costs side of the calculation. It is not
clear why composting is thought likely to
only occur in conjunction with biogas
Case Study 4
(Mou Iik and Srivas tava I9 75)
This is a thorough piece of research that
concentrates on the State of Gujarat where
about 28% of all Indian biogas plants are
located. The study details the location,
ownership pattern, and running problems of
biogas plants in the State and discusses a
number of the important issues in the
operation of the biogas scheme. The
economic aspects of biogas are covered and
an attempt is made to answer the question:
“If farmers are rational and the economics
are as good as claimed by the KVIC experts,
why does the demand for biogas plants not
increase rapidly?’
Economic and financial analyses are
carried out on nine plant designs ranging in
size from 60 to 1250 ftJ/day (1.7 to 35
The data used are largely from the
. __,_
Khadi Village industries Commission and
might well be thought to be overly optimistic. This problem with the data is recognized
by the authors and some attempt is made to
adjust the KVIC figures on the basis of
survey data. The main adjustment is in the
amount of gas that is likely to be produced
from the plants per year, because it was
found that the actual output was less than
the design capacity suggested by the KVIC.
A number of costs and benefits are listed.
Explicit account is taken of the costs of the
ancillary investment associated with pipes
and appliances and with the costs of keeping
the gas-holder well painted. The gas is
valued in terms of the market cost of a thermally equivalent amount of kerosene. The
dung and slurry are given money values
based on data supplied by the KVIC, but no
indication is given of the basis on which the
figures were calculated (the slurry is said to
be 1.875 times more valuable than the
equivalent amount of dung). Different life
spans were assumed for various parts of the
plant: the civil construction was assumed to
last 40 years; the gas holder IOyears; and the
pipes etc. 30 years. Various indicators of
investment worth are calculated on the basis
of interest rates of 10, 13, and 15%.The only
plant to have a negative net present value
was the 60-ft3 plant when the interest rate
was 15%.
A number of points are of interest.
No mention is made in the analysis of the
costs associated with the labour used to run
the plant, nor is any cost attributed to the
water used. The inclusion of these two costs
would have reduced the practicality of the
Just over 40% of the benefits of the
scheme arise from the difference in the
values of the dung and slurry.
The method used for the valuation of the
dung and slurry is not made explicit and it is
therefore not clear how the opportunity
costs are derived. It is, however, suggested
that agricultural wastes are the normal fuel
in the area and that these have negligible
costs to the farmer. If this were the case,then
the benefits of the biogas plant would be the
differential value between dung and slurry
as a fertilizer (and questions might be asked
about methods of improving the value of the
dung through aerobic fermentation), the
value of the saving of the agricultural wastes
(which might bz small), and the value of the
gas. The gas might be valued in terms of the
heat equivalent of the agricultural wastes, or
it might be valued in terms of a thermally
equivalent amount of kerosene. But, if
kerosene were used. this would raise the
question of whether the agricultural waste
might also have been valued according to its
opportunity cost in terms of kerosene (an
adjustment might be made to take into
account the fact that biogas can be used for
lighting and for driving engines whereas
agricultural wastes cannot). The problem
can be shown simply:
Inputs: dung (as fertilizer) + agricultural
wastes (as fertilizer). Outputs: dung (as
fertilizer) + agricultural wastes (as heat).
Inputs: dung (as fertilizer). Outputs: slurry
(as fertilizer) + gas (heat).
By subtracting the ‘without biogas’
situation from the situation ‘with biogas’the
net effect is: Inputs: dung (as fertilizer) +
agricultural waste (as fertilizer). Outputs:
slurry (as fertilizer) + gas (as heat) agricultural waste (as heat). This is
equivalent to saying that the benefits of gas,
slurry, and agricultural waste (as fertilizer)
are gained for the loss of the dung as
fertilizer and the loss of heat from the
agricultural waste.
The financial ana!ysis differs only from
the economic analysis in the subsidy to the
farmer for the purchase of the biogas plant.
This would again appear to illustrate a
possible confusion between the analysis of
the project from the point of view of
opportunity costs and from the point of view
of cash transactions. The financial analysis
does bring out the point that the actual
returns as perceived by the farmer can be
quite small and that this could restrict the
rate of acceptance of a project that is
‘socially’ beneficial.
This is the only study that explicitly
compares the returns to biogas with the
returns that can be achieved by other rural
investments such as buffalo, tractors, and
lift irrigation. However, it is not stated
whether the analyses were carried out on a
comparable basis.
The fact that very little of the data used in
the analysis was obtained from actual
weakens the
conclusions that are based on it. This is a
useful ‘back of the envelope’ calculation, but
it could not be considered as an
authoritative statement as to the viability of
biogas in India.
Case Study 5
(Berger I9 76)
This is an extremely thorough anaiysis of
a IOO-ft3/day (2.8-m3/ day) plant under
Nepalese conditions. It involves a complete
statement of costs and benefits (though
water is excluded) that includes a more than
usually detailed discussion of the possible
valuations that can be placed on gas, dung,
and slurry. The possible values for the gas
from a lOO-ft3plant range from Rs836/year
(valued in terms of charcoal) to Rs5055
when valued in terms of gasoline. The
annual value of the gas in terms of electricity
is Rs2263. The value of the fertilizer from
the plant is taken as the nitrogen value of the
slurry minus the nitrogen value of the dung
(i.e. the dung is assumed to have been
burned in the situation without the biogas
plant). A variety of other assumptions are
discussed that show the variation in the
nitrogen value of the dung or slurry depending on the way the dung/slurry is used
in the fields.
A simplified method is used for the
treatment of capital: the plant is depreciated
over 25 years in equal annual amounts and
the interest is taken as the interest on half the
capital value per year (i.e. 5000 divided by 2,
times 15% equals 375 per year). This is a
commonly used approxihation, but it does
produce an annual capital cost that is
substantially less than the amount that is
produced using the annuity method
described previously. The annuity method
produces an annual cost of Rs773 per year
for 25 years at 15% with no scrap value.
The net result of the analysis shows that
annual benefits exceed annual cost by
Rs5545. These would have been reduced to
Rs347 had the annuity method been used for
the valuation of capital.
The analysis assumes that the cow dung
would previously have been burned, though
the author points out that those who might
consider using the biogas plants are
currently burning wood. Furthermore, it is
assumed that 100 ft’ of gas would actually
be produced per day - this might be
considered far too optimistic. But the
analysis is set out so clearly that it would not
be difficult to do as the author suggests “to the extent that individual farm
are different
from the
assumption the analysis can be modified.”
The only slight problem with the chosen
format is that it is not obvious at first sight
that the benefits from the slurry are net of
the costs of the nitrogen that could have
been obtained merely from using the dung
directly on the fields.
The Social and Economic
Determinants of the Demand
for Biogas
The answer to a number of important
macroquestions cannot be supplied by the
kind of microanalysis discussed so far. In
particular, the introduction of biogas on a
wide scale has implications
macroplanning such as the allocation of
government investment and the effects on
the balance of payments etc. 15Furthermore,
many of the factors that determine the rate
of acceptance of biogas plants, such as credit
facilities and technical backup services, are
likely to have to be p!anned as part of a
general macropolicy. The importance of
15Makhijani and Poole (1975) suggest that
current biogas designs would use up 20% of
India’s annual production of steel and 25% of her
cement production if biogas plants were applied
to 10 million hectares of cultivatable land per
year (p. 88). Mardon (1976) shows (p. 9) that at
the planned rate of biogas implementation of
100000 by 1978. it would take 300 years to satisfy
the energy needs of the 570 of the population who
own 5-6 cattle in India.
biogas from the point of view of the
allocation of research and development
funds also requires a more macroview of the
the techniques for the
production of biogas are likely to undergo
substantial change in the coming years
(particularly reduction of capital costs and
ease of operation) it is still possible to
broadly desc,;be the social and physical
characteristics of rural areas in which biogas
plants are most likely to be viable (or are
least likely to fail). The prevalence of these
give a more
macroindication of the possible importance
of biogas in the country’s
development, fuel, and fertilizer policies.
From the framework provided by social
cost-benefit analysis it can be assumed that
biogas will be most viable in those situations
in which the necessary inputs have a low
opportunity cost, where the efficiency of the
operation of the plant is ‘adequate,’ and
where the alternatives to the outputs from
biogas p!ants have a high opportunity cost.
Surveys would have to be carried out to
determine the physical location of those
areas that contain these characteristics.
Necessary Inputs have a Low
Opportunity Cost
This is most likely to occur where: (I)
agriculture is such that sufficient amounts of
material from which methane gas can be
produced are available with an opportunity
cost that is at least no higher than when it is
used as a fertilizer or fuel; (2) industries exist
(e.g. paper production, distilleries) that
produce, as by-products, large amounts of
from which methane can be
produced; (3) and/ or there is no social
restriction on the use of human waste; (4)
cow dung is traditionally
and (5) water is available and can easily be
fed into the digester; (6) capital is available
with a sufficiently low opportunity cost as a
result of either the adequate supply of
or because
of the
lack of
be expected
for its alternative use -it
that methane generation
is to
only become an attractive opportunity when
certain other investments, such as irrigation,
have been carried out; and (7) labour is
available and willing to undertake the work
of operating the methane generator on a
continuous basis.
Efficiency of the Operation of the
Plant is Adequate’6
This is most likely to be possible where:
(1) there is uniform input of gas producing
material (the plants seem to be simplest to
run when only one type of input, such as
cow dung, is used); (2) technical advice and
technical skills are available both for the
construction of the plant and in troubleshooting - this might be ‘most likely to
occur near towns; (3) smal!er size plants Can
be avoided - there is less problem of shock
loading with larger plants and with larger
piants it becomes possible to employ and
train full-time operators; (4) the surrounding air temperature does not fall
below the level at which methanogenic
bacteria operate adequately (above 15 “C
for the bacteria commonly found in
generators); (5) plant design is adequate e.g. no places in which blockages can occur
or water condense; and (6) maintenance and
good operating prac:ice is likely to be
carried out.
Alternatives to the Outputs from
Biogas Plants have a High
Opportunity Cost
This is likely to occur where: (1) there is a
physical limit to the amount of fuel and
fertilizer available .from an alternative
source -- this might occur as a result of the
cost of transport or from policy decisions as
to the rate and geographical direction of
expansion in the larger scale fuel and
fertilizer industries17; (2) and/or there is a
scarcity of wood; (3) and/or dung is being
burned as a fuel; and (4) there is insufficient
water to make use of chemical fertilizers; (5)
there is insufficient cash to purchase other
fuels and fertilizers; (6) the use for the gas is
near the generator (or simple and economic
compression facilities exist); and (7) the cost
of handling the slurry is not large enough to
reduce the net value of the slurry to
unacceptable levels (some plant locations
make slurry handling particularly difficult
and expensive).
Some of these characteristics can be
modified by government policy and
expenditure, but it should be relatively easy
to identify the relevant area, particularly on
the basis of the third set of characteristics where alternatives to biogas and slurry are
costly. Which areas are isolated from
alternatives by high transport costs? Which
areas are experiencing unacceptable levels
of depletion of forest resources? Where is
burned’? Answers to such
questions would be available from a survey
of rural energy needs and current fuel and
fertilizer practices.
The areas in which biogas might be most
via$le can be further narrowed down by
considering the factors in the second group
that affect running efficiency. Areas with
low night or winter temperatures can be
excluded; areas can be identified in which
technical advice is available or in which the
government is prepared to make it available
through rural development policies or
policies to help certain groups; and locations
(ouch as intensive animal rearing units) can
be identified that are known to have large
amounts of uniform outputs from which gas
can be generated.
The availability of inputs, within the areas
identified so far, will be determined more by
the social groups concerned than by purely
geographical characteristics. Any number of
social groups can be considered, but eight
groups are suggested by way of illustration:
agricultural or other business, with intensive
animal or crop production; cooperatives
formed to produce biogas; large existing
l”Finlay (1976) is an excellent trouble170ECD (1968, p. 59) - a significant
shooting guide to the problems encounteredin percentageof farmers surveyed for this report
the construction and maintenance of biogas state that lack of accessto fertilizer outlets was
the main reasonfor not using chemicalfertilizer.
social groups that are able to cooperate such as large families, communal cooking
groups, and cooperatives formed for other
purposes; large farmers; small farmers;
landless labourers; traditional collectors of
cow dung; and women.
Unless the output from the biogas plants
can be sold or bartered, investment in biogas
will only be viable to those groups who have
both accessto the various inputs and have
sufficient use for the outputs. This link
between inputs and the need for the output is
particularly important with the gas itself. If
there is no way of the plant owner profitably
using the gas, then it might be better to
compost the inputs rather than go to the
trouble of producing methane. A number of
studies assume that the gas can be
compressed and transported in cylinders at
reasonable cost (Prasad et al. 1974, p. 1358;
Makhijani 1976p. 17: Philippines da la Salla
University). But, compression, whether into
cyiinders or through an extensive system of
pipes, involves a ‘higher’ level of technical
‘know-how’ than the rest of the biogas
system. The compression of methane will
therefore not be a practical proposition in
many developing countries.
Until the problems of gassale and delivery
have been overcome, it is likely that biogas
investment will have to be concentrated on
the first four of the eight groups described
previously. Only they are able to bring
together the necessary inputs and are able to
gain the benefits from the gas either through
cooking or through the use of machines
(pumps for irrigation etc.). But, as has been
pointed out in a number of studies of India
(Prasad et al. 1974; Government ol India
1976; Mardon
1976) the
distribution of income is such that
currently only the relatively well-off can
afford biogas investments. This means that
only a small proportion of the village’s
energy needscan be met in this way, and that
there is a likelihood that dung and other
wastes will be denied those people who
traditionally collected them. If income
distribution considerations are part of the
government’s objectives, then a strategy of
introducing biogas plants (or any other rural
technology) only to the richest groups may
well be unacceptable. The remaining option
is to encourage the use of larger (community
scale) plants on some more cooperative
basis. But, as the ICAR report puts it: “At
present not a single community biogas plant
exists in the country. The concept will
remain just a theoretical slogan unless the
attendant socio-economic and technical
problems are investigated in some pilot
plants in the public sector, located in
zones in the
country.” (G overnment of India (ICAR)
1976, p. 42; seealso Moulik and Srivastava
1975, p. 50).
If sale of the gas is possible, a wider range
of social groups could be involved in the
generation of methane, but these other
groups (such as the last four groups in the
previous list) would find it difficult to
assemble the other necessary inputs without
some form of cooperation and assistance.
For instance, where dung is currently
burned, the introduction of biogas will
require the most behaviour changes in
activities normally undertaken by women,
such as cooking and the making of dung
cakes, but women as a group are likely to
lack access to capital. An interesting
suggestion in this context is that loans be
made directly to these women to give them
an interest in and control over such changes,
and to provide a means of directly raising
their economic and social status (personal
Prof Scarlet Epstein,
Institute of Development Studies).
The Government’s own policies, then, are
likely to be the largest factor in determining
the distribution of biogas plants within the
areas narrowed down according to the
characteristics described previously. The
provision of capital is one such policy
variable. But the extent to which the lack of
capital will be a constraint to the future
adoption of blogas plants cannot be inferred
from the current situation because past
practice involves both much higher capital
costs per plant than are likely to apply in
future, and because only the family-sized
plants have been promoted. The Gujarat
Study (Moulik and Srivastava 1975) has a
number of useful suggestions about possible
methods of finance in India, but the
rural credit are much wider
than the problems that particularly apply in
the financing of biogas plants and they are
not the subject of this study. A second major
area of government policy that could
influence the rate of acceptance of biogas
plants concerns the provision of technical
backup services to the plant. Again, it is
difficult to specify the characteristics of this
backup, but considerable experience has
now been gained with the working of
agriculture extension agents and the iessons
learned might be used to develop a model for
the provision of technical ‘extension’
An Approach to Research
Two ideas are central to the arguments in
this paper: first, research into biogas
techniques must be placed in the wider
context of policies for rural development
and rural energy needs; and second, research
and development of a particular set of
techniques must not be abstracted from the
social and political context in which the
techniques are to be applied. It follows from
this that before starting on research on
biogas, governments
(and research
organizations) must first decide how
important biogas is likely to be in meeting
the needs of rural peoples. At the most
general level this involves establishing the
range of other possible areas of village Iif:
that might also benefit from research and
innovation; more specifically it requires an
examination of rural energy needs, current
practices for satisfying these needs, and
alternative means of satisfying them in the
If biogas does appear initially to offer
advantages over other alternatives, research
and development must be directed toward
meeting a specific set of objectives. In
particular, the objective of maximizing the
production of methane gas must be set
against the objective of improving the
distribution of income (and therefore
energy) and the need to further the interests
of particular groups within society. This
trade-off will determine whether research is
to concentrate on those plants that would be
most easily accepted (which may be the
plant that meets the needs of the rich farmer
group) or on those plants that are likely to
satisfy the energy needs of a wider range of
social groups (when the community plant
might be favoured).
The appropriateness of a particular
package of biogas technology will vary from
location to location depending on the
objectives chosen and on the availability of
resources. This suggests that research
priorities will have to be specified in terms of
a process for conducting the research rather
than in terms of individual pieces of
research. It would seem
appropriate in ihese circumstances to
develop a process in which the views of
villagers and the views of the researchersare
combined in the development of research
activities. This will involve an iterative
process in which, in principle, viliagers
specify their needs,technologists specify the
technical options available, and the villagers
restate their needsin relation to the options.
An implication of this would he that the
location of the research would largely be in
specific rural areas rather than in the
laboratories of urban research institutes.
This does not mean that sensible lists of
technical research cannot already be drawn
rup (Moulik and Srivastava 1975, p. 53-58;
Sathianathan 1975,p. 164-172;Government
of India (lC4R) 1976,’p. 41-42), but it does
mean that much of this work will only be
useful if the views of the potenial users are
given an important role in the development
o-f the technology. Three broad areas of
economic and technical research already
, stand out.
Cost Reduction
The current designs for biogas production
at the village level appear to be considerably
more expensive than they need be. This high
cost not only reduces the numbers of plants
operating but it also reduces the advantages
that village-level biogas plants have over
larger-scale technologies (Prasad et al. 1974,
p. 1355; Disney 1976). In particular, many
current designs involve very .much higher
amounts of capital per unit of output than
more conventionai means of suppiying
energy (Disney 1976, p. 9).
Cost reductions can be acldeved through
the use of other materials (plastics or locally
available wood, water, and brick), through
more efficient designs, and by increasing the
efficiency of the fermentation process
(Prasad et al. 1974, p. 1355-l 356, discuss
methods of reducing construction costs),
Ideally, it should be possible to simulate the
likely balance between the costs and benefits
that would be associated with any change in
plant design or operating procedures. For
instance, the net benefit of increasing the
temperature of the fermentation process
might be compared with the net benefits of a
longer material retention time, insulation of
the plant from the surrounding air, agitation
of the material being digested, preprocessing
of the inputs, or the addition of supplements
such as urea and urine, etc. Similarly, the
unit cost of gas production might be
tabulated for a variety of different plant
scales and designs. I-however, neither the
biochemistry of the continuous process nor
the costs associated with such changes seem
to have been estabiished with sufficient
accur-acyto enable such precise comparisons
to be made.
The Need for More
Appropriate Data
The data that currently exist on the
viability of biogas plants are not only very
unreliable (with considerable variation in
the values assumed by different studies for
similar parameters), but are obtained from a
narrow range of possible plant designs and a
narrow range of socioeconomic and
agricultural zones. The practicality of
biogas plants will vary considerably amn3ng
different locations according to the balance
among i the various costs and benefits
operatifig in each location. It is therefore
essentiil that evaluations are carried out in a
much wider range of circumstances and
using a number of improved designs. In
Korea, all 23000 plants said to be operating
are all the same size (about 35 ftJ/day,
Mardon 1976, p. 16); in India most plants
are between 150 and 200 ftJ/day (even
though pianls up to 5000 ftj/day are offered
by the KVlC (Moulik and Srivastava 1975);
and in the Philippines the most successful
plant is said to be the 1000 ftJ/day batch
process plant (Philippines de la Salla
University; Mardon 1976, p. 18-19).
Such studies might adopt a method of
analysis similar to the one outlined here, and
particular attention should be given to
questions of the social acceptability of the
production techniques. The researchers
should involve the people who will be
affected by biogas investment as much as
possible both in the research design and in
carrying out the research itself. Attention
should also be given to the acceptability of
the gas for cooking and to the acceptability
of handiing dung and other wastes.
Research on CommunityScale Plants
The income distribution
in most
developing countries, combined with the
current costs of biogas plants, has led to the
fear that family-sized biogas plants wiil
worsen the income distribution of rural
areas and will in any case only satisfy a very
small proportion of total rural energy needs.
If income distribution and the need to satisfy
the fuel requirements of a larger proportion
of the community are important it is argued
that community-based plants are the only
option. 18
The number of community-based plants
(as opposed to large piants run by
institutions or agricultural industries) is
thought to be very small, and there ‘s
therefore a particular need to research ihe
issues surrounding such plants in varying
social situations.
l*Farvar and Bajrachanya (1975) stressthat
for reasons of income distribution
of community
community plants are really the only types of
biogas plants that would be promoted. This view
is also put forward by Government of India
(ICAK) 1976, Moulik and Srivastava 1975, and
Prasad et al. 1974.
Biogas Systems in Asia:
A Survey
S. K. Subramanian
Nearly 70”; of India’s biogas plants,
u,hich now total more than 36000, were built
during the fuel and fertilizer crises of 1975
76. H owe\scr. the Indian Council of
Agricultural Research (ICAR) had begun
anaerobic cow dung (‘gobar’ in Hindi)
fermentation as early as 1938-39(Pate1 1975;
Sathianathan 1975). Significant biogas
plant use began in I95 I, when the gasholder
and digester were combined into one
semicontinuous unit. The design most
commonly used was introduced in 1954 by
the Khadi Viiiage Industries Commission
(KVIC) and incorporated a device to stir the
slurry and break up the scum;
The KVIC design offers capacities of
between I .5 and 85 rn3 of gas output per day
at an estimated cost of between U.S.$260
and 4400 (KVIC 1975). However, most
plants are for domestic purposes and have 37 rn3 capacities. To reduce costs, these
designs are unheated and unstirred, and thus
The study reported in this chapter was
undertaken by the Management Development Institute (MDI) at the request of
IDRC. and was recently published as a
monograph by MDI. Their permission to
include this edited version in our book is
gratefully acknowledged.
A grant from the Indian Council of Social
Science Research. New Delhi, made
possible visits to over 70 Indian biogas
establish,ments and discussions with
officials, extension agencies, plant owners,
banks. etc.. while IDRC financed visits to
biogas plants and meetings with officials in
Thailand. Indonesia, the Philippines, South
Korea. and Japan.
require a depth of nearly 4.5 m and a
retention period of about 55 days. In the
larger (85-rnJ) units 4OY$oft he gas generated
1s consumed for heating (Gobar Gas
Research Station, Ajitmal).
KVIC is experimenting with larger 140 +
m3 units to gain experience for still larger
ones. Projects by other groups include the
Rural Electrification
125-m-’ unit, Karimagar District, Andhra
Pradesh (probably incorporating a 3.5-kW
gas pump to recirculate contents three
times) and a 420-m3 digester for the Delhi
Dairy Corporation.
Other Designs and Approaches
Early KVIC attempts to use split bamboo
as a digester construction material in West
Bengal failed becausethey were attacked by
rats. Some IO years ago, narrow (1-2.5 cm)
earthen rings were successfully used to build
a digester in Kalimpong, and three
prefabricated rings, 1 m x 2 m in diameter,
are currently used at a housing cooperative
in Sangli, Maharashtra, for a 3-mJ digester,
and four for a 4.5-m3 digester.
A number of new designs have been
published by the Gobar Gas Research
Station, Ajitmal (Rambux Singh 1973). Qne
uses agricultural waste insulation and an
external water jacket heated by a solar
heater that delivers 1.5 litres of water per
minute at 60 OC. It is claimed that by using
this process slurry retention time has been
reduced from 50 to 55 days to between 15
and 18days, and gas production has risen by
300% even in the winter.
The steel gasholders of the current !ndian
design were found to work best despite .+eir
high cost (35Ycot the total capital costs) and
problems (corrosion).
Painting once a year (or monthly with
engine oil) is recommended to prevent corrosion; at Urlikanchan, Maharashtra, 3
Iitres of engine oil are added to the top of the
digester each month for this purpose.
Alternative ferrocement gasholders were
found by the Indian Institute of Technology,
Madras, to be too heavy (producing gas
pressure of 20 cm of water) and to have poor
strength and flexibility; furthermore, they
could not be easily leak-proofed if a hole was
bored through them. An experimental
gasholder of woven bamboo, aluminum foil,
and a polythene covering was tried, but it
collapsed in a dust storm. Other local
constructiotl materials are being tested at
the Indian Institute of Scienceat Bangalore.
Early experiments in India with negative
pressures (-Icm water) were not successful.
Night Soil
The KVIC recommends connecting
toilets to cow-dung digesters, as this
conserves expenditures on septic tanks. The
Gandhi Samarak Nidhi Institution also
advocates night-soil-based plants and 55
such units operate in Maharashtra alone.
Examples include a 14-m3 plant using the
night soil of 187 inmates of a leprosy home
near Poona; two (IO and 4 m3) units at the
Parasakthi College for Women near
Tenkasi, Tamil Nadu, a large pilot project of
the National Environmental Engineering
Institute based on 1000 inmates at the
Nagpur Central Jail; and a 5.5-m3
experimental plant in the Ratnagiri bus
station, Maharashtra.
Other Wastes
Experiments on the anaerobic digestion
of grass, water hyacinths, and rice straw
have been conducted,
the latter
Operation and Maintenance
In the Indian design the dung:water ratio
is I: I. The design uses a relatively small and strawboard mill wastes have been
amount of water and there is good mixing studied by the National Environmental
due to the high height:width ratio (6:l) and Engineering Institute, Nagpur. In another
because the gas bubbles that rise from the case, using Hungarian technology, the
bottom of the piant prevent settling out of National Sugar Institute, Kanpur, treated
the sludge (Mardon 1976). Of the plants semiwet bagasse mixed with 3% cow dung
visited during this study, 89% were in opera- and 5% urea, using city sewage as an
tion; some for over 10 years. This improved initiator. This process has operated since
situation was due to the extension of service 1963, and involves a battery of i2 batch
facilities. Recommended maintenance pro- digesters (6 m x 3-m diameter). After an
cedures include daily feeding and agitation, initial 3-4 day aerobic treatment, the wastes
and annual painting of the gasholder. Plant are digested for about 40 days. A similar
failures were due to masonry construction project using a wet process at the Aarey
defects, failure to paint the gas holders, im- Milk Colony near Bombay produced gas at
proper feeding, lethargy of plant owners, a rate much below the design capacity and
proved to be uneconomical.
and changes of ownership.
Winter Operation
The low winter temperatures (0 OC) of
Northern India can cause gas production to
drop by 20-30%. Farmers cover gasholders
with plastic sheets after sunset during the
cold months, and the addition of molasses,
algae, urea, or urine is claimed to increase
gas production. Authorities in Haryana
State recommend building plants one size
larger than needed to overcome the problem
of low gas output.
As most of the units are family-size, the
gas is used for cooking: per capita
consumption was observed to be about
0.2 mJ/day and much loss seemed due to
inefficient burners. Specially designed
burners can overcome the low pressure and
flame propagation of CO,-diluted methane,
although coal-gas, LPG, and homemade
tin-can burners are also used. At Parasakthi
College, Tenkasi (Tamil Nadu), two plants
(17 and 28 m3) feed an efficient boiler,
generating enough steam to cook for 730
Biogas use ‘r-rdiesel engines using a biogas
to diesel fuel ratio of 85: 15 has increased.
The institutes of Technology at Madras
and Bombay, and the Indian Oil Company
Research Centre at Faridabad
researching other engine applications.
The Tulsi Shyam Temple at Gujarat has
(since 1966)used a 85-m3 plant based on 300
cattle to run an engine that drives a water
pump and a flour mill, in addition to
generating 7.5 kVA of power for 4 hours at
industrial uses of biogas are limited, but
examples include small-scale KVIC soap
and safety-match projects, and a water
heater at a laundry near Bombay (42 m3).
There is a growing interest and emphasis
on the manurial value of the digested sludge,
and it has even been suggested that biogas
plants would be more correctly named ‘biofertihzer plants. If the digester is close to the
fields, the slurry is fed directly into the
irrigation channel, but most often there
must be severai drying pits, which are used
in turn. Many farmers add grass, straw, and
bagasse to the slurry pit, thereby speeding
up the composting process to about 3
months (instead of 9-12). in contrast to
farmyard manure, slurry breeds no white
ants and contains no weed seeds; a further
advantage is that, unlike dung, the slurry is
not stolen for fuel.
Vegetable farmers use digester slurry
alone; others mix it with chemical fertilizers.
Experiments at the Lalit Garden near
Calcutta, West Bengal, comparing vegetable
growth using compost, chemical fertilizers,
and slurry found taste and size, especially of
peas, best with the slurry. Weight of root
vegetables increased by nearly 300% with
night-soil slurry fertilizer compared with
normal irrigation practices at the Central
Jail Nagpur, Maharashtra. Similar success
with Napier and Tara grass crops was
reported by the V.S. St. John’s Secondary
School, Gannavaram, Andhra Pradesh, and
with sugarcane at Digras, Maharashtra, and
Katur, Andhra Pradesh. in both these cases
the plants were built essentiahy for the value
of the sludge.
Slurry is claimed to be ideal for nurseries
and it has beenused to correct the overuse of
chemical fertilizer in rice fields (Dinikaki,
West Bengal). Slurry has also been used
successfully as a direct fish feed (after
dilution) in West Bengal.
Integrated Systems
India has no system that attempts to
integrate the slurry with the growing of
algae, which in turn can be used to feed fish
etc. Thick colloidal cow-dung slurry does
not easily separate into sludge and a
supernatant clear layer, possibly because of
the restricted use of water for dilution in
India. Even night-soil slurry has to be
gravel-filtered at the Nagpur Central Jail
where it is proposed to use the filtrate to feed
algae and fish, and to irrigate crops. Using
the Slurry, Auroville Centre, Pondicherry,
Southern India, plans to grow water
hyacinth for banana plantation mulch.
Community Plants
Although there are a number of large
biogas plants in India, none can be said to be
a truly community plant. However, one
Indian ‘mini’ community system operated
between 1969 and 1970 in Khiroda
Panchayat, near Bhusaval, Maharashtra. in
this system several public toilets fed three
digesters (5.5, 14, and 25 m3) and the gas
provided light for two city streets. Failure of
the system was attributed to the transfer of
the key operators and the electrification of
the village.
KVIC and the Rural Electrification
Corporation, together with the Council of
Scientific and industrial Research, are
planning community plants at Digras,
Maharashtra, and Karimnagar, Andhra
Pradesh. The Digras plant will be fed by 20
animals and 10 community toilets; each
family will be charged one rupee per month
for the use of the toilets, generating an income of $1 I /month, and the gas will be sold
to ten families. The slurry will be given back
to those who supplied the dung in
proportion to the number of cows owned.
The Karimnagar plant (125 m3), based on
300 cattle, will provide gas to half the village
(30 families), and run five 3:5-kW pumps,
The system will be admidistered by the
village and will employ two labourers.
Three semicommunity operations exist at
the VSF Cooperatives, KCP Sugar Factory,
at Vuyuru,
Madhavpram Dairy in Madras, and at the
Kasturba Gram Krishi Kshetra, Indore. The
first, a 35-m3 plant based on 70 buffalo and
calves (and ten baskets of sugar press mud
daily), was built for $2200 (using a 25%
government grant). This system supplies gas
to 14 families for 3 hours daily at
$2.25/month, one full-time labourer is
employed and the system is considered
uneconomic. The second (14 m3) supplies
seven houses(50 occupants) with 11 hours a
day (nine in winter) for $1.75/month, again
a full-time labourer is employed. The last, a
70-m3 plant, supplies gas to 40 families 24
hours a day during February-July, and 14
hours a day for the remaining months. It is
based on 200 cattle that produce 1600 kg of
dung daily.
Minimum Number of Animals
The number of animals neededto support
small domestic digesters is a key criterion in
assessing the acceptability of biogas systems. It is usually suggested that five cattle
are required for a 1.5-m3 plant; this could
put the technology out of reach of most
Indians. However, this survey suggests that
some 1.5-3 m3 plants can be operated on two
cattle or on an attached toilet and one
animal because per capita demand with
good burners can be as low as 0.08 mj/day.
Dung output depends on the animals’ feed
and breeding, but the quantum of gas output
is currently considered to be about 0.06
m3/ kg of dung. The KVIC now recommends
2-3 animals for a 2-m3 unit, and 3-4 for a 3m3 plant.
Extension and Credit
The KVIC has played a key role in the
extension of biogas. It has technical staff
posted in all states to offer free expert
advice. in response to an increasing
workload the ‘supervision charge scheme’
was devised, and about 400 approved local
artisans canvass potential customers, assist
in construction, and help secure loans in
return ‘for a set fee from the KVIC (usually
about $20 plus $2.50 per toilet). Today most
State Boards also have their own technical
staff, and since 1973some State Agricultural
Departments have been mobilized for the
biogas program. in Haryana the entire
District Government has been involved.
Until 1973 the KVIC gave grants of 5070% to institutions (100% for “backward”
areas); individuals got a $35-42 grant
according to need, and an interest-free loan
of up to $285 repayable over up to 10 years.
This meant that $88-94 had to be provided
by the plant owner who ended up paying
between 24 and 52% of the total
construction cost.
Commercial banks and State Agricultural Departments entered the field in 1973,
and the subsidy was li.nited to 250/, with
KVIC giving the grant to institutions and
the Ministry of Agriculture making grants
to individuals. in 1976-77 the subsidy was
reduced to 2070, and 5% decreases are
planned for each of the next 2 years, after
which it may be withdrawn. Banks provide
the balance (up to 100% of the remaining
cost) with a 4-year loan at 12-14% annual
interest, on the basis of a mortgage, or
personal and third-party
and “backward-area”
installations will likely continue to receive
liberal subsidies.
An interesting example of the provision of
extension services in the cooperative sector
is the Cooperative Sugar Factory at Sangli,
Maharashtra. Biogas units have been built
for sugarcane grower shareholders through
a building cooperative using prefab
structures. The mill guarantees bank loans
and the advance is reco,veredfrom payments
against the sugarcane crop. The scheme has
existed for 2 years and will soon be extended
to nonshareholders.
Recently the Government has launched
the “All India Coordinated Project” on
biogas for an integrated development of
technology. The Government, the Reserve
Bank, and other management institutions
tion. For a 20-day retention at 30 “C, the
manual (institute of Agricultural Engineering and Utilization, 1976) estimates gas
The Republic of Korea
production at 2OO-240%of digester volume.
Nearly 27000 small digesters have been But actual gas production for the 5.5-m”
installed in Korea since 1969 through the model was 0.3 m3 in January and 2 rn3 in
efforts of the Office of Rural Development September, or a 36C+&of-given-capacitypeak
(ORD). However, the cold winters and lack production.
of cattle make Korea’s experience with
Winter Operation
biogas quite different from india’s. ORD
Most farmers do not operate the digesters
estimates that the country’s severe winter
results in national average fuel requirements between December and March, when
of 43Yc,for heating and 53% for cooking temperatures are as low as -17 OC, and gas
(ORD 1976). A rural household consumes production is even inadequate for cooking.
3.5 tonnes of farm products, 2.3 tonnes of The gasholders are covered with straw
firewood, 200 coal briquettes, and 20 litres during these winter months. Vinyl covers
of kerosene each year. Home heating is by were tried but were ineffective and furthermore the sophistication of heating the
the traditional ‘ondol’ under-the-floor
system for which most of the rice straw, digester was not justified for the small
barley waste, and wood are burned: much plants. Operation is more favourable in the
deforestation and loss of compost material warmer South.
has resulted.
Research, development, and extension
Large-Scale Heated Digester
are handled by the Institute of Agricultural
ORD has embarked on the development
Engineering and Utilization, the Rural of village-scale digesters. A 40-family,
Guidance Bureau (under ORD) at Suweon, 155-m3 digester was operated at the Liveand the College of Agriculture. ORD also stock Experiment Station, Suweon, t. .r -.
conducts experiments and is working on a the Korea-UK Farm Machinery Project
large heated digester under the Korea-UK during 1976. The plant is based on 2.4
Farm Machinery Project.
tonnes of dung from poultry and 170 cattle
and has a retention time of about 40 days.
Scale of Operation and Design
The dung is mechanically mixed with water
All the field units are of household size and urine in a separate unit (solid:liquid
and consist of a rectangular underground ratio of 1:2); 33-40% of the gas generated is
concrete tank with an overflow, a feed pipe, used to heat the digester to maintain a
and a mixing tank; digester capacity is 5.5 or temperature of 35 “C year rocnd. in winter,
8.0 rn.3. The O.I-cm-thick PVC gasholder hot supernatant liquor from the digester is
(later models have four compartments) rests used to melt the ice that forms in the mixing
inside the digester. The rectangular design is unit.
being changed to a circular one and a steel
The primary digester is about 6 m in
gasholder may be adopted because of the diameter, and is almost totally underrise in price of PVC; the PVC holder costs ground; the top surface is well insulated. A
$55 (as opposed to $65 for steel) and it dete- secondary digester (6-m diameter by 4 m)
riorates in sunlight. The price of the whole supports the gasholder, which has a capacity
5.5-m’ unit is low at about $140. Wooden of 110 m3. The gas pressure is lo-15 cm of
gasholders with a plastic lining were aban- water column. The gas is compressed and redoned because they leaked.
circulated by a ‘bubble gun’ through the
primary digester, and this breaks up any
Operation and Maintenance
scum builc‘llp. Experiments are continuing
Cattle or pig dung (sometimes with night on the use of biogas in kerosene engine apsoil) is fed monthly or weekly at a I : 1 dilu- plications and home heating.
are concerned about meeting the 1978target
of 100000 plants.
The total construction cost of the plant
Minimum Number of Animals
was $16000; $9600 for structures, $4060 for
A 4.5-m” digester is said to need the total
steel pipes and the gas holder, and $2400 for waste output from 8 cows, 23 pigs, or 630
machmery and instruments. The ORD is fowl according to the design manual of
planning eight more units that will be Institute of Agricultural Engineering and
located in villages.
Utilization, But often only two cows and a
-..--I.. *I..
fe-wpigs supply
*-==.-holdplants; therefore, gas output is often handicapped.
Other Developments and Inputs
The Institute of Agricultural Engineering
Extension and Credit
and Utilization is experimenting with PVC
In each subregion (“@n”) the Rural
and concrete fixed-dome digesters. The
Office of the ORD (Ministry of
College of Agriculture at Suweon is working
on a two-stage digester of reinforced plastic Agriculture and Fisheries) provides techinsulated with paddy husk. The plant is de- nical extension and financial loan assistance
signed primarily for pig manure at 15 to farmers, but there is no regular loan
dilution and a 30-day retention; interest- system and the 33-5070 Government grant
ingly the gas passesthrough an algal culture system has been discontinued. Most of the
to use up the CO, (Lee and Kim 19’75).A biogas construction in Korea is undertaken
primary school in Kyong Ju-Shi is operating by the farmers themselves.
Rapid urbanization and the shortage of
a plant with night soil and the army has
waste slowed the construction of
shown interest in this, although there appears to be psychological inhibitions against family units in rural areas during 1975 and
its use. The digestion of vegetable wastes has barely 4000 were built. In 1976 the ORD
installation to
received scant attention in Korea (Lee and abandoned family-unit
concentrate on the development of villageKim 1975).
size units, gas storage and purification,
power generation, etc.
No farmer is totally dependent on biogas:
it supplies only 3-6Yb of home heating, and
The Philippines
less than half the cooking needs(43-45s as a
family of five needs 0.7 m3 of gas for 3 hours
Fuel is not a major problem in the
daily). Biogas saves about 226 hours of Philippines as firewood is plentiful. Conhousework per family per year (ORD 1976). sequently, interest in biogas stems from its
Thus each house has a cooking fire (Mardon pollution control and public health applica1976) in addition to an unmodified LPG tions. Pigs (and some buffalo) provide most
burner for biogas. However, heating, of the animal wastes, but despite some poscooking, and power will be provided by the sible psychological inhibitions the National
larger village-sized plants.
Housing Authority (NHA) is also promoting night-soil digestion, and one digester
is already operating. Techniques to avoid
Slurry in boxes is carried by hand to the night soil overdilution and to screen out
nearby fields, mixed with compost, and harmful detergents have been developed.
applied; the sludge acts as soil conditioner. Further units are being considered for a
However, at the Livestock Experiment Manila hospital and for the proposed
Station pilot digester, the slurry is diluted Palawan Island Resort.
after settling and pumped directly to the
The major research activity is centred at
fields. No special emphasis is placed on the the National Institute of Science and Techmanurial value of the slurry, and it is not nology (NIST), at the University of the
taken into account in the evaluation of the Philippines at Los Ba’i%os,and Maya Farms.
pilot plant. Future plans involve the The greatest potential seems to be in the
building of oxidation and algal fish ponds. digestion of agricultural wastes as their
volume is estimated to be 1000times greater
than livestock wastes.
Field Experience
Nearly 100 Taiwanese-type units have
been built under NIST guidance. They cost
over $690, and need the manure from 5 to 10
pigs diluted at 1:3. These units produce
enough gas to supply the cooking needs of
five people. Some other plants have been
built by individuals on their own initiative.
A NIST-prepared
of 10
methanogenic isolates is recommended as a
starter. In the Philippines, the digested
slurry is not used to any extent as a fertilizer.
The Chan-type digester from the South
Pacific has been adopted lately as it allows
more room for maintenance. Prefab
digesters and galvanized iron gasholders
have also been experimented with by NIST.
New digesters both for integrated systems
and for agricultural wastes (straw, banana
leaves water hyacinth, etc.) are being
worked on. Findings include the discovery
that banana leaves or straw that have previously been used for mushroom growth are
more readiiy digested.
Integrated Systems
The University of the Philippines at Los
Baliios has an integrated biogas system that
uses slurry to grow Chlorella, fish, and rice.
Waste from 10 pigs is diluted in the proportion of I:4 and fed to two digesters in series
(2l-day retention). The gas would be sufficient to meet the cooking needs of five
people. The slurry is settled out in two settling tanks, positioned in series,diluted, and
channeled to the algal fish pond and the rice
fields. A windmill stirs the algal culture and
transfers liquids.
Maya Farms
Maya Farms (40 miles south of Manila) is
the largegt Asian biogas establishment, with
48 large (2.5~3~3 m) batch plants based on
7500 pigs (soon to be increased to 15000).
Every other day, a digester is fed with 5
tonnes of a dung (dilution 1:1)and some predigested slurry as a starter. The contents are
then stirred mechanically every day for 2
minutes. The gas (rich in CO,) produced in
the first 3 days is purged; after this, gas
production is ideal for 23 days, at 60-80 cm
water column. On an experimental basis,
paddy straw is mixed with the dung.
In addition, five continuous digesters of
Indian and Taiwanese designs are operated,
producing gas at pressures as high as 45 cm
water column; straw digestion experiments
with these plants failed due to clogging.
Daily gas production for the whole farm is
about 560 m3, although the technical
capacity is 840 m3, but this would require
more animals. The gas storage capacity is
about 140 m3.
The gas is used in a canteen (daily per
capita consumption is 0.1 m3), a meat
processing plant, and a soup cannery. The
gas also powers a 625 litre/minute water
pump with an old 35-kW car engine, which
has a consumption of 0.6 mJ/ kW/ h of gas.
Similarly, a I IO-kW car engine has operated
a 60 kV.4 generator (1800 rpm) 4-5 hours a
day for 6 months to run four freezers.
Generally, biogas-run gasoline engines were
found to give higher-than-rated rpm’s,
though with diesels the rpm’s are lower.
Refrigerators with a gas consumption of
0.08 mJ/hour, water heaters, lamps,
burners, etc. are also on display. The
burners are LPG models with the holes enlarged to 0.3-cm diameter. Steam generation has been abandoned because of low
The sludge from the few continuous
digesters is fed directly into irrigation water,
but the batch-digested sludge is settled out
for 10 days, dried (with heat from biogas in
rainy weather), and used as soil conditioner
for submarginal soils. The liquid from the
settling lagoon along with wash water is
aerated for 7 days at 75 psig of pressure. A
windmill will soon be used for the air compression in the aeration process. After BOD,
COD, salt, and water plant growth analyses,
the solution is used to fertilize the rice crop
and grow Chlorefla, which feed both fish
and animals; feed waste is also added to the
fish pond. The treated solution has been an
effective fertilizer despite a low (l-2%)
nitrogen content (except for an excess
introduction of copper) and in fact, over-
fertilization has often resulted. Reduction of
the treatment cycle of the digested
slurry/sludge is being attempted.
A network of satellite farms has been
established to minimize the risk of infection
among the animals. The satellites consist of
units of 25 young pigs that are supplied together with biogas technology to smaller
farms in the area; 25 such units at eight local
farms exist at present.
Extension and Credit
Responsibility for extension work is
divided among the National Housing
Authority., the Engineering Battalion of the
Military, the Community Development Department, and others, although the NHA
has a coordinating role in new settlements.
The Development Bank of the Philippines
recently began to give loans to pig farmers
for biogas plants at 6% interest; approximately $412 (3000 pesos) is given for single
and $550 (4000 pesos) for twin digesters.
The Division of Agricultural Economics,
Ministry of Agriculture, built a demonstration plant as early as 1965(Deemark 1975),
but subsequent development has been
hindered by a shortage of livestock wastes.
Currently the Department of Animal Husbandry at Kasetsart University, the Department of Health, and the Applied Scientific
Research Corporation are all developing
various biogas systems. The major constraints to the development of biogas
appear to be the convenience of wood and
charcoal as a fuel and the lack of dung.
The sanitation centre in Sara-Buri near
Bangkok, in collaboration with the Faculty
of Public Health, Mahidol University, is
working on night-soil biogas research.
Kasetsart University has also built four
family-sized units on the campus for research and demonstration purposes. NO
community-size plants are contemplated.
Galvanized Iron Gasholders
Early galvanized iron (G. I.) gasholders
built by the Division of Agricultural
Economics, Ministry of Agriculture have
had to be replaced by the more expensive
steel models. But during the survey three
satisfactory G.I. units were found in Lopbury Province, and in Ban Mee District galvanized iron coated with asphalt was used to
build a 1.5-m (diam.) gasholder that has performed satisfactorily since 1973,for only 600
bhatt ($30), or half the normal cost. Leaks in
another G.I. drum built for 400 bhatt ($20)
were repaired with white lead and boat
caulking cement. and one such plant has
operated for 12 years. Interestingly, _L this
latter case, shade was claimed to improve
digester performance.
Industrial Wastes
The Applied Scientific Research Corporation of Thailand is developing an
anaerobic digestion process to treat about
1500 m3 per day of high-BOD distillery
waste (potential dai!y production should be
about 47500 m3 of biogas). This is primarily
to treat waste and the methane will be flared
off because impurities (especially H,S) rule
out distillery use, and scrubbing & considered too expensive.
Field Experience
Extension and Credit
There are now nearly 225 family-sized
(2.8-m3) units based on cattle or pig wastes
in Thailand. The loading is 20-40 kg/day at
1:I- 1:1.5 dilution; feeding varies from daily
to monthly. The gas produced meets the
cooking needs of between 5 and 7 people
using homemade or LPG burners. Some
farmers use the digested slurry on their
gardens, but most use chemical fertilizer
(ESCAP 1975).
Sanitation is the most important consideration in biogas installation in Thailand
and consequently the Health Department
has responsibility for its promotion,
primarily to control disease carriers like
fruit and house flies (Mardon 1976). Coordination is carried out by the Rural Development Department.
A sanitation officer from one of nine
centres travels to the villages. Farmers who
are interested in a plant make an initial payment, after which, delivery is arranged.
Most plants have been built in Saraburi
province with some units being installed in
the homes of headmen for publicity. The
iron molds used for gasholder construction
unfortunately limit digester size.
A government subsidy scheme has now
been withdrawn, but the Agricultural Banks
are contemplating loans for biogas as part of
a fertilizer scheme.
Only twelve units are said to be in operation in Indonesia as firewood is plentiful in
most areas and animal wastes are not.
Muslim opposition to pig dung use may also
be a limiting factor as it forced the Indonesian Board of Voluntary Services
(BUTSI) to move a plant built at Bakum.
Oil-drum demonstraticn units (digesters
made of one or more empty oil drums)
include a train of six double-drum poultry
manure units at Denpasar. Two units are
used at Petung - one heated with compost
around the drum and another based on
night soil. Various dilutions are being tried
and the use of solar heating is being investigated by a civil engineer at Yogyakarta. A
7.8-m3 rectangular unit produces cooking
fuel at Atuag. In 1976 Community Aid
Abroad (Australia) assisted a Bogar school
to set up a $600(250000 rupiyah) 3-m3 Chan
digester based on 25% of the dung of 20
cattle - lack of water forced operation at a
dilution of 1:2. Predigested slurry is used as
a starter, and feeding is once every 2 days.
The school has also experimented with a
three-drum unit. There is great reluctance to
use the gas, however.
The Development Technology Centre
(DTC) at the Bandung Institute of Technology recently set up a 1 x 2 x 0.95 m rectangular unit with an overhead gasholder
and a water-jacketed top at the Buruacljak
dairy farm, Lebang. Feeding is weekly (dilution 1:2), and a glass window allows obser-
vation. Several triple oil-drum digesters are
also operated by the DTC at Lembang.
Removable connecting joints allow better
The village technology unit of BUTS1
hopes to promote biogas by demonstrating
its feasibility to village chiefs.
The Bogor Biological Institute will soon
launch a biogas program based on agricultural wastes -. 3 1million tonnes of corn and
paddy stalks (four times the animal wastes)
are available yearly (Sudirjo and Kismomihardjo 1975).
Night-soil use is likely to be accepted for
digestion though the use of slurry as manure
has yet to be, especially in Bali, where a
witch doctor attributed sickness to its use.
Small digesters are said to have operated
in the Tohaku region for many years. Recently several institutions, including the
National Institute of Animal Industry at
Chiba, the Public Works Research lnstitutes, the Fermentation Research Institute
at Anage, M/S Hitachi Plant Construction,
the Ministry of Agriculture, and the Agency
for Industrial Science and Technology
(MITI) have worked on anaerobic digestion
of rural, urban, and industrial wastes for
pollution control. Japan is the only country
in the region to have adopted high-ternperature digestion (in the thermophilic range) of
some wastes.
Livestock Wastes
The growing Japanese pollution problem
has resulted in a spate of antipollution laws
and methods of meeting them. Since 1973 a
multiinstitutional, nation-wide effort has attempted to reduce the pollution problems of
animal wastes. The energy crisis added
further impetus to this effort. Digester
experiments include a 20Glitre unit (60 cm
in diameter, made of fibre-reinforced plastic
insulated with 5 cm glass fibre) based on the
wastes of one pig diluted I:3 with a 16-day
retention period. A 160-W submerged pump
Recently, digestion of distillery waste has
been discontinued because it does not
remove the brown colour pollutant. The
lOOO-2000-m3units at the Chiba Distillery,
for example, have been converted into
aerators, with the concentrated sludge being
discharged into the sea.
agitates the contents, and a temperature of
35 “C is maintained. Gas production is
about 20 litres of 62% methane per day. Dry
matter content of the slurry is 5.2%; organic
matter content is 3.6% wet and 71.83% dry
basis; nitrogen content is 0.32% wet and
6.19% dry basis (Yagi 1975).
A similar larger unit (5 m3; 1.5 kW pump)
based on 25 pigs at the Kagawa Prefecture is
being researched by the Ministry of Agriculture. Digested slurry from this unit is fed
directly to the fields (Yagi 1975).
A large Kochi Prefecture digester, which
is insulated with vinyl, maintains a yearround temperature of 30 “C. Poultrydropping digestion experiments have resulted in toxic ammonium ion accumulations (over 300 ppm). No night soil
digestion has been tried.
The Nippon Veterinary and Zootechnical
College, Department of Animal Hygiene,
found mesophilic digestion more efficient
than thermophilic (Kamata and Uchida
1972). and the Public Works Research Institute found that sewage sludge contains
heavy-metal contamination that is likely to
render it useless as a fertilizer for edible
Urban Wastes
These wastes are currently either incinerated or used as land fill. But, as part of the
MIT1 “Sunshine Project” Hitachi Plant
Construction has been investigating their
optimum anaerobic fermentation (Takatani
et al. 1975)and has concentrated, since April
1975, on thermophilic digestion only. The
experimental units are small (1 litre, 100
litres, and 1 m3), but a 1200-m3plant is being
planned. The dilution is 1:2 and the C/N
ratio is kept at l/20 with ammonium carbonate. The retention periods are 25 and 7
days, respectively, for mesophilic and
thermophilic digester. The latter has a
loading rate approximately 2.4 times
greater, and in winter one-third of the gas is
used for heating.
Other Countries
Iudustrial Wastes
The Fermentation Research Institute,
Inage, has been promoting thermophilic
anaerobic digestion of industrial wastes (i.e.
distillery, butanol, yeast, antibiotic, and
paper mill waste) since the Second World
War. BOD removal is 70-90%, and the
sludge is used for fertilizer.
Both thermophilic and mesophilic digestion produce the same amount of biogas per
unit of volatile solid, but the former allows
reduction of the retention period to 5-7 days
and loading rates 2.5 times greater
(mesophilic loading rate is 2-3 g/ litre/day
versus 5-6 for thermophilic digestion). This
allows a considerable reduction in digester
size (Sonoda et al. 1965).
Twelve distilleries
have anaerobic
digesters; five working under government
supervision treated 200 million litres of
wastes in 1966 and recovered 170000 m3 of
gas for fuel (Fermentation Research Institute 1974).
As alternative fuel sources in Bangladesh
are limited, the Council of Scientific and Industrial Research and the Bangladesh
Academy of Rural Development have built
some demonstration plants. Polythene-bag
designs are, in an attempt to reduce costs,
being tried at the Bangladesh University of
Engineering and Technology at Dacca
(Islam 1976). Long-range plans include
combined water hyacinth - cow dung
digestion and village-size plants are also
being considered as they may be suitable for
the particular social structure (Eusuf 1975).
Biogas is extensively used for cooking,
lighting, fertilizer, and for small internal
combustion engines (Fang Chen 1976). As
of September 1975, over 200000 family-size
(lo-m3 capacity, generating about 5 m3 of
biogas per day) digesters were operating in
the province of Szechuan (ESCAP 1975).
They are built essentially underground, of
brick, cement, and pebbles, with no moving
parts; the gas pressure is kept constant automatically by changing water levels. A key
factor is the size of the door connecting the
fermentation tank to the outlet chamber.
The summer temperature is about 23 OC,
winter temperature about 10 “C.
Maintenance is carried out once a year
(People’s Publishing House 1974). The feed
* is a mixture of urine (3OYo),night soil (IO%),
and water (5OYo);vegetable matter is decomposed for 10 days before inclusion. Lime
solution or grass ashes arc added to maintain a pH of 7-8. The burners are made of
soil and carbon ash, with a biogas:air ratio
of 1:10. Free-standing biogas lamps are used
( Production Team of Tang Ngan 1973).
The ResearchOffice for Parasitic Disease
Prevention and the Revolution Committee
of the Mien Chu District Communicable
Diseases Prevention Office have found that
the best pathogen control method is to
remove digested sludge from the middle of
the digester to allow worms and eggs to
settle in the digester. Physical and chemical
destruction of the worms and parasitic eggs
is then carried out after 6 months retention
when the digester is fully emptied (Research
Office for Parasitic Disease Prevention,
Province of Szechuan 1973).
1975). Demonstration units at military dairy
farms, universities, and integrated rural development centres are built at Government
Problems include low winter temperatures, waterlogged hilly areas, and high steel
costs. The Appropriate
Development Organization designs and
builds lo-m3 fixed-dome digesters based on
Chinese technology (cost per unit about
$590, or 5600 rupees, ESCAP 1976).
Sri Lanka
A 2.8-m3 demonstration plant was built
by the Industrial DevelopmPnt Board,
Ministry of Industries during 1973-74. Research and development were carried out
concurrently by the Peradeniya and
Katubedda campuses on vegetable material
digestion (especially of salvina and water
hyacinth). The G,‘-Frnment plans to introduce subsidies -c ‘I mrzge the use of nonnd the Asian Rural
conventional I uc.
Energy Rese,*.T
c*i “‘eoject Experimental
Station to be . 2;
In a rural village near
Hambantota, VI .I~ was UNDP-assisted,
will also havf airrgas generators.
A cheaper and more compact generator
with no moving parts ix being developed by
IDB and is named the “Lakgen.” The main
components will be two underground brick
static tanks, with one at a higher elevation
than the other, so that gas pressure is maintained by the slurry. In addition to low
initial cost and easeof operation, the gas will
Since the first biogas plant in 1970, the be available at a higher pressure than in the
Development and Consultancy Services of current units (Industrial Development
ihe Butwal I-ethnical Institute and the Board, Sri Lanka 1976).
Energy Research and Development Group
under Tribuvan University have contributed
to the construction of 1002%m3 digesters in
In 1973 there were said to be nearly 7500
1975 alone (cost: $400 each). Major
biogas units on the island, most
problems include transportation and steel
costs, water access,and the provision of loan based on 12 hogs. The design includes a
unique manual mixing device made out of
credit when fixed assets are limited.
PVC pipe tied to a piece of rope. The
digested sludge is used as fertilizer, with a
small part being used for Chlorella cultivaThe Government has built nearly 100 tion (Chung PO 1973).
units, some capable of producing 11 m3 ot
Integrated systems, combining a bag
biogas per day. Gasholders are free if the digester with algal and fish cultivation, are
farmers build their own digesters (ESCAP said to originate from Taiwan (ESCAP
diversity of successful systems suitable to
conditions in the country of application.
This should be recognized in planning future
programs, as emphasis on uniform design
may seriously reduce the potential usefulness of the technology, which may well
depend on being adapted to the detailed
features of the location in which they are to
be used.
The number of viable systems may be
Interest of the International
even larger than those established to date.
Agencies in Asia
Thus experimentation under local condiBiogas systems‘are now receiving atten- tions must be fostered, although it may well
tion from several international agencies be possible in the context of highiy stanfollowing the crisis in the supply of energy dardized (perhaps mass-produced) parts,
and fertilizer. After its 1974 Colombo components, and materials.
Declaration, the Economic and Social Commission for Asia and the Pacific (ESCAP)
Technological Aspects of
held biogas workshops (in New Delhi,
the Region’s Experience
August 1975, on Technology and Economics, and in Manila, October 1975, on
The socioeconomic factors governing
Fermentation Technology), and began publishing newsletters. The Energy Division of biogas systems are very much interrelated
ESCAP will survey the potential of energy with the technical factors. Higher digestion
resources in the region (including biogas) in efficiencies reduce plant cost; whereas,
cheaper and more easily accessible inputs
UNIDO has asked UNDP to finance a and the efficient use of outputs can bring the
global project on biogas plants during 1978- system within reach of a larger section of the
1982, but the future of the project is still un- rural community. Some of the technological
certain. UNICEF and WHO have also ex- findings of the survey are presented here.
pressed interest.
Under UNEP’s Rural Energy Project, Biochemical and Other Operational
pilot digesters are being built in Senegal and
Sri Lanka, in cooperation with the Brace
Research Institute, McGill University,
Poor Digestibility of the Dung
Canada, and Oklahoma State University,
One kilogram of volatile solids could proUSA. A 84-m3unit in Sri Lanka is to run a 6duce
0.75-1.0 m3 of biogas at normal
k W generator for lighting and welter supply.
UNEP is presently considering a request temperature and pressure (NTP), depending
from Kenya for assistance in harnessing on the quantity of carbohydrates. fats, and
solar, wind, and biogas energy in a rural proteins in the feed. Although dung contains
area. Through its International Referral 75% volatile solids (dry basis), the digestion
System (IRS) UNEP is also facilitating in- efficiency is only about 20% (cow dung proformation exchange on the subject. The duces 0.09-0.2 m3 biogas/kg, volatile
World Bank has expressed interest, but be- matter; sewage sludge 0.4-0.6 m3, Mohan
lieves that a thorough analysis of the tech- Rao 1974). Most of the lignin-bound
nical, economic, and financial feasibilities cellulose is not digested.
The efficient digestion of cellulose demust yet be made.
pends on its rate of hydrolysis into sugars.
Some Generalizations
This can be achieved by heating under presWithin the limited efforts made so far in sure, or treatment with acids and bases.
Asia, there is already a considerable However, these are not applicable at small
1975). The digester bag is a light, massproduced bag of 0.55-mm hypalon laminated with neoprene and reinforced with
nylon, with a PVC inlet and outlet. Small
circular (5-30 m3) and large rectangular bags
(50- 100 m3) are available from Fortune Industrial Corporation, Taipei.
scales,and the only possibility at this scale is
a relatively low-cost cellulase enzyme. But
there has been little research on hydrolysis
even though it is largely responsible for the
slowness of the digestion process (see
Chapter 1). Hitachi and the Nomura Research Institute, Kangawa, Japan, are
studying some problems of hydrolysis in
paper-waste digestion, but it should be
noted that increased digestion reduces the
humus content of digested slurry though the
nitrogen content remains largely the same.
Use of Other Inputs
The field experience of the Asian region
(except Japan) is essentially confined to the
treatment of livestock wastes and night soil;
however, the digestion of fresh and dry plant
residues, algae, and various marine, agricultural, and biological wastes is being investigated in pilot plants. For example the
National Sugar Institute, Kanpur, India,
has been operating a complex agricultural
waste system. Water hyacinth and algal research has been initiated in Bangladesh,
India, and the Philippines and nearly 1.9 ml
of biogas/ g of water hyacinth have been obtained; cadmium and nickel contamination
actually increases production (NASA
1974). Evidence exists that pretreatment of
agricultural wastes (such as chopping,
soaking, decaying, or mushroom cultivation) assists their digestion (NIST, the
Philippines). Where livestock wastes are
scarce, the digestion of these other organic
wastes becomesvery relevant, but little data
currently exists. More research in this field is
Frequency $f Feeding
On the basis ozthe Indian and Chinese experience, the secret of successful biogasplant operation lies in the daily feeding
cycle. Ideally, but impracticably, the feeding
should be continuous. The National
(NEERI), India, resorts to feeding three
mimesa day, and some farmers in india
practiced twice daily feeding in winter.
Outside these two countries, however, daily
feeding has not been generally adopted.
Organic Loading
The production of gas is also dependent
on the weight of volatile solids added per
digester volume per day. The size of the
digester consequently depends on the
loading, which in turn depends on dilution,
time, and temperature
digestion. Maximum loading is 2-3
kg/mJ/day in mesophilic digestion, and 5-6
kg in thermophilic. Loading rates in India
(mesophilic) are around 1.6-2 kg/ mJ/day.
Loadings of 3.17 and 3.2 kg/ m’/day have
been achieved (Yagi 1975; NEERI, India).
A further 2-300% increase is possible if t he
sludge concentration is increased to over
10% (Fermentation Research Institute;
Sonoda et al. 1965). Loadings of 1.8-7.6 kg
for mesophilic and I .8-l 8.8 kg for
digestion are reported
(Kamata and Uchida 1972). Hitachi of
Japan (urban wastes) reports rates of
0.77-4.7 kg and 1.73-12.6 kg for rnesophilic
and t hermophilic digest ion, respectively
(Takatani et al. 1975).
and Retention Time
These two factors are interdependent and
the experience of the region varies considerably. India uses a I : 1 dilution and a 50-day
retention period though a 30-day retention
possible even with existing
iS thought
designs. At Maya Farms f Philippines) a 1:1
dilution is standard, with a 45-day retention
soon to be reduced to 23 days; spend slurry
starter is used. The University of the Philippines uses a 1:4 dilution and a 21-day retention; NIST dilutions are 1:2-l :3. The Agricultural College at Suweon, Korea, normally practices 3O-day digestion at I:5 dilution;
this is reduced in their heated digester,
which receives feed at 1:2 dilution for a 20day retention. The National Institute of
Animal Husbandry (Japan) dilutes feed to
I :3 for a retention period of 16days. Hitachi
dilutes urban wastes with sewage sludge
(1:2); mesophilic digestion takes 25 days,
thermophilic takes 7 days.
Higher loading rates would reduce
digester volume, cut down on the heat load
and water requirements, and minimize
sludge disposal problems.
Mesophilic and Thermophilic
In the region, high-rate thermophilic
digestion is practiced only in Japan for industrial wastes that are discharged at high
temperatures. At Hitachi, the gas generated
under both mesophilic and thermophilic
conditions was similar at between 320 and
340 ml/g of volatile solid, but the organic
loading rate was 3.8-3.9 g/ml/day for mesophilic digestion and 9 g/ml/day for thermophilic - a 2.3-fold increase resulting in a
reduced retention time
(Takatami et al. 1975). Since April 1975,
Hitachi has used the thermophilic process
In a comparison of mesophilic and
thermophilic digestion, the Department of
Animal Hygiene, Nippon Veterinary and
Zootechnical College, found the thermophilic process superior during the first halfperiod, and the mesophilic more efficient
over the second. They concluded that the
cheaper mesophilic process was more
appropriate, taken overall, for the treatment
of pig feces (Kamata and Uchida 1972).
Furthermore, the ammonium
reported to be toxic.
ion was
Removal of Toxic Materials
The harmful effect of certain toxic
materials is known to cause the failure of
diges ers.
Hi’t h concentrations of ammonia, lignin,
certain essential oils (orange peel), H,S,
highly saturated alcohols, and some
unsaturated alcohols have been found to be
toxic (Fermentation Research Institute,
Japan). Soluble sulfides formed by the reduction of sulfates affect the digestion of
yeast wastes (removal results in a 40% rise in
gas production, Sonoda and Seiko 197:+).
The toxic effect of unsettled undiluted slurry
has also been noted and may be due to high
BOD levels.
Kill Rates of Pathogens
A retention of more than 14 days above
35 “C seems to remove most pathogens.
Reduction of the hardier parasite eggs
appears possible through physical separation. Chinese studies report an 80-98%
reduction in parasite egg concentration with
an improved effluent storage chamber
(McGarry 1976), and NEERI, India, claims
Carbon/Nitrogen Ratio
The optimum C/N ratio is usually given 1 99% pathogen removal using oxidationas 1:30, but the N and C content of the feed pond after-treatment of the sludge (a 30-day
reduced hookworm
varies with the age and growth of the feed retention
plants, and the diet, age etc., of the animals.
Moreover, what is measured chemically is incidence by 70%). Chinese studies show
not what is available to the bacteria. For that for digestion periods of lo-90 days, the
instance, the digestion of some vegetable hardiest egg was Ascarid (roundworm). Egg
wastes failed due to a lack of nitrogen viability ranged from 63 to 79% and
(NIST, the Philippines). Nitrogen-supple- remained at 47% even after 100 days. Paramenting additives include ammonium typhoid B. bacilla survived for 44 days, and
carbonate (Hitachi, Japan where the C/N schistemes up to 37 days (Research Office
ratio is maintained at 1:20, Takatani 1975), for Parasitic Disease Prevention 1973).
Anaerobic digestion, therefcre, compares
3% urea (National Sugar Institute, India),
cattle urine, molasses, oil cakes, or algae. A well with any other feasible techniques for
22% rise in gas production was said to result handling night soil and is clearly better than
from a 1%addition of algae (National Dairy existing malpractices in excreta disposal.
Research Institute, India). But some Indian But, very low retention times can be a source
scientists have doubts about the importance of trouble (McGarry 1976).
of nitrogen in anaerobic digestion because
Fertilizer Value of the Slurry and
the slowness of the process means a lower
its Loss through Drying
nitrogen demand. Algae benefits may be
Data on the fertilizer value of digested
partly due to easy digestibility, while the
slurry are scarce and vague; anaerobic
urea may only increase CO,-production.
digestion does not create fertilizer, as some
claim. Total waste solids are reduced, the
nutrients concentrated, and the form of
some of the nitrogen changed, but total
nitrogen in the slurry is essentially conserved. The problem is to determine how
much more nutritive the slurry is in comparison with the original material put into
the digeste;, not just when it comes out of
the digester, but also at the point of its end
use. There is often a confusion of the meaning of slurry and sludge in certain publications: the first is simply the digester effluent;
the second. the settled-out effluent with
much of the liquid removed.
Fresh cattle dung contains 3.5% nitrogen
(dry basis), 74% in organic form, 26% in the
more assimilable ammoniacal form. Digested slurry contains 50% organic and 50%
ammoniacal nitrogen (Hart 1963) - a 24%
increase in the latter. Similarly, Acharya
(1956) and Idnani and Varadarajan ( 1974)
report that 15% of the dung nitrogen is
converted into the ammoniacal form. The
digested slurry contains [email protected] its
nitrogen as ammonia; in fresh dung
ammonia makes up only 3.5-5.570of its total
nitrogen. NDRI, India, found 16-18a/oofthe
slurry ammoniacal.
China reports
approximately a 10yo ammonia nitrogen
concentration increase (People’s Publishing House, Peking 1974).
But Leui ( 1975) claims that slurry
contains 60-75Y0 of physiologically active
nitrogen as ammonia, 25% as amino acids,
with the balance difficult to utilize. And
United Aircraft Corporation (USA) found
that digestion of cattle wastes increases
crude protein by 100% and amino acid
content of the digested product by 400yo
(Coe and Turk).
The ammoniacal transformation therefore appears to be dependent on food composition, and this could expain the variable
data. For example, experiments on the
digestion of cow dung, with the addition of
various carbohydrates and lye nitrogen (as
nut cake), found that 43-63% of the total
nitrogen was transformed into ammonia.
With other organic materials (in combination with the dung), conversion into
ammonia varied from 11.8% with bagasse,
to 23.2% with legume leaves; it was 9. lyt,
with dung alone (Idnani and Varadarajan
The drying of the sludge volatilizes over
97% of the ammoniacal nitrogen (ICAR,
India) and results in a net loss of 18%of the
total nitrogen. Dried sludge residue contains
1.78Yonitrogen; if the ammoniacal nitrogen
were conserved, the figure would be 2.16$&
studies on N-availability
confirm this: with 30 mgN/ 100 g of soil, in 5
months the extent of nitrification was 2 1.370
for fresh slurry, 16i.370for compost, and
18.6yo for sun-dried slurry (Idnami and
Varadarajan 1974). Volatilization and loss
are attributed to alkalinity and the pH
during fermentation rises from 7.2 to 8.3,
presumably becauseof ammonia accumulation.
To avoid the loss of the ammoniacal
nitrogen, it would seem best to apply the
slurry wet and plough it under; however,
ICAR experiments on wheat, marua, etc.,
shtiw that sun-dried slurry makes better
manure (Idnani and Varadarajan 1974).
Fertilizers were applied to crops on an
equivalent nitrogen basis (125 kgN/ ha for
wheat); farmyard manure fertilized yields
were taken as 100. The results for wet
digested slurry were 103, for dry 113. When
only 30 kgN/ha were applied, ammonium
sulfate produced a yield of 137 (Berger
studies with 60
1976). Nitrification
mgN/300 g of soil showed that after 3
months 7.4vo of the nitrogen in the digested
slurry, 4.7yo of the N in the farm manure,
and 87% of the ammonium sulfate nitrogen
was nitrified.
Thus, the ‘chemical fertilizer’ ammonium
sulfate is at least four times as available and
effective as manure and slurry. With both of
the latter, the nitrogen is only one-third
available, with the rest becoming available
during the second and third years after
application (carryover effect). Chemical
nitrogen is applied yearly and has little
carryover (ESCA P 1975).
Damage to the soil through the repeated
use of only chemical fertilizer is well known:
during the survey, some fertilizer experts
stated that organic and chemical fertilizers
were complementary; and ICAR experi-
ments show that incorporating chemical
fertilizers into the slurry resulted in manures
superior to either alone (Idnani and
Vara.darajan 1974).
Composting organic wastes with slurry
results in a compost that is ready for
application in 3 months, one-quarter of the
usual composting time. This does not
conserve nitrogen well, and ICAR experiments show that nitrogen efficiency is only
20-30% as opposced to 30-5096 in regular
composting (Idnani and Varadarajan 1974).
In experiments on facilitating the handling of the very dilute (90% water) slurry,
Idnani and Chawla devised a filter bed of
green leaves or straw from which semisolid
residue could more easily be removed. The
resulting mixtures were a better manure
than compost.
The University of the Philippines at Los
Ba?os claims that daily feeding of digested
slurry to fields results in a plant nitrogen
intake only slightly lower than with urea;
Maya Farms even reported overfertilization with slurry alone. But-the situation is
complicated by carry-over el’fects and rates
of application. Such remarkable results
through the useof digested slurry/ sludge are
thought by some to be due to the action of
humic acid on plant roots and to the
presence of various micronutrients. Anaerobic digestion causes only a 20-30% loss of
organic matter, thus plant residues (humus)
are conserved.
For optimum utilization of digester
nutrients, the feasibility of establishing an
integrated farming system incorporating
aquaculture as well as agriculture should be
considered in view of the high yields and
short life-cycle of biomass in. water.
Similarly, ways to use colloidal cow-dung
slurry should be explored. Finally, the
possible contamination with heavy (and
other) metals and pathogens should not be
overlooked in studies of the fertilizer value
of sludge and slurry,.
Design Aspects
Agitation of the digester contents is often
recommended to ensure intimate contact
between the microorganisms and their food
and to increase the rate of decomposition by
releasing small trapped gas bubbles from the
microbial cell matrix. It also helps to break
up scum. Certain authorities claim that
loading could be increased by four times in
well-stirred, high-rate digesters (seeChapter
1). Most family digesters now in use make
no provision for agitation. But there are
exceptions such as the 140-m3digester being
designed in India, the large-scale pilot unit
in Korea, the batch digesters in the
Philippines, and all units in Japan. Some
Japanese models use a submerged pump for
both agitation and mixing (Yagi 1975). Gas
recirculation was reported to be both
beneficial (India and Korea) and of little
benefit (Japan).
Some experiments show that stirring may
have only a temporary benefit on gas
production. The Public Works Research
Institute, Japan, found that continuous
mixing produced only 5% more biogas than
once-a-day agitation. But Hitachi, which
adopted intermittent agitation in small-scale
trials, uses continuous agitation in the larger
models. According to them, the effect of
agitation may not be apparent in small-scale
digesters though it does break up the scum.
The current practice in industrialized
nations is toward continuous mixing (see
Chapter 1).
Winter Operation and Heating
the Digester
Reports on the operation of gas plants in
winter are not consistent. Research
institutions report a reduction of 60-729; in
gas production during winter, but the State
of Haryana, india, reports only a reduction
of 25-33yo. The problems of winter are
overcome by setting up a plant one size
larger than normal requirements, feeding
larger inputs into the digester, covering the
gasholder with plastic sheets, using hot
water for feed preparation, and adding
various materials like urea, urine, molasses,
an.d oil cakes. Korean units experience a gas
production drop of &5-90% during their
severe winter. Part of the inconsistency may
be explained by the greater efficiency of the
iargkr scale plants under identical operating
conditions (Hitachi Plant Engineering and
Construction Co. Ltd. 1975).
Systematic investigations on digester
heating have been carried out only in Korea
and Japan. The heat could be supplied by
burning biogas or by recovering waste heat
from gas-operated engines. Percentages
varying from 25 to 47.7% of gas produced
have been needed to keep digesters in the
mesophilic range; again, the different scales
of operation may account for the divergenic
in amount of gas needed.
Experiments using solar heat are in
progress in India at the Gobar Gas Research
Institute, and in Indonesia. Another
possibility, that of using heat liberated
during aerobic composting to conserve
heat in anaerobic digestion, was suggested
as early as 1952 (Lessage and Abiet 1952).
at between $255 and $1420 each, as
inexpensive as was first expected.
In the Indian design, the steel gasholder
makes up 30-4070 of the total cost. With
proper maintenance (i.e. annual painting)
they have operated for over 10 years.
Wooden gasholders in Korea failed and
were replaced by PVC holders, which also
developed cracks due to weathering. Later
PVC designs have four independent
chambers so that damage to any one does
not affect the others. Ferrocement is
relatively heavy and inferior to steel in
strength and flexibility, but work on
building low-cost gasholders from local
materials is being continued in India. An
external water jacket may be useful for a
night-soil digester. Transparent gasholders
have been suggested to increase solar
The need for twin digesters is being radiation, but there is some doubt about
questioned: both NEERI (India) andLhe how tolerant methanogenic bacteria are to
University of the Philippines at Los Banos light.
use twin digester systems and have conUtilization of Gas
cluded that a single digester could serve their
Methane suffers a major storage problem
needs just as well.
Defects that had caused failure of the as it does not liquefy under pressure at
digesters seen during the survey essentially ambient temperatures (critical temperature
concerned the masonry work and did not arid pressure: -82.5 OC,46.0 bar). To store or
pertain to the design. The Indian design at transport the energy equivalent of 13 litres
times suffers from choking of the feed inlet. of gasoline as compressed gas at 2000 psi, a
A problem with the Taiwanese design is the 1.6 x 0.27 m cylinder weighing 60 kg is
development of difficult-to-repair leaks in required (Meynell 1976).
The low pressure of biogas and the low
both the water seal and digester compartflame
propagation speed of methane (66
Among the other designs, the Chinese cm/set), which is further inhibited by COz,
model with its built-in gas dome and lack of call for specially designed biogas appliances.
moving parts has attracted attention in Watson House Laboratory recommends
Pakistan, Korea, and Sri Lanka. In addition that biogas burners have a total flame port
to delivering the gas at increased pressure, it cross-section area 300 times the injector
is easy to construct in rural areas, and cross-sectional area. Suitable flames can be
dispenses with expensive steel gasholders. obtained with orifices of 0.96 and 1.04 mm
But the annual maintenance and sludge with a gas pressure of 2.5-20 tirn of water.
removal could prove bothersome. The The heat output range varies from i 30 to 430
Indian design, in contrast, has been in con- kg-Cal/ cm2 of port area/hour. Indian
tinuc,us operation for over 10years in some burners with a 60% efficiency use large 6well-run plants. The other development is m-m ports and the premix flame is short.
the bag digester: two brands, hypalon and
Biogas lights are generally inefficient, but
butylon (Dunlop, New Zealand) are the Chinese report the brightness of a
available. They have not proven as light nor, standing lamp to be greater than that of a
;‘ ,*iz
;);p$ 1’.‘;“‘,;“~y;, ;,,”i, ;
,_,,\ _,/
about biogas-related investments. Such
decisions are made on both technical
considerations and information about social
and economic issues. As we have shown,
technical data are often not available with
sufficient accuracy and vary in different
situations. This section describes those
results of the survey of the Asian experience
that relate to the different social and
economic conditions,
problems, and
achievements of the region. Clearly, such a
review cannot be a comprehensive treatment
of all the issues involved, and this is not
hanging one (Production Team’ of ‘T’ang
Ngan 1973).
Findings on the utilization of biogas
engines are not consistent. A compression
ratio of 13-l 5 is recommended for biogas use
in engines (compared with a ratio of 6 or 7
for gasoline engines). Carbon dioxide
increases anti-knock characteristics and
does not have to be removed. India has had
success toward adapting diesel engines to
biogas using a biogas:diesel fuel ratio of
85: 15. Engines running on biogas can go five
times longer without an oil change.
Few examples exist of other systems of
energy conversion using biogas, but
Parasakthi College in Southern India
effectively uses biogas to run a steam boiler
for cooking purposes. Other recommended
uses for large-scale operations involve the
separation and use of CO2 to make calcium
carbonate, to promote algal growth, and to
make dry ice for local health services,
refrigeration, etc. (Pathak and Colah 1976;
Prasad et al. 1974).
Future Research and
Technological problems to consider, in
rough priority of order, include: the design
of efficient burner and gas-use equipment,
such as refrigerators and gas distribution
systems; design of biogas-operated engines;
and studies to determine the best use for
slurry and sludge. Fermentation kinetics
studies are necessary to find the optimum
dilution, retention time, organic loading,
etc. As well, improvement studies of dung
digestion through enzyme action and other
pretreatments should be undertaken.
Digester design itself should be scrutinized; different climatic conditions should
be assessedand designed for; and the use of
energy should be
solar and wind
investigated. Finally, the use of industrial
wastes from agro-based industries, and the
isolation and ‘education’ of bacteria for
operating at low temperatures might be
Social and Economic Issues
Decisions are being made all the time
On the Indian experience, people who
have so far been able to benefit from biogas
plants have been in or above the middleclass levels. For instance, the survey carried
out in Gujarat by the Indian Institute of
Management, Admedabad, revealed that
nearly 67% of the owners were of medium
socioeconomic status, and only 2670 were
from the low-income group (Moulik and
Srivastava 1975). The individual families
who owned gas plants had, on an average, 10
ha of land and 10 head of cattle. In another
survey by the Dena Bank in Gujarat, most
owners had an annual income of more than
US $1100 (many over US $2&00), and their
\?/a~agriculture (Deila
primary occup,,.,..
Bank 1975). They were all literate and nearly
40vo had subsidiary occupations such as a
business or a service. Others of equivalent
social status had no such subsidiary
Similarly, in the State of Haryana, which
has the largest number of biogas plants, five
villages were surveyed. Of the 12 biogas
plants in these villages, five were in one
village - this was attributed to the enterprising character of its inhabitants. Out of a
sample of 835 households, 68 1 had animais;
a rough breakdown follows: landless 39%
(farm labourer 36%, business 1.570,services
1.5%); marginal farmers (up to 1 ha) 13.5%;
small-scale farmers (l-2 ha) 15.370; lower
medium (2-4 ha) 17.5%; upper medium (4-6
ha) 5.5%; and large (over 6 ha) 9.2%. The
biogas plant owners are from the last three
categories, which represent only about one-
114”. <#j
% ,.;’
, ,:.
_ _‘_
The need for gas was important to the
installation of biogas at Tuisisham Temple
in the Gir Forest, Gujarat. Located in the
jungle and having no power source, its 300
head of cattle feed a digester that generates
electricit? and the power to lift water. The
Temple uses wood for cooking because of
The motivation for biogas plant instaiiathe need for mass cooking on short notice:
this highlights the need for an alternative
tion varies with countries, but it is difficult
fuel. If, ail of the biogas from a community .i,i
to avoid the conclusion, at least on Indian
experience, that past demand for biogas has plant were used for irrigation or small
been generated by external inducement.
industry, and electricity came to the area,
Occasional kerosene s&cities, irregular
alternate uses for the biogas would have to
supplies of petroleum, and scarcity of be found. Some large farms with power are
firewood due to intensive cultivation, as well nevertheless interested in biogas for lift
as the problems of burning firewood during
irrigation or for greater independence from
the rainy season,etc., may well have induced the rural power systems,in which power cuts
some individuals in India to install biogas are common.
plants. According to the Dena Bank survey
Biogas is also successfulin delta areasthat
(1975), about 93% of the owners installed have an adequate number of cattle, but no
their units for cooking. The smokey flame forests to supply firewood. Multiple
from the traditional fuel (cattle dung) cropping in these areas creates a demand for
blackens the kitchen and utensils and affects fertilizer. The relative success of biogas in
the eyes. According to a survey of 56 gobar the Kr-I<hna and Cauvery deltas appears to
gas plants in Uttar Pradesh, biogas use has conClr.i:i this.
reduced the eye infections of housewives,
The educated and well-to-do in Andhra
saved time, increased the life of utensils, and Pradesh were reported to be attracted by the
improved the cleanliness of the house and convenience of the gas, but in contrast the
the dress of the women (Sathianathan 1975). lower middle class workers are motivated by
Although there is an increasing awareness the value of the digested slurry as fertilizer.
of the high value of the digested slurry as Other factors that are generally not
manure, the value of the gas seemsto have considered by individuals are important to
been the prime attraction in the past. The state or national governments and these
main advantage as fertilizer is perceived to include environmental and deforestation
be that the digested slurry can be.used to control, public health advantages, and cost
speed up the process of composting other savings through recycling of refuse.
wastes and thereby increase the volume of
The key motivation in Thailand stems
from the desire to use the gas as an
compost produced.
When the disposal of dung becomes a alternative to expensive charcoal, although
problem, as in large urban areas, the biogas pollution control could be a motivation for
plant is seen as coming to the rescue. The piggeries, particularly in southern Thailand.
digested slurry from the biogas units in Most units in Indonesia are demonstration
Madras and Bombay i:; fed into the city plants and wood is plentifu!; only
drainage systems. Some families close to overpopulzred Java, which is facing a
Calcutta built their digesters in response to deforestation problem, might be interested
complaints from their neighSoL;rsabol!t the in biogas for fuel. In the Philippines, the gas
smell izf the dung and the number of ilie:; and is again the main attraction, but the easy
mosquitoes. Others were attracted by the availability
of firewood means that
saving on septic tanks by connecting toilets pollution control is likely to be the motive in
to the biogas units (one firm in Sang& the future. In Korea, gas is used for cooking
Maharashtra, supplies prefabricated toilets and to save compost materials like straw and
along with the biogas units).
forest products from being burned for
“‘+-sj,;. ” &id’ ,of the population. This situat.lon is
gtineraiiy repeated in other Asian nations.
Essentially, it is the rich who have ins&led
biogas plants. Many factors have made it
difficult for the poor to use biogas plants.
fuel. Poiiuti& ‘control is the prime ftirce in
the propagation of biodigesters in Japan.
Few people drew attention to the
advantagks -arising from the release of
household iabour for the other productive
activities (as in the case of a Korean farmer
who increased silk output). Although
apparently insignificant, this may be, or
could become, an important motivating
The survey showed that many reasons
quoted for nonadoption were often too
simplistic: a complex set of interactions are
involved and these vary between areas and
countries. Many of the reasons for
nonadoption are associated with three main
First, people just do not have sufficient
resources (of capital, input materials, land,
or time) to run plants efficiently. Lack of
cattle, due either to different agricultural
systems or the increase of mechanization, is
seen as a major constraint to biogas
adoption in Thailand, Korea, and among
the poor sections of Indian society. This is in
spite of the fact that enterprising people
have managed to run 2.8-m3/day plants on
one animal and a calf. Inadequate water
supply is another input that prevents the
spread of plants. Lack of spaceeither for the
plant or for slurry pits is similarly often cited
as a constraint (Dena Bank 1975). Lack of
cash liquidity forms another barrier to the
purchase of biogas plants. The absolute size
of the capital required, together with
problems of cash flow, certainly rule out
biogas for the poorer sections of society.
Second, the returns to investment are seen
as too low in relation to other uses of the
resources. The returns to the individual are
often thought to be inadequate becausethey
are notional rather t.han in cash; they are in
the form of savings in the use of other
resources rather than in direct sales. The
value of using the digested slurry is often
only available to the farmer who can use it
on his own land, but in India 70% of
household milch cows are owned by landless
people (National Dairy Research Institute,
Karnal, India). Low returns also occur when
the gas is not particularly valued because of
the availability of other sources of heat,
particularly wood; the Imlonesian program
may be limited in this way. in Korea, where
90% of villages are said to be supplied with
cheap electricity (US $2 per family per
month), the cost of the alternative biogas,
with a capital cost of US $150 and low gas
production in the cold winter, may be less
The spatial arrangements of communities
form the third major problem area
preventing the adoption of biogas. Returns
to biogas often become negative if the
digested slurry has to be transported a
considerable distance to the fields, or if the
gas is produced at some distance from where
it is to be used (such as in kitchens);
similarly, problems arise if the cattle are
moved away from the digester in summer
(Dena Bank 1975).
To these three sets of problems must be
added the important phenomena such as the
limited diffusion of the technical knowledge
and experience to run plants (particularly in
Indonesia) and the lack of the institutional
infrastructure, credit, and extension faciiities. In Indonesia, plants were resisted due to
the Muslim’s attitudes to the use of pig
manure. Lack of spare parts and technical
problems, however, are not as important as
they once were.
Night Soil
In India the psychological and religious
barriers to the use of night soil vary
considerably. There are instances like in
Haryana where 30% of plants have toilets
attached to digesters, where some have been
discontinued due to pressure from elderly
parents. Some households whose plants run
on the combined digestion of night soil with
cattle dung are unwilling to admit such use.
Religious sentiments exist against the use of
such gas for cooking food offered during
worship either at home or in the temple.
It is difficult to correlate these sentiments
with either education or religion: a ladies’
college in the South could convince an
orthodox family to use night soil by arguing
that fire has no unholiness; but the college
itself was forced by the students to use the
/F< ~,,:
gas only in the chemistry laboratory.
Religious feelings cause objections in parts
of Uttar Pradesh, West Bengal, and the
South, but other religious areas like Gujarat
and Maharashtra have shown a great
willingness to use night soil.
In Thailand, the Philippines, and Korea,
there are likely to be psychological barriers,
but BUST1 considered that such resistance
is not likely in Indonesia, where human
waste is currently used as fish feed. Such
psychological inhibitions can disappear in
However, night-soil use does warrant
certain precautions: worms and parasites
are more commonly present in night soil
than in cow dung, and sludge brought out
undigested can have an offensive smell and
cause health problems.
Community Plants
The idea of community plants provides a
possible means for bringing the benefits of
biogas systems within reach of poorer
sections of the rural population. Although
no community plants are now operating in
India, a number of large-scale units for
schools, villages, prisons, and other
institutions are being considered. However,
these cannot be strictly classified as
community plants as a single institution
owns the inputs, and controls the outputs.
During the survey there was little positive
response to community plants because
many felt that Indians were too individualistic. Most cooperative ventures succeeded
only as long as there was positive leadership,
such as the case of the Khiroda Panchayat
(Maharashtra) community plant using night
soil for street lighting that failed after the
transfer of its most enthusiastic worker. The
Harigan Cooperative in Mahishal, Maharashtra, has decided against community
biogas in its developments even though such
a plant was used initially. Suggestions for
the promotion of community plants are
receiving considerable attention, and the
community plant as a commercial venture is
being considered, as is the formation of a
biogas corporation at the national or state
Positive aspects of a community plant
include large-scale efficiency for rural power
generation, industry, water pumping, etc. A
number of social and technical problems
would have to ‘be soived. however. People
would have to pool animal livestock waste
resources, use community !atrines, avoid
excessive use of water, and not add
disinfectant to the wastes. Technica!
problems would involve the equitable
distribution of the biogas produced (as well
as the slurry). Above and beyond this would
be the cost and problems of management.
The collection of input wastes might pose
a problem, and as well, cow dung is used for
domestic fuel, in brick kilns, in rural house
construction, etc. Thus it is essential to study
these alternate usesand the seasonal fluctuations of their supply. There is some concern
that the demand for biogas might deny its
availability to existing users: the poor thus
deprived of dung, and unable to use gas,
might turn to wood, and so cause
Thailand, Indonesia, and the Philippines
have no community plants and are faced
with problems. such as lack of cooperative
spirit, similar to those in India. However,
the National Housing Authority of the
Philippines is planning large plants for new
settlements, and Korea will build eight large
digesters in selected villages.
As mentioned earlier, the extension
program in India was greatly stimulated by
the energy crisis. The State of Haryana
alone set up over 12000 plants in about 2
years and it leads ail other states in extension
work. Factors contributing to this success include planning and execution at the
grass-roots level, an intensive media
campaign, a fair price structure, and
accessible bank loans. The Haryana State
Government considers the influence of successful plants crucial in the creation of new
demand; thus, farmers who own digesters
are asked to demonstrate them to prospective owners. Similar methods are working in Sangli District in Maharashtra, and in
the Punjab.
Anot.her important factor in extension
work has been the ‘approved supervisor,’
_ I,
who is an artisan trained and authorized by
the KVIC to act as a biogas agent. Although
primarily a salesman, he is sometimes much
more: he helps arrange loans for the
construction of the digester, sometimes
employing trained village youths for the job;
afterward he counsels :inJ advises technically on the day-to-day operaiion of the
digesters, being much more visible and
accessible than any po+ernm&t official
could be. At present thc:rc art;
over 400
“/ .I-’
trained supervisors.The need for local .%&&hop facilities,
standardization, and easy availability of
spare parts was stressed by many owners
during the survey. It is still difficult to get
biogas burners and lamps tested in approved
government laboratories.
The opportunity to examine extension
activities in other Asian countries was
severely limited: clearly it is difficult to draw
conclusions from this scattered set of
observations. The identification of visible
‘leaders’ to initiate a processof diffusion was
quite successful in some cases. However,
there was no clear evidence of whether these
diffusion processes reached ‘downward’
toward the poorer strata of society or merely
‘sideways’ to similarly well-off families. One
issue is perhaps clear: to be effective, the
extension of knowledge about biogas plants
must operate very closely with services
providing accessible and usable credit or
subsidies, and with technical services
providing the necessary equipment and
guarantees of maintenance and trouble-free
Credit and Subsidy
Credit procedures for biogas plants are
complicated by the very low resale value of
the plants and the consequent reliance on
third party guarantees. Furthermore, advances for biogas units by Indian banks are
based on Government cost estimates that
are uniform for the entire country. According to severai owners these are lower
than the actual costs in some areas because
of rocky soil or simply higher iabour and
material costs. Interest rates on these advances are high, and there is much ‘running
around’ involved. Because of this, a number
of middle-class owners consider loans a
burden and prefer to raise their own funds.
There was a phase of interest-free loans early
on, but with present rates up to 14%, many
owners wanted a reduction in interest rates
and extension of the repayment period to
IO years. Some even suggest that interest
should only be charged in the case of
Although family plants can operate with
two animals, most banks insist that the
borrower have at least five or six animals,
with a minimum of 2 ha cultivated agricuitural land. It is clear from these conditions
that the credit system is meant for the
wealthier classes, and it also indirectly reflects who the biogas owners are.
From the banks’ point of view the rate of
interest is the sameas that normally applied
to other agricultural advances (4% over
bank rate), and most of the borrowers are
relatively well-to-do.
The question of the length of the loan repayment period is complicated by the KVIC
experience that 95% of the IO-year loans
approved have defaulted. Nevertheless
many banks advance the loan on personal
guaiantees without insisting on other securities.
The former outright subsidy of US $34
(Rs 300) was replaced by one equal to 25% of
the plant cost - this has now been reduced
to 20%. Some marginal farmers and employed plant owners said they were attracted
by the subsidy, but the Dena Bank survey in
Gujarat reported that subsidy was not a
major factor in attracting plant owners. In
fact, according to the banks the subsidy
should be withdrawn completely because
the benefits go to the well-to-do; if continued, it should be confined to the marginal
farmers. Curiously, the subsidy is not given
for plants totally operated on night soil.
In the view of the Reserve Bank of India
(1976), biogas plants of ail sizes are
profitable, and the continuation of subsidies can be supported if they are considered as an income transfer both from the
present generation to the future one (for the
conservation of fossil fuels) and as a transfer from urban to rural areas (Sanghi et al.
1976). During the survey it was found that
the motivation to invest in biogas was rarely
based purely on economic grounds.
Very little information is available on the
credits and subsidies in other countries.
Thailand has abolished subsidies, and the
withdrawal of the subsidy in Korea has
drastically affected further installation of
digesters. On the other hand, Sri Lanka is
planning to establish financial subsidies and
in Pakistan the government supplies gasholders free to farmers who build their own
Other Benefits
The main objective of biogas investment
in rural Asia should be to improve the distribution of income by serving the needsof a
wide range of social groups. Depending on
the local socioeconomic conditions, biogas
investment will have its own order of priorities: for example, it may become an
attractive opportunity oniy when certain
other investments such as irrigation have
been carried out (seeChapter 2). The present
ownership pattern reveals that biogas
systems can be afforded only by the relatively well-off. Technical as well as socioeconomic considerations should dictate the
operation of large community plants.
During the survey some officials referred
to the indirect social benefits resulting from
the biogas extension program. These inProblems of Evaluation
cluded a spirit of self-reliance, an increased
diffusion of metallurgical and technical
A considerable number of potential
skills, and a general rise in the standard of benefits from biogas systems have been sugliving and cleanliness.
gested, but the problem remains of evaluating the conditions under which these
Some General Implications
benefits can be reaped.
Macroevaluations often assumethat most
It is clear that biogas system: in the Asian
region could provide fuel and fertilizer sub- of the available inputs will go directly into
stitution, waste recycling, pollution control, the digester, but microevaluations suggest
and improvement of sanitary conditions. that alternative uses of inputs and seasonal
What is not so clear is how significant these fluctuations limit their availability in
contributions are now, and will be in the practice. Local factors like climate,
future. Nor is it clear who benefits from the cropping pattern, terrain, and social
exploitation of the technology. Extreme practices will influence not only technical
positions are taken by both the enthusiasts design, but also costs and benefits. Most of
lack reiiabie data at
and the sceptics, with both sides tending to the present evaiuations
the microlevel and suffer from an underdisregard the facts.
However, a number of people have estimation of the costs and overestimation
attempted to make serious, objective assess- of the benefits (Taylor 1976). Some evaluaments of the social and economic potential tions also make the error of ‘double
of the technology in Asia. Most relate to counting.’ For example, when organic
India and the system most widely used there wastes are added to the slurry pit and com- the family-sized plant based on cattle posted with digested slurry, we cannot apdung waste. Little information is available portion the total value of the compost to the
trom other countries (see Chapter 2; also biogas plant becausehad the organic wastes
Berger 1976; ICAR 1976; Moulik and been composted, they would have given an
Srivastava 1975;Prasad et al. 1974; Reserve almost equal quantity of manure. The
Bank of India 1976; and Sanghi and Dey biogas plant speeded up the composting
1976), and even the best of these studies ig- process: the value to a rural economy of this
nore or misinterpret some of the social and speeding up of the process is a complex
economic issuesdiscussed here. Even when question.
taken together, these papers provide almost
Present evaluations of biogas in India
no guidance for judgements and policies in assign greater value to the slurry than to the
gas (ICAR 1976; Moulik and Srivastava
other Asian countries.
i 19
built, or any combination of such factors
(Taylor 1976).
1975), and this is so even without considering the organic humus benefits. ‘The ICAR
study presumesthat the value of the manure
is 2.3 times the value of the gas. Even Disney
(1976) does not give the sludge any value
other than as nitrogen even though his study
is on the economics of fertilizer production.
The best way to assessthe true value of the
slurry would be to measure the extra output
of crops, algae, or fish (e.g. slurry is 13%
more effective than farmyard manure -Berger 1976;Idnani and Varadarajan 1974).
Ambiguities in the data abound; if we
want to find out what the fuel value of
biogas is, we must measure methane
content, but many assessmentsof increased
gas production ignore this fact. Dung
output will vary widely with the breed of
animal, its food etc., and this is why
generalizations on the number of animals to
support a given size of plant are often misleading. There can be no generalizations
about the price of inputs as they will vary
with season and location. Even family
wealth cannot be defined in terms of land
ownership as the land may be infertile or
their actual standard of living may continue
to be low.
.,, Summary
So far only a small number of known
designs have been built and tested. There
remains great scope for improvements and
cost reduction, yet even the existing system
is reported to be highly cost-efficient. A
good cost analysis must find not only the
different alternatives for fuel and fertilizer,
but also pose the question: Is biogas the best
use of the available resources?Social and environmental benefits, the depletion of nonrenewable resources, and fluctuations occurring outside the system have rarely been
taken into consideration (an energy crisis
can considerably alter the benefits of
The possible role of Government subsidy
must be viewed from the overall context of
socioeconomic development and selfreliance. A baseline may be needed, and a
scenario could describe what would happen
in a particular village under various assumptions if electrification took place, or if
biogas piants were- msraliea, a sugar mill
Asian biogas systemsare characterized by
great divcrsity,,<ven though only a limited
number %[email protected]&tually been built. Most are
used for family cooking, although other uses
are on the increase. But even so, burner and
appliance efficiency has still received inadequate attentipn.
The greatest benefits from biogas systems
are to be derived from the manurial value of
the slurry; however, this fact is not well
known outside India and China. Even in
India, the ability of biogas digesters to
convert part of the organic nitrogen of the
feed to ammoniacal nitrogen has not been
exploited. The benefits of organic humus
and .nitrogen ‘carry-over’ effects of the
sludge have still to be investigated and no
reliable data yet exist on the increase in crop
yields that biogas slurry can produce.
Design and operational improvements
must be conducted and the optimum use of
the outputs determined. The digestion of
cellulosic materials (especially agricultural
waste), with the resultant acceleration of the
digestion process and reduction of capital
costs, would gain a wider acceptance for
biogas, particularly in regions that do not
have cattle.
The public health control aspects of
anaerobic digestion compare with any other
feasible night-soil handling techniques. The
hardier parasitic eggs are best controlled by
physical separation. There is considerable
scope for industrial and urban waste
treatment as well as for the recycling of
livestock waste through the use of biogas.
Motivation for biogas installation varies
- governments seem to be most interested
in biogas applications for environmental
control, foreign exchange savings, and
control of deforestation.
Family-size units are owned for the most
part, by the well-to-do, as a host of reasons
have made it difficult for poorer people to
have biogas plants. Nonadoption can be due
to psychological and practical problems
,.associated with the handling of various
wastes and slurry, or simply a lack of necessary resources (i.e. capital, input materials,
land, time). Some prefer to invest elsewhere.
Biogas systems can succeed in areas where
inputs have low opportunity costs, the alternatives have high opportunity costs, and
where pl$.nts can be operated with adequate
efficient ;.
To bc/ effective, an extension program
must operate very closely with systems of
credits, subsidies, and technical services.
Subsidies can be viewed as a transfer
payment from the urban rich to the rural
poor, or as a transfer payment to the future
generations for the conservation of fossil
The main objective of biogas investment
in most parts of rural Asia should be the distribution
of income
and needs to a wide
The criterion of attractive returns on investment matters very little if the necessary
capital and means are not available to the
villager. Further, the eldaluations do not
take into account the sociizl and other latent
costs of the depletion of nonrenewable
resources. An analysis has to consider not
merely the different alternatives for meeting
fuel, fertilizer, and other needs but also
whether investments in biogas are the best
use of available resources. The economics of
biogas systems is highly location-specific
and it is essential to identify rural zones with
the right potentials and socioeconomic
environment to maximize the returns to the
individual, the rural community, and the
nation as a whole.
The timely funding from both the International Development Research Centre and the
Indian Council of Social Science Research, and
range of social groups. Large community
plants are most likely t(? achieve this, and
their greater efficiency would
treatment of various types of wastes. Power
the consent and encouragementof Dr B.K.
Madan, Chairman, ManagementDevelopment
rural industry are conceivable uses for
biogas in a village - even the waste heat
could be effectively used. Such a unit could
be run as a commercia1
but the
operation of such community plants is
plagued with many social and technical
Institute, enabled me to undertake this study. My
thanks are due to all of them. I am particuiarly
Deemark, Dr W.D. Han, Dr Ashok Jain, Mr
H.R. Sreenivasan, and Dr G.P. Sudirjo, whtl
took great pains in arranging my suriey visits,
and to the many people in different countries who
freely shared their experience and educated me
on the subject. Thi,s work greatly benefited from
the advice and encouragement of Mr R. Martin
Bell and Dr C.H.G. Oldham.
Appendix 1
Total gas
dest- carbonydrate
added royed
Green garbage T = 37 *C, stirred
(76.5% garbage e = 30 days
LR = 0.077 Ib/ftJ/
+ sludge)
day nonacclimatized
Green garbage T = 37*C, stirred
e = 30 days
( 100%)
LR = 0.154 lb
Paper pulp
T = 37 “C
(50% sludge
8 = 30 days
50% pulp)
Green garbage
Kraft paper
Garden debris
Chicken manur
Steer manure
Sewage sludge
T = 37 OC
30 days
3L digester
LR = 0.77 lb
digestion: typical gas yields
2.4 l/
Properties of
component in
feed (%I
74.3% vs
destroyed 1
90.3% cellulose
64.7Y$ total
solids destroyed 1
No. 67 (1967).
Appendix 2
digester rates: some models and results
Temperature effect
(So = 200 mg,/l)
1st order
Stearic acid
I- = 10 “C E = 72
-I- = 20 oc
( 1974)
T= 3OOC E = 11.8
T = 37 OC
Palmitic acid
Myristic acid
(So = 1375 mg/l)
0.023 day-r
0.004 day-1
0.0077 day-1
k-s = 417 mg/l
9 = 0.77 day-1
k = 143 mg/l
9’ = 1 day-l
k, = 105 mg/l
4 = 0.95 day-l
Oleic acid
(So = 1835 mg/l)
(So = 1835 mg/l)
Acetic acid
(S,, = 1568 mg!!
0 = 1.5-12 day)
(So = 13744 mg/l
8= 6-30 days
Domestic waste
digested with
sewage sludge
(35-60 “C
t9= 4-20 days
3.33 g/l day feed)
Elephant grass
(batch digester,
1st order
Cow manure
(So = 3800 15300 mg/l
= 10 days)
extracted from
river bed
(So = 20000 mg / 1)
(Batch digester)
= 3180 mg/l
= 4.0 day-’
= 1816 mg/l
= 5.0 day-r
= 154,333,869 mg/l
= 8.7, 4.8, 4.7 day-l
T = 37 OC
T = 37 oc
T = 37 OC
T = 35, 30, 25 OC
( 1970)
= 7530 mg/l COD
9’ = 5.4 day-l
T = 37 OC
1st order’
kfinal /day’
k = 0.06 day-l
= 0.0526
( Y=G( !-c-~~))
1st order
k = 0.125 day-r
k -0.12
= 0.23
= 0.31 I
(k = mm gas pressure/h/50
with reference to potential
gas production
on residence time. Broadly (a) 8<
IO days: (b) 8 > 10 days.
Novak &
37 ‘-‘C
Zero order
35 oc
40 oc
45 oc
50 oc
55 oc
60 “C
T = 32 OC
T = 22 OC
( 1966)
T = 35 OC
Gaddy et
al. 1974
T= 15W
T = 20 w
ml sludge fed)
Cellulose digestion
System and
of cellulose
Batch, mixed two 2000 mg/l
pure cultures
isolated from
sewage digester
38 OC, mesophilic
3120 mg/l
Batch, mixed
culture from
sewage digester
25 OC, mesophilic
No. 1 filter
Digesting rate
(I)260 mg/l/day
(2)660 mg/l/day
in sewage
132 mg/l/day
(1) 149 ,ng/l/day
(2) 426 mg/l/day
(1) 744 mg/l
(2) 2980 mg/l
20000 mg/l
5.9- 48 to 216
in fibrous
6.5 mg/l/day
river sludge
41200 mg/i
Batch, mixed
culture from
rumen fluid
4OOC, thermophilic
6.5, No. 2
filter paper
mixed culture,
sewage sludge
kraft pulp milled
13744 mg/l
Ttio exp. were made
with different strains
Batch, pure
culture isolated
from soil and
manure, 55 OC,
25 OC
pH was mainralr:[email protected]
Two experiments were
made with different
Range of rates of samples
from monthly monitorings
11400 mg/ l/day
pH was maintained with
mglh day
Enrichment culture used
Appendix 3
Suggestions for studies in core technology
Plant-scale studies
1. collection of data and evaluation of
existing designs, including reasons for
2. studies/ trials and evaluations (technical
and economic) of design modifications
(e.g. gas holder design, materials of construction (with corrosion, reliability,
cost), mixing, and heating/ preheating/
3. batch-scale plant: study of efficiency,
4. EVOP studies on se!ected plants
forward follow5. other topics should i .11;1e
ing laboratory/ pilot-jcale work as set out
Basic factors relating to process operation
and efflciencv
1. rates, yields, and limiting steps as a function of feed material and preparation
2. collect data on effect of operating conditions and constraints on performance
2. rate limitations due to microbiological
3. shocks, inhibitions with local pollutants
4. pathogen kill studiesI
1. development of cheap/robust/appropriate instruments for metering inputs,
outputs; temperature; pH; gas metering
2. simple control schemes/strategies
Gas treatment/ handling/use (many pro blems are problems of using known technology, establishing good practice, etc.)
1. purification - design and testing of convenient methods
2. evaluation of potential by-product utilization
3. piping materials (standards, etc.)
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