A Study from the Town of Surahammar

A Study from the Town of Surahammar
Food Waste Disposers – Effects on Wastewater Treatment Plants
A Study from the Town of Surahammar
Tina Karlberg
Erik Norin
[Scrambled words:]
Sludge quality
Sorting at source
Nitrogen emissions
Water consumption
Screened matter
Published by VAV AB
VA-FORSK is the waste and sewage technology research and development program of
the municipalities in Sweden. The program is financed entirely by the municipalities,
which is unique as previously government funds have always been used for this type of
operation. The annual subscription for the program is currently 1.05 SEK per
inhabitant per member municipality. The fee is voluntary and the interest shown by the
municipalities has been very great. Almost all municipalities participate in the
program, which means that annual budget is slightly more than eight million Swedish
VA-FORSK was started jointly by the Association of Local Authorities and VAV
(Swedish Water Works Association). The association started in 1990. The program
emphasizes applied research in the field of municipal waste and sewage. The project is
run throughout the entire range of the technical water and sewage field under the
Drinking water
Sewage system
Wastewater treatment
Financial and organization
Education and information
VA-Forsk is led by a committee appointed jointly by VAV and the Association of
Local Authorities. The committee is answerable to the board of VAV. Currently, the
committee consists of the following persons:
Ola Burström, Chairman
Professor Peter Balmer
CEO Roger Bergström
Unit manager Bengt Göran Hellström
Local government commissioner Nina Jarlbäck
Technical manager Peeter Maripuu
VA manager Stefan Marklund
KS & KF member Håkan Mattsson
KS member Åsa Möller
Department head Peter Stahre
Section manager Jan Söderström
GRYAAB, Gothenburg
Stockholm Vatten AB
Municipality of Luleå
VA plant of Malmoe
Association of Local Authorities
Asle Aasen, coopted member
Research manager Jan Falk, secretary
NORVAR, Norway
The authors are solely responsible for the content of the report and does not represent
the opinion of VAV.
Phone: +46-8-677 25-70
Fax: +46-8-677-25-75
VAV AB is a service company of Svenska Vatten- och Avloppsverksföreningen
(Swedish Water Works Association).
Food Waste Disposers – Effects on Wastewater Treatment Plants
A Study from the Town of Surahammar
Tina Karlberg
Erik Norin
Published by VAV AB
The VA-FORSK report series
Report title:
Food Waste Disposers – Effects on Wastewater
Treatment Plants, a Study from the Town of Surahammar
Report designation
No. in the VA-FORSK series:
ISSN number:
ISBN number:
Tina Karlberg, Erik Norin, VBB VIAK AB
VA-FORSK project No.:
Project name:
Food Waste Disposers –
Wastewater Treatment Plants
Project funding:
Report can be ordered from: AB Svensk Byggtjänst, Litteraturtjänst, 113 87 Stockholm,
tel. +46-8-457-11-00
Scope of the report
Number of pages:
Search keywords:
Waste treatment, waste sludge, wastewater, biogas,
closed cycle, food waste disposer, food waste,
environmental influence, digester gas, digestion
The report describes the effects of a large-scale
installation of food waste disposers in the town of
Surahammar, and their effect on Haga wastewater
treatment plant. No negative conclusions were reached
during the follow-up period.
Target groups:
Municipal civil servants
Environmental authorities
Publishing year:
Price 1999:
SEK 200, not including VAT
Cover art:
Kim Gutekunst
The Swedish authorities have long maintained a restrictive attitude toward food waste
disposers. During recent years, several municipalities have shown interest in using
waste disposers in their waste disposal systems. Before disposers are installed, it is
important to consider the issues surrounding, among other things, the sewage treatment
in general, the limitations of the sewage system, and the design of the wastewater
treatment plants.
In the municipality of Surahammar, food waste disposers have been introduced as a
sorting at source alternative for the organic household waste. At the end of 1998, there
were waste disposers in approx. 40 percent of the households connected to the
municipal sewage system. The prognosis was that the percentage of households using
food waste disposers would approach 50%. The report shows the results of a case study
performed during 1998. The conditions at Haga wastewater treatment plant have been
studied and compared to reference operational data from the period prior to the
installation of the disposers. Furthermore, the results from investigations (sewage
system, local resident survey) performed by Surahammar KommunalTeknik are
Based on the study performed, the following conclusions, among others, have been
 The sewage system has exhibited no problems during the investigation period. No
overflow was detected during 1998.
 No service interruptions have occurred at the wastewater treatment plant.
 During 1998, a slight increase in the amount of screened matter was detected.
 It has not been possible to detect any increase in the amount of incoming nitrogen,
phosphorous or BOD from the water analyses. However, an increase in the ratio
BOD/N shows that the composition of the wastewater has changed during the
investigation period.
 The biological step does not seem to be affected; the aeration demand has not
 The increase in gas production seems to correspond to the theoretical biogas
potential of the waste, but no effects on the sludge treatment have been detected
 The emissions of N, P and BOD have not increased at Haga wastewater treatment
 The fluctuations in the measured data, as well as a decrease in the load, makes it
difficult to reach conclusions that are very certain. This is why results/observations
which in certain cases point in different directions cannot be fully explained.
 In order for a study of this type to provide anything further, the investigation period
must be considerably longer and characterize the situation during stable operating
The waste disposers have had no effects other than positive on the operations at Haga
wastewater treatment plant. Neither has the sewage system exhibited any problems. In
order to be able to reach more certain conclusions and to gather more knowledge, both
a longer investigation period and a process-specific experimental program is needed.
The project at hand should be regarded as a background study. An interesting question
in this context is whether Surahammar is particularly well suited to exploit the use of
waste disposers, or whether the project has succeeded because of careful planning.
Food waste disposers have historically seen very restricted use in Sweden. The reasons
for this are among other things the fear of problems with the sewage system
(sedimentation and congestion), increased oxygen consumption at the wastewater
treatment plant, and the risk of an increased influx of unwanted material and objects to
the wastewater treatment plants. Furthermore, there has been a fear of increased
emissions of oxygen consuming substances and nutritive salts from the wastewater
treatment plants.
There are a number of reasons why a future increase in the use of food waste disposers
could be expected.
1. The risk of receiving unwanted, non-degradable objects (e.g. bottle caps and
similar) with the wastewater is less for modern food waste disposers than for older
2. The advertised deposit tax must result in a decrease in the deposited amounts.
Merged digestion with the municipal sludge is one way of reducing the levels,
particularly if the rest product can be marketed for use in agriculture.
3. An increased interest in the production of biogas has made the alternative of
organic waste digestion more attractive.
4. In many cases, there exists today a surplus capacity in the digestion step at
municipal wastewater treatment plants in Sweden. Thus, there is often space
available to introduce additional organic matter, without having to make any
significant new investments.
5. The sorting at source of waste is costly. The combined transport of waste with the
wastewater could save resources.
In Surahammar, food waste disposers have been installed in approx. 1,500 households
(December of 1998). About 40 percent of the households are connected to the
municipal sewage system and Haga wastewater treatment plant. Within the
municipality, food waste disposers constitute one of three existing sorting at source
alternatives for the household food waste; the remaining two are home composting and
the use of a designated refuse containers with centralized handling.
By studying the conditions at Haga wastewater treatment plant before and after the
installation of food waste disposers, we wish to increase the knowledge of how the use
of disposers affect the processes and operations of a municipal wastewater treatment
Food waste disposers have been used in household applications since the 1930s. The
first generation food waste disposers were designed to effectively grind both food and
bone remnants. Other accounts even state that it was no problem grinding both cutlery
and beer bottles and other items one wished to get rid of in a trouble free and easy way.
[Cartoon strip:]
"What are you going to do with all the trash in the sink, Ernie?"
"Grind it in the kitchen disposer, Spencer."
"What about the dishes? What are you…"
"I'm too tired to bother with them now. I'll do them later."
Today's food waste disposers are designed to grind only soft food waste. The disposer
shreds rather than grinds the waste, and is unable to grind hard bones and materials
that are too tough, such as fish skins.
2.1 The Development in Sweden
In Sweden, the Environmental Protection Agency, municipalities and VAV have
consistently maintained a very restrictive attitude toward food waste disposers, in the
beginning because the biological treatment stage at sewage works was not added and a
large increase in emissions was predicted if food waste disposers were allowed. There
have also been concerns as to how the sewage system would handle the increasing load
of suspended matter, regarding risks of congestion and hydrogen sulfide production,
among other things.
Later, the authorities and the environmental public opinion have been skeptical toward
food waste disposers also for a different reason. It is believed that it in principle is
wrong to first mix the food waste with water and later to separate out the useful matter
from the sludge. The low acceptance of sewage sludge has also contributed to an
unwillingness to mix in the pure food waste.
In conclusion, much of the criticism can be traced to the fact that primarily the effects
on the wastewater treatment have been considered, where the cost certainly could
increase, but also the revenue. The effects on waste handling with reduced emissions
from transportation, less problems at the landfills and an improved working
environment have not been discussed to the same extent.
Today the view on food waste disposers has become somewhat less categorical and
many municipalities are interested in the technology. The reason for this, among other
things, is the following:
Sorting at source and treatment of household waste has proved more expensive and
more complicated than what was predicted at first. Therefore, interest in other
technological alternatives has awakened, such as suction systems and food waste
Many wastewater treatment plants have a surplus capacity in their digestion tanks,
or could create a surplus capacity with minor modifications.
The biogas technology has undergone a renaissance.
It is primarily among the municipally owned companies, who singularly have the
responsibility for both waste and sewage management, that interest has increased.
Within these organizations, it is easier to see and discuss the advantages and
disadvantages, and possible cost effects, of integrated systems for waste and sewage.
Food waste disposers are not in any way considered for use in areas where wastewater
treatment plants are not equipped with digestion tanks and thus are incapable of gas
Most municipalities allow the use of food waste disposers after dispensation has been
given. Generally, very few dispensations are granted, but this could also be because the
dispensation requirement is unknown to most subscribers. In certain municipalities, a
higher fee is assessed from households that request dispensation and install food waste
disposers. Stockholm (Stockholm Vatten AB) is one example of this.
Food waste disposers may, based on the sales statistics of Disperator, exist in less than
1 percent of the Swedish households.
2.2 The Situation Abroad
In the United States, there are food waste disposers installed in 49 percent of the
households connected to the municipal sewage system. Over 95 percent of American
cities allow the use of food waste disposers, but a few cities have prohibited
installation due to lacking capacity, or for other reasons (Wicke, 1987). In 1992, more
than 90 municipalities had requirements regarding the installation of food waste
disposers for new construction, among those Detroit, Indianapolis and many
municipalities and towns in California. The total number of disposers in active use has
been estimated at 45 million. ISE (In-Sink-Erator) is the world leader with an
approximate 80 percent share of the US market.
Food waste disposers are less common in European countries. In Great Britain and
Holland, scarcely 5 percent of the households have disposers (de Konig & van der
Graaf, 1996; Mortensson, 1996).
New York City expressly prohibited the use of food waste disposers in the beginning
of the 1970s (Wicke, 1987). The reason for this was among other things an overflow of
untreated wastewater. Therefore, it was desired not to introduce unnecessarily large
amounts of organic material by permitting the installation of disposers.
This ban in New York apparently provoked a large number of studies in the United
States, praising the food waste disposer in the same unreflecting way some European
studies have vilified it.
A survey of the literature shows that the issue of food waste disposers is, or has been,
much fought over in many Western countries, Holland, Denmark, Australia and Japan
among others. It can further be concluded that several of the disposer investigations in
circulation are purely commissioned reports, initiated and funded by a disposer
manufacturer, even though they have been conducted at a university or an R&D
3.1 Comprehensive Functional Description
Food waste disposers are manufactured by a number of different companies. They are
mounted underneath the kitchen counter and are connected to the sink (diameter of 90
mm). A dishwasher may be connected to the disposer as well, and the outlet from the
disposer is connected to a U-shaped drain trap. Both continuously working disposers
and batch fed disposers are available.
When a batch fed disposer is used, the food waste is first fed into the disposer. After
flushing with water, the disposer is turned on by shutting a stopper in the run position.
A continuous disposer is turned on with a manual switch mounted on the counter while
the waste is flushed into the disposer by the water.
In the most commonly used disposer on the Swedish market, the waste is flushed onto
a rotating disk with a number of 3-4 mm wide holes in it. The disk rotates at approx.
1,400 rpm. Outside the rotating part sits a fixed, saw-toothed shredder with approx. 2mm wide slotted openings. By centrifugal force the waste is thrown onto the rotating
shredder, passes it through the holes and into the outlet pipe. Thus, it can be concluded
that modern food waste disposers really do not incorporate a traditional grinding
The limited hole diameter of the disposer causes harder, non-shreddable materials to
stay in the upper part of the disposer. Such material must be removed manually when
the disposer is turned off. Furthermore, the electrical supply to the motor is broken if
Other facts
Retail price about SEK 3,000, not including VAT
Water flushing requirement 5-7 liters per minute
Annual energy requirement 3-4 kWh/household
Water consumption 3-6 l/household and day
Particles size of the food ground waste is an interesting question for several reasons
(sewage system sedimentation, intake screen and sedimentation basin processes, etc.).
However, there are differing opinions on the actual size that the particles from the
disposer have. Disperator states that their disposers (ISE) normally produce particles of
up to 3-5 mm. During a thesis study conducted on an ISE disposer at Mälardalens
högskola (college), it was found that particles of up to 20 mm were common and that
also pieces up to 40-50 mm could be found after the disposer (Jenny Nilsson, 1998).
This occurred most often in the case of waste fractions such as onion and potato peel.
In a Japanese study aimed at characterizing the waste fractions from a disposer
manufactured by SinkMaster, a dispersion was obtained of between 2 and 5 mm.
3.2 Grindable Waste
The question of how much of organic household waste is grindable has been
investigated or discussed earlier in some projects. Data from two Swedish reports
(Lagerkvist &Karlsson, 1983; Nilsson et al., 1990) is shown in Table 1.
Both reports indicate that approx. 20 percent of the food waste suitable for composting
is not suitable for grinding in a food waste disposer. This could for example be skin
from chicken and sausage, pork chop bones, avocado pits, and other hard materials.
Nilsson et al. (1990) further show that an additional 24 percent of the material suitable
for composting is not ground since it is wrapped in packaging, which is not cleaned
out, and put directly into the general refuse.
Table 1: Amounts and fraction of grindable waste, according to two Swedish reports:
1) Lagerkvist & Karlsson, 1983 and 2) Nilsson et al., 1990.
Suitable for composting
Grindable (directly)
Indirectly grindable
Not grindable
household waste
100 %
78 %
22 %
100 %
56 %
24 %
20 %
A current national study (Olsson & Retzner, 1998), based on a “pick” analysis of
household waste from six Swedish municipalities, showed that the food waste made up
approx. 40 % of the household waste. From the report data, it can further be obtained
that one person today generates about 75 kg of food waste annually. Assuming 20
percent of this cannot be ground down, 60 kg/person and year remains. We further
assume that, for various reasons (see above), an additional 10-15 % is lost ("indirectly
grindable") by being put with the general refuse. Based on this reasoning we assume
the following specific amounts of grindable and non-grindable food waste:
Amount of food waste that is ground: 50 kg/person and year (67 %).
Amount of food waste put with the general refuse: 25 kg/person and year (33 %).
The assumption is based on general data and has not been adjusted to the conditions in
the municipality of Surahammar. However, from Olsson & Retzner (1998) it can be
concluded that there is a limited variation in the generated amounts of waste between
different municipalities. In Surahammar, the intention is to make their own estimate of
what portion of the food waste is ground, as opposed to put with the general refuse.
International data varies on how great the grindable portion is. This could partly be due
to the differing food preparation patterns of different countries, and partly that the
amount data also internationally is based on assumptions or investigations using small
data sets. One example of this is given by de Koning & van der Graaf (1996) who
assume that the grindable amount is 44 kg/person and year.
The amount of waste to be collected from the households decreases if food waste
disposers are installed. The work involved in vehicle transportation could therefore be
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me time estimate that the household water consumption does not change because of
disposer use. These conditions could be described in terms of an increase in the
contaminant amount and content and a flow which remains constant.
The specific contaminant contributions from wastewater and food waste respectively
can be described based on general standard rules. These are shown in Table 2 together
with the percent increase in the contaminant load resulting from the fact that 25 and 50
percent respectively of the individuals connected sort their waste at source using food
waste disposers (fwd).
In the table below is also the ratio BOD7/N shown for the different fractions. This ratio
is shown since it can be used as a "flow independent" indicator, showing if and to what
extent the waste affects the composition of the wastewater. The ratio BOD7/N is in this
respect a slightly more efficient standard of value than the ratio BOD7/P, because
nitrogen is input from the food waste in a relatively smaller amount than phosphorous.
(BOD7= oxygen consumption over seven days, N=nitrogen, P=phosphorous)
From the numbers in Table 2, the consequences of 25 and 50 percent respectively of
the connected individuals using food waste disposers may be calculated. In the case of
25 percent connected, a 12 percent increase in the BOD amount would be expected and
a 2 percent increase in the amount of phosphorous and nitrogen. A 50 percent
connection means that the increase is doubled, i.e. the BOD amount increases by
roughly 25 percent and the amount of phosphorous and nitrogen by roughly 4 percent.
The conclusion is that the input from waste of nutritive salts and oxygen consuming
substances into the wastewater is expected to be moderate also for a considerable
systems expansion. The theoretical load increase for Surahammar, based on regional
waste data, is shown later in the report (Table 6).
In addition, the effects from the increase in wastewater contaminant content do not
have to imply a corresponding increase in the load at the plant. For example, the
production of digester gas increases proportionally to the BOD increase only if the
incoming BOD can be transferred to the digestion tanks to the extent that BOD is
transferred from the wastewater. Varying decomposition and separating processes in
the pipelines and at the wastewater treatment plant could lead to different conditions.
Table 2: Theoretical contaminant contributions from sewer and food waste disposers
(fwd). General sewer and waste data has been used.
Dry matter
Glow losses g/person
phosphorous day
and 175
amount at
50 % fwd
and 122
and 48
and 2.1
and 13.5
and 4
and 3
and 0.6
and 7.2
and 5
and 0.07
and 3.1
and 61
25 %
and day
and day
1) Data obtained mainly from the Swedish Environmental Protection Agency, 1995.
2) The amounts refer to 67 % of what is generated in the households as food waste.
Data from Olsson & Retzner, 1998; RVF, 1996 (averages) and Wicke, 1987. Data
from the latter refers to the waste BOD content, which in the investigation indicates
a very large spread in the data for the BOD7 content of the ground waste. Different
sources claim between 10 and 38 g of BOD7 per person and day.
If the input of heavy metals into the waste is examined, it is observed that the effects
could become large for certain parameters. The degree of sorting at source determines
entirely the amount of heavy metals in the contaminant input from the food waste
disposer. Both the solid sewage products as well as the solid waste should contain – for
optimal sorting at source – only heavy metals coming from the consumed foodstuff.
4.1 Haga Wastewater treatment plant
Haga wastewater treatment plant is the main plant for Surahammar. All wastewater
from the towns of Surahammar and Ramnäs and the village of Haga is routed to the
Description of the Wastewater treatment plant
Haga wastewater treatment plant was designed for mechanical, biological and chemical
treatment of wastewater. In the mechanical stage, coarser solid contaminants (screened
matter) are separated out by a cleaning screen (mesh size 3 mm), while sand is
separated out in a sand trap. The primary sedimentation then follows. The biological
stage is designed for the activated sludge process and the chemical treatment takes
place through joint precipitation in the biological stage. Spent pickling baths (ferrous
sulfate) from Surahammar Steel works are used as the precipitation chemical.
Following the joint precipitation, a (final) sedimentation takes place before the treated
wastewater is routed to the recipient, Kolbäck creek, via a 200-m long outlet pipe.
During parts of the year, the outgoing flow is used in the irrigation of energy grasses
on the plant property. The wastewater treatment plant is also equipped with a
postprecipitation assembly, currently not in use.
The produced sewage sludge, extracted from the primary sedimentation basins at the
plant, is led directly to a digestion plant for stabilization. The digestion stage consists
of two digestion tanks connected in series, where the first one undergoes total mixing
and is heated, while the second could be considered more of an intermediate storage
(with gas extraction, however). Sewage sludge from Virsbo wastewater treatment plant
is also collected and treated at Haga, and is fed to the plant via the thickeners. The
digested sludge does not undergo mechanical dewatering, but is transferred to sludge
drying beds that also function as intermediate storage. An elementary flowchart of the
wastewater treatment plant is shown in Appendix 1.
Connection and Design
The wastewater consists mainly of household wastewater. The two main industries
connected to the plant, Surahammar Steel Works and Adtranz, only contribute smaller
amounts of permeates from ultra filtration plants. Design data for Haga wastewater
treatment plant is shown in Table 3.
Table 3: Original design data for Haga wastewater treatment plant. (pe=population
equivalent, qdim=design flow)
Total load
approx. 435
Specific load
Current Load and Treatment Results
The connection to the wastewater treatment plant, according to the environmental
reports from the past couple of years, has been approx. 9,500 population equivalents
while the actual number of connected individuals has been approx. 8,830. Measured
load data for incoming wastewater for 1995, 1996, 1997 (environmental report data) is
shown in Table 4.
Table 4: Load during 1995, 1996 and 1997.
Average flow
Total nitrogen
If we reduce the industrial share of the load (approx. 670 pe with the specific
contaminant amounts normally associated with a population equivalent: 70 g of
BOD7/pe,d; 14 g of N/pe,d and 2.5 g of P/pe,d), we obtain the following data for the
approx. 8,830 individuals connected:
49 g of BOD7/p,d (Standard household from the Swedish Environmental Protection
Agency: 48 g/p,d)
2.1 g of P/p,d (Standard household from the Swedish Environmental Protection
Agency: 2.1 g/p,d)
13.5 g of N-tot/p,d (Standard household from the Swedish Environmental
Protection Agency: 13.5 g/p,d)
From this we conclude that the specific load at Haga wastewater treatment plant almost
exactly follows the standard figures for household wastewater from the Swedish
Environmental Protection Agency (Swedish Environmental Protection Agency, 1995).
This illuminates the difficulties with the concepts of individual and population
equivalent for the issue of what specific BOD7 is to be used for the description of an
The contaminant content of the incoming and outgoing wastewater for 1995, 1996 and
1997 is shown in Table 5.
Table 5: Contaminant content of the incoming and outgoing wastewater (in  out) for
1995, 1996 and 1997.
Total nitrogen
75  3.1
3.7  0.27
110  4.9
4.3  0.3
102  4.9
4.1 0.27
96  4.3
4  0.3
23  18
27  20.8
27  19.4
26  19
According to the current requirements, the residue of the outgoing wastewater must not
exceed 15 mg of BOD7/l and 0.5 mg of phosphorous/l. These requirements have been
met with good margin during the past couple of years. No requirements exist for the
reduction of nitrogen. Table 5 shows the following average reduction for 1995-1997:
96 % BOD7
92 % phosphorous
27 % total nitrogen
Sludge Management
Following the treatment (digestion and drying in beds), the majority of the sewage
sludge has been used on cultivated lands (growing of energy crops). Sewage sludge is
now also used in the production of construction soil. Screened matter from the
preliminary treatment is transported to Vafab's (Västmanlands refuge collection
authority) waste treatment plant in Västerås for landfill, while sand from the sand traps
is spread out on the drying beds. The sludge quality has been varying during the past
couple of years. The heavy metals led, cadmium and zinc have been close to the
current limit values.
During a period in 1997, experiments were conducted on the digestion of ensilage with
sewer sludge. During the reference operations preceding the grass digestion
experiments, the production of digester gas amounted to approx. 3,000 m3/d. With the
addition of ensilage, the digester gas production increased by approx. 370 m3/d. The
specific production in both cases amounted to approx. 0.5 m3 per kg of added wet
matter (VS = wet substance).
The methane gas produced at the digestion plant is used internally in the production of
electricity and heat. During 1995-1996, the average gas production has been 335 m3/d.
From the current specific gas production (0.5 m3 per kg of added VS, wet substance), it
is shown that the daily raw sludge has corresponded to approx. 960 kg of dry matter
(TS = dry substance) per day (at 70 percent VS of TS).
This corresponds to 350 tons of TS per year, or barely 9,000 m3 of raw sludge annually
at a TS content of 4 percent. Spread over the stated population equivalents contributing
sludge (Haga 9,500 pe and Virsbo approx. 2,000 pe), we obtain a specific sludge
production of approx. 85 g of TS/pe,d, which is low compared to the standard data. (If
we calculate the pe values using traditional methods, the corresponding sludge
calculation yields 120 of TS/pe,d which is similar to the standard data.)
Sewage System and Overflow
The amount of separate systems in the wastewater sewage system is roughly 96
percent. The drainage water coming from a large number of properties is connected to
the wastewater treatment plant, which means that significant amounts of melted and
rain water reach the plant during snow melting and rain. In relation to the pure water
consumption within the municipality, the incoming amounts of wastewater during the
past years have been 30-50 percent higher on an annual basis. During the past year, a
considerable amount of work has been put into searching for and fixing water leaks.
This has resulted in a reduced flow to the wastewater treatment plant, but the effects
have not been quantified.
Overflow in the sewage system and at the treatment plant only occurs as an exception
because the sewage disposal system of Surahammar has a considerable surplus
capacity. Overflow may occur at the C5 main pumping station and at the wastewater
treatment plant. The overflow points at the plant lie after the intake screens (overflow
occurs at flows exceeding 4 x qdim) and after the primary sedimentation (overflow
occurs at flows exceeding 2 x qdim). During 1995 and 1996 no overflow took place at
all, while during 1997 an overflow of approx. 28,000 m3 occurred at the C5 pumping
station (sewage system work) and 2,000 m3 at the plant.
If the incoming wastewater during a high-flow period contains 50 mg of BOD7/l and a
total overflow of 2,000 m3, this results in an increase in the BOD7 emissions by 96 kg
of BOD7 annually. Using food waste disposers at the level corresponding to October of
1998, this number would theoretically increase by approx. 8 percent to 104 kg
annually. This should be compared with the BOD emissions for the treated wastewater,
which is approx. 7,000 kg annually.
4.2 Waste Management in Surahammar
Surahammar KommunalTeknik AB is responsible for the management of both waste
and wastewater for the municipality. Since 1997, there are new refuse collection
regulations in place for the municipality.
Sorting at Source of Waste
The new refuse collection regulations state that, using a charge system, the households
may choose from three alternatives for their food waste sorting at source. At the
households, so-called general refuse (everything but packaging, hazardous waste, and
bulky waste) is conventionally collected by truck every other week.
This fraction of the household waste is transported to the Vafab waste treatment plant
in Västerås.
For organic waste suitable for composting, three alternatives have been offered at
different fees:
1. Food waste disposer (see Chapter 4.2.3)
2. Home composting
3. Special container for organic waste
The alternative of home composting means that the occupants themselves compost
their waste using so-called warm composting. The households are expected to purchase
and pay for the upkeep of their compost containers, and to make sure that the compost
product is used in an environmentally friendly way. Home composting carries the
lowest charge of the various alternatives.
The container collection alternatives mean that the households receive an additional
waste container. Emptying of the containers takes place once or twice a week,
depending on the season. The organic waste is then transported to a central treatment
plant for composting or digestion.
Experiments with Food Waste Disposers – the Housing Cooperative Skivlingen
In 1993, a first experiment with food waste disposers was started in an apartment block
belonging to the housing cooperative Skivlingen in Surahammar. The disposers were
installed in 32 of the 39 apartments belonging to the cooperative in connection with
changing the mains for the block. The installation of disposers was a part in a larger
sorting at source program for Skivlingen. The total effect on the waste side was that the
refuse collection could be reduced from emptying six 400 liter containers twice a
week, to emptying three once a week (SKT, 1992).
Among other things, the experiment was followed up by surveys in 1993 and 1995. At
both times, the apartment occupants were mostly satisfied with their disposers. In the
second survey, 96 percent were satisfied while 22 percent claimed they had
experienced some problems with the disposers. Most of the time the incorrect items
had ended up in the disposers, and there had been difficulty in removing objects or
food scraps. On the average, the disposer is used 3-4 times a day. 75 percent and 78
percent respectively of the apartment occupants responded to the surveys at these two
An important part of the experiment was to examine the effects on the sewer system
belonging to Skivlingen. The investigation began with flushing and video-filming of
the pipes joining two identical apartment blocks at Skivlingen 1 and Skivlingen 2.
Skivlingen 1, which did not receive any food waste disposers, would serve as the
reference object, while Skivlingen 2 was the object where disposers were installed.
Little over a year following the installation, the pipes were video-filmed for the first
time after flushing. No differences could be detected between Skivlingen 1 and 2. Two
years later, another inspection was performed when the service line for Skivlingen 2
was flushed using high pressure.
The personnel could not detect any accumulation of particles, sludge or grease this
time either. The flushing initiative was taken by the cooperative, wanting an additional
inspection of the pipes.
Another observation made is that the water consumption at the cooperative Skivlingen
was reduced by 25 percent during 1993-1996. However, Surahammar
KommunalTeknik does not wish to reach the conclusion that this is due to the
disposers, since fittings in the apartments were changed out during the same period.
Food Waste Disposers on a Large Scale
Since the experiment at the housing cooperative Skivlingen turned out well,
Surahammar KommunalTeknik decided to introduce food waste disposers as an
alternative in the new refuse collection charge. The alternative means that the
households either purchase the disposers themselves and are responsible for the
installation, or that the disposers are installed by Surahammar KommunalTeknik for a
fixed charge spread out over an eight year period. If the latter is chosen, video-filming
of the sewage service line is also performed before the installation, free of charge.
The installation of the waste disposers started in May of 1997, and in October of 1998
roughly 1,100 households, mainly one-family households, had made their choice and
had had their disposers installed. Within the field of activity of Haga wastewater
treatment plant, there are 3,700 households of which the majority received the offer for
a food waste disposer through Surahammar KommunalTeknik. The connection in
October of 1998 thus meant that 30 percent of the households had disposers installed.
The prognosis was then that approx. 1,900 households (roughly 50 percent of the
households within the field of activity of Haga wastewater treatment plant) would be
equipped by the end of the summer of 1999.
The installation of food waste disposers is an operation that has a certain drawn-out
effect on the wastewater treatment plant. Per capita, the rate of expansion has been
about 60 households per month. The load on the treatment plant can be expected to
increase at the same rate. In the investigation, this has been considered when relevant.
Based on the waste and wastewater data, we have calculated how great an effect the
current installation of disposers will have on the load increase at the Haga wastewater
treatment plant. Note that we in this calculation have used standard figures obtained
from the regional waste management company Vafab, which we believe will be more
valid for the conditions in Surahammar. It is therefore not possible to directly compare
the general information of Table 2 with Table 6.
Vafab normally calculates the household based waste amounts as follows:
Food waste (gross) from single homes: 5.5 kg/week
Food waste (gross) from apartment block households: 2.5 kg/week
Using the same grinding factor as earlier, 67 percent, we obtain that the 1,100
connected households (900 single houses and 200 apartments) in Surahammar together
deliver 520 kg of waste per day to the sewage system. The wastewater is assumed, as
shown earlier, to correspond to approx. 5,000 m3/d. The load increase that 1,100
households with disposers are assumed to create in Surahammar is shown in Table 6.
Table 6: Theoretical load increase due to food waste disposers in 1,100 households in
Surahammar. Regional waste data has been used.
Dry matter
Org. matter
P total
Average 95-97 October of
Load increase
The investigation is based mainly on a comparison of the conditions in the sewage
system and at the treatment plant before and after the installation of waste disposers in
Surahammar during the spring of 1998.
5.1 Selection of Samples
During normal operation, the sample collection at Haga wastewater treatment plant has
been performed according to the instructions of the current inspection program. This
means the following sample collection routines:
Incoming wastewater: BOD7, CODcr, Tot-P, Tot-N, NH4-N
Outgoing wastewater: BOD7, CODcr, TOC, Tot-P, Tot-N, NH4-N
Treated sewage sludge: pH, TS, Glow losses (GF), Tot-P, Tot-N, NH4-N, CaO,
heavy metals and organic environmentally damaging substances
The incoming wastewater is sampled over a 24-hour period, 13 times a year, while the
outgoing water is sampled the same way 25 times a year. The sludge sampling is
performed as collection samples twice a year. For the above analyses, there thus exists
historical data for a large number of years stretching back in time. At the same time, it
could be concluded that, aside from normal analysis, only a limited historical data set
exists for the remaining parts of the treatment plant.
With regards to additional water inspections, additional samples were obtained during
a period of two months, according to the points of Table 7. The samples were obtained
as weeklong samples, which was considered more relevant than the day samples. The
following parameters have been analyzed: BOD7 (filtered), BOD7 (unfiltered), Tot-P,
Tot-N, NH4-N, N-org and suspended matter.
During the entire follow-up period, also pH, temperature, flow and oxygen content
have been followed in the biological step. By paying particular attention to the
phosphorous emissions during the period, the need for a change in the precipitation
chemical dosage has been followed indirectly.
Table 7: Description of sampling points for additional wastewater sample.
Sample type
Incoming wastewater
Before the fine-mesh screen at existing
sampling point according to the
inspection program
Mechanically purified water
Outgoing from the sand trap,
alternatively incoming to the primary
Primary sedimentation of
Outgoing from the primary
sedimentation, before the admixing of
precipitation chemical
Biologically and chemically
Outgoing from the intermediate
treated water
sedimentation, at the existing point
according to the inspection program
Unfortunately it was noted that the water from this sampling point was affected by
the surplus sludge, which is collected via the primary sedimentation. This data can
therefore not be used.
The study of the sludge has consisted in part of analyses of the sludge quality, and in
part by measuring the amount of sludge, sand and screened matter. In addition, the
conditions in the digestion tank have been observed, including the amount of produced
For the sludge analyses the following parameters were determined: TS, glow losses
(VS), Tot-P, Tot-N, NH4-N, TOC and heavy metals. The definition of the sampling
points are given in Table 8.
Following the initial batch of analyses, the sludge sampling for S1 was interrupted.
This was because no reference values could be obtained for the sampling point. For
sampling point S2, the sludge quality was followed through the normal sludge
sampling according to the inspection program.
Table 8: Description of selected sampling points for sewage sludge.
Sample type
Raw sludge
Digested sludge
Outgoing sludge from primary
Sludge from digestion tank
5.2 Observations
Aside from the investigations, ordinary observations regarding the operation are an
important part of the follow-up. This part has been conducted within the framework of
the normal plant supervision.
Operations data is shown for most of the parameters running up until December
of 1998 when the study was concluded. A period with additional water sampling (week
samples) took place during May-June. Analysis data from these did not deviate from
the recurring and normal day samples. The week sampling was therefore discontinued
after this.
It should further be noted that Table 6 only illustrates the theoretical load conditions
for October of 1998. Since the expansion rate has been constant, this data represents
the average load for the fall of 1998 which has been exploited in comparative studies
of the gas production during different years.
In this results chapter, both data registered during the follow-up period and
observations are shown and discussed. The presentation follows the path of the
wastewater and concludes with the results from the latest survey. A broader discussion
of the results is presented in Chapter 7, "Comments and Conclusions".
6.1 Sewage system
The inspection of the sewage system is something into which Surahammar
KommunalTeknik has put a lot of effort. Above all, all the service lines for the
buildings choosing disposers were inspected prior to the installation, to determine if the
service line had any defects, which would mean that the connection of waste disposers
would be unsuitable. Only one area in Surahammar has so far been deemed unsuitable
in this regard.
During the expansion period, also the general sewage system, through an expanded
inspection program, has been inspected continuously through video-filming and
recurrent flushes, among other things. The effort was above all concentrated to
stretches that beforehand were deemed prone to congestion. Until October of 1998, no
signs of congestion or accumulation of sludge or particles could be detected by the
inspections. These observations are consistent with previous experience (refer to
Chapter 4.2.2).
During emergency overflow in the sewage system or at the treatment plant, and when
effluence is diverted past one or more treatment stages, impure or partly purified
wastewater can be routed directly into the recipient. This is a strong reason why
authorities have a skeptical attitude toward food waste disposers. In Surahammar,
overflow normally occurs to a very limited extent, both in the sewage system and at the
treatment plant. A fact illustrating this is that no overflow occurred until October of
1998, in spite of it being an unusually rainy year.
6.2 Incoming Wastewater
The incoming flow into Haga wastewater treatment plant decreased slightly during
1998 compared to earlier years. An explanation for this is that Surahammar
KommunalTeknik put a lot of effort into renovating the lines (fixing leaks) during the
year. At one of the points, there has been an estimated decrease in the overflow from
the storm water sewage system to the wastewater sewage system by 30,000 m3 per
The changes in the flow conditions, which have precisely affected the follow-up period
compared to earlier years, to a certain extent makes the evaluation a bit more difficult.
That one without this interruption could trace the cause of a change in the water
consumption to the waste disposers is still uncertain.
If the households that have disposers installed would decrease their water consumption
by 10 percent, it would mean a decrease in the flow by approx. 65 m3/day. That
amount corresponds to roughly 1 percent of the average flow.
A reduction in the leakage into the sewage system, unless the seepage water is
contaminated ground or drainage water, should not affect the incoming contaminant
amounts. On the other hand, the contaminant content of the wastewater should
The load increase, as we might be expected after the addition of food waste, could not
be traced in the analyses of the incoming water. One contributing factor for this is that
the contaminant levels and amounts vary greatly from day to day. It is then not strange
that a decrease by a few percent would be undetectable, which is the case with nitrogen
and phosphorous.
Stranger is that a calculated increase in the BOD7 amount by 15 percent has not been
visible. The incoming amounts of BOD7, with a moving average added, are shown in
Figure 2. The diagram reflects the large variations caused by the current method of
measuring (water sampling and flow measurements). The BOD load at the treatment
plant does not seem to be increasing during 1998. The variation in the incoming BOD
amount lies well within the variational limits of earlier years.
If the BOD7 result is considered separately from other observations, the following
possible explanations for the missing load increase, among other things, can be found:
 The expected increase in the BOD load is hidden by natural variations.
 The analyses and/or samples have not been reliable, because of:
 Incorrect methods,
 That the waste enters the treatment plant in batches and thus cannot be caught by
the sampling,
 That the day sampling, performed between Tuesdays and Wednesdays, has
produced an incorrect picture of the waste amounts if the bulk of the food waste is
generated during weekends.
 The incoming water contains less BOD7 than expected, because of:
 The waste contains less organic matter than assumed,
 Food waste disposers are used less often than assumed,
 Other sources of contamination have reduced their BOD7 contributions, or that the
conditions have changed in other ways (an effect of, for example, reduced leakage
of "dirty water", due to renovations of the sewage system),
 That considerable denitrification or decomposition for other reasons take place in
the sewage system after the disposers were put into operation.
The summary points to two scenarios: either food waste does not enter the treatment
plant, or the food waste does enter, but in such a way that it does not show up in the
analyses of the water BOD7 content. Which sequence of events lie behind this has not
been determined within the framework of the project. However, what may be
concluded is that observations of, for example, the amount of screened matter and
production of gas, support the theory that food waste arrives at the treatment plant.
Figure 2. BOD7 amounts (kg/d) in the incoming wastewater at Haga wastewater
treatment plant, between December of 1994 and December of 1998. Moving average
(15 intervals) is shown.
As was discussed earlier, the ratio of BOD7 and total nitrogen can be used as a flow
independent value parameter for changes in the wastewater composition. A change in
the ratio could in any case indicate whether the incoming wastewater is affected by the
food waste. Using this projection as a starting point, it is possible to reach the
conclusion from Figure 3 that food waste disposers are in use and that they do affect
the composition of the wastewater.
The progression of the curve for the ratio BOD7/N indicates a change in the load at the
same rate as the installation of disposers. Furthermore, it can be determined that the
ratio BOD7/N before the installation agrees with the theoretical value shown in Table
2, i.e. approx. 3.7. On the other hand, the ratio is not in agreement with the later period.
The ratio is 4.5-4.6 in the diagram, while the theoretical prediction yields approx. 4.2.
It would be easy to trace the deviation to the use of incorrect data for the food waste
composition. This could indicate that the waste contains more BOD7 or less nitrogen
than has been assumed.
[Graph: "The installation of disposers began here"]
Figure 3. Progression of the ratio of BOD7 and nitrogen in the incoming wastewater,
between 1994 and December of 1998. Moving average (15 intervals) is shown.
If the results shown in the last diagrams are combined, two conflicting indications are
found. In one case (BOD7 amounts, kg/d), the effect of the disposers cannot be
confirmed. The second parameter (the ratio BOD7/N) shows that the effect of the
disposers has been greater than what would be expected from a theoretical projection.
As we are unable to confirm an increase in the incoming BOD amount, it can be
concluded that a constant BOD7 amount in the incoming wastewater, in combination
with a reduced amount of incoming nitrogen, would cause this result. One explanation
of this phenomenon would be that denitrification takes place in the sewage system.
To confirm whether anything unexpected happens to the BOD7 and nitrogen content of
the wastewater, a comparison has been made with the incoming phosphorous amount.
Certainly, a tendency toward an increased influx of phosphorous is evident during
1998, but it must at the same time be kept in mind that historically the influx of
phosphorous has varied considerably more than the 4.5 percent increase we expect
6.3 Screened Matter and Sand
The screened matter from the cleaning screen is collected in a container and a value for
the produced amount is obtained from the weighing performed when it is hauled away.
It can be assumed that the weighing of the screened matter does not provide an exact
picture of the production. The water content and organic content are examples of
parameters that more than likely vary from time to time. These parameters in particular
should also be affected by whether the wastewater contains any food waste or not.
Since no reference data exists for "normal" screened matter, no study of the screened
matter properties has been conducted for the case at hand. Even if reference data from
other treatment plants exists, it is doubtful it could be used in this study, due to the
variation in the screened matter quality for different plants.
In the current study, we have therefore focused on observing the amounts of screened
matter. Figure 4 shows the weighed amounts of screened matter removed from Haga
wastewater treatment plant between 1996 and 1998.
Figure 4. Average amounts of removed screened matter (kg/d). The respective average
amounts for the period 1996-97 and the latter part of 1998 have been indicated.
It can be concluded from the graph that the amount of screened matter from Haga
wastewater treatment plant has increased since the disposers were installed. The
average amount for 1996-97 was 26 kg/d, while the corresponding value for the period
between March and December of 1998 was 46 kg/d, an increase by 20 kg/d. If it is
assumed that the increase is due to food waste getting caught in the screen and,
somewhat simplified, that the specific weight of the waste in the screened matter is the
same as it is in the kitchen, barely 4 percent of the incoming waste gets caught in the
screens. Even if the assumption contains large errors, it can be concluded that a lesser
portion of the waste in the wastewater is caught in the screens.
That a small amount of food waste is caught in the screens is in itself not surprising
due to the fact that relatively large particles may leave the disposer (refer to Section
3.1). (A general theory of sorting at source claims that the food waste in some
households is flushed down the toilet to reduce cost. However, there is nothing
indicating that this kind of abuse would have increased in Surahammar after the new
sorting at source systems were introduced.)
If a fine-meshed cleaning screen is used as the first treatment stage, it could then be
expected that more screened matter is produced because of the increased use of food
waste disposers – and thereby also an increase in the handling cost (removal and
landfill). The increased amount in this case, 20 kg/d, corresponds to over 7 tons
annually. This effect can be reduced if some of the screened matter is let through by
running the screens at shorter delay time intervals between cleanings.
Using a coarser screen could be another solution. Both methods run contrary to the
prevailing ones. One method for reducing the negative effects from an increased
amount of screen matter is to extract the energy content from the organic matter before
the final disposal of the screened matter.
Regarding the quality of sand from the sand trap, the service personnel have noticed no
changes compared with before.
6.4 Effects Within the Wastewater treatment plant
Approximately 50 percent of the incoming BOD7 is separated inside the wastewater
treatment plant before the biological stage. Even though the load on the primary
sedimentation stage is relatively low (surface load: 1.67 m3/m2,h at the designed flow
rate), this must be regarded as a rather high separation rate compared to what has been
measured at plants with a similar design and load (systems where all the surplus sludge
is extracted in the primary sedimentation stage). (Unfortunately, reference data from
Haga wastewater treatment plant prior to the disposer installations does not exist.) If
this assumption were correct, it would then be easy to explain a high BOD7 separation
level in the primary sedimentation stage by claiming that food waste particles are
efficiently sedimented. However, what may not be determined is how much of the food
waste is separated in the primary sedimentation stage, and how much continues into
the treatment plant in dissolved form. An interpretation of the study performed by
Nilsson et al. indicates that approx. 75 percent of the measured food waste BOD7
would be in the form of particles, and 25 percent in dissolved form. Figure 5 shows the
separation prior to the biological stage.
The effects on the biological treatment stage can be estimated only from collected data.
The oxygen for the tanks is supplied by large aerators and turbo compressors. The
turbo compressors are controlled by oxygen meters in the tanks, at a predetermined set
point. It is therefore not possible to extract a load change from the oxygen level
measurements alone. However, the power consumption of the turbo compressors can
be used in detecting changes in the performed aeration work. Since the service
personnel have not been able to measure any changes in the power consumption, it
would seem that food waste does not affect the biological step to any greater extent.
Assuming that food waste really does enter the treatment plant, this could happen if
food waste is separated mainly in the primary sedimentation.
After primary sedimentation
Figure 5. BOD amounts in the incoming wastewater and after the primary
sedimentation (prior to the biological step), respectively.
It documented that the increased electrical energy level required to decompose food
waste in a biological stage, mainly corresponds to the increase in energy that can be
extracted from the digestion tank (Mortensson, 1996). However, the information is
based on conditions in Denmark, and even if it has not been stated explicitly, it is
probable that those conditions are valid for the case when no sedimentation takes place
prior to the aerobic treatment stage. Otherwise, a factor such as the age of the sludge in
the biological step could be crucial to the energy result. Low sludge ages is prerequisite
to a higher gas extraction, and at the same time a lowered aeration energy
During the course of the study, we have also been unable to detect any significant
increase in the phosphorous content of the incoming wastewater. Therefore, no need
has been perceived to increase the precipitation chemical dosage at Haga wastewater
treatment plant. For precipitation chemicals, it has been recommended to use preprecipitation in connection to waste disposer use. The advantage to this is that the preprecipitation gives rise to a more efficient BOD separation prior to the biological stage,
which in all likelihood should have a positive effect on the energy balance of the
treatment plant (reduced aeration demand, increased gas production).
6.5 Sludge
A central question concerning the use of waste disposers is how much of the waste can
be transferred to the digestion stage to be transformed into biogas. We have been able
to conclude that the amount of screen matter increases, but that theoretically only a
smaller portion should be separated there. Despite the fact that both primary sludge and
bio/chemical sludge is extracted from the primary sedimentation, the changes in the
amounts due to the food waste are not large enough so that the service personnel at
Haga wastewater treatment plant have been able to notice any difference in the
Separated sludge is pumped in the treatment plant to a conventional sedimentation
thickener, which has operated normally during 1998.
The samples collected from digested sludge have shown no deviations when analyzed
(except for metals) compared to samples from prior reference years. The sludge metal
content has decreased noticeably during the past year. The most important reason
behind this is that new condensation water purification equipment has been installed at
the thermal plant. The remodeling has in part taken place parallel to the installation of
disposers. The detected reduction in metal content should therefore be attributed to this
effort, and not the food waste disposers.
According to the Waste Management Research Unit at Griffith University (1994), the
amount of sludge can be expected to increase by 4 percent at a connection level of 25
percent. Similar figures are presented by de Koning & van der Graaf, 1996. If we
assume that also the dewatered sludge increases by 4 percent, it would mean that the
annual amount of sludge removed from Haga wastewater treatment plant would
increase from 570 m3/year to approx. 590 m3/year.
6.6 Digestion and Gas Production
The gas production is measured continuously by a gas flow meter which measures all
the gas produced. (The gas motor is measured separately, the gas boiler consumption is
not measured.) The variation in the gas production is relatively high, and when
compared to historical data it can be concluded that similar variations are normally
occurring during the year. For example, an annually recurring "summer dip" can be
clearly seen.
When studying the figures for gas production, one must take into consideration periods
of change in the operation of the digestion plant because of digestion tests. The last
time operations were altered was during the spring of 1998 when digestion tank 2 was
not running at maximum potential due to by-passing (flow through the bottom valve).
The digestion plant was put into full operation again during August-September, which
is why the entire first portion of 1998 yields uncertain and above all lower gas
production values. The complete picture for the 1994-1998 gas production is presented
in Figure 6.
Figure 6. Gas production for the period between January of 1995 and December of
1998. A moving average (50 intervals) and a trend line shown.
It is obvious that an increase in the gas production can be seen during the latter part of
1998. In order to confirm that this is a lasting trend, it will be necessary to follow the
development over a longer time. Since there are recurrent variations during the year, it
is important that comparisons are made with the corresponding periods for different
years. The average gas production is therefore presented in Table 9 for the months of
September-December of 1995, 1996, 1997 and 1998.
Table 9. Average gas production for September-December of 1995, 1996, 1997 and
Average Sep.-Dec.
The above view also indicates a pronounced increase in gas production. A comparison
between the current period in 1998 and the average periods of 1995 and 1997 shows
that the increase is relatively large. However, it is questionable whether 1997 would be
a relevant year because of the digestion tests that were performed. But, even if we limit
ourselves to the 1995-1996 period and compare this to 1998, an approximate 70 m3/d
increase in the production is obtained on average.
At a methane content of 65 percent, the increased amount of gas corresponds roughly
to an annual energy of 160,000 kWh.
A comparison with the biogas potential of the food waste shows that the result is of the
correct magnitude. For direct digestion of a corresponding amount of food waste in a
digestion tank, the energy efficiency is 180,000-210,000 kWh/year at a specific gas
production of 600-700 liters of biogas per kg of incoming dry substance (VS). The
difference between the various production figures can be best explained by
decomposition and separation of organic material taking place in different subprocesses prior to the digestion.
Aside from the introduction of food waste disposers, nothing in the operation of the
wastewater treatment plant has been found to explain such a pronounced increase in
gas production. The service personnel have reported that the “down”- times, digestion
tank temperatures, sludge pumping routines and other relevant operating parameters
remain the same.
6.7 Outgoing Wastewater – Quality
One negative effect the use of waste disposers could result in an increased output of
nutritive salts to the recipient. With such a small load increase as in the case of
Surahammar, the risks associated with increased emissions should be low. One reason
for this is that the purification stage for the phosphorous and BOD7 reduction within
certain limits can be adjusted to maintain the reduction capacity during a load increase.
However, the same nitrogen treatment potential is not available in a treatment plant
lacking active nitrogen purification. If a greater part of the waste nitrogen is in particle
form, it could be expected that the increase in the outgoing nitrogen amount is limited
and considerably smaller than the increase in the incoming amount. This is particularly
true for a treatment plant such as Haga in Surahammar, where the digested sludge is
not dewatered.
The results from the analyses at Haga wastewater treatment plant confirm that the
emissions of nitrogen, phosphorous and BOD7 are largely unaffected. During 1998, the
outgoing wastewater levels have remained at the same average level as during the
previous years. Naturally, this result is reasonable in light of the fact that no increase in
the incoming wastewater levels could be seen either. Table 10 presents the outgoing
levels from Haga wastewater treatment plant.
Table 10: Contaminant levels of outgoing wastewater during 1998, compared to
average values for 1995-1997.
Average 1995-1997
Average 1998
6.8 Survey
Surahammar KommunalTeknik conducted a survey in August of 1998, aimed at the
households who had disposers installed during 1997/1998. The survey was designed to
find out what the households thought about using food waste disposers, if they had
performed as intended and if the tenants were satisfied with the installation work
performed by SKT. The survey was sent out to 258 house-owners (one-family
households) and when the last day for replying had passed, 60 percent of the persons
asked had answered.
To the question of how households considered the food waste disposer functioned
practically, ninety-six percent answered “very well” or “rather well”. Every fifth
person answered yes to the question if they had had any problems with the disposer.
Typical problem areas are blockages in the pipes within the building, materials caught
in the disposer and problems due to incorrect installation, e.g. the disposer has vibrated
loose. It can be concluded that the result of the survey gave positive answer to
Surahammar KommunalTeknik, regarding both the project as a whole and the actions
of the company during the implementation period. Furthermore, it can be concluded
that there have been problems in certain buildings, but when these problems have
occurred alterations or the changes to poorly functioning piping, for example elbow
bends, have rectified the problem. For more information on the function of the
disposers, and the attitudes of the users, please contact Surahammar KommunalTeknik.
We have several times in this report described the problems with finding trends in the
data material, showing large fluctuations also during normal operation. For the
parameters nitrogen and phosphorous – for which we can already state that the load
increase should be small – it is convenient to claim that the increase is obscured by
normal variations. It is more peculiar that an increase in the amount of BOD7 cannot be
found in the incoming wastewater, since this theoretically should be 15 percent. Here
also is an example of contradicting results as the increase in the gas production
indicates an increased influx of organic matter during the period.
The increase in the ratio BOD7/N could indicate the same thing.
It is obvious that there are weak points in the data material, as well as many possible
explanations for the fact that the results and observations in some cases point in
different directions. For example, one circumstance that may give rise to inadequate
data is that water samples have been collected during a Tuesday/Wednesday 24-hour
period. It is conceivable that the households have prepared more and larger meals
during weekends – thus creating more food waste – compared to a day in the middle of
the week. In such a case, the water analyses do not reflect the real load on the treatment
plant. One fact which complicates this reasoning is that precisely this aspect was
investigated by the week-long samples collected during the summer of 1998. Nothing
was then found which indicated the described effect.
Haga wastewater treatment plant is a plant with surplus capacity. A load increase by a
few percent thus does not appreciably affect the operation or the purifying performance
of the plant. At plants with higher loads, a lasting increase in the BOD7 load by 5-10
percent could make it more difficult to meet the emissions requirements. At treatment
plants incorporating biological reduction of nitrogen and/or phosphorous, the added
waste could be positive since it means an addition of easily degraded carbon, while at
the same time the nitrogen and phosphorous contents are relatively low. However, it
must not be forgotten that emissions, due to emergency overflow, could increase when
food waste disposers are installed in the sewage system. This problem must be put in
proportion to other problems that potentially could be solved through disposers.
The project has not included any studies of the sewage system, nor issues regarding the
attitudes of the users. As we summarize the most important conclusions below, we
would also like to point to some results from investigations performed by Surahammar
KommunalTeknik. The following conclusions pertain to a follow-up period ending in
December of 1998:
Concerning the sewage system and the use of disposers
No interruptions of the service, congestion, or other problems with the sewage
system have been detected.
No overflow has occurred during 1998.
A satisfactory majority of the households state that they are pleased with waste
disposers as a alternative for food waste sorting at source.
Interruption or pipe congestion has occurred in a few buildings, but most of the
problems have been easy to remedied.
Concerning effects on the wastewater treatment plant
No interruptions of the service have occurred at the wastewater treatment plant.
An increase in the amount of screened matter was found during 1998. The increase
is estimated at 4 percent of the added theoretical food waste amount.
No increase in the amount of incoming nitrogen, phosphorous or BOD7 has been
found for certain, which may be due to many factors. However, an increase in the
ratio BOD7/N indicates that the wastewater composition has changed because of
the installation of disposers.
There is nothing to indicate that the connection of disposers affect the functioning
of the biological stage; no changes in the aeration demand have been found. If food
waste enters the treatment plant, this indicates that the bulk of the BOD7
originating from the solid waste is separated in the primary sedimentation.
The service personnel have not noticed any changes in sludge handling (extraction
of surplus sludge, pumping, thickening, etc.) due to disposers.
Since the introduction of disposers, the increase in gas production corresponds to
the theoretical biogas potential of the food waste.
The contaminant emissions from Haga wastewater treatment plant have not
increased, which therefore has not prompted any changes in the operation of the
plant (e.g. increased use of precipitation chemicals).
Fluctuations in the measured data and analyzed samples, in combination with a
moderate load increase, make it difficult to reach sure conclusions from a relatively
short assessment period. The sampling method used probably has had a large
significance on the results.
For a study of this kind to provide any further information, the follow-up period
must be considerably longer and characterize the situation during stable operating
It can however be concluded that this project with disposer has so far had no dramatic
consequences for Surahammar KommunalTeknik. The food waste disposers have not
affected the operation of Haga wastewater treatment plant other than in a positive
direction. As was discussed earlier, there are still questions remaining on how the
waste affects the various process stages. In order to learn more and reach more definite
conclusions, a longer follow-up period and experimental programs directed to specific
processes are needed (e.g. for the sewage system or treatment steps at the plant). In
view of this, the present project should be regarded as introductory study covering
wider areas of approach.
An interesting question in this context is whether Surahammar is particularly well
suited to exploit the use of disposers. It is evident that Surahammar KommunalTeknik
has been critical in its evaluation of various stretches of the sewage system within the
municipality, and has stated that certain areas cannot use disposers. But, is the
remaining sewage system then uniquely suited for the task of transporting food waste?
Haga wastewater treatment plant could be viewed in the same way, and it could be
questioned whether this is a special plant compared to other treatment plants in
Sweden. Certainly, the treatment plant does have a certain surplus capacity, a primary
sedimentation stage and a digestion tank assembly. But there is, for example, no preprecipitation, regarded by many as a prerequisite for achieving an efficient and energysaving operation when food waste is added to the sewage disposal system. The
conclusions reached are that in Sweden we still have a limiting understanding of
disposers and their effect on treatment plants.
However, what this study does show is that Haga wastewater treatment plant in
Surahammar, and also plants similar to this one, is able to receive food waste via the
wastewater without running the risk of crucial interruptions.
Whether food waste disposers from an environmental point of view are positive or
negative instruments in the waste management is a complex issue. Waste management
in general, the limitations of the sewage system, and the process design of the
wastewater treatment plant, are among many issues central to an municipality
concerned with questions of waste handling. In some municipalities issues such as the
purity of the sewage sludge and the market demand food waste counter the possibility
of marketing it, could also be decisive.
Mortensson, O., 1996. Ishøj kommune & I/S Avedøre Kloakverk.
Account of Conditions and Consequences Associated with the Use of Food
Waste Disposers. Ole Mortensson Rådgivende Ingeniørfirma ApS.
De Koning, J & van der Graaf, J.H.J.M, 1996. Kitchen Food Waste Disposers. Effects
on Sewer System and Waste Water Treatment.
Disperator AB. Sorting at Source using Food Waste Disposers.
Informative material. (No date.)
K-Konsult, 1987. Introducing Food Waste Disposers into Restaurant Kitchens.
Commissioned by Stockholm VA plant.
Lagerkvist, A. & Karlsson, N., 1980. Integrated Transport System for Household
Waste Sorted at Source – a Study of Consequences. Research report 1.
Department for refuse technology. Högskolan i Luleå (Luleå College).
McKane, Mark, 1992, The Effects of Food Waste Disposers on the Environment.
Swedish Environmental Protection Agency, 1995. What does the Household Waste
Contain? Swedish Environmental Protection Agency report 4425.
Nilsson, P., Hallin, P-O, Johansson, J., Karlén, L., Lilja, G., Petersson, B.Å &
Pettersson, J., 1990. Sorting at Source using Food Waste Disposers. A Case
Study in the Municipality of Staffanstorp. Reforsk R&D report 145.
SKT, 1992. Experimental Sorting at Source for the Apartment Building Skivlingen 2.
Project Description Draft. Surahammar KommunalTeknik.
Tofte, K.M. & Thomsen, K., 1994. Use of Food Waste Disposers in the Municipality
of Herning.
Waste Management Research Unit, Griffith university, 1994. Economic and
Environmental Impacts of Disposal of Kitchen Organic Waste using Traditional
Landfill, Food Waste Disposer, Home Composting.
Wicke, Charles A., 1987. The Effect of the Household Garbage Disposer on the
Personal Correspondence 1998
Baker, John. Disperator AB.
Leander, Jörgen. Vafab, Västerås.
Nilsson, Jenny. Mälardalens högskola (Mälardalen College).
Reports published in the VA-FORSK series since 1995
Rings on the Water – the VA Plants and Agenda 21
Contamination Transport in Sewage Disposal Systems. Calculation
Possibilities using MouseTrap
Alternative Sewage Disposal Systems in Bergsjön and Hamburgsund.
Subreport from the ECO-GUIDE Project
Evaluation of Biological Separation of Phosphorous at Öresundsverket in
Helsingborg – Process Technological and Microbiological Aspects
Internal Control at VA Plants. Workbook for Creation and Implementation of
Internal Control Programs for the Working Environment at VA Plants
Regional VA Collaboration – Potential and Principles
Increase in Drinking Water Hardness using Chalk-Carbon Dioxide, an
Alternative to Lime-Carbon Dioxide – Full Scale Trials at the Öxsjö Plant in
Wetland Clearing at Landsbro Wastewater Treatment Plant
Detergents – Effects on Treatment Plants and Environment
Evaluation of VAV Leakage Statistics
Tree Roots and Sewage Pipes. An In-depth Investigation into Root Problems
of New Sewage Pipes
Renovation of Water Lines. Guidelines for Choosing Method, Design and
New Chemicals – A Challenge for Municipal Treatment Plants. Preliminary
CD-ROM in the VA Field
Quality Assurance and Delivery Assurance for Drinking Water Distribution
Experiment Report of Biological Separation of Phosphorous at Jämshög
Treatment Plant. Municipality of Olofström
Organic Waste as Plant Nutrient Resource. Potential and Proposal for Research
and Development Efforts
Root Penetration of Sewage Lines. An Investigation of Extent and Cost for the
Municipalities in Sweden
Human Urine Sorted at Source in Closed Cycle. Three Part Preliminary Study
VA Viewed in a New Way – Service Contractors from a Few Municipalities
Drainage Basin Based Organizations as Active Planners
Judgment Basis for Irrelevant Water in Sewage Systems. Methodology Guide
Effects from Melting Snow on Sewage Systems in Urban Areas
Heavy Metal and Organic Environmentally Harmful Substance Purification of
Sewage Sludge
Effects of Chemicals in the VA Context. A Compilation of Data
Oxygen Gas in Combination with Aeration in Pilot Tests of Nitrogen
Treatment at Västerås Wastewater Treatment Plant
Exporting Swedish VA expertise
Literature Database on the Internet for Ground Water in Urban Areas
Competing in the VA Business
Evaluation of VA Solutions in Ecological Villages
Active Support for House-Owners in New Construction of VA Systems
Bioculture Dosages for a Congested Infiltration System
Biogas Plants in Sweden
VA Services in a New Form – About Competition and Structural Change in
Accessibility of Phosphorous to Plants in Different Types of Sludge, Fertilizer
and Ash
Drinking Water and Corrosion – A Guide for Water Works
Alternative Sewage Disposal Systems in Bergsjön and Hamburgsund,
Summarizing Final Report of the ECO-GUIDE Project
Analysis of Sewage Disposal Systems with Computer Models. Application
Examples for the MOUSE System
Finding Leaks using Pressure Impulse Measurements – Transient Method
Modeling of Ecological Storm Water Management
Dewatering of Sewage Sludge using Methods Close to Nature – Experiences
from a Full-Scale Test in Lövånger
The Connection Between Cost and Fees in Municipal VA Operations
Customer Oriented Quality Development in VA Operations. Report from a
Preliminary Study
Seepage and Drainage Water in Waste Water Systems
Dewatering Lagoons for Sludge from Separate Wells
Reports published in the VA-FORSK series
Pressure Impulses in Water Conduit Systems – A Few Examples
The Effect of Pressure Impulsing on Water Conduit Systems
Analysis of Reported Cost According to the DRIVA Cost Comparison for
Purification Potential of Slow Sand Filters
Contact Filtering of Surface Water – An Advancing Technology
Evaluation of the WEF CD Based Course "Operations Training – Wastewater
Treatment Course"
Nordic Conference on Nitrogen Purification and Biological Phosphorous
Toluene in Sewage Sludge – A Study of Linghed Treatment Plant
Long-Term Effects from Large-Scale Sewage Infiltration. Experiences from
Structure for VA Pipe Line Systems
Ozone Treatment, Followed by Slow Sand Filtering during the Production of
Drinking Water
Nitrification Inhibition in Swedish Municipal Wastewater – Investigations
using the Screening Method and Pure Cultures of Nitrification Bacteria
Cationic Polyacrylamides – Impact on the Microbiology of the Ground
Environmental Pipe Line Systems for Sewage Disposal Systems – A Guide
The Drinking Water Situation in Sweden
Systems Analysis VA – Hygiene Study
How Should an LCA Report be Interpreted?
VA-FORSK Reports 1992-1998
International Compilation of Experience from the Field of Ecological Storm
Water Management
Environmental Contaminants in Drinking Water
Process Model for Water Works – Application of Weasel on Seven Swedish
Water Works
Customer Surveys for the VA Field – Guide and Proposed Survey
Irrigation of Energy Forest using Biologically Treated Wastewater
Mapping the Retention of Phosphorous and Metals in Municipal Sludge
Dumps – Model Area Avan in Gävle
Development of a Biosensor for Denitrification Inhibition
VA Services and Waste Management in Local Agenda 21 in Scania –
Experiences from 20 Municipalities
Food Waste Disposers – Effects on Wastewater Treatment Plants. A Study
from Surahammar
VAV publications may be ordered from
113 87 Stockholm
Phone: +46-8-457-??-00
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