Technews
National Dairy Development Board
For Efficient Dairy Plant Operation
July-August 2003
No.45
SOME ASPECTS OF DAIRY PLANT
EFFLUENT TREATMENT
This bulletin includes technical and latest development on products,
systems, techniques etc. reported in journals, companies’ leaflets and
books and based on studies and experience. The technical
information on different issues is on different areas of plant
operation. It is hoped that the information contained herein will be
useful to readers.
The theme of information in this issue is Some Aspects of Dairy
Plant Effluent Treatment. It may be understood that the
information given here is by no means complete.
In this issue:
•
•
•
•
•
Introduction
Pretreatment
Primary Treatment
Secondary Treatment
Reducing Pollution Load
1. INTRODUCTION
Effluents generated from dairy processing facilities can present
difficult treatment problems because they contain large amount of
carbohydrates, protein, fat and mineral salts. The effluent can
produce distinct odours and heavy pollution of water if the
discharge is not properly treated. Organic matter of these wastes
must be treated by biological stabilization process before it is
discharged into a body of water. Improper waste disposal is a
hazard to humans and to aquatic life.
Consider the following:
∗ The organic matter in the effluent provides a food source for
rapid microbial growth, with a result in reducing the dissolved
oxygen contained in the water. Water normally contains
approximately 8 parts per million (ppm) of dissolved oxygen. A
minimum standard for fish life is 5 ppm of dissolved oxygen,
below which fish can suffocate.
∗ Further, such a system would be unsuitable as a source of
potable water.
∗ Milk proteins, solution of detergents and some sanitizers
contribute to the phosphorous and nitrogen load of the effluent.
Both elements promote unwanted growth of algae in lakes and
slow-running waters.
∗ Surfactant present in the effluent tend to form a foam on water
surfaces, thus impeding the uptake of oxygen into water. As a
result of low oxygen concentration in water, fish may die.
Therefore, surfactants must be biodegradable.
Therefore, there is increasing demand by regulatory authorities and
the public for better effluent treatment by industry. Processors and
regulatory authorities are responsible for the disposal of waste
materials promptly and completely. Accumulation of effluent even
for short periods of time, can attract insect and rodents, produce
odours, and become a public nuisance.
Several factors, such as quantity, pollutant strength and nature of
constituents of effluent have both economic and environmental
consequences concerning treatability and disposal. Significant
characteristics that determine the cost for effluent treatment are the
relative strength of the effluent and the daily volume of discharge.
Table 1 provides general characteristics of dairy plant effluent(1).
Table 1: Usual characteristics of untreated dairy effluent
Index
Average value
a
3
1-2
Specific quantity of effluent, (m /tonne )
a
0.8-2.5
Specific BOD5, (kg/tonne )
BOD5, (mg/l)
500-2000
COD / BOD5
1.3-2.2
Phosphorous total, (mg/l)
10-100
Suspended solids total (SS), (mg/l)
100-1000
pH
2-12
Temperature, (oC)
25-35
a. tonnes processed milk
Spent cleaning compounds and sanitizers are discharged into waste
treatment facilities. The toxicity of sanitizers, if present in high
concentration, can inhibit biodegradable process.
The major concerns for treatment of this effluent are pH
fluctuations and possible long term exposure to trace heavy metals.
However, these effects can be controlled and waste minimized
through appropriate plant design and optimal concentration use of
cleaning compounds and sanitizers.
An optimum waste management programme would include waste
prevention techniques and utilization of waste products,
pretreatment, primary treatment, secondary treatment, tertiary
treatment, if necessary, and disposal of treated effluent.
This issue of Technews covers only some important aspects of
dairy plant effluent and its treatment and does not detail the
treatment processes and plants.
2. PRETREATMENT
Pretreatment may include coarse and fine screening, hydraulic and
load balancing (flow equalization), skimming and pH control.
Screening: Screening is designed to remove suspended particles from
the effluent, in order to protect the remainder of the treatment plant
from damage by gross solids and to protect subsequent treatment
stages from solids overload. Screening might be done in two stages:
coarse screening, to remove solids of 20 mm size and above; and fine
screening, to remove solids of size 0.25 mm and above.
Coarse screens could be either static or mechanically raked. In either
case it is important that the velocity of the flow through the chamber
is between 0.3 m per second to 0.8 m per second. Fine screens,
installed after coarse ones, can be static, brushed or rotating drum
screens (not used where there are high levels of fat). Provision should
be made for the high-pressure or steam-cleaning of fine screens.
Flow Equalization: Biological treatment processes operate best under
constant and consistent organic load. It is therefore essential that
adequate provision is made for balancing both pollution load and
flows. This is done in an equalization tank. This unit is characterized
by a varying flow into and a constant flow from the tank.
Equalization tanks can be lagoons, steel construction tanks, or
concrete tanks, often without a cover. Consideration should be given
to mixing and aerating the contents where the potential for
biodegradation of the waste exists.
Skimming: This process is frequently incorporated if large floatable
solids are present. These solids are collected and transferred into
some disposal unit. Separation of solids is frequently increased by the
addition of lime and alum, ferric chloride (FeCL3), or a selected
polymer. Paddle flocculation may follow the addition of flocculant.
pH Control: Control of pH is necessary to ensure that the effluent
does not damage the structure, equipment or pipe work. pH values
above 11 and below 5 may cause damage in some types of biological
effluent treatment plants. Effluent with pH below 6 may attack
concrete used in the system. Further, most biological processes
operate best within the pH range 6.5 – 8.5 and with constant pH even
outside this range. It is usually recommended to neutralize the
effluent to pH values within the limits of 6-9 prior to treatment.
The pH is controlled by dosing hydrochloric acid (HCl), sulphuric
acid (H2SO4), nitric acid (HNO3) or carbon dioxide. It should be
remembered that a sulphate concentration in excess of 1000 mg/l will
attack concrete unless it is sulphate resistant(2). Hence care must be
taken during neutralization, especially if H2SO4 is used. For CO2, flue
gases can be used, or a separate CO2 supply can be arranged. Using
CO2 in place of strong mineral acids has several benefits(3): it is cost
saving, it is safer and easier to store, it allows precise control of pH, it
is non-corrosive.
Nutrient Balancing: Biological treatment processes can be inhibited
if the balance of available nutrients is insufficient for the microbes to
break down the organic matter in an efficient manner. Dairy effluent
may have an excess of phosphorous and a deficiency of nitrogen or
potassium. Generally, the ratio between biochemical oxygen demand
(BOD), nitrogen and phosphorous should be 100:5:1 to facilitate
microbial breakdown(1).
Nutrient deficiency can be overcome by adding urea (or other source
of nitrogen) and phosphoric acid. It is important that the available
nitrogen and phosphorous are measured at the entry to the biological
treatment and not prior to other physical/chemical processes.
3. PRIMARY TREATMENT
The main purpose of primary treatment is to remove particles from
the effluent. Sedimentation and flotation techniques are commonly
used.
Sedimentation: Grit particles like sand, gravels, clay etc.
generally enter the waste stream from the truck– and tank–
washing area or through the corrosion of concrete and paved
surfaces. If allowed to pass through the process, it can cause
serious damage to pumps and other equipment and, with sludge,
can cause pipe clogging.
Flotation: In this process, fat, oil, grease (FOG) and other
suspended matter are removed from effluent. This process is
effective in removing FOG from effluent.
Many small plants still prefer static grease traps designed on the
basis of flow. A retention time of about 30 minutes or more is
provided and the accumulated FOG is removed manually.
Biological dosing system can be used with good effect in
degrading FOG, but may be inactivated on inhibited by cleaning
chemicals. There are however Grease Digesters which combine
the attributes of grease traps and dosing systems. The combined
system separates FOG accumulation and biodegradation. The
FOG – degrading bacteria are self-regenerating and are not
inactivated by cleaning chemicals(4). The main problem in this
system is the possibility of accumulated FOG being subjected to
higher temperature and becoming emulsified. Recently, dissolved
air flotation (DAF), which removes suspended matter by using
small air bubbles, has become more popular (Fig 1).
Fig 1: Dissolved air flotation (DAF)
Typical design parameters for a DAF unit are(1):
∗ Upward flow velocity: upto 7.2 m per h
∗ volumetric retention time at maximum inflow: 20-30 minutes
∗ recycle rate: 20-35% of inflow
∗ air/solid ratio: 0.005-0.05 kg air per kg solid to be removed.
Flocculating agents are commonly used to pretreat effluent prior to
treatment by a DAF unit. This system can remove more than 50%
of the chemical oxygen demand (COD) load. The technique
requires high investment and operating costs.
DAF systems maintain a concentration of bacteria that are kept
alive within the system to biodegrade pollutants in the effluent.
The high costs of the system can be offset by selling the collected
FOG to renderers for secondary applications, and reusing in some
plant operations the treated effluent after it has been run through
filtration and disinfecting processes.
4. SECONDARY TREATMENT
The primary purpose of secondary treatment is to continue the
removal of organic matter and to produce an effluent low in BOD
and suspended solids (SS). This may include biological treatment
to degrade the dissolved organic matter and chemical treatment to
remove phosphorous and nitrogen or to aid in the flocculation of
solids.
Biological treatment processes can be aerobic (activated sludge
and trickling filters or biological filtration) or anaerobic.
Activated Sludge: This process is widely used.
principle is shown in Fig 2.
Its operating
Oxygen
Input
Aeration
stage
Sludge
separation
Treated
effluent
preaeration
stage
(optional)
Sludge recycle
Fig 2 : Basic activated sludge process
Primary treatment is optional. It is very effective for the removal
of all organic matters in the effluent. Several configurations of
aeration tank have become popular in the treatment of dairy
effluent. Some modified activated sludge plants have high BOD
removal efficiency (95 to 98%). The sequencing batch reactor
(SBR) is a relatively recent variation of the activated sludge
system, that accommodates the entire process in a single tank but
at different times, with steps including filling, aerating, settling,
drawing off floating scum and removing treated effluent(5).
Biological Filtration: Biological or trickling filters reduce BOD
and SS by bacterial action and biological oxidation as effluent
passes in a thin layer over stationary media (usually rocks)
arranged above an over-drain. Primary treatment should precede
this process if the effluent suspended solids concentration exceeds
100 mg/l(6).
A very common form of biofilter used in the treatment of dairy
effluent is the high-rate biofilter. The media, usually in the form of
opern-textured plastic, can be either random pack or modular.
High-rate biofilters are normally loaded above 0.6 kg BOD / cubic
m and generally remove 50-70% of the applied BOD(1).
The effluent is distributed over the media surface at a minimum
flow rate of 1.5 cubic m per sq. m plan area per hour. This ensures
that no clogging of the media occurs and discourages insect life.
The important parameters in the operation of a biofilter are(1):
• loading rate – is it essential to maintain the loading rate to
ensure that the filter media do not get clogged.
• fats and grease – the presence of FOG in concentration above
50 mg/l can result in the coating of the biological film; this can
lead to uncontrolled anaerobic activity and significant odours in
extreme cases.
• BOD applied – excessive loading rates (shock loads) can result
in clogging of the media; prolonged BOD loading can give rise
to odour problems.
• pH – adequate control of pH is required to maintain the
efficiency of the biofilter and to prevent damage to the media
and support structure.
• temperature – reduced efficiency will occur if the temperature
within the biofilter drops below 80C.
The outflow from high-rate biological filters is usually not of
sufficiently high quality and needs further treatment, such as in
activated sludge system.
5. REDUCING POLLUTION LOAD
Effluent treatment and disposal are both costly. Moreover,
products, chemicals and other materials contributing to effluent
pollution are also expensive. Hence, dairy plants should aim to
minimize the pollution load in the raw effluent. Some measures to
reduce products loss were suggested in Technews Issues 40 and 41
(Sept-Oct and Nov-Dec 2002). Additionally, following steps may
help in reducing pollution of effluent.
Equipment
•
Ensure that flow and transportation of raw materials and
products is by as short a path as possible, in order to minimize
the adherence of residues.
• All plants should be easy to clean. Avoid dead spaces, which
require additional cleaning and disinfecting.
• Ensure, by automatic control of the flow paths, that there is no
cross-contamination between the products and the cleaning and
disinfection chemicals.
.1 F
OxamylEnsure that raw materials, products, additives and
Production
•
•
•
•
Ensure that raw materials, products, additives and auxiliary
chemicals are not lost by splashing, leaking valves or pipe
connections, or overflowing containers.
Optimize operations with the aim of minimizing the residues
deposited on product-contacting surfaces, e.g. preheating in the
production of UHT and concentrated milk reduces the deposits
on heat-exchange surfaces.
Try to utilize whey, e.g. by producing dried whey powder for
suitable uses.
Never discharge centrifugal sludge into the effluent.
Hygienic operations
•
Remove product residues from product-contacting surfaces, e.g.
by blowing out with filtered compressed air or rinsing with a
small volume of water or, for cream residues, rinsing with
warmed skim milk, which makes it easier to regain and use the
cream that is rinsed off.
•
•
•
•
•
Avoid unnecessary dilution of chemical solutions with rinsing
water, which increases the consumption of detergents or
sanitizers.
Use mixed detergents instead of pure chemicals. Suitable
additives can markedly improve the efficiency of alkalis and
acids in many cases. As a result, the chemical pollution of the
effluent by solutions of no further use is decreased.
Regenerate used cleaning solutions by sedimentation,
centrifugation or membrane filtration techniques in order to
extend their useful life whenever practical. The sludge may be
disposed of or utilized for biogas production.
Disinfect closed systems by heating instead of applying
chemicals, if it is safe and the consumption of energy seems
acceptable.
Avoid using chemicals that are dangerous to aquatic systems or
may disturb effluent treatment. If in doubt, consult the producer
or the distributor of the detergent/sanitizer.
Conditioning of water and effluent
•
•
•
Reduce the water hardness only as far as necessary for the
intended use. Water-softening should be done by ion exchange
or reverse osmosis. Other physical methods cannot be
recommended since, to date, too little is known about their
effectiveness.
Use as much condensate from evaporators as possible in order
to reduce water hardness, but do not overlook possible
microbiological risks.
Neutralize the surplus alkalis from cleaning agents by using
carbon dioxide or boiler-flue gas but not mineral acids. This
reduces inorganic pollution of the wastewater. Further
advantages of utilizing boiler-flue gas are less sulphuric dioxide
exhaust and low running costs. However, special installations
are required.
With efficient pollution reduction measures, the aim should be to
reduce the BOD of the raw effluent to less than 200 mg/l of milk.
References:
1.
Byrne, R.J. (2003). Design and operation of dairy effluent
treatment plants, in Encyclopedia of Dairy Science, Vol. 2, Eds.
H. Roginski, J.W. Fuquay and P.F. Fox, Academic Press,
London, 733-743
2.
Anonymous (1980). Guide for Dairy Mangers on Wastage
Prevention in Dairy Plants, IDF Bulletin No.124, 5-6
3.
Anonymous (1998). Gas ends acid reign, Dairy Industries
International, December, 39
4.
Burton, C. (1997). FOG clearance, Dairy Industries
International, December, 41
5.
Gregerson, J. (2001). Waste not. Food Engineering, May, 46-50
6.
Marriott, N.G. (1999). Principles of Food Sanitation, Aspen
Publishers, Maryland, 187-206
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National Dairy Development Board
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