xvEPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Anaerobic Lagoons
DESCRIPTION
An anaerobic lagoon is a deep impoundment,
essentially free of dissolved oxygen, that promotes
anaerobic conditions. The process typically takes
place in deep earthen basins, and such ponds are used
as anaerobic pretreatment systems.
Anaerobic lagoons are not aerated, heated, or mixed.
The typical depth of an aerated lagoon is greater than
eight feet, with greater depths preferred. Such depths
minimize the effects of oxygen diffusion from the
surface, allowing anaerobic conditions to prevail. In
this respect, anaerobic lagoons are different from
shallower aerobic or facultative lagoons, making the
process analogous to that experienced with a single-
stage unheated anaerobic digester, except that
anaerobic lagoons are in an an open earthen basin.
Moreover, conventional digesters are typically used
for sludge stabilization in a treatment process, whereas
lagoons typically are used to pretreat raw wastewater.
Pretreatment includes separation of settlable solids,
digestion of solids, and treatment of the liquid portion.
Anaerobic lagoons are typically used for two major
purposes:
1) Pretreatment of high strength industrial wastewaters.
2) Pretreatment of municipal wastewater to allow
preliminary sedimentation of suspended solids as a
pretreatment process.
Anaerobic lagoons have been especially effective for
pretreatment of high strength organic wastewaters.
Applications include industrial wastewaters and rural
communities that have a significant organic load from
industrial sources. Biochemical oxygen demand
(BOD) removals up to 60 percent are possible. The
effluent cannot be discharged due to the high level of
anaerobic byproducts remaining. Anaerobic lagoons
are not applicable to many situations because of large
land requirements, sensitivity to environmental
conditions, and objectionable odors. Furthermore, the
anaerobic process may require long retention times,
especially in cold climates, as anaerobic bacteria are
uneffective below 15° C. As a result, anaerobic
lagoons are not widely used for municipal wastewater
treatment in northern parts of the United States.
Process
An anaerobic lagoon is a deep earthen basin with
sufficient volume to permit sedimentation of settlable
solids, to digest retained sludge, and to anaerobically
reduce some of the soluble organic substrate. Raw
wastewater enters near the bottom of the pond and
mixes with the active microbial mass in the sludge
blanket. Anaerobic conditions prevail except for a
shallow surface layer in which excess undigested
grease and scum are concentrated. Sometimes
aeration is provided at the surface to control odors.
An impervious crust that retains heat and odors will
develop if surface aeration is not provided. The
discharge is located near the side opposite of the
influent. The effluent is not suitable for discharge to
receiving waters. Anaerobic lagoons are followed by
aerobic or facultative lagoons to provide required
treatment.
The anaerobic lagoon is usually preceded by a bar
screen and can have a Parshall flume with a flow
recorder to determine the inflow to the lagoon. A
cover can be provided to trap and collect the methane
gas produced in the process for use elsewhere, but this
is not a common practice.
Microbiology
Anaerobic microorganisms in the absence of dissolved
oxygen convert organic materials into stable products
such as carbon dioxide and methane. The degradation
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process involves two separate but interrelated phases:
acid formation and methane production.
During the acid phase, bacteria convert complex
organic compounds (carbohydrates, fats, and proteins)
to simple organic compounds, mainly short-chain
volatile organic acids (acetic, propionic, and lactic
acids). The anaerobic bacteria involved in this phase
are called "acid formers," and are classified as
nonmethanogenic microorganisms. During this phase,
little chemical oxygen demand (COD) or biological
oxygen demand (BOD) reduction occurs, because the
short-chain fatty acids, alcohols, etc., can be used by
many microorganisms, and thereby exert an oxygen
demand.
The methane-production phase involves an
intermediate step. First, bacteria convert the short-
chain organic acids to acetate, hydrogen gas, and
carbon dioxide. This intermediate process is referred
to as acetogenesis. Subsequently, several species of
strictly anaerobic bacteria (methanogenic
microorganisms) called "methane formers" convert the
acetate, hydrogen, and carbon dioxide into methane
gas (CFLJ through one of two major pathways. This
process is referred to as methanogenesis. During this
phase, waste stabilization occurs, represented by the
formation of methane gas. The two major pathways of
methane formation are:
1) The breakdown of acetic acid to form methane and
carbon dioxide:
CH3COOH
2) The reduction of carbon dioxide by hydrogen gas
to form methane:
CO2 + 4H2
Equilibrium
When the system is working properly, the two phases
of degradation occur simultaneously in dynamic
equilibrium. That is, the volatile organic acids are
converted to methane at the same rate that they are
formed from the more complex organic molecules.
The growth rate and metabolism of the methanogenic
bacteria can be adversely affected by small fluctuations
in pH substrate concentrations, and temperature, but
the performance of acid-forming bacteria is more
tolerant over a wide range of conditions. When the
process is stressed by shock loads or temperature
fluctuations, methane bacteria activity occurs more
slowly than the acid formers and an imbalance occurs.
Intermediate volatile organic acids accumulate and the
pH drops. As a result, the methanogens are further
inhibited and the process eventually fails without
corrective action. For this reason, the methane-
formation phase is the rate-limiting step and must not
be inhibited. For the design of an anaerobic lagoon to
work, it must be based on the limiting characteristics of
these microorganisms.
Establishing and maintaining equilibrium
The system must operate at conditions favorable for the
performance of methanogenic bacteria. Ideally,
temperatures should be maintained within the range of
25 to 40 C. Anaerobic activity decreases rapidly at
temperatures below 15 C , when water temperature
drops below freezing, and biological activity virtually
ceases. The pH value should range from 6.6 to 7.6,
but should not drop below 6.2 because methane
bacteria cannot function below this level. Sudden
fluctuations of pH will inhibit lagoon performance.
Alkalinity should range from 1,000 to 5,000 mg/L.
Volatile acid concentration is an indicator of process
performance because the acids are converted to
methane at the same rate that they are formed if
equilibrium is maintained. Volatile acid concentrations
will be low if the lagoon system is working properly.
As a general rule, volatile acid concentrations should
be less than 250 mg/L. Inhibition occurs at volatile
acid concentrations in excess of 2,000 mg/L. Table 1
presents optimum and extreme operating ranges for
methane formation. The rate of methane formation
drops dramatically outside these extreme ranges. In
addition to adhering to the above guidelines, sufficient
nutrients such as nitrogen and phosphorus must be
available. Concentrations of inhibitory substances,
including ammonia and hydrogen when a high
concentration of sulfate ions are present, and
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TABLE 1 IDEAL OPERATING RANGES
FOR METHANE FERMENTATION
Parameter
Temp. ( C)
PH
Alkalinity (mg/L as
Optimum
30-35
6.6-7.6
2,000-3,000
Extreme
25-40
6.2-8.0
1,000-5,000
CaCO3)
Volatile acids (mg/L
as acetic acid)
50-500
2,000
Source: Andrews and Graef, 1970.
concentrations such as calcium, should be kept to a
minimum. Excessive concentrations of these inhibitors
produce toxic effects. Depending on its form, ammonia
can be toxic to the bacteria as well as affect its
concentration. Concentration of free ammonia in
excess of 1,540 mg/L will result in severe toxicity, but
concentrations of ammonium ion must be greater than
3,000 mg/L to produce the same effect. Maintaining a
pH of 7.2 or below will ensure that most ammonia will
be in the form of ammonium ion, so higher
concentrations can be tolerated with little effect. Table
2 provides guidelines for acceptable ranges of other
inhibitory substances.
TABLE 2 CONCENTRATIONS OF
INHIBITORY SUBSTANCES
Substance
Sodium
Potassium
Calcium
Magnesium
Sulfides
Moderately
Inhibitory (mg/L)
3,500-5,500
2,500-4,500
2,500-4,500
1,000-1,500
100-200
Strongly
Inhibitory mg/L)
8,000
12,000
8,000
3,000
>200
Source: WEF and ASCE (1992), reprinted from Parkin, G.F.
and Owen, W.F. (1986).
APPLICABILITY
Type of wastewater
Anaerobic lagoons are used for treatment of industrial
wastewaters, mixtures of industrial/domestic
wastewaters with high organic loading, and as a first
stage in municipal lagoons. Typical industries include
slaughterhouses, dairies, meat/poultry-processing
plants, rendering plants, and vegetable processing
facilities.
Typically, anaerobic lagoons are used in series with
aerobic or facultative lagoons, enhancing the operation
of both types of lagoons as aerobic or facultative
lagoons providing further treatment of the effluent.
Initial treatment in an anaerobic lagoon often renders
the waste more amenable to further treatment and
reduces the oxygen demand.
Anaerobic lagoons often are used in small or rural
communities where space is plentiful but costs are a
concern. Low construction and operating costs make
anaerobic lagoons a financially attractive alternative to
other treatment systems, although sludge must
occasionally be removed.
ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of anaerobic
lagoons are listed below:
Advantages
More effective for rapid stabilization of strong
organic wastes, making higher influent organic
loading possible.
Produce methane, which can be used to heat
buildings, run engines, or generate electricity,
but methane collection increases operational
problems.
Produce less biomass per unit of organic
material processed. Less biomass produced
equates to savings in sludge handling and
disposal costs.
Do not require additional energy, because they
are not aerated, heated, or mixed.
Less expensive to construct and operate.
Ponds can be operated in series.
Disadvantages
Require a relatively large area of land.
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Produce undesirable odors unless provisions
are made to oxidize the escaping gases. Gas
production must be minimized (sulfate
concentration must be reduced to less than
100 mg/L) or mechanical aeration at the
surface of the pond to oxidize the escaping
gases is necessary. Aerators must be located
to ensure that anaerobic activity is not inhibited
by introducing dissolved oxygen to depths
below the top 0.6 to 0.9 m (2 to 3 feet) of the
anaerobic lagoon. Another option is to locate
the lagoon in a remote area.
Require a relatively long detention time for
organic stabilization due to the slow growth
rate of the methane formers and sludge
digestion.
Wastewater seepage into the groundwater may
be a problem. Providing a liner for the lagoon
can prevent this problem.
Environmental conditions directly impact
operations so any variance limits the ability to
control the process, (e.g. lagoons are sensitive
to temperature fluctuations).
DESIGN CRITERIA
The design of aerobic lagoons is not well defined and
a widely accepted overall design equation does not
exist. Numerous methods have been proposed, but the
results vary widely. Design is often based on organic
loading rates and hydraulic detention times derived
from pilot plant studies and observations of existing
operating systems. States in which lagoons are
commonly used often have regulations governing their
design, installation, and management. For example,
state regulations may require specific organic loading
rates, detention times, embankment slopes (1:3 to 1:4),
and maximum allowable seepage (1 to 6 mm/d).
Optimum performance is based on many factors,
including temperature and pH. Other important factors
to consider include:
Organic loading rate
Typical acceptable loading rates range between 0.04
and 0.30 kg/m3/d (2.5 to 18.7 Ib BOD5/103 ft3/d),
varying with water temperature.
Detention time
Typical detention times range from 1 to 50 days,
depending on the temperature of the wastewater.
Lagoon dimensions
Because anaerobic lagoons require less surface area
than facultative lagoons since the oxygen transfer rate
is not a factor, their design should minimize the surface
area-to-volume ratio. Typical surface areas range from
0.2 to 0.8 hectares (0.5 to 2 acres). The lagoon
should be as deep as practicable, as greater depth
provides improved heat retention. A depth of 2.4 to
6.0 m (8 to 20 feet) can be used; however, depths
approaching 6.0 m (20 feet) are recommended to
reduce the surface area and to conserve heat in the
reactor (lagoon). The lagoon should be designed to
reduce short circuiting and should incorporate a
minimum freeboard of 0.9 m (3 feet).
Construction of lagoon bottom
Groundwater seepage may be a concern. The lagoon
should be lined with an impermeable material such as
plastic, rubber, clay, or cement.
Control of surface runoff
Lagoons should not receive significant amounts of
surface runoff. If necessary, provision should be made
to divert surface water around the lagoon. Table 3
summarizes general design criteria for anaerobic
lagoons.
PERFORMANCE
System performance depends on loading conditions,
temperature conditions, and whether the pH is
maintained within the optimum range. Table 4 shows
expected removal efficiencies for municipal
wastewaters. In cold climates, detention times as great
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TABLE 3 DESIGN CRITERIA
Criteria
Range
Optimum water
temperature
(C):
PH
Organic loading:
Detention Time:
Surface Area:
Depth
30 - 35 degrees (Essentially
unattainable in municipal
systems)
6.6 to 7.6
0.04-0.30 kg/m3/d (2.5-18.7
lbs/103 ftVd ((temperature
dependent)
1 to 50 days (temperature
dependent)
0.2 to 0.8 hectares (0.5 to 2 acres)
2.4 to 6.0 meters (8 to 20 feet)
(depths approaching 6.0 meters
[20 feet] preferred)
Source: Metcalf & Eddy, Inc., 1991.
as 50 days and volumetric loading rates as low as 0.04
kg BOD5/m3/d (2.5 lbs/103 ft3/d) may be required to
achieve 50 percent reduction in BOD5. Table 4 shows
the relationship of temperature, detention time, and
BOD reduction. Effluent TSS will range between 80
and 160 mg/L.
The effluent is not suitable for direct discharge to
receiving waters. Lagoon contents that are black
indicate that the lagoon is functioning properly.
OPERATION AND MAINTENANCE
Operation and maintenance requirements of a lagoon
are minimal. A daily grab sample of influent and
effluent should be taken and analyzed to ensure proper
TABLE 4 FIVE-DAY BOD REDUCTION
AS A FUNCTION OF DETENTION TIME
AND TEMPERATURE
Temperature
(deg. C)
10
10-15
15-20
20-25
25-30
Detention
time (d)
5
4-5
2-3
1-2
1-2
BOD
reduction (%)
50
30-40
40-50
40-60
60-80
Source: World Health Organization, 1987.
operation. Aside from sampling, analysis, and general
upkeep, the system is virtually maintenance-free.
Solids accumulate in the lagoon bottom and require
removal on an infrequent basis (5-10 years), depending
on the amount of inert material in the influent and the
temperature. Sludge depth should be determined
annually. Table 1 depicts optimum and extreme
operating ranges for methane formation. Rates outside
of these extreme ranges will decrease the rate of
methane formation.
COSTS
The primary cost associated with constructing an
anaerobic lagoon is the cost of the land, earthwork
appurtenances, required service facilities, and the
excavation. Costs for forming the embankment,
compacting, lining, service road and fencing, and piping
and pumps also need to be considered. Operating
costs and power requirements are minimal.
REFERENCES
Other Related Fact Sheets
Other EPA Fact Sheets can be found at the following
web address:
http ://www. epa. gov/owm/mtb/mtbfact.htm
1. Andrews, J. F., and S.P. Graef, 1970.
Dynamic Modeling and Simulation of the
Anaerobic Digestion Process. Advances in
Chemistry, Vol. 105, 126-162. American
Chemical Society, Washington D.C.
2. Eckenfelder, W. W., 1989. Industrial Water
Pollution Control, 2nd ed., McGraw-Hill,
Inc., New York.
3. Joint Task Force of the Water Environment
Federation and the American Society of Civil
Engineers, 1992. Design of Municipal
Wastewater Treatment Plants Volume II:
Chapters 13-20. WEF Manual of Practice
No. 8. ASCE Manual and Report on
Engineering Practice No. 76. Alexandria,
Virginia.
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4. Liu, Liptak, and Bouis, 1997. Environmental
Engineers' Handbook, 2nd ed., Lewis
Publishers, New York.
5. Metcalf& Eddy, Inc., 1991. Wastewater
Engineering: Treatment, Disposal, Reuse,
3rd ed., McGraw-Hill, New York.
6. Parkin, G.F. and Owen, W.F., 1986.
Fundamentals of Anaerobic Digestion of
Wastewater Sludge. Journal of
Environmental Engineering, 112 (5).
7. Peavy, Rowe and Tchobanoglous, 1985.
Environmental Engineering. McGraw-Hill,
Inc., New York.
8. U.S. EPA, 1980. Innovative and Alternate
Technology Assessment Manual. EPA/430/9-
78-009. Cincinnati, Ohio.
9. U.S. EPA, 1979. Municipal Environmental
Research Laboratory Office of Research and
Development. Process Design Manual for
Sludge Treatment and Disposal.
EPA625/1-79-011. Cincinnati, Ohio.
10. World Health Organization, 1987.
Wastewater Stabilization Ponds, Principles
of Planning and Practice, WHO Technical
Publication 10, Regional Office for the Eastern
Mediterranean, Alexandria, Virginia.
ADDITIONAL INFORMATION
Richard H. Bowman, P.E.
West Slope Supervisor
Colorado Dept of Public Health and Environment
Water Quality Control Division
222 South 6th Street, Room 232
Grand Junction, CO 81502
E. Joe Middlebrooks, Ph.D., P.E., DEE
Environmental Engineering Consultant
360 Blackhawk Lane
Lafayette, CO 80026-9392
Gordon F. Pearson
Vice President
International Ecological Systems & Services, IESS
P.O. Box 21240
B-l Oak Park Plaza
Hilton Head, SC 29925
Sherwood Reed, Principal
Environmental Engineering Consultants (EEC)
50 Butternut Road
Norwich, VT 05055
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection Agency.
Office of Water
EPA 832-F-02-009
September 2002
For more information contact:
Municipal Technology Branch
US EPA
1200 Pennsylvania Ave, NW
Mail Code 4204M
Washington, D.C. 20460
* 2002 if
THE YEAR OF
CUBAN WATER
Mohamed Dahab, Ph.D, P.E., DEE
University of Nebraska-Lincoln
W348 Nebraska Hall
Lincoln, NE 6588-0531
1MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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