xvEPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Facultative Lagoons
DESCRIPTION
Facultative waste stabilization ponds, sometimes
referred to as lagoons or ponds, are frequently used
to treat municipal and industrial wastewater in the
United States. The technology associated with
facultative lagoons has been in widespread use in the
United States for at least 90 years, with more than
7,000 facultative lagoons in operation today. These
earthen lagoons are usually 1.2 to 2.4 m (4 to 8 feet)
in depth and are not mechanically mixed or aerated.
The layer of water near the surface contains dissolved
oxygen due to atmospheric reaeration and algal
respiration, a condition that supports aerobic and
facultative organisms. The bottom layer of the lagoon
includes sludge deposits and supports anaerobic
organisms. The intermediate anoxic layer, termed the
facultative zone, ranges from aerobic near the top to
anaerobic at the bottom. These layers may persist for
long periods due to temperature-induced water-
density variations. Inversions can occur in the spring
and fall when the surface water layer may have a
higher density than lower layers due to temperature
fluctuations. This higher density water sinks during
these unstable periods, creates turbidity, and
produces objectionable odors.
The presence of algae in the aerobic and facultative
zones is essential to the successful performance of
facultative ponds. In sunlight, the algal cells utilize
CO2 from the water and release Q2 produced from
photosynthesis. On warm, sunny days, the oxygen
concentration in the surface water can exceed
saturation levels. Conversely, oxygen levels are
decreased at night. In addition, the pH of the near
surface water can exceed 10 due to the intense use of
CO2 by algae, creating conditions favorable for
ammonia removal via volatilization. This
photosynthetic activity occurs on a diurnal basis,
causing both oxygen and pH levels to shift from a
maximum in daylight hours to a minimum at night.
The oxygen, produced by algae and surface
reaeration, is used by aerobic and facultative bacteria
to stabilize organic material in the upper layer of
water. Anaerobic fermentation is the dominant
activity in the bottom layer in the lagoon. In cold
climates, oxygenation and fermentation reaction rates
are significantly reduced during the winter and early
spring and effluent quality may be reduced to the
equivalent of primary effluent when an ice cover
persists on the water surface. As a result, many states
in the northern United States and Canada prohibit
discharge from facultative lagoons during the winter.
Although the facultative lagoon concept is land
intensive, especially in northern climates, it offers a
reliable and easy-to-operate process that is attractive
to small, rural communities.
Common Modifications
A common operational modification to facultative
lagoons is the "controlled discharge" mode, where
pond discharge is prohibited during the winter months
in cold climates and/or during peak algal growth
periods in the summer. In this approach, each cell in
the system is isolated, then discharged sequentially.
A similar modification, the "hydrograph controlled
release" (HCR), retains liquid in the pond until flow
volume and conditions in the receiving stream are
adequate for discharge.
A recently developed physical modification uses
plastic curtains, supported by floats and anchored to
the bottom, to divide lagoons into multiple cells
and/or to serve as baffles to improve hydraulic
conditions. Another recent development uses a
floating plastic grid to support the growth of
duckweed (Lemna sp.) plants on the surface of the
final cell(s) in the lagoon system, which restricts the
penetration of light and thus reduces algae (with
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sufficient detention time > 20 days), improving the
final effluent quality.
APPLICABILITY
The concept is well suited for rural communities and
industries where land costs are not a limiting factor.
Facultative lagoons can be used to treat raw,
screened, or primary settled municipal wastewater
and biodegradable industrial wastewaters.
ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of facultative
lagoons are listed below:
Advantages
Moderately effective in removing settleable solids,
BOD, pathogens, fecal coliform, and ammonia.
Easy to operate.
Require little energy, with systems designed to
operate with gravity flow.
The quantity of removed material will be relatively
small compared to other secondary treatment
processes.
Disadvantages
Settled sludges and inert material require periodic
removal.
Difficult to control or predict ammonia levels in
effluent.
Sludge accumulation will be higher in cold climates
due to reduced microbial activity.
Mosquitos and similar insect vectors can be a
problem if emergent vegetation is not controlled.
Requires relatively large areas of land.
Strong odors occur when the aerobic blanket
disappears and during spring and fall lagoon
turnovers.
Burrowing animals may be a problem.
DESIGN CRITERIA
Waste stabilization pond systems are simplistic in
appearance, however, the reactions are as
complicated as any other treatment process. Typical
equipment used in facultative lagoons includes lining
systems to control seepage to groundwater (if
needed), inlet and outlet structures, hydraulic
controls, floating dividers, and baffles. Many existing
facultative lagoons are large, single-cell systems with
the inlet constructed near the center of the cell. This
configuration can result in short-circuiting and
ineffective use of the design volume of the system. A
multiple-cell system with at least three cells in series
is recommended, with appropriate inlet and outlet
structures to maximize effectiveness of the design
volume. Most states have design criteria that specify
the areal organic loading (kg/ha/d or Ibs/acre/d)
and/or the hydraulic residence time. Typical organic
loading values range from 15 to 80 kg/ha/d (13 to 71
Ibs/acre/d). Typical detention times range from 20 to
180 days depending on the location. Detention times
can approach 200 days in northern climates where
discharge restrictions prevail. Effluent biochemical
oxygen demand (BOD) < 30 mg/L can usually be
achieved, while effluent TSS may range from < 30
mg/L to more than 100 mg/L, depending on the algal
concentrations and design of discharge structures.
A number of empirical and rational models exist for
the design of simple and series constructed facultative
lagoons. These include first-order plug flow, first-
order complete mix, and models proposed by
Gloyna, Marais, Oswald, and Thirumurthi. None of
these has been shown to be clearly superior to the
others. All provide a reasonable design as long as the
basis for the formula is understood, proper
parameters are selected, and the hydraulic detention
and sludge retention characteristics of the system are
known. This last element is critical because short
circuiting in a poorly designed cell can result in
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detention time of 40 percent or less than the
theoretical design value.
PERFORMANCE
Overall, facultative lagoon systems are simple to
operate, but only partially reliable in performance.
BOD5 removal can range up to 95 percent.
However, the TSS range may exceed 150 mg/L.
Removal of ammonia nitrogen can be significant (up
to 80 percent), depending on temperature, pH, and
detention time in the system. However, the removal
cannot be sustained over the winter season. Due to
precipitation reactions occurring simultaneously with
the daily high pH (alkaline) conditions in the lagoon,
approximately 50 percent phosphorus removal can
be expected. Removal of pathogens and coliforms
can be effective, depending on temperature and
detention time.
Limitations
Limitations may include the inability of the process to
meet a 30 mg/L limit for TSS due to the presence of
algae in the effluent, particularly during warm
weather, and not meeting effluent criteria consistently
throughout the year. In cold climates, low
temperatures and ice formation will limit process
efficiency during the winter. Odors may be a
problem in the spring and fall during periods of
excessive algal blooms and unfavorable weather
conditions.
OPERATION AND MAINTENANCE
Most facultative lagoons are designed to operate by
gravity flow. The system is not maintenance intensive
and power costs are minimal because pumps and
other electrically operated devices may not be
required. Although some analytical work is essential
to ensure proper operation, an extensive sampling
and monitoring program is usually not necessary. In
addition, earthen structures used as impoundments
must be inspected for rodent damage.
COSTS
Cost information for facultative lagoons varies
significantly. Construction costs include cost of the
land, excavation, grading, berm construction, and
inlet and outlet structures. If the soil is permeable, an
additional cost for lining the lagoon should be
considered.
REFERENCES
Other Related Fact Sheets
Other EPA Fact Sheets can be found at the following
web address:
http ://ww. epa. gov/owm/mtb/mtbfact.htm
1. Middlebrooks, E.J., et al., 1982.
Wastewater Stabilization Lagoon Design,
Performance and Upgrading, McMillan
Publishing Co., New York, NY.
2. Pano, A. and Middlebrooks, E.T., 1982.
Ammonia Nitrogen Removal in Facultative
Wastewater Stabilization Ponds. Water
Pollution Control Federation Journal, 54 (4)
344-351.
3. Reed, S.C., et al., 1995, 2nd Ed. Natural
Systems for Waste Management and
Treatment, McGraw Hill Book Co., New
York, NY.
4. Reed, S.C., 1985. Nitrogen Removal in
Wastewater Stabilization Ponds, Water
Pollution Control Federation Journal.
57(1)39-45.
5. U.S. EPA, 1983. Design Manual -
Municipal Wastewater Stabilization Ponds,
EPA-625/1-83-015, US EPA CERI.
Cincinnati, OH.
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6. WPCF, 1990. MOP FD-16, Natural
Systems for Wastewater Treatment, Water
Pollution Control Federation, Alexandria,
VA.
ADDITIONAL INFORMATION
Richard H. Bowman, P.E.
West Slope Supervisor
Colorado Department of Public Health and
Environment
Water Quality Control Division
222 South 6th Street, Room 232
Grand Junction, CO 81502
Earnest F. Gloyna, P.E.
Consulting Engineer
701 Brazos, Suite 550
Austin, TX 78701
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
Leroy C. Reid, Jr., Ph.D
1273 Annapolis Drive
Anchorage, AK 99508-4307
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-02-014
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Ave., NW
Mail Code 4201M
* 2002_*
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