United States
Environmental Protection
                          Wastewater Technology Fact Sheet
                          Facultative Lagoons
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

sufficient detention time > 20 days), improving the
final effluent quality.


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.


Some advantages and disadvantages of facultative
lagoons are listed below:


   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


   Settled sludges and inert material require periodic

   Difficult to control or predict ammonia levels in

   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

   Burrowing animals may be a problem.


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

detention time  of 40  percent  or  less than  the
theoretical design value.


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 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


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.

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


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)

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.

5.     U.S.  EPA,  1983.   Design  Manual  -
       Municipal Wastewater Stabilization Ponds,
       EPA-625/1-83-015,  US   EPA   CERI.
       Cincinnati, OH.

6.     WPCF,  1990.   MOP FD-16,  Natural
      Systems for Wastewater Treatment, Water
      Pollution Control Federation, Alexandria,


Richard H. Bowman, P.E.
West Slope Supervisor
Colorado Department of Public Health and
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
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

              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
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