United States
Environmental Protection
                          Wastewater Technology Fact Sheet
                          Aerated, Partial Mix Lagoons
Partial mix  lagoons are commonly  used to treat
municipal and industrial wastewaters. This technology
has been widely used in the United States for at least
40 years. Aeration is provided by either mechanical
surface  aerators  or submerged diffused aeration
systems.    The  submerged   systems  can  include
perforated tubing or piping, with  a variety of diffusers

In aerated lagoons, oxygen is supplied mainly through
mechanical or diffused aeration  rather than by algal
photosynthesis.      Aerated  lagoons  typically are
classified by the amount of mixing provided. A partial
mix system provides only enough aeration to satisfy the
oxygen  requirements of the  system and does not
provide energy to keep all total suspended solids (TSS)
in suspension.

In some cases, the initial cell in a system might be a
complete mix unit followed by partial mix and settling
cells.  Most energy in complete mix systems is used in
the mixing function which requires about 10 times the
amount of energy needed for an  equally-sized partial
mix system to treat municipal wastes. A complete mix
wastewater treatment system is similar to the activated
sludge treatment process except that it does not include
recycling of cellular material, resulting in lower mixed
liquor suspended solids concentrations, which requires
a longer hydraulic detention time than activated sludge

Some solids in partial  mix  lagoons  are kept  in
suspension to contribute to overall  treatment.  This
allows for anaerobic fermentation of the settled sludges.
Partial mix lagoons are also called facultative aerated
lagoons and are generally designed with at least three
cells in series, with total detention time dependent on
water temperature.  The lagoons are constructed  to
have a water depth of up  to 6 m (20 ft) to ensure
                        maximum oxygen transfer efficiency when using diffused
                        aeration.   In  most cases,  aeration is  not applied
                        uniformly over the entire system. Typically, the most
                        intense aeration (up to 50 percent of the total required)
                        is used in the first cell.  The final cell may have little or
                        no aeration to allow settling to occur. In some cases, a
                        small separate settling pond is provided after the final
                        cell.   Diffused aeration equipment typically provides
                        about 3.7 to 4 kg O2/kW-hour  (6 to 6.5 Ibs O2/hp-
                        hour) and mechanical surface aerators are rated at 1.5
                        to 2.1 kg (ykW-hour (2.5  to 3.5 Ibs  (yhp-hour).
                        Consequently,  diffused systems are somewhat more
                        efficient, but  also  require  a  significantly greater
                        installation and maintenance effort.

                        Aerated lagoons can reliably  produce an effluent with
                        both biological oxygen demand (BOD) and TSS < 30
                        mg/L if provisions for settling are included at the end of
                        the system. Significant nitrification will occur during the
                        summer  months if adequate dissolved oxygen  is
                        applied.   Many systems  designed only  for  BOD
                        removal  fail to meet discharge standards during the
                        summer because of a shortage of dissolved oxygen.
                        Nitrification of ammonia and BOD removal  occur
                        simultaneously and systems can become oxygen limited.
                        To achieve nitrification in heavily loaded systems, pond
                        volume and aeration capacity beyond that provided for
                        BOD removal are necessary. Oxygen requirements for
                        nitrification are more demanding than for BOD removal.
                        It is generally assumed that 1.5 kg of oxygen is required
                        to treat  1  kg of BOD.   About 5  kg of O2  are
                        theoretically required to convert 1 kg of ammonia to


                        An aerated lagoon is well suited for municipal and
                        industrial wastewaters  of low to  medium strength.
                        While such systems are somewhat land intensive,they
                        require much less area than a facultative lagoon and can
                        provide a better level of treatment.   Operation and

management requirements are also less than  those
required for activated sludge and similar technologies.

A physical modification to  an aerated lagoon uses
plastic curtains supported by floats and anchored to the
bottom to divide  existing lagoons into multiple cells
and/or serve as baffles to improve hydraulic conditions.
A recently developed  approach suspends a row  of
submerged diffusers from flexible floating booms which
move in a cyclic pattern during aeration activity. This
serves to treat a larger volume with each aeration line.
Effluent  is periodically  recycled within the  system  to
improve performance.  If there is sufficient depth for
effective oxygen transfer, aeration is used to upgrade
existing facultative ponds and is sometimes  used on a
seasonal basis  during  periods of  peak wastewater
discharge to the lagoon (e.g. seasonal food processing

   phosphorous in facultative ponds do not occur in
   aerated ponds.

   Aerated  lagoons  may  experience  surface ice

   Reduced rates of biological activity occur during
   cold weather.

   Mosquito and  similar insect vectors can be a
   problem if vegetation on the dikes and berms is not
   properly maintained.

   Sludge accumulation rates will be higher in cold
   climates because low temperature inhibits anaerobic

   Requires energy input.

Advantages and disadvantages of aerated, partial mix
lagoons are listed below:


   Require less land than facultative lagoons.

   Require much less  land than  facultative ponds,
   depending on the design conditions.

   An aerated lagoon can usually discharge throughout
   the winter while discharge may be prohibited from
   an ice-covered facultative  lagoon in the same

   Sludge disposal may be necessary but the quantity
   will be relatively small compared to other secondary
   treatment processes.


   Aerated lagoons  are not as effective as facultative
   ponds   in   removing   ammonia  nitrogen   or
   phosphorous, unless designed for nitrification.

   Diurnal changes  in  pH and alkalinity that affect
   removal  rates  for   ammonia  nitrogen   and
Equipment  typically required for aerated lagoons
includes the following: lining systems, inlet and outlet
structures, hydraulic controls, floating  dividers  and
baffles, aeration equipment.

Every system should have at least three cells in series
with each cell lined to prevent adverse groundwater
impacts. Many states have design criteria which specify
design loading, the hydraulic residence time,  and the
aeration requirements. Pond depths range from 1.8 to
6 m (6 to 20 ft), with 3 m (10 ft) the most typical (the
shallow depth systems usually are converted facultative
lagoons). Detention times range  from 10 to 30 days,
with 20 days the most typical (shorter detention times
use higher intensity aeration).  The design  of aerated
lagoons for BOD removal  is based on  first-order
kinetics and the complete mix hydraulics model. Even
though  the  system  is  not  completely  mixed,  a
conservative design will result.  The model commonly
used is:
                C  = effluent BOD

       C = influent BOD
       KT = temperature dependent rate constant

       K20 = rate constant at 20 C

       K20 = 0.276 d'1 at 20 C

        = temperature coefficient (1.036)

       V   V   (T-20)
       J^T - ^20

       T = temperature of water

       t = total detention time in system

       n = number of equal sized cells in system

Detention times in the settling basin or portion of a basin
used for settling of solids should be limited to two days
to limit algae growth.  The  design of inlet and outlet
structures should receive careful attention.


BOD removal can range up to 95 percent. Effluent
TSS can range from 20 to 60 mg/L, depending on the
design of the settling basin and the concentration of
algae in the effluent.  Removal of ammonia nitrogen in
aerated lagoons is usually less effective  than  in
facultative lagoons because of shorter detention times.
Nitrification of ammonia can occur in aerated lagoons
or if the system is specifically designed for that purpose.
Phosphorus  removal  is also less  effective than in
facultative lagoons because of more stable pH and
alkalinity conditions.  Phosphorus removals of about 15
to 25 percent can be expected with aerated lagoons.
Removal of coliforms and fecal coliforms  can  be
effective, depending on detention time and temperature.
Disinfection may be necessary if effluent limits are less

The  aerated lagoon  system  is simple to  operate and
reliable  in performance for BOD  removal.   TSS
removal can be influenced by the presence of algae in
the lagoon, but generally is acceptable. The service life
of a lagoon is estimated at 30 years or more.

Depending upon  the  rate  of  aeration  and  the
environment, aerated lagoons may experience ice
formation on the water surface during cold weather
periods. Reduced rates of biological activity also occur
during cold weather.  If properly designed, a system
will continue to function and  produce acceptable
effluents under these conditions.  The potential for ice
formation on floating aerators may encourage the use of
submerged diffused aeration in very cold climates. The
use of  submerged perforated tubing  for  diffused
aeration requires maintenance and cleaning on a routine
basis  to maintain design aeration rates.  There are
numerous types of submerged aeration equipment that
can be used in warm or cold climates, which should be
considered in  all designs.   In  submerged diffused
aeration,  the routine application  of HC1 gas in the
system is used to dissolve accumulated material on the
diffuser units.

Any earthen structures used as impoundments must be
periodically  inspected.  If left  unchecked,  rodent
damage   can cause  severe  weakening of  lagoon


Typically,  operation occurs by gravity flow in and out
of the lagoon. Energy would be required if pumps are
necessary for either influent or  effluent.  Energy is
required for the aeration devices, with the amount
depending on the intensity of mixing desired. Partial
mix systems require between 1 and 2 watts per cubic
meter (5  and 10 horsepower per million gallons) of
capacity, depending on the depth and configuration of
the system.
       E = 6598 (HP)
       E = electrical energy, kWh/yr

       HP = aerator horsepower, hp

Construction costs associated with partially mixed
aerated lagoons include cost of the land, excavation,
and inlet and outlet structures. If the soil where the
lagoon is constructed is permeable, an additional cost
for lining should be expected. Excavation costs vary,
depending on whether dirt must be added or removed.
Compacting costs run between $3 to $5 per cubic
yard; synthetic lining material costs about $0.50 to $1
per square foot.

Operating costs of partially aerated lagoons include
power surface or diffused aeration equipment and
maintenance of these units.


Other Related Fact Sheets

Other EPA Fact Sheets can be found at the following
web address:


1.      Manual of'Practice FD-13, 1988. Aeration,
       WPDF, ASCE.

2.      Middlebrooks,EJ.,etal., 1982. Wastewater
       Stabilization Lagoon Design, Performance
       and Upgrading, McMillan Publishing  Co.,
       New York, NY.

3.      Reed,  S.C., et al.,  1995, 2d ed.  Natural
       Systems for  Waste Management  and
       Treatment,  McGraw Flill  Book Co., New
       York, NY.

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

5.      WPCF,  1990.    MOP  FD-16,  Natural
       Systems for Wastewater Treatment, WPCF,
       Alexandria, VA.
Richard H. Bowman, PE
West Slope Supervisor
Colorado  Department   of  Public  Health  and
Water Quality Control Division
222 South 6th Street, Room 232
Grand Junction, CO 81502

Glen T. Daigger, Ph.D., P.E., DEE
Senior Vice President
100 Inverness Terrace East
Englewood, CO 80112-5304

John Hinde
Air Diffusion Systems
28846-C Nagel Court
P.O. Box 38
Lake Bluff, IL 60044

E. Joe Middlebrooks, Ph.D., PE. 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

Linvil G Rich
Alumni Emeritus Professor
Clemson University
P.O.Box 1185
Clemson, SC 29633

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-02-008
               September 2002
                                                           For more information contact:

                                                           Municipal Technology Branch
                                                           U.S. EPA
                                                           Mail Code 4204M
                                                           1200 Pennsylvania Ave., NW
                                                           Washington, D.C., 20460
                                                                   * 2Q02 *_
                                                                   CLEAN WATER.
                                                       Excellence in compliance through optimal technical solutions
                                                       MUNICIPAL TECHNOLOGY