vvEPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Aerated, Partial Mix Lagoons
DESCRIPTION
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
attached.
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
treatment.
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
nitrate.
APPLICABILITY
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
wastes).
ADVANTAGES AND DISADVANTAGES
phosphorous in facultative ponds do not occur in
aerated ponds.
Aerated lagoons may experience surface ice
formation.
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
reactions.
Requires energy input.
DESIGN CRITERIA
Advantages and disadvantages of aerated, partial mix
lagoons are listed below:
Advantages
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
climate.
Sludge disposal may be necessary but the quantity
will be relatively small compared to other secondary
treatment processes.
Disadvantages
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:
where:
Ce=C0/[l+(KT)(t)/n]n
C = effluent BOD
-------
C = influent BOD
OPERATION AND MAINTENANCE
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.
PERFORMANCE
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
than<200MPN/100mL.
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.
Limitations
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
embankments.
Energy
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)
1.026
where:
E = electrical energy, kWh/yr
HP = aerator horsepower, hp
-------
COSTS
ADDITIONAL INFORMATION
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.
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. 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
Environment
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
CH2MHILL
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
Principal
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 *_
TmYEAROF
CLEAN WATER.
1MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY
------- |