vvEPA
United States
Environmental Protection
Agency
EPA832-R-93-005n
September 1993
Fort Deposit, Alabama
Constructed Wetland Treatment System
Case History
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SYSTEM DESCRIPTION
A
s designed, the Fort Deposit wet-
land treatment system includes
the following main components:
• An 8.9-acre aerated pond
• Two 7.5-acre constructed wetland
cells
• A 0.1-acre post-aeration pond
The town s existing stabilization pond
was modified to provide more effective
pre-treatment. The modifications
included relocating the influent and
effluent points and adding floating
mechanical aerators. Seven acres of the
pond were aerated, leaving the remain-
ing area to serve as a settling basin.
These modifications improve 5-day
biochemical oxygen demand (BOD5)
and ammonia nitrogen (NH3-N)
removal efficiency, reduce organic
and solids loading to the wetland cells,
and provide additional flexibility in
the overall treatment process.
The wetland cells are configured side
by side. Each cell covers 7.5 acres and
has an aspect ratio (length to width) of
4.6:1. The cell floors are slightly sloped
for easy draining during maintenance.
Although most of the 15 acres of wet-
land cells are less than 2 feet deep, each
cell has three "deep zones," which are
4 feet deep and about 20 feet wide.
The deep zones remain free of rooted
marsh vegetation, thus allowing effluent
to be redistributed through the system
and providing atmospheric aeration.
The deeper water in these zones also
furnishes year-round habitat for aquatic
life, particularly mosquito fish and
wetland birds.
The parallel operation of the two
wetland cells gives the town the ability
to direct all flow through a single cell
during wetland resting and maintenance
periods. Moreover, the rate of flow
to each cell can be varied to allow
flexibility in operations and to aid in
testing or research.
The treated effluent enters a post-
aeration pond after passing through the
wetland cells. This system component
is used to meet the effluent dissolved
oxygen limits specified in the permit.
This 75.000-gallon earthen pond is
equipped with a floating mechanical
aerator. Final effluent flow rate from
the post-aeration pond is continuously
measured by a Parshall flume.
Aerated
Lagoon
The Fort Deposit constructed
wetland treatment system
uses an aerated lagoon for
pretreatment followed by two
parallel wetland cells.
Wetland
Cell 1
Wetland
Cell 2
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Outlet weir structures allow
water level control for
adjustment of hydraulic
retention time.
Dense stands of submerged
cattail stems and leaves
serve as growth media for
microorganisms that feed on
impurities in the influent. The
natural transfer of atmospheric
oxygen to these microbes is
essential in removing organic
matter and ammonia from
the wastewater.
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BACKGROUND
The town of Fort Deposit, located
south of Montgomery, Alabama,
has a population of slightly more
than 1,500. Until 1985, the town's waste-
water was treated in a 10-acre waste
stabilization pond and consistently met
discharge limits. In 1985, a new discharge
permit was issued by the Alabama
Department of Environmental Manage-
ment. This permit required the town
to meet more stringent standards based
on water quality limitations in the
receiving water. Since the town's
stabilization pond was unable to meet
the new standards, an administrative
order requiring the town to upgrade
its system was issued.
An engineering analysis of treatment
alternatives was conducted by the
environmental consulting firm
CH2M HILL to compare a variety of
conventional and innovative technolo-
gies. On the basis of an evaluation of
environmental benefits, reliability, and
cost, treatment by constructed wetlands
was selected as the most cost-effective
approach for compliance with the new
permit limitations.
The use of constructed wetlands to
remove impurities in wastewater and
to consistently achieve treatment levels
that meet permit requirements was an
emerging technology in 1985. To assist
with funding their new system, the town
applied for and was awarded a $610,000
U.S. Environmental Protection Agency
(EPA) Innovative/Alternative
Technology grant for its wetland project.
This additional funding, coupled with
low construction and maintenance costs
associated with the wetland system.
reduced the financial impact of the
upgrade on the community and provided
it with a system that would require only
slightly more maintenance than the
existing stabilization pond.
Post-aeration is essential for
compliance with the effluent
standard for dissolved oxvgen.
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ANCILLARY BENEFITS
In addition to improving the quality
of the effluent discharged to the
receiving stream, the creation of
the Fort Deposit constructed wetland
treatment system has significantly
increased wildlife. This new habitat
provides cover and food for various
types of wetland-dependent vertebrate
and invertebrate life including a
variety of ducks and wading birds and
their prey.
As a result of the wetland's success
and the desire of others to adopt similar
technology, the town is receiving
visitors from other areas of the state
and the nation.
The Fort Deposit wetlands
continue to diversify as new
plant species colonize the cells.
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In the Fort Deposit wetland system,
wastewater is treated by the naturally
occurring bacteria and fungi that
colonize the sediments on the bottom
of the cells and the stems and leaves of
the wetland vegetation below the water
level. These microorganisms help trans-
form and remove organic matter and
nutrients that might otherwise degrade
adjacent surface waters.
The vegetation in the two wetland
cells was selected to simulate a natural
wetland and included an initial planting
of 68,000 cattail and bulrush plants.
Influent from the aerated pond is
distributed to the cells by pipes with
1-inch holes drilled at 10-foot intervals.
This method of distributing influent
starts the flow through the treatment
system and reduces the buildup of solids
at the head of the wetland cells.
The system is designed so that the
effluent takes up to 30 days to flow
through the wetland cells. The actual
retention time varies seasonally to
account for changes in the reaction
rate of microorganisms in the cells.
Because the microorganisms react more
quickly at higher temperatures, the
retention time can be decreased during
the summer and still provide the
required contact time for effective
removal of impurities. Conversely,
during the winter's colder temperatures,
the reaction rate of the microorganisms
is lower and the retention time is
increased by raising water levels.
Aluminum stop logs, located in three
outlet structures along the width of each
wetland cell, control cell water depth
and promote the flow of effluent
through the treatment system.
After treatment by the wetland cells,
effluent is conveyed to the post-aeration
pond, where it receives supplemental
aeration from a floating aerator.
Influent distribution to the
wetland cells is enhanced by
perforated pipes on a rip-rap
slope across the width of the
wetland cells.
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ACKNOWLEDGEMENTS
The Waterworks and Sewer Board of
the Town of Fort Deposit
Henry Crenshaw, Chairman
Leo Goldsmith, Board Member
WO. Ward, Board Member
David Edwards, Manager
Consulting Engineers
Dennis A. Sandretto,
CH2M HILL
Project Manager
Robert L. Knight,
CH2M HILL Project
Environmental Scientist
Alabama Department of
Environmental Management
Truman Green,
Chief, Municipal Branch,
Water Division
U.S. Environmental Protection Agency
Robert Freeman,
Municipal Grants Program,
Region IV
This brochure was prepared by
CH2M HILL for the
U.S. Environmental Protection Agency.
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PERFORMANCE
Construction of the cells began in
June 1989, with planting starting
during May 1990. By August
1990, the vegetation provided almost
complete cover, and operation of the
wetland cells began. Since then, with
only one exception for NH3, the Fort
Deposit constructed wetland treatment
system has consistently achieved permit
compliance and has caught the attention
of others seeking a low cost, dependable
natural treatment system. Because of
its outstanding contribution to water
resource conservation, the Fort Deposit
system received several awards including
the Alabama 1991 Governor's Conserva-
tion Achievement Award, the Alabama
Engineering Excellence Award, and
the Grand Award from the American
Consulting Engineers Council.
Month
December
1991 January
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June
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21
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Deep zones in the wetlands
provide open water for ducks
and wading birds, enhance
flow distribution in the
wetland cells, serve as a sump
for settling solids, and provide
additional hydraulic residence
time in the wetland cells.
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