Enwwra^Protocfcr Agmcy
Constructed Wetlands for
Wastewater Treatment and
Wildlife Habitat
Fort Deposit, AL: Constructed Wetland Treatment
System Case History
Fort Deposit, Alabama
Canttrutfed W«U«nd Twrtnwm SyiMnt
Background
System Description
Operations and Management
Performance
Ancillary Benefits
Acknowledgements
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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 wastewater 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 Management. 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
technologies. 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
oxygen.
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System Description
As designed, the Fort Deposit
wetland 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 remaining area
to serve as a settling basin. These modifications improve 5-day biochemical oxygen demand (BODS) 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 wetland 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 Fort Deposit constructed wetland treatment system uses an aerated
lagoon for pretreatment followed by two parallel wetland cells.
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.
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Operations and Management
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 transform 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.
Outlet weir structures allow water level
control for adjustment of hydraulic
retention time.
Influent distribution to the wetland cells is
enhanced by perforated pipes on a rip-rap
slope across the width of the wetland
cells.
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.
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Dense stands of submerged cattail stems
and leaves serve as growth media for A n , . i ,1 *T j n f£\ t • ±u
After treatment by the wetland cells, effluent is conveyed to the
microorganisms matjeed on impurities in . .
the influent. The natural transfer of post-aeration pond, where it receives supplemental aeration
atomospheric oxygen to these microbes is from a floating aerator.
essential in removing organic matter and
ammonia from the wastewater.
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Performance
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.
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 Conservation Achievement
Award, the Alabama Engineering Excellence Award,
and the Grand Award from the American Consulting
Engineers Council.
Month BOD5 TSS
Nitrogen
In Out In Out TKN In NHsOut
1990August 102 5 137 10
September 27 8 101 18
October 30 3 168 18
November 27 3 127 10
December 15 4 71 9
1991 January 20 5 52 10
February 13 4 18 4
March 26 7 40 8
April 22 10 97 15
May 21 9 52 20
June 29 10 72 25
July 33 7 69 10
August 56 7 183 7
September 24 4 87 12
October 30 8 125 18
November 32 4 106 7
December 33 12 64 16
1992January 39 4 83 19
February 22 4 32 4
March 34 4 58 5
20.0
11.0
19.0
14.0
10.0
8.0
11.0
19.0
10.0
80.0
5.0
21
20.0
10.0
6.0
11.0
11.5
10.0
6.7
10.0
0.57
0.66
0.78
0.93
2.60
1.10
0.74
0.89
0.70
0.35
0.94
6.43
0.90
0.99
0.75
0.21
0.87
0.38
0.15
0.22
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April 31 4 119 3 12.0 0.51
Wetland effluent BOD5 and total suspended solids
(TSS) are consistenly in compliance with permit limits
despite variable inflow quality to the wetland cells.
Total Kjeldahl nitrogen (TKN) is mineralized in the
wetland cells to NH3 and then nitrified to achieve the
low discharge limits.
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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 wetland1 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.
Fort Deposit
Wetland Design Criteria
Average Daily Flow 0.24 mgd
Influent Quality
BODs
TSS
TN
NH3-N
Effluent Criteria
BODs
TSS
NH3-N
pH
40 mg/L
lOOmg/L
20 mg/L
10 mg/L
10(18)amg/L
30 mg/L
2(5)a mg/L
6-9 units
The Fort Deposit wetlands continue to
diversify as new plant species colonize the
cells.
Areas
Lagoon 10 acres
Wetland Cells (2) 7.5 acres each
()a winter limits December-April
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Acknowledgements
The Waterworks and Sewer Board of the Town of Fort Deposit
Henry Crenshaw, Chairman
Leo Goldsmith, Board Member
W.O. 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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