vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-024
September 2000
Waste water
Technology Fact Sheet
Free Water Surface Wetlands
DESCRIPTION
Free water surface (FWS) wetlands are defined as
wetland systems where the water surface is exposed
to the atmosphere. Most natural wetlands are FWS
systems, including bogs (primary vegetation
mosses), swamps (primary vegetation trees), and
marshes (primary vegetation grasses and emergent
macrophytes.) The observation of water quality
improvements in these natural wetlands for many
years led to the development of constructed
wetlands in an effort to replicate the water quality
and habitat benefits of natural wetlands in a
constructed ecosystem. The majority of FWS
constructed wetlands designed for wastewater
treatment are marshes, but a few operating examples
of bogs and swamps exist. In FWS treatment
wetlands, water flows over a vegetated soil surface
from an inlet point to an outlet point. In some
cases, water is completely lost to evapotranspiration
and seepage within the wetland. A diagram of FWS
wetland is shown in Figure 1.
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Inlet
Manifold
Membrar
Imperme
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1
Vegetation ^ ^
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able Soils Rootl"£
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Medium
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Source: Adapted from drawing by S.C. Reed, 2000.
FIGURE 1 FREE WATER SURFACE
WETLAND
There are relatively few examples of the use of
natural wetlands for wastewater treatment in the
United States. Because any discharge to a natural
wetland must satisfy National Pollutant Discharge
Elimination System (NPDES) limits, these wetlands
are typically used for advanced wastewater
treatment (AWT) or tertiary polishing. The design
goals for constructed wetlands range from an
exclusive commitment for basic treatment functions
to systems which provide advanced treatment
and/or combine with enhanced wildlife habitat and
public recreational opportunities. The size of the
FWS wetland systems ranges from small on-site
units designed to treat septic tank effluents to large
units with more than 16,188 hectares (40,000
acres). A large system is being used to treat
phosphorus from agricultural storm water drainage
in south Florida. Operational FWS wetlands
designed for municipal wastewater treatment in the
United States range from less than 3785 liters per
day (1,000 gallons per day) to more than 75,708
nrVday (20 million gallons per day).
Constructed FWS wetlands typically consist of one
or more shallow basins or channels with a barrier to
prevent seepage to sensitive ground waters and a
submerged soil layer to support the roots of the
selected emergent macrophyte vegetation. Each
system has appropriate inlet and outlet structures to
ensure uniform distribution and collection of the
applied wastewater. The most commonly used
emergent vegetations in constructed FWS wetlands
include cattail (Typha spp.), bulrush (Scirpus spp.),
and reeds (Phragmites spp.). In systems designed
primarily for treatment, it is common to select only
one or two species for planting. The plant canopy
formed by the emergent vegetation shades the water
surface, preventing growth and persistence of algae,
and reduces wind-induced turbulence in the water
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flowing through the system. Perhaps most
important are the submerged portions of the living
plants, the standing dead plants, and the litter
accumulated from previous growth. These
submerged surfaces provide the physical substrate
for the periphytic-attached growth organisms
responsible for much of the biological treatment in
the system. The water depth in the vegetated
portions of these systems ranges from a few inches
to two feet or more.
The influent to these wetlands spreads over a large
area of shallow water and emergent vegetation. The
subsequent low velocity and essentially laminar flow
provides for very effective particulate removal in the
front part of the system. This parti culate material,
characterized as total suspended solids (TSS),
contains Biochemical Oxygen Demand (BOD)
components, fixed forms of total nitrogen (TN) and
total phosphorus (TP), and trace levels of metals
and more complex organics. The oxidation or
reduction of these particulates releases soluble
forms of BOD, TN, and TP to the wetland
environment, which are available for adsorption by
the soils and removal by the active microbial and
plant populations throughout the wetland. Oxygen
is available at the water surface, microsites on living
plant surfaces, and on root and rhizome surfaces,
allowing some aerobic activity the wetland. It is,
however, prudent to assume that the bulk of the
liquid in the FWS wetland is anoxic or anaerobic.
The lack of oxygen can limit the biological removal
of ammonia nitrogen (NH3/NH4 - N) via
nitrification, but the FWS wetland is still effective
for removal of BOD, TSS, trace metals, and some
complex organics because the treatment of these
occurs under both aerobic and anoxic conditions.
If nitrogen removal and/or enhancement of wildlife
habitat is a project goal, consideration should be
given to alternating shallow water emergent
vegetated zones with deeper (greater than 1.83
meters or six feet) water zones containing selected
submerged vegetation. Deeper water zones provide
a completely exposed water surface for atmospheric
re-aeration and submerged vegetation provides an
additional source of oxygen for nitrification. The
deeper water zones will also attract and retain a
large variety of wildlife, particularly ducks and other
water birds. This concept, in use at Arcata,
California, and Minot, North Dakota, can provide
excellent treatment on a year-round basis in warm
climates and on a seasonal basis in colder climates
where low temperatures and ice formation occur.
The hydraulic residence time (HRT) in each of the
open water zones should be limited to about three
days at design flow to prevent the re-emergence of
algae. Such systems should always start and end
with shallow emergent vegetation zones to ensure
retention and treatment of parti culate matter and to
minimize wildlife toxicity in the open water zones.
The use of FWS constructed wetlands has increased
significantly since the late 1980's. The systems are
widely distributed in the United States and are found
in about 32 states.
Common Modifications
In the United States, it is routine to provide some
preliminary treatment prior to a FWS wetland. The
minimal acceptable level is the equivalent of primary
treatment which can be achieved with septic tanks,
with Imhoff tanks for smaller systems, or with deep
ponds with a short HRT. About 45 percent of
operational FWS wetland systems use facultative
lagoons for preliminary treatment, but these systems
have also been used behind other treatment systems.
For example, some of the largest FWS systems, in
Florida and Nevada, were designed for tertiary
effluent polishing and receive effluent from
mechanical AWT plants.
Non-discharging, total retention FWS systems have
been used in arid parts of the United States where
the water is completely lost through a combination
of seepage and evapotranspiration. These systems
require that attention be paid to the long term
accumulation of salts and other substances which
might become toxic to wildlife or plants in the
system. While it is impossible to exclude wildlife
from FWS wetlands, it is prudent to minimize their
presence until the water quality approaches
secondary levels of treatment. This can be
accomplished by limiting open water zones to the
latter part of the system and using dense stands of
emergent vegetation in the front part of the wetland.
Selecting vegetation with little food value for
animals or birds may also help. In colder climates or
where large land areas are not available for wetland
removal of nitrogen, a smaller wetland system can
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be designed for BOD/TSS removal. Nitrogen
removal can be achieved with a separate process.
Wetland systems in Kentucky and Louisiana
successfully use an integrated gravel trickling filter
for nitrification of wastewater ammonia. Seasonally
operated FWS wetlands are also used in very cold
climates, in which the wastewater is retained in a
lagoon during the winter months and discharged to
the wetland at a controlled rate during the warm
summer months.
APPLICABILITY
FWS wetlands require a relatively large land area,
especially if nitrogen or phosphorus removal is
required. The treatment is effective and requires
little in the way of mechanical equipment, energy,
and skilled operator attention. Wetland systems can
be a most cost effective treatment alternative where
suitable land is available at reasonable cost. They
also provide enhanced habitat and recreational
values. Land requirements and costs tend to favor
application of FWS technology in rural areas.
FWS wetland systems reliably remove BOD,
Chemical Oxygen Demand (COD), and TSS. With
a sufficiently long FtRT, they can also produce low
levels of nitrogen and phosphorus. Metals are also
removed and a reduction in fecal coliforms of about
a one log can be expected. In addition to municipal
wastewaters, FWS systems are used to treat mine
drainage, urban storm water, combined sewer
overflows, agricultural runoff, livestock and poultry
wastes, landfill leachates, and for mitigation
purposes. Because the water is exposed and
accessible to humans and animals, the FWS concept
of receiving partially treated wastewater may not be
suited for use in individual homes, parks,
playgrounds, or similar public facilities. A gravel
bed subsurface flow (SF) wetland is a choice for
these applications.
ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of FWS
wetlands are listed below:
Advantages
FWS wetlands offer effective treatment in a
passive manner, minimizing mechanical
equipment, energy, and skilled operator
requirements.
FWS wetlands may be less expensive to
construct, and are less costly to operate and
maintain than conventional mechanical
treatment systems.
Year-round operation for secondary
treatment is possible in all but the coldest
climates. Year-round operation for
advanced or tertiary treatment is possible in
warm to moderately temperate climates.
Wetland systems provide a valuable addition
to the "green space" in a community, and
include the incorporation of wildlife habitat
and public recreational opportunities.
Wetland systems produce no residual
biosolids or sludges requiring subsequent
treatment and disposal.
The removal of BOD, TSS, COD, metals,
and persistent organics in municipal
wastewaters can be very effective with a
reasonable detention time. The removal of
nitrogen and phosphorus can also be
effective with a significantly longer
detention time.
Disadvantages
The land area required for FWS wetlands
can be large, especially if nitrogen or
phosphorus removal are required.
The removal of BOD, COD, and nitrogen
are biological processes and essentially
continuously renewable. The phosphorus,
metals, and some persistent organics
removed by the system are bound in the
wetland sediments and accumulate over
time.
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• In cold climates low winter temperatures
reduce the rate of removal for BOD and the
biological reactions responsible for
nitrification and denitrifi cation. An
increased detention time can compensate for
this, but the increased wetland size required
in extremely cold climates may not be cost
effective or technically feasible.
• The bulk water in most constructed FWS
wetland systems is essentially anoxic,
limiting the potential for rapid biological
nitrification of ammonia. Increasing the
wetland size and, therefore, the detention
time, may compensate for this, but may not
be cost effective. Alternate methods for
nitrification in combination with a FWS
wetland have performed successfully.
Mosquitoes and other insect vectors can be
a problem.
• The bird population in a FWS wetland can
have adverse impacts if an airport is nearby.
FWS constructed wetlands can remove fecal
coliforms by at least one log from typical
municipal wastewaters. This may not be
sufficient to meet discharge limits in all
locations and supplemental disinfection may
be required. The situation is further
complicated because birds and other wildlife
in the wetland produce fecal coliforms.
DESIGN CRITERIA
Published models for the pollutant removal design
of FWS wetland systems have been available since
the late 1980's. More recent efforts have produced
three textbooks containing design models for FWS
wetlands (Reed, et al., 1995; Kadlec & Knight,
1996; Crites & Tchobanoglous, 1998) All three
models are based on first order plug flow kinetics
but provide different results based on the use of
different databases. The Water Environment
Federation (WEF) presents a comparison of the
three approaches in the Manual of Practice on
Natural Systems (WEF, 2000.) Another
comparison is found in the U.S. EPA design manual
on wetland systems (U.S. EPA, 2000.) This
manual also includes design models developed by
Gearheart and Finney. The designer of a FWS
wetland system should consult these references and
select the method best suited for the project under
consideration. A preliminary estimate of the land
area required for an FWS wetland can be obtained
from Table 1 of typical areal loading rates presented
below. These values can also be used to check the
results from other references.
The pollutant requiring the largest land area for
TABLE 1 TYPICAL AREAL LOADING
RATES
Constituent
Hydraulic
Load (in/d)
BOD
TSS
NH3/NH4
asN
NO3 as N
TN
TP
Typical Target Mass
Influent Effluent Loading
Cone. Cone. Rate
(mg/L) (mg/L) (Ib/ac/d)*
0.4 - 4**
5-100 5-30 9-89
5-100 5-30 9-100
2-20 1-10 1-4
2-10 1-10 2-9
2-20 1-10 2-9
1-10 0.5-3 1-4
removal determines the necessary treatment area for
the wetland, which is the bottom surface area of the
wetland cells. The wastewater flow must be
uniformly distributed over the entire surface for that
area to be 100 percent effective. This is possible
with constructed wetlands by careful grading of the
bottom surface and the use of appropriate inlet and
outlet structures. Uniform distribution of
wastewater is more difficult when natural wetlands
are used for treatment or polishing. The existing
configuration and topography are typically retained
in these natural wetlands, which can result in
significant short circuiting of flow. Dye tracer
studies in such wetlands have shown that the
effective treatment area can be as little as 10 percent
of the total wetland area. The total treatment area
should be divided into at least two cells for all but
the smallest systems. Larger systems should have at
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least two parallel trains of cells to provide flexibility
for management and maintenance.
Wetland systems are living ecosystems. The life and
death cycles of the biota produce residuals which
can be measured as BOD, TSS, nitrogen,
phosphorus, and fecal coliforms. As a result,
regardless of the size of the wetland or the
characteristics of the influent, there will always be a
residual background concentration of these
materials in wetland systems. Table 2 summarizes
these background concentrations.
Because removal of BOD and various nitrogen
forms is temperature dependent, the temperature of
TABLE 2 "BACKGROUND"FWS
WETLAND CONCENTRATIONS
Constituent
Concentration Range
BOD5 (mg/L)
TSS (mg/L)
TN (mg/L)
NH3/NH4 as N (mg/L)
NO3 as N (mg/L)
TP (mg/L)
Fecal Coliforms
(MPN/100mL)
1 -10
1 -6
1 -3
<0.2
50 - 500
the wetland must be known for proper design. The
water temperature in large systems with a long HRT
(greater than 10 days) will approach the average air
temperature except during subfreezing weather in
the winter. Methods to estimate the water
temperature for wetlands with a shorter HRT (less
than 10 days) can be found in the references cited.
Because living plants and litter provide significant
frictional resistance to flow through the wetland , it
is necessary to consider the hydraulic aspects of
system design. Manning's equation is generally
accepted as the model for the flow of water through
FWS wetlands. Descriptive information is found in
the references cited. Flow resistance impacts the
configuration selected for the wetland cell: the
longer the flow path, the higher the resistance. To
avoid hydraulic problems, an aspect ratio (L:W) of
4:1 or less is recommended.
PERFORMANCE
A lightly loaded FWS wetland can achieve the
"background" effluent levels shown in Table 2. In
general, an FWS constructed wetland is designed to
produce a specified effluent quality. Table 1 can be
used to estimate the size of the wetland necessary to
produce the desired effluent quality. The design
models in the referenced publications provide a
more precise estimate of required treatment area.
Table 3 summarizes actual performance data for 27
FWS systems from a recent Technology Assessment
(U.S. EPA, 2000).
In theory, the performance of a wetland system can
be influenced by hydrological factors. High
TABLES SUMMARY OF
PERFORMANCE FOR 27 FWS
WETLAND SYSTEMS
Constituent Mean Influent Mean Effluent
(mg/L) (mg/L)
BOD5
TSS
TKN as N
NH3/NH4 as N
NO3 as N
TN
TP
Dissolved P
Fecal Coliforms
(#/100mL)
70
69
18
9
3
12
4
3
73,000
15
15
11
7
1
4
2
2
1320
Source: U.S. EPA, 2000.
evapotranspiration (ET) rates may increase effluent
concentrations, but may also increase the HRT in
the wetland. High precipitation rates dilute the
pollutant concentrations but also shorten the HRT
in the wetland. In most temperate areas with a
moderate climate, these influences are not critical
for performance. Hydrological aspects only need to
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be considered for extreme values of ET and
precipitation.
OPERATION AND MAINTENANCE
The routine operation and maintenance (O&M)
requirements for FWS wetlands are similar to those
for facultative lagoons. They include hydraulic and
water depth control, inlet/outlet structure cleaning,
grass mowing on berms, inspection of berm
integrity, wetland vegetation management, mosquito
and vector control (if necessary), and routine
monitoring.
The water depth in the wetland may need
adjustment on a seasonal basis or in response to
increased resistance from the accumulating plant
litter in the wetland channel. Mosquitoes may
require control, depending on local conditions and
requirements. The mosquito population in the
treatment wetland should be no greater than in
adjacent natural wetlands.
Vegetation management in FWS wetlands does not
include the routine harvest and removal of the
harvested material. Plant uptake of pollutants
represents a relatively minor pathway, so harvest
and removal on a routine basis does not provide a
significant treatment benefit. Removal of
accumulated litter may become necessary if there
are severe restrictions to flow. Generally, this will
only occur if the wetland channels have been
constructed with very high aspect ratios
(L:W > 10:1). Vegetation management may also
include wildlife management, depending on the type
of vegetation selected for the system. Animals such
as nutria and muskrats have been known to
consume all emergent vegetation in FWS
constructed wetlands.
Routine water quality monitoring is required for all
FWS systems with an NPDES permit. The permit
specifies the monitoring requirements and frequency
of monitoring. Sampling for NPDES monitoring is
usually limited to untreated wastewater and the final
system effluent. Since the wetland component is
usually preceded by some form of preliminary
treatment, the routine monitoring program does not
document wetland influent characteristics. Periodic
samples of the wetland influent should be obtained
and tested for all but the smallest systems to provide
the operator with an understanding of wetland
performance and a basis for adjustments, if
necessary.
COSTS
The major items included in the capital costs for
FWS wetlands are similar to those for lagoon
systems, including land, site investigation, site
clearing, earthwork, liner, rooting media, plants,
inlet and outlet structures, fencing, miscellaneous
piping, engineering, legal, contingencies, and
contractor's overhead and profit. The liner can be
the most expensive item. For example, a plastic
membrane liner can approach 40 percent of
construction costs. In many cases, compaction of
the in-situ native soils provides a sufficient barrier
for groundwater contamination. Table 4
TABLE 4 CAPITAL AND O&M COSTS
FOR 100,000 GAL/D FWS WETLAND
Item
Cost ($)*
Land Cost
Site
Investigation
Site Cleaning
Earthwork
Liner
Soil Planting
Media
Plants
Planting
Inlets/Outlets
Subtotal
Engineering,
legal, etc.
Total Capital
Cost
O&M Costs
($/year)
Native Soil
Liner
16,000
3,600
6,600
33,000
0
10,600
5,000
6,600
16,600
98,000
56,800
154,800
6,000
Plastic
Membrane Liner
16,000
3,600
6,600
33,000
66,000
10,600
5,000
6,600
16,600
164,000
95,100
259,100
6,000
* June 1999 Costs, ENR CCI = 6039
Source: Water Environment Federation, 2000.
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summarizes capital and O&M costs for a
hypothetical 378,500 liters per day (100,000 gallon
per day) FWS constructed wetland, required to
achieve a 2 mg/L ammonia concentration in the
effluent. Other calculation assumptions include the
following: influent NH3 = 25 mg/L; water
temperature = 20°C (68°F); water depth = 0.46
meters (1.5 ft); porosity = 0.75; treatment area =1.3
hectares (3.2 ac); and land cost = $12,355/hectare
($5,000/ac).
Table 5 compares the life cycle costs for this
wetland to the cost of a conventional sequencing
batch reactor (SBR) treatment system designed for
TABLE 5 COST COMPARISON FOR
FWS WETLAND AND CONVENTIONAL
WASTEWATER TREATMENT
Cost Item
Capital Cost ($)
O&M Cost ($)
Total Present Worth
Process
Wetland
259,000
6,000/yr
322,700
SBR
1,104,500
106,600/yr
2,233,400
Costs* ($)
Cost per 1000 gal
treated ** ($)
0.44
3.06
*Present worth factor 10.594 based on 20 years at 7
percent interest
**Daily flow rate for 365 d/yr for 20 yr, divided by 1000 gal.
Source: Water Environment Federation, 2000.
the same flow and effluent water quality.
REFERENCES
Other Related Fact Sheets
Wetlands: Subsurface Flow
EPA 832-F-00-023
September, 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Crites, R.W. and G. Tchobanoglous, 1998,
Small and Decentralized Wastewater
Management Systems, McGraw Flill Co.,
New York, NY.
2. Kadlec, R.H. and R. Knight, 1996,
Treatment Wetlands, Lewis Publishers,
Boca Raton, FL.
3. Reed, S.C.; R.W. Crites; and E.J.
Middlebrooks, 1995, Natural Systems for
Waste Management and Treatment - Second
Edition, McGraw Hill Co, New York, NY.
4. U.S. EPA, 2000, Free Water Surface
Wetlands for Wastewater Treatment: A
Technology Assessment, U.S. EPA, OWM,
Washington, D.C.
5. U.S. EPA, 2000, Design Manual
Constructed Wetlands for Municipal
Wastewater Treatment, U.S. EPA, CERI,
Cincinnati, OH.
6. Water Environment Federation, 2000,
Natural Systems for Wastewater Treatment,
MOP FD-16, WEF, Alexandria, VA.
ADDITIONAL INFORMATION
Billmayer Engineering
JJ. Billmayer
2191 Third Avenue East
Kalispell, MT 59901
City of Ouray
Carl Cockle
P.O. Box 468
Ouray, CO 81427
Joseph Ernest
Associate Engineer
P.O. Box 5015
Freemont, CA 94537-5015
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Humbolt State University
Dept. of Environmental Resource Engineering
Dr. Robert Gearheart
Arcata, CA 95522
Mississippi Gulf Coast Regional Wastewater
Authority
William Rackley
3103 Frederick Street
Pascagoula, MS 39567
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C. 20460
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