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United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-154 Nov. 1984
&ER& Project Summary
Technology Assessment of
Wetlands for Municipal
Wastewater Treatment
Henry C. Hyde, Roanne S. Ross, and Francesca Demgen
An assessment was made of the
technical and economic feasibility of
using wetland systems for municipal
wastewater treatment facilities. Various
types of natural and artificial wetland
systems-are currently being used for
wastewater treatment, including marsh-
es, shallow ponds, cypress domes,
cypress strands, and swamps. All of the
wetlands reviewed for this assessment
were either fresh or brackish systems
used to treat domestic wastewater to
various degrees. A wastewater wetland
may also provide secondary benefits
such as wildlife habitats and recreational
and educational opportunities.
The research completed on waste-
water wetland systems is extensive.
Results of bench- and pilot-scale
projects have produced a number of
full-scale systems, including artificial
and natural marshes, peatiands, swamps,
marsh/pond/meadows, and bogs.
A wetland system can provide primary,
secondary, or advanced wastewater
treatment. The need for open land or
existing wetland areas makes this
technology most applicable in areas
outside of congested urban centers.
Capital costs, operational costs, and
energy requirements are significantly
less than those for conventional treat-
ment alternatives. Land availability and
proximity to the wastewater source are
the major variables affecting cost.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the tech-
nology assessment that is fully docu-
mented in a spearate report of the same
title (see Project Report ordering
information at back).
Introduction
The use of wetlands for wastewater
treatment has both historical and ecologi-
cal bases. Riverine wetlands often occur
where a stream enters a river, lake, or bay.
Here the velocity of the tributary water is
reduced, and the silt carried by the stream
is deposited in the marsh, swamp, or
peatland. Vegetation develops in this
fertile, shallow area and acts as a filter for
water entering the marsh and larger
bodies of water. The wetland is a sink for
nutrients and organic debris. The tributary
waters or wastewater continually supply
the wetland with nutrients, creating a
highly productive system that can also
support a wide variety of wildlife.
Wetlands serve as important rearing
grounds for animals and fish that live m
the streams above and in the rivers, lakes,
and bays below. The wetlands of this
country's great rivers have been using
this living filter mechanism to treat
domestic wastewaters for many years,
with little recognition of their role in
meeting the nation's clean water goals.
A wetland may be defined as an area
that is covered periodically or perma-
nently with water or varying depths and
that supports hydrophilic vegetation.
Wetlands may be fresh or saline, and
some are associated with larger bodies
of water. Table 1 describes various types
of natural and artificial wetlands.
The primary components of a wetland
treatment system are influent wastewater
and a shallow, mostly vegetated basin
with or without a point source discharge.
The submerged and emergent plants.
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their associated microorganisms, and
the wetland soils are responsible for most
of the treatment effected by the wetland.
Wastewater wetland systems can provide
primary, secondary, or advanced treat-
ment.
Technology Assessment
Procedure
An extensive literature review was
conducted on research findings and
investigations, including 15 full-scale
projects. Many volumes of data from
operating wetland systems were con-
densed into a summary format to assess
the technical and economic viability of
such systems for municipal wastewater
treatment.
Treatment Mechanisms
Wastewater treatment in a wetland
system is accomplished through bio-
logical, physical, and chemical reactions.
Though the processes involved are fairly
well understood, questions remain as to
the roles of individual reactions in the
treatment process. To further complicate
any attempt at modeling the treatment
process, the significance assigned to
each treatment reaction varies with
specific project conditions. The major
components of the wetland system that
perform the wastewater treatment are
algae, macrophytes (larger, rooted plants),
bacteria, zooplankton, and the substrate
(bottom soils).
A wetland environment can be used
to accomplish various levels of treatment.
The desired level of influent treatment will
determine the flow path. Comminuted,
aerated, raw sewage can be treated
through a multi-cellular artificial system.
The water progresses through a series of
plots until the desired quality is reached.
Primary or secondary effluent can also
serve as wetland influent. Process flow
diagrams for natural and artificial wetlands
appear in Figure 1.
Performance
The literature review indicated much
confusion in the reporting of performance
data for natural and artificial wetland
systems used to treat municipal waste-
water. Also, the basis on which perform-
ance data are reported is not standard-
ized. For example, some articles report
performance data as a function of time,
whereas others report them as a function
of distance. Usually, no indication is
given of how time and distance are
interrelated. Furthermore, the data for
most of the natural systems are extremely
site specific and cannot be generalized.
In spite of the above limitations,
reported removal ranges for constituents
of concern in wastewater appear in Table
2. A review of the data indicate that the
performance of wetlands with respect to
most wastewater constituents is not well
defined.
Process Advantages and
Limitations
Like a 11 wastewater treatment systems,
wetlands have specific advantages and
limitations. Their important advantages
are described as follows:
• Performance of the system depends
heavily on the proliferation of
selected plants that exist in many
areas of the country, thus allowing
use of wetlands nationwide.
• Operation and maintenance costs
are generally well below those of
conventional treatment systems.
• In addition to wastewater treatment,
wetlands can provide wildlife habitats,
Table 1. Types of Wetlands
Classification and Type
open spaces, recreational areas,
educational opportunities, and stream
flow augmentation.
• Treated effluents from the wetland
can be available for reuse and are
compatible with projects involving
aquaculture and silviculture.
• Many categories of pollutants are
removed within a single system.
• The treatment mechanisms (parti-
cularly the soil and vegetation) are
relatively stable and allow the
system to withstand shock loadings.
Important limitations of wetland systems
are as follows:
• The amount of land required by
wetlands may restrict their use in
congested urban areas.
• Treatment process efficiencies are
not completely defined and thus
make precise design criteria difficult
to establish.
• The wetland plant species vary in
their requirements and may be
Description
Natural:
Riverine
Lacustrine
Palustrine
Tidal
Artificial:
Marshes
Ponds
Marsh/Pond
Trench
Wetlands located adjacent to rivers or
streams (e.g., marshes, shallow ponds, or
wet meadows).
Wetlands adjacent to or near lakes (e.g.,
marshes, shallow ponds, or wet meadows).
Wetlands isolated from open bodies of water
(e.g., bogs and cypress domes).
Wetlands subject to tidal action with
various flooding regimes.
Shallow depths with emergent plants such as
cattails and bulrushes covering nearly the
entire surface. Basin may be sealed.
Somewhat deeper than marshes, with an open
water surface. May contain submerged plants
and plants on the banks.
A combination of the two preceding components.
Narrow ditches, lined or unlined, that are
planted with vegetation, usually bulrushes.
Table 2. Reported Ranges of Contaminant Removal Efficiency
Removal Efficiency (%)
Constituent
Suspended solids
BODs
COD
Nitrogen (as N)
Phosphorus (as P)
Refractory organics
Heavy metals
Natural
Primary
Treatment
90
84
Wetland
Secondary
Treatment
30-90
70-96
50-80
40-90
10-61
20-90
Artificial
Primary
Treatment
50-90
50-90
30-98
20-90
Wetland
Secondary
Treatment
0-48
0-60
28-34
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Evapotranspiration
\ \
Q
Influent
Effluent
Possible Influents
1. Aerated Raw Sewage
2. Primary Effluent
3. Secondary Effluent
Percolation
Wetland Vegetation Types
1. Submergents
2. Emergents
3. Floating
4. Phreatophytes
A. Natural Wetland
Effluent Disposal
1. Non-Point Discharge
2. Percolation
3. Point Discharge
Evapotranspiration
t f
Evapotranspiration
A
I I
Percolation
Possible Influents
1. Aerated Raw Sewage
2. Primary Effluent
3. Secondary Effluent'
Wetland Vegetation Types Effluent Variations
1. Submergents
2. Emergents
3. Floating
4. Phreatophytes
Flow Pattern Variations
1. Smaller multiple cells.
2. Cells subdivided by internal levees.
3. Islands to direct flow
B. Artificial Wetland
Figure 1. Process flow diagrams for natural and artificial wetlands.
1. Point Discharge
2. Non-Point Discharge
3. Percolation
limited by physical conditions such
as sunshine, temperature, and
water depth.
The possibility of vector breeding and
pathogen transmission are signifi-
cant public health concerns.
Significant problems may exist in
obtaining discharge permits from
water quality enforcement agencies.
Design Considerations
Specific design criteria for developing
wetland wastewater treatment systems
are limited compared with those for
conventional wastewater treatment
processes. In wetland systems, removal
efficiencies are a function of naturally
occurring parameters that cannot be
easily controlled, such as temperature.
sunlight, and native plant species. Thus
the direct control exercised at a conven-
tional treatment plant is impossible with
a wetland system. This difference is a
major reason that the design approach for
wetlands is different than for conven-
tional systems.
Several basic steps must be followed
when developing a design for a wetland
treatment system:
1. Base goals for a wetland system on
waste discharge requirements and
potential for reuse or concurrent
beneficial use (e.g., recreation).
2. Evaluate the availability and suit-
ability of existing wetlands.
3. Analyze local conditions with regard
to water budgeting, water application
method, and composition of soils.
4. Determine the hydraulic loading and
application method.
5. Select the vegetation.
6. Design levees and containment
structures.
7. Design distribution system to insure
good circulation.
Each situation is unique and requires a
site-specific evaluation, including a pilot
test before the full-scale design is
developed. Design objectives, environ-
mental setting, native plant species,
climatic conditions, etc., are important
considerations for a natural wetland
system, and they cannot be generalized
as can most conventional systems.
Artificial wetland systems are more
suitable for generalized design criteria.
Such criteria have been published in the
literature based on bench-scale, pilot-
scale, and full-scale research. Examples
of design criteria for artificial wetlands
are listed in Table 3.
Operational Considerations
Wetlands treatment systems have low
operation and maintenance require-
ments. Typical tasks include the following:
Daily
Periodic
Water quality monitor-
ing
Flow records and
meter upkeep
Visual inspection of
flow structures (weirs,
flumes, pipes, etc.!
Erosion control
Levee repair
Pump Maintenance
Distribution line
maintenance
Vehicle main-
tenance
One of the important questions regard-
ing wetlands operation and maintenance
is vegetation control and harvesting. Of
the 13 full-scale projects investigated.
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Table 3. Design Criteria for Artificial Wetlands Using Primary or Secondary Effluents
Design Criterion or Parameter
Type of
System
Trench
Marsh
Marsh/Pond
Marsh
Pond
Lined
trench
Detention Depth of
time, days flow, m (ft)
Range Typical Range Typical
6-15
8-20
4-12
6-12
4-20
10 0.3-0.5
(1.0-1.5)*
10 0.15-0.6
(0. 5-2.9 '/
6 0.15-0.6
(0.5-2.0)*
8 0.5-1.0
(1.5-3.0)*
6 —
0.4
(1.3)*
0.25
(0.75)'
0.25
1 (.075)*
0.6
(2.0)*
—
Loading rate,
ha/ 1000 m3 /day
(acres/mgd)
Range Typical
1.2-3.1
(11-29)*
1.2-12
(11-112)"
0.65-8.2
(6.1-76.7)*
1.2-2.7
(11-25)*
0.16-0.49
(1.5-4.6)*
2.5
(23)*
4.1
(38)'
2.5
(23)*
1.4
(13)*
0.20
(1.9)*
*Systems with reeds or rushes
"Systems with reeds, rushes, or others
none reported major vegetation harvest-
ing. A major reason for harvesting
vegetation is to resolve problems such as
poor circulation that yields inefficient
treatment, mosquito breeding, nuisance
odors from stagnation, etc. Nuisance
odors can be avoided by maintaining
aerobic conditions through circulation of
water within the system.
Vector production can be minimized
through proper management techniques.
A successful method for controlling
mosquito larvae has been the introduction
of Gambusia spp. (mosquito fish). Ade-
quate circulation is also important to
minimize stagnant conditions.
Operation and maintenance tasks can
be minimized through proper design, and
they can generally be accomplished by a
well trained operator. The time required
for major tasks such as levee repair or
vegetation harvesting varies with the
season.
A major advantage of a wetland system
is the low operational energy require-
ments. If the topography of the site allows
gravity flow from the treatment plant,
operational energy demands will be
minimal. If the pumping requirements are
significant, however, energy consumption
can approach that of a conventional
treatment system.
Conclusions
Wetland systems are most suitable for
communities with wastewater flows of
less than 7,570 mVday (2 mgd) that
require secondary, advanced secondary,
or advanced treatment.
Climate is not a limiting factor, since
wetlands exist in most parts of the
continental United States. The specific
wetland type and the plant species are
selected based on the local climate and
environmental setting. For example,
Florida has extensive areas of swamps,
cypress domes, and strands, whereas
Wisconsin, Minnesota, and Michigan
have numerous bogs and peatlands.
Wetland systems are land-intensive
and thus are not generally applicable to
major urban areas with high land costs.
Wetland systems are cost-effective,
energy efficient, and capable of reliably
meeting secondary and advanced waste-
water treatment requirements in con-
junction with pretreatment systems.
Natural treatment systems such as
wetlands have the benefit of low operation
and maintenance requirements, but
control over the process is limited. This
drawback may be reduced as design
parameters become more established
through additional experience with
wetland systems.
A community may justify the risk of
limited process control by taking into
account the favorable aspects of a
wetland system and the known risks
involved with conventional plants. Com-
pared with a conventional plant, a
wetland system has considerable lower
annual costs and energy requirements,
which are major incentives for a small
community.
The use and reliability of wetland
systems for municipal wastewater treat-
ment should increase as additional
successful experience is gained in the
future.
Recommendations
The following recommendations con-
cern implementation of wastewater
treatment technology for wetlands and
are based on the foregoing conclusions.
1. Construction and design of artificial
wetland basins. Various methods of
basin construction need to be
evaluated to determine the most
cost-effective method. The design of
the basin should minimize energy
requirements and facilitate main-
tenance.
2. Engineering design criteria. Research
projects need to test design criteria
(surface and organic loadings) for
both artificial and natural wetlands.
This information is necessary for
design of the different wetland types
in various geographical locations
(warm and cold climates).
3. Impacts on natural wetlands. Much
controversy exists over the impacts
of introducing wastewater into a
natural wetland. Research is needed
to determine the significance of
these impacts.
4. Labor requirements. Limited in-
formation is available regarding
labor requirements for operation
and maintenance. Operating facil-
ities need to document actual labor
requirements so that other agencies
can accurately estimate labor de-
mands and operational procedures.
5. Removal efficiencies. Limited data
are presently published on the
removal efficiency of the following
parameters for artificial wetlands:
Total solids, dissolved solids, sus-
pended solids, total organic carbon
(TOC), chemical oxygen demand
(COD), nitrogen, heavy metals,
coliforms, and pathogens. For nat-
ural wetlands, limited published
data exist on the removal efficiencies
for refractory organics, pathogens,
and coliforms.
6. Costs. Existing documentation of
cost is poor. Accurate accounts of
construction and operation and
maintenance costs need to be kept
and reported by operating facilities.
7. Information transfer. Publication of
successful project information in
widely read professional publications
is needed to inform wastewater
agencies of opportunities in wetland
wastewater treatment. Guidance
documents published by the U.S.
Environmental Protection Agency
(EPA) for distribution to state and
regional regulatory and funding
agencies and wastewater manage-
ment agencies are needed to promote
the understanding of wetland treat-
ment technology.
8. Regulations. The design of any
wetland wastewater treatment
system must be consistent with all
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appropriate state and federal regu-
lations and with guidelines from the
EPA Construction Grants Program.
The full report was submitted in
fulfillment of Contract No. 68-03-3106 by
WWI Consulting Engineering under the
sponsorship of the U.S. Environmental
Protection Agency.
•USGPO: 1984-559-111-10731
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Henry C. Hyde is currently with Henry Hyde & Associates, Sausalito. CA 94965;
Roanne S. Ross is with Waste and Water International Consulting Engineers.
Emeryville. CA 94608; and Francesco Demgen is with Demgen A quatic Biology,
Valleyjo. CA.
Jon H. Bender is the EPA Project Officer (see below).
The complete report, entitled "Technology Assessment of Wetlands for Municipal
Wastewater Treatment,"(Order No. PB 85-106 896; Cost: $13.OO, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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