Analyze how concepts were applied to problem solving questions by writing a response /assessment for each part to demonstrate your understanding of such.
A HANDBOOK OF
CONSTRUCTED WETLANDS
a guide to creating wetlands for:
AGRICULTURAL WASTEWATER
DOMESTIC WASTEWATER
COAL MINE DRAINAGE
STORMWATER
in the Mid-Atlantic Region
Volume
1
ACKNOWLEDGMENTS
Many people contributed to this Handbook. An interagency Core Group provided the initial impetus for the Handbook, and later provided
guidance and technical input during its preparation. The Core Group comprised:
Carl DuPoldt, USDA – NRCS. Chester, PA
Robert Edwards, Susquehanna River Basin Commission,
Harrisburg, PA
Lamonte Garber, Chesapeake Bay Foundation, Harrisburg. PA
Barry Isaacs, USDA – NRCS, Harrisburg, PA
Jeffrey Lapp. EPA, Philadelphia, PA
Timothy Murphy, USDA – NRCS, Harrisburg, PA
Glenn Rider, Pennsylvania Department of Environmental
Resources, Harrisburg. PA
Melanie Sayers, Pennsylvania Department of Agriculture, Harrisburg, PA
Fred Suffian, USDA – NRCS Philadelphia, PA
Charles Takita, Susquehanna River Basin Commission, Harrisburg, PA
Harold Webster, Penn State University, DuBois, PA.
Many experts on constructed wetlands contributed by providing information and by reviewing and commenting on the Handbook. These
Individuals included:
Robert Bastian. EPA .WashinSton, DC
Robert Knight, CH2M HILL, Gainesville, FL
Fran Koch, Pennsylvania Department of
William Boyd, USDA – NRCS. Lincoln, NE
Environmental Resources, Harrisburg, PA
Robert Brooks, Penn State University,
University Park, PA
Eric McCleary, Damariscotta, Clarion, PA
Donald Brown, EPA, Cincinnati, OH
Gerald Moshiri, Center for Wetlands and
Eco-Technology Application, Gulf Breeze,
Dana Chapman, USDA – NRCS, Auburn, NY
FL
Tracy Davenport, USDA -NRCS, Annapolis,
John
Murtha, Pennsylvania Department of
MD
Environmental Resources, Harrisburg. PA
Paul DuBowy, Texas A & M University,
Robert Myers, USDA – NRCS, Syracuse, NY
College Station, TX
Kurt Neumiller, EPA, Annapolis, MD
Michelle Girts, CH2M HILL, Portland, OR
Richard Reaves, Purdue University, West
Robert Hedin, Hedin Environmental,
Lafayette, IN
Sewickley, PA
William
Sanville, EPA, Cincinnati, OH
William Hellier. Pennsylvania Department of
Environmental Resources, Hawk Run, PA
Dennis Sievers, University of Missouri,
Columbia, MO
Robert Kadlec, Wetland Management
Services, Chelsea, MI
Earl Shaver, Delaware Department of
Natural Resources and Environmental
Douglas Kepler, Damariscotta. Clarion, PA
Control, Dover, DE
Robert Kleinmann, US Bureau of Mines,
Pittsburgh, PA
Daniel Seibert, USDA – NRCS, Somerset, PA
Jeffrey Skousen, West Virginia University,
Morgantown. WV
Peter Slack, Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Dennis Verdi, USDA – NRCS, Amherst, MA
Thomas Walski, Wilkes University, WilkesBarre, PA
Robert Wengryznek, USDA – NRCS, Orono,
ME
Alfred Whitehouse, Office of Surface
Mining. Pittsburgh, PA
Christopher Zabawa, EPA, Washington, DC.
This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental Protection
Agency-Region III, in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided with
nonpoint source management program funds under Section 319 of the Federal Clean Waler Act.
The findings. conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA – NRCS,
EPA – Region III, the Commonwealth of Pennsylvania, or any other state in the northeastern United States concerning the use of constructed
wetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specific
programs and restrictions in this regard.
VOLUME 1
TABLE OF CONTENTS
CHAPTER 1. INTRODUCTION . . . . . . . . . . . . . . . . . . …………………………………………._…………………………………………., .5
CHAPTER 2. CONSTRUCTED WETLANDS AS ECOSYSTEMS ………………………………………………………………. 7
What Are Wetlands? ……… . . …………………………………………………………………………………………………. 7
Wetland Functions and Values.. ……………………………………………………………. . ……………………………. 7
Components of Constructed Wetlands …………………………………………………………………………………..
8
Water. …………………………………………………………………………………………………. ……………………….
Substrates, Sediments, and Litter ……………………………………………………………………………………
Vegetation …………………………………………………………………………………………………………………….
Microorganisms ……………………………………………………………………………………………………………..
Animals.. …………………………………………………… ………………………………………………………………..
8
8
8
9
9
Aesthetics and Landscape Enhancement ………………………………………………………………………. 10
CHAPTER 3. CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS ……………………………………………….. 11
How Wetlands Improve Water Quality …………………………………………………………………………….
11
Advantages of Constructed Wetlands.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Limitations of Constructed Wetlands ………………………………………………………………………. 11
Types of Constructed Wetlands ………………………………………………………………………………………….. 12
13
Surface Flow Wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subsurface Flow Wetland ……………………………………………………………………………………………
Hybrid Systems …………………………………………………………………………………………………………..
Winter and Summer Operation …………………………………………………………………………………………..
Creation of Hazard …………………………………………………………………………………………………………….
Change and Resilience …………. . …………………………………………………………………………………………..
13
13
13
14
14
CHAPTER 4. GENERAL DESIGN OF CONSTRUCTED WETLANDS ………………………………………………………. 17
Design Considerations ……………………………………………………………………………………………………….. 17
Planning ………………………………………………………………………………………………………………………….. 17
Site Selection …. ……………………………………………………………………………………………………………….. 18
Land Use and Access.. ……………………………………… ………………………………………………………… 18
Land Availability ………………………………………………………………………………………………………… 18
Topography ………………………………………………………………………………………………………………… 19
Environmental Resources.. ………………………………………………. …………………………………………. 19
Permits and Regulations ……………………………………………………………………………………………………. 19
Structures ……………………………………………………. ……………………………………………………………….. 20
20
Cells …………………………………………………………………………………………………………………………..
20
Liners …………………………………………………………………………………………………………………………
Flow Control Structures ………………………………………………………………………………………………. 20
Inlets …………………………………………………………………………………………………………….. 21
Outlets ………………………………………. …………………………………………………………………………. 22
System Lifetimes ……………………………………………………………………………………………………
23
Chapter 5. HYDROLOGY …………………………………………………………………………………………………………………….
Climate and Weather ………………………………………………………………………………………………………….
25
25
Hydroperiod ………………………………………………………………… ……… …………………………………………… 25
Hydraulic Residence Time …………………………………………………………………………………………………
Hydraulic Loading Rate ……………………………………………………………………………………………………..
Groundwater Exchange ………………………………………………………………………………………………………
Evapotranspiration …………………………………………………………………………………………………………….
26
26
26
26
Water Balance …………………………………………………………………………………………………………………..
26
Chapter 6. SUBSTRATES.. …………….. . ………………………………………………………………………………………………….
Soil …………………………………………………………………………………………………………………………………..
Sand and Gravel ………………………………………………………………………………………………………………..
Organic Material …………. …………………………………………………………………………………………………..
29
29
39
Chapter 7, VEGETATION …………………………… . ……………………………………………………………………………………..
Selecting plants …………………………………………………. …………………………………………………………….
Surface Flow Wetlands …………………………………………………………………………………………………
Subsurface Flow W e t l a n d s ……………………………………………………….. . …………………………………
31
31
31
34
34
34
34
34
35
35
36
30
Sources of Plants …………………………………………………………………………………………… . …………………
Seeds …………………………………………….. . ………………………………………………………………………….
Wetland Soil …. ……………………………………………………………………………………………………………
Rhizomes,Tubers. and Entire Plants ………………………………………………………………………………
When To Plant.. …………………………………………………………………………………………………………………
Site Preparation ………………………………………………………………………………………………………………..
How To Plant …………………………………………………………………………………………………………………….
Surface Flow Wetlands.. ……………………………………. . ……………………………………………………….. 36
Subsurface Flow Wetlands ………………………………………………………….. . . ……………………………. 36
Establishing and Maintaining Vegetation …………………………………………………………………………….. 36
Chapter 8. CONSTRUCTION ………………………………………………………………………………………………………………
Construction Plans …………………………………………………………………………………………………………….
Pre-Construction Activities ………………………………………………………………………………………………..
Construction Activities ………………………………………………………………. . …………………………………..
Inspection, Startup, and Testing …………………………………………………………………………………………..
39
39
39
39
Chapter 9: OPERATION, MAINTENANCE, AND MONITORING ……………………………………………………………
Operation and Maintenance ………………………………………………………………………………………………..
Operation and Maintenance Plan ……………………………………………………… .:. ……………………….
Hydrology …………………………………………………………………………………………………………………..
Structures ……………………………………………………………………………………………………………………
Vegetation …………………………………………………………………………………………………………………..
Muskrats …………………………………………………………………………………………………………………….
Mosquitoes ………………………………………………………………………………………………………………….
Monitoring ……………………………………………………………………………………………………………………….
Monitoring Plan.. …………………………………………………………………………………………………………
Monitoring for Discharge Compliance.. ………………………………………………………………………….
Monitoring for System Performance ………………………………………………………………………………
Monitoring for Wetland Health ……………………………………………………………………………………..
41
41
40
41
41
41
42
42
42
43
43
43
43
44
REFERENCES .I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . … . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . …………………….
45
PHOTOGRAPHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . …………………….. 47
LIST OF TABLES
..
Table 1. Emergent plants for constructed wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
LIST OF FIGURES
Figure 1. Surface flow and subsurface flow constructed wetlands ………………………………………………………… .12
Figure 2. Inlet and outlet designs ……………………………………………………………………………………………………….. 21
Figure 3. Influent splitter box ……………………………………………………………………………………………………………… 22
Natural processes have always cleansed
water as it flowed through rivers, lakes, streams,
and wetlands. In the last several decades,
systems have been constructed to use some of
these processes for water quality improvement.
Constructed wetlands are now used to improve
the quality of point and nonpoint sources of
water pollution, including stormwater runoff,
domestic wastewater, agricultural wastewater,
and coal mine drainage. Constructed wetlands
are also being used to treat petroleum refinery
wastes, compost and landfill leachates, fish
pond discharges, and pretreated industrial
wastewaters, such as those from pulp and paper
mills, textile mills, and seafood processing. For
some wastewaters, constructed wetlands are the
sole treatment; for others, they are one component in a sequence of treatment processes.
One of the most common applications of
constructed wetlands has been the treatment of
primary or secondary domestic sewage effluent.
Constructed wetland systems modelled after
those for domestic wastewater are being used to
treat the high organic loads associated with
agriculture. A large number of wetlands have
been constructed to treat drainage from active
and abandoned coal mines and more than 500
such systems are operating in Appalachia alone.
The use of constructed wetlands to control
stormwater flows and quality is a recent application of the technology and the number of such
systems is increasing rapidly.
The treatment of wastewater or stormwater
by constructed wetlands can be a low-cost, lowenergy process requiring minimal operational
attention. As a result of both extensive research
and practical application, insight is being gained
into the design, performance, operation, and
maintenance of constructed wetlands for water
quality improvement. Constructed wetlands can
be sturdy, effective systems. However, to be
effective, they must be carefully designed,
constructed, operated, and maintained.
This Handbook has been prepared as a
general guide to the design, construction, operation, and maintenance of constructed wetlands
for the treatment of domestic wastewater, agricultural wastewater, coal mine drainage, and
stormwater runoff in the mid-Atlantic region,
The Handbook is not a design manual. The use
of constructed wetlands to improve water quality
is a developing technology. Much is not yet
understood, and questions remain regarding the
optimal design of wetland systems and their
longevity. As our experience with these systems
increases, the information offered here will be
replaced by more refined information. The
Handbook should be used with this clearly in
mind.
The Handbook is divided into five volumes.
This, the first, provides information common to
all types of constructed wetlands for wastewater
and runoff. It is to be used in conjunction with
an accompanying volume that provides information specific to a particular type of wastewater or
runoff. The other volumes in the series are
Volume 2: Domestic Wastewater, Volume 3:
Agricultural Wastewater, Volume 4: Coal Mine
Drainage, and Volume 5: Stormwater Runoff.
While constructed wetlands are being used to
treat other kinds of wastewater, such as industrial wastewaters, a discussion of these applications is beyond the scope of this Handbook.
However, the information presented here may be
useful in developing other applications.
A number of conferences on constructed
wetlands have been held recently. The proceedings of these conferences include experimental
and operational data from wetland systems built
to treat a number of different kinds of wastewaters and runoff, and present detailed discussions
of process kinetics and system design. Proceedings from three well-known conferences are:
Moshiri, G. A. (ed.) 1993. Constructed Wetlands
for Water Quality Improvement. CRC Press, Boca
Raton, FL. 632 pp.
V O L U M E 1: G E N E R A L C O N S I D E R A T I O N S
5
Cooper, P. F., and B. C. Findlater (eds.) 1990.
Constructed Wetlands in Water Pollution Control.
Proceedings of the International Conference on
the Use of Constructed Wetlands in Water Pollution Control. Cambridge, UK, 24-28 September.
WRc, Swindon, Wiltshire, UK. 605 pp.
Hammer, D. A. (ed.) 1989. Constructed Wetlands
for Wastewater Treatment: Municipal, Industrial
and Agricultural. Lewis Publishers. Chelsea, MI.
831 pp.
Conferences and published’ information continue
to become available as more constructed wetland
systems are built and monitored.
V
O L U M E
1 :
GE
N E R A L
C
O N S I D E R A T I O N
CHAPTER 2
CONSTRUCTED WETLANDS AS ECOSYSTEMS
Constructed wetlands for water treatment
are complex, integrated systems of water,
plants, animals, microorganisms, and the
environment. While wetlands are generally
reliable, self-adjusting systems, an understanding of how natural wetlands are structured and
how they function greatly increases the likelihood of successfully constructing a treatment
wetland. This chapter provides an overview of
the major components of wetland ecosystems
and of the most important processes that affect
water treatment.
WHAT ARE WETLANDS?
Wetlands are transitional areas between land
and water. The boundaries between wetlands
and uplands or deep water are therefore not
always distinct. The term “wetlands” encompasses a broad range of wet environments,
including marshes, bogs, swamps, wet meadows,
tidal wetlands, floodplains, and ribbon (riparian)
wetlands along stream channels.
All wetlands – natural or constructed, freshwater or salt – have one characteristic in common: the presence of surface or near-surface
water, at least periodically. In most wetlands,
hydrologic conditions are such that the substrate
is saturated long enough during the growing
season to create oxygen-poor conditions in the
substrate. The lack of oxygen creates reducing.
(oxygen-poor) conditions within the substrate
and limits the vegetation to those species that are
adapted to low-oxygen environments.
The hydrology of wetlands is generally one
of slow flows and either shallow waters or
saturated substrates. The slow flows and shallow water depths allow sediments to settle as the
water passes through the wetland. The slow
flows also provide prolonged contact times
between the water and the surfaces within the
wetland. The complex mass of organic and
VOLUME 1
inorganic materials and the diverse opportunities
for gas/water interchanges foster a diverse community of microorganisms that break down or transform a wide variety of substances.
Most wetlands support a dense growth of
vascular plants adapted to saturated conditions.
This vegetation slows the water, creates microenvironments within the water column, and provides
attachment sites for the microbial community.
The litter that accumulates as plants die back in
the fall creates additional material and exchange
sites, and provides a source of carbon, nitrogen,
and phosphorous to fuel microbial processes.
WETLAND FUNCTIONS
AND VALUES
Wetlands provide a number of functions and
values. (Wetland functions are the inherent
processes occurring in wetlands; wetland values
are the attributes of wetlands that society perceives as beneficial.) While not all wetlands
provide all functions and values, most wetlands
provide several. Under appropriate circumstances.
constructed wetlands can provide:
water quality improvement
flood storage and the desynchronization of storm
rainfall and surface runoff
cycling of nutrients and other materials
habitat for fish- and wildlife
passive recreation, such as bird watching
and photography
active recreation, such as hunting
education and research
aesthetics and landscape enhancemerit.’
G ENERAL C O N S I D E R A T I O N S
7
COMPONENTS OF
CONSTRUCTED WETLANDS
A
constructed wetland consists of a properlydesigned basin that contains water, a substrate, and,
most commonly, vascular plants. These components
can be manipulated in constructing a wetland. Other
important components of wetlands, such as the
communities of microbes and aquatic invertebrates,
develop naturally.
W
ATER
Wetlands are likely to form where landforms
direct surface water to shallow basins and where a
relatively impermeable subsurface layer prevents the
surface water from seeping into the ground. These
conditions can be created to construct a wetland. A
wetland can be built almost anywhere in the landscape by shaping the land surface to collect surface
water and by sealing the basin to retain the water.
Hydrology is the most important design factor in
constructed wetlands because it links all of the
functions in a wetland and because it is often the
primary factor in the success or failure of a constructed wetland. While the hydrology of constructed wetlands is not greatly different than that of
other surface and near-surface waters, it does differ in
several important respects:
l
l
l
small changes in hydrology can have fairly significant effects on a wetland and its treatment effectiveness
because of the large surface area of the water and
its shallow depth, a wetland system interacts
strongly with the atmosphere through rainfall and
evapotranspiration (the combined loss of water by
evaporation from the water surface and loss
through transpiration by plants)
the density of vegetation of a wetland strongly affects
its hydrology, first, by obstructing flow paths as the
water finds its sinuous way through the network of
stems, leaves, roots, and rhizomes and, second, by
blocking exposure to wind and sun.
S U B S T R A T E S, SE D I M E N T S,
AND
LITTER
Substrates used to construct wetlands include soil, sand, gravel, rock, and organic
materials such as compost. Sediments and litter
then accumulate in the wetland because of the
low water velocities and high productivity
typical of wetlands. The substrates, sediments,
and litter are important for several reasons:
l
l
they support many of the living organisms in
wetlands
substrate permeability affects the movement of
water through the wetland
many chemical and biological (especially
microbial) transformations take place within
the substrates
l
l
l
substrates provide storage for many
contaminants
the accumulation of litter increases the amount
of organic matter in the wetland. Organic matter
provides sites for material exchange and microbial attachment, and is a source of carbon, the
energy source that drives some of the important
biological reactions in wetlands.
The physical and chemical characteristics of
soils and other substrates are altered when they
are flooded. In a saturated substrate, water
replaces the atmospheric gases in the pore
spaces and microbial metabolism consumes the
available oxygen. Since oxygen is consumed
more rapidly than it. can be replaced by diffusion
from the atmosphere, substrates become anoxic
(without oxygen). This reducing environment is
important in the removal of pollutants such as
nitrogen and metals.
V
E
G
ET A T I O N
Both vascular plants (the higher plants) and
non-vascular plants (algae) are important in
constructed wetlands. Photosynthesis by algae
increases the dissolved oxygen content of the
water which in turn affects nutrient and metal
Constructed wetlands attract waterfowl and
wading birds, including mallards, green-winged
teal, wood ducks, moorhens, green and great
blue herons, and bitterns. Snipe, red-winged
blackbirds, marsh wrens, bank swallows, redtailed hawks, and Northern harriers feed and/or
nest in wetlands.
A ESTHETICS
AND
L ANDSCAPE
ENHANCEMENT
While wetlands are primarily treatment
systems, they provide intangible benefits by
increasing the aesthetics of the site and enhancing the landscape. Visually, wetlands are unusually rich environments. By introducing the
element of water to the landscape, constructed
wetlands, as much as natural wetlands. add
diversity to the landscape. The complexity of
shape, color, size, and interspersion of plants,
and the variety in the sweep and curve of the
edges of landforms all add to the aesthetic
quality of the wetlands. Constructed wetlands
can be built with curving shapes that follow the
natural contours of the site, and some wetlands
for water treatment are’ indistinguishable, at first
glance, from natural wetlands.
V O L U M E 1: G E N E R A L C O N S I D E R A T I O N S
reactions. Vascular plants contribute to the
treatment of wastewater and runoff in a
number of ways:
they stabilize substrates and limit
channelized flow
l
they slow water velocities, allowing suspended materials to settle
they take up carbon, nutrients, and trace
elements and incorporate them into plant
tissues
they transfer gases between the atmosphere
and the sediments
leakage of oxygen from subsurface plant
structures creates oxygenated microsites
within the substrate
their stem and root systems provide sites
for microbial attachment
theycreate litter when they die and decay.
Constructed wetlands are usually planted
with emergent vegetation (non-woody plants
that grow with their roots in the substrate
and their stems and leaves emerging from
the water surface). Common emergents
used in constructed wetlands include
bulrushes, cattails, reeds, and a number of
broad-leaved species.
M
ICROORGANISMS
A fundamental characteristic of wetlands is
that their functions are largely regulated by
microorganisms and their metabolism (Wetzel
1993). Microorganisms include bacteria, yeasts,
fungi, protozoa, rind algae. The microbial
biomass is a major sink for organic carbon and
many nutrients. Microbial activity:
l
l
transforms a great number of organic and
inorganic substances into innocuous or
insoluble substances
alters the reduction/oxidation (redox) conditions of the substrate and thus affects the
processing capacity of the wetland
is involved in the recycling of nutrients.
Some microbial transformations are aerobic
(that is, they require free oxygen) while others
are anaerobic (they take place in the absence of
free oxygen). Many bacterial species are facultative anaerobes, that is, they are capable of functioning under both aerobic and anaerobic conditions in response to changing environmental
conditions.
Microbial populations adjust to changes in
the water delivered to them. Populations of
microbes can expand quickly when presented
with suitable energy-containing materials. When
environmental conditions are no longer suitable,
many microorganisms become dormant and can
remain dormant for years (Hilton 1993).
The microbial community of a constructed
wetland can be affected by toxic substances,
such as pesticides and heavy metals, and care
must be taken to prevent such chemicals from
being introduced at damaging concentrations.
A
N
I
M
A
L
S
Constructed wetlands provide habitat for a
rich diversity of invertebrates and vertebrates.
Invertebrate animals, such as insects and worms.
contribute to the treatment process by fragmenting detritus and consuming organic matter. The
larvae of many insects are aquatic and consume
significant amounts of material during their
larval stages, which may last for several years.
Invertebrates also fill a number of ecological
roles; for instance, dragonfly nymphs are important predators of mosquito larvae.
Although invertebrates are the most
important animals as far as water quality improvement is concerned, constructed wetlands
also attract a variety of amphibians, turtles,
birds, and mammals.
VOLUME 1: GENERAL C ONSIDERATIONS
9
CHAPTER 3
CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS
A constructed wetland is a shallow basin
filled with some sort of substrate, usually soil or
gravel, and planted with vegetation tolerant of
saturated conditions. Water is introduced at one
end and flows over the surface or through the
substrate, and is discharged at the other end
through a weir or other structure which controls
the depth of the water in the wetland.
HOW WETLANDS IMPROVE
WATER QUALITY
A wetland is a complex assemblage of water,
substrate, plants (vascular and algae), litter
(primarily fallen plant material), invertebrates
(mostly insect larvae and worms). and an array
of microorganisms (most importantly bacteria).
The mechanisms that are available to improve
water quality are therefore numerous and often
interrelated. These mechanisms include:
ADVANTAGES OF
CONSTRUCTED WETLANDS
Constructed wetlands are a cost-effective and
technically feasible approach to treating wastewater and runoff for several reasons:
l
. operation and maintenance expenses (energy
and supplies) are low
l
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l
l
l
l
l
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chemical transformation
adsorption and ion exchange on the surfaces of
plants, substrate, sediment, and litter
l
breakdown and transformation of pollutants by
microorganisms and plants
The most effective treatment wetlands are
those that foster these mechanisms. The specifics for the various types of wastewater and
runoff are discussed in the wastewater-specific
volumes.
they provide habitat for many wetland organisms
they can be built to fit harmoniously into the
landscape
they provide numerous benefits in addition to
water quality improvement, such as wildlife
habitat and the aesthetic enhancement of open
spaces
they are an environmentally-sensitive
approach that is viewed with favor by the
general public.
LIMITATIONS OF
CONSTRUCTED WETLANDS
uptake and transformation of nutrients by
microorganisms and plants
predation and natural die-off of pathogens.
wetlands are able to tolerate fluctuations in
flow
In addition:
settling of suspended particulate matter
filtration and chemical precipitation through
contact of the water with the substrate and
litter
operation and maintenance require only
periodic, rather than continuous, on-site labor
. they facilitate water reuse and recycling.
l
l
wetlands can be less expensive to build than
other treatment options
There are limitations associated with the use
of constructed wetlands:
l
VOLUME 1: GENERAL
they generally require larger land areas than
do conventional wastewater treatment systerns. Wetland treatment may be economical
relative to other options only where land is
available and affordable.
CONSIDERATIONS
17
l
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l
performance may be less consistent than in
conventional treatment. Wetland treatment
efficiencies may vary ‘seasonally in response to
changing environmental conditions, including
rainfall and drought. While the average performance over the year may be acceptable, wetland
treatment cannot be relied upon if effluent
quality must meet stringent discharge standards
at all times.
Also, the use of constructed wetlands for wastewater treatment and stormwater control is a fairly
recent development. There is yet no consensus on
the optimal design of wetland systems nor is there
much information on their long-term performance.
the biological components are sensitive to toxic
chemicals, such as ammonia and pesticides
There are several types of constructed wetlands:
surface flow wetlands, subsurface flow wetlands,
and hybrid systems that incorporate surface and
subsurface flow wetlands. Constructed wetland
systems can also be combined with conventional
treatment technologies. The types of constructed
wetlands appropriate for domestic wastewater,
agricultural wastewater, coal mine drainage, and
stormwater runoff are discussed in the wastewaterspecific volumes.
flushes of pollutants or surges in water flow may
temporarily reduce treatment effectiveness
they require a minimum amount of water if they
are to survive. While wetlands can tolerate
temporary drawdowns, they cannot withstand
complete drying.
TYPES OF CONSTRUCTED
WETLANDS
Water level is above the ground surface; vegetation is rooted and emerges
above the water surface: waterflow is primarily above ground
WETLAND PLANTS AND WATER
SOIL
LINER
Surface Flow Wetland
NATIVE SOIL
Water level is below ground; water flow is through a sand or gravel bed; roots
penetrate to the bottom of the bed
WETLAND PLANTS
SOIL. SAND. AND GRAVEL
LINER
Subsurface Flow Wetland
NATIVE SOIL
Figure 1. Surface flow and subsurface flow constructed wetlands
(from Water Pollution Control Federation 1990).
V OLUME 1 : GENERAL C ONSIDERATIONS
SURFACE
FL O W W ETLANDS
A surface flow (SF) wetland consists of a
shallow basin, soil or other medium to support the
roots of vegetation, and a water control structure
that maintains a shallow depth of water (figure 1).
The water surface is above the substrate. SF
wetlands look much like natural marshes and can
provide wildlife habitat and aesthetic benefits as
well as water treatment. In SF wetlands, the nearsurface layer is aerobic while the deeper waters
and substrate are usually anaerobic. Stormwater
wetlands and wetlands built to treat mine drainage
and agricultural runoff are usually SF wetlands.
SF wetlands are sometimes called free water
surface wetlands or, if they are for mine drainage,
aerobic wetlands. The advantages of SF wetlands
are that their capital and operating costs are low,
and that their construction, operation, and maintenance are straightforward. The main disadvantage
of SF systems is that they generally require a larger
land area than other systems.
S UBSURFACE FL O W W
ETLANDS
A subsurface flow (SSF) wetland consists of a
sealed basin with a porous substrate of rock or
gravel. The water level is designed to remain below
the top of the substrate. In most of the systems in the
United States, the flow path is horizontal, although
some European systems use vertical flow paths. SSF
systems are called by several names. including
vegetated submerged bed, root zone method, microbial rock reed filter, and plant-rock filter systems.
Because of the hydraulic constraints imposed
by the substrate, SSF wetlands are best suited to
wastewaters with relatively low solids concentrations and under relatively uniform flow conditions.
SSF wetlands have most frequently been used to
reduce 5-day biochemical oxygen demand (BOD5)
from domestic wastewaters.
The advantages cited for SSF wetlands are
greater cold tolerance, minimization of pest and
odor problems, and, possibly, greater assimilation
potential per unit of land area than in SF systems.
It has been claimed that the porous medium
provides greater surface area for treatment contact
than is found in SF wetlands, so that the treatment
responses should be faster for SSF wetlands which
can, therefore, be smaller than a SF system designed for the same volume of wastewater. Since
the water surface is not exposed, public access
problems are minimal. Several SSF systems are
operating in parks. with public access encouraged.
The disadvantages of SSF wetlands are that
they are more expensive to construct, on a unit
basis. than SF wetlands. Because of cost, SSF
wetlands are often used for small flows. SSF
wetlands may be more difficult to regulate than SF
wetlands, and maintenance and repair costs are
generally higher than for SF wetlands. A number of
systems have had problems with clogging and
unintended surface flows.
H
Y B R I D
S
Y S T E M S
Single stage systems require that all of the
removal processes occur in the same space. In
hybrid or multistage systems, different cells are
designed for different types of reactions. Effective
wetland treatment of mine drainage may require a
sequence of different wetland cells to promote
aerobic and anaerobic reactions. as may the remoral of ammonia from agricultural wastewater.
WINTER AND SUMMER
OPERATION
Wetlands continue to function during cold
weather. Physical processes, such as sedimentation. continue regardless of temperature, providing
that the water does not freeze. Many of the reactions take place within the wetland substrate,
where decomposition and microbial activity
generate enough heat to keep the subsurface layers
from freezing. Water treatment will continue
under ice. To create space for under-ice flow,
water levels can be raised in anticipation of freeze,
then dropped once a cover of ice has formed.
V OLUME 1: GENERAL C O N S I D E R A T I O N S
Rates of microbial decomposition slow as
temperatures drop and the wetland may need to
be made larger to accommodate the slower
reaction rates. For agricultural wetlands, which
rely on microbial activity to break down organic
wastes, it may be prudent to store the wastewater in the pretreatment unit during the cold
months for treatment during the warm months.
The high flows that are common in winter and
spring because of snowmelt, spring rains, and
high groundwater tables can move water so
quickly through a wetland tEat there is not
enough retention time for adequate treatment.
Because removal rates are much higher during
warm weather, the agricultural wetland can
often be smaller than if the water were treated
year-round.
Wetlands lose large amounts of water in the
summer through evapotranspiration. The adequacy of flow in the summer must be considered
since it will affect water levels in the wetland and
the amount of wetland effluent available for
recycling (if this is part of the design). A supplemental source of water may be required to maintain adequate moisture in the wetland.
CREATION OF HAZARD
The question of hazard arises from the fact
that, in ecological terms, everything must go
somewhere. Wetlands are able to degrade,
transform, or assimilate many contaminants,
such nitrogen, and are sinks for some materials.
For persistent materials, such as phosphorous
and metals, wetland sinks may become sources
if not properly constructed and managed. The
extent to which wetlands retain contaminants
such as phosphorous and metals is an important
unknown factor, as are the conditions under
which wetlands may release stored contaminants. Bioaccumulation and biotoxicity in
treatment wetlands is not clearly documented
nor understood.
Persistent compounds can be a concern,
depending on the constituents in the wastewater. For instance, mine drainage contains metals
and stormwater carries hydrocarbons deposited
on paved surfaces. Heavy metals are often
sequestered in wetland sediments that may be
washed out of wetlands during storms, thereby
providing only a lag time in pollutant dispersal.
Transport of toxic materials in this way is a
concern, as is the transport of phosphorus, an
extremely important factor in the over-enrichment of surface waters. The question of hazard
underscores the importance of designing and
operating constructed wetlands properly and
monitoring them periodically.
CHANGE AND RESILIENCE
All ecosystems change over time. Wetlands for
wastewater treatment can be expected to change
more quickly than most natural wetlands because
of the rapid accumulation of sediment, litter, and
pollutants. Some natural variability is also inherent in all living systems and is to be expected.
The change in species composition as ecosystems mature is known as succession. In
general, species diversity increases as ecosystems mature. Diversity (the number of species
within a habitat, such as a wetland) is often
considered a measure of ecosystem resilience
(the ability of the system to accept disturbance):
as the number of species increases, so does the
complexity of the interactions of the different
species with each other and with their environment; the greater the number of interactions, the
more resilient the system is as a whole and the
broader its capacity to adapt to change.
In wastewater treatment wetlands, the
stresses of high wastewater loadings can lead to
dominance by a few aggressive, highly tolerant
species, such as cattail and common reed, which
may eventually eliminate other species. If
wildlife habitat values are important to the
V OLUME 1 : GENERAL C O N S I D E R A T I O N S
project, intervention to maintain diversity may
be necessary. If habitat values are not important,
changes can be allowed to proceed without
interference as long as the wetland continues to
treat the water to acceptable levels.
Any ecosystem, natural or constructed, has
limits to its ability to accept disturbance. The
performance of constructed wetland systems
may change over time as a consequence of
changes in the substrate and the accumulation of
pollutants in the wetland. Constructed wetlands
must be monitored periodically for evidence of
stress so that remedial action, if necessary, can
be taken.
V OLUME 1: GENERAL C O N S I D E R A T I O N S
15
CHAPTER 4
GENERAL DESIGN OF CONSTRUCTED WETLANDS
DESIGN CONSIDERATIONS
Despite a large amount of research and published information, the optimal design of constructed wetlands for various applications has not
yet been determined. Many constructed wetland
systems have not been adequately monitored or
have not been operating long enough to provide
sufficient data for analysis. Among the systems
that have been monitored, performance has varied
and the influences of the diverse factors that affect
performance, such as location, type of wastewater
or runoff, wetland design, climate, weather,
disturbance, and daily or seasonal variability, ‘have
been difficult to quantify.
In general, wetland designs attempt to mimic
natural wetlands in overall structure while fostering those wetland processes that are thought to
contribute the most to the improvement of water
quality. Mitsch (1992) suggests the following
guidelines for creating successful constructed
wetlands:
keep the design simple. Complex technological
approaches often invite failure.
design for minimal maintenance.
design the system to use natural energies, such as
gravity flow.
design for the extremes of weather and climate,
not the average. Storms, floods, and droughts are
to be expected and planned for, not feared.
design the wetland with the landscape, not
against it. Integrate the design with the natural
topography of the site.
avoid over-engineering the design with rectangular basins, rigid structures and channels, and
regular morphology. Mimic natural systems.
give the system time. Wetlands do not necessarily become functional overnight and several
years may elapse before performance reaches
optimal levels. Strategies that try to short-circuit
the process of system development or to overmanage often fail.
l
design the system for function, not form. For
instance, if initial plantings fail, but the overall
function of the wetland, based on initial objectives, is intact, then the system has not failed.
PLANNING
A conceptual planning phase is essential.
Wetlands can be designed in a variety of system
types and configurations to meet specific wastewater needs, alternative sites are often available, and
a variety of local, native plant species can be
chosen. Every site is unique and the design of a
constructed wetland system will be site-specific.
The planning phase consists of characterizing
the quantity and quality of the wastewater to be
treated, determining the discharge standards to be
met, selecting the site, selecting system type and
configuration, and specifying the design criteria to
be met by the detailed engineering plans. Economic factors include the land area required. the
type of water containment, the control and transport of water through the system, and vegetation.
Setting and prioritizing the objectives of the
wetland system is key to the creation of a successful system. The characteristics of a local natural
wetland should be used as a model for the constructed wetland, modified to fit the needs of the
project and the specifics of the constructed wetland site.
A constructed wetland should be designed to
take advantage of the natural features of the site
and to minimize its disturbance. Wetland shape is
dictated by the existing topography, geology, and
land availability. The number of cells depends on
topography, hydrology, and water quality. On
level sites, cells can be created with dikes. On
sloping sites, cells can be terraced.
A site-sensitive design that incorporates
existing features of the site reduces the amount
V OLUME 1: GENERAL C O N S I D E R A T I O N S
37
of earthmoving required and increases the visual
attractiveness of the site. Earth grading and
shaping can blend newly created landforms into
the existing landscape. Basins and channels can
be curved to follow the natural contours of the
site. Various types of vegetation can be planted in
and around a constructed wetland to reduce
erosion, screen views, define space, control
microclimate, and control traffic patterns.
Planning should be oriented toward the
creation of a biologically and hydrologically
functional system. Plans should include clear
goal statements and standards for success. The
possible future expansion of the operation
should be considered.
Plans should include detailed instructions
for implementing a contingency plan in case the
system does not achieve its expected performance within a specified time. Plans should be
reviewed and approved by the appropriate
regulatory agencies.
SITE SELECTION
Selecting an appropriate location can save
significant costs. Site selection should consider
land use and access, the availability of the land,
site topography, soils, the environmental resources
of the site and adjoining 1and, and possible effects
on any neighbors. The site should be located as
close to the source of the wastewater as possible,
and downgradient if at all possible so that water
can move through the system by gravity. While a
wetland can be fitted to almost any site, construction costs can be prohibitively high if extensive
earthmoving or expensive liners are required.
A site that is well suited for a constructed
wetland is one that:
l
is conveniently located to the source of the
wastewater
Provides adeqyate space
l
is gently sloping, so that water can flow through
the system by gravity
l
contains soils that can be sufficiently compacted to
minimize seepage to groundwater
l
is above the water table
l
is not in a floodplain
l
does not contain threatened or endangered species
l
does not contain archaeological or historic resources.
LAND U
SE AND
ACCESS
Access is an important consideration, The wetland should be placed so that the water can flow by
gravity. If the odors or insects could be a problem, as
with some agricultural wastewaters, the wetland
should be placed as far from dwellings as possible.
The site should be accessible to personnel, delivery
vehicles, and equipment for construction and maintenance.
For agricultural and some domestic wastewaters,
the wetland may be installed on private land. The
landowner must be carefully chosen. It is essential
that the landowner is cooperative and fully understands the limitations and uncertainties associated
with a developing technology such as constructed
wetland treatment.
The current and future use and values of adjoining
land also will affect the suitability of a site for a
constructed wetland. The opinions of area residents
and those of environmental and public interest groups
should be considered. A large buffer zone should be
placed between the wetland and neighboring property.
The wetland should not be placed next to the edge of
the property.
L A N D A V A I L AB I L IT Y
The effectiveness of a constructed wetland in
treating wastewater or stormwater is related to the
retention time of the water in the wetland. The
usefulness of a constructed wetland may therefore be
limited by the size of the wetland needed for adequate
retention time. The site selected should be large
enough to accommodate present requirements and any
future expansion.
V O L U M E 1: GENERAL C O N S I D E R A T I O N S
T O P O G RA P H Y
Landform considerations include shape, size,
and orientation to the prevailing winds. While a
constructed wetland can be built almost anywhere,
selecting a site with gradual slopes that can be easily
altered to collect and hold water simplifies design
and construction, and minimizes costs.
Previously drained wetland areas, including
prior converted (PC) agricultural sites, may be wellsuited for a constructed wetland since the topography is usually conducive to gravity flow. The
appropriate regulatory agencies must be contacted
before disturbing any PC site.
Since the best location for a constructed wetland is
a low, flat area where water flows by gravity, it is
important to ensure that the area is not already a
wetland: not all wetlands have standing water throughout the year. The Natural Resources Conservation
Service (NRCS), the US Fish and Wildlife Service, or
state regulatory personnel should be contacted to
determine whether or not a site contains jurisdictional
wetlands.
EN V I R O N M E N T A L R E S O U R C E S
such as the Agricultural Stabilization and Conservation Service (ASCS) crop compliance photography and county soil survey information, can be
useful in identifying hydric soils and drained
wetlands that may be difficult to detect otherwise.
Surface and groundwater considerations
include possible flooding and drainage problems,
location and depth of aquifers, and the location,
extent, and classification of receiving waters such
as streams and groundwater. A constructed wetland should not be sited on a floodplain unless
special measures can be taken to limit its impact
on the floodway. Floodplain elevations can often
be determined from sources such as Federal Flood
Insurance maps or from the Federal Land Management Agency. Landuser input may be the best
source of information for assessing previous
hydrologic conditions.
US Fish and Wildlife Service and state natural
resource agencies should be contacted regarding
the potential for significant habitat, or habitat for
rare or endangered species. The possible presence
of archaeological resources should be verified.
PERMITS AND REGULATIONS
To avoid damaging important resources
on the site, the presence or absence of significant
environmental resources must be determined.
Sources of information that can be helpful in selecting a site include the US Geological Survey Topographic Quadrangle maps, and National Aerial
Photography Program (NAPP) and and National High
Altitude Photography Program (NHAPP) photographs. Geographical information system (GIS) maps
are also available.
The National Wetlands Inventory (NWI) maps
and the County Soil Survey with the list of county
hydric soils should be checked for possible locations
of existing wetlands. However, the NW1 maps are
based on aerial photography and may not show small
wetlands or the less obvious wetlands (wet meadows, vernal pools, and some forested wetlands) and
the NW1 information should be field-checked by a
wetlands scientist. Historical aerial photography,
V OLUME
The appropriate agency(ies) must be contacted
to determine the regulatory requirements for a
proposed constructed wetland and its discharge.
Work in a waterway or natural wetland requires a
permit. Discharges to natural waters also require a
permit. In some zoned communities, zoning
approval may be required.
Any stormwater plan must meet local and state
stormwater regulations. Some local ordinances
have incorporated stormwater provisions which
must be complied with. Stormwater regulations
vary from place to place and should be consulted
before developing a stormwater management plan.
The regulatory status of a proposed stormwater
wetland, and its relationship to streams and any
nearby natural wetlands, must be discussed with
the state and/or federal wetland permitting agency
before site plans are decided upon.
1: GENERAL C ONSIDERATIONS
19
STRUCTURES
CELLS
Wetlands can be constructed by excavating
basins, by building up earth embankments (dikes),
or by a combination of the two.
Dikes must be constructed of soils with adequate fine-grained material that will compact into
arelatively stable and impervious embankment.
The dikes should be high enough to contain the
expected volume plus ample freeboard to accommodate occasional high flows as well as the
buildup of litter and sediment over time. To
ensure long-term stability. dikes should be sloped
no steeper than 2H:lV and riprapped or protected
by erosion control fabric on the slopes. An emergency spillway should be provided.
If multiple cells are used, divider dikes can be
used to separate cells and to produce the desired
length-to-width ratios. On steep sites, they can be
used to terrace cells. Dikes can also be used to
control flow paths and minimize short-circuiting.
Finger dikes are often used to create serpentine
flow paths and can be added to operational systems to mitigate short-circuiting. Finger dikes can
be constructed of soils, sandbags, straw bales, or
treated lumber.
Bottom slopes are generally not critical. An
exception may be mine drainage wetlands that use
subsurface flow through deep beds of compost to
induce sulfate reduction; these cells should slope
about 1 – 3% upstream. Bottoms should be relatively level from side to side.
Muskrats can damage dikes by burrowing into
them. Although muskrats generally prefer to start
their burrows in water than is more than 3 ft deep,
they can be a problem in shallower waters. Muskrats can be excluded by installing electric fence
low to the ground or by burying muskrat-proof
wire mats in the dikes during construction.
LINERS
Constructed wetlands must be sealed to avoid
possible contamination of groundwater and also to
20
prevent groundwater from infiltrating into the
wetland. Where on-site soils or clay provide an
adequate seal, compaction of these materials may
be sufficient to line the wetland. Sites underlain
by karst, fractured bedrock, or gravelly or sandy
soils will have to be sealed by some other method.
It may be necessary to have a laboratory analyze
the construction material before choosing a
sealing method. On-site soils can be used if
they can be compacted to permeability of
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