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,
DOMESTIC WASTEWATER
-------
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,
Hirrisburg, 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, Susquehaana 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 .Washington. DC
William Boyd, USDA - NRCS. Lincoln. NE
Robert Brooks, Penn State University,
% University Park, PA
Donald Brown, EPA, Cincinnati, OH
Dana Chapman, USDA - NRCS, Auburn, NY
Tracy Davenport, USDA - NRCS. Annapolis,
MD
Paul DuBowy, Texas A & M University,
College Station, TX
Michelle Girts, CH2M HILL, Portland. OR
Robert Hedin, HedSn Environmental,
Sewlckley, PA
William Hellier, Pennsylvania Department of
Environmental Resources, Hawk'Run, PA
Robert Kadlec, Wetland Management
Services, Chelsea, MI
Douglas Kepler, Damariscolta, Clarion, PA
Robert Kleinmann, US Bureau of Mines,
Pittsburgh, PA
Robert Knight, CH2M HILL, Gainesville, FL
Fran Koch, Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Eric McCleary, Damariscotta, Clarion, PA
Gerald Moshiri, Center for Wetlands and
Eco-Technology Application, Gulf Breeze,
FL
John Murtha. Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Robert Myers, USDA - NRCS, Syracuse, NY
Kurt Neumiller, EPA, Annapolis, MD
Richard Reaves, Purdue University, West
Lafayette, IN
William Sanville, EPA, Cincinnati. OH
Dennis Sievers, University of Missouri,
' Columbia, MO
Earl Shaver, Delaware Department of
Natural Resources and Environmental
Control, Dover, DE
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, Wilkes-
Barre,.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
nonpoinl source management program funds under Section 3.19 of the Federal Clean Water 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.
PS5384RSEZ
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VOLUME 2
CONTENTS
CHAPTER 1. INTRODUCTION ................. ........................ ป ........................................... - ........ : ................... : ..... ' 3
CHAPTER 2. USING CONSTRUCTED WETLANDS TO TREAT DOMESTIC WASTEWATER ......... . ............... 5
Introduction ... ........................... ............................ ................................. ................... '' .........
Contaminant Removal Processes ..................... ; [[[ ............. ..... ...........
Advantages and Limitations of Constructed Wetlands ............... . ........ : ........ - ...................................... 6
Creating Effective Constructed Wetlands .................... . ......... ....... .....,..ป ........ ......... .......... ......... 6
. Types of Constructed Wetlands .............. .. ........... .. .............. ........ ...ป....- ................................ : ......... 7
.Wastewater Characteristics ........ ....................... ...... ซ .................. ............ ; ..... ' ....................... ; ...........
Water Quality ............................ ........... ....................... [[[ ....... "" ฎ
Water Quantity .................................... , ............................................. ; ..... :-" ............. ' ................
Pretreatment ..... ......... .. ......... ............ ............. [[[ .' ..........
System Configuration ............ . ..... .... ................ ; .......... ...................... ...... , ........ ; ...............................
Length-to-Width Ratio ............... ..... ..... . ....... ซ ............ ....... ..... ........................................ .......... 9
Compartmentalization ............... . ......... : ........................ ............................................ ...... ' ..... .""
Step-feeding ...... . ......... ; ..... ป ......................... .......... ................ ........ ; ..... "' ....................... in
Recycling ........................ ....... ................................... v .............. : .............. ; ....... """ ................
CHAPTER 3. PERFORMANCE EXPECTATIONS . ..... . ...................................... ป .......... ...... .......................... **
Introduction : .................... < ...... ................ ........ ................................................. ............................
Biochemical Oxygen Demand and Total Suspended Solids ..................... . ................................... .- **
Nitrogen ................ ...ป ........ ' ......... ....................... ..... : ...................................... ..... "'" ....... ' ..............
Phosphorus ........... ....... . ............... r ................................ "" ...... ' ......... ' ................ ' ............... " ...............
Toxics ..'. ....... . .......... ........ ........................ " ................ ; ....... ............... " ........................................
Pathogens ....... . ..................... . ................ ................................ "" .................... " ........... , ......
CHAPTER 4. SURFACE FLOW WETLANDS ................. .... ..... :....-.-.. ....... : ................................... .......................
Wetland Design .... ............. , ......... ป ........... ................ ; ............................. - ....... - ........ ;
Configuration ............................. . [[[ ...................................... "'"
Water Depth ...... '. ........ .' .............................. ....... ....... - [[[ J
. ...................... . ........... 18
Biochemical Oxygen Demand ......................... , ...................................... ..................................
Total Suspended Solids ......... . ............... ......... ................ ..................... - ......... " ....... ; ........
Nitrogen ............ , .................. .......... ............................ ............................. ' ................................ V"
CHAPTER 5. SUBSURFACE FLOW WETLANDS
' Introduction .............. .. ...........
21
Wetland Design '
Darcy's Law ; '
Media Types - ' _
Length-to-Width Ratio .; > '
Bed Slope - * v."' '. '
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LIST OF TABLES
Table 1. Removal mechanisms in constructed wetlands ป,ป 5
Table 2. Advantages and limitations of constructed wetland treatment of domestic wastewater .... 6
Table 3. Guidelines for creating constructed wetlands....: '
Table 4. Summary of municipal constructed wetland operational data ; ป ^
Table 5. Design summary for surface flow wetlands ....: ป. -
Table 6. Design summary for subsurface now wetlands -. :
LIST OF FIGURES
.9
.9
Figure 1. Configuration options for constructed wetlands.
Figure 2. Flow patterns for constructed wetlands ป "-
Figure 3. BODS mass loading and removal rates in wetland.treatment systems ^
FiRUie 4. Nitrogen transformations : ; "
Figure 5. Total nitrogen mass loading and removal rates in wetland treatment systems .15
.VOLUME 2: DOMESTIC WASTEWATER"
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CHAPTER 1
INTRODUCTION
This volume focuses on the use of constructed
wetlands to treat domestic wastewater. It is to be
used in conjunction with Volume 1: General Consid-
erations, which provides general information on
wetland hydrology, soils, and vegetation, and on the
design, construction, operation, and maintenance of
wetland systems.
Constructed wetlands can provide an inexpen-
sive and easily managed means otremoving 5-day
biochemical oxygen demand, particulates, nutrients,
and bacteria from domestic wastewater. Constructed
wetlands for domestic wastewater have found a wide
range of applications, ranging from large municipal
systems to single family homes. Constructed wet-
lands can provide year-round treatment but are
readily adaptable to seasonal or occasional uses, for
instance, at parks, camps, and schools. Some
systems have focused oh maximizing the amount of
wastewater treated cin the smallest amount of land
possible while other systems have focused on
polishing pretreated effluents with larger wetlands
that provide wildlife habitat and aesthetics in
addition to water quality improvement. Constructed
wetlands can be used to upgrade the performance of
existing facilities or as a component of new waste-
water treatment systems.
A number of documents have been published
recently on the use of constructed wetlands in
treating domestic wastewater. These publications
include: .
Center for Environmental Resource Management.
1993. Proceedings Subsurface Flow Constructed
Wetlands Conference. University of Texas-El Paso,
El Paso, TX.
EC/EWPCA. 1990. European Design and Operations
Guidelines for Reed Bed Treatment Systems. P. F.
Cooper (ed.), Proceedings International Conference
on the Use of Constructed Wetlands in Water Pollu-
tion Control. Pergamon Press, Oxford, UK.
Reed, S. G. 1993. Design of Subsurface Flow Con-
structed Wetlands For. Wastewater Treatment: a
Technology Assessment. EPA 832-R-93-Q01. EPA
Office of Wastewater Management, Washington, DC.
Environmental Protection Agency. 1988. Design
Manual: Constructed Wetlands and Aquatic Plant
Systems for Municipal Wastewater Treatment.
EPA/625/1-88/022. Center for Environmental
Research, Cincinnati, OH.
Reed, S. C., E. J. Middlebrooks, and R. W. Crites.
. 1994. Natural Systems for Waste Management
and Treatment. 2nd edition. McGraw-Hill Book
Company, New York City, NY.
Water Pollution. Control Federation. 1990.
Natural Systems for Wastewater Treatment,
Manual of Practice FD-16, Chapter 9. Alexandria,
VA.
Water Science and Technology, Volume 29. 1994.
The National Small Flows Clearinghouse
(NSFC) at West Virginia University, Morgantown,
West Virginia (telephone 1-800-624-8301) provides
technical assistance and information to small
communities.
The Environmental Protection Agency (EPA)
has sponsored a project to collect and catalog
information from wastewater treatment wetlands
into a computer database. The Wetlands Treat-
ment Database (North American'Wetlands for
Water Quality Treatment Database)(Knight, R. L.,
R. W. Ruble, R. H. Kadlec, and S. C. Reed 1994) is
available on 3.5" diskette. To order, contact: Don
Brown, USEPA (MS^347), Cincinnati, OH 45268;
phone: (513) 569-7630; fax: (513) 569-7677; ,
e-mail: brown.donald@epamail.epa.gov.
While much experience has been gained in the
design of constructed wetland systems for domes-
tic wastewater, much is not yet understood and
many of the relationships between design and
performance have not been clearly established.
Constructed wetland technology continues to be
refined as more systems are installed and moni-
tored over longer periods qf time. The guidance
presented here should be considered as today's
"state of the art" and will likely be modified as
our understanding of these systems grows.
VOLUME 2: DOMESTIC WASTEWATER
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VOLUME 2: DOMESTIC WASTEWATER ..
i
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CHAPTER 2
USING CONSTRUCTED WETLANDS TO TREAT
DOMESTIC WASTEWATER
INTRODUCTION
Domestic wastewaters contain large amounts of
nutrients, participates, and organic matter that
must be removed before the water can be dis-
charged. Constructed wetlands are highly effective
in removing 5-day biochemical oxygen demand
(BODS) and total suspended solids (TSS) from
pretreated domestic wastewater. Removal efficien-
cies for nitrogen, particularly ammonia, vary
considerably, depending on system design, reten-
tion time, and the oxygen available for nitrifica-
tion. Phosphorus removal may be limited in the
long-term, although good removal may be seen
during the first several years. The numbers of
pathogenic bacteria and viruses are significantly
decreased during passage through constructed
wetlands. Removal capabilities are discussed in
Chapters. "
CONTAMINANT REMOVAL
PROCESSES
Wetlands remove contaminants through a series
of interacting physical, chemical, and biological
processes, including filtration, sedimentation,
adsorption, precipitation and dissolution, volatil-
ization, and biochemical interactions (table 1).
The suspended solids that remain after pretreat-
ment are removed in the wetland mainly by sedi-
mentation and filtration. These physical processes
also remove a significant portion .of other waste-
water constituents, such as BOD5, nutrients, and
pathogens, that are associated with the solids.
Adsorption is the principal removal mechanism
for dissolved pollutants such as phosphorus and
dissolved metals. Adsorption is promoted by the
large amount of surface area provided by the
sediments, vegetation, soils, and litter.
Table 1. Removal mechanisms in constructed wetlands
(after Brix 1993).
Wastewater Constituent
Biochemical oxygen demand
Suspended solids
Nitrogen
Phosphorus
Pathogens
RemovalJMeGhanisms
Microbial degradation (aerobic and anaerobic)
Sedimentation (accumulations of organic matter/sludge
on sediment surfaces)
Sedimentation/filtration '
Chemical ammonification followed by microbial nitrification
and denitrification
Plant uptake- .
Volatilization of ammonia
Soil sorption (adsorption-precipitation reactions with aluminum,
iron, calcium, and clay minerals in the soil)
Plant uptake .
Sedimentation/filtration
Natural die-off
Attack-by antibiotics excreted from the roots of wetland plants
Predation by invertebrates and other microbes
VOLUME 2: DOMESTIC WASTEWATER
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Soluble organic compounds are, for the most
part, degraded by microbes, especially bacteria, that
grow on the surfaces-of the plants, litter, and the
substrate. The oxygen needed to support aerobic
microbial processes is supplied by diffusion from
the atmosphere, by photosynthetic oxygen produc-
tion within the water column, and, to some extent,
by leakage of oxygen from the roots of the -vegeta-
tion. Sbme anaerobic microbial degradation also
occurs.
ADVANTAGES AND LIMITATIONS
OF CONSTRUCTED WETLANDS
When properly designed, constructed wetlands
offer a number of advantages, including low cost,
simplicity of operation, and effective removal of
BODsandTSS (table 2). .When sized adequately,
constructed wetlands are also tolerant of fluctuating
flows and variable water quality. For instance, at
the Des Plaines Rive.r Wetlands Demonstration
Project, effluent concentrations of TSS, nitrate, and
total phosphorus remained low and steady although
influent concentrations were often quite high and.
varied significantly with time, (Hey et al. 1994).
Constructed wetland treatment is constrained
by a number of limitations, including relatively
large land requirements and a degree of uncer-
tainty not found in more conventional
approaches (table. 2). '
CREATING EFFECTIVE
CONSTRUCTED WETLANDS
Suggestions for creating an effective con-
structed wetland are given in table 3. Since the
objective of using a constructed wetland is to
simplify the handling of wastewater, the system ,
should be made as easy to operate as possible while
ensuring reliable treatment.- Building a slightly
larger system may be more expensive to construct
but may be more reliable and less costly to operate
than a smaller system. Attention.to several factors
will help to ensure successful wetland treatment:
Adequate pretreatment. Pollutant loads in raw
wastewater can exceed the ability of a wetland
to.treat or assimilate them. Wetland treatment
is suitable for waters that have received primary
or secondary treatment.
Adequate retention time. A wetland treats
wastewater through a number of biological
(largely microbial), physical, and chemical
processes. The water must remain in the
Table 2. .Advantages and limitations of constructed wetland treatment of domestic wastewater.
Advantages
Excellent removal of BODS and TSS
Good removal of nutrients, depending on system
design
Ability to handle daily or seasonally variable
loads
Low energy and maintenance requirements
Simplicity of operation
limitations .
Variable treatment efficiencies due to the effects of
season and weather
Uncertainty as to treatment effectiveness under all
conditions ' ,
Sensitivity to high ammonia levels
Larger land area requirement than for conventional
treatment .
Potential for mosquito production
VOLUME 2: DOMESTIC WASTEWATER
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wetland long enough for biological and chemical
transformations to take, place and for sedimenta-
tion and deposition to occur. The wetland must
be built large enough to provide the necessary
retention time.
Supplemental water. If a constructed wetland is
to remain healthy, it must remain relatively wet.
Wetland plants are generally tolerant of fluctuat-
ing flows, but they cannot withstand complete
drying. For this reason, either a fairly regular
supply of wastewater must be assured or a
supplemental source of water must be provided.
Proper management. Constructed wetlands are
"high management, low maintenance" systems.
They must be actively managed if they are to
perform well. "Management" means watching
the wetland for signs of stress or disease and'
adjusting water levels or-waste water input
streams accordingly. While wetlands are low
maintenance systems, they are not maintenance-
free. For instance, distribution systems must be
cleaned periodically to avoid plugging and
uneven distribution of flow, and valves and
piping must be checked to detect and correct
blockages or leaks.
TYPES OF CONSTRUCTED
WETLANDS
Domestic wastewater can be treated with surface
flow (SF) or subsurface flow-(SSF) wetlands.
The advantages of SF wetlands are that .
their design and construction are straight-
forward. Operation and maintenance are simple.
Because the water surface is unconstrained, SF
wetlands are able to handle wide variations in flow.
SF systems can provide excellent removal of BODS
and TSS and some installations have achieved good
removal of ammonia and total nitrogen. SF systems
are similar to natural marshes and can provide
wildlife habitat as well as wastewater treatment.
SF wetlands are discussed in Chapter 4.
. In.SSF wetlands, the water level is.ihtended to
remain below the surface of the substrate. Since the
water is not exposed to the atmosphere, potential
problems with insects, odors, and safety are
Table 3. Guidelines for creating constructed wetlands.
Know what you are dealing with:
Wetlands must have water:
Size the wetland generously:
Give the plants a chance:
Don't overload the wetland:
Protect the wetland from toxics:
Keep an eye on what is happening:
Get interdisciplinary help:
Sample the wastewater
Know what pretreatment will accomplish
Know the water budget
An undersized wetland cannot perform well
Allow time for establishment
Avoid shock loadings
Application rates must not exceed treatment rates
Limit, the toxics entering the wetland
Keep herbicides out of thie wetland
Monitoring is needed to assure continued performance
Environmental engineer
Water quality specialist
Plant materials specialist or biologist
State agencies
VOLUME 2: DOMESTIC WASTEWATER
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avoided. There is debate over the most effective
length-to-width ratio, type of vegetation, and type
and size of medium. A few recent European designs
have incorporated vertical flow and batch loading in
an attempt to promote more effective wetting and
drying cycles, and to entrain more oxygen for nitri-
fication (Bastian and Hammer 1993). Because of the
hydraulic constraints .imposed by the media, SSF
wetlands are best suited to the treatment of wastewa-
ters under relatively uniform now conditions. There
have been problems with surface flow and apparent
plugging. SSF wetlands are discussed in Chapter 5.
WASTEWATER .
. CHARACTERISTICS
To design the wetland .treatment system, an
accurate assessment of contaminant loadings is
needed (loading = contaminant concentration x
water volume). To calculate loadings, data are
needed on the average water quality; the max-
imum concentrations, and the largest and smallest
volumes that may occur. Maximum concentra-
tions will probably occur in late summer when
losses due to evapotranspiration are greatest.
The highest flows can.be expected during, the wet
season, but pqllutant concentrations may be
lower at this time because of dilution. The design
should be'based on the highest contaminant
loadings.'
WATER,QUALITY
For design, water'quality analyses generally
include:
. ...
alkalinity
5-day biochemical oxygen demand (BOD5)
total suspended solids (TSS)
total dissolved solids (TDS)
dissolved oxygen
nitrate plus nitrite nitrogen (NO2 + NO3-N)
ammonia nitrogen (NH3-N).'
total phosphorus
heavy metals (for instance, lead, mercury,
chromium, zinc)
. refractory organics
total or fecal coliform bacteria.
The design of the wetland is usually based on
the removal of BOD (usually measured as 5-day
biochemical oxygen demand, BOD5) or nitrogen
(measured as total Kjeldahl nitrogen or nitrate
nitrogen). Concentrations of ammonia (NH3 +
NH -N, un-ionized ammonia + the ammonium
ion) should be evaluated because of the toxicity
of ammonia to wetland plants.
WATER QUANTITY
An accurate estimate of the volume of waste-
water is needed, including the expected average,
maximum, and minimum flows. The level of
detail required (daily/monthly, or seasonal flows)
will be projectrspecific. The frequency and
duration of freezing conditions must be estimated
to determine if storage or special operating prac-
tices will b
-------
microbial community and other biological
components of the wetland.
SYSTEM CONFIGURATION x
Various configurations are possible for the
constructed wetland and for incorporating it into a
treatment system (figures 1 and 2). System con-
figuration includes length-to-width ratio (some-
times called "aspect"), compartmentalization, and
the location of single or multiple discharge points.
The configuration should take advantage of the
natural topography of the site to minimize'excava-
tion and grading costs. While wetlands are often
designed as rectangles, wetlands can be built in
almost any shape to fit the topography of the site.
Stabilization
Pond
err
Imhoff
Tank
Commlnuter
-D-
*
nr
V/n
ป
Sludge
Drying Bed
Septic
Tanks
D-i
D-
Q-
c
cw
h
CW: Constructed Wetland .
Figure 1. Configuration options for
constructed wetlands
. (from Steiner and Freeman 1989).
Plug flow
c. Rectrculatlon
d. "Jelly roll" step feed-recireulaUon cell
CW: Constructed Wetland
Figure 2. Flow patterns for constructed wetlands
(from Steiner and Freeman 1989).
Whatever the.configuration, care must be taken to
ensure equal flow distribution at the inlet and to
avoid short-circuiting of flow to the outlet.
LENGTH-TO-WIDTH RATIO
Many SF constructed wetland have, been
designed with an overall length-to-width ratio of
about 3:1 to 4:ll This ratio has been thought to
lower excessive loading at the inlet of early sys-
tems built with high length-to-width ratios and to
provide good removal of BODsand TSS. The
concern was that in a longer, narrower wetland the
upper end might become overloaded while the
VOLUME 2: DOMESTIC WASTEWATER
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lower end might lack adequate nutrients. How:
ever, the relationships between length-to-width
ratios and performance has not been adequately .
studied and an optimal length-to-width ratio has
not been determined.
For SSF wetlands, the length-to-width ratio of
the wetland cell is an important consideration in
the hydraulic design since the maximum potential
hydraulic gradient is related to the available depth
of the bed divided by the length of the now path.
Cell length can be limited by hydraulic capacity if
surface flow is to be avoided. Therefore, SSF
wetlands may have length-to-width ratios larger or
smaller than 1:1. It is thought that most of the
removal takes place in the vicinity of the influent
area and that systems with relatively high BOD5
discharge requirements 'can therefore achieve
adequate BODsahd TSS reductions with relatively
short length-to-width ratios (Reed 1993). In'many
of the early SSF systems that were designed with a
.ratio of 10:1 or more and a total depth of 2 ft
(0.6 m), surface flow has developed. The surface
flow is thought to result from inadequate hydrau-
lic gradients (Reed. 1993).
COMPARTMENTALIZATION
Compartmentalizing the wetland with
several'cells arranged in series or in parallel is
suggested because it allows flows to be redistrib-
uted through the system as necessary for mainte-
nance or repair. Cells arranged in parallel
facilitate the maintenance of plant communities
(because of the greater edge length relative to
surface area) and allow-different plant popula-
tions and any associated.plant diseases or
pathogens to be isolated. Ideally, cells can be
arranged to permit operation in series or in
parallel, with alternate discharge points and
' interconnections. . . '
suggested as a means of distributing organic
loads along the length of the wetland, thereby
lessening .the organic loading on the upper end
of the cell. The additional capital and operating
costs of step-feeding need to be weighed against
.the potential benefits gained.
RECYCLING
It may be advantageous to recycle all or a
portion of the wetland effluent. Recycling can be
used to dilute influent BODS and solids. Recy-
cling may increase dissolved oxygen concentra-
tions and detention time, which in turn may
enhance nitrification and nitrogen removal.
Recycling is also an efficient way to maintain
adequate flow during low-flow periods.
The disadvantages of recycling are the
increased construction costs and increased
operation (pumping) costs. A wrap-around
design may help to minimize these costs. Also,
recycling may slowly increase salinities as
. evapotranspiration removes water from the
,. system. The added costs of recycling must be
weighed against the potential benefits gained.
STEP-FEEDING .
Step-feeding, which is the use of multiple
input points along the length of a cell, has been
10
VOLUME 2: DOMESTIC WASTEWATER
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CHAPTER 3
PERFORMANCE EXPECTATIONS
INTRODUCTION
Perhaps more is known about the domestic
wastewater applications of constructed wetlands
than for any other use. A database on wastewater
treatment wetlands has been compiled by EPA's
Risk Reduction Engineering Laboratory (Knight et
al. 1994). The database contains data from 323
.wetland cells at 178 locations in the United States
and Canada, and includes information on natural
and constructed wetlands, and งF, SSF, and hybrid
systems. The majority of the systems .in the
database have been installed recently and have as
yet produced little operational data. Performance
data for 11 SF and 2 SSF domestic wastewater
constructed wetlands that have been operating
long enough to produce useful data are summa-
rized in table 4.
BIOCHEMICAL OXYGEN
DEMAND AND TOTAL
SUSPENDED SOLIDS
BODS is removed by filtration and sedimenta-
tion of particulate matter and by microbial degra-
dation of soluble BODS. The organic matter in
treatment wetlands provides a source of energy for
populations of bacteria, fungi, and aquatic
macroinvertebrates similar to those in conven-
tional activated sludge and trickling filter treat-
ment plants. In SSF wetlands, physical removal
of BODS is believed to occur rapidly through
settling and entrapment of particulate matter in
the void spaces in the media;
Both SF and SSF wetlands are extremely
efficient at assimilating BOD5 and TSS (table 4).
Table 4. Summary of municipal constructed wetland operational data
. (adapted from Knight et al. 1993).
BOD5 (mg/L) TSS (mg/L) NHa-N (mg/L) TP (mg/L)
In Qut %. in Out %. In Out %_ _in Out %
Surface Flow Wetlands .
Benton-cattail
Benton-woolgrass
Cobalt
Gustine 1A
Gustine IB
Gustine 1C
Gustine. ID
Gustine 2A
Kelly Farm
Moodna Basin
Norwalk
26
26
21
130 -
130
145
141
151
-
53
229
10
12
5 "
50
27
24
30
45
-
18
9
62
52
78
62
79
83
78
70
-
66
96
57
57
36
73
81
88
98
100
'
34
232
11
16
28
40
23
. 57
20
34
' .-.
12
33
82
73
23
46
72
36
79
66
-
64
86
7.7
7.7
2.9
17.0
16.3
18.4
19.6
18
8.4
20.4
-
7
6
1
16
17
.20
22
23
0
11
-
.9
.4
.0
.1
.9
.4
.9
.2
.1
.4
inc
16
65
6
inc
inc
inc
inc
99
44
-
4.5 4.2 7.
4.5 4.0 12
'1.7 0.8 54
'-.'-'
- - - .
...
- - - .
...
...
...
-
Subsurface Flow Wetlands
Kingston
Monterey
56
38
9
15
84
60
83
32
3
7
96
.78
22
9.3
16
8
.7
27 '
7
3.4 2.1 38
-
%: percent reduction
inc: increase
VOLUME 2: DOMESTIC.WASTEWATER
11
-------
Constructed -wetland treatment systems commonly
receive inflow BODS concentrations of 10 to 100
mg/L, depending on the degree of pretreatment
(Knight et al. 1993). For the constructed wetland
systems summarized in table 4, removal efficien-
cies for BOD, were generally greater than 60% for .
both SF and SSF wetlands. These efficiencies were
realized in spite of widely varying retention times,
configurations, input concentrations, and wetland
plant communities (Water Pollution Control Fed-
eration 1990).
' Knight et al. (1993) found that typical
BODsmass removal efficiencies were near 70%
' or more at mass loading rates up to 250 Ib/ac/day
(280 kg/ha/day) for SF and SSF wetlands. Lower
removal efficiencies occurred, especially when
mass loadings were less than 45 Ib/ac/day
(50 kg/ha/day). A linear regression of 324 munici-
pal, stormwater, and other data records used to
examine the predictability of BODS outflow concen-
tration as a function of BOD^ inflow concentration
and hydraulic loading (figure 3) produced the
following relationship: , .
BODOUT = 0.097*HLR + 0.192*BODIN
R2 = 0.72:
where
BODOUT = BOD outflow concentration, mg/L
BODIN = BOD inflow concentration, mg/L
HLR = hydraulic loading rate, cm/day.
While average annual removal rates were
generally high, rates sometimes varied considerably
Figure 3. BODS mass loading and removal
rates in wetland treatment'systems
. .{from Knight etal.'1993).
on a monthly or seasonal basis. To maximize the
removal of BOD5 and TSS, the growth of plants
(particularly underground tissues) and the accumu-
lation of litter should be encouraged. Plants and
plant litter provide organic carbon and attachment
sites for microbial growth, as well as promoting
filtration and sedimentation.
In wetlands, BOD is produced within the
system by the decomposition of algae and fallen
plant litter. As a result, wetland systems do not
completely remove BOD and a residual BODS
from 2 to 7 mg/L is often present in the wetland
, effluent (Reed 1993). This internal production of
BOD decreases efficiencies at very low inflow
concentrations.
The potential for wetlands to assimilate. TSS is
similar to the potential for BODS removal (table 4).
Removal rate and efficiency are consistently high
up to loading rates of 135 Ib/ac/day (.150 kg/h/day)
(Knight et at 1993). Removal efficiencies for TSS
are also closely related to input concentration, with
lower efficiencies measured at low input concentra-
tions. Cooper et al. (1993) found that TSS removals
.increased with increasing accumulation of plant
detritus in the litter layer.
NITROGEN
The organic nitrogen entering a treatment
wetland is usually associated with particulate
.material, such as algae (especially when pretreat-
ment ponds are used) and organic wastewater
solids. Plant detritus generated within the wetland
can also be source of organic nitrogen..
In wetlands, nitrogen occurs in a number of
forms, the most important of which are nitrogen gas
(N2), nitrite (NO2), nitrate (NO3), ammonia (NH3),
and ammonium (NH4+). The forms of nitrogen most
often regulated are ammonia and total nitrogen
(TN). Un-ionized ammonia can be toxic to fish and
.other aquatic life while excess nitrogen contributes
to the over-enrichment of natural waters.
In contrast to the simplicity of BOD and TSS
removal, the chemistry of nitrogen removal is
VOLUME 2: DOMESTIC WASTEWATER
-------
complex (figure 4). The removal processes in-
clude some that require oxygen (aerobic reactions)
and others that take place in the absence of oxygen
(anaerobic reactions). Decomposition and miner-
alization processes in the wetland convert a
significant part of organic nitrogen to ammonia.
Ammonia is then oxidized to-nitrate by nitrifying
bacteria in aerobic zones (nitrification) and ni-
trates are converted to nitrogen gas by denitrifying
bacteria in anoxic zones (denitrification); the gas
is released to the atmosphere. The sequencers:
mineralization:
organic nitrogen-> ammonia
. , nitrogen
aerobic or anaerobic
reaction
nitrification: .
ammonia nitrogen -> nitrate
nitrogen
denitrification:
nitrate nitrogen -> nitrogen
gas
aerobic reaction
anaerobic reaction;
requires a carbon
source as food for the
bacteria
Nitrogen Gas
Nitrous Oxide Gas
SOIL
, AIR
WATER.-
Ozygen
t
6^"Aซobic^rAmmonia"+ Nitrate VNttnne* _'
- * . Llnward ' ".VI -_."-""
. .. .
AnerobicSoil
Diffusion
_ *.r
Fixation ' *f*
Organic N
Upward
Diffusion
.Ammonia
Nitrogen Gas
Nitrous Oxide Gas'
Denilrification
Downward
Diffusion
Mineralizalionl
Nitrate
Leaching ซ '
Figure 4. Nitrogen transformations
(after Gambrel and Patrick 1978, cited in
Mitsch and Gosselink 1986).
Since nitrification is an aerobic process,
rates are controlled by the availability of oxygen
to the nitrifying bacteria. The process of nitrifi-
cation is usually limited by the availability of
dissolved oxygen availability, and also by
temperature and retention time (Knight et al.
1993). Denitrification is usually very rapid and
the loss of nitrogen gas to the atmosphere
represents a limitless sink. Decaying plant litter
may provide anoxic sites for denitrification
(Crumpton et al. 1993)
Some nitrogen is taken up by plants and .
incorporated into plant tissue, but this removal
pathway is of limited importance in wetlands in
the northeastern United States because the
above-ground parts of most emergent plants die.
back yearly and because below-ground tissue
increases only very slowly (Brix 1993). Most of
the nitrogen bound in plant tissue is returned to
the wetland when the plants die and decay.-
Many constructed wetlands, both SF and
SSF, are unable to meet typical NPDES limits for
ammonia. Reed and Brown (1992) believe that
the factor responsible in both cases is the insuf-
ficient availability of oxygen to support the
activity of the nitrifying .organisms. In the SF
case, the water may be too deep and the vegeta-
tion too dense for wind and turbulence of the
water surface to allow for significant aeration.
One attempt to correct this problem combines
overland flow with a SF wetland. The water
depth in the overland flow segment is less than
2 inches (5 cm) deep, which allows for effective
aeration and nitrification. Hammer (1992)
suggests a marsh-pond-marsh sequence to .
increase the available oxygen: the water passes
through an SF wetland area to convert organic
nitrogen to ammonia, then through a pond (a
.deeper, open water area) for nitrification of
ammonia to nitrate and subsequent denitrifica-
tion to nitrogen gas, then through another
wetland area to complete the denitrification of
nitrate. In SSF systems, the roots of the vegeta-
tion may not penetrate deeply enough and an
anaerobic layer develops at the bottom of the
wetland. For this reason, SF wetlands may
VOLUME 2: DOMESTIC WASTEWATER
13
-------
have higher nitrogen removal capacities than
SSF wetlands. '.
Of 12 SSF systems in the southern United
States, most showed a marginal or negative
ammonia removal rate (in half of the systems
output ammonia levels were near or higher than
input levels) regardless of detention time in the
system (Reed'1993). However, two systems
showed very high removal rates at hydraulic
retention times (HRT) comparable to the other
systems. The difference is believed to be lack of
algae, availability of oxygen, and sufficient HRT
in the two systems, so that high levels of nitrifi-
cation can occur. In both cases, the roots pen-
etrated to the bottom of the bed. In the 12
systems with poor nitrogen-removal, roots did
not penetrate the entire depth of the bed; flow
therefore passed through the beds below the
root zones where oxygen, was not likely to be
present.
The depth of the media in most systems in
the United States is about 2 ft (0.6 m) but in most
cases the plant roots only penetrate to about 1 ft
(0.3 m). Deeper penetrations were observed when
nutrient levels in the water were low or when
plants were located at the sides, of the cells where
there was less flow than in the main portion of the
bed. The use of parallel .cells operated on a batch-
type fill and draw basis to allow atmospheric
oxygen to be introduced into the substrate has also
btfen used to increase ammonia removal.
TN removal has been highly correlated to
loading.rates as high as. 10 kg/ha/day, with re-
moval efficiencies typically between 75 and 95%
(Water Pollution Control Federation 1990). With
loading rates between 10 and 80 kg/ha/day, total
nitrogen removal efficiency varies widely, with
some systems showing high values and others
much lower values. TN removal efficiency is
highly dependent on HRT and decreases signifi-
cantly at design HRTs of less than about 5 days
(Water Pollution Control Federationl990). Phipps
and Crumpton (1994) found that seasonal varia-
tions in nitrate and organic nitrogen loads had
significant effects on the effectiveness of con-
structed wetlands as sinks for TN: during periods
of high nitrate loading, the wetlands were nitro-
gen sinks while the wetlands were nitrogen
sources during periods of low nitrate loading.
Knight et al. (1993) found that, unlike BODS,
removal efficiencies for TN declined at mass
loading rates above 20 kg'/ha/day. Also, mass
removal efficiencies were more consistent at lower
mass loading rates than they were for BQDS. A
regression predicting TN outflow concentration
base&on hydraulic loading rate (HLR) and TN,
inflow concentrations was developed from 213
records in the EPA database (figure 5). The
multiple linear regression can be expressed as:
TNOUT = 0.28*HLR + 0.33*TNIN
R2=0.54
where
TNOUT = TN outflow concentration,
mg/L
TNIN = TN inflow concentration,
mg/L
HLR = Hydraulic loading rate,
cm/day.
Based on this equation, TN removal is more
dependent on the effect of high HLRs than is
BODS removal (Knight et al. 1993),
TN can be generated in wetlands through
nitrogen fixation, in which: certain plants convert
atmospheric nitrogen into.the organic form.
Many wetland plants are able to fix nitrogen and
natural background concentrations of TN are
generally in the range of 0.5 to 3 mg/L. Appar-
ently because of this natural nitrogen fixation
process, TN removal efficiency decreases when
TN input concentrations approach background.
(Water Pollution Control Federation 1990).
PHOSPHORUS
In the short term, phosphorus is a highly
mobile element in wetlands that is involved in
many biological and soil/water interchanges.
Dissolved phosphorus may be present in organic
or inorganic forms and is readily transferred
VOLUME 2: DOMESTIC WASTEWATER
-------
between the two forms. It has been assumed
that microbes,-algae, and vascular plants cycle
phosphorus annually, with uptake during the
growing season and gradual release to the water
column on death and decay. However, data on
the annual recycling of phosphorus are still
limited. Harvesting the above-ground portions of
vascular plants at the end of the growing season
to remove phosphorus was shown to be ineffec-
tive because much of the phosphorus had been
gradually translocated to the roots and rhizomes
before then (Mitsch and Gosselink 1986).
The long-term removal of phosphorus by
wetlands is limited. The major sink for phospho-
rus in most wetlands is the soil. Phosphorus may
be buried in organic form in peats or chemically
adsorbed in complexed forms with aluminum,
iron, or calcium (Faulkner and Richardson 1989).
Soil adsorption can result in significant removal
of dissolved phosphorus for a while after system ,
startup, but removal then decreases as adsorption
sites become filled. The length of the removal
period depends on the chemical adsorption
capacity of the sediments and can be estimated
through laboratory analyses.
Removal at the SF and SSF wetland systems
listed in table 4 ranged from 7% to 95%. The -
extra removal seen in some wetlands may be
explained by the export of organisms with their
associated phosphorus loads or by chemical
T3
"S
o
1 "
>
O
E
-------
solids. In general, pathogenic microorganisms
are highly host-specific and do not survive long
apart from the host.
Constructed wetlands can provide high
percentage removals of pathogens and have
been shown to be capable of removing bacterial
and viral indicators at efficiencies of 90% to
99% at HRTs of three to six days (Ives 1988). At
HRTs of three to six days, constructed wetlands
are thought to be at least equivalent and, in
most cases, more effective than conventional
wastewater treatment systems in removing
disease-causing bacteria and viruses.
Constructed wetlands treatment typically
decreases total coliform levels to 103 total
coliform/100 ml or less when undisinfected
secondary wastewaters from conventional
treatment.systems are being treated. SF wetland
systems are generally- capable of a one* to two-
log reduction in fecal coliforms (EPA 1993),
which in many cases is not enough to routinely
satisfy discharge requirements. Peak flows
resulting from intense rainfall also disrupt
removal efficiencies for fecal coliforms. As a
result, many systems use some form of final
disinfection.
Removal of bacteria and viruses by wetlands
is increased by densely vegetated cells and by
longer retention times. Storage in wetlands, as
in ponds, can be an effective means of reducing
bacteria and viruses.
Where virus removal is of concern, the
design can be adjusted to optimize viral re-.
moval. Since suspended solids (algal cells and
colloidal clay particles) play a key role in
removing viruses from the water column, the
design should incorporate means to supply the
'necessary adsorption sites, for instance, by
encouraging the accumulation of fine sediments
and the growth of unicellular algae. The pro-
duction of adsorption sites and removal of
virus-laden particles should occur sequentially
through a series of densely vegetated cells (to
promote sedimentation of. particles and floccu-
lated bacteria) and open water cells (to promote
the growth of algae). Because viruses are charged
particles and respond to flocculants, at facilities
that pretreat with septic tanks, most viruses
become attached to the solids and remain in the
tank septage.
'16
VOLUME 2: DOMESTIC WASTEWATER
-------
CHAPTER 4
SURFACE FLOW WETLANDS
WETLAND DESIGN
Guidelines for designing a SF constructed
wetland are given in table 5.
CONFIGURATION
The configuration should take .advantage of the
natural topography of the site to minimize
excavation and grading costs. The configuration
should allow water to move through the wetland
by gravity. While treatment wetlands are often
designed as rectangles, wetlands can be built as
semi-circular or irregular shapes to fit the
topography of the site. Using curved shapes also
eliminates right-angled corners, which tend to
be "dead water" areas. If a shape other than a
rectangle is used, the widest portion should be
located at the inlet end to facilitate equal flow
distribution.
For large wetlands, dividing the wetland into
side-by-side cells should be considered. Divid-
ing a wide wetland into parallel cells lessens the
likelihood of preferential flow paths and short-
circuiting, and promotes the contact of the
wastewater with- the surfaces in the wetland. It
also facilitates maintenance since,one set of cells
can be taken out of operation temporarily.
If the removal of nitrogen and ammonia is a
major objective, including a deeper (2-3 ft)
open water pond in the middle of a longer
wetland cell should be considered to increase
nitrification and denitrification.
WATER DEPTH
The design should plan for 3 to 8 inches of
surface -water, with a maximum of 18 inches.
Deeper water may be advisable in winter to
accommodate the slower reaction rates during
cold weather and to guard against freezing. The
wetland may have to be divided lengthwise into
a series of cells to prevent the water in any of
the cells from being deeper than desired. Each
Configuration
Flow
Bottom slopes
Water depth
Vegetation
Construction
Table 5. Design summary for surface.flow wetlands.
Fit the wetland to the site
Divide large wetlands into side-by-side cells
./
By gravity, as much as possible
Side-to-side elevations: level
Inlet to outlet slopes: almost flat (0.5 - 1.0%)
3-8 inches, depending on the plants selected
18 inches maximum
Complete coverage is more important than the species used
Use at least two or three different species .
Wetland must be sealed to limit infiltration and exfiltration
Water table must be below or excluded from the wetland
VOLUME 2: DOMESTIC WASTEWATER
17
-------
of cells will then discharge to a downstream cell
of the same width. The maximum length of each
cell is based on the slope of the hottom of the
cell (which should not exceed 0.5 to 1.0%) and
the water depth suitable for the wetland vegeta-
tion (which is generally 18 inches or less). The
number and length of the subdivisions will
depend on the length of the cells and the slope
of the bottom. ,
The bottom of the cells-should be flat from
side to side to assure an even distribution of
water across the cells and to prevent channeling.
SIZING
Procedures for sizing SF wetlands for the
removal of BOD5, TSS, and nitrogen are still
preliminary. It has been widely presumed that
simple first order chemical reaction rates apply
for pollutant removal and that constructed
wetlands roughly follow plug-flow in their
internal hydrology. EPA's Design Manual:
Constructed Wetlands and Aquatic Plant Sys-
tems for Municipal Wastewater Treatment (1988)
and the Water Pollution Control Federation
Manual of Practice, Natural Systems for Waste-'
water Treatment (1990) assume that the reduc-
tion of a specific water quality parameter, such
as BOD , is a first order reaction and that con-
centration decreases exponentially with increas-
ing retention time within the wetland.
However, several recent studies have shown
that the movement of water through constructed
wetlands is considerably more complex than that
described by standard flow equations (Kadlec et
al. 1993. Kadlec 1994). The flow through a
wetland is related to the morphology of the cell,
the pattern of vegetation density, and the balance
between evapotranspiration and precipitation.
In constructed wetlands, mixing characteristics
are intermediate between plug flow and well-
mixed, flows are typically in the transition zone
between laminar and turbulent, and hydrologic
conditions change continuously with changes in
the weather and the seasons. Factors such as
obstructions to flow, the development of chan-
neling, recirculation patterns, and the presence
of stagnant areas cause further deviations from
calculated theoretical flows. Contact times are
not often as great as the theoretical residence
time calculated from the wetland empty volume
and the volumetric flow rate. As a final, compli-
cating factor, the chemistry of wetlands is com-
plex, involving interrelated biological reactions
and mass transfers. These factors and the lack of
good information on factors such as reaction rate
constants have probably led to many systems
being under-designed.
BIOCHEMICAL OXYGEN DEMAND .
The standard equation for BODS removal
for an unrestricted flow system (EPA 1988,
Water Pollution Control Federation 1990)
assumes that BOD5 removal is described by a
first-order model:
where
Ce/Co = exp (-Krt) .
C = effluent BOD5,mg/L
C = influent BOD5, mg/L
n _ *
(4.1)
AA.1AAMVU*. Jป V
- temperature-dependent first-order
reaction rate constant, days-
t = hydraulic residence time, days.
Flow through vegetated SF wetlands is com-
plex and equation 4,1 must be modified to account
for a number of the factors that affect flow through
wetlands. Reed et al. (1994) suggest the following:
Ce/C0=F exp (-0.7 . KT . V7' l = void fraction. ,
The values to be used for KT, F, AV, and in
VOLUME 2: DOMESTIC WASTEWATER
-------
designing constructed wetlands for domestic
wastewater have not been confirmed. A typical
value that is often used for'JC,. at 20ฐC is (D.0057
days -1 (EPA 1988). However, experimental data
on the values to be used in designing-con^
structed wetlands have been difficult to obtain
because of the logistic and economic difficulties
experimenting with wetlands on a scale large
enough to be appropriate. The wetland should
be designed generously to accommodate these
uncertainties.
The hydraulic residence-time (t) can be
represented by:
t = LWd/Q (4.3)
where
L = length
W = width
d = depth . -
Q = average flow rate [(flow in +
flow out)/2]
The Water Pollution Control Federation
(1990) recommends a minimum wetland area of.
about 28 to 37 ac per million gallons per day
(mgd) of wastewater (3 to 4 ha/1000mVday.). A
maximum BODS loading rate of about 90 lb/ac/
day (100 kg/ha/day) is recommended to help to
prevent the occurrence of nuisance problems.
NITROGEN
Adequate HRT as influenced by hydraulic
loading rate (HLR) and length-to-width ratio
appears to be an important factor affecting
TN removal efficiency, with lower removal
efficiencies at HLRs greater than 3 inches/day
(8 cm/day) and length-to-width ratios less than
2:1 (Water Pollution Control Federation 1990).
Total nitrogen removal efficiency through nitrifi-
cation/denitrification is temperature-dependent,
with lowered removal efficiencies below about
48ฐ F (10ฐ C). Knight (1986) found no decrease in
TN removal efficiency in a volunteer wetland at
temperatures above 50ฐ F (12ฐ C). The Water
Pollution Control Federation (1990) suggests that,
to attain a TN removal efficiency of 50% or more,
a minimum wetland area of about 37 ac per mgd
of wastewater (4 ha/1000m3/day) should be
provided. .
TOTAL SUSPENDED SOLIDS
Constructed wetlands are generally effective
at reducing the concentration of TSS. Removal.
efficiencies are similar to .those for BODS and
design for BOD5 should accomplish similar TSS
effluent levels. It is important to maintain
shaded conditions with dense vegetation near
the inflow and outflow to limit the growth of
algae, which can add to TSS levels.
VOLUME 2: DOMESTIC WASTEWATER
19
-------
20
VOLUME 2: DOMESTIC WASTEWATER
-------
CHAPTERS
SUBSURFACE FLOW WETLANDS
INTRODUCTION ,
The design information provided in this
chapter is a summary of the information in
Subsurface Flow Constructed Wetlands for
Wastewater Treatment: A Technology Assessment
(Reed 1993). Reed based his recommendations
on the performance of 14 municipal, domestic,
hospital, and industrial systems that have pro-
vided detailed data and that are thought to be
representative of constructed wetland systems in
the United States. Many of these systems are in
the South and West, and most have been operat-
ing for less than five years. Only a limited
number of systems in the Mid-Atlantic states
have provided operational data.
WETLAND DESIGN
Guidelines for designing a SSF wetland are
given in table 6.
.DARCY'S LAW
" The intent of the SSF wetland treatment
concept is to maintain the flow below the surface
of the media in the bed. The design of SSF wet-
lands has generally been based on Darcy's Law,
which describes the flow regime in a porous
medium. However, many of the systems designed
with Darcy's Law have developed unintended
surface flow and may have been under-designed.
Configuration
Flow
Bottom slopes
Inlet
Outlet
Vegetation
Construction
Table 6. Design summary for subsurface flow wetlands.
. Fit the wetland to the site .
Divide large wetlands into side-by-side cells.
By gravity, as much as possible
Subsurface flow design based on Darcy's Law
Side-to-side elevations: level
Inlet to outlet slopes: almost flat (0.5 - 1.0%)
Surface manifold with adjustable outlets
- Perforated subsurface manifold connected to adjustable outlet
Complete coverage .is more important than the species used
Use at least two or three different species
Porous media must be clean
Wetland must be sealed to limit infiltration and exfiltration
Water table, must be below, or excluded from the wetland
VOLUME 2: DOMESTIC WASTEWATER
21
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Darcy's Law assumes laminar flow, a con-
stant and uniform'flow (Q), and lack of short-
circuiting, conditions that do not exist in con-
structed wetlands. Darcy's Law is thought to
provide a reasonable approximation of the
hydraulic conditions in an SSF bed if small to
moderate size gravel (<1.5 inches, or <4 cm) is
used as the medium, the system is properly
constructed to minimize short-circuiting, the
system is designed to depend on a minimal
hydraulic gradient, and the flow (Q in equation
5.1) is considered to be the "average" flow
KQj + On J/2] in the system to account for any
gains or losses due to precipitation, evaporation,
or seepage.
Darcy's Law is typically defined with equa-
UonS.l:
Q=k,AS (5.1)
where
Q = flow per unit time, (fWday, gal/day,
mVday, etc.) . . .
k = hydraulic conductivity of a unit
area of the medium perpendicular
to the flow direction, (fWftVday,
gal/day, mVnWday, etc.)
A = total cross-sectional area, perpen-
diculartoflow(ft2,rn2,etc)
S = hydraulic gradient of the water 1
surface in the flow system
(slope of the water table)
(dh/dL, ft/ft, m/m).
Systems in .the United States and Europe
with successful hydraulic performance do so
either with a sloping bottom and/or adjustable
outlet structures which allow the water level to
be lowered at the end of the bed. A sloped
bottom or lowering the water level at the end of
the bed produces the pressure head required to
overcome resistance to flow through the media
, and thus maintains subsurface flow.
Clogging has occurred in some systems. The
clogging is believed to result from the introduc-
tion of fine particulate material into the medium
because of improper construction procedures.-
Neyertheless, it is judicious to provide a large
safety .factor against clogging. A value
-------
for 2 ft (0.6 m) deep beds and to about 0.75:1 for 1
ft (0.3 m) deep beds. Using such a low value for
hydraulic gradient will help to maintain near-
laminar flow in the bed and validate the use of
Darcy's Law in the design of the system. Since this
approach ensures a relatively wide.entry zone, it
will also result in low. organic loading on the cross-
sectional area and thereby lessen concerns over
clogging.
BED SLOPE
The bottom of the cell can be flat or slightly
sloping from top to bottom. The top surface of the
medium should be. level regardless of the slope of
the bottom. A level surface facilitates plant man-
agement and minimizes surface flow problems.
Once surface flow develops on a downward sloping
surface, flow may not penetrate the medium even
though the true water level within the medium is
well below the surface.
SIZING
BIOCHEMICAL OXYGEN DEMAND
SSF systems are generally sized for BOD5 re-
moval. In SSF systems, the physical removal of
BODS is believed to occur rapidly through settling
and entrapment of particulate matter in the void
spaces in the gravel or rock media (Reed 1993).
Data from 14 SSF systems in the United States
indicate that BOD5 removal improves only slightly
after 1 to 1.5 days HRT, up to an HRT of 7.5 days.
The removal data for these systems can be reason-
ably approximated by a first order plug flow rela-
tionship up to about ฑ2 days. BOD5 removal there-
after is limited and is believed to be influenced by
the. production of residualBODj within the system.
This is compatible with the hypothesis that BODS is
removed rapidly in the front part of wetland
systems.
Most of the existing systems in the United
States and Europe have been designed as attached
growth biological reactors using the same equa-
tions as those used for SF wetlands (equations
4.1 - 4.3). The plug flow model is presently in
general use and seems to provide a general
approximation of performance.. It is believed
that the plug flow rate constant for SSF wetlands
is higher than for facultative lagoons or SF
wetlands because the surface area available on
the media in SSF wetlands is much higher than
in the other two cases. This surface area-sup-
ports the attached growth microorganisms that
are believed to provide most of the treatment
responses in the system. At an apparent organic
loading of 98 Ib/ac/day (110 kg/ha/day), the rate
constant for the SSF wetland (1.104 d'1) is about
an order of magnitude higher than that for
facultative lagoons, and about double the value
.often used for SF wetlands.
The "t", or hydraulic residence time (HRT)
factor in equation 4.1 can be defined as:
t = nLWd/Q (5.2)
where
n = effective porosity of media (% as a
decimal)
L = length of bed (ft, m) .
W = width of bed (ft, m)
d = average depth of liquid in bed (ft, m)
Q = average flow through bed (ftVday,
mVday). .
The Q value in equation 5.2 is the average
flow in the bed {(Qjn + Qout)/2] to account for
precipitation, seepage, evapotranspiration, and
other gains and losses of water during transit in
the bed. This is the same value used in Darcy's
Law for hydraulic design.
The "d" value in the equation is the average
depth of liquid in the bed. If, as recommended
previously, the design hydraulic gradient is
limited to 10% o'f the potential available, then
the average depth of water in the bed will be
equal to 95% of the total depth of the treatment
media in the bed.
VOLUME 2: DOMESTIC WASTEWATER
23
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Since the term LW in equation 5.2 is equal to
the surface area of the bed, rearrangement of
terms permits the.calculation of .the 'surface area,
'(AJ required to achieve the necessary level of
BODS removal:
, = Qln(Ce/C0)/-kTdn (5.3)
where
At = bed surface area (ft2, m2)
other terms as defined previously.
, The final design and sizing of the SSF bed for
BOD5 removal is an iterative process:
1. Determine the media type, vegetation, and
depth of bed to be used.
2. By field or laboratory testing, determine the
porosity (h) and "effective" hydraulic conduc-
tivity (k,) of the media to be used.
3. Use equation 5.3 to determine the required
surface area (A,) of the be'd for the desired
levels of BODS removal.
4. Depending on site topography, select a pre-
liminary length-tp-width ratio; 0.4:1 up to 3:1
are generally acceptable.
5. Determine bed length (L) and width (W) for
the previously assumed length-to-width ratio,
and the results of step 2. .
6. Using .Darcy's Law. (equation 5.1) with the
previously recommended limits (ks
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NITROGEN
The major pathway for nitrogen removal in
SSF wetlands is biological nitrification followed
by denitrification. The controlling factor in
ammonia removal is the availability of oxygen in
the substrate. In continually saturated beds,
leakage of oxygen from the roots of plants is the
major source of oxygen.
Two systems demonstrating excellent ammo-
nia removal have plant roots (and therefore some
available oxygen) throughout the profile, and
sufficient HRT to complete the reaction. Data
from these systems suggest a two-stage system in
which the BODg is decreased to about 20 mg/L,
followed by nitrification with oxygen supplied
by the vegetation. The limiting factor-in this
case is the rate at which the plants can provide
oxygen. The extent to which plants can provide
oxygen is unknown at this time. The remaining
BODg in the second stage would then be avail-
able for denitrification. .
Alternative methods for nitrification include
shallow overland flow, mechanical aeration after
BODS removal, providing open water zones for
surface reaeration, and using parallel cells
operated on a batch*type fill and draw basis to
allow atmospheric oxygen to be introduced into
the substrate.
VOLUME 2: DOMESTIC WASTEWATER .25
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26
VOLUME 2: DOMESTIC WASTEWATER
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REFERENCES
Bastian, R. K., and p. A. Hammer. 1993. The use
of constructed wetlands for wastewater treatment
and recycling, pp 59-68 in Constructed Wetlands
for Water Quality Improvement. G. A. Moshiri
(ed.). CRC Press, Boca Raton, FL,
Brix,H. 1993. Wastewater treatment in con-
structed wetlands: system design, removal
processes, and treatment performance, pp 9-22
in Constructed Wetlands for Water Quality
Improvement, G. A. Moshiri (ed.). CRC Press,
Boca Raton, FL.
Center for Environmental Resource Management.
1993. Proceedings Subsurface Flow Constructed
Wetlands Conference. University of Texas-El
Paso, El Paso, TX. 306 pp. .
Cooper, C.M., S. Testa, III, and S. S. Knight. 1993.
Evaluation of ARS and SCS Constructed Wet-
land/Animal Waste Treatment Project at
Hernando, Mississippi. National Sedimentation
Laboratory Research Report No. 2, Oxford, MS.
55 pp.
Crumpton, W. G., T. M. Isenhart, and S. W. Fisher.
1993. Fate of non-point .source loads in freshwa-
ter wetlands: results from experimental wetland
mesocosms. pp 238-291 in Constructed Wetlands
for Water Quality Improvement, G. A. Moshiri
(ed.); CRC Press, Boca Raton, FL.
EC/EWPCA. 1990. European.Design and Opera-
tions Guidelines for Reed Bed Treatment Sys-
tems. P. F. Cooper (ed.) Proceedings Interna-
tional Conference on the Use of Constructed
Wetlands in Water Pollution Control. Pergamon
Press, Oxford, U.K.
EPA (Environmental Protection Agency). 1988.
Design Manual: Constructed Wetlands and.
Aquatic Plant Systems "for Municipal Wastewater
Treatment. EPA/625/1-88/022. Center for Envi-
ronmental Research, Cincinnati, OH. 83 pp.
EPA (Environmental Protection Agency). 1993.
Created and Natural Wetlands for Controlling
Nonpoint Source Pollution. CRC Press, Boca
Raton, FL/ 216 pp. -
Faulkner, S. P., and C.J. Richardson. 1989. Physi-
cal and chemical characteristics of freshwater
wetland soils, pp 41-72 in Constructed Wetlands '
. for Wastewater Treatment, D. A. .Hammer (ed.).
Lewis Publishers, Chelsea, MI.
Gambrel, R. P., and W. H. Patrick, Jr. 1978. Chemi-
cal and microbiological properties of anaerobic
soils and sediments, in Plant Life in Anaerobic
Environments, D. D. Hook and R. M. m. Crawford
(eds.) Lewis Publishers, Chelsea, MI.
Hammer, D. A (ed.). 1989. Constructed Wetlands
for Wastewater Treatment: Municipal, Industrial
and Agricultural. Lewis Publishers, Chelsea, MI.
831 pp.
Hammer, D. A. 1992. Designing constructed
wetland systems to treat agricultural nbnpoint
source pollution. Ecological Engineering 1:49-
82^
Hey, D. L., K. R. Barrett, and C. Biegen. 1994. The
hydrology of four experimental constructed
marshes. Ecological Engineering 3:319-343.
Ives, M. 1988. Viral dynamics in artificial wet-
lands, pp 48-54 in G. A. Allen and R. A.
Gear-heart (eds.). Proceedings of a Conference on
Wetlands for Wastewater Treatment and Resource
Enhancement. August.2-4, Humboldt State
University, Arcata, CA..
Kadlec. R. H. 1994. Detention and mixing in free
water wetlands. Ecological'Engineering 3:345-
380.
Kadlec, R. H., W. Bastiaens, and D. T. Urban. 1993.
Hydrological design of free water surface treatr
ment wetlands, pp 77-86 in Constructed Wet-
lands for Water Quality Improvement, G. A.
Moshiri (ed.). CRC Press, Boca Raton, FLf
VOLUME 2: DOMESTIC WASTEWATER
27
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Knight, R.U. 1986. Florida Effluent Wetlands -
Total Nitrogen. CH2M HILL Wetland Tech. Ref.
Doc. Ser., No 1.
Knight, R L., R. Wl Ruble, R. H. Kadlec, and S.C.
Reed. 1994. Wetlands Treatment Database
(North American Wetlands for Water Quality
Treatment Database). USEPA, Risk Reduction
Engineering Laboratory, Cincinnati, OH. The
database is available on 3.5" diskette, and
requires DOS 3.3 or higher, 640K of memory,
and 4MB of free disk space. To order,,contact:
Don Brown, USEPA (MS-347), Cincinnati, OH
45268; phone: (513) 569-7630; fax: (513) 569-
7677; e-mail: brown.donald@epamail.epa.gov.
Mitsch, W. j., and J. G. Gosselink. 1986.. Wet-
lands. Van Nostrand Reinhold, New York, NY.
539 pp.
Phipps, R. G., and W. G. Crumpton. 1994. Fac-
tors affecting nitrogen loss in experimental
wetlands with different hydrologic loads.
Ecological Engineering 3:399-=408.
Reed, S.C- 1993.. Design of.Subsurface Flow
Constructed Wetlands.For Wastewater Treat-
ment: a Technology Assessment. .EPA 832-R-
93-001. EPA Office of Wastewater Management,
Washington, DC.
Reed, S., andD.'S. Brown. 1992.' Constructed
wetland design - the first generation. Water
Environment Research 64(6):776-781.
Reed, S., F. J. Middlebrooks, and R.W. Crites.
1994. Natural Systems'for Waste Management
and Treatment. 2nd ed. McGraw-Hill, New
York.
Steiner, G. R., and R. J. Freeman, Jr. 1989. Con-
figuration and substrate design considerations
for constructed wetlands wastewater treatment.
pp 363-377 in Constructed Wetlands for Waste- .
water Treatment, D. A. Hammer (ed.).- Lewis
Publishers, Chelsea, MI.
Watson, J.T., and].,A-Hobson. 1989. Hydraulic
. design considerations and control structures for
constructed wetlands for wastewater treatment.
pp 379-391 in Constructed Wetlands for Wastewa-
ter Treatment, D. A. Hammer (ed.). Lewis Pub-
lishers, Chelsea, MI. .
Water Pollution Control Federation. 1990. Natural
Systems for Wastewater Treatment. Manual of
Practice FD-16, Chapter 9. Alexandria, VA.
28
VOLUME 2: DOMESTIC WASTEWATER
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ABBREVIATIONS AND CONVERSION FACTORS
MULTIPLY
ac,acre
cfs, cubic foot per second
cfs, cubic foot per second
cm, centimeter
cm/sec, centimeter per second
ฐF, degree Fahrenheit
ft. foot
ft2, square foot
ft3, cubic foot
ft/mi, foot per mile
fps, foot per second .
g/mz/day, gram per square meter per day
gal, gallon
gal, gallon
gpm, gallon per minute
ha, hectare
inch
kg, kilogram
kg/ha/day, kilogram per hectare per day
kg/m2, kilogram per 'square meter
L, liter
L, liter
Ib, pound .
Ib/ac, pound per acre
m, meter
m2, square meter
m3, cubic meter
m3, cubic meter .
m3/ha/day, cubic meter per hectare per day
. mm, millimeter
.mi, mile
BY
0.4047
448.831 .
2.8317 X 10"2
0.3937
3.28x10-*
5/9(ฐF-32)
0.305
9.29 x 10'2
2.83 x lO'2
0.1895
18.29
8.92
3.785
3.785 x 10'3
6.308 x ID'2
2.47
. 2.54
2.205
0.892
0.2
3. 531 x 10'2
0.2642
0.4536
1.121
3.28 .
10.76
1.31
264.2
106.9
/ 3.94 xlO'2
1.609
TO -OBTAIN
ha, hectare
gpm, gallon per minute
m3/s, cubic meter per second
inch
fps, foot per second
ฐC, degree Celsius
m, meter
m2- square meter
m3, cubic meter
m/km, meter per kilometer
m/min, meter per minute
Ib/ac/day, pound per acre per day
L; liter :
m3, cubic meter
L/s, liter per second
ac, acre
cm, centimeter ,
Ib, pound
Ib/ac/day, pound per acre per day
lb/ft2, pound per square foot
ft3, cubic foot
gal, gallon
kg, kilogram
kg/ha, kilogram per hectare
ft, foot .
ft2, square foot
yd3, cubic yard
gallon, gal
gallon per day per acre, gpd/ac
inch
kilometer, km
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For sale by the US. Government Printing Office-
Superintendent of Documents, MaU Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-053000-8-
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