oo
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
AGRICULTURAL WASTEWATER
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ACKNOWLEDGMENTS
Many penpl.. contributed to this Handbook. A,i In.«r:u.,u<:y Core Group provido.l the initial impetus for the Handbook, and inter orovid*H
guidnnce and technical input during its preparation. The Core Group comprised:
Carl DuPoldl. USDA - NRCS. Chester. PA
Robert Edwards, Susquuhanna River Basin Commission.
Harrisburg. PA
Lainonle 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. Harrisbura. PA
Harold Webster, Penn State University, DuBois. PA.
Wet'andS C011tributed fay Providing information and by reviewing and commenting on the Handbook. These
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, Hedin Environmental,
Sewickley. PA
William Hellier, Pennsylvania Department of
Environmental Resources, Hawk Run. PA
Robert Kadlec. Wetland Management
Services. Chelsea, MI
Douglas Kepler, DamariscoUa, 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 Neumilfer, 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.^.
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 Water Act.
The findings, conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA - NRCS,
EPA - Region HI. 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.
PS5384RSGZ
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VOLUME 3
TABLE OF CONTENTS
CHAPTER 1. INTRODUCTION
• 3
CHAPTER 2. USING CONSTRUCTED WETLANDS IN AGRICULTURE
Contaminant Removal Processes '.'
Advantages and Limitations of Constructed Wetlands
Creating Effective Constructed Wetlands ' 6
Types of Constructed Wetlands .""
Wastewater Characteristics ..'."."...'
Water Quality , _
Water Quantity ; "" •
Prertreatment .....
Discharge Option , ', '""""
CHAPTERS. PERFORMANCE EXPECTATIONS ] u
Introduction ;.... _ ' '""
Biochemical Oxygen demand and Total Suspended Solids n
Nitrogen
Phosphorus
Pathogens
_, . •"" " • 14
Toxics
CHAPTER 4. SURFACE FLOW WETLANDS ; 17
Wetland Design ; '
Configuration .-. _
Water Depth '
Sizing „ ZZZZZZZZZZZZZZZ 18
Presumptive Method for BOO 18
Field Test Method for BOD '. ZZ.1Z....Z....ZZ... 20
Presumptive Method for Nitrogen . 21
CHAPTER 5. SUBSURFACE FLOW WETLANDS 23
Introduction . ' "•
Wetland Design \ _ '""",". 23
Darcy's Law. 23
Media Types .^Z.Z.ZZ 24
Length-to-Width Ratio ",' 24
Bed Slope .......Z.......Z . 25
Sizing ; ZZZZZZZZZZZZZ.25
Biochemical Oxygen Demand ' 25
Total Suspended Solids .ZZZ" 26
Nitrogen _ 26
REFERENCES '
"*
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LIST OF TABLES
Table 1. Removal mechanisms in constructed wetlands
Table 2. Advantages and limitations of constructed wetland treatoent'of [[[ 5
domestic wastewater ....................................
Table 3. Guidelines for creating constructed wetlands ................. • ........ ............................................... ' ............... 5
Table 4. Summary of agricultural constructed wetland operationaTdata .............................. ' .............................. 6
Table 5. Design summary for surface flow wetlarids ........ [[[ • n
Table 6. Design summary for subsurface flow wetlands ............................................... " ..................... ' ............ 17
[[[ 23
LIST OF FIGURES
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CHAPTER 1
INTRODUCTION
This volume focuses on the use of con-
structed wetlands to treat agricultural wastewa-
ter. It is to be used in combination with
Volume 1: General Considerations, which pro-
vides information on wetland hydrology, soils
and vegetation, and on the design, construction.
operation, and maintenance of constructed
wetland systems.
Constructed wetlands can provide an inex-
pensive and easily operated means of removing
organic matter, particulates, nutrients, and
bacteria from agricultural wastewater. Agricul-
tural wastewaters suitable for wetland treatment-
include milkhouse wastewaters, runoff from
concentrated livestock areas, and effluents from
settling tanks and manure treatment lagoons.
Interest in using constructed wetlands to
treat agricultural wastewaters has been
prompted by the success of constructed wet-
lands in removing organic matter, particulates,
and nutrients from municipal wastewaters.
However, the use-of constructed wetlands in
agriculture is a fairly recent development and
the number of systems that have been installed"
is still small. While the data show that properly
designed wetlands can be effective in treating
agricultural wastewater,.much is not yet under-
stood and many of the relationships between
design and performance have not been clearly
established. The use of constructed wetlands in
agriculture will be modified and refined as more
systems are installed and monitoring data are
gathered over longer periods of time. The guid-
ance presented here should be considered as
today's "state of the art" and will likely be modi-
fied as our understanding of these systems grows.
As with other agricultural waste management
practices, constructed wetlands are one compo-
nent of an overall agricultural waste management
system (AWMS). This Handbook discusses the
contributions that constructed wetlands can make
to an AWMS, the performance that can reasonably
be expected, and the factors that are important in
the design of effective constructed wetlands. Step-
by-step procedures in designing constructed wet-
lands are given.
This volume incorporates the guidance pre-
sented in the Soil Conservation Service (SCS, now
the Natural Resources Conservation Service) 'con-
structed Wetlands for Agricultural Wastewater
Treatment Technical Requirements (1991), which is
an important reference for those interested in usin°
constructed wetlands in agriculture. °
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CHAPTER 2
USING CONSTRUCTED WETLANDS IN AGRICULTURE
CONTAMINANT REMOVAL
PROCESSES
The most important removal mechanisms in
agricultural constructed wetlands are physical
sedimentation and filtration, and biological assimila-
tion, breakdown, and transformation (table I).
The suspended solids that remain in the effluent
from pretreatment unit are removed in the wetland
by sedimentation and filtration. These physical
processes also remove a significant portion of other
wastewater constituents,such as biochemical
oxygen demand (BOD), nutrients, and pathooenc
(Brix 1993). Soluble organic compounds are° for
the most part, degraded by microbes, especially
bacteria, that are attached to the surfaces of pla'nts
litter, and the substrate.
ADVANTAGES AND LIMITATIONS
OF CONSTRUCTED WETLANDS
Constructed wetland treatment of agricultural
wastewater offers a number of advantages (table 2),
Table 1. Removal mechanisms in constructed wetlands
(after Brix 1993).
Wastewater ConstihiPnf
Biochemical oxygen demand
Suspended solids
Nitrogen
Phosphorus
Pathogens
Removal
Microbial degradation (aerobic and anaerobic)
Sedimentation (accumulation of organic matter/slud°e
sediment surfaces)
Sedimentation/filtration
on
by microbial nitrification and
Plant uptake
Volatilization of ammonia
Soil sorption (adsorption-precipitation reactions with aluminum
iron, calcium, and clay minerals in the soil) "«««n.
Plant uptake
Sedimentation/filtration
Natural die-off
Table\2. Advantages and limitations of constructed wetland treatment.
Advantages
• are capable of providing a high level of
treatment
• can reduce or eliminate odors
• are inexpensive to operate
• are largely self-maintaining
Limita{iong
are affected by season and weather, which may
reduce treatment reliability
are sensitive to high ammonia levels
may hold potential for mosquitoes and other insect
pests
are able to handle variable wastewater loadings * require a continuous supply of water
• reduce the amount of area needed for land
application
require dedicated, single land use
• may be more expensive to construct than other
treatment options
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including odor control and simplicity of opera-
tion. Constructed wetland treatment is con-
strained by a number of limitations, including
variability in treatment effectiveness and the
sensitivity of wetland plants to high ammonia
concentrations. The advantages and limitations
of a constructed wetland as an alternative to
other treatment options must'be understood and
weighed before deciding to install a constructed
wetland.
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 as possible to operate
while ensuring reliable treatment. Building a
slightly larger system may be more expensive to
construct but may be less costly and more
reliable to operate than a smaller system. At-
tention to several factors will help to ensure
successful wetland treatment:
• Adequate pretreatment. Pollutant loads in
agricultural wastewaters often greatly exceed
the ability of a wetland to treat or assimilate
them. A wetland can be severely damaged by
a wastewater that is too concentrated, for
instance, one that contains high levels of
ammonia. Pretreatment to lower pollutant
loads is essential to avoid overloading the
wetland.
• Adequate retention time. A wetland treats
wastewater through a number of biological
(largely microbial). physical, and chemical
processes. The water must remain in the
wetland long enough for biological and
chemical transformations to take place and for
Table 3. Guidelines for creating constructed wetlands.
Know what you are dealing with
Size the wetland generously
Wetlands must have water
Give the plants a chance
Don't overload the wetland
Don't kill the wetland
Effluent disposal must be addressed
Keep an eye on what is happening
Get interdisciplinary help
Sample the wastewater
Know what pretreatment will accomplish
Too small a wetland cannot perform well
Know the water budget
Provide a supplemental source of clean water
Allow time for establishment
Avoid shock loading
Keep ammonia levels to 100 mg/L or less
Application rates must not exceed treatment rates -
Keep raw milk out of the wetland
Keep herbicides out of the wetland
Use other recognized practices
Monitoring is important
Environmental engineer
Water quality specialist.
Plant materials specialist/biologist/extension agent
State agencies
VOLUME 3: AnRinn.TttPAi WASTCWATFB
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sedimentation and deposition to occur. The
wetland must be built large enough to pro-
vide the necessary retention time.
Supplemental water. If a constructed wet-
land is to remain healthy, it must remain
relatively wet. Wetlands are generally
.tolerant of fluctuating flows, but they cannot
withstand complete drying. For this reason,
either a slow release of wastewater must be
assured or a supplemental source of water
must be provided. Supplemental water can
be used to dilute the wastewater to accept-
able levels and also to assure that the wet-
land stays wet. Enough water should be
supplied to the wetland to maintain a slow
flow of water, since stagnant water can lead
to problems with odors and mosquitoes.
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 input streams accordingly. While wet-
lands are low maintenance systems, they are
not maintenance-free. For instance, the
pretreatment unit must be cleaned periodi-
cally to keep excessive solids from entering
the wetland, and valves and piping must be
checked to detect and correct blockages or
leaks.
TYPES OF CONSTRUCTED
WETLANDS
Most wetlands used in agriculture are
surface flow (SF) wetlands. The advantages of
SF wetlands are that their design and construc-
tion are straightforward. Operation and main-
tenance are simple. Because the water surface
is unconstrained, SF wetlands are able to
accept wide variations in flow. SF wetlands
can provide excellent removal of 5-day bio-
chemical oxygen demand (BOD5) and total
suspended solids (TSS). Good removal of ammo-
nia and total nitrogen has been achieved at some
wetlands. SF wetlands are discussed in Chapter 4.
In subsurface flow (SSF) wetlands, the water
level must.r.emain below the surface of the sub-
strate if the wetland is to perform well. The
design and construction of SSF wetlands are
therefore more complicated than for SF wetlands
and SSF wetlands must be managed and moni-
tored much more closely. There have been prob-
lems with unintended surface flow and apparent
plugging. Because of the hydraulic constraints
imposed by the media, SSF wetlands are best
suited to relatively uniform flows. Their use in
agriculture has thus been limited. SSF wetlands
may be appropriate for field drain discharges and
research on such systems is being conducted.
SSF systems are discussed in Chapter 5.
WASTEWATER CHARACTERISTICS
To design the wetland correctly, an accurate
assessment of contaminant loadings is needed
(loading = contaminant concentration x waste-
water volume). To calculate loadings, data are
needed on the average water quality, the maximum
concentrations, and the largest and smallest
volumes that may occur. Loadings may vary
throughout the year as the volumes of water
change in response to climatic factors, such as
rainfall and evaporation. Maximum concentra-
tions will probably occur in the late summer and
fall, when water losses due to evapotranspiration'
are greatest. The highest flows can be expected
during the wet season, but pollutant concentra-
tions may be lower at this time because of dilu-
tion. The design should be based on the highest
pollutant loadings.
WATER QUALITY
The characteristics of agricultural wastewater'
vary, depending on the specifics of the agricultural
operation, and should be determined by laboratory
analysis before the wetland is designed. The Soil
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Conservation Service (SCS. nou- the Natural
Resources Conservation Service) recommends the
following analyses for agricultural wastewater
(SCS 1991):
5-day biochemical oxygen demand (BOD )
total solids (TS) " 3
total Kjeldahl nitrogen (TKN)
nitrate nitrogen (NO3-N)
ammonia nitrogen (NH3 + NH^-N)
total phosphorus (TP).
The design of the wetland is usually based on
the removal of BOD (usually measured as 5-day
biochemical oxygen demand, BODS) or nitrogen
(measured as total Kjeldahl nitrogen or nitrate
nitrogen). Concentrations of ammonia (NH3 +
NH4-N, un-ionized ammonia + the ammonium ion)
should be evaluated because of the toxicity of
ammonia to \vetland plants.
Additional analyses may be needed. For
instance, if high salinities could occur, chloride
concentrations should be measured to determine
the salinities that the vegetation will be exposed
to; salinities in the brackish range will suggest that
salt-tolerant vegetation should be planted. The
likelihood of toxic compounds, and high or low
pHs that could affect the biological components
of the wetland should be considered.
WATER QUANTITY
An accurate estimate of the volume of waste-
water is needed, including the expected average,
maximum, and minimum wastewater flows, An
accurate figure for the volume of water to be
treated must be determined: too small a wetland
will perform poorly; a large wetland may require
supplemental water to maintain the wetland
during the dry season. The maximum expected
flow must be determined. If the maximum ex-
pected flow is larger than the capacity of the
wetland, a bypass will be required. The minimum
expected flow should be calculated to determine
the volume of supplemental clean water that may
be needed.
PRETREATMENT
Raw agricultural wastewaters are character-
ized by very high concentrations of BODS,
nutrients, and total and dissolved solids.3 To
avoid overloading \vetland removal capabili-
ties, raw wastewaters must be pretreated to
lower the concentrations of these contami-
nants. Pretreatment is also necessary to lower
nutrient and organic loads (particularly ammo-
nia) to avoid damaging the wetland vegetation.
Pretreatment to lo\ver total organic loading
also helps to control mosquito populations
(Wieder et al. 1989). Pretreatment can be made
through settling tanks and basins, flotation
tanks, filters, or mechanical separators, either
singly or in combination. The Agricultural
Waste Management Field Handbook (SCS
1992) discusses various pretreatment options.
In the northeastern United States, percent
removals in BODsby settling tanks are gener-
ally around 70%. To protect the vegetation,
SCS (1991) suggests a target concentration of
100 mg/L ammonia for the effluent from the
pretreatment unit. However, Reaves et al.
(1995) found that while concentrations of
200 -.300 mg/L ammonia damaged the growth
of young cattail shoots in a wetland used to
treat swine effluent, mature cattails did not
seem to be affected. Reaves et al, (1995)
theorize that the ammonia may be present as
the non-toxic ammonium ion (NH/) rather
than the toxic ammonia ion (NH3).
Pretreatment must remove solids. Most
settleable, floating, and non-biodegradable
solids, such as plastics and grease, must be
removed before the water enters the wetland or
the wetland will eventually clog. Pretreatment
to remove solids also avoids potential clogging
of pipelines, gates, and valves. Since bacteria"
and viruses adsorb on solids, pretreatment to
decrease solids also decreases the numbers of
bacteria and viruses (Ives 1988). SCS (1991)
suggests 1,5 00 mg/L total solids or less as a
target concentration for the effluent from the
pretreatment unit.
8
VOLUME 3- Anorr-MtTtiD»t U/»
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Fats are a particular problem since they float
on the surface of the water, causing a scum that
blocks gas transport and rapidly depletes dis-
solved oxygen. Raw milk can suffocate a wetland
and must be kept out of the wetland.
An organic filter (a bed of chipped mixed
wood bark) is being tested as a means of reducing
odors and ammonia concentrations at a veal
operation (Murphy et al. 1993). The effluent from
the settling tanks passes through the filter by
subsurface flow before .entering the wetland. The
bed has completely eliminated odors; ammonia
removals have been variable.
In addition to pretreatment, dilution may be
necessary for som.e wastewaters. The wetland
system can be designed to recycle the wetland
effluent for use in diluting the effluent from the
pretreatment unit.
DISCHARGE OPTIONS
There are several options for the wetland
effluent:
• storage for later land application
• discharge to an infiltration area
• recycle through the wetland.
Where possible, the effluent should be re-
cycled. Recyling the treated effluent from the
wetland is an efficient way to dilute influent BOD
and suspended solids. Recycling decreases the
. potential for odors and may possibly increase
dissolved oxygen concentrations, which in turn
may enhance nitrification. Recycling is also an
efficient way to maintain adequate flows during
low-flow periods.
The disadvantages of recycling are the in-
creased construction costs and increased operation
(pumping) costs. Also, recyling may slowly
increase salinities as evapotranspiration removes
water from the system.
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CHAPTERS
PERFORMANCE EXPECTATIONS
INTRODUCTION
Data on agricultural systems are limited, both
in the number of systems that have been built and
in the length of time the systems have been operat-
ing. However, a number of constructed wetland
systems are being used to treat domestic wastewa-
ters, which contain a array of contaminants similar
to those in agricultural wastewater. Information
from domestic systems is thus useful in assessing
the potential of constructed wetlands to .treat
agricultural wastewater.
BIOCHEMICAL OXYGEN
DEMAND AND TOTAL
SUSPENDED SOLIDS
Wetlands provide a number of mechanisms for
removing BOD5 and TSS, and constructed wet-
lands are,extremely efficient at removing these
contaminants. At seven agricultural systems that
have recently begun operating, removals range
from about 60% to more than 90% (table 4). In a
survey of 324 municipal, industrial, stormwater,
and other constructed wetlands, Knight et al.
Table 4. Summary of agricultural constructed wetland operational data
BODs (mg/L)
In Out %.
TSS (mg/L)
In Out
NH3-N (mg/L)
In Out %.
Dairya
Dairy0
Dairy0
Swine1'
Swine6
Swine*
Chicken
manure?
Field drain
effluent11
37
36
37
1,343
1,688
1,998
354
64
45
45
45
47
45
1,320
1,550
13 65
9 75
11 70
375 68
339 81
397 62
138 61
6 91
28 38
80
80
20 56
24 47
230 83
190 88
137
109
125
700
7,216
1,338
282
105
118
118
118
94
88
1,060
1,300
44 68
56 49
47 62
281 65
151 89
558 42
75
9
•17
10
9
31
36
155
130
73
91
86
92
92
'67
59
85
90
6.5
6.1
7.5
243
208
343
64
54.7
94
94
94
112
112
0.4 93
0.3 95
1.2 84
68 70
110 51
90 SO
29 54
3.5 94
23 76
3 97
2 98
36 68
41 63
0.3 0.3
TN(mg/L)
In Out
654 201 56
476 215 57
890 275 36
92 38 57
70 6 91
104 41 61
104 6 94
104 5 95
295 <95 68
320 64 80
23 21 6
TP(mg/L)
In Qui
13.5
14.4
14.2
56
75
62
14
25.8
66
66
66
28
27
22
25
0.02
3.3 76
3.8 74
5.7 60
24 58
22 78
23 49
S 66
6.2 76
30 55
23 65
15 77
18 36
16 41
<4 >80
5 80
0.02 0
%: percent removal .
a surface flow, milkhouse wash water + runoff + rainfall, Mississippi, 2 years of data (Cooper et al. 1993)
b surface flow, bam washwater + yard runoff, Indiana, 1 year of data (arcsine means) (Reaves et al. 1994)
c surface flow, milking parlor washwater + yard runoff + rainfall, Oregon, 6 months of data (Skarda et al. 1994)
d surface flow, effluent diluted with stormwater pond effluent. Alabama, 1 year of data (Hammer et al. 1993)
e surface flow, lagoon effluent, flow rates of 2610 gpd/1094 gpd/540 gpd (lines 1-3). Alabama. 3 months of data
(McCaskey et al. 1994) or _ or
f surface flow, lagoon effluent, marsh-pond-marsh system, Mississippi, 16 months of data (Cathcart, Hammer.
and Triyono 1994). .
g subsurface flow, dilute chicken manure, Czechoslovakia, 1 year of data (Vymazal 1993]
h subsurface flow, cropland tile drain effluent, Pennsylvania, 4 years of data (Taylor et al. in preparation)
-------
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BOD, Mass Loading Rate (kg/ha/d)
Figure 1. BOD, mass loading and removal rates in
wetland treatment systems
(from Knight et al. 1993).
(1993) found that BOD, mass removal efficiencies
were generally 70% or more at mass loading rates
up to 250 Ib/ac/day (280 kg/ha/day), which is
considerably higher than the 65 Ib/ac/day recom-
mended by SCS (1992) as the maximum BOD,
loading rate for agricultural wetlands. A linear
regression of the 324 data records used to examine
the predictability of BOD, outflow concentration as
a function of BOD, inflow concentration and
hydraulic loading produced the following (figure 1)
(Knight et al. 1993):
BODOUT = 0.097*HLR + 0.192*BODIN
Rz'= 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
usually high, rates sometimes varied considerably
on a monthly or seasonal basis (Knight et al. 1993).
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 BOD of
5 to 7 mg/L is often present in wetland effluent
(EPA 1993). This internal production of BOD
decreases removal efficiencies at very low inflow
concentrations.
TSS is removed primarily by sedimentation and
Miration, and removal is enhanced as the density of
surfaces within the wetland increases. Cooper et al
(1993) found that TSS removals increased as plant
litter accumulated.
To maximize the removal of BOD5 and TSS, the
growth of plants (particularly underground tissues)
and the accumulation of litter should be encouraged.
Plants and plant litter provide organic carbon and
attachment sites for microbial growth, and promote
filtration and sedimentation. Because of the impor-
tance of microbial processes in removing BOD ,
adequate residence time must be provided. The SCS
(1991) recommends a hydraulic residence time of at
least 12 days.
NITROGEN
In contrast to the simplicity of BOD and TSS
removal, the chemistry of nitrogen removal is com-
plex (figure 2). Nitrogen occurs in a number of
forms, including organic and inorganic compounds,
and nitrogen gas. In wetlands, the important forms
include nitrogen gas (N2j, nitrate (NO/), nitrite (NO/)',
ammonia (NH3), and ammonium (NH/).
. In wetlands, the removal of nitrogen involves a
series of reactions (Mitsch and Gosselink 1986).
Decomposition and mineralization processes .convert
a significant part of organic nitrogen to ammonia.
Ammonia is oxidized to nitrate by nitrifying bacteria
in aerobic zones (nitrification) and nitrates are
converted to nitrogen gas by denitrifying bacteria in
anoxic zones (denitrification); the gas is released to
the atmosphere. The sequence is:
mineralization:
organic nitrogen -> ammonia aerobic or anaerobic
nitrogen reaction
nitrification:
ammonia nitrogen -> nitrate aerobic reaction
nitrogen
denitrification:
nitrate nitrogen -> nitrogen gas anaerobic reaction,
requires a carbon source
as food for the bacteria
Since nitrification is an aerobic process, rates are
controlled by the availability of dissolved oxygen to
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-------
CHAPTER 4
SURFACE FLOW WETLANDS
WETLAND DESIGN
Guidelines to designing a SF constructed
wetland are given in table 5. The design assumes
that the wastewater has been pretreated to reduce.
BOP5 by 70%, TSS to less than 1,500 mg/L, and
ammonia to less than 100 mg/L.
CONFIGURATION
The configuration should take advantage of the
natural topography of the site to minimize excava-
tion and grading costs. The configuration should
allow water to move through the wetland by
gravity. The wetland should not be placed where
excess solids could be washed into the wetland;
for instance, sites next to solids;settling pads
should be avoided. 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 rect-
angle is used, the widest portion should be located
at the inlet end to facilitate equal flow distribu-
tion.
For large wetlands, dividing the, wetland into
side-by-side cells should be considered. Dividing
a wide wetland into parallel cells lessens the
likelihood of preferential flow paths and short-
circuiting and promotes the contact of the waste-
water 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.
Pretreatment
Configuration
Flow
Bottom slopes
Water depth
Vegetation
Construction
Table 5. Design summary for surface flow wetlands.
Reduction of BOD5 by 70%
Reduction of solids to <1,5 00 mg/L
Reduction of ammonia to <100 mg/L
Fit the wetland to the site
Divide large wetlands into side-by-side cells
3y gravuy, 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
-------
WATER DEPTH
The design should plan for 3 to 8 inches c:
surface xvater, with a maximum of 18 inches.
Deeper water may be advisable in winter to accom-
modate the slower reaction rates during cold
weather and to guard against freezing. The \vet-
land 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 ceil
will then discharge to a downstream cell of the
same width. The maximum length of each cell is
based on the slope of the bottom of the cell (which
should not exceed 0.5 to 1.0%) and the water
depth suitable for the wetland vegetation (which
is generally 18 inches or less). The number and
length of the subdivisions \vill 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 hydrol-
ogy. However, several recent studies have shown
that the movement of water thrtiugh 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. Constructed
wetlands exhibit mixing characteristics intermedi-
ate 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 channeling, recirculation patterns,
and the presence of stagnant areas cause further
deviations from calculated theoretical flows.
Contact times are not often as great as the theoreti-
cal residence time calculated from the wetland
empty volume and the volumetric flow rate. As
a final complicating factor, the chemistry of
wetlands is complex, 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 underdesigned.
Agricultural wetland systems are usually sized
to remove the necessary amount of BOD5. The
SCS (1992) uses two methods to determine the size
of a constructed wetland for a specific site: the
presumptive method and the field test method.
The presumptive method is used when the
pretreatment system has not yet been installed and
the concentration of BOD, in the pretreatment unit
can only be estimated. The presumptive method
first calculates a surface area and then determines
the resulting hydraulic residence time, adjusting
the size as necessary to achieve the required
residence times.
The field test method uses known, measured
BOD5 concentrations in the effluent from the
pretreatment unit to calculate hydraulic residence
times, from which the required surface area is then
calculated.
Removal efficiencies for TSS are similar to
those for BODS and design for BOD5 removal
should achieve similar TSS removal.
For wetlands designed to remove ammonia as
well as BOD5, the size of the wetland should be
based on the removal of the ammonia. Since
ammonia removal is a less efficient process than
BOD, removal, ammonia removal requires a larger
wetland area than does BOD5 removal.
PRESUMPTIVE METHOD FOR BOD.
This method assumes that a certain amount of
BOD, is present in the wastewater and that a
certain amount is removed by the pretreatment
system. It then uses the remaining BOD, load with
-------
a standard wetland areal loading rate to determine
the surface area needed for adequate treatment.
1.
2.
3.
Determine the production of BOD. per day:
Livestock production of BOD5 is as follows:
production (Ib)
Average ,1000 Ib animal
weight Hbl unit (AU1 per day
Dairy cows 1300
Beef cattle 750
Swine 200
Poultry - layers 4
Poultry - broilers 2.2
1.6
1.4
2.1
3.7
5.1
4.
BODS= BOD5/1000 Ib AU x number of
animals x average weight/1000 Ib
Add 10% BOD5to account for that in waste
hay and feed:
Adjusted BOD3= BODsx 1.1
Determine BOD3 remaining after pretreatment.
if known.
A well-managed settling/flotation tank for
milkhouse wastewater will remove 70% of
BOP5. Use a removal factor of 0.30 (70%
removal):
BOD5 remaining in tank effluent = adjusted
BOD5x0.30
If using an anaerobic lagoon, use a rate of 40%
rate (60% removal):
BODS remaining in lagoon effluent = adjusted
BOD5x0.40
Determine the water surface area (SA) for the
constructed wetland:
SA = BOD3 loading / recommended areal BODS
loading rate
SCS recommends an areal BOD5 loading rate of
65 Ib BOD5 /ac/day. It is known that treatment,
and therefore areal loading rate, are affected by
climate but no research or field data are
available that can be used to quantify the
influence of different climate conditions.
A standard value is therefore used.
Determine the overall dimensions. The
optimal length-to-width ratio has not yet been
determined. The SCS (1992) recommends a
ratio of 3:1 to 4:1. For a length-to-width ratio
of3:l:
W = width of constructed wetland
L= length = 3VV
Then SA = 3W x VV = 3W*
Determine the hydraulic residence time (t) in
days. Data needed are average water depth
(D). porosity (P), and daily flow rate (Q).
t = SAxD xP/Q (4.1)
where: t = hydraulic residence time, days
SA = surface area of constructed
wetland, ft2 (length x width)
D = average water depth in
constructed wetland, ft
Q = average daily flow rate. ftVday
P = porosity, percent as a decimal.
The Q value is the average flow in the bed,
calculated from flow through the bed plus
gains and losses from precipitation and evapo-
transpiration. Published values are usually
available for precipitation .and evapotranspira-
tion for local conditions. Large rainfall and
snowmelt volumes can greatly affect Q and
must be considered in the design; some con-
structed .wetlands have failed because high
flows were not factored into the design.
The porosity (P) is the ratio of the volume
occupied by water to the volume occupied by
plants and water combined. The following
porosity values have been determined:
cattails (Typha spp.) 0.95 (SCS 1992)
bulrush 0.86
(Scirpus yalidus]
0.86
woolgrass
(S. cyperinus]
common reed 0.98
(Phragmites)
rushes (Juncus spp.) 0.95
(Watson and
Hobson 1989)
(Watson and
Hobson 1989)
{Watson and
Hobson 1989)
(Watson and
Hobson 1989).
-------
The volume occupied by underground plant
structures (roots and rhizomes) increases over time
(Reed 1993) and porosity gradually decreases. Tin;
wetland should be designed conservatively to
allow for the diminished porositv.
The SCS recommends a hydraulic residence
lime of at least 12 days since this residence time
has been found empirically to provide adequate
removal of BOD5.
FIELD TEST METHOD FOR BOD.
The field test method uses data from samples
collected in the pretreatment unit to calculate
hydraulic residence times via the following
equation:
t = 2.7 (In C - In Ce + In F) / 1.1 '™>'. or (4.2)
t « (In C. - In Ce + In F) / 65K,.
where
t = hydraulic residence time, days
C( = constructed wetland influent BOD5
concentration, mg/L
Ce = desired constructed wetland effluent
BODj concentration, mg/L
In = natural logarithm
F = fraction of BODS that is not removed as
settleable solids near the head of the
wetland, expressed as a decimal fraction,
(soluble BODj/total BOD5)
T = water temperature, °C
Kj. = temperature-dependent reaction rate
constant, days •'
The values for C, and F are determined from
samples of the supernatant (the liquid above the
solid layer) in the pretreatment unit. A composite
sample (several samples combined) should be
collected within the unit. Ideally, samples should
be collected and analyzed during various seasonal
conditions. Because BOD. concentrations in these
systems can vary widely, and because an adequate
safety factor must be assured, the highest sample
value should be used to design the system.
The temperature of the water (T) is controlled
by local climatic conditions. The lowest water
temperature under which the wetland will be
expected to perform should be used for the design
In constructed wetlands in the northern states, if
the wetland does not freeze completely, the
wetland will continue to function and water
temperatures under the ice can be estimated as
40°F (5"C)(Bovd 1991). However, at low tempera-
tures, removal rates will be lower and the wetland
will have to be larger to accommodate the slower
rates. Alternatively, the wastewater can be stored
in the pretreatment unit during the cold seasons.
In this case, a higher value for water temperature
can be used and the wetland made smaller, de-
pending on BOD5load. wastewater volume, and
local temperatures.
The values to be used for F and K_ in design-
ing constructed wetlands have not been con- °
firmed. The value often used for F in domestic
systems is about 0.52. This should be the lower
limit for F unless research determines otherwise.
If organic material is adequately removed by
pretreatment. the value of F can be increased, but
it should not be more than 0.90. For an agricul-
tural waste treatment lagoon, the value ofF may-
equal 0.90. A value for K,. of 0.0057 (l.lpM)is~
often used. However, experimental data on the
values to be used in designing constructed wet-
lands have been difficult to obtain because of the .
logistic and economic difficulties in experiment-
ing with wetlands on a scale large enough to be
appropriate. The wetland should be sized gener-
ously to accommodate these uncertainties.
The hydraulic characteristics of the con-
structed wetland should provide the required
hydraulic residence time of at least,12 days. The
hydraulic design is calculated using the average
depth of water and average daily flow rate into the
wetland to find an arrangement that results in the
required hydraulic residence time:
SA=t/(DxP/Q) (4.3)
where
SA = surface area, ft2 (length x width)
t = hydraulic residence time, days
D = average water depth in the
constructed wetland, ft
P = porosity, percent as a decimal
Q = average daily flow rate, ftVday.
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PRESUMPTIVE METHOD FOR NITROGEN
The presumptive method can be modified to
design wetlands for nitrogen removal. To treat
ammonia to concentrations in the wetland
effluent of less than 10 mg/L (the usual discharge
limit). Hammer (1992) recommends that influent
nitrogen not exceed 9 Ib/ac/day as TKN (10 kg/
ha/day). To size the constructed wetland for
TKN:
1. Determine the production of ammonia:
Ammonia
production (Ib)
Average 1000 Ib animal
weight fib) unit fAU1 per dav
Dairy cows 1300 1.6
Beef cattle 750 1.4
Swine 200 2.1
Poultry - layers 4 3.7
Poultry - broilers 2.2 5.1
1. Calculate the concentration of nitrogen (as
TKN) per day:
Assuming that TKN is 150% of ammonia:
mg/L TKN = mg/L ammonia x 1.5
2. Calculate the daily load of TKN:
TKN (Ib/day) = mg/L TKN x fWhr of influent
x 680 (conversion factor)
3. Determine the surface area needed:
Surface area (SA)(ac) = Ib/day TKN +
9 Ib TKN ac/day.
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-------
CHAPTER 5
SUBSURFACE FLOW WETLANDS
INTRODUCTION.
The use of SSF wetlands in agriculture has
been limited because of the high probability that
they will be clogged by water containing more
than about 30 mg/L solids.
The design information provided in this
chapter is a summary of the information in Subsur-
face Flow Constructed Wetlands for Wastewater
Treatment: A Technology Assessment (Reed 1993).
Reed based his recommendations on the perfor-
mance of 14 municipal, domestic, hospital, and
industrial systems that have provided 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 operating for less than five
years. Only a limited number of systems\in the
Northeast have provided operational data.
WETLAND DESIGN
Guidelines to designing a SSF constructed
wetland are given in table 6. The design assumes
that the wastewater has been pretreated to reduce
BOD,by 70%, TSS to less than 1,500 mg/L, and
ammonia to less than 100 mg/L.
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.
Pretreatment
Configuration
Flow
Bottom slopes
Inlet
Outlet
Vegetation
Construction
Tables. Design summary for subsurface flow wetlands.
Reduction of BODS by 70%
Reduction of solids to <1,5 00 mg/L
Reduction of ammonia to <100 mg/L
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 ;
Medium(a) must be clean
Wetland must be sealed to limit infiltration and exfiltration
Water table must be below or excluded from the wetland
-------
Darcy's Law assumes laminar flow, a constant
and uniform flow (Q). and lack of short-circuiting.
conditions that do not exist in constructed wet- °.
lands (see Volume I). Darcy's Law is thought to
provide a reasonable approximation of the°hydrau-
lic conditions in an SSF bed if small to moderate
size gravel (
-------
lie 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 management 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 designed for BOD5
removal. In SSF systems, the physical removal
of BOD5 is believed to occur rapidly through
settling and entrapment of particulate matter in
the void spaces in the gravel or rock media
(Reed 1993).
Most of the existing systems in the United
States and Europe have been designed as at-
tached growth biological reactors using the same
equations as those used for SF wetlands (equa-
tions 4.1 - 4.3). The plug flow model is pres-
ently 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
supports the attached growth microorganisms
that are believed to provide most of the treat-
ment 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 factor in
equation 4.1 can be defined as:
t = nLWd/Q
(5,2)
where
n = porosity (% as a decimal)
L = length of bed (ft.'m)
W = width of bed (ft, m)
d = average water depth (ft, m)
Q = average flow rate through bed
(ftVday, mVday).
The Q value in equation 5.2 is the average
flow in the bed [(Q,0 + Q.J/2]'. calculated from
flow through the bed plus gains and losses from
precipitation and evapotranspiration. This is
the same value used in Darcy's Law for hydrau-
lic design.
The "d" value in the equation is the average
depth of liquid in the bed. If- the design hydrau-
lic gradient is limited to 10% of the potential
available, as recommended above, 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.
Since the term LVV in equation 5.1 is equal to
the surface area of the bed, rearrangement of •
terms permits the calculation of the surface area
(A ) required to achieve the necessary level of
BOD, removal:
A, = LxW = Qln(C./C0)/-kTdn (5.3)
where
Af = bed surface area (ft2)
other terms as defined previously.
The depth of the media selected will depend
on the design intentions for the system. If the
vegetation is intended as a major source of oxygen
for nitrification in the system, then the depth of
-------
the bed should not exceed the potential root
penetration depth for the plant species chosen.
This will ensure the availability of some oxygen
throughout the bed profile but may require man-
agement practices which assure root penetration to
these depths.
The design and sizing of the SSF bed for BODS
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 (n) and "effective" hydraulic conduc-
tivity (kj of the media to be used.
3. Use equation 5.3 to determine the required
surface area of the bed for the desired levels of
BOD5 removal.
4. Depending on site topography, select a pre-
liminary length-to-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
-------
plants can provie o ^ ^
plants can provide oxy en ^^
Pe. The ^^^^e ?or denitrification. i
ould then be ava, able t ri{ication include
the substrate.
-------
8
-------
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ABBREVIATIONS AND CONVERSION FACTORS
MULTIPLY
ac, acre
cfs, cubic foot per second
cfs. cubic fool per second
cm, centimeter
cm/sec, centimeter per second
°F. degree Fahrenheit
ft, foot
ft2, square foot
ft3, cubic foot
ft/mi, foot per mile
fps, foot per second
g/m2/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, lite*
Ib, pound
Ib/ac, pound per acre
m, meter
m2, square meter
m3, cubic meter
m3, cubic meter
nvVha/day, cubic meter per hectare per day
mm, millimeter
mi, mile
BY
0.4047
448.831
2.8317 x 10'2
0.3937
3.28 X 10"2
5/9(°F-32)
6.305
9.29 x 10'2
2.83 x 10'2
0.1895
18.29
8.92
3.785
3.785 x 10'3
6.308 x 10'2
2.47
2.54
2.205
0.892
0.2
3. 531 x lO'2
0.2642
0.4536
1.121
3.28
10.76
1.31
264.2
106.9
3.94 x 10'2
1.609
TO OBTAIN
ha, hectare
gpm, gallon per minute
m3/s, cubic meter per second
inch
fps, foot per second
°C, 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
Ib/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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