xvEPA
United States
Environmental Protection
Agency
Office Of Water
(4204)
EPA 832-R-93-008
July 1993
Subsurface Flow
Constructed Wetlands For
WasteWater Treatment
A Technology Assessment
Printed on Recycled Paper
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Acknowledgements
ACKNOWLEDGEMENTS
Mr. Sherwood C. Reed, P.E., of Environmental Engineering Consultants was the
principal author and editor of this document. Many others also contributed to its
development. Instrumental in providing input into this document were the workshop
participants who met in New Orleans in September, 1992, and provided technical and
editorial comments. Special thanks to Robert E. Lee, Chief, Municipal Technology
Branch, and Robert Bastian, Work Assignment Manager -of the U.S. EPA Office of
Wastewater Enforcement and Compliance, for their input and support and to
Engineering-Science, Inc. which provided management and production support during
this effort.
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Executive Summary
EXECUTIVE SUMMARY
Interest in, and the utilization of, constructed wetlands for treatment of a
variety of wastewaters has grown rapidly since the mid 1980s. However, a lack of
consensus has resulted in the use of different, and often conflicting, criteria and
guidance for the design of these systems. Therefore a better understanding of the
internal renovative processes in these systems was essential for the future application
'of this promising technology, and to ensure reliable and cost-effective design
procedures. A consensus on the report contents was reached via several review
cycles by an internationally recognized panel of experts and via discussions at a two-
day workshop in September 1992.
Two types of constructed wetlands are in common use: the first type, the free
water surface (FWS) wetland, exposes the water surface in the system to the
atmosphere. The second type, the subsurface flow (SF) wetland, maintains the water
level below the surface of gravel or other media placed in the wetland bed. This
report is concerned with the SF wetland type.
This report verifies that SF constructed wetlands can be a reliable and cost-
effective treatment method for a variety of wastewaters. These have included.
domestic, municipal, and industrial wastewaters as well as landfill leachates.
Applications range from single family dwellings, parks, schools, and other public
facilities to municipalities and industries. It can be a low-cost, low-energy process
requiring minimal operational attention. As such the concept is particularly well suited
for small to moderate sized facilities where suitable land may be available at a
reasonable cost. Significant advantages include lack of odors, lack of mosquitoes and
other insect vectors, and minimal risk of public exposure and contact with the water
in the system.
The process can remove BOD5and suspended solids to very low concentrations
and produce the equivalent of tertiary effluent. Interim design guidelines for BOD5
removal are provided in the report. Nitrogen removal to very low levels is possible if
sufficient detention time and oxygen to support the necessary nitrification reactions
are present. Many of the early systems were deficient in both respects. Corrective
action is possible, and the report presents tentative methods for appropriate design.
A limited data base supports the capability of the SF wetland process for effective
removal of metals and other priority pollutants. However, the process has limited
capacity for removal of phosphorus as presently conceived, and supplemental
treatment may be necessary. A one- or two-log reduction in fecal conforms can be
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Executive Summary
reliably achieved with this process, lower levels may require post disinfection.
In addition to design guidelines, the report provides an assessment of concept
applicability and the research needs for a better understanding of the process.
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Preface
PREFACE
This report was sponsored by the U.S. EPA Office of Wastewater Enforcement
and Compliance under the direction of Mr. Robert K. Bastian, and Mr. Robert E. Lee,
of the Municipal Technology Branch.
It is the intent of this report to present current (1993) information and guidance
on design, construction, performance, operation and maintenance of subsurface flow
constructed wetlands used for treatment 'of domestic and municipal wastewaters.
Subsurface flow constructed wetlands are in relatively common use in Europe,
Australia, and in a number of states in the United States. However, there has been
no apparent consensus on design procedures or performance expectations. A two-
step procedure was used for the preparation of this report to develop a consensus
among knowledgeable experts.
The first step was to submit a draft report prepared by Mr. Sherwood Reed,
E.E.C., to selected experts for their detailed review. These individuals included: Mr.
Donald Brown, U.S. EPA RREL; Dr. Dennis George, Tennessee Tech University; Mr.
Michael Ogden, Southwest Wetlands Group: Dr. Richard Gersberg, San Diego State
University; Mr. Ronald Crites, Nolte & Associates; Dr. Robert Knight, CH2MHNI; Dr.
George Tchobanoglous, University of California-Davis; Mr. Michael Mines, Tennessee
Valley Authority; Dr. Frank Saunders, McCulley, Frick & Gilman, Inc.; and Dr. Peter
Jenssen, Jordforsk, Norway.
The second step was accomplished at a two-day workshop held in New
Orleans,.-LA on September 24 and 25, 1992. Participants in that meeting included
invited experts, representatives from US EPA Region VI, state and local officials, and
local design firms with experience with this technology. These participants are listed
in Appendix A of this report. The workshop commenced with a detailed presentation
of the contents of the draft report. This was followed by intensive discussion by
participants on all of the major topics of concern. Additional time was taken at the
meeting to identify research needs for further optimization of this technology. A
revised version of the text was then circulated to all participants and previous
reviewers for their comments.
The preparation of this final report considers all of the comments and
suggestions received from all sources. As a result, this report represents a consensus
among the experts participating in this effort on design, construction, and operation
and maintenance of subsurface flow constructed wetlands. Current and future
iv
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Preface
research will undoubtedly improve understanding of the basic concepts involved and
lead to more sophisticated design models. The design procedures and related
guidance in this report are therefore considered to be valid, but subject to further
improvement.
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Table of Contents
TABLE OF CONTENTS
PAGE
ACKNOWLEDGEMENTS i
EXECUTIVE SUMMARY ii
PREFACE iv
CHAPTER 1 INTRODUCTION 1-1
CHAPTER 2 BACKGROUND 2-1
CHAPTER 3 PERFORMANCE EVALUATIONS 3-1
BOD5Removal 3-2
TSS Removal 3-8
Nitrogen Removal 3-10
Phosphorus Removal 3-16
Fecal Coliform Removal 3-17
CHAPTER 4 DESIGN CONSIDERATIONS 4-1
Hydraulics and Hydrology 4-1
Bed' Clogging 4-1
Hydraulic Design 4-3
Aspect Ratio 4-5
Bed Slope 4-5
Media Types 4-6
Inlet Structures 4-7
Outlet Structures 4-B
BOD5Removal 4-9
Nitrogen Removal 4-14
Vegetation Selection and Management 4-19
CHAPTER 5 CONSTRUCTION DETAILS -5-1
costs 5-2
CHAPTER 6 ON-SITE SF WETLAND SYSTEMS 6-1
Louisiana Method 6-1
TVA Method 6-3
Plug Flow Model for On-Site Systems 6-6
vi
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Table of Contents
CHAPTER 7 OTHER POTENTIAL APPLICATIONS 7-1
Stormwater Systems 7-1
Landfill Leachates 7-2
Mine Drainage 7-2
Agricultural Runoff 7-2
CHAPTER 8 RESEARCH NEEDS 8-1
High Priority Research Needs 8-1
Medium Priority Research Needs 8-2
Low Priority Research Needs 8-2
CHAPTER 9 CONCLUSIONS 9-1
CHAPTER 10 REFERENCES 10-1
APPENDIX
A - List of participants in New Orleans workshop.
B - Process details for systems listed in Table 2.
vii
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Table of Contents
LIST OF TABLES
Table 1. Historical Design Approaches, European and
U.S. Sources 2-4
Table 2. Data Sources for Performance Evaluation 3-1
Table 3. Comparison of First Order Plug
Flow Rate Constants 3-8
Table 4. Ammonia Removal in SF Wetlands 3-13
Table 5. Typical Media Characteristics for SF Wetlands 4-6
Table 6. Performance of Vegetated and Unvegetated SF
Wetland Beds 4-12
Table 7. Potential Oxygen From Vegetation at Santee, CA 4-15
LIST OF FIGURES
Figure 1. BOD5lnput Versus Output 3-2
Figure 2. BOD5Removal Versus Hydraulic Residence Time 3-3
Figure 3. System Aspect Ratio Versus BOD5Removal 3-4
Figure 4. BOD5Loading VS BOD5Removal, Mass Basis 3-5
Figure 5. Suspended Solids, Input Versus Output 3-8
Figure 6. Suspended Solids Removal Versus HRT 3-9
Figure 7. Suspended Solids Removal Versus System Aspect Ratio 3-9
Figure 8. Ammonia Input Versus Output 3-11
Figure 9. Ammonia Removal Versus HRT ... 3-12
Figure 10. TKN Mass Loading Versus Mass Removal 3-14
Figure 11. Effluent Ammonia Versus TKN Mass Loading 3-15
Figure 12. Phosphorus Input Versus Output 3-16
Figure 13. Lithium Tracer Study, Carville, LA . . 4-10
Figure 14. Cost Distribution for SF Constructed Wetlands 5-3
Figure 15. Actual Cost Distribution for an SF System in Louisiana 5-4
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List of Abbreviations
ac
ac/mgd
BOD5
OC
Ce
-cm
cm/d
CO
COD
cso
d
dh/dL
EPA
ET
ft
FWS
gal
gal/d
LIST OF ABBREVIATIONS
Cross-sectional area
Acres
Acres per million gallons per day
5-day biochemical oxygen demand
Degrees Celsius
Concentration effluent
Centimeters
Centimeters per day
Concentration influent
Chemical oxygen demand
Combined sewer overflow
Days
Hydraulic gradient
Environmental Protection Agency
Evapotranspi ration
Feet
Free water surface
Gallons
Gallons-per day
ix
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List of Abbreviations
gm
gpd
ha
HRT
in/d
kg
kg/d
kg/ha/d
ks
Ib/ac/d
L
L:W
m
Grams
Gallons per day
Hectare
Hydraulic residence time
Inches per day
Kilograms
Kilograms per day
Kilograms per hectare per day
Hydraulic conductivity
Pounds per acre per day
Total bed length
Length to width
Meters
m
m2
mVd
M3/d/PE
mg/L
ml
Square meters
Square meters per population equivalent
Cubic meters per day
Cubic meters per day per population equivalent
Million gallons per day
Milligrams per liter
Milliliters
mm
Millimeters
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List of Abbreviations
N Nitrogen
n Porosity
N H3 Ammonia
NPDES National Pollutant Discharge Elimination System
02/ m2/d Oxygen- per square meter per day
0 L Organic loading
p.E. Professional engineer
PE Population equivalent
PVC Polyvinyl chloride
Q Flow
RREL Risk Reduction Engineering Laboratory
SF Subsurface flow
TKN Total Kjeldahl nitrogen
TSS Total suspended solids
TVA Tennessee Valley Authority,
U.S. United States of America
U.S. EPA United States Environmental Protection Agency
W Bed width
WPCF Water Pollution Control Federation
X I
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Chapter 1
CHAPTER 1
INTRODUCTION
This report describes the design, construction and performance of subsurface
flow constructed wetlands as used in the United States for wastewater treatment.
Utilization of this technology has grown very rapidly in the past five years (1988-93),
and it is clear from the discussion in the next section of the report that there has been
no general consensus on design of these systems or their performance expectations.
In recognition of that situation, various offices within U.S. EPA, as well as other
agencies and groups, have sponsored several efforts to better understand wetland
systems and their capabilities and limitations for wastewater treatment. These efforts
have included: a detailed, multi-year monitoring program at several constructed
wetlands in -Kentucky (sponsored by EPA headquarters, the National Small Flows
Clearinghouse, EPA Region IV, and TVA); an inventory of all constructed wetlands
used for wastewater treatment in the U.S. (sponsored by EPA RREL); site visits with
evaluations and reports at selected operating subsurface flow wetlands in the U.S.
(sponsored by EPA RREL); a brief performance evaluation at three subsurface flow
systems in Louisiana (sponsored by EPA Headquarters); detailed performance
monitoring and evaluation at two subsurface flow systems, also in Louisiana
(sponsored by EPA RREL); creation of a detailed data base covering design, operating
and performance data for natural and constructed wetland systems treating municipal
and industrial wastewater and storm water runoff in the U.S. (sponsored by EPA
Corvallis); and several workshops and seminars sponsored by EPA Region VI.
All of these sources were utilized in the preparation of this report and the
discussion and evaluations which are' included. The focus of this report is the
subsurface flow type wetland. Information on other types of wetland systems has
has been included for comparative purposes.
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Chapter 2
CHAPTER 2
BACKGROUND
Wetlands are defined as land where the water surface is near the ground
surface lung enough each year to maintain saturated soil conditions, along with the
related vegetation. Marshes, bogs, and swamps are all examples of naturally
occurring wetlands. A "constructed wetland" is defined as a wetland specifically
constructed for the purpose of pollution control and waste management, at a location
other than existing natural wetlands. There are two basic types of constructed
wetlands, the free water surface wetland and the subsurface flow wetland. Both
types utilize emergent aquatic vegetation and are similar in appearance to a marsh.
The free water surface (FWS) wetland typically consists of a basin or channels
with some type of barrier to prevent seepage, soil to support the roots of the
emergent vegetation, and water at a relatively shallow depth flowing through the
system. The water surface is exposed to the atmosphere, and the intended flow path
through the system is horizontal.
The subsurface flow (SF) wetland also consists of a basin or channel with a
barrier to prevent seepage, but the bed contains a suitable depth of porous media.
Rock or gravel are the most commonly used media types in the U.S. The media also
support the root structure of the emergent vegetation. The design of these systems
assumes that the water level in the bed will remain below the top of the rock or gravel
media. The flow path through the operational systems in the U.S. is horizontal.
The SF type of wetland is thought to have several advantages over the FWS
type. If the water surface is maintained below the media surface there is little risk of
odors, odors, exposure, or insect vectors. In addition, it is believed that the media
provides greater available surface area for treatment than the FWS concept so the
treatment responses may be faster for the SF type, which therefore can be smaller in
area than a FWS system designed for the same wastewater conditions. The
subsurface, position of the water and the accumulated plant debris on the surface of
the SF bed offer greater thermal protection in cold climates than the FWS type.
Subsurface flow constructed wetlands first emerged as a wastewater treatment
technology in Western Europe based on research by Seidel (1) commencing in the
1960s, and by Kickuth (2) in the late 1970s and early 1980s. Early developmental
work in the United States commenced in the early 1980s with the research of
Wolverton, et al. (3) and Gersberg et al. (4).
2 -I
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Chapter 2
The SF concept developed by Seidel included a series of beds composed of
sand or gravel supporting emergent aquatic vegetation such as cattails (Typha),
bulrush (Scirpus), and reeds (Phragmites), with Phragmites being the most commonly
used. In the majority of cases, the flow path was vertical through each cell to an
underdrain and then onto the next cell. Excellent performance for removal of BOD5
TSS, nitrogen, phosphorus, and more complex organics was claimed. Pilot studies of
the concept in the United States were marginally successful, and it has not been
utilized in recent years in this country.
Kickuth proposed the use of cohesive soils instead of sand or gravel; the
vegetation of preference was Phragmites and the design flow path was horizontal
through the soil media. Kickuth's theory suggested that the growth, development and
death of the plant roots and rhizomes would open up flow channels, to a depth of
about 0.6 m (2 ft) in the cohesive soil, so that the hydraulic conductivity of a clay-like
soil would gradually be converted to the equivalent of a sandy soil. This would permit
flow through the media at reasonable rates and would also take advantage of the
adsorptive capacity of the soil for phosphorus and other materials. Very effective
removal of BOD5 TSS, nitrogen, phosphorus, and more complex organics was
claimed. As a result, by 1990 about 500 of these "reed bed" or "root zone" systems
had been constructed in Germany, Denmark, Austria, and Switzerland. The types of
systems in operation include on-site single family units as well as larger systems
treating municipal and industrial wastewaters. Many of the -early systems were
designed (5) with a criterion of 2.2 nfof bed surface area per population equivalent
(PE). A PE in European terms is equivalent to the organic loading from one person,
or approximately 0.04 kg/d BODJn typical primary effluent. That is equal to a
surface organic loading of about 180 kg/ha/d (162 Ib/ac/d). The more recently
constructed systems in Europe (15) have been designed for 5 to 10 nf/PE (40 - 80
kg/ha/d). The hydraulic loading at 5 m2/PE (at an assumed 0.2 m3/d/PE) would be
about 4 cm/d (1.6 in/d), which, in a commonly used term in the U.S., is equivalent to
23 acres/mgd of design flow, and would provide a hydraulic residence time (HRT) of
about six days. For comparison, FWS wetlands in Europe are typically designed at 10,
mVPE, which results in a surface area about twice that required for the SF type (39).
Commencing in 1985, a number of "reed bed" systems were constructed in
Great Britain based on Kickuth's concepts, but in many cases gravel was used as the
bed media rather than cohesive soil (5) due to concerns regarding soil hydraulic
conductivity. Many of these beds were built with a sloping bottom (0.5 to 1%) and
a flat surface. The purpose of the sloping bottom was to provide sufficient hydraulic
gradient to ensure subsurface flow in the bed. The flat upper surface would allow
temporary flooding as a weed control measure to kill undesirable plants. Some of
2-2
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Chapter 2
these systems also had an adjustable outlet which permitted easy maintenance of the
desired water level in the bed (19).
Wolverton's work in Louisiana began with experimental bench scale trays in a
greenhouse containing rock or gravel media and supporting a stand of emergent
aquatic vegetation (3). The trays were filled with wastewater, and then drained after
a certain number of hours (range 12 to 48 hours). In essence the procedure was a
fill and draw batch type process. Excellent performance was demonstrated for BOD5
TSS, and NH,, and moderate performance for phosphorus with a one-day HRT (3)
The typical organic loading during these experiments (at one-day HRT) was about 58
kg/ha/d (52 Ib/ac/d), and the hydraulic loading was about 8 cm/d (3.5 in/d). Design
criteria based on this work (16) included one day HRT, about five acres of bed surface
area per mgd, and up to 15:1 aspect ratio (L:W). These 'criteria, or variations, have
been widely applied and, as of 1991, there were about 60 systems in operation or
in various stages of design in the south central U.S., based on these values. These
systems range from on-site single family units to large-scale municipal systems (up
to 4 mgd) (16).
Gersberg's work (4) was conducted over a period of several years in Santee,
CA, in large-scale, continuous flow, field experiments using 0.76 m (2.5 ft) deep
gravel beds. The removal of BOD5 TSS, and NH, was correlated with the depth of
root penetration for the plant varieties (Typha, Scirpus, Phragmites) with the best
removals occurring with the deepest root penetration (Scirpus then Phragmites). The
organic loading was approximately 55 kg/ha/d (49 Ib/ac/d and the hydraulic loading
about 5 cm/d (2 in/d). This hydraulic loading is equal to 18 ac/mgd using the common
U.S. term. The HRT in this system was about six days, compared to only one day
from Wolverton's (3) work and possibly up to six days in the new European systems
(15).
Beginning in the mid 1980s, the Tennessee Valley Authority (TVA) began a
program of research and technical assistance on constructed wetlands for treatment
of a variety of waste streams (municipal wastewater, acid mine drainage, agricultural
wastes and runoff, etc.) (9). Their criteria for subsurface flow wetlands designed for
wastewater treatment, originally derived from the work of Kickuth (18), have been
modified significantly in subsequent years. By 1991 there are probably at least 80
subsurface flow systems, in operation in a number of states, based on criteria and
assistance provided by TVA (20). These systems range in size from on-site single
family units to larger municipal systems (3785 mVd, 1 mgd).
The organic loading on one of the early TVA systems (Benton, KY) was 81
kg/ha/d (72 Ib/ac/d) and the design hydraulic loading 14 cm/d (5.6 in/d) (3.6 ac/mgd).
2-3
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Chapter 2
The theoretical HRT at this hydraulic loading would be about 2 d. At one of the more
recently constructed systems (Bear Creek, AL) the organic loading is about 4 kg/ha/d
(3.4 Ib/ac/d) with a hydraulic loading of about 3 cm/d (1.2 in/d)(31 ac/mgd). The
theoretical HRT, at design flow, would be about three days. The current TVA
recommendations (17) for small-scale systems using a septic tank and a bed depth of
0.5 m (1.5 ft) are 62 kg/ha/d (56 Ib/ac/d) organic loading and 5 cm/d (1.8 in/d)
hydraulic loading (20 ac/mgd). The theoretical HRT at this hydraulic loading is about
4 d. Table 1 summarizes the various design approaches discussed above.
Table 1. Historical Design Approaches, European and U.S. Sources
Source Organic Loading Hydraulic Loading Area Hrt
(kg/ha/d)a (cm/d)b (m2/m3/d)c (d)
Europe
Boon (1985)
Cooper (1990)
180
80
9
4
11
24
2.6
6
U.S.
Wolverton (1983) 58 8 13
Gersberg (1.985) 55 5 20
TVA
Benton, KY (1989)
BearCr, AL (1991)
81
4
14
3
7
33
2
3
a. kg/ha/d x 0.892 = Ib/ac/d
b. cm/d x 0.394 = in/d
c. nf/mVd x 2.091 = ac/mgd
A comparison of the data presented in Table 1 indicates there has been no
consensus regarding design criteria for SF wetlands. This obviously has a significant
impact on system costs and may affect concept feasibility. The U.S. EPA, and others,
undertook the investigative tasks described in the Introduction to this report, and the
results of those studies are discussed in the next section.
2-4
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Chapter 3
CHAPTER 3
PERFORMANCE EVALUATIONS
This section compares performance expectations for the SF. concept to actual field
results obtained by those studies sponsored by the U.S. EPA and others, which were
described previously. The data from those sources are not specifically identified in the
graphical presentations which follow but are listed in Table 2 below, and described
in more detail in Appendix B. The fourteen systems listed in Table 2 are believed to
be representative examples of the systems in operation in the United States. They
were selected for this analysis since each had a significant body of relatively reliable
input/output water quality data which characterizes the performance of the wetland
component in the system. Many other operating systems only have limited effluent
data collected to verify their compliance with NPDES requirements. All of the data
shown in the subsequent graphical presentations are averages over the available
period of record for each of these systems. None of these systems have been
operating longer than five years.
Table 2. Data Sources for Performance Evaluation
Location
Greenleaves, LA
Degussa Co., MS
Bear Creek, AL
Monterey, VA
Denham Springs, LA
Benton, LA
Haughton, LA
Carville, LA
Mandeville, LA
Benton, KY
Hardin, KYa
Hardin, KYb
Utica, MS"
Utica, MSd
Wastewater
Type
Municipal
Industrial
Domestic
Municipal
Municipal
Municipal
Hospital
Municipal
Municipal
Municipal
Municipal
Municipal
Municipal
Municipal
Design Flow
(mVd )°
564
6737
59
83
6548
378
380
465
4633
685
236
186
189
416
Treatment Area
(ha)f
0.44
0.89
0.20
0.02
6.15
0.61
0.61
0.26
1.85
1.46
0.32
0.32
0.61
0.81
a. phragmites bed, b. Scirpus bed, c. North system, d. South system
e. mVd x (2.64 x I06= mgd. f. Ha x 2.47 = acres.
3- 1
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Chapter 3
BOD, REMOVAL
The physical removal of BODJs believed to occur rapidly through settling and
entrapment of particulate matter in the void spaces in the gravel or rock media.
Soluble BODJs removed by the microbial growth on the media surfaces and attached
to the plant roots and rhizomes penetrating the bed. Some oxygen is believed to be
available at microsites on the surfaces of the plant roots, but the remainder of the bed
can be expected to be anaerobic.
Compared to other forms of wastewater treatment, both SF and FWS wetland
systems, are unique in that BODJs actually produced within the system due to the
decomposition of plant litter and other naturally occurring organic materials. As a
result, the systems can never achieve complete BOD5 removal and a residual BOD5
from 2 to 7 mg/L is typically present in the effluent.
Figure 1 presents BOD5 input versus BOD5output data for the systems listed
in Table 2. All effluent values are well below the 20 mg/L reference level which is a
common permit requirement, and this can be achieved regardless of the input concen-
4 0
3O
20-
10
20 mg/L
+
+ -
4- +
1O 2O 3Q 4O 5O 6O
BOD NPUT (m0/U
Figure 1. BOD5 Input Versus Output.
3 - 2
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Chapter 3
tration (within the range shown). Data from similar systems in Europe show
essentially the same relationship with input BOD5concentrations up to 150 mg/L (39).
The low values, in the lower left portion of the graph, are influenced by the presence
of the residual BOD5discussed above, which limits final effluent levels to a range of
2 to 7 mg/L.
_
£
_i
>
§
EC
8
CD
1UU
90
80
70
60
5O
40
30
2O
r*
• • *
• •
H ' '
-
-
...
'
I 1 1 1 1 I I
345
HRT (d>
8
Table 2
Systems
A Other
Systems
Figure 2. BOD5 Removal Versus Hydraulic Residence Time
A comparison of dissolved and total BOD5and COD for influent and effluent at
two sites in Louisiana suggest that most of the BOD5 leaving those systems was
residual organics from the system and not of wastewater origin (27).
Figure 2 presents BOD5removals versus the calculated HRT for these same
systems. It seems clear that after one- to one- and one half-day HRT, the removal of
BODJs not strongly dependent on HRT since removal improves only slightly
thereafter, up to an HRT of 7.5 d. The 60 to 65 percent removal values shown at the
one-day HRT are not due to poor BOD5 removal, but rather to relatively low input
levels (see Appendix B for data). The lowest removal, at 20 percent, is due to an
input BOD5( 5 mg/L) which was already at the threshold for residual. The fourteen
3 - 3
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Chapter 3
systems listed in Table 2 are designated by square symbols in the figure; three
additional systems, from other sources, are also included to extend the range of HRT
shown.
The BOD5 removal values for these data can be reasonably approximated by a
first order plug flow relationship up to about ± 2 d. The BOD5removal thereafter is
limited and is believed to be influenced by the production of residual BOD5in the
system, as mentioned previously. This is compatible with the hypothesis that BOD5
is removed rapidly in the front part of these systems, and suggests that the removals,
indicated on the figure, at the longer detention times may have actually been obtained
at an earlier point in the bed prior to sampling and testing of the final effluent.
It has been suggested in the past that these SF wetland systems must have a
high aspect ratio (L:W) to ensure maintenance of plug flow conditions and high levels
of performance (16). A common recommendation indicated that the L:W should be
at least 10:1 and that lesser levels might impair removal efficiency. Figure 3 tests
that hypothesis by comparing the average BOD5 removal achieved to the aspect ratio
of the various systems listed in Table 2.
1OO
9O
80
7O
6O
50
40
30
20
10
O
_i_
_i_
_i_
6 8 1O 12
ASPECT RATIO UW
14
16
18
20
Figure 3. System Aspect Ratio Versus BOD5Removal.
3 - 4
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Chapter 3
In these cases the aspect ratio varied from less than 2:1 to over 17:1. As
illustrated by the figure, there does not appear to be any relationship between aspect
ratio and BOD5removal capabilities. The very low removal associated with the
highest L:W (17:1) in the plot is due to the very low input BOD5( < 5 mg/L) and is
not related to the aspect ratio. In such cases, further significant BOD5 removal
cannot be expected regardless of aspect ratio or HRT. In this case, any reduction of
the wastewater from an input value of 5 mg/L was probably replaced, in part, by the
residual BOD5from plant detritus.
Other natural treatment systems, such as facultative lagoons and land treatment
systems, have displayed a near linear relationship between mass organic loading and
mass removal rates, up to relatively high loading rates.
Figure 4 illustrates this relationship for SF wetlands and confirms that a linear
relationship does exist between these two parameters. The lvalue for the curve fit
is 0.97, indicating an excellent correlation.
100
5 9O
! so
Jf
70
* 60
8 so
I 30
* 20
Z 1O
y = O.653x + O292
r»= 0.970
1 ' i !_
O 10 2O 30 40 50 6O 7O 8O 9O 1OO 11O 120 130 1*0 150
BOO MASS LOADMO fcfl/ha/d)
Figure 4. BOD5 Loading VS BOD5 Removal, Mass Basis.
3 - 5
-------
Chapter 3
Caution is necessary when discussing mass organic loadings on these wetland
systems. The values shown on Figure 4, and similar values which can be found in
other references, are not the actual areal organic loading on the system, but rather are
the "apparent" organic loadings obtained by dividing the daily organic load by the total
surface, area of the system. That approach implies that the organic load is applied
uniformly over the entire surface area of the system. As discussed previously, much
of the input solids and BOD5are probably removed rapidly near the front end of the
system, so the actual organic loading 'on this zone is much higher than on the rest of
the system. This situation also prevails in FWS wetlands, most facultative lagoons,
and most overland flow land treatment systems where the influent is typically applied.
at the head of the treatment unit. In slow rate land treatment systems where
sprinklers are used to distribute the wastewater over the entire treatment surface the
"apparent" organic loading may be equal to the actual organic loading. A few FWS
wetland systems designed for further polishing of highly treated tertiary effluent are
dealing with low levels of essentially soluble BOD5and in these cases the "apparent"
organic loading may approximate the actual areal organic loading.. In overland flow
systems dealing with high concentrations of organic solids, sprinklers are also used
to ensure a more uniform distribution over the available treatment area. In wetland
systems, this purpose could be achieved with step feed of the influent at more than
one point along the flow path to ensure a more uniform loading.
This non-uniform application of organic wastes complicates development of an
accurate and precise design model for BOD5removal, since it is likely that the actual
removal rates may vary along the flow path, and, concurrently, residual BODJs being
produced from decomposing plant detritus. The situation is further complicated in
that the only data currently available are input/output data from a limited number of
systems, some of which have a longer HRT than is probably necessary to achieve the
measured effluent.
The development of the ultimate design model must wait for collection of a
sufficient body of reliable data describing the internal performance within these
systems. In the interim, some rational design approach is necessary to ensure that
these systems are cost-effective and can reliably produce the expected effluent
quality. The same situation prevails in the design of overland flow land treatment,
facultative lagoons, and similar aquatic concepts. In these cases, it is the consensus
opinion that a first order plug flow model provides an acceptable and rational basis for
design. Advances have been made with lagoon systems, and more complex models
accounting for dispersion and mixing have been developed. However, the first order
plug flow model still prevails as the most commonly used.
3 -6
-------
Chapter 3
A first order plug flow model for BOD5 removal also has been used by a number
of engineers for design of these SF wetland systems - in the United States, Europe,
and Australia (6,7,8,9,10,12,13,14,15,19,20). The general form of the model is
presented below:
—- -e• T
(i)
Where: Ce = Effluent BOD5(mg/L)
c = Influent BOD5(mg/L)
K° = temperature dependent rate constant (d-1)
t T = hydraulic residence time (d)
KT = K20(1 .06) (T-20)
K = rate constant at 20o C, (d"1)
20 = 1.104 d'1
T = temperature of liquid in the system ( °C)
A rate constant K20equal to 1.104 d"1for use in equation (1) for the design of
SF constructed wetlands has been proposed and published in several sources (6,7,8).
Independent support for that value can also be found in other published literature (19).
This value is believed to be conservative and is associated with an "apparent" organic
loading on the system of about 110 kg/ha/d ( = 98 Ib/ac/d). The highest "apparent"
organic loading shown on Figure 4 is about 143 kg/ha/d (= 128 Ib/ac/d) and the
associated "apparent" rate constant would be about 1.385 d"1 In theory, it should
be possible to use this higher value for design, but the more conservative 1.104 d"1
is recommended since there is some independent support for that value. Collection
of additional performance data in the future may permit further optimization and
possibly a significant increase in the 1.104 d'Vate constant.
It is believed that the plug flow rate constant for these SF wetlands is higher
than those for facultative lagoons or for FWS wetlands because the surface area
available on the media in the SF wetlands is much higher than in the other two cases.
This surface area supports the development, and retention, of attached -growth
microorganisms which are believed to provide most of the treatment responses in the
system. Table 3 compares the rate constants for these three treatment concepts, at
an "apparent" organic loading of 110 kg/ha/d (98 Ib/ac/d). The rate constant for the
SF wetland is about an order of magnitude higher than facultative lagoons, and about
double the value for FWS wetlands.
3 -7
-------
Chapter 3
Table 3. Comparison of First Order Plug Flow Rate Constants (7,11,40)
Treatment Process
Rate Constant3
Subsurface Flow Wetland
Facultative Lagoon
Free Water Surface Wetland
1.104
0.117
0.501
a. At an apparent organic loading rate of about 110 kg/ha/d
TSS REMOVAL
Suspended solids removal is very effective in SF constructed wetlands. Most
of the removal probably occurs within the first few meters of travel distance from the
inlet zone.
Figure 5 presents the input TSS versus output TSS for the sites listed in
Table 2, with 20 mg/L shown as a reference index. Except for one excursion
(believed due to extensive surface flow and short circuiting), all of the systems
produce a final effluent with less than 20 mg/L TSS regardless of the input level (up
to 118 mg/L). The systems with high TSS input values (< 50 mg/l) typically have
facultative lagoons and the high solids are due to algae carry-over from the lagoon.
4O
20
2O mo/I-
Figure 5. Suspended Solids, Input Versus Output.
3-8
-------
Chapter 3
Figure 6 compares the TSS removal rate to the HRT for the systems
examined. The relationship is similar to that shown on Figure 2 for BODJn that after
an HRT of one day there is little improvement in removal of suspended solids.
a
100
90
80
70
60
50
30
20
10
2 3 -4 5 6
HYORAULC RESCeNCE TIVE. KRT (cfi
Figure 6. Suspended Solids Removal Versus HRT.
100
90
80
« 70
£ »
o so
S 4C
5 30
20
10
0
2 4 6 8 10 12 14 • 16 18 2O
ASPECT RATIO UW
Figure 7. Suspended Solids Removal Versus System Aspect Ratio.
3-9
-------
Chapter 3
Figure 7 compares the TSS removal rate to the aspect ratio of the various
systems. These results are similar to those shown in Figure 3 for BOD5and indicate
no relationship between the system aspect ratio and TSS removal.
A kinetic design model is not available for TSS removal. However, based on
the relationships illustrated in Figures 1 to 7, it is apparent that TSS removal follows
the same pattern as BOD5. This suggests that when a system is designed for a
particular level of BOD5 removal, the TSS removal will be comparable as long as
subsurface flow is maintained in the bed.
NITROGEN REMOVAL
The removal of non-ionized ammonia is typically the major nitrogen parameter
of concern due to its toxicity for fish and other aquatic animals, and to its added
oxygen demand on receiving streams. Many of the earliest SF wetland systems were
only required to remove BOD5and TSS and have done so successfully. In some
cases, their permits have since been revised to require ammonia removal. Many of
the new systems also have ammonia limits (depending on receiving water.
requirements).
The nitrogen entering wetland systems can be measured as organic nitrogen
and ammonia (expressed as TKN), or as nitrate, or a combination of both nitrogen
measurements. Septic tanks, primary treatment systems, and facultative lagoon
effluents do not usually contain nitrate, but can have significant levels of organic N
and ammonia. During the warm summer months, facultative lagoons can have low
levels of ammonia in the effluent, but often contain high concentrations of organic N
associated with the algae leaving with the effluent. Aerated secondary treatment
system effluents typically have low levels of organic N but contain significant
concentrations of ammonia and nitrate. Systems with high intensity or long-term.
aeration can have most of the nitrogen in the nitrate form.
The organic N entering a SF wetland is typically associated with particulate
matter such as organic wastewater solids and/or algae. The plant detritus and other
naturally occurring organic materials in the wetland can also be a source for organic
N. Decomposition and mineralization processes in the wetland will convert a
significant part of this organic N to ammonia.
Biological nitrification followed by denitrification is believed to be the major
pathway for ammonia removal in both types of constructed wetlands, as they are
3-1 0
-------
Chapter 3
presently operated (4,7,10). Plant tissue analysis at several locations indicate that a
single annual harvest of the plant material might account for ten percent or less of the
nitrogen- removed by the system (7,22,23). A more frequent harvesting program
might increase this potential, but it would also increase the costs for operation of the
system..
Figure 8 presents ammonia input versus output data for the systems included
in this study which have a continuous discharge and without recycle. The line on the
graph indicates the condition when input equals output. One half of the systems are
at or above that line, indicating that there is a net production of ammonia as the
wastewater passes through the bed. The source of this "extra" ammonia is believed
to be from the anaerobic decomposition of the organic nitrogen trapped in the bed as
particulate matter. Since the bed is anaerobic, there is then insufficient oxygen
available in the bed to oxidize this ammonia to nitrate. Figure 9 presents ammonia
removal for some of the systems versus HRT. The results for several systems
(Denham Springs, LA for example, NH, in = 0.7 mg/L, NH, out = 10 mg/L, removal
= - 1370%) are below the lower limit on the graph and are therefore not shown. One
point (the open square in the upper right corner) represents data from Santee, CA; the
remainder are data from the Table 2 systems.
IN = OUT
6 8 1O 12
MPUT OII0/U
14
Figure 8. Ammonia Input Versus Output.
3-1 1
-------
Chapter 3
i
*
|
i
I
1UU
80
60
40
20
0
-20
-40
-60
-80
— tf
-------
Chapter 3
achieved. At Bear Creek, the entire flow passed through the Typha root zone and 80
percent ammonia, removal was produced in 3.9 d. This strongly supports the
hypothesis that the plant roots in these SF wetland beds are the primary source of
oxygen needed for nitrification of ammonia and other biochemical responses, and that
it is therefore essential to bring the wastewater into direct contact with the root zone.
Table 4 summarizes the site conditions and the ammonia removal
performance of the 14 systems used in this analysis. It is clear from an examination
of these data that systems with no algae, with longer detention times, and with deep
root penetration produce the best ammonia removal results. The two systems in the
list (Santee and Bear Creek) which incorporate all three factors produced the best
results of all.
Table 4. Ammonia Removal in SF Wetlands.
Location
Denham Springs, LA
Haughton, LA
Carville, LA
Benton, KY
Mandeville, LA
Greenleaves, LA
Hardin, KY (bulrush)
Hardin, KY (reeds)
Utica, MS (north)
Utica, MS (south)
Degussa Corp., AL
Monterey, VA
Bear Creek, AL
Santee, CA (Scitpus)
Ammonia
Removal
%
-1328
-554
-22
-45
-50
-14
18
2
57
45
45
6
80
94
Algae
Present
Yes
Yes
Yes
Yes
No
No
No
No
Yes
Yes
No
No
No
No
HRT
d
1
4.5
1.4
5
0.7
1
3.3
3.3
5
3.7
1
0.9
3.9
7
Bed
Depth
m
0.61
0.76
0.76
0.61
0.61
0.61
0.61
0.61
0.64
0.64
0.61
0.91
0.30
0.61
Root3
Depth
%
50
50
50
40
50
50
50
40
60
60
50
30
100
100
a. Root depth expressed as a percentage of total bed depth.
The early investigators, and designers in the U.S. and Europe, believed that
oxygen from the plant roots would be available throughout the SF wetland bed.
Implicit in that belief is the assumption that the plant roots would penetrate the full
3- 13
-------
Chapter 3
depth of the bed. A procedure advocated by Kickuth (5) to achieve that result was
to lower the water level in the bed during the fall of the year, for three successive
years, to induce maximum root penetration. That procedure has not been attempted
in the U.S. to date.
The media depth in most of the beds in the U.S. is about 0.6 m (2 f-t), but in
most cases, the plant roots have been observed to penetrate only to 0.3 m (1 ft) or
less. Deeper penetrations have been observed when nutrient levels in the water are
low or when the plants are located at the sides of the cells and other possible "dead
spots" with less flow than the main portion of the bed. As a result, about half of the
flow in most U.S. systems occurs in a zone where oxygen is not likely to be present
and nitrification cannot occur. The results shown for most of the data points in Figure
9 are typical expectations for the current mode of SF wetland system operation and
bed configuration. To ensure significant nitrification in these systems, it will be
necessary to develop an appropriate oxygen source. Further discussion on that topic
is provided in a later section of this report.
Figure 10 compares the mass removal rate for total Kjeldahl nitrogen (TKN) to
the mass loading (most of the systems in Table 2 have no TKN data). TKN is
essentially equal to total nitrogen (TN) in these cases since there is typically little
nitrate entering or leaving the SF systems. The line shown on the figure is the result
of a regression analysis for FWS wetland systems for these same parameters (24).
I
1
10
16 18 2O
FWS
Figure 10. TKN Mass Loading Versus Mass Removal
3-14
-------
Chapter 3
There is no apparent relationship between mass loading of TKN and removal for the
six SF systems where data were available. All but one of these data points are
significantly below the FWS line, indicating that these SF systems are less efficient
at TKN (or TN) removal than FWS systems. The difference is again believed to be the
availability of oxygen. In FWS systems, the water surface is exposed to the
atmosphere and some direct oxygen transfer is possible. The only data point on
Figure 10 which is close to the line is the Bear Creek, AL system, where (as discussed
previously) oxygen from the plant roots is apparently available in this shallow bed
(0.3m) to support nitrification.
It has been suggested that the effluent ammonia from these wetland systems
is related to the mass loading of TKN or TN on the system (41). Figure 11 presents
this comparison for six of the Table 2 SF systems where sufficient data were
available. The regression curve of best fit is also shown, but the low r'value
indicates that the correlation is not very significant. A missing element in this
approach is the residue from decomposition of plant matter in the system since the
TKN loading is only a measure of nitrogen in the entering wastewater. Figure 11
suggests that a TKN mass loading of less than 2 kg/ha/d (1.8 Ib/ac/d) would be
necessary to consistently achieve an effluent ammonia of 2 to 3 mg/L in SF wetlands
as they are currently operated with an oxygen deficiency.
20
18
16
14
12
10
8
6
4
2
0
0310
4 6 8 1O 12 14
TKN MASS LOADING (k g/ha/d)
16
18 20
Figure 11. Effluent Ammonia Versus TKN Mass Loading
3-1 5
-------
Chapter 3
Many of the older systems in the Gulf States used soft tissue flowering plants
for aesthetic reasons. Their decomposition is very rapid in the fall and after a frost,
and measurable increases in effluent BOD5and ammonia can be observed (25). The
same effect is likely when significant surface flow is allowed on these systems;
surface flow will result in a much more rapid breakdown of plant detritus and release
of ammonia to the water. If the surface of the bed is maintained in a dry condition
(except during rainfall events), plant detritus will decompose much more slowly.
PHOSPHORUS REMOVAL
Figure 12 presents phosphorus input versus output data for the systems where
data were available. The sloping line on the figure represents the condition where
input equals output. Two of the systems fall on that line and several others show
marginal removal capabilities. Only one data point shows excellent removal - the
Bear Creek, AL system. The media in this case are fine river gravel, and the presence
of oxides of iron and aluminum associated with this media may be responsible for the
95 percent removal of phosphorus observed during the first year of system operation.
» 4 e «
PHOSPHORUS MVT (mg/U
Figure 12. Phosphorus Input Versus Output
3-16
-------
Chapters
Phosphorus removal in most constructed wetland systems is not very effective
because of the limited contact opportunities between the wastewater and the soil.
Some experimental and developmental work has been undertaken using expanded clay
aggregates and the addition of iron and aluminum oxides; some of these treatments
may have promise but the long-term expectations have not been defined. Some
systems in Europe use sand instead of gravel to increase the phosphorus retention
capacity, but selecting this media results in a larger system because of the reduced
hydraulic conductivity of sand compared to gravel. If significant phosphorus removal
is a project requirement, then very large land areas or alternative treatment methods
will probably be required.
FECAL COLIFORM REMOVAL
These SF wetland systems are, in the general case, capable of a one- to two-
log reduction in fecal conforms, which in many cases is not enough to routinely satisfy
discharge requirements which often specify < 200/100 ml. Peak flows in response
to intense rainfall events also disrupt removal efficiencies for fecal conforms. As a
result, most of the systems listed in Table 2 utilize some form of final disinfection.
One exception is the lightly loaded system in Bear Creek, AL, which uses a small
sized-river gravel as the bed media. At Bear Creek the fecal conforms are reduced
from 8 x 104/100 ml to 10/100 ml or less on an average basis.
3-1 7
-------
Chapter 4
CHAPTER 4
DESIGN CONSIDERATIONS
The major concerns in the design of SF constructed wetlands include:
• Hydraulic and hydrological conditions,
• BOD5and TSS removal mechanisms,
• Nitrogen removal efficiency,
• Vegetation selection and management,
• Construction details, and
HYDRAULICS & HYDROLOGY
A basic intent of the SF wetland treatment concept is the maintenance of flow-
beneath the surface of the media in the bed. However, a significant number of
operating SF constructed wetlands are exhibiting varying degrees of surface flow on
top of the media bed. Since these systems were designed for complete subsurface
flow, this condition represents a potential design deficiency. It has been suggested
that this surface flow is due to clogging of the void spaces in the bed either by the
vegetative roots and associated plant materials or by the accumulation of suspended
solids separated from the wastewater stream. However, most of these systems have
been in operation for less than five years and surface flow was observed at many
soon after flow commenced.
BED CLOGGING
A preliminary, unpublished 1990 investigation by Reed suggested a relationship
between the organic loading on the cross section of the bed at the entry zone and
observed surface flow on the SF wetland bed. The assumption was that clogging at
the entry zone was causing the surface flow. A cross sectional loading of < 0.5 kg
4-1
-------
Chapter 4
BOD5/m2/d (0.1 Ib BOD5/ft2/d) was associated with no observed surface flow.
Systems with observed surface flow had higher organic loadings, > 0.5 kg/nf/d.
Unfortunately, this apparent relationship has now been published in a number of
sources and has become part of some design guidelines (17). Subsequent
investigations at the original sites have indicated that the observed surface flow at
most of the systems can be explained by inadequate hydraulic design and inattention
to the requirements of Darcy's Law (described below). The organic loading approach
may have some merit in that a reduced organic loading on the cross section should
certainly reduce the potential for clogging. There are, however, no data available at
present to support the selection of a specific cross sectional organic loading. The net
affect of this approach is to increase the cross sectional area of the bed and thereby
reduce the aspect (L:W) ratio. That same result can be achieved, in a more rational
manner, by proper application of Darcy's Law.
Pits have been excavated in six of these SF systems (two in Kentucky, four in
Louisiana) to observe any clogging substances, and to determine their characteristics
(26,27,28). At only one Kentucky site, by June 1990, a persistent gelatinous
substance had almost completely clogged the void spaces in the first 25 percent of
the bed. Laboratory tests indicated that this material was about 80 percent inorganic,
with that fraction composed of silica, clay minerals', and limestone dust. The clogging
at this site occurred rapidly during the first year of operation and did not significantly
expand in area in subsequent years. Pits excavated at the same locations in 1992
showed no evidence of gelatinous substances anywhere in the bed. It is likely that
this isolated case of severe clogging may have been due to the overloaded condition
of the bed during its first year of operation.
At the four sites in Louisiana, and the second site in Kentucky, the suspended
matter in the rock voids was not gelatinous in character and washed easily from the
rock surfaces (27,281. It was similar in appearance to a mixed liquor sample, with a
slight odor in some cases. At three of the four sites in Louisiana, the solids present
occupied less than two percent of the void space available for flow of water; in the
worst case the solids present occupied about six percent of the available void spaces
at a location close to the inlet pipe. In all cases, these solids were at least 80 percent
inorganic material. Plant roots and related detritus were not encountered below
depths of about 0.3 m (1 ft) in any of these systems (27,281. As a result, at these
five sites it does not appear that accumulation of TSS or plant detritus were
responsible for clogging or for any surface flow which may have occurred.
At all six of the sites investigated, the rock media were delivered by truck over
unpaved roads, in all kinds of weather, and, as described by the operators, the trucks
tended to follow the same pathway entering and leaving the wetland bed. It is quite
4-2
-------
Chapter 4
possible that a large portion of the inorganic solids observed in the void spaces of the
media was due to soil from the truck tires, and soil, rock dust and fines from the rock
media transported in the truck. At the formerly partially clogged Kentucky site, these
inorganic materials may have then trapped algal solids entering the bed, resulting in
formation of the gelatinous material creating the clogging. The fact that the clogged
zone never expanded beyond the first 25 percent of the bed, and that the gelatinous
material has now disappeared, suggests that construction activity may have been the
primary cause of the clogging instead of a continuing biochemical reaction.
HYDRAULIC DESIGN
When subsurface flow conditions are expected in the SF wetland bed it is
common practice to use Darcy's Law, which describes the flow regime in a porous
media. Darcy's Law is typically defined with equation (2).
Q = ksA S (2)
Where: Q = flow per unit time, nf/d (ftVd), or (gal/d), etc.
k = hydraulic conductivity of a unit area of the medium
perpendicular to the flow direction, m3/ m2/ d
(ftVftVd), or (gal/d), etc.
A = total cross-sectional area, perpendicular to flow, m2
(ft2).
S = hydraulic gradient of the water surface in the flow
system dh/dL, m/m, (ft/ft).
(All units must be consistent)
Darcy's Law is not strictly applicable to subsurface flow wetlands because of
physical limitations in the actual system. It assumes laminar flow conditions, which
may not be the case when large rock or very coarse gravel are used as the media.
Turbulent flow will occur in these coarse media when the hydraulic design is based
on a high hydraulic gradient. Darcy's Law also assumes that the flow (Q) in the
system is constant and uniform, but in the actual case in a SF wetland the input
versus output Q may vary due to precipitation, evaporation, and seepage; and short
circuiting of flow may occur due to unequal porosity or poor construction. Ail of
these factors limit the theoretical applicability of Darcy's Law, but it remains as the
only reasonably accessible model for design of these SF systems. If small to
moderate sized gravel (< 4 cm) is used as the media, if the system is properly
constructed to minimize short circuiting, if the system is designed to depend on a
4-3
-------
Chapter 4
minimal hydraulic gradient, and if the Q in equation (2) is considered to be the
"average" flow ([Qin+ Qoutl/2) in the system to account for any gains or losses due
to precipitation, evaporation or seepage, then Darcy's Law can provide a reasonable
approximation of the hydraulic conditions in these SF beds.
Some of the constraints on Darcy's-Law can be reduced by conducting
predesign tests with the actual media to be used to determine the "effective"
hydraulic conductivity under various flow and hydraulic gradient conditions, and to
ensure that laminar flow conditions prevail. These tests are recommended for large-
scale projects and/or for repetitive use of the same media on a number of small-scale
projects. The test can use a flume or trough of reasonable length (< 6 m) and
reasonable cross sectional area (depends on size of media to be tested, but generally
< 0.2 m2). The inlet end of the trough is capable of being raised above the datum
to produce the desired test slope. The gravel is contained within perforated plates in
the trough, and manometers are installed at appropriate locations to measure the head
differential (dh) for calculation of the hydraulic gradient (dh/dL). Clean water is used
in the test, and the inflow adjusted so that the gravel is saturated at the head of the
flume but with no surface flow. During' the test the outflow (Q) is measured with a
stop watch and a conveniently sized container, and the depth of the wetted zone (A)
at the perforated outflow plate is measured. It is possible with these data to then
calculate the "effective" hydraulic conductivity for design of the system, and to
calculate the Reynolds number to ensure laminar flow conditions.
It is believed that the surface flow observed on many of the operational SF
systems in the U.S. is the result of an inadequate hydraulic gradient provided by the
system's design and selected configuration (13). Many of the problem systems in the
U.S. have been constructed with a very high aspect ratio (L:W), and without any
bottom slope or water level controls at the outlet works. In some cases, the outlet
ports in the effluent manifold were at or near the top of the bed, thereby negating the
development of any significant hydraulic gradient in the bed and ensuring the
occurrence of surface flow from the beginning of operations. Systems in the U.S. and.
Europe with successful hydraulic performance (i.e.,: maintenance of subsurface flow)
do so with either a sloping bottom and/or adjustable outlet works 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 then produces the pressure head required to
overcome resistance to flow through the media and the maintenance of subsurface
flow conditions. An adjustable outlet provides greater flexibility and control and is the
recommended approach.
4-4
-------
Chapter 4
Aspect Ratio
The aspect ratio (L:W) of the wetland bed is a very important consideration in
the hydraulic design of SF wetland systems, since the maximum potential hydraulic
gradient is related to the available depth of the bed divided by the length of the flow
path. Many of the early systems designed with an aspect ratio of 10:1 or more and
a total depth of 0.6 m (2 ft) have an inadequate hydraulic gradient and surface flow
is inevitable. The hydraulic gradient (S factor in equation 2) defines the total head
available-in the system to overcome the resistance to horizontal flow in the porous.
media.
For example, in a SF bed 200 m long and 0.6 m deep, if the water level is at
the surface of the media at the influent end and near the bottom of the bed at the
effluent end (water depth = 0.2 m), the available hydraulic gradient would be
0.4m/200m or 0.002. If this wetland bed were 100 m wide (L:W =2:1) and used
a gravel media with a hydraulic conductivity (ks) of 10,000 m3/m2/d, the maximum
subsurface flow, based on equation (2), would be 800 mVd (0.21 mgd). Using the
same volume of gravel in a bed 450 m long results in a bed width of about 45 m (L:W
= 10:1) and a hydraulic gradient of 0.0009; the maximum subsurface flow in this
case would be 162 mVd (0.04 mgd), which is 20 percent of the flow allowed by the
shorter bed. If the design flow were actually 800 mVd in the second case, then
surface flow on top of the bed would be unavoidable, even though the bed contains
exactly the same volume of media.
Bed Slope
SF systems in Europe (29) have been constructed with up to 8 percent slope
on the bottom of the bed to maintain an acceptable hydraulic gradient. However, it
is not practical and probably not possible with SF systems to precisely design and
construct the bed for a specific hydraulic gradient due to variabilities in the media
used and in construction techniques, and the potential for longer term partial clogging.
In addition, the construction of a bed with a sloping bottom provides no flexibility for
'future adjustments. Greater flexibility and control is possible with an adjustable outlet'
which permits control of the water level over the entire design depth of the bed. In
this case, the bottom of the bed could be flat or with a very slight slope to ensure
drainage, when required. However, because of the hydraulic gradient requirements,
the aspect ratio (L:W) will have to be relatively low (in the range of 0.4:1 to 3:1 ) to
provide the flexibility and the reserve capacity for future operational adjustments.
4-5
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Chapter 4
Media Types
Table 4 presents a summary of typical characteristics for the types of media
which have been used in SF constructed wetlands. Essentially all of the operational
SF constructed wetlands in the U.S. have used media ranging from medium gravel to
coarse rock. The values in Table 4 are intended as preliminary information only.
Following selection of a media type and size; the hydraulic conductivity and porosity
of the material should be determined in the field or laboratory, prior to system design,
The recent, trend in the Gulf States toward the use of larger sizes of rock is
believed due to the impression created by the surface flow conditions on many of the
early systems. It was apparently thought that the surface flow was caused by
clogging and that the use of a coarser rock with larger void spaces and a higher
hydraulic conductivity would overcome the problem. In most cases the problem has
not been overcome since the hydraulic gradient provided is too small. The use of
smaller rock sizes has a number of advantages in that there is more surface area
available on the media for treatment, and the smaller void spaces are more compatible
with development of the roots and rhizomes of the vegetation, and the flow
conditions should be closer to laminar. When turbulent flow occurs in the coarser
media listed in Table 5, the "effective" hydraulic conductivity will be less than the
values listed in the table.
Table 5. Typical Media Characteristics for SF Wetlands
Type
Coarse Sand
Gravelly Sand
Fine Gravel
Medium Gravel
Coarse Rock
Effective
Size
D 10m m
2
8
16
32
128
na
Porosity
%
32
35
38
40
45
ksb
Hydraulic
Conductivity
mVmVd
1,000
5,000
7,500
10,000
100,000
a. The porosity is used to determine the actual flow velocity in the void spaces, and in equations (3) and
(5) to determine the size of the SF bed. Porosity is equal to Void Volume/Total Volume, and is
expressed as a percent.
b. Assuming non-turbulent, near laminar flow conditions, with clean, water.
mVmVd x 24.6 = gpd/ft2.
4-6
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Chapter 4
The hydraulic conductivity (ks) values in Table 5 assume that the media and the
water flowing through it are clean so that clogging is not a factor. As discussed in
a previous section, some clogging can occur in these systems, especially near the inlet
zone where most of the suspended solids will be removed. As noted previously, the
observed clogging represented less than 6 percent of the void spaces in the systems
investigated. The majority of the material (>80%) was inorganic and believed to be
the residue from construction activities, and should not, therefore, have a cumulative
impact on hydraulic conductivity. It is, however, necessary to provide a large safety
factor against these contingencies and adoption of an approach similar to that used
in the design of land treatment systems (30) is proposed. It is therefore
recommended that a value < 113 of the "effective" hydraulic conductivity (ks) be
used for design. The initial design, for the same reasons, should not utilize more than
70 percent of the potential hydraulic gradient available in the proposed bed. These
two limits, combined with an adjustable outlet for the bed discharge should ensure
a more than adequate safety factor in the hydraulic design of the system. These two
limits will also have the practical effect of limiting the aspect ratio of the bed to < 3:1
for 0.6 m (2 ft) deep beds and to about 0.75:1 for 0.3 m (1 ft) deep beds. Using
such a low value for hydraulic gradient will help maintain near laminar flow in the bed
and further validate the use of Darcy's Law for design of these systems. Since this
approach ensures a relatively wide entry zone, it will also result in a low organic
loading on the cross sectional area and thereby reduce concerns over clogging.
In addition to the internal hydraulic concerns discussed above, it is necessary
to have adequate inlet and outlet structures for the bed to assure proper distribution
and collection of flow and maximum utilization of the media provided in the bed.
Inlet Structures
The inlet devices at operational systems include surface and subsurface
manifolds, an open trench perpendicular to the flow direction, and simple, single point
weir boxes. The manifold designs include a variety of features. In some cases
perforated pipe is used for both surface and subsurface installations. In one case the
subsurface- manifold utilized two to three valved outlets in the cell. A surface
manifold developed by TVA uses multiple, adjustable outlet ports (31,32). This allows
the operator to make adjustments for differential settlement of the pipe and to
maintain uniform distribution of the wastewater. The proponents of subsurface inlet
manifolds claim they are necessary to avoid the build-up of algal slimes on the rock
surfaces and resulting clogging adjacent to a surface manifold. The disadvantages of
a subsurface manifold are the inability for future adjustment and the limited access for
4-7
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Chapter 4
maintenance. In one case, a buried manifold became clogged with turtles (entered the
piping system from the preliminary treatment lagoon) and had to be removed.
A surface manifold, with adjustable outlets, seems to provide the maximum
flexibility for future adjustments and maintenance and is recommended. Use of a
coarse rock (8 to 15 cm [3 - 6"]) in this entry zone, coupled with an adequate
hydraulic gradient for the bed, should ensure rapid infiltration and prevent ponding and
algae development. In continuously warm and sunny climates, shading of this entry
zone with either vegetation or a structure may also be necessary. In cold winter
climates, some thermal protection for an above-surface manifold will probably be
necessary.
Outlet Structures
Outlet structures in use at operational SF wetland systems include subsurface
manifolds, and weir boxes or similar gated structures. The perforated subsurface
manifold is the most commonly used device; however, the location of that manifold
in the bed has varied considerably. In a few cases it has been located in a shallow
trench, below the bottom of the bed, permitting complete drainage of the bed and
development of the maximum hydraulic gradient for the system. In many cases, the
manifold and/or the outlet ports have been located above the bottom of the bed, and
in some cases the outlet ports have been located near the top of the bed. As
indicated previously, this latter practice results in surface flow on the bed.
In most cases, the subsurface outlet manifold connects directly to the final
discharge pipe, and/or to a concrete channel used for final disinfection. Some system
designs in Europe and those in the U.S. derived from that practice (31,32), connect
the subsurface manifold to an adjustable outlet for water level control. Flow then
proceeds to either discharge or disinfection.
The use of an adjustable outlet was previously recommended to maintain an
adequate hydraulic gradient in the bed. This device can also have significant,
operational and maintenance benefits. The surface of the bed can be flooded to
encourage, development of newly planted vegetation and to suppress undesirable
weeds, and the water level can be lowered in anticipation of major storm events and
to provide additional thermal protection against freezing during winter operations in
cold climates.
The use of a perforated subsurface manifold connected-to an adjustable outlet
would seem to offer the maximum flexibility and reliability as the outlet device for SF
wetland systems. Since the manifold is buried and inaccessible following
4-8
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Chapter 4
construction, careful grading and subbase compaction would be required during
construction and clean-out risers should be provided in the line.
BOD, REMOVAL
'5
When process kinetics were given any consideration, most of the existing
systems in the U.S. and. Europe were designed as an attached growth biological
reactor using a first order plug flow model, shown previously as equation (1):
0)
The effluent BOD5(C,) in equation 1 is, as previously discussed, influenced by
the production of residual BOD5within the wetland from decomposition of plant
detritus and other naturally occurring organics. This residual BODJs typically in the
range of 2 to 7 mg/L. As a result, equation (1) should not be used for designs for a
final BOD5< 5 mg/L.
It has been argued that plug flow kinetics do not apply to SF constructed
wetland systems because the dye and tracer studies which have been performed do
not exhibit the ideal plug flow response. Figure 13 presents the results of a tracer
study, using lithium chloride (an inorganic, conservative tracer), conducted in 1980
at the operational SF wetland system at Carville, LA (27). Essentially 100 percent of
the tracer was accounted for in the effluent, so this can be considered a valid study.
It clearly did not exhibit ideal plug flow responses, but the curve is much closer to
plug flow conditions than to the complete mix alternative. The centroid of the curve
indicates an HRT of 48 hours, which is identical to the theoretical detention time
calculated with the actual flow, measured porosity, and wetland cell dimensions.
There was no surface flow during this test. Data from similar tests at other sites
show an even closer resemblance to plug flow.
The shape of the curve on Figure 13 is similar to those observed with
facultative ponds and similar wastewater treatment concepts where plug flow
conditions are also assumed as the basis for design (33). Models are available for
these systems which attempt to define conditions between plug flow and complete
4-9
-------
Chapter 4
mix, but the difficulty in defining the necessary parameters has resulted in minimal use
of these alternatives.
2-
1-
O toooooooooop
O 2O
MAK AT *8 Ml
C8nHOf>, HRT • 48 HR
100% U MCOVDtY
4O 6O 80
TWE (how*)
1OO 12O
Figure 13. Lithium Tracer Study, Carville; LA.
The plug flow model is presently in general use. and it seems to provide a
reasonable approximation of performance in these SF constructed wetlands.
However, more sophisticated models have also been proposed for SF wetlands. One
model includes a plug flow segment followed by three continuously stirred (complete
mix) reactors in series (43). When sufficient data are available to validate these
alternative models, they may replace the current approach. In the interim, the use of
the plug flow model is recommended for design.
The "f or hydraulic residence time (HRT) factor in equation (1) can be defined
as:
t-
nLWd
(3)
4-1 0
-------
Chapter 4
Where: n = effective porosity of media (see Table 5) % as a
decimal
L = length of bed, m (ft)
W = width of bed, m (ft)
d = average depth of liquid in bed, m (ft)
Q = average flow through the bed, nf/d (ftVd)
The Q value in equation (3) is the average flow in the bed to account for
precipitation, seepage, evapotranspiration and other gains and losses of water during
transit of the bed. This is the same value used in Darcy's Law for hydraulic design.
Published values are usually available for an estimate of precipitation and ET losses
at a particular site.
The d value in equation (3) is the average depth of liquid in the bed. If, as
recommended previously, the design hydraulic gradient is limited to 10 percent of the
potential available, -then the average depth of water in the bed will be. equal to 95
percent of the total depth of the treatment media in the bed.
The temperature dependence of the rate constant in equation (1) is defined as:
(4)
Where: KT = rate constant at temperature T, d
K20 = rate constant at 20 °C, d'1
= 1.104
0 = 1.06
Combining equations (1), (3), and (4) produces:
-1
j) IQ
• ~— CT
(5)
4-11
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Chapter 4
Since the term LW in equation (5) is equal to the surface area of the bed,
rearrangement of terms in equation (5) permits the calculation of the surface area (A,)
required to achieve the necessary level of BOD5removal:
_ p[ln(C0/CJ]
(6)
Where: As = bed surface area, m2(ft2)
Other terms defined previously
The depth of media selected will depend on the design intentions for the
system. If the vegetation is intended as a major oxygen source for nitrification in the
system, then the depth of the bed should not exceed the potential root penetration
depth for the plant species to be used. This will ensure availability of some oxygen
throughout the bed profile, but may require management practices which assure root
penetration to these depths. Table 6 presents results from the pilot system in Santee,
CA (4) showing the relationship between root penetration and performance. The root
depths shown in Table 6 are considered to be near the maximum practical limit to be
expected. The design approach in Europe has also assumed a maximum depth of 0.6
m for Phragmites (15).
Table 6. Performance of Vegetated and Unvegetated SF Wetland Beds (4)
Bed
Type
Bulrush, Scirpus
Reeds, Phragmites
Cattails, Typha
No Vegetation
Root
Depth, m
0.8
0.6
0.3
0.0
Final
BOD5
5
22
30
36
Effluent Qualitv
TSS
4
8
6
6
ma/La
N H3
2
5
18
22
a. Primary effluent input (BOD5= 118 mg/L, SS = 57 mg/L, NH3 = 25 mg/L)
There is one operational system in the U.S. (Monterey, VA) with a media depth
of 0.9 m (3 ft); the most commonly used depth is 0.6 m (2 ft). One system (Bear
4-12
-------
Chapter 4
Creek, AL) using Typha in fine gravel is obtaining excellent performance with a depth
of 0.3 m, which matches the root penetration listed in Table 6 for that plant.
The final design and sizing of the SF bed for BOD5 removal is an
iterative process:
1. Determine the media type, vegetation, and depth of bed to be used.
2. Determine by field or laboratory testing the porosity (n) and "effective" hydraulic
conductivity (ks) of the media to be used.
3. Determine the required surface area of the bed, for the desired level of BOD5
removal, with equation 6.
4. Depending on site topography, select a preliminary aspect ratio (L:W); 0.4:1 up to
3: 1 are generally acceptable.
5. Determine bed length (L) and width (W) from the previously assumed aspect ratio,
and results of step 2.
6. Using Darcy's Law (equation 2) with the previously recommended limits (ks < 1/3
"effective" value, hydraulic gradient S < 10% of maximum potential), determine
the flow (Q) which can pass through the bed in a subsurface mode. If this Q is
less than the actual design flow, then surface flow is possible. In that case it is
necessary to adjust the L and W values until the Darcy's Q is equal to the design
flow.
7. It is not valid to use equation 5 with effluent BOD5(Ce) values below 5 mg/l. As
previously discussed, these wetland systems export a BOD5 residual due to
decomposition of the natural organic detritus in the system.
8. In cold climates it is necessary to assume a design temperature for BOD5to first
determine the required surface area. Thermal calculations are then necessary to
determine the winter heat losses and bed temperature conditions during the design
HRT. Further iterations of this procedure are necessary until the assumed
temperature and the temperature determined by the heat loss calculations
converge.
4-13
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Chapter 4
Suspended Solids Removal
A kinetic model is not available for suspended solids removal; based on the data
presentations in Figures 5 and 6, it is unlikely that such a model will be developed.
It would appear that with an HRT of about 1 d the TSS will be removed to a level of
about 10 mg/L. As a rule of thumb it can be assumed that if the system is designed
for a certain level of BOD5removal, the TSS removal will be comparable as long as
significant long-term surface flow does not occur. Long-term surface flow can result
in short circuiting and the addition of TSS to the surface flow stream in the form of
algae and plant detritus.
NITROGEN REMOVAL
As indicated in the discussion related to Figures 8 and 9, the major pathway for
nitrogen removal in SF wetland systems is biological nitrification followed by
denitrification (4,7,8). Based on the data presentations in Figures 8 and 9, it is clear
that ammonia removal is not very effective in most of the operational SF systems
studied. The limiting factor in ammonia removal via nitrification is believed to be the
availability of oxygen in the media profile. This constraint is apparent regardless of
the age or operational history of the system. The only two systems demonstrating
excellent ammonia removal in Figure 9 have plant roots (and therefore available
oxygen) throughout the profile, and sufficient HRT to complete the reactions. The
majority of the systems constructed in the Gulf States in recent years have too brief
an HRT ( < 2d) and inadequate root penetration and development to depend on the
plant roots as an oxygen source for significant nitrification. In these cases, other
oxygen sources will be necessary.
There is no consensus on how much oxygen can be furnished by the vegetation
in SF wetlands or on the oxygen transfer efficiency of various plant species. There
is consensus that these emergent aquatic plants transmit enough oxygen to their roots
to keep alive. The disagreement occurs over how much excess oxygen is then
available to support biological activity in the root zone. Published estimates have
ranged from zero to 45 gm 02/m2/d of wetland surface area (4,7,34). This oxygen
is not believed to be diffused throughout the subsurface profile, but is likely to be
available-only on the surfaces of the smaller roots, within the root zone, in the bed.
These aerobic microsites on the root hairs provide potential contact surfaces for the
nitrification of ammonia. These microsites probably occur throughout the length of
the bed, but the oxygen demand for BOD5 removal is likely to limit the potential for
nitrification in the front part of the system.
4-1 4
-------
Chapter 4
An alternative oxygen source, other than the plant roots, is not apparent in the
completely submerged, but fully rooted, SF bed. Since these systems, as shown in
Table 6, do provide significant ammonia removal (which is correlated to root depth),
it seems likely that there is some oxygen available on the surfaces of the plant roots.
Since it takes about 5 mg of oxygen to convert 1 mg of ammonia to nitrate, it is
possible to estimate the oxygen that was provided in the Santee system to achieve
the removals shown in Table 6. These results are summarized in Table 7.
The values in Table 7 do not define the actual oxygen production of the plants,
but only indicate the amount of oxygen necessary to account for the known removal
of ammonia at the Santee project. It seems likely that the plant roots were the source
of this oxygen since no other source is apparent nor is any other alternative ammonia
removal process likely. These values from Santee are also consistent with the
performance of the Bear Creek system (see Figure 9 and related discussion). The
average value of 7.5 gm 02/mVd is also consistent with, and near the low end of, the
range reported by European investigators (7,341.
Table 7. Potential Oxygen from Vegetation at Santee, CA
Plant Type Root Depth Available Oxygen
m g m / m3/ da g m / m2/ db
Bulrush rush (Scirpus)
Reeds (Phragmites)
Cattails (Typhal
0.76
0.60
0.30
7.5
8.0
7.0
5.7
4.8
2.1
Average 7.5
a. Available oxygen per unit volume of actual root zone.
b. Available oxygen per unit surface area of 0.76 m deep wetland bed.
The example below illustrates the application of these tentative relationships:
Assume: Q = 378 mVd, NH, in = 20 mg/L, NH, out = 2 mg/L
Plant type: cattails, Bed depth = 0.3 m
Average available oxygen 7.5 gm/nf/d (from Table 6)
BODJn = 75 mg/L, BOD5level for start of nitrification = 20 mg/L
Media porosity = 0.4
4-15
-------
Chapter 4
HRT for BOD5 = ln(75/20)/1.104 = 1.2 d (equation 3)
Area for BOD5 removal to 20 mg/L = 0.4 ha (equation 6)
Nitrification:
Oxygen available = (0.3 m)(7.5 gm/m3/d) = 2.25 gm/nf/d
Oxygen required = (20 - 2)(378)(5) = 34,065 gm/d
Nitrification area required = (34,065)7(2.25) = 15,140 m2= 1.5 ha
Total HRT = 1.2 + 4.8 = 6.0 d Total Area = 0.4 + 1.5 =1.9 ha (4.7 ac)
(47 ac/mgd)
The example presumes a two-stage system in which the BODJs reduced to
about 20 mg/L followed by nitrification with oxygen supplied by the vegetation. The
remaining BODJn this second stage would be available for denitrification and the final
effluent BOD5should be at background levels for these systems. The results of this
example are consistent with the Santee performance and other locations where plant
supplied oxygen is believed to be responsible for nitrification. This suggests that
nitrification via this pathway is possible when both fully developed root systems and
sufficient detention time are available. The limiting factor in this case is the rate at
which the plant can provide oxygen. However, there, are, at present, limited
independent data to support this tentative procedure derived from the experience at
Santee, which should be used with caution for design until further research results are
available. It will also be necessary to identify the seasonal and temperature influences
on the procedure to achieve successful application in cold climates.
Adoption of this approach for nitrification would then require at least a six-day
HRT, and the cost of the rock or gravel media could be prohibitive. Alternative.
methods for oxygen supply and nitrification, therefore, deserve consideration.
These alternative methods may include: mechanical aeration after BOD5
reduction, provision of open water zones for surface reaeration, use of the overland
flow land treatment concept for nitrification, and the use of parallel nitrification cells
operated on a batch type fill and draw basis to return atmospheric oxygen to the
media profile. Another approach, which is under construction for both FWS and SF
cells in Kentucky, will superimpose a vertical flow, recirculating bed, composed of fine
gravel, for nitrification on top of the existing system. In this case the recirculating
filter will be located at the influent end of the bed. The effluent from the nitrification
4-16
-------
Chapter 4
bed will mix with the normal untreated flow in the wetland cell where the nitrates
produced should be denitrified. With this approach the wetland cell need only be
sized for BOD5 removal requirements since nitrification is expected to occur in the
recirculating bed. The concept offers promise for retrofit of the many existing
systems which are having difficulty meeting their discharge limits for ammonia. The
concept' may also be more cost-effective than a minimum of a six-day HRT for
wetlands where plant-available oxygen is assumed as the sole source for nitrification.
The hydraulic design of such a system will have to include consideration of the recycle
flow within the horizontal portion of the bed. Data collection is planned for a 12-
month period at the Kentucky site and should lead to optimization of the concept for
application elsewhere.
Three other design models for estimating ammonia removal in constructed
wetland systems are available in the literature. The WPCF Manual of Practice (8)
presents a model for ammonia removal based on a regression analysis of both FWS
and SF systems, for annual average conditions with no provision for temperature
correction:
(O..01)«?)
[ (1.527) (InNHJ -(1.050) (lnNH0) +1.69]
(6)
Where: A s = wetland surface area required for ammonia removal,
ha
N |-| = effluent ammonia, mg/L
N H e = influent ammonia, mg/L
s 6 = "average" flow through system, nf/d
Based on pilot scale work with SF wetlands, Bavor (12) has presented the
following equation for ammonia removal in those systems:
_ (Q) (lnNH0/NHe)
s
(7)
4-1 7
-------
Chapter 4
Where: A = required surface area, m2
K-j- = temperature dependent rate constant, d"1
= K20(1.03)(T-
K2Q = 0.107d-1
d = average depth of liquid in the bed, m
n = effective porosity of bed media, % as a decimal.
Q = average flow through the system, mVd
Based on a regression analysis of a limited number of systems, Hammer and
Knight (41) have proposed the following equation for ammonia removal in constructed
wetland systems:
A s = (0.001831) (NHJ (Q)
N H0+0. 1 6063
(8)
Where: ^ = required wetland surface area, ha
NI_|S = Influent ammonia concentration, mg/L
NHe = desired effluent ammonia concentration, mg/L
Q = average flow through the system, mVd
Equations 6, 7, and 8 produce comparative results down to an ammonia
effluent concentration of about 2 mg/L. However, all three equations will predict a
total treatment area at least twice that resulting from the initial procedure presented,
which depends on the oxygen produced by the vegetation. The total HRT for the
initial example was six days; the HRT for the same conditions, using equations 6, 7,
and 8, ranges from 13 to 19 days. This may be because all three procedures were
derived from site conditions where oxygen may have been slightly to significantly
deficient in the water flowing through the system. None of these equations will
provide an accurate prediction of ammonia removal for the SF systems shown on
Figures 8 and 9. This is believed to be due to the almost complete lack of oxygen in
most of the systems and, conversely, to the presence of available oxygen in the two
systems which demonstrated excellent ammonia removal Equations 6, 7, and 8 may
provide a reasonable order-of-magnitude estimate of ammonia removal performance
4 - 18
-------
Chapter 4
of many of the existing SF constructed wetlands which do not have sufficient oxygen
from the plants or other sources.
VEGETATION SELECTION AND MANAGEMENT
Most of the constructed wetlands in the U.S utilize one or more of the plant
species listed in Tables 6 and 7. About 40 percent of the operational SF systems use
only Scripus. Phragmites is the most widely used species in the European systems.
A number of systems in the Gulf States also used a number of flowering plants for
aesthetic reasons. These soft tissue plants decompose very rapidly and can affect
water quality in the effluent. Many locations adopted a routine fall harvest to remove
these plants before they died or suffered frost damage. There have been some
attempts to create a plant diversity similar to that present in a natural marsh; this
approach is more expensive and the intended diversity can be difficult to maintain.
Any of the three species listed in Tables 6 and 7 are suitable for use in SF
systems. If the plant is expected to provide a significant treatment function, then the
depth of the bed should not exceed the potential root penetration depth. The
Phragmites used in many European systems offer several advantages for a low
maintenance treatment system. They will grow and spread faster than bulrush; their
roots should go deeper than cattails; and they are not a food source for muskrats and
nutria which have been a problem for cattail and bulrush wetlands. However, the
habitat values for a Phragmites system are probably less than for other plant species.
A number of systems in the Gulf States utilize an annual harvest, regardless of
the plant species used. In contrast, routine annual harvesting is not practiced in
Europe or at most other systems in the U.S. It may be useful to remove undesirable
weeds during the early part of the growing season for the first few years of operation.
Flooding of the bed surface after the initial planting can help reduce weed infestation.
A routine annual harvest of the entire system provides minimal benefits and is not
recommended. It is also suggested that the use of soft tissue flowering plants be
avoided and thereby eliminate the need for their annual harvest and related
maintenance.
Water level management in the SF bed is not only helpful for weed control, but
can also be used to induce deeper root penetration. Based on experience in Europe,
it is claimed that if the water level in the bed is gradually lowered in the fall of each
year the roots will penetrate to greater depths. A three year period is considered
necessary for Phragmites roots to reach their 0.6 m potential depth. Although this
approach has not been tried in the U.S. it should be successful, but it may have to be
4-19
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Chapter 4
repeated every year for the operational life of the system. The alternative is a root
zone where the major mass of roots are limited to the top ± 0.25 m in the portions
of the bed where nutrient concentrations are high.
The Typha roots penetrated to the full depth of the shallow bed ( 0.3 m) at the
constructed wetland system in Bear Creek. This system was lightly loaded, and since
it served a public high school it received essentially no wastewater flow at night,
during the weekends, and during the summer months. These dormant periods when
the nutrient concentrations would be at low levels in the bed are believed to be a
contributing factor to the rapid penetration of the root system. It might be possible
to replicate this experience at other locations with continuous flow if the system were
divided into two or more parallel cells and if the initial flow were less than the ultimate
design capacity. In this case, the wastewater flow could be alternated between the
cells during the summer and fall months and result in enhanced root penetration in the
temporarily dormant cell.
At the Santee pilot system, the plant roots penetrated to the depths shown in
Table 6 without any special manipulation of water levels or other similar management
activities. The six-day HRT combined with the warm climate and continuous growing
season at this site probably ail contributed to low nutrient conditions in the latter part
of the bed, thereby inducing root growth and penetration.
As root development commences, nitrification should be enhanced; this should
be rapidly followed by denitrification (as long as a carbon source is available). The
resulting loss of nitrogen to the atmosphere further reduces the availability of nutrients
in the bed and may promote further progressive root development in the portions of
the bed where the nitrifying organisms can successfully compete for the available
oxygen. It is likely that root development would still be limited in the front part of
such a bed where the oxygen demand for BOD5 removal would limit the development
of the nitrifiers. In this area much of the nitrogen would still be in the ammonia form
and the plant roots would not have to penetrate deeply to obtain sufficient nutrients.
A critical difference between the Santee project and many of the operating
systems in the Gulf States is the six-day HRT at the former versus the one- to two-
day HRT at-the latter systems. The use of new plant species, harvesting and other
vegetation management activities are unlikely to improve ammonia removal
performance at these short detention time systems.
Deep penetration of the roots in these short detention time beds may be
possible if most of the flow is actually occurring on top of the bed. In this case there
would be minimal flow through most of the bed profile, resulting in low nutrient levels
4-20
-------
Chapter 4
and deeper root penetration. The deeper root penetration in this case would not result
in improved treatment since the roots are not in contact with the bulk of the
wastewater. Hydraulic improvements are necessary for such systems to maintain
flow throughout the full bed profile, but the short detention time will still be a limiting
factor.
4-21
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Chapter 5
CHAPTER 5
CONSTRUCTION DETAILS
It is prudent for all but the smallest systems to divide the surface area required
for treatment into two or more cells, in parallel, to provide flexibility for operation and
maintenance. Each cell should be provided with an access ramp for maintenance
equipment.
The hydraulic performance of- these constructed wetlands can be significantly
influenced by improper construction activities. Initial excavation and grading must be
carefully controlled to avoid low spots and preferential flow down one side of the cell,
or erratic cross flow within the cell. Past experience has shown that even with
careful initial grading, the cell profile can be disrupted by uncontrolled truck traffic
bringing the gravel or rock media into the bed. It is suggested that the native soils at
the bottom of the SF wetland cell be compacted to the same degree required for a
highway subgrade to withstand damage from trucks and other equipment, and then
construction vehicle access to the cell should be limited during wet conditions. The
liner, or other impermeable barrier (if needed) goes on top of the compacted soil, and
the bed media is placed on top of the liner. If a membrane liner is used, a layer of
sand-is suggested to prevent puncture of the liner.
Selection of media type and size is critical to the successful performance of the
system. Unwashed crushed stone has been used on a large number of projects.
Truck delivery of such material during construction can lead to problems due to
segregation of the fines in the truck during transit, and then the deposition of all of
those fines in a single spot when the load is dumped. This can result in numerous
small blockages in the flow path and internal short circuiting within the system.
Washed stone or gravel is preferred. Coarse aggregates for concrete construction are
commonly available throughout the U.S. and would be suitable for construction of SF
wetland systems.
Appropriate inlet and outlet devices have been discussed in a previous section.
It is essential that both devices provide for uniform distribution and/or collection.
Some method for controlling the water level in the bed should be utilized following the
effluent manifold.
The vegetation on most of the existing SF wetland systems has been planted
by hand, with the initial spacing ranging from 0.3 to 1.2 m (1 - 4 ft). The use of
individual root/rhizome material with a growing shoot at least 0.2 m (8 in) in length
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Chapter 5
is suggested. The use of locally available plants is recommended since they have
already adapted to the environmental conditions; however, these plants are also
available from commercial suppliers. The root/rhizome material should be placed in
the gravel or rock media, at a depth equal to the expected operational water level.
The growing shoot should project above the surface of the media. If mature, locally
available plants are used, they can be separated into individual root/rhizome/shoot
units; in these cases the mature stem should be cut back to < 0.3 m. (< 1 ft) before
planting.
The water level in the bed should be maintained slightly above the media
surface during planting and for several weeks thereafter to suppress weed
development and promote growth of the planted species. An initial, moderate
application of commercial fertilizers will also enhance plant development. Wastewater
applications can probably commence six to eight weeks after planting if vigorous
growth is observed. An early spring planting is preferred whenever possible. The
plants, and therefore system performance for nitrogen, may not begin to reach
maturity and equilibrium until late in the second growing season..
COSTS
A recent survey (13) indicates that the capital costs for SF wetland systems
averaged around $200,000 per hectare ($87,000/ac) and the FWS systems were
about $50,000 per hectare ($22,000/ac). The major cost difference of the two
systems is in the expense of procuring the rock or gravel media, hauling it to the site,
and placing it. Although the construction cost per hectare is higher for SF wetlands,
the design flow rates at currently operating SF systems are also much higher than at
the FWS type. As a result, for the systems included in the survey, the unit cost is
$163/m3($0.62/gal) of wastewater treated for the SF type, and $206/m3($0.78/gal)
for the FWS type. However, many of these early SF systems were much smaller than
more recently designed SF systems, so the current unit costs are likely to be higher.
As shown in Table 3, the BOD5rate constant for SF wetlands is about double
the value for FWS wetlands. However, that does not mean that FWS wetlands are
double the size of SF wetlands for the same conditions and performance expectations.
The two systems may operate at different water depths, and the increased "porosity"
(space available for water flow) in the FWS case compensates to some-degree for the
reduced rate constant. In the typical case, the FWS wetland might be about 70
percent larger than a SF wetland to achieve the same BOD5 performance. Figure 14
presents the cost distribution for SF wetlands as derived by Conley, et al. (15).
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Chapter 5
8
o
60
4O
30
20
C 10
8!
LAND COST CLEARING ROCK MEDIA CONSTRUCT
MAJOR COST COMPONENTS
MISC.
Figure 14. Cost Distribution for SF Constructed Wetlands.
The land costs are quite low in this case but procurement and placement of the
rock media represents about 53 percent of the total construction costs. This is an
expense not required by the FWS wetland alternative. This cost distribution does not
include the cost for any collection or pumping systems in the community or any
preliminary treatment.
Figure 15 presents the actual construction costs for a small (0.4 ha) SF system
in southern Louisiana (35). The actual costs are referenced to the left axis and the
percent of total costs to the axis on the right. The cost of the rock media is
comparable to that shown on Figure 14. Rock is not locally available at this site so
the material was barged from Arkansas to Louisiana and then trucked to the site. A
lower cost might be expected if rock or gravel media were available locally. The land
costs in this case were high since the system is located in and serves a large.
residential subdivision.
The most cost-effective choice between SF and FWS constructed wetlands will
then depend on the local costs for land and for the SF media. If the land costs are as
low as shown in Figure 14, the FWS concept may be favored, assuming the additional
land needed was available; if the land is as expensive as shown on Figure 16 the SF
concept may be more economical, assuming a source of low-cost media is locally
available.
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Chapter 5
100
s
e
8
LAND CONSTRUCT ROCK IvEDlA PLANTS
MAJOR COST CCM=ONENTS
3 S/ao-e % of total
Figure 15. Actual Cost Distribution for an SF System in Louisiana.
If significant ammonia nitrification or nitrogen removal is required, then the FWS
concept with an HRT of 8 to 10 d may still be more economical since about 6-d HRT
is required for an SF system depending only on plant available oxygen. If a
recirculating filter is used in combination with the SF concept, the total HRT might be
in the range of 2 d (for warm climates) and it might then be the more cost-effective
alternative.
The select/on of the more cost-effective alternative will depend on treatment
goals, the availability and cost of land in the area, and on the cost of the media for
the SF alternative. Other factors include mosquito and positive odor control offered
by the SF concept, and less concern over public access. These factors would favor
the use of the SF concept in close proximity to public facilities and dwellings.
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Chapter 6
CHAPTER 6
ON-SITE SF WETLAND SYSTEMS
The SF wetland concept is particularly well suited for on-site applications
because of the advantages (odor and vector control, public access issues) of the
process. Although there is no general consensus on design, a large number of on-site
SF wetland systems have been constructed and placed in operation in Louisiana,
Arkansas, Kentucky, Mississippi, Tennessee, Colorado, and New Mexico. These
systems serve single-family dwellings, public facilities and parks, apartments, -and
commercial developments.
In general these systems can provide better than secondary levels of treatment
and in some states a surface discharge has been permitted (disinfection may be
required depending on local conditions). A standard multi-compartment septic tank
is the typical preliminary treatment device. Guidelines for design of these small SF
wetland systems can be obtained from state agencies in Louisiana, Arkansas,
Kentucky and the TVA (17,36). Many on-site systems have also been designed using
the same first order plug flow model described previously. Systems of this type in the
southwestern U.S. are typically designed for an effluent with < 10 mg/L BOD5, < 10
mg/L TSS, and 10 mg/L total nitrogen, with an HRT in the wetland bed of > 6 d (42).
A review of the various design methods in current use suggests that the TVA
method and the plug flow model offer the most rational approach to design; these two
and the approach used in Louisiana will be discussed in detail.
LOUISIANA METHOD
The method used in Louisiana (36) is derived from the-work of Wolverton, et
al. (3), and as applied to larger scale municipal systems in the region. The system
typically involves a single narrow trench, excavated in clay, or lined in more permeable.
soils. The HRT in the bed is one to two days (assuming the total specified bed
volume is actively contributing to treatment). The performance in existing systems'
has been somewhat variable, but effluent expectations are typically in the range of
BOD5< 30 mg/L, TSS < 30 mg/L, and no limits on nitrogen.
The trenches in the Louisiana systems typically contain a 0.6 m (2 ft) depth of
crushed stone, with a layer of smaller gravel on top. The trench bottoms are typically
flat and the outlet does not allow adjustment of water level in the bed. Ammonia
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Chapter 6
removal is not an issue with these on-site systems and is not required. Ornamental
flowering plants are often used as the vegetation of choice in these systems. The bed
outlet maintains the water level at mid-depth of the bed to provide a water source for
the ornamental plants.
This constraint probably reduces the "active" volume in the bed which is
potentially contributing to treatment. A better approach might be to use an adjustable
outlet and more drought resistant plants such as Phragmites. The septic tank effluent
is applied to these beds by a perforated pipe located at about the mid-point of depth,
and extending about 1/3 of the length of the trench. It is likely that the actual HRT
in these systems, at design flow, is less than the anticipated one to two days because
of the hydraulic constraints imposed by the inlet and outlet devices.
The specific guidelines for these systems in Louisiana are (36):
1. Systems 1.5 m3/d (400 gpd) or less:
• Three-compartment septic tank, 1-.9 m3 (500 gal) in first compartment,
0.9 m3 (250 gal) each in second and third compartment.
• Wetland trench dimensions: 0.46 m (1.5 ft) deep, 0.61 m (2 ft) wide and
32 m (105 ft) long, or 0.9 m (3 ft) wide and 21 m (70 ft) long. The top 150
mm (6 in) is maintained in a dry state; therefore the required treatment volume
is 5.9 m3 (210 ft3) for design flow < 1.5 m3/d.
• The rock in the treatment zone should be clean, washed rock ranging from 25
to 76 mm ( 1 to 3 in) in size. The top layer can be the same material or smaller
in size.
• The trench should be lined with a plastic film at least 12 mil in thickness or other
equivalent material.
• The effluent pipe for the bed will be a perforated pipe laid as a header across the
full width of the bed, so that the high water level in the bed is maintained at a
depth of 305 mm (1 ft) at all times.
• The influent pipe to the wetland bed will be a perforated pipe, extending
3 m (10 ft) into the bed on the centerline, with the invert of the pipe at the liquid
level in the bed as defined above.
• Aquatic plants approved for use include:
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Chapter 6
Arrow Arum (Peltandra virginica) Arrowhead (Sagittaria latifolia)
Cattails (Typha latifolia Reed (Phragmites)
Pickerelweed (Pontederia cordata) Canna Lily (Canna flaccida)
Calla Lily (Zantedeschia aethiopica) Bulrush (Scirpus americanus)
Elephant Ear (Calocafia esculenta) Ginger Lily (Hedychium coronatum)
• Typical plant density is about 60 plants per bed, evenly spaced, with a
maximum of 0.46 m (1.5 ft) between individual plants. Soil should be washed
from roots before planting and roots must be placed below the design liquid level
in the bed. Dead vegetation must be regularly removed from the bed and
additional plants installed as necessary.
• Adjacent surface runoff must be excluded from the bed by appropriate grading
around the site.
• The bed should be 15 m (50 ft) from any well or potable water line, and at least
3 m (10 ft) from any property line.
2. Systems larger than 1.5 m3/d (400 gpd):
The three compartment septic tank will have a total volume equal to 2.5 times the
design daily' flow. The wetland bed must be increased in volume by 1.4 m3 ( 50 ft3)
for every 0.4 m3/d (100 gpd) of flow above 1.5 m3. Other requirements are the
same as above.
TVA METHOD
The TVA concept (17) uses a design approach which considers the factors
controlling the hydraulic performance of the bed, and the organic loading (kg
BOD5/m2/d) on the entry zone cross sectional area to avoid potential clogging. The
total area required for treatment is then divided into two equal cells in series. The
transfer structure, between the cells, is equipped with an adjustable flow device to
control the water level in the first cell. Both surface manifolds with adjustable outlets
and buried manifolds have been used for the inlet structures.
The second cell in the TVA systems is the same size as the first cell and is unlined
to permit seepage of the treated wastewater into the ground. The present procedure
for design of these cells does not take into account the permeability of the soil
beneath the unlined cell, but in the general case this second cell provides an
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Chapter 6
infiltration area comparable to that required for conventional leaching beds or
trenches.
The specific TVA guidelines are (17):
• Determine design flow (Q) from the home, a typical rate is 0.45 m3/d (120 gpd)
per bedroom; state or local requirements will govern.
• Determine the daily organic loading (OL). A value of 0.045 kg BOD5/person/d
(0.1 Ib/d) in the effluent leaving the septic tank is acceptable.
• Determine the total surface area (As) of the bed using the previously determined
design flow and a surface loading of 31.9 nfof bed area per nf/d of flow (1.3
ftVgpd) for a bed depth of 0.3 m (1 ft). A bed 0.46 m (1.5 ft) deep should be
designed for a loading of 21.3 nf/mVd (0.87 ftVgpd). The shallower depth is
preferred if site conditions permit.
• The cross-sectional area (A,) is determined by comparing the impact of Darcy's
Law, and organic loading criteria, and adopting the larger of the two cross
sectional areas.
Organic loading factor: L0 = 4.097 nf/kg BOD5/d (20 ft2/lb BOD5/d)
Cross-sectional area: Ac=(LJ(OL)
Darcy's Law: A c = Q / ksS
Where: A = cross-sectional area of bed, m2(ft2).
Qc = design flow, mVd (ftVd).
k = hydraulic conductivity
s = 259 m3/d/m2(850 ft3/d/ft2)
S = hydraulic gradient.
= 0.005 for flat bottom.
= 0.01 to 0.02 for 1 to 2 percent bottom slope.
The bed width (W) can be determined since the depth has been selected, and the
cross-sectional area determined above.
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Chapter 6
• The total bed length (L) can be determined by dividing the previously calculated
surface area by the bed width. A two-cell system would have a length of L/2
for each cell.
• An impermeable liner or low permeability clay bottom is necessary for the first
cell in a two-cell system. Synthetic liners using 20-30 mil polyethylene or
polyvinyl chloride are acceptable.
• Washed river gravel is the preferred bed media, using pea gravel sizes up to 1.3
cm (0.5 inch) in diameter. The inlet and outlet zones of each cell should use 5
to 10 cm ( 2 to 4 inch) stone for a 0.6 m (2 ft) length around and over the
influent and effluent manifolds.
• Schedule 40 PVC pipe, with a 10 cm (four-inch) diameter is acceptable for inlet
and outlet manifolds, with 2.5 cm (one-inch) drilled holes.
• The outlet manifold should connect to either a swiveling standpipe or a flexible
hose in a manhole to permit water level control in the bed.
• Surface water must be diverted away from the bed.
• The top of the bed should be mulched.
• Local plant species should be used on the bed. The preferred species include:
cattail, sedge, rush, soft stem bulrush, and reeds. Decorative, flowering plants
can be used around the edges of the bed.
An evaluation of this TVA design method reveals several concerns.. It does not
take into account the temperature influence on performance when applied in cold
climates. It assumes that infiltration and percolation into the ground will be
acceptable but it does not include any field measurements of the actual hydraulic
capacity of the in-situ soils. It does not provide a method for estimating nitrogen
removal in the system for locations where ammonia or nitrogen control is required.
The HRT in these systems will probably range from 3 to 4 days at design flow, which
may not be sufficient to deal with the ammonia levels leaving most septic tanks. In
summary, the TVA approach is probably conservative and acceptable where in-ground
disposal is the final discharge pathway and where nitrogen limits do not apply to such
a discharge. In these cases, some field measurements to determine the actual
hydraulic capacity of the soil is also recommended. The TVA approach may not be
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Chapter 6
sufficiently conservative for design of systems with a surface discharge in locations
with cold winter climates and should be used with caution in these cases.
PLUG FLOW MODEL FOR ON-SITE SYSTEMS
The plug' flow model has been presented and discussed in detail in previous
sections of this report. A summary listing of simplified criteria and procedures is given
below to demonstrate the application to small scale on-site systems:
• Determine the design flow; 0.23 mVd (60 gpd) is a reasonable assumption for per
capita flow for residential systems. State or local criteria will govern.
. Use a multi-compartment septic tank. Use one tank for single-family dwellings;
use two or more tanks in series for larger scale (> 10,000 gpd) projects. The total
volume of the tank(s) should be at least twice the design daily flow.
• Assume that the BOD5(C,) leaving the septic tank(s) is conservative > 100
mg/L.
• Assume that the wetland effluent BOD5(C,) will not exceed 10 mg/L.
• Use clean, washed gravel as the treatment media in the bed, size range 1.25 - 2.5
cm (0.5 - 1 inch), with a total depth of 0.6 m (2 ft). For design assume the
"effective" water depth in the bed is 0.55 m (1.8 ft). Reasonable estimates are:
hydraulic conductivity (k,) = 1500 mVmVd (5000 ftVftVd; porosity = 0.38. If
a large number of systems are to be installed using the same materials, field or
laboratory testing for hydraulic conductivity (k,) and porosity (n) is recommended.
• Reeds (Ph rag mites) are the preferred plant species.
• Estimate the summer and winter water temperatures to be expected in the bed.
In the summer and in year-round warm climates 20°C is reasonable. In cold
winter climates, a winter water temperature of = 6° C is a reasonable assumption.
• Determine the bed surface area with:
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Chapter 6
^u>«^£Lln(c'/c-n
(6)
• As a safety factor, use a rate constant K20 which is 75 percent of the base value
(1.104 d"1). So, for design of small on-site systems K20= 0.828 d"1.
• At 20°C, and with the other factors defined above, this equation reduces to:
Metric: As= 13.31(Q) = m2 (Q in mVd)
U.S unit As = 4.07(Q) = ft2 (Q in ftVd)
At 6°C:
Metric: As = 30.1(Q) = m2 (Q in nf/d)
U.S units As= 9.2(Q) = ft2 (Q in ftVd)
. Adjustments for other temperatures, other media types, etc., should use the basic
design equations.
• Adopt an aspect ratio (L:W) of 2:1, calculate bed length (L) and width (W) since
the surface area was determined above. In the general case, an aspect ratio of
2:1, or less, with a bed depth of 0.6 m (2 ft) will satisfy the Darcy's Law
constraints on hydraulic design of the bed, so hydraulic calculations are not
required. If site conditions will not permit use of an L:W of 2:1 for the bed, and
a 0.6 m bed depth, then hydraulic calculations as described previously will be
necessary.
This approach will give an HRT of about 2.8 d (at 20°C) in the bed, which is
more than adequate for BOD5 removal to < 10 mg/L. If nitrogen removal to 10 mg/L
is required, the size of the system should be doubled to produce an HRT of about six
days. Nitrogen removal during the winter months, in cold climates may require an
HRT of about 10 days. In these cases, heat loss calculations should be performed to
assure that the bed is adequately protected against freezing.
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Chapter 6
• Construct the bed as a single cell for single-family dwellings. Use multiple cells
(at least two) in parallel for larger systems.
• Use clay or a synthetic liner to prevent seepage from the bed.
• Construct the bed with a flat bottom and a perforated effluent manifold at the
bottom of the bed. A perforated inlet manifold a few inches above the bottom
of the bed is adequate for most small systems. These inlet and outlet zones
should use 2.5 to 5 cm (one- to two-inch) washed rock for a length of about
1 m (3 ft), and for the full depth of the bed.
• The effluent manifold should connect to either a swiveling standpipe or a
flexible hose for discharge, to allow control of the water level in the bed.
• The inlet and effluent manifolds should have accessible cleanouts at the surface
of the bed.
The system as described above should produce an effluent with BOD5< 10
mg/L, TSS < 10 mg/L, and TN < 10 mg/L, and should therefore be suitable for either
surface or in-ground discharge. Percolation tests, basin infiltration tests, or other field
or laboratory techniques should be used to determine the hydraulic capacity of the
native soils for in-ground discharge systems. The position of the groundwater table
in the proposed disposal area must also be determined. The larger the system, the
more sophisticated this testing should be. Because of the very clean nature of the
wetland effluent, it should be possible to design the final disposal bed or trenches at
about one third to one half the size that would be required for direct septic tank
disposal
For example, a typical conventional on-site system for a family of four (1 mVd,
300 gpd) might include a 4 m3(1000 gal) septic tank and a 46 m2(500 ft2)
infiltration area in a sandy loam soil. Addition of a wetland component with a 6 d
HRT would require about 28 m2(300 ft2) of area. If appropriate credit for the higher
level of treatment is allowed, the total area for the wetland cell and the infiltration bed
could be less than 46 m2( < 500 ft2).
An evaluation of this design approach suggests that it is both more conservative
and more flexible than the other two. It allows adjustment in the design for different
temperature conditions, bed configurations, and either surface or subsurface discharge
options. It is appropriate for single-family dwellings and larger systems serving public
buildings and commercial establishments. As a result, this approach is recommended
for design of on-site systems.
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Chapter 7
CHAPTER 7
OTHER POTENTIAL APPLICATIONS FOR SF CONSTRUCTED WETLANDS
Constructed wetlands are in use or have been proposed for the treatment of
landfill leachate, agricultural runoff, feed lot runoff, acid mine drainage, drainage from
coal and ash piles, stormwater runoff, and combined sewer overflows (CSO). In most
of the operational systems, the wetland concept in use is most often the free water
surface type (FWS). This section will briefly examine the potential for the use of
subsurface flow wetlands (SF) for these applications.
The SF concept involves submerged flow in a bed of gravel or crushed stone,
and the design intent is to maintain that condition at all times to realize the treatment
potential and to avoid surficial short circuiting. Implicit in that approach are the
assumptions that the flow will be relatively steady state, and that the wastewater
does not contain large quantities of inorganic solids which might lead to rapid clogging
of the pore spaces in the bed.
STORMWATER SYSTEMS
The SF system can accommodate the diurnal flow pattern for domestic,
municipal, and industrial wastewater flow but are not well suited hydraulically for
treatment of stormwater or CSO discharges where the peak flow may be several
orders of magnitude higher than the "average" flow. It would be uneconomical to
provide a gravel bed large enough to contain, in a subsurface mode, the peak storm
event. The use of sediment traps ahead of the wetland could reduce the impact of
inorganic solids, and the construction of sufficient freeboard to contain the storm flow
above the SF bed with subsequent steady-state discharge through the bed may sound
possible. However, it is unlikely that the full bed volume would be involved in
treatment of the ponded water; preferential discharge in a zone near the outlet is a
more likely flow path. In this case the bulk of the media may not contribute to
treatment during the major storm events. As a result, there seems to be negligible
advantages for the use of gravel media, so a FWS constructed wetland is probably the
preferred choice for stormwater and CSO discharges. A FWS wetland may tend to
serve as a batch type plug flow reactor in response to storm events. The major
treatment responses (other than sedimentation) will probably occur in the standing
water between the storm events. That treated water will then be displaced by the
incoming flow from the next storm event.
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Chapter 7
LANDFILL LEACHATES
SF constructed wetlands are being used for treatment of landfill leachates. In
these cases the flow is relatively uniform and low in inorganic suspended solids and
the SF wetland can be used to advantage. Collection of additional data is necessary
for a better understanding of the capabilities of the wetland for removal of the
complex organic and inorganic materials which may be found in leachates.
'MINE DRAINAGE
SF wetlands can also be used for treatment of mine drainage, where again the
flow is relatively uniform and the treatment does not result in the precipitation and
deposition of large quantities of inorganic solids in the bed. FWS wetlands are more
commonly used in the eastern states to treat drainage from coal mines, as well as coal
and ash piles In these cases, the precipitation and storage of iron and manganese in
the wetland cell is the major treatment goal.
AGRICULTURAL RUNOFF
Agricultural runoff is also subject to intermittent peak events and high inorganic
sediment loads, so FWS wetlands would again seem best suited for these cases.
Discharges from feed lots and other sources of high organic strength wastewater can
be treated by either SF or FWS constructed wetlands. In both cases, preliminary
treatment for solids and BOD5reduction is necessary.
In summary, the use of SF wetlands is probably best suited, because of the
hydraulic constraints imposed by the media, for the treatment of wastewaters with
relatively low solids concentrations, under relatively uniform flow conditions, and in
locations where the advantages of the subsurface flow mode (ie: no insect vectors,
higher rate of treatment, etc) are important considerations.
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Chapter 8
CHAPTER 8
RESEARCH NEEDS
Research needs to further advance and understand the SF wetland concept
were discussed at an EPA workshop held in New Orleans, LA on September 24, and
25, 1992. Participants in that workshop are listed in Appendix A. These research
needs were categorized as "high", "medium" and "low" priority needs, and are
summarized below.
HIGH PRIORITY RESEARCH NEEDS
A better understanding of the nitrogen removal and nitrogen transformations
occurring in these SF systems is necessary. This should lead to the development of
rational temperature and possibly seasonally dependent design models for nitrogen
removal.
Additional data collection is necessary on the spatial responses to BOD5within
the SF wetland bed to permit development and validation of improved design models
to replace the interim procedures now in use.
Further research is needed on identifying the oxygen needs and sources in these
SF systems. The role of the plant roots in providing this oxygen is especially
important.
The use of plant types other than reeds, rushes, and cattails needs to be
investigated to determine if optimum species exists. The need for routine plant
harvest also deserves study.
Most operational SF wetlands demonstrating successful ammonia removal have
an HRT of about six days or more. The system at Benton, LA has apparently
demonstrated high ammonia removals with an HRT of < 1 d. The factors responsible
for this performance at Benton need to be defined.
Although recent studies indicate minimal clogging in the beds investigated, the
effort needs to be continued to determine the long-term risks of clogging.
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Chapter 8
Efforts should continue to collect reliable performance data from full-scale
operating systems to confirm and supplement the results from laboratory, greenhouse,
and pilot-scale research.
MEDIUM PRIORITY RESEARCH NEEDS
Intermittent or batch type flow to alternating beds might enhance the oxygen
status and therefore the ammonia removal capability. The approach should be tested;
and then demonstrated if promising.
A design model for pathogen removal in these systems would be useful, with
the dependency on time and temperature defined.
The response to complex organic and inorganic compounds in industrial and
agri-chemical wastes needs definition.
LOW PRIORITY RESEARCH NEEDS
The use of specialized media for improved phosphorus removal could be
developed and demonstrated.
The removal mechanisms for metals in these systems could be better defined.
The role of the plant roots (other than serving as an oxygen source) in
maintaining desirable bed conditions needs study.
The management of variable flows from communities which suffer from high
infiltration/inflows in their sewer systems needs consideration.
Treatment of stormwater, CSO discharges, animal wastes, and leachates needs
further consideration.
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Chapter 9
CHAPTER 9
CONCLUSIONS
1 . The subsurface flow (SF) constructed wetland concept can offer high
performance levels for BOD5and TSS at relatively low costs for construction and
operation and maintenance. It is particularly well suited for small to moderate
sized installations where suitable land and media are available at a reasonable.
cost.
2. The odor and vector control offered by the SF concept make it attractive for
systems which are in close proximity to the public. These uses range from single
family dwellings to larger developments and public facilities.
3. The cost effectiveness of SF wetland systems as compared to free water surface
(FWS) wetlands for the same water quality goals will depend on the local
availability of land and the cost for land and for the media used in the SF concept.
4. Ammonia removal in most of the present generation of operating SF systems is
deficient. The reason is believed to be the lack of oxygen in the bed profile and
a too brief HRT to complete the nitrification reactions.
5. Effective ammonia removal has been reliably established in a few SF wetland
systems. The common elements in those systems are full penetration of the plant
roots and an HRT > 3 d.
6. Removal of BOD5and TSS is not related to the aspect ratio (L:W) of the system.
7. Surface flow has been observed at a number of operational SF systems. This is
believed to be largely due to inadequate hydraulic design of the systems and not
to clogging of the pore spaces in the media. The water level in the bed can be
effectively controlled with adjustable outlet structures.
8. It is likely that some oxygen is available from the plant roots to support
nitrification reactions. Effective use of that oxygen source requires complete
development of the root zone in the bed profile and sufficient detention time.
Neither condition is present in most operational SF systems. Further research is
necessary to optimize these relationships.
9 -1
-------
Chapter 9
9. Methods appear to be available to induce and maintain root penetration in order
to enhance this oxygen source for nitrification. About a six-day HRT would be
necessary for significant nitrification with a fully developed root zone and warm
weather conditions. This approach cannot be used as a retrofit for most existing
systems since there is not enough area available to increase the HRT to 6 d.
10. Use of a recirculating nitrification filter in combination with the SF wetland bed
seems to 'offer promise for successful ammonia control and continued high levels
of BOD5and TSS removal. This combination may be more cost-effective than an
FWS wetland designed for the same performance level.
11. The removal of BODJn SF wetlands shows a linear relationship to the BOD5
mass loading up to levels of at least 140 kg/ha/d (125 Ib/ac/d).
12. A first order plug flow kinetic model provides a reasonably accurate estimate of
BOD5removal performance, and is recommended for use as an interim approach
until more sophisticated models can be developed and validated.
13- Darcy's Law provides a reasonable approximation of the hydraulic performance
in these SF systems as long as the limitations are recognized and accommodated.
14. The hydraulic conductivity (ks) and porosity (n) of the media to be used in these
systems should be tested in the field or laboratory prior to final design.
15. To provide an adequate safety factor, no more than one-third of the measured
"effective" hydraulic conductivity, and no more than 10 percent of the maximum
potential hydraulic gradient should be used for the hydraulic design of these
systems. These constraints will tend to limit the aspect ratio to < 3:1 for beds
0.6 m deep.
16. SF systems of all sizes should include a final adjustable outlet to permit control
of the water level in the bed.
17. Larger systems (Q = > 5,000 gpd) should consider the use of multiple wetland
cells in parallel to improve control and flexibility in operations.
18. The limited data available on removal of fecal conforms indicate that these
systems are capable of about a one- or two-log reduction with sufficient HRT. In
most cases that will not be sufficient to reach the commonly applied limit of
200/100 ml, so some form of final disinfection may be necessary.
9 - 2
-------
Chapter 9
19. Some form of preliminary treatment, at least to the primary level, is typically used
for SF wetland systems in both the U.S. and Europe. Primary treatment using
septic or Imhoff tanks is suitable for small to moderate sized systems. Many
existing SF systems follow facultative lagoons since they were typically added as
a polishing step. Facultative lagoons are an acceptable form of preliminary
treatment but can add large concentrations of algae. In these cases a variable
level draw-off in the, lagoon may help reduce the algal load on the wetland
component.
9-3
-------
Chapter 10
CHAPTER 10
REFERENCES
1. Seidel, K. ( 1966). Reinigung von Gewassern durch hohere Pflanzen Deutsche
Naturwissenschaft, 12.: 298-297.
2. Kikuth, R. (1977). Degradation and incorporation of nutrients from rural
wastewaters by plant rhizosphere under liminic conditions. Utilization of Manure
by Land Spreading, Comm. of the Europ. Communitite, EUR 5672e, London, 235-
243.
3. Wolverton, B.C., R.C. McDonald, W.R. Duffer, (1983). Microorganisms and
Higher Plants for Wastewater Treatment. J. Environmental Quality, 12(2): 236-
242.
4. Gersberg, R.M., B.V. Elkins, S.R. Lyons, C.R. Goldman, (1985). Role of Aquatic
Plants in Wastewater Treatment by Artificial Wetlands. Water Research, 20:363-
367.
5. Boon, A.G. (1985). Report of a Visit by Members and Staff of WRC to Germany
to Investigate the Root Zone Method for Treatment of Wastewaters. Water
Research Centre, Stevenage, England.
6. U.S. EPA, (1988). Design Manual-Constructed Wetlands and Aqua tic Plant
Systems for Municipal Wastewater Treatment. EPA 625/111-88/022, U.S. EPA
CERI, Cincinnati, OH.
7. Reed, S.C., E.J. Middlebrooks, R.W. Crites, (1988). Natural Systems for Waste
Management & Treatment. McGraw Hill, New York, NY.
8. WPCF (1990). Natural Systems for Wastewater Treatment. Manual of Practice
FD-16, Reed, S.C., ed., Water Pollution Control Federation, Alexandria, VA.
9. Hammer, D.A., Editor, (1989). Constructed Wetlands for Waste water
Treatment-Municipal, Industrial and Agricultural. Lewis Publishers, Chelsea, Ml.
10. Cooper, P.F., Findlater, B.C., Editors, (1990). Constructed Wetlands in Water
Pollution Control. Pergamon Press, New York, NY.
10-1
-------
Chapter 10
11. Neel, J.K., J.H. McDermott, C.A. Monday, (1961). Experimental Lagooning of
Raw Sewage. Jour. Water Pollution Control Fed., 33(6)603-641.
12. Bavor, H.J., D.J. Roser, P.J. Fisher, I.C. Smalls, (1988). Joint Study on Sewage
Treatment Using Shallow Lagoon-Aquatic Plant Systems. Water Research
Laboratory, Hawkesbury Agricultural College, Richmond, NSW, Australia.
13. Reed, S.C., D. Brown (1992). Constructed Wetland Design -The First
Generation. Jour. WEF, 64(6),776-781.
14. Conley, L.M., R.I. Dick; L.W. Lion, (1991). An Assessment of the Root Zone
Method of Wastewater Treatment. Jour. WPCF 63(3),239-247.
15. European Design and Operations Guidelines for Reed Bed Treatment Systems.
Cooper, P.F., Ed., EC/EWPCA Emergent Hydrophyte Treatment Systems Expert
Contact Group, WRc Swindon, Swindon England, A-5, 1990.
16. Jones, A.A., B.C. Wolverton, (1990). Technology Designed for Outer Space, An
Outstanding Success on Earth -Microbial Rock Plant Filter An Emerging And
Promising Low Cost, Low O & M Wastewater Treatment Bio-Technology. Sixth
Regional Municipal Technology Forum on Microbial Rock Plant Filter Treating
Municipal Wastewater, Louisiana State University, Baton Rouge, LA.
17. Steiner, G.R., J.T. Watson, K.D. Choate, (1991). General Design, Construction,
and Operation Guidelines for Small Constructed Wetlands Waste water Treatment
Systems, in: Proceedings, Constructed Wetlands for Water Quality Improvement -
An International Symposium, Lewis Publishers, (In Press).
18. Steiner, G.R., J.T. Watson, D.A. Hammer, D.F. Harker, (1987). Municipal
Waste water Treatment with Artificial Wetlands -A TVA/ Kentucky Demonstration.
in: Aquatic Plants for Water Treatment and Resource Recovery, Reddy & Smith
ed., 923-932, Magnolia Publishing., Orlando, FL.
19. Cooper, P.F., J.A. Hobson, (1990). Sewage Treatment by Reed Bed Systems:
the Present Situation in the United Kingdom in: Constructed Wetlands for
Wastewater Treatment: Municipal, Industrial and Agricultural, D. Hammer, ed.,
153-171, Lewis Publishers, Chelsea, Ml.
20. Watson, J.T. (1992). Constructed Wetlands for Municipal Wastewater
Treatment: State-of-the-Art. Presented at: Symposium Epuration Des Eaux Usses
Par Les Plants: Perspectives D'Avenir Au Quebec, Montreal, Quebec, March 20,
1992.
10-2
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Chapter 10
21. Tchobanoglous, G., F.L. Burton (1991). Wastewater Engineering- Treatment,
Disposal and Reuse. Third edition, McGraw Hill Inc., New York, NY.
22. Herskowitz, J., S. Black, W. Lewandowski, (1987). Listowel Artificial Marsh
Treatment Project, in: Aquatic Plants for Water Treatment and Resource
Recovery, Reddy & Black, ed., 247-254, Magnolia Publishing, Orlando, FL.
23. Gearheart, R.A., B.A.Finney, S. Wilbur, J. Williams, D. Hull (1985). The Use of
Wetland Treatment Processes in Water Reuse, in: Future of Water Reuse, Vol2,
617-638, AWWA Research Foundation, Denver, CO.
24. Knight, R.L., R.H. Kadlec, S.C. Reed (1991). Database: North American Wetlands
for Water Quality Treatment. Unpublished Internal EPA report, prepared for U.S.
EPA ERL, Corvallis, OR.
25. Little, J. (1991). Operator, Deneham Springs, LA. Personal Communication.
26. Kadlec, R.L., J.T. Watson (1991). Hydraulics and Solids Accumulation in a Gravel
Bed Treatment Wetland. in: Proceedings, International Symposium on
constructed Wetlands for Water Quality Improvement, Lewis Publishers, in press.
27. Reed, S.C. (1991). Constructed Wetlands Characterization-Carville & Mandeville
LA. Unpublished Internal EPA Report, prepared for U.S. EPA RREL, Cincinnati,
OH.
28. Reed, S.C. (1992). Constructed Wetlands Characterization- Green/eaves &
Hammond, LA. Unpublished Internal EPA Report, prepared for U.S. EPA RREL,
Cincinnati, OH, (in preparation).
29. Brix, H. (1987). Treatment of Waste water in the Rhizosphere of Wetland
Plants- The Root-Zone Method. Water Science Technology, vol 19, 107-115.
30. U.S. EPA, (1981). Process Design Manual- Land Treatment of Municipal
Wastewater. EPA 625/i -81-501, U.S. EPA CERI, Cincinnati, OH.
31. Watson, J.T., Reed, S.C., R.H. Kadlec, R.L. Knight, A.E. Whitehouse, (1989).
Performance Expectations and Loading Rates for Constructed Wetlands, in:
Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and
Agricultural, D. Hammer, ed., 319-351, Lewis Publishers, Chelsea, Ml.
10 - 3
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Chapter 10
32. Steiner, G.R., R. J. Freeman, (1989). Configuration and Substrate Design
Considerations for Constructed Wetlands Wastewater Treatment, in: Constructed
Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural, D.
Hammer, ed., 363-391, Lewis Publishers, Chelsea, Ml.
33. U.S. EPA, (1983). Design Manual-Municipal Wastewater Stabilization Ponds
EPA 625/1-83-015, U.S. EPA, CERI, Cincinnati, OH.
34. Lawson, G.J. (1985). Cultivating Reeds for Root Zone Treatment of Sewage.
Report 965, Institute of Terrestrial Ecology, Cumbria, England.
35. McHugh, K., McHugh & Associates, (1991). Personal Communication.
36. Am berg, L.W. (1988). Rock-Plant Filter, An Alternative for Septic Tank Effluent
Treatment. Presented at: Louisiana Public Health Association Conference, April
1988.
37. Ogden, M. Southwest Wetlands Group. Personal Communication.
38. U.S. EPA (1980). Design Manual-Onsite Wastewater Treatment and Disposal
Systems: EPA 625/1 -80-012, U.S. EPA CERI, Cincinnati, OH.
39. Brix, Hans, (1992). Constructed wetlands for municipal wastewater treatment
in Europe, Proceedings: Wetland Systems in Water Pollution Control. University
of New South Wales, Sydney, Australia.
40. Reed, S.C., E.J. Middlebrooks, R.W. Crites (1994). Natural Systems for Waste
Management & Treatment. 2nd Edition, McGraw Hill, New York, NY, in press.
41. Hammer, D.A., R.L. Knight, (1992). Designing constructed wetlands for nitrogen
removal, in: Proceedings, Wetland Systems in Water Pollution Control. University
of New South Wales, Sydney, Australia.
42. Ogden, M., Southwest Wetlands Group (1992). Personal Communication.
43. Kadlec, R.H. (1993). Personal Communication.
10-4
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APPENDIX A
List of Participants in U.S. EPA Workshop, September 24/25, 1992
Mr. Robert E. Lee
Municipal Technology Branch
Office of Wastewater Enforcement
& Compliance
U.S. EPA
Washington, DC 20460
Mr. Robert K. Bastian
Municipal Technology Branch
Office of Wastewater Enforcement
& Compliance
U.S. EPA
Washington, DC 20460
Mr. Donald Brown
U.S. EPA RREL
Cincinnati, OH 45268
Mr. Michael Mines
TVA
400 West Summit Hill Dr.
Knoxville, TN 37902
Sherwood C. Reed
Environmental Engineering Consultants
RR 1, Box 572
Norwich, VT 05055
Mr. Ronald W. Crites
Nolte & Associates, Inc.
1750 Creekside Oaks Drive, Suite 1200
Sacramento, CA 95833
Mr. Oscar Cabra
U.S. EPA Region VI
12th Floor, Suite 1200
Dallas, TX 75202-2733
Mr. Ancil Jones
U.S. EPA Region VI
12th Floor, Suite 1200
Dallas, TX 75202-2733
Mr. Robert freeman
Cobb County Water District
680 South Cobb Drive
Marietta, GA 30060
Dr. Dennis George
College of Engineering
Tennessee TechUniversity
Cooksvilie, TN 38505
Mr. Michael Ogden
South West Wetlands Group
1590 San Mateo Land
Santa Fe, NM 87501.
Dr. Richard Gersberg
Gr Sch of Public Health
San Diego State University
San Diego, CA 92182
Dr. Marty Tittlebaum
857 High Plains Ave
Baton Rouge, LA 70810
Mr. Dick Smith
U.S. EPA Region VI
12th Floor, Suite 1200
Dallas, TX 75202-2733
Mr. Ron Rodi
Rodi & Songy, Inc.
5000 One Perkins PI., Suite 9A
Baton Rouge, LA 70808
Mr. Robert Crawford
State of Louisiana D.E.Q.
P.O. Box 82263
Baton Rouge, LA 70884-2263
A-l
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APPENDIX B
Site Details for the Systems Listed in Table 2
(Water Quality Data Are Average Values Over the Period of Record at Each Site)
GREENLEAVES SUBDIVISION (Mandeville, LA)
Flow: 0.149 mgd, Area: 1.1 ac, L = 457 ft, W = 105 ft, L:W = 4.4:1
bed depth = 2 ft, HRT = 1 d, Preliminary treatment: 2 cell lagoon, floating
aerators in first cell
Wastewater type: domestic, plant type: bulrush.
Input output
mg/L mg/L
BOD5 36 12
TSS 42 1 0
DEGUSSA CORPORATION (Mobile, Al)
Flow: 1.78 mgd, Area 2.2 ac, L = 475 ft, W = 28 ft, L:W =17:1
bed depth = 2 ft, HRT = 1 d, Preliminary treatment: oxidation ditch
Wastewater type: organic industrial, plant type: bulrush
BOD5 5 4
TSS 23 4
COD 287 245
TKN 22 18
NH3 4.2 2.3
B-1
-------
APPENDIX B
PHILLIPS HIGH SCHOOL (Bear Creek, AL)
Flow: 0.0155 mgd, Area: 0.502 ac, L = 175 ft, W = 125 ft, L:W = 1.4:1
bed depth = 1 ft, HRT 3.9 d, Preliminary treatment: extended aeration
Wastewater type: school/domestic, plant type: cattails.
BOD5 13 1
TSS5 60 3
TKN 22 2.6
NH3 10 2
N 03 26 6
TN 48 9
TP 5 0.23
FC 80,000 10
MONTEREY, VA (Monterey, VA)
Flow: 0.022 mgd, Area: 0.056 ac, L = 74 ft, W = 33 ft, L:W = 2.2: 1,
bed depth = 3 ft, HRT = 0.9 d, Preliminary treatment: Imhoff tank,
Wastewater type: municipal (high I & I), plants: cattails, bulrush.
BOD5 39 15
TSS 32 7
NH3 9.3 8.7
DENHAM SPRINGS, LA (Denham Springs, LA)
Flow: 1.73 mgd, Area = 15.2 ac, L = 1050 ft, W + 210 ft, L:W = 5:1
bed depth = 2 ft, HRT = 1 d, Preliminary treatment: Facultative lagoon,
Wastewater type: municipal, plants: bulrush, Canna lillies.
Input output
mg/L mg/L
BOD5 25 10
TSS 48 14
NH3 0.7 10
FC 52,000 3,800
B-2
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APPENDIX B
BENTON, LA (Benton, LA)
Flow: = 0.100 mgd, Area: 1.5 ac, L = 900 ft, W = 58 ft, L:W = 15.5:1
bed depth = 2 ft, HRT = 21 d, Preliminary treatment: Facultative lagoon +
recirculation
Wastewater type: municipal, plants: bulrush, canna lillies.
BOD5 18 6
TSS 57 4
NH3 0.6 2.8
HAUGHTON, LA (Haughton, LA)
Flow: = 0.100 mgd, Area: 1.5 ac, L = 934 ft, W: = 72 ft, L:W = 13:1,
bed depth = 2.5 ft, HRT = 4.5 d, Preliminary treatment: facultative lagoon,
Wastewater type: municipal, plants: bulrush, canna lillies.
BOD5 12.5 2
TSS 47 14
NH3 1.1 7.2
CARVILLE, LA (US PHS Hospital, Carville, LA)
Flow: 0.1228 mgd, Area: 0.64 ac, L = 528 ft, W = 62 ft, L:W = 8.5:1
bed depth = 2.5 ft, HRT = 1.4 d, Preliminary treatment: 3 cell lagoon, air in 1 st
cell
Wastewater type: hospital/domestic, plants: arrowhead.
BOD5
TSS
VSS
COD
TKN
NH3
N03
TP
20
93
65
107
8.6
4.8
0
2.3
8
17
8
44
7.1
5.1
0
2.3
B-3
-------
APPENDIX B
MANDEVILLE, LA (Mandeville, LA)
Flow: 1.224 mgd, Area: 4.56 ac, L = 470 ft, W = 207 ft, L:W = 2:1
bed depth = 2 ft, HRT = 0.7 d, Prelimtreat= 3 cell aerated (Hinde tubing)
lagoon, Wastewater type: municipal, plants: bulrush.
BOD5 4 1 10
TSS 59 7
VSS 39 5
COD 79 53
TKN 5 3
NH3 1.4 2.1
N03 4.4 0.8
TP 3 4
BENTON, KY, CELL 3 (Benton, KY)
Flow: 0.1811 mgd, Area: 3.6 ac, L = 1092 ft, W = 144 ft, L:W = 7.6:1
bed depth = 2 ft, HRT = 5 d, Prelim treat.: facultative lagoon,
Wastewater type: municipal, plants: bulrush & weeds.
BOD5 26 9
TSS 56 4
TKN 14.1 9.5
NH3 5.1 7.4
N03 0.3 0.4
TN 14.4 9.8
TP 4.4 3.4
B-4
-------
APPENDIX B
HARDIN, KY, phragmites side (Hardin, KY)
Flow: 0.0624 mgd, Area = 0.79 ac, L = 475 ft, W = 72 ft, L:W = 6.6:1,
bed depth: 2 ft, HRT = 3.3 d, Preliminary treatment: contact stabilization plant,
Wastewater type: municipal, plants: reeds.
B O D5 51 9
TSS 118 17
TKN 20.7 12.1
NH3 10.1 9.9
NO, 0.5 0.3
TN 21.2 12.5
TP 4.9 2.2
HARDIN, KY, scirpus side (Hardin, KY)
Flow: 0.0492 mgd, Area = 0.79 ac, L = 475 ft, W = 72 ft, L:W = 6.6:1,
bed depth: 2 ft, HRT = 4.2 d, Preliminary treatment: contact stabilization plant,
Wastewater type: municipal, plants: bulrush.
Input output
mg/L mg/L
BOD5 51 4.1
TSS 118 9.4
TKN 10.7 9.7
NH3 10.1 8.3
N O3 0.5 0.3
TN 21.2 10
TP 4.9 2.4
B-5
-------
APPENDIX B
UTICA, MS, NORTH SYSTEM (Utica, MS)
Flow: 0.05 mgd, Area: 1.5 ac, L = 280 ft, W = 140 ft, L:W = 2: 1,
bed depth = 2.1 ft, HRT = 5 d, Preliminary treatment: Facultative lagoon,
Wastewater type: municipal, plants: bulrush & cattails.
BOD, 38 14
TSS 52 23
NH3 6.7 2.9
N03 . 0.3 0.2
TP 5.8 2.4
FC 2,308 700
UTICA, MS, SOUTH SYSTEM (Utica, MS)
Flow: 0.11 mgd, Area: 2 ac, L = 315 ft, W = 158 ft, L:W = 2:1,
bed depth = 2.1 ft, HRT = 3.7 d, Preliminary treatment: Facultative lagoon,
Wastewater type: municipal, plants: bulrush & cattails.
BOD, 31 11
TSS" 32 11
NH3 5.6 3.1
N03 0.3 0.2
TP 4.3 2.6
FC 1,272 628
B-6
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MUNICIPAL TECHNOLOGY BRANCH
SUBSURFACE FLOW
CONSTRUCTED WETLANDS
FOR WASTE WATER
TREATMENT:
A TECHNOLOGY ASSESSMENT
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