A HANDBOOK OF
CONSTRUCTED WETLANDS
                   a guide to creating wetlands for:
                    .AGRICULTURAL WASTEWATER
                       DOMESTIC WASTEWATER
                        COAL MINE DRAINAGE
                            STORMWATER

                     in the Mid-Atlantic Region
                           Volume,
             DOMESTIC WASTEWATER

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                                               ACKNOWLEDGMENTS
Many people contributed to this Handbook. An Interagency Core Group provided the initial impetus for the Handbook, and later provided
guidance and technical input during its preparation.  The Core Group comprised:         ^
Carl DuPoldt, USDA - NRCS, Chester. PA
Robert Edwards, Susquehanna River Basin Commission,
  Hirrisburg, PA
Lamonte Garber, Chesapeake Bay Foundation, Harrisburg, PA
Barry Isaacs, USDA - NRCS, Harrisburg, PA
Jeffrey Lapp, EPA, Philadelphia, PA -   .
Timothy Murphy, USDA - NRCS. Harrisburg. PA
Glenn Rider, Pennsylvania Department of Environmental •
  Resources, Harrisburg, PA
                 Melanie Sayers, Pennsylvania Department of Agriculture, Harrisburg, PA
                 Fred Suffian, USDA - NRCS, Philadelphia, PA
                 Charles Takita, Susquehaana River Basin Commission, Harrisburg, PA
                 Harold Webster, Penn State University, DuBois, PA.
 Many experts on constructed wetlands contributed by providing information and by reviewing)and commenting.on the Handbook. These
 individuals included:   .                        .                       .                                        •
 Robert Bastian, EPA .Washington. DC
 William Boyd, USDA - NRCS. Lincoln. NE
 Robert Brooks, Penn State University,
 % University Park, PA
 Donald Brown, EPA, Cincinnati, OH
 Dana Chapman, USDA - NRCS, Auburn, NY
 Tracy Davenport, USDA - NRCS. Annapolis,
   MD
 Paul DuBowy, Texas A & M University,
   College Station, TX
 Michelle Girts, CH2M HILL, Portland. OR
 Robert Hedin, HedSn Environmental,
   Sewlckley, PA
 William Hellier, Pennsylvania Department of
   Environmental Resources, Hawk'Run, PA
 Robert Kadlec, Wetland Management
   Services, Chelsea, MI
 Douglas Kepler, Damariscolta, Clarion, PA
 Robert Kleinmann, US Bureau of Mines,
   Pittsburgh, PA
 Robert Knight, CH2M HILL, Gainesville, FL
 Fran Koch, Pennsylvania Department of
  Environmental Resources, Harrisburg, PA
 Eric McCleary, Damariscotta, Clarion, PA
 Gerald Moshiri, Center for Wetlands and
  Eco-Technology Application, Gulf Breeze,
  FL
 John Murtha. Pennsylvania Department of
  Environmental Resources, Harrisburg, PA
 Robert Myers, USDA - NRCS, Syracuse, NY
 Kurt Neumiller, EPA, Annapolis, MD
 Richard Reaves, Purdue University, West
  Lafayette, IN
 William Sanville, EPA, Cincinnati. OH
 Dennis Sievers, University of Missouri,
'  Columbia, MO
 Earl Shaver, Delaware Department of
  Natural Resources and Environmental
  Control, Dover, DE
 Daniel Seibert, USDA - NRCS, Somerset, PA
 Jeffrey Skousen, West Virginia University,
  Morgantown, WV
 Peter Slack. Pennsylvania Department of
  Environmental Resources, Harrisburg, PA
 Dennis Verdi, USDA - NRCS, Amherst, MA
 Thomas Walski, Wilkes University, Wilkes-
  Barre,.PA
 Robert Wengryznek, USDA - NRCS, Orono,
 .ME
. Alfred Whitehouse, Office of Surface
  Mining; Pittsburgh. PA
 Christopher Zabawa, EPA, Washington, DC.
  This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental Protection
  Agency-Region III, in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided with
 •nonpoinl source management program funds under Section 3.19 of the Federal Clean Water Act.
  The findings, conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA - NRCS.
  EPA - Region III, the Commonwealth of Pennsylvania, or any other state in the northeastern United States concerning the use of constructed
  wetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specific
 • programs and restrictions in this regard.
                                                                                                                 PS5384RSEZ

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                                        VOLUME 2
                                        CONTENTS
CHAPTER 1. INTRODUCTION ................. ........................ •ป ........................................... - ........ : ................... : ..... ' 3
CHAPTER 2. USING CONSTRUCTED WETLANDS TO TREAT DOMESTIC WASTEWATER ......... . ............... 5
         Introduction ... ........................... ............................ • ................................. • ................... '•' .........
         Contaminant Removal Processes ..................... ; [[[ • ............. • ..... ...........
         Advantages and Limitations of Constructed Wetlands ............... . ........ : ........ - ...................................... 6
         Creating Effective Constructed Wetlands .................... . ......... • ....... .....,..ป ........ — — ......... • .......... •• ......... 6
       .  Types of Constructed Wetlands .............. .. ........... .. .............. • ........ ...ป....- ................................ — ••: ......... 7
         .Wastewater Characteristics ........ ....................... • ...... ••ซ• .................. • ............ ; ..... ' ....................... ; ...........
              Water Quality ............................ • ........... • ....................... [[[ • ....... "" ฎ
              Water Quantity .................................... , ............................................. ; ..... :•-" ............. ' ................
          Pretreatment ..... ......... .. ......... — • ............ • ............. • [[[ .' ..........
          System Configuration ............ . ..... .... ................ ; .......... ••• ...................... — ...... •, ........ ; ............................... •
              Length-to-Width Ratio ............... ..... ..... . ....... ซ ............ • ....... • ..... •• ........................................ • .......... 9
              Compartmentalization ............... . ......... : ........................ • ............................................ • ...... •' ..... .""
              Step-feeding ...... . ......... ; ..... ป ......................... • .......... • ................ — — ........ ; ..... "' .......................    in
              Recycling ........................ — • ....... • ................................... v .............. : .............. ; ....... """ ................
 CHAPTER 3. PERFORMANCE EXPECTATIONS . ..... . ...................................... ••••ป .......... • ...... • .......................... **
          Introduction : .................... •••••< ...... • ................ ........ ................................................. • ............................
          Biochemical Oxygen Demand and Total Suspended Solids ..................... . ................................... •.-•• **•
          Nitrogen ................ ...ป ........ — '• ......... • ....................... • ..... : ...................................... ..... "'" ....... ' ..............
          Phosphorus ........... ....... •. ............... r ................................ "•" ...... ' ......... ' ................ ' ............... " ...............
          Toxics ..'. ....... . .......... • ........ • ........................ "•—• ................ ; ....... — ............... ••"• ........................................
          Pathogens ....... . ..................... .— ................ •— ................................ ""• .................... " ........... ,—• •— • ......
 CHAPTER 4. SURFACE FLOW WETLANDS ................. .... ..... :....-.-.. ....... : ................................... • .......................
          Wetland Design .... ............. , ......... ป ........... • ................ ; ............................. - ....... - ........ ;
             Configuration ............................. —. [[[ ...................................... "'"
             Water Depth ...... '. ........ .' .............................. • ....... • ....... •-•• [[[ J
                                                                 .           ...................... . ........... 18
             Biochemical Oxygen Demand ......................... , ...................................... ••— ..................................
             Total Suspended Solids ......... . ............... •— ......... •— ................ • ..................... — - ......... " ....... ; ........
             Nitrogen ............ , .................. .......... • ............................ — ............................. ' ................................ V"
  CHAPTER 5.  SUBSURFACE FLOW WETLANDS
          ' Introduction .............. .. ...........
                                                                                              21
           Wetland Design	•	•	'	
             Darcy's Law	•	•	;	'	  •
             Media Types	••	•	•	••	-•	'	_
             Length-to-Width Ratio .;	>	•	'	
             Bed Slope	-	•	*	v."'	'.	'	

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                           LIST OF TABLES

Table 1. Removal mechanisms in constructed wetlands	ป,ป	•••	•	•	5
Table 2. Advantages and limitations of constructed wetland treatment of domestic wastewater .... 6
Table 3. Guidelines for creating constructed wetlands....:	•	'•	•	•
Table 4. Summary of municipal constructed wetland operational data	;	•	ป• ^
Table 5. Design summary for surface flow wetlands ....:	ป.	•	-	
Table 6. Design summary for subsurface now wetlands	-.	:	
                            LIST OF FIGURES
                                                                                     .9
                                                                                     .9
Figure 1. Configuration options for constructed wetlands.	•	
Figure 2. Flow patterns for constructed wetlands	ป	•••"-
Figure 3. BODS mass loading and removal rates in wetland.treatment systems	^
FiRUie 4. Nitrogen transformations	:	;	•	"	
Figure 5. Total nitrogen mass loading and removal rates in wetland treatment systems	.15
                               .VOLUME 2: DOMESTIC WASTEWATER"

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                                        CHAPTER 1
                                     INTRODUCTION
   This volume focuses on the use of constructed
wetlands to treat domestic wastewater. It is to be
used in conjunction with Volume 1: General Consid-
erations, which provides general information  on
wetland hydrology, soils, and vegetation, and on the
design, construction, operation, and maintenance of
wetland systems.
    Constructed wetlands can provide an inexpen-
sive and easily managed means otremoving 5-day
biochemical oxygen demand, particulates, nutrients,
and bacteria from domestic wastewater. Constructed
wetlands for domestic wastewater have found a wide
range of applications, ranging from large municipal
systems to single family homes. Constructed wet-
lands can provide year-round treatment but are
readily adaptable to seasonal or occasional uses, for
instance, at parks, camps, and schools. Some
systems have focused oh maximizing the amount of
wastewater treated cin the smallest amount of land
possible while other systems have focused on
polishing pretreated effluents with larger wetlands
that provide wildlife habitat and aesthetics in
addition to water quality improvement. Constructed
wetlands can be used to upgrade the performance of
existing facilities or as a component of new waste-
water treatment systems.
    A number of documents have been published
 recently on the use of constructed wetlands in
treating domestic wastewater. These publications
 include:   .
 Center for Environmental Resource Management.
 1993. Proceedings Subsurface Flow Constructed
 Wetlands Conference. University of Texas-El Paso,
 El Paso, TX.
 EC/EWPCA. 1990.  European Design and Operations
 Guidelines for Reed Bed Treatment Systems. P. F.
 Cooper (ed.), Proceedings International Conference
 on the Use of Constructed Wetlands in Water Pollu-
 tion Control. Pergamon Press, Oxford, UK.
 Reed, S. G. 1993. Design of Subsurface Flow Con-
 structed Wetlands For. Wastewater Treatment: a
 Technology Assessment. EPA 832-R-93-Q01. EPA
 Office of Wastewater Management, Washington, DC.
 Environmental Protection Agency. 1988. Design
 Manual: Constructed Wetlands and Aquatic Plant
 Systems for Municipal Wastewater Treatment.
 EPA/625/1-88/022. Center for Environmental
 Research, Cincinnati, OH.
 Reed, S. C., E. J. Middlebrooks, and R. W. Crites.
. 1994. Natural Systems for Waste Management
 and Treatment. 2nd edition. McGraw-Hill Book
 Company, New York City, NY.
 Water Pollution. Control Federation. 1990.
 Natural Systems for Wastewater Treatment,
 Manual of Practice FD-16, Chapter 9.  Alexandria,
 VA.
 Water Science and Technology, Volume 29.  1994.
     The National Small Flows Clearinghouse
 (NSFC) at West Virginia University, Morgantown,
 West Virginia (telephone 1-800-624-8301) provides
 technical assistance and information to small
 communities.
     The Environmental Protection Agency (EPA)
 has sponsored a project to collect and catalog
 information from wastewater treatment wetlands
 into a computer database. The Wetlands Treat-
 ment Database (North American'Wetlands for
 Water Quality Treatment Database)(Knight, R. L.,
 R. W. Ruble, R. H. Kadlec, and S. C. Reed 1994) is
  available on 3.5" diskette. To order, contact: Don
  Brown, USEPA (MS^347), Cincinnati, OH 45268;
  phone: (513) 569-7630; fax: (513) 569-7677; ,
  e-mail: brown.donald@epamail.epa.gov.
     While much experience has been gained in the
  design of constructed wetland systems for domes-
  tic wastewater, much is not yet understood and
  many of the relationships between design and   •
  performance have not been clearly established.
  Constructed wetland technology continues to be
  refined as more systems are installed and moni-
  tored over longer periods qf time. The guidance
  presented here should be considered as today's
  "state of the art" and will likely be modified as
  our understanding of these systems grows.
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VOLUME 2: DOMESTIC WASTEWATER                          ..  •                •
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                                       CHAPTER 2
                 USING CONSTRUCTED WETLANDS TO TREAT
                              DOMESTIC WASTEWATER
           INTRODUCTION

   Domestic wastewaters contain large amounts of
nutrients, participates, and organic matter that
must be removed before the water can be dis-
charged. Constructed wetlands are highly effective
in removing 5-day biochemical oxygen demand
(BODS) and total suspended solids (TSS) from
pretreated domestic wastewater. Removal efficien-
cies for nitrogen, particularly ammonia, vary
considerably, depending on system design, reten-
tion time, and the oxygen available for nitrifica-
tion. Phosphorus removal may be limited in the
long-term, although good removal may be seen
during the first several years.  The numbers of
pathogenic bacteria and viruses are significantly
decreased during passage through constructed
wetlands.  Removal capabilities are discussed in
Chapters.                   "     •
                   CONTAMINANT REMOVAL
                             PROCESSES

                 Wetlands remove contaminants through a series
              of interacting physical, chemical, and biological
              processes, including filtration, sedimentation,
              adsorption, precipitation and dissolution, volatil-
              ization, and biochemical interactions (table 1).
                 The suspended solids that remain after pretreat-
              ment are removed in the wetland mainly by sedi-
              mentation and filtration. These physical processes
              also remove a significant portion .of other waste-
              water constituents, such as BOD5, nutrients, and
              pathogens, that are associated with the solids.
                 Adsorption is the principal removal mechanism
              for dissolved pollutants such as phosphorus and
              dissolved metals. Adsorption is promoted by the
              large amount of surface area provided by the
              sediments, vegetation, soils, and litter.
                        Table 1. Removal mechanisms in constructed wetlands
                                         (after Brix 1993).
      Wastewater Constituent
   Biochemical oxygen demand
   Suspended solids

   Nitrogen
   Phosphorus



   Pathogens
        RemovalJMeGhanisms
Microbial degradation (aerobic and anaerobic)
Sedimentation (accumulations of organic matter/sludge
   on sediment surfaces)

Sedimentation/filtration                     '

Chemical ammonification followed by microbial nitrification
   and denitrification
Plant uptake-                                         .
Volatilization of ammonia

Soil sorption (adsorption-precipitation reactions with aluminum,
   iron, calcium, and clay minerals in the soil)
Plant uptake                                   .

Sedimentation/filtration
Natural die-off
Attack-by antibiotics excreted from the roots of wetland plants
Predation by invertebrates and other microbes
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    Soluble organic compounds are, for the most
part, degraded by microbes, especially bacteria, that
grow on the surfaces-of the plants, litter, and the
substrate. The oxygen needed to support aerobic
microbial processes is supplied by diffusion from
the atmosphere, by photosynthetic oxygen produc-
tion within the water column, and, to some extent,
by leakage of oxygen from the roots of the -vegeta-
tion. Sbme anaerobic microbial degradation also
occurs.
 ADVANTAGES AND LIMITATIONS
   OF CONSTRUCTED WETLANDS
    When properly designed, constructed wetlands
 offer a number of advantages, including low cost,
 simplicity of operation, and effective removal of
 BODsandTSS (table 2). .When sized adequately,
 constructed wetlands are also tolerant of fluctuating
 flows and variable water quality.  For instance, at
 the Des Plaines Rive.r Wetlands Demonstration
 Project, effluent concentrations of TSS, nitrate, and
 total phosphorus remained low and steady although
 influent concentrations were often quite high and.
 varied significantly with time, (Hey et al. 1994).
     Constructed wetland treatment is constrained
 by a number of limitations, including relatively
  large land requirements and a degree of uncer-
  tainty not found in more conventional
  approaches (table. 2).          '
          CREATING EFFECTIVE
      CONSTRUCTED WETLANDS

      Suggestions for creating an effective con-
   structed wetland are given in table 3. Since the
   objective of using a constructed wetland is to
   simplify the handling of wastewater, the system ,
   should be made as easy to operate as possible while
   ensuring reliable treatment.- Building a slightly
   larger system may be more expensive to construct
   but may be more reliable and less costly to operate
   than a smaller system. Attention.to several factors
   will help to ensure successful wetland treatment:
   • Adequate pretreatment. Pollutant loads in raw
     wastewater can exceed the ability of a wetland
     to.treat or assimilate them. Wetland treatment
     is suitable for waters that have received primary
     or secondary treatment.
   • Adequate retention time. A wetland treats
     wastewater through a number of biological
     (largely microbial), physical, and chemical
     processes. The water must remain in the
         Table 2. .Advantages and limitations of constructed wetland treatment of domestic wastewater.
            Advantages
   Excellent removal of BODS and TSS
   Good removal of nutrients, depending on system
     design
   Ability to handle daily or seasonally variable
     loads
   Low energy and maintenance requirements

   Simplicity of operation
             limitations   .
Variable treatment efficiencies due to the effects of
  season and weather
Uncertainty as to treatment effectiveness under all
  conditions      '                            ,
Sensitivity to high ammonia levels
Larger land area requirement than for conventional
  treatment       •         .
Potential for mosquito production
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 wetland long enough for biological and chemical
 transformations to take, place and for sedimenta-
 tion and deposition to occur. The wetland must
 be built large enough to provide the necessary
 retention time.

• Supplemental water. If a constructed wetland is
 to remain healthy, it must remain relatively wet.
 Wetland plants are generally tolerant of fluctuat-
 ing flows, but they cannot withstand complete
 drying. For this reason, either a fairly regular
 supply of wastewater must be assured or a
 supplemental source of water must be provided.
• Proper management. Constructed wetlands are
 "high management, low maintenance" systems.
 They must be actively managed if they are to
 perform well. "Management" means watching
 the wetland for signs of stress or disease and'
 adjusting water levels or-waste water input
 streams accordingly. While wetlands are low
 maintenance systems, they are not maintenance-
 free. For instance, distribution systems must be
 cleaned periodically to avoid plugging and
 uneven distribution of flow, and valves and
        piping must be checked to detect and correct
        blockages or leaks.
             TYPES OF CONSTRUCTED
                      WETLANDS

          Domestic wastewater can be treated with surface
      flow (SF) or subsurface flow-(SSF) wetlands.
          The advantages of SF wetlands are that  .
      their design and construction are straight-
      forward. Operation and maintenance are simple.
      Because the water surface is unconstrained, SF
      wetlands are able to handle wide variations in flow.
      SF systems can provide excellent removal of BODS
      and TSS and some installations have achieved good
      removal of ammonia and total nitrogen. SF systems
      are similar to natural marshes and can provide
      wildlife habitat as well as wastewater treatment.
       SF wetlands are discussed in Chapter 4.
         . In.SSF wetlands, the water level is.ihtended to
      remain below the surface of the substrate. Since the
      water is not exposed to the atmosphere, potential
      problems with insects, odors, and safety are
                          Table 3. Guidelines for creating constructed wetlands.
   Know what you are dealing with:


   Wetlands must have water:

   Size the wetland generously:

   Give the plants a chance:


   Don't overload the wetland:

   Protect the wetland from toxics:


   Keep an eye on what is happening:

   Get interdisciplinary help:
Sample the wastewater
Know what pretreatment will accomplish

Know the water budget

An undersized wetland cannot perform well

Allow time for establishment
Avoid shock loadings

Application rates must not exceed treatment rates

Limit, the toxics entering the wetland
Keep herbicides out of thie wetland

Monitoring is needed to assure continued performance

Environmental engineer
Water quality specialist
Plant materials specialist or biologist
State agencies
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  avoided. There is debate over the most effective
  length-to-width ratio, type of vegetation, and type
  and size of medium. A few recent European designs
  have incorporated vertical flow and batch loading in
  an attempt to promote more effective wetting and
  drying cycles, and to entrain more oxygen for nitri-
  fication (Bastian and Hammer 1993). Because of the
  hydraulic constraints .imposed by the media, SSF
  wetlands are best suited to the treatment of wastewa-
  ters under relatively uniform now conditions. There
  have been problems with surface flow and apparent
   plugging. SSF wetlands are discussed in Chapter 5.
                WASTEWATER  .
         .    CHARACTERISTICS

       To design the wetland .treatment system, an
    accurate assessment of contaminant loadings is
    needed (loading = contaminant concentration x
    water volume). To calculate loadings, data are
    needed on the average water quality; the max-
    imum concentrations, and the largest and smallest
    volumes that may occur. Maximum concentra-
    tions will probably occur in late summer when •
    losses due to evapotranspiration are greatest.
     The highest flows can.be expected during, the wet
    season, but pqllutant concentrations may be
    lower at this time because of dilution. The design
    should be'based on the highest contaminant
    loadings.'      •     •
    WATER,QUALITY
    For design, water'quality analyses generally
    include:
                               .           ...
           alkalinity
           5-day biochemical oxygen demand (BOD5)
           total suspended solids (TSS)
           total dissolved solids (TDS)
           dissolved oxygen
           nitrate plus nitrite nitrogen (NO2 + NO3-N)
           ammonia nitrogen (NH3-N).'
           total phosphorus
      heavy metals (for instance, lead, mercury,
          chromium, zinc)
     . refractory organics
      total or fecal coliform bacteria.
   The design of the wetland is usually based on
the removal of BOD (usually measured as 5-day
biochemical oxygen demand, BOD5) or nitrogen
(measured as total Kjeldahl nitrogen or nitrate
nitrogen). Concentrations of ammonia (NH3 +
NH -N, un-ionized ammonia + the ammonium
ion) should be evaluated because of the toxicity
of ammonia to wetland plants.
 WATER QUANTITY
 An accurate estimate of the volume of waste-
 water is needed, including the expected average,
 maximum, and minimum flows. The level of
 detail required (daily/monthly, or seasonal flows)
 will be projectrspecific.  The frequency and
 duration of freezing conditions must be estimated
 to determine if storage or special operating prac-
 tices will b
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microbial community and other biological
components of the wetland.

     SYSTEM CONFIGURATION x

   Various configurations are possible for the
constructed wetland and for incorporating it into a
treatment system (figures 1 and 2). System con-
figuration includes length-to-width ratio (some-
times called "aspect"), compartmentalization, and
the location of single or multiple discharge points.
The configuration should take advantage of the
natural topography of the site to minimize'excava-
tion and grading costs. While wetlands are often
designed as rectangles, wetlands can be built in
almost any shape to fit the topography of the site.
          Stabilization
            Pond
                                 err
                  Imhoff
                  Tank
     Commlnuter
         -D-












*
nr
V/n
ป

                          Sludge
                        Drying Bed
           Septic
           Tanks
           D-i
           D-
           Q-
c
cw
h
             CW: Constructed Wetland .
       Figure 1. Configuration options for
              constructed wetlands
       . (from Steiner and Freeman 1989).
                                              Plug flow
                                                                c. Rectrculatlon
                                      d. "Jelly roll" step feed-recireulaUon cell
                                         CW: Constructed Wetland
                              Figure 2. Flow patterns for constructed wetlands
                                     (from Steiner and Freeman 1989).
Whatever the.configuration, care must be taken to
ensure equal flow distribution at the inlet and to
avoid short-circuiting of flow to the outlet.

LENGTH-TO-WIDTH RATIO
    Many SF constructed wetland have, been
designed with an overall length-to-width ratio of
about 3:1 to 4:ll  This ratio has been thought to
lower excessive loading at the inlet of early sys-
tems built with high length-to-width ratios and to
provide good removal of BODsand TSS. The
concern was that in a longer, narrower wetland the
upper end might become overloaded while the
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     lower end might lack adequate nutrients. How:
     ever, the relationships between length-to-width
     ratios and performance has not been adequately   .
     studied and an optimal length-to-width ratio has
     not been determined.
        For SSF wetlands, the length-to-width ratio of
     the wetland cell is an important consideration in
     the hydraulic design since the maximum potential
     hydraulic gradient is related to the available depth
     of the bed divided by the length of the now path.
     Cell length can be limited by hydraulic capacity if
     surface flow is to be avoided.  Therefore, SSF
     wetlands may have length-to-width ratios larger or
     smaller than 1:1. It is thought that most of the
     removal takes place in the vicinity of the influent
     area and that systems with relatively high BOD5
      discharge requirements 'can therefore achieve
      adequate BODsahd TSS reductions with relatively
      short length-to-width ratios (Reed 1993).  In'many
      of the early SSF systems that were designed with a
     .ratio of 10:1 or more and a total depth of 2 ft
      (0.6 m), surface flow has developed. The surface
      flow is thought to result from inadequate hydrau-
      lic gradients (Reed. 1993).


      COMPARTMENTALIZATION
          Compartmentalizing the wetland with
      several'cells arranged in series or in parallel is
      suggested because it allows  flows to be redistrib-
      uted through the system as necessary for mainte-
      nance or repair.  Cells arranged in parallel
      facilitate the maintenance of plant communities
      (because of the greater edge length relative to
      surface area) and allow-different plant popula-
      tions and any associated.plant diseases or
      pathogens to be isolated. Ideally, cells can be
      arranged to permit operation in series or in
      parallel, with alternate discharge points and
     ' interconnections.   .            .      '
 suggested as a means of distributing organic
 loads along the length of the wetland, thereby
 lessening .the organic loading on the upper end
 of the cell.  The additional capital and operating
 costs of step-feeding need to be weighed against
.the potential benefits gained.
 RECYCLING
    It may be advantageous to recycle all or a
 portion of the wetland effluent. Recycling can be
 used to dilute influent BODS and solids. Recy-
 cling may increase dissolved oxygen concentra-
 tions and detention time, which in turn may
 enhance nitrification and nitrogen removal.
 Recycling is also an efficient way to maintain
 adequate flow during low-flow periods.
     The disadvantages of recycling are the
 increased construction costs and increased
 operation (pumping) costs.  A wrap-around
 design may help to minimize these costs. Also,
 recycling may slowly increase salinities as
 . evapotranspiration removes water from the
,. system. The added costs of recycling must be
 weighed against the potential benefits gained.
       STEP-FEEDING    .
           Step-feeding, which is the use of multiple
       input points along the length of a cell, has been
10
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                                        CHAPTER 3
                          PERFORMANCE EXPECTATIONS
            INTRODUCTION

    Perhaps more is known about the domestic
wastewater applications of constructed wetlands
than for any other use.  A database on wastewater
treatment wetlands has been compiled by EPA's
Risk Reduction Engineering Laboratory (Knight et
al. 1994). The database contains data from 323
.wetland cells at 178 locations in the United States
and Canada, and includes information on natural
and constructed wetlands, and งF, SSF, and hybrid
systems. The majority of the systems .in the
database have been installed recently and have as
yet produced little operational data. Performance
data for 11 SF and 2 SSF domestic wastewater
constructed wetlands that have been operating
long enough to produce useful data are summa-
rized in table 4.
      BIOCHEMICAL OXYGEN
        DEMAND AND TOTAL
         SUSPENDED SOLIDS

   BODS is removed by filtration and sedimenta-
tion of particulate matter and by microbial degra-
dation of soluble BODS. The organic matter in
treatment wetlands provides a source of energy for
populations of bacteria, fungi, and aquatic
macroinvertebrates similar to those in conven-
tional activated sludge and trickling filter treat-
ment plants. In SSF wetlands, physical removal
of BODS is believed to  occur rapidly through
settling and entrapment of particulate matter in
the void spaces in the  media;
   Both SF and SSF wetlands are extremely
efficient at assimilating BOD5 and TSS (table 4).
Table 4. Summary of municipal constructed wetland operational data
. (adapted from Knight et al. 1993).
BOD5 (mg/L) TSS (mg/L) NHa-N (mg/L) TP (mg/L)
In Qut %. in Out %. In Out %_ • • _in Out • %
Surface Flow Wetlands .
Benton-cattail
Benton-woolgrass
Cobalt
• Gustine 1A
Gustine IB
Gustine 1C
Gustine. ID
Gustine 2A
Kelly Farm
Moodna Basin
Norwalk
26
26
21
130 -
130
145
141
151
-
53
229
10
12
5 "
50
27
24
30
45
-
18
9
62
52
78
62
79
83
78
70
-
66
96
57
57
36
73
81
88
98
100
'
34
232
11
16
28
40
23
. 57
20
34
' .-.
12
33
82
73
23
46
72
36
79
66
-
64
86
7.7
7.7
2.9
17.0
16.3
18.4
19.6
18
8.4
20.4
-
7
6
1
16
17
.20
22
23
0
11
-
.9
.4
.0
.1
.9
.4
.9
.2
.1
.4

inc
16
65
6
inc
inc
inc
inc
99
44
• -
4.5 4.2 7.
4.5 4.0 12
'1.7 0.8 54
•'-.'-'
- - - • .
...
- - - .
...
...
...
-
Subsurface Flow Wetlands
Kingston
Monterey
56
38
9
15
84
60
83
32
3
7
96
.78
22
9.3
16
8

.7
27 '
7
3.4 2.1 38
-
%: percent reduction
inc: increase











                                     VOLUME 2: DOMESTIC.WASTEWATER
                                              11

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 Constructed -wetland treatment systems commonly
 receive inflow BODS concentrations of 10 to 100
 mg/L, depending on the degree of pretreatment
 (Knight et al. 1993). For the constructed wetland
 systems summarized in table 4, removal efficien-
 cies for BOD, were generally greater than 60% for  .
 both SF and SSF wetlands. These efficiencies were
 realized in spite of widely varying retention times,
 configurations, input concentrations, and wetland
 plant communities (Water Pollution Control Fed-
 eration 1990).
   '  Knight et al. (1993) found that typical
 BODsmass removal efficiencies were near 70%
' or more at mass loading rates up to 250 Ib/ac/day
 (280 kg/ha/day) for SF and SSF wetlands. Lower
 removal efficiencies occurred, especially when
 mass loadings were less than 45 Ib/ac/day
 (50 kg/ha/day). A linear regression  of 324 munici-
 pal, stormwater, and other data records used to
 examine the predictability of BODS outflow concen-
 tration as a function of BOD^ inflow concentration
 and hydraulic loading (figure 3) produced the
 following relationship:  ,  .
     BODOUT = 0.097*HLR + 0.192*BODIN
                 R2 = 0.72:
  where
     BODOUT = BOD outflow concentration, mg/L
       BODIN = BOD inflow concentration, mg/L
          HLR = hydraulic loading rate, cm/day.

      While average annual removal rates were
  generally high, rates sometimes varied considerably

      Figure 3. BODS mass loading and removal
         rates in wetland treatment'systems
            . .{from Knight etal.'1993).
on a monthly or seasonal basis. To maximize the
removal of BOD5 and TSS, the growth of plants
(particularly underground tissues) and the accumu-
lation of litter should be encouraged. Plants and
plant litter provide organic carbon and attachment
sites for microbial growth, as well as promoting
filtration and sedimentation.
    In wetlands, BOD is produced within the
system by the decomposition of algae and fallen
plant litter.  As a result, wetland systems do not
completely remove BOD and a residual BODS
from 2 to 7 mg/L is often present in the wetland
, effluent (Reed 1993).  This internal production of
BOD decreases efficiencies at very low inflow
 concentrations.
    The potential for wetlands to assimilate. TSS is
 similar to the potential for BODS removal (table 4).
 Removal rate and efficiency are consistently high
 up to loading rates of 135 Ib/ac/day (.150 kg/h/day)
 (Knight et at 1993). Removal efficiencies for TSS
 are also closely related to input concentration, with
 lower efficiencies measured at low input concentra-
 tions. Cooper et al. (1993) found that TSS removals
.increased with increasing accumulation of plant
 detritus in the litter layer.
                 NITROGEN

     The organic nitrogen entering a treatment
  wetland is usually associated with particulate
  .material, such as algae (especially when pretreat-
  ment ponds are used) and organic wastewater
  solids. Plant detritus generated within the wetland
  can also be source of organic nitrogen..
     In wetlands, nitrogen occurs in a number of
  forms, the most important of which are nitrogen gas
  (N2), nitrite (NO2), nitrate (NO3), ammonia (NH3),
  and ammonium (NH4+). The forms of nitrogen most
  often regulated are ammonia and total nitrogen
  (TN).  Un-ionized ammonia can be toxic to fish and
  .other aquatic life while excess nitrogen contributes
  to the over-enrichment of natural waters.
      In contrast to the simplicity of BOD and TSS
  removal, the chemistry of nitrogen removal is
                                VOLUME 2:  DOMESTIC WASTEWATER

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complex (figure 4). The removal processes in-
clude some that require oxygen (aerobic reactions)
and others that take place in the absence of oxygen
(anaerobic reactions). Decomposition and miner-
alization processes in the wetland convert a
significant part of organic nitrogen to ammonia.
Ammonia is then oxidized to-nitrate by nitrifying
bacteria in aerobic zones (nitrification) and ni-
trates are converted to nitrogen gas by denitrifying
bacteria in anoxic zones (denitrification); the gas
is released to the atmosphere. The sequencers:
mineralization:
  organic nitrogen-> ammonia
   .  •       ,      nitrogen
             aerobic or anaerobic
             reaction
nitrification:  .
  ammonia nitrogen -> nitrate
                   nitrogen

denitrification:
  nitrate nitrogen -> nitrogen
                 gas
             aerobic reaction
             anaerobic reaction;
             requires a carbon
             source as food for the
             bacteria
     Nitrogen Gas
     Nitrous Oxide Gas
   SOIL
                      , AIR
                     WATER.-
                                  Ozygen
                                   •t
                   6^"Aซobic^rAmmonia"+ Nitrate VNttnne* _'

                     - • * .    Llnward '   ".VI -_•."-""
                     . .. .
                  AnerobicSoil
                     Diffusion
    _     —*.r
Fixation   '    *f*
    • Organic N
             Upward
             Diffusion
             .Ammonia
      Nitrogen Gas
      Nitrous Oxide Gas'
                       Denilrification
                                    Downward
                                    Diffusion
                              Mineralizalionl
                                      Nitrate
                             Leaching ซ  '
        Figure 4. Nitrogen transformations
     (after Gambrel and Patrick 1978, cited in
           Mitsch and Gosselink 1986).
    Since nitrification is an aerobic process,
 rates are controlled by the availability of oxygen
 to the nitrifying bacteria. The process of nitrifi-
 cation is usually limited by the availability of
 dissolved oxygen availability, and also by
 temperature and retention time (Knight et al.
 1993). Denitrification is usually very rapid and
 the loss of nitrogen gas to the atmosphere
 represents a limitless sink. Decaying plant litter
 may provide anoxic sites for denitrification
 (Crumpton et al. 1993)
    Some nitrogen is taken up by plants and   .
 incorporated into plant tissue, but this removal
 pathway is of limited  importance in wetlands in
 the northeastern United States because the
 above-ground parts of most emergent plants die.
 back yearly and because below-ground tissue
 increases only very slowly (Brix 1993).  Most of
 the nitrogen bound in plant tissue is returned to
 the wetland when the plants die and decay.-
    Many constructed wetlands, both SF and
 SSF, are unable to meet typical NPDES limits for
 ammonia.  Reed and Brown (1992) believe that
 the factor responsible in both cases is the insuf-
 ficient availability of oxygen to support the
 activity of the nitrifying .organisms. In the SF
 case, the water may be too deep and the vegeta-
 tion too dense for wind and turbulence of the
 water surface to allow for significant aeration.
 One attempt to correct this problem combines
 overland flow with a SF wetland. The water   •
 depth in the overland flow segment is less than
 2 inches (5 cm) deep,  which allows for effective
 aeration and nitrification. Hammer (1992)
 suggests a marsh-pond-marsh sequence to  .
 increase the available oxygen: the water passes
 through an SF wetland area to convert organic
 nitrogen to ammonia,  then through a pond (a
.deeper, open water area) for nitrification of
 ammonia to nitrate and subsequent denitrifica-
 tion to nitrogen gas, then through another
 wetland area to complete the denitrification of
 nitrate.  In SSF systems, the roots of the vegeta-
 tion may not penetrate deeply enough and an
 anaerobic layer develops at the bottom of the
 wetland. For this reason, SF wetlands may
                                        VOLUME 2: DOMESTIC WASTEWATER
                                                                                        13

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have higher nitrogen removal capacities than
SSF wetlands.         '.    •
    Of 12 SSF systems in the southern United
States, most showed a marginal or negative
ammonia removal rate (in half of the systems
output ammonia levels were near or higher than
input levels)  regardless of detention time in the
system (Reed'1993). However, two systems
showed very  high removal rates at hydraulic
retention times (HRT) comparable to the other
systems. The difference is believed to be lack of
algae, availability of oxygen, and sufficient HRT
in the two systems, so that high levels of nitrifi-
cation can occur. In both cases, the roots pen-
etrated to the bottom of the bed. In the 12
systems with poor nitrogen-removal, roots did
not penetrate the entire depth of the bed; flow
therefore passed through the beds below the
root zones where oxygen, was not likely to be
 present.
    The depth of the media in most systems in
 the United States is about 2 ft (0.6 m) but in most
 cases the plant roots only penetrate to about 1 ft
 (0.3 m). Deeper penetrations were observed when
 nutrient levels in the water were low or when
 plants were located at the sides, of the cells where
 there was less flow than in the main portion of the
 bed. The use of parallel .cells operated on a batch-
 type fill and draw basis to allow atmospheric
 oxygen to be introduced into the substrate has also
 btfen used to increase ammonia removal.
     TN removal has been highly correlated to
 loading.rates as high as. 10 kg/ha/day, with re-
 moval  efficiencies typically between 75 and 95%
 (Water Pollution Control Federation 1990).  With
 loading rates between 10 and 80 kg/ha/day, total
 nitrogen removal efficiency varies widely, with
 some systems showing high values and others
 much lower values.  TN removal efficiency is
 highly dependent on HRT and decreases signifi-
 cantly at design HRTs of less than about 5 days
  (Water Pollution Control Federationl990).  Phipps
  and Crumpton (1994) found that seasonal varia-
  tions in nitrate and organic nitrogen loads had
  significant effects on the effectiveness of con-
structed wetlands as sinks for TN: during periods
of high nitrate loading, the wetlands were nitro-
gen sinks while the wetlands were nitrogen   •
sources during periods of low nitrate loading.
    Knight et al. (1993) found that, unlike BODS,
removal efficiencies for TN declined at mass
loading rates above 20 kg'/ha/day. Also, mass
removal efficiencies were more consistent at lower
mass loading rates than they were for BQDS. A
regression predicting TN outflow concentration
base&on hydraulic loading rate (HLR) and TN,
inflow concentrations was developed from 213
records in the EPA database (figure 5). The
multiple linear regression can be  expressed as:
    TNOUT = 0.28*HLR +  0.33*TNIN
             R2=0.54
where
    TNOUT = TN outflow concentration,
             mg/L
      TNIN = TN inflow concentration,
             mg/L
       HLR = Hydraulic loading rate,
              cm/day.
     Based on this equation, TN removal is more
 dependent on the effect of high HLRs than is
 BODS removal (Knight et  al. 1993),
     TN can be generated in wetlands through
 nitrogen fixation, in which: certain plants convert
 atmospheric nitrogen into.the organic form.
 Many wetland plants are able to  fix nitrogen and
 natural background concentrations of TN are
 generally in the range of 0.5 to 3 mg/L.  Appar-
 ently because of this natural nitrogen fixation
 process, TN removal efficiency decreases when
 TN input concentrations approach background.
  (Water Pollution Control Federation 1990).


               PHOSPHORUS

     In the short term, phosphorus is a highly
  mobile element in wetlands that is involved in
  many biological and soil/water interchanges.
  Dissolved phosphorus may be present in organic
  or inorganic forms and is readily transferred
                             VOLUME 2: DOMESTIC WASTEWATER

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 between the two forms. It has been assumed
 that microbes,-algae, and vascular plants cycle
 phosphorus annually, with uptake during the
 growing season and gradual release to the water
 column on death and decay. However, data on
 the annual recycling of phosphorus are still
 limited. Harvesting the above-ground portions of
 vascular plants at the end of the growing season
 to remove phosphorus was shown to be ineffec-
 tive because much of the phosphorus had been
 gradually translocated to the roots and rhizomes
• before then (Mitsch and Gosselink 1986).
    The long-term removal of phosphorus by
 wetlands is limited. The major sink for phospho-
 rus in most wetlands is the soil.  Phosphorus may
 be buried in organic form in peats or chemically
 adsorbed in complexed forms with aluminum,
 iron, or calcium (Faulkner and Richardson 1989).
 Soil adsorption can result in significant removal
 of dissolved phosphorus for a while after system  ,
 startup, but removal then decreases as adsorption
 sites become filled. The length of the removal
 period depends on the chemical adsorption
 capacity of the sediments and can be estimated
 through laboratory analyses.
    Removal at the SF and SSF wetland systems
 listed in table 4 ranged from 7% to 95%. The -
 extra removal seen in some wetlands may be
 explained by the export of organisms with their
 associated phosphorus loads or by chemical
T3
"S

o
••1 "
>
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E

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     solids. In general, pathogenic microorganisms
     are highly host-specific and do not survive long
     apart from the host.
        Constructed wetlands can provide high
     percentage removals of pathogens and have
     been shown to be capable of removing bacterial
     and viral indicators at efficiencies of 90% to
     99% at HRTs of three to six days (Ives 1988). At
     HRTs of three to six days, constructed wetlands
     are thought to be at least equivalent and, in
     most cases, more effective than  conventional
     wastewater treatment systems in removing
      disease-causing bacteria and viruses.
         Constructed wetlands treatment typically
      decreases total coliform levels to 103 total
      coliform/100 ml or less when undisinfected
      secondary wastewaters from conventional
      treatment.systems are being treated. SF wetland
      systems are generally- capable of a one* to two-
      log  reduction in fecal coliforms (EPA 1993),
      which in many cases is not enough to routinely
      satisfy discharge requirements.  Peak flows
      resulting from intense rainfall also  disrupt
      removal efficiencies for fecal coliforms.  As a
      result, many systems use some form of final
      disinfection.
          Removal of bacteria and viruses by wetlands
      is increased by densely vegetated cells and by
      longer retention times. Storage in wetlands, as
      in ponds, can be an effective means of reducing
      bacteria and viruses.
          Where virus removal is of concern, the
       design can be adjusted to optimize viral re-.
       moval. Since suspended solids (algal cells and
       colloidal  clay particles) play a key role in
       removing viruses from the water column, the
       design should incorporate means to supply the
      'necessary adsorption sites, for instance, by
       encouraging the accumulation of fine sediments
       and the growth of unicellular  algae.  The pro-
       duction of adsorption sites and removal of
       virus-laden particles should occur sequentially
       through a series of densely vegetated cells (to
       promote sedimentation of. particles and floccu-
       lated bacteria) and open water cells (to promote
the growth of algae). Because viruses are charged
particles and respond to flocculants, at facilities
that pretreat with septic tanks, most viruses
become attached to the solids and remain in the
tank septage.
'16
                                   VOLUME 2: DOMESTIC WASTEWATER

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                                       CHAPTER 4
                            SURFACE FLOW WETLANDS
          WETLAND DESIGN
   Guidelines for designing a SF constructed
wetland are given in table 5.
CONFIGURATION
   The configuration should take .advantage of the
natural topography of the site to minimize
excavation and grading costs. The configuration
should allow water to move through the wetland
by gravity. While treatment wetlands are often
designed as rectangles, wetlands can be built as
semi-circular or irregular shapes to fit the
topography of the site. Using curved shapes also
eliminates right-angled corners, which tend to
be "dead water" areas. If a shape other than a
rectangle is used, the widest portion should be
located at the inlet end to facilitate equal flow
distribution.
   For large wetlands, dividing the wetland into
                     side-by-side cells should be considered. Divid-
                     ing a wide wetland into parallel cells lessens the
                     likelihood of preferential flow paths and short-
                     circuiting, and promotes the contact of the
                     wastewater with- the surfaces in the wetland. It
                     also facilitates maintenance since,one set of cells
                     can be taken out of operation temporarily.
                         If the removal of nitrogen and ammonia is a
                     major objective, including a deeper (2-3 ft)
                     open water pond in the middle of a longer
                     wetland cell should be considered to increase
                     nitrification and denitrification.
                     WATER DEPTH
                         The design should plan for 3 to 8 inches of
                     surface -water, with a maximum of 18 inches.
                     Deeper water may be advisable in winter to
                     accommodate the slower reaction rates during
                     cold weather and to guard against freezing. The
                     wetland may have to be divided lengthwise into
                     a series of cells to prevent the water in any of
                     the cells from being deeper than desired. Each
    Configuration


    Flow

    Bottom slopes


    Water depth


    Vegetation


    Construction
Table 5. Design summary for surface.flow wetlands.

       Fit the wetland to the site
       Divide large wetlands into side-by-side cells
                                                     ./
       By gravity, as much as possible

       Side-to-side elevations: level
       Inlet to outlet slopes: almost flat (0.5 - 1.0%)

       3-8 inches, depending on the plants selected
       18 inches maximum

       Complete coverage is more important than the species used
       Use at least two or three different species .

       Wetland must be sealed to limit infiltration and exfiltration
       Water table must be below or excluded from the wetland
                                     VOLUME 2: DOMESTIC WASTEWATER
                                                                     17

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of cells will then discharge to a downstream cell
of the same width. The maximum length of each
cell is based on the slope of the hottom of the
cell (which should not exceed 0.5 to 1.0%) and
the water depth suitable for the wetland vegeta-
tion (which is generally 18 inches or less).  The
number and length of the subdivisions will
depend on the length of the cells and the slope
of the bottom. ,•
    The bottom of the cells-should be flat from
side to side  to assure an even distribution of
water across the cells and to prevent channeling.


                  SIZING

    Procedures for sizing SF wetlands for the
 removal of BOD5, TSS, and nitrogen are still
 preliminary. It has been widely presumed that
 simple first order chemical reaction rates apply
 for pollutant removal and that constructed
 wetlands roughly follow plug-flow in their
 internal hydrology. EPA's Design Manual:
  Constructed Wetlands and Aquatic Plant Sys-
  tems for Municipal Wastewater Treatment (1988)
  and the Water Pollution Control Federation
  Manual  of Practice, Natural Systems for Waste-'
  water Treatment (1990) assume that the reduc-
  tion of a specific water quality parameter, such
  as BOD  , is a first order reaction and that con-
  centration decreases exponentially with increas-
  ing retention time within the wetland.
      However, several recent studies have shown
  that the movement of water through constructed
  wetlands is considerably more complex than that
  described by standard flow equations (Kadlec et
  al.  1993. Kadlec 1994). The flow through a
  wetland is related to the morphology of the cell,
   the pattern of vegetation density, and the balance
   between evapotranspiration and precipitation.
   In  constructed wetlands, mixing characteristics
   are intermediate between plug flow and well-
   mixed, flows are typically in  the transition zone
   between laminar and turbulent, and hydrologic
conditions change continuously with changes in
the weather and the seasons. Factors such as
obstructions to flow, the development of chan-
neling, recirculation patterns, and the presence
of stagnant areas cause further deviations from
calculated theoretical flows. Contact times are
not often as great as the theoretical residence
time calculated from the wetland empty volume
and the volumetric flow rate. As a final, compli-
cating factor, the chemistry of wetlands is com-
plex, involving interrelated biological reactions
 and mass transfers. These factors and the lack of
 good information on factors such as reaction rate
 constants have probably led to many systems
 being under-designed.


 BIOCHEMICAL OXYGEN DEMAND  .
     The standard equation for BODS removal
 for an unrestricted flow system (EPA 1988,
 Water Pollution Control Federation 1990)
  assumes that BOD5 removal is described by a
  first-order model:
  where
     Ce/Co = exp (-Krt)  .

C  = effluent BOD5,mg/L
C  = influent BOD5, mg/L
 n               _    *
                                          (4.1)
            — AA.1AAMVU*. — —	Jป	V •
            - temperature-dependent first-order
              reaction rate constant, days-
          t = hydraulic residence time, days.
      Flow through vegetated SF wetlands is com-
   plex and equation 4,1 must be modified to account
   for a number of the factors that affect flow through
   wetlands. Reed et al. (1994) suggest the following:

         Ce/C0=F exp (-0.7 . KT . V7' • l •  = void fraction. ,
       The values to be used for KT, F, AV, and  in
                                VOLUME 2: DOMESTIC WASTEWATER

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designing constructed wetlands for domestic
wastewater have not been confirmed. A typical
value that is often used for'JC,. at 20ฐC is (D.0057
days -1 (EPA 1988).  However, experimental data
on the values to be used in designing-con^
structed wetlands have been difficult to obtain
because of the logistic and economic difficulties
experimenting with wetlands on a scale large
enough to be appropriate. The wetland should
be designed generously to accommodate these
uncertainties.
    The hydraulic residence-time (t) can be
represented by:
                t = LWd/Q             (4.3)
where
        L = length
       W = width
        d = depth            •  .  -
       Q = average flow rate [(flow in +
           flow  out)/2]
    The Water Pollution Control Federation
(1990) recommends a minimum wetland area of.
about 28 to 37 ac per million gallons per day
(mgd) of wastewater (3 to 4 ha/1000mVday.). A
maximum BODS loading rate of about 90 lb/ac/
day (100 kg/ha/day) is recommended to help to
prevent the occurrence of nuisance problems.
NITROGEN
   Adequate HRT as influenced by hydraulic
loading rate (HLR) and length-to-width ratio
appears to be an important factor affecting
TN removal efficiency, with lower removal
efficiencies at HLRs greater than 3 inches/day
(8 cm/day) and length-to-width ratios less than
2:1 (Water Pollution Control Federation 1990).
Total nitrogen removal efficiency through nitrifi-
cation/denitrification is temperature-dependent,
with lowered removal efficiencies below about
48ฐ F (10ฐ C). Knight (1986) found no decrease in
TN removal efficiency in a volunteer wetland at
temperatures above 50ฐ F (12ฐ C).  The Water
Pollution Control Federation (1990) suggests that,
to attain  a TN removal efficiency of 50% or more,
a minimum wetland area of about 37 ac per mgd
of wastewater (4 ha/1000m3/day) should be
provided.                                 .
TOTAL SUSPENDED SOLIDS
    Constructed wetlands are generally effective
at reducing the concentration of TSS. Removal.
efficiencies are similar to .those for BODS and
design for BOD5 should accomplish similar TSS
effluent levels. It is important to maintain
shaded conditions with dense vegetation near
the inflow and outflow to limit the growth of
algae, which can add to TSS levels.
                                    VOLUME 2: DOMESTIC WASTEWATER
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20
VOLUME 2: DOMESTIC WASTEWATER

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                                       CHAPTERS
                         SUBSURFACE FLOW WETLANDS
           INTRODUCTION    ,

   The design information provided in this
chapter is a summary of the information in
Subsurface Flow Constructed Wetlands for
Wastewater Treatment: A Technology Assessment
(Reed 1993). Reed based his recommendations
on the performance of 14 municipal, domestic,
hospital, and industrial systems that have pro-
vided detailed data and that are thought to be
representative of constructed wetland systems in
the United States.  Many of these systems are in
the South and West, and most have been operat-
ing for less than five years. Only a limited
number of systems in the Mid-Atlantic states
have provided operational data.
                                    WETLAND DESIGN
                             Guidelines for designing a SSF wetland are
                          given in table 6.
                         .DARCY'S LAW

                           "  The intent of the SSF wetland treatment
                          concept is to maintain the flow below the surface
                          of the media in the bed. The design of SSF wet-
                          lands has generally been based on Darcy's Law,
                          which describes the flow regime in a porous
                          medium. However, many of the systems designed
                          with Darcy's Law have developed unintended
                          surface flow and may have been under-designed.
   Configuration


   Flow


   Bottom slopes


   Inlet

   Outlet

   Vegetation


   Construction
Table 6. Design summary for subsurface flow wetlands.

     .  Fit the wetland to the site          .
       Divide large wetlands into side-by-side cells.

       By gravity, as much as possible
       Subsurface flow design based on Darcy's Law

       Side-to-side elevations: level
       Inlet to outlet slopes: almost flat (0.5 - 1.0%)

       Surface manifold with adjustable outlets

     - Perforated subsurface manifold connected to adjustable outlet

       Complete coverage .is more important than the species used
       Use at least two or three different species

       Porous media must be clean
       Wetland must be sealed to limit infiltration and exfiltration
       Water table, must be below, or excluded from the wetland
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                                                                         21

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   Darcy's Law assumes laminar flow, a con-
stant and uniform'flow (Q), and lack of short-
circuiting, conditions that do not exist in con-
structed wetlands. Darcy's Law is thought to
provide a reasonable approximation of the
hydraulic conditions in an SSF bed if small to
moderate size gravel (<1.5 inches, or <4 cm) is
used as the medium, the system is properly
constructed to minimize short-circuiting, the
system is designed to depend on a minimal •
hydraulic gradient, and the flow (Q in equation
5.1) is considered to be the "average" flow
KQj + On J/2] in the system to account for any
gains or losses due to precipitation, evaporation,
or seepage.
    Darcy's Law is typically defined with  equa-
UonS.l:
                 Q=k,AS                (5.1)
where
         Q =  flow per unit time, (fWday, gal/day,
              mVday, etc.)         .     .      .
         k =  hydraulic conductivity of a unit  •
              area of the medium perpendicular
              to the flow direction, (fWftVday,
              gal/day, mVnWday, etc.)
         A =  total cross-sectional area, perpen-
            •  diculartoflow(ft2,rn2,etc)
         S =  hydraulic gradient of the water    1
              surface in the flow system
              (slope of the water table)
              (dh/dL, ft/ft, m/m).
    Systems in .the United States and Europe
 with successful hydraulic performance do so
 either with a  sloping bottom and/or adjustable
 outlet structures which allow the water level to
 be lowered at the end of the bed.  A sloped
 bottom or lowering the water level at the end of
 the bed produces the pressure head required to
 overcome resistance to flow through the media
, and thus maintains subsurface flow.
     Clogging  has occurred in some systems. The
 clogging is believed to result from the introduc-
 tion of fine particulate material into the  medium
 because of improper construction procedures.-
                                                       Neyertheless, it is judicious to provide a large
                                                       safety .factor against clogging. A value 
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for 2 ft (0.6 m) deep beds and to about 0.75:1 for 1
ft (0.3 m) deep beds. Using such a low value for
hydraulic gradient will help to maintain near-
laminar flow in the bed and validate the use of
Darcy's Law in the design of the system. Since this
approach ensures a relatively wide.entry zone, it
will also result in low. organic loading on the cross-
sectional area and thereby lessen concerns over
clogging.
BED SLOPE
    The bottom of the cell can be flat or slightly
sloping from top to bottom.  The top surface of the
medium should be. level regardless of the slope of
the bottom.  A level surface facilitates plant man-
agement and minimizes surface flow problems.
Once surface flow develops on a downward sloping
surface, flow may not penetrate the medium even
though the true water level within the medium is
well below the surface.
                   SIZING

BIOCHEMICAL OXYGEN DEMAND
    SSF systems are generally sized for BOD5 re-
moval.  In SSF systems, the physical removal of
BODS is believed to occur rapidly through settling
and entrapment of particulate matter in the void
spaces in the gravel or rock media (Reed 1993).
Data from 14 SSF systems in the United States
indicate that BOD5 removal improves only slightly
after 1 to 1.5 days HRT, up to an HRT of 7.5 days.
The removal data for these systems can be reason-
ably approximated by a first order plug flow rela-
tionship up to about ฑ2 days. BOD5 removal there-
after is limited and is believed to be influenced by
the. production of residualBODj within the system.
This is compatible with the hypothesis that BODS is
removed rapidly in the front part of wetland
systems. •
    Most of the existing systems in the United
States and Europe have been designed as attached
growth biological reactors using the same equa-
tions as those used for SF wetlands (equations
4.1 - 4.3). The plug flow model is presently in
general use and seems to provide a general
approximation of performance.. It is believed
that the plug flow rate constant for SSF wetlands
is higher than for facultative lagoons or SF
wetlands because the surface area available on
the media in SSF wetlands is much higher than
in the  other two cases. This surface area-sup-
ports the attached growth microorganisms that
are believed to provide most of the treatment
responses in the system. At an apparent organic
loading of 98 Ib/ac/day (110 kg/ha/day), the rate
constant for the SSF wetland (1.104 d'1) is about
an order of magnitude higher than that for
facultative lagoons, and about  double the value
.often used for SF wetlands.
    The "t", or hydraulic residence time (HRT)
factor  in equation 4.1 can be defined as:
                t = nLWd/Q             (5.2)
where
        n = effective porosity of media (% as a
            decimal)
        L = length of bed (ft, m)  .
       W = width of bed (ft, m)
        d = average depth of liquid in bed (ft, m)
       Q = average flow through bed (ftVday,
            mVday).                        .
    The Q value in  equation 5.2 is the average
 flow in the  bed {(Qjn + Qout)/2] to account for
 precipitation, seepage, evapotranspiration, and
 other  gains  and losses of water during transit in
 the bed. This is the same value used in Darcy's
 Law for hydraulic design.
    The "d" value in the equation is the average
 depth of liquid in the bed.  If,  as recommended
 previously, the design hydraulic gradient is
 limited to 10% o'f the potential available, then
 the average depth of water in the bed will be
 equal  to 95% of the total depth of the treatment
 media in the bed.
                                    VOLUME 2: DOMESTIC WASTEWATER
                                               23

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          Since the term LW in equation 5.2 is equal to
      the surface area of the bed, rearrangement of
      terms permits the.calculation of .the 'surface area,
      '(AJ required to achieve the necessary level of
      BODS removal:
                ,        = Qln(Ce/C0)/-kTdn     (5.3)
      where
              At = bed surface area (ft2, m2)
                   other terms as defined previously.
         , The final design and sizing of the SSF bed for
      BOD5 removal is an iterative process:
      1. Determine the media type, vegetation,  and
         depth of bed to be used.
      2. By field or laboratory testing, determine the
         porosity (h)  and "effective" hydraulic conduc-
         tivity (k,) of the media to be used.
       3. Use equation 5.3 to determine the required
         surface area (A,) of the be'd for the desired
         levels of BODS removal.
      4. Depending on site topography, select a pre-
         liminary length-tp-width ratio; 0.4:1 up to 3:1
         are generally acceptable.
       5. Determine bed length (L) and width (W) for
         the previously assumed length-to-width ratio,
         and the results of step 2.  .
       6. Using .Darcy's Law. (equation 5.1) with the
         previously recommended limits (ks 
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NITROGEN

   The major pathway for nitrogen removal in
SSF wetlands is biological nitrification followed
by denitrification. The controlling factor in
ammonia removal is the availability of oxygen in
the substrate. In continually saturated beds,
leakage of oxygen from the roots of plants is the
major source of oxygen.
   Two systems  demonstrating excellent ammo-
nia removal have plant roots (and therefore some
available oxygen) throughout the profile, and
sufficient HRT to complete the reaction.  Data
from these systems suggest a two-stage system in
which the BODg is decreased to about 20 mg/L,
followed by nitrification with oxygen supplied
by the vegetation. The limiting factor-in this
case is the rate at which the plants can provide
oxygen. The extent to which plants can provide
oxygen is unknown at this time. The remaining
BODg in the second stage would then be avail-
able for denitrification.       .
   Alternative methods for nitrification include
shallow overland flow, mechanical aeration after
BODS removal, providing open water zones for
surface reaeration, and using parallel cells
operated on a batch*type fill and draw basis to
allow atmospheric oxygen to be introduced into
the substrate.
                                    VOLUME 2:  DOMESTIC WASTEWATER •                        .25

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26
                                  VOLUME 2: DOMESTIC WASTEWATER

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                                        REFERENCES
Bastian, R. K., and p. A. Hammer. 1993. The use
  of constructed wetlands for wastewater treatment
  and recycling, pp 59-68 in Constructed Wetlands
  for Water Quality Improvement. G. A. Moshiri
  (ed.). CRC Press,  Boca Raton, FL,
Brix,H. 1993. Wastewater treatment in con-
  structed wetlands: system design, removal
  processes, and treatment performance, pp 9-22
  in Constructed Wetlands for Water Quality
  Improvement, G.  A. Moshiri (ed.).  CRC Press,
  Boca Raton, FL.
Center for Environmental Resource Management.
  1993. Proceedings Subsurface Flow Constructed
  Wetlands Conference. University of Texas-El
  Paso, El Paso, TX. 306 pp.   .
Cooper, C.M., S. Testa, III, and S. S. Knight. 1993.
  Evaluation of ARS and SCS Constructed Wet-
  land/Animal Waste Treatment Project at
  Hernando, Mississippi. National Sedimentation
  Laboratory Research Report No. 2, Oxford, MS.
• 55 pp.
Crumpton, W. G., T. M. Isenhart, and S. W. Fisher.
  1993. Fate of non-point .source loads in freshwa-
  ter wetlands: results from experimental wetland
  mesocosms.  pp 238-291 in Constructed Wetlands
  for Water Quality Improvement, G. A. Moshiri
  (ed.); CRC Press,  Boca Raton, FL.
EC/EWPCA. 1990.  European.Design and Opera-
  tions Guidelines for Reed Bed Treatment Sys-
  tems. P. F. Cooper (ed.) Proceedings Interna-
  tional Conference on the Use of Constructed
  Wetlands in Water Pollution Control.  Pergamon
  Press, Oxford, U.K.
EPA (Environmental Protection Agency). 1988.
  Design Manual: Constructed Wetlands and.
  Aquatic Plant Systems "for Municipal Wastewater
  Treatment. EPA/625/1-88/022.  Center for Envi-
  ronmental Research, Cincinnati, OH.  83 pp.
EPA (Environmental Protection Agency). 1993.
  Created and Natural Wetlands for Controlling
  Nonpoint Source Pollution.  CRC Press, Boca
  Raton, FL/ 216 pp.       -

Faulkner, S. P., and C.J. Richardson.  1989. Physi-
  cal and chemical characteristics of freshwater
  wetland soils,  pp 41-72 in Constructed Wetlands '
 . for Wastewater Treatment, D. A. .Hammer (ed.).
  Lewis Publishers, Chelsea, MI.
Gambrel, R. P., and W. H. Patrick, Jr. 1978.  Chemi-
  cal and microbiological properties of anaerobic
  soils and sediments, in Plant Life in Anaerobic  •
  Environments, D. D. Hook and R. M. m. Crawford
  (eds.) Lewis Publishers, Chelsea, MI.
Hammer, D. A (ed.). 1989.  Constructed Wetlands
  for Wastewater Treatment: Municipal, Industrial
  and Agricultural. Lewis Publishers, Chelsea, MI.
  831 pp.
Hammer, D. A. 1992. Designing constructed
  wetland systems to treat agricultural nbnpoint
  source pollution. Ecological Engineering 1:49-
  82^
Hey, D. L., K. R. Barrett, and C. Biegen.  1994.  The
  hydrology of four experimental constructed
  marshes. Ecological Engineering 3:319-343.
Ives, M.  1988. Viral dynamics in artificial wet-
  lands,  pp 48-54 in G. A. Allen and  R. A.
  Gear-heart (eds.).  Proceedings of a Conference on
  Wetlands for Wastewater Treatment and Resource
  Enhancement.  August.2-4, Humboldt State
  University, Arcata, CA..
Kadlec. R. H. 1994. Detention and mixing in free
  water wetlands. Ecological'Engineering 3:345-
  380.

Kadlec, R. H., W. Bastiaens, and D. T.  Urban. 1993.
  Hydrological design of free water surface treatr
  ment wetlands, pp 77-86 in  Constructed Wet-
  lands for Water Quality Improvement, G. A.
  Moshiri (ed.). CRC Press, Boca Raton, FLf
                                     VOLUME 2: DOMESTIC WASTEWATER
                                             27

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      Knight, R.U. 1986. Florida Effluent Wetlands -
        Total Nitrogen. CH2M HILL Wetland Tech. Ref.
        Doc. Ser., No 1.
      Knight, R L., R. Wl Ruble, R. H. Kadlec, and S.C.
        Reed. 1994. Wetlands Treatment Database
        (North American Wetlands for Water Quality
        Treatment Database).  USEPA, Risk Reduction
        Engineering Laboratory, Cincinnati, OH. The
        database is available on 3.5" diskette, and
        requires DOS 3.3 or higher, 640K of memory,
        and 4MB of free disk space. To order,,contact:
        Don Brown, USEPA (MS-347), Cincinnati, OH
        45268; phone: (513) 569-7630; fax: (513) 569-
        7677; e-mail: brown.donald@epamail.epa.gov.

       Mitsch, W. j., and J. G. Gosselink.  1986.. Wet-
        lands. Van Nostrand Reinhold, New York, NY.
        539 pp. •
       Phipps, R. G., and W. G. Crumpton. 1994. Fac-
        tors affecting nitrogen loss in experimental
        wetlands with different hydrologic loads.
        Ecological Engineering 3:399-=408.
       Reed, S.C- 1993.. Design of.Subsurface Flow
        Constructed Wetlands.For Wastewater Treat-
       • ment: a Technology Assessment. .EPA 832-R-
        93-001. EPA Office of Wastewater Management,
        Washington, DC.
       Reed, S., andD.'S. Brown. 1992.'  Constructed
        wetland design - the first generation. Water
        Environment Research 64(6):776-781.
       Reed, S., F. J. Middlebrooks, and R.W. Crites.
         1994.  Natural Systems'for Waste Management
        and Treatment.  2nd ed. McGraw-Hill, New
        York.
       Steiner, G. R., and R. J. Freeman, Jr. 1989. Con-
        figuration and substrate design considerations
        for constructed wetlands wastewater treatment.
        pp 363-377 in Constructed Wetlands for Waste- .
        water Treatment, D. A. Hammer (ed.).- Lewis
        Publishers, Chelsea, MI.
Watson, J.T., and].,A-Hobson.  1989. Hydraulic
.  design considerations and control structures for
  constructed wetlands for wastewater treatment.
  pp 379-391 in Constructed Wetlands for Wastewa-
  ter Treatment, D. A. Hammer (ed.). Lewis Pub-
  lishers, Chelsea, MI.            .
Water Pollution Control Federation. 1990. Natural
  Systems for Wastewater Treatment. Manual of
  Practice FD-16, Chapter 9. Alexandria, VA.
28
                                   VOLUME 2: DOMESTIC WASTEWATER

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ABBREVIATIONS AND CONVERSION FACTORS
MULTIPLY
ac,acre
cfs, cubic foot per second
cfs, cubic foot per second
cm, centimeter
cm/sec, centimeter per second
ฐF, degree Fahrenheit
ft. foot
ft2, square foot
ft3, cubic foot
ft/mi, foot per mile
fps, foot per second .
g/mz/day, gram per square meter per day
gal, gallon
gal, gallon
gpm, gallon per minute
ha, hectare
inch
kg, kilogram
kg/ha/day, kilogram per hectare per day
kg/m2, kilogram per 'square meter
L, liter
L, liter
Ib, pound • • .
Ib/ac, pound per acre
m, meter
m2, square meter
m3, cubic meter
m3, cubic meter .
m3/ha/day, cubic meter per hectare per day
. mm, millimeter
.mi, mile
BY
0.4047
448.831 .
2.8317 X 10"2
0.3937
3.28x10-*
5/9(ฐF-32)
0.305
9.29 x 10'2
2.83 x lO'2
0.1895
18.29
8.92
3.785
3.785 x 10'3
6.308 x ID'2
2.47
. 2.54
2.205
0.892
0.2
3. 531 x 10'2
0.2642
0.4536
1.121
3.28 .
10.76
1.31
264.2
106.9
/ 3.94 xlO'2
1.609
TO -OBTAIN
ha, hectare
• gpm, gallon per minute
m3/s, cubic meter per second
inch
fps, foot per second
ฐC, degree Celsius
m, meter
m2- square meter
m3, cubic meter
m/km, meter per kilometer
m/min, meter per minute
Ib/ac/day, pound per acre per day
L; liter :
m3, cubic meter •
L/s, liter per second
ac, acre
cm, centimeter ,
Ib, pound
Ib/ac/day, pound per acre per day
lb/ft2, pound per square foot
ft3, cubic foot
gal, gallon
kg, kilogram
kg/ha, kilogram per hectare
ft, foot .
ft2, square foot
yd3, cubic yard
gallon, gal
gallon per day per acre, gpd/ac
inch
kilometer, km

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