IN THE UNITED STATES

      Sherwood C. Reed*  and Donald- S. Brown**
        Environmental Engineering Consultants
      ^ RR I, Box 572,  Norwich, VT  05055
        Risk Reduction Engineering Laboratory
        U.S. Environmental Protection Agency
               Cincinnati, OH  45268
              Contract No. 68-CO-0027
              Work Assignment Manager

                  Donald S. Brown
    Water and Hazardous Waste Research  Division
       Risk Reduction Engineering Laboratory
               Cincinnati, OH  452S8
               CINCINNATI, OH  45268
To Be Presented  at the IAWPRC Specialist Conference
   "Wetland Systems  in Water Pollution Control"
     Sydney,  Australia, Nov. 30 - Dec. 3, 1992


The work described in this paper has been funded by the United
States Environmental Protection Agency.;through the Agency's Risk
Reduction Engineering Laboratory, Cincinnati, Ohio. - However, this
paper has not been subject to the Agency's review and therefore
does not necessarily reflect the views of the Agency, and no
official endorsement should be inferred. ;<-    >-     :



                          by: Sherwood C. Reed, P. E..         '

                       .  Environmental Engineering Consultants
                         RR  1, Box 572

                         Norwich, VT 05055  U.S.A.

                         Donald Brown

                         US  EPA RREL

                         Cincinnati, OH 45268 U.S.A.

iim* Hec!r?] hundn:d 9rave1 bed. °r subsurface flow (SF) wetland systems exist in the
United States ranging in size from single family dwellings to municipal systems

tS npr? FIJI0!8 S0J° n'°°° m /CL Most of these systems have been constructed in
the period 1988 to 1992 without the benefit of a consensus on design, construction Sr

operational procedures. The U.S. Environmental Protection Agency (EPA) commence I a

continuing series of studies in 1989 to identify the critical issues and to dewlSp

S^r^iE0:?8 ,?rUeH1a fy th1S C0ncept' These efforts have incl"d<* a defied
S yi<            tS and Performance evaluations, and special  data collection and
evaluation programs at selected sites. Two of these special studies were ' coajlitod

             mmer     9?1f and tw° are Presently underway. This  paper 1s based  n the
     The focus ^of the paper is on the capability of these  systems to  remove  biochemical

  nnth6mand (B?S5)l t0tal susPended solids CTSS), and ammonia nitrogen ^(NH ,1s N^
since these are the major water quality parameters controlled by the  regulatory

"                    A'0'31 °f U fUl1 scal« ^rational systems  we re^IeS to develop
                    .data so"rces are not Identified in the graphical presentations  but

                                                                     *        ™
                  Table 1. Data Sources for Performance Evaluation
Green leaves, LA
Degussa Co., MS
Bear Creek, AL
Monterey, VA
Denham Springs, LA
Benton, LA
Haughton, LA
Carvllle, LA
Mandeville, LA
Benton, KY
Hardln, KY*
Hardln, KY*
Utica, MS8
Utica. MS"
Design Flow
. 564
Treatment. Area
. oy
• ell
1 AR
* Phragmites bed, * Scirpus bed, 8 North system,  Q  South system


BOD Removal                                 ^

     Input versus output BOD5 data for the  14  systems listed in Table 1 are shown 1n
Figure 1. All effluent values are well below the typical 20 mg/L effluent standard and
this has been achieved regardless of  the  Input concentration (within the range sihown)
The low values 1n the lower  left corner of  the graph Illustrate a minor limitation of'
these systems. These systems will typically export an effluent BOD in the range of 5
mg/L due to decomposition of natural  organic materials in the system.
                   +  +
               +     +
          1O     2O    30    40    60    60

                 »OO M>UT


* •
• .
• •
1 * '
	 1 	 1 	 1 	 I 	 1 	 | 	 t 	
> 1 2 3 4 5 6 7 t>
           Fig.  1  BOD5  input  vs  output
                                                     Fig. 2 BOD removal vsi HRT
     Figure 2 presents the removal of BOD versus  the  actual  hydraulic residence time
(HRT) in these systems. The  removal  of  BOD  1s strongly dependent on HRT up to about 1 d
but improves only  slightly thereafter,  up to an HRT of 7.5 d.  The 60 to 65 percent
removals at about  1 d HRT are  not  due to ineffective  removal capability but rather to
relatively low input levels.

     It has been suggested that these SF wetland  systems  should  be constructed with a
high aspect ratio  (L:W) to insure  plug  flow conditions and high  levels of performance
Figure 3 tests that hypothesis with  L:W from less than 2:1 to over 17:1. As
demonstrated by the figure there does not appear to be any relationship between aspect
ratio and BOD removal. The very low  removal associated with the  highest L:W in the plot
is due to the very low BOD input-at  this-system (< 5  mg/L) and not due to any
relationship with  aspect ratio.

     Figure 4 illustrates the  relationship  between mass organic  loading rates and mass
removal rates in SF constructed wetlands. A linear relationship  is confirmed by the
relatively high r£ value.








A    V
              e   a  10   12  14

                ASPECT RATIO  LlW
                         16   18  20
                                              O  10 20 30 40 SO 00 70 80 90 100110 (20130140 ISO

                                                       MO UU* LCMDMft
       Fig 3.  Bod Removal  vs Aspect Ratio
                                                Fig 4.  BOD removal vs BOD loading


     A first order plug flow model for BOD removal has Seen suggested for design of
these systems in Australia, Europe and the U.S. (Bavor, H.J. et al, 1988, Reed, 1933
Boon, A.G. 1985, US EPA, 1988, WPCF, 1990, Conley, L.M., et al.,^1991).

     A plug flow rate constant was calculated for each of the systems listed in Table
1. These results are plotted versus organic loading on Figure 5. The relationship
between the rate constant and the organic loading has an r * of 0.95 indicating an
excellent correlation.

     For comparative purposes, the same relationship for facultative lagoons, as
derived by Neel, et al (1961) is also shown on Figure 5. Preliminary work with data
from free water surface wetlands (FWS) indicates a similar relationship with the curve
fall ng about midway between facultative lagoons and SF wetlands. An extension of this
Sit !Tfy analys1^ locates that an FWS wetland might be up to 75% larger than an SF
wetland for comparable flow and BOD removal goals. The choice between the two concepts
may then depend on the availability and cost of land and the cost for the SF media in
tne local area.
wPti«iL1S btl1e^d. ^at the rate constant for SF wetlands is higher than that for FWS
wetlands or facultative lagoons because the media 1n the SF wetland provides more
specific surface area and opportunity for retention of the organisms which contribute
the biological treatment responses. In open water, continuous flow reactors  1t 1s
usually necessary _to provide for sludge recycle and a high "sludge age" to obtain

Z i



                                              FACULTATIVE LAGOON
                  O      20     40     60     80     100    12O    14O     16O

                                   ORGANIC LOADING (Kg/ha/d)

              Fig. 5 Plug Flow Rate Constant vs Organic Loading

comparable reaction rates.  The relationship for SF wetlands  shown on Figure 5 suggests
that these systems can be reliably designed for organic loadings  up to at least 1IDO
 H( u u  Tt!ls prov1des support for use of a rate constant in  the  neighborhood of l.o  d~1
which has been used in the plug flow models mentioned above.

     The relationship shown on Figure 5 also suggests that many of the existing-systems
are larger than they need to be for effective BOD removal. This has probably occurred
because the designers made conservative estimates for input BOD and the  actual input
BOD has been significantly less.                                                 '


                           results or a tracer  study,  using  lithium  chloride, ^oriducted
                                              m -vcrrm i c ,  ;ur> .      '
                                                                           caci i CTn
the four such studies conducted  under the  EP& program,  Issentially 100 percent of the
tracer was accounted for  1n the  effluent so it can be considered a valid study. The
centrold of the curve at  48 hr  1s  Identical to the theoretical  HRT for the system. It
clearly does not exhibit  ideal  plug  flow responses but is similar to curves for "lagoons
and other  reactors designed with plug flow kinetics.  The plug flow model seems to give
reasonably accurate estimates of system performance.  As additional data is obtained the
use of alternative models such  as  a  series of continuously stirred reactors  may be
adopted, but the plug flow model is  likely to continue 1n use for the near term.
                   HAK AT »« HR
                         cannon. HRT » 41 m

                            100* u Btcovm

               40     60

                  TKE fexn)
                           —I	•	1—
                           80    100
                                                                               2O ing/L

                                                       -J	1	1_
          Fig  6. Lithium Tracer Study
                                                       10  20  30  40  50 60  70  60  90 .00 11O 120
                                                             «u»w»»eo MUM HHIT  50 mg/L) have facultative lagoons for preliminary
treatment and the high solids  are due to algal carry-over from the lagoon. The;'removal
and decomposition of these solids in  the SF  bed  has an impact  on the  NH,  status 1n the
system  as discussed  in the next section of this  paper.

     The relationship  between  TSS removal and HRT in the system is similar to the BOD ''
results shown on Figure 2. After  about 1 day HRT there is little significant   *
Improvement. The relationship  between TSS removal and  aspect ratio is also similar to
the BOD results shown  on  Figure 3,  indicating that  there is no correlation between
system  aspect ratio  and TSS removal.

     Concerns have been expressed over the potential for gradual clogging of  these beds
with TSS or dead root/rhizome  material. The  special EPA studies have  excavated  pits in
four systems in Louisiana ranging in  age from 2  to  5 years and minimal clogging has
been observed in all cases. In three  cases,  the  solids represent less than two  percent
of the  available void  spaces;  1n  the  worst case  the solids approached six percent of
the available void spaces. In  all cases, these trapped solids  were composed of  at least
80 percent Inorganic matter. .Discussions with the operators and review of the
construction histories Indicate that  these materials may have  been delivered  to the
site during construction  either as  fines with the media or as  soil on the tires of the
trucks  and other equipment, therefore,- further accumulation may be minimal

     The surface flow  which can be  observed  on many of these systems  has  been
attributed to clogging of the  media.  A more  likely  explanation is provision of  an
Inadequate hydraulic gradient  in  the  hydraulic design  of the system.  Many of  these
early systems had L:W  ratios approaching 10:1, had  a flat bottom on the bed,  and had
the outlet ports in  the effluent  manifold near the  top of the  bed.


   The ma^or concern is the removal of unoxidized  ammonia (NH3) to               _




,4 *
i (I
' » *

a POJ^TS eeLow -100 «)
           Fig 8.  NH3 Input vs Output

Fig 9. NH3 Removal vs HRT
oxygen to oxidize this ammonia to nitrate. Support for this hypothesis is shown on
Figure 10 which presents ammonia removal versus HRT,

     Most c  the systems shown on Figure 9 display a marginal or a negative ammonia
removal rr  regardless of detention time. Two of the systems, shown as open squares  in
thlfigure   .splay veVy high removal rates at comparable HRT levels. The difference for
these ?wo - ata sets is that the root zone was fully developed in the bed so thai  plant
available   '.ygen could support nitrification. In one case (Bear Creek, AL) the bed is
only 03m  .eep and supports a stand of  Typha, with the roots penetrating to th.»  bottom
 f the'fine  gravel bed (Watson, 1990). Ammonia removal at this system  reaches  80
percent with an HRT of 3.9 d. The second case is the Sdrpus bed at the Santee, CA
pilot system, where the roots also penetrated to the bottom  of the 0.76 m bed  and
ammonia  removals of 94 percent were achieved with an HRT of  7d with primary treated
wastewater as  Input (Qersberg, et al, 1985).

     Many of the early investigators and designers assumed that the plant roots would
always penetrate to their full potential  depth so the entire depth  of  the bed  would  be
an  active  root  zone with at  least some  oxygen available. The recent EPA  studies
 indicate that  root/rhizome penetration  is limited to about 0.3 m  regardless  of the
plant  species  used. Deeper roots can  be found ne.ir  the edges of the bed  and  other  dead
soots" for flow but in  general the  active root zone is 0.3 m or  less in  most systems in
the u S  As a  result, a significant portion  of the  flow at these  systems never comes in
contact  with the root zone and  has  no opportunity to utilize the  available  oxygen for
 nitrification.  At  the Bear Creek and  Santee  systems shown on Figure 9 all  of the flow
 comes in contact with the  root  zone and there is sufficient  residence time  to complete
 the nitrification  reactions.  There is  no consensus on how much oxygen might be
 available at the plant  roots to support nitrification.  Estimates range from zero to
 about 45 gm 09/m2/d  (Cooper, et a!  1990).  Based on the  experience at Santee and at the
 Bear Creek site it seems likely that  some plant  produced oxygen is available  in the
 root zone, on the surfaces of the root hairs.  It can be  calculated from the ammonia
 removal  data at these sites that the available oxygen is about 7.5 gm/m /d in the
 active root zone for Typha,  Scirpus,  and Phragmites. Assuming a 0.6 m deep active root
 zone this would translate to 4.5 gm 02/m2/d of wetland surflcial area, which  1s; near


  the low end of  available oxygen values rin t&e ,literatuirea^u> ""                '.     -

        Based on  the work at Santee and in Europe the maximum potential  root zone depths
  are: Typha 0.3  m,  Scirpus 0.8 m, Phragmites 0.6 m.  Using these values and the 7.5
  g/m-Vd oxygen availability, it is possible to show that nitrification  to low levels  of
  ammonia ($ 2 mg/L) will require at least 5 to 6 days residence time in the system
  during the warm weather growing seasoii.

      Other models are available for predicting ammonia removal (Bavor,  1988, WPCF,  1990)
 in SF constructed wetlands, but these predict  even larger land areas  than the previous
 case.  The reason is believed to be that these models were derived  from systems which
 were generally deficient in oxygen and do not  reflect the potential nitrification if  the
 root zone is fully developed.  These models may adequately describe the performance of
 the present generation of systems in the U.S.  which tend to have short  detention times
 and inadequate root zone development.

     However, the costs of  the  land and  the media for the SF  bed  in a system with six
                                                 basis te
            i?t unsa*urated vertical flow reactors  for nitrification such as
    o      +    I   a"d/eci Bating sand filters has been common practice  in
wastewater treatment for a number of years. The use of vertical flow wetland eel 'Is
been demonstrated  and discussed  in the literature (Cooper, it al 1990^

     A vertical flow recirculating filter composed  of fine gravel  has been desianed for
                         "     IC§ntUCky WMch W3S  haVln* Prob^eL Beting the 9'
             lS-S^" ^ IC§ntUCky WMch W3S haVln* Probe     eting the
fS^1^iSr4SJ^F^^rS^  2£ 5^5 $3 twos70seKt? S
high hydraulic conductivity.  Based on nitrification experience with other attach S
growth processes it appears  that  about  1230 m* of this secific surfa
                                                              surace area
required to oxidize 1 kg of ammonia nitrogen per day with a possible recycle ratio of
up to 3:1.  The  depth of the bed to be used is 0.6 m!  The ni?r1? ca?ion bed Jm L
superimposed on top of the existing SF. constructed, wet! and? at the head of thllll
                                     .            ,                     o
  ltr  rWi }l ^Z  the  T irculated ^fluent from the SF cell to the ?op of S
nnt^C; I   nitrified  percolate  is expected to drain through the media and mix with the
untreated wastewater in the bed. Denitrification of the nitrate 1s then expected £
occur. This system was under construction 1n April  1992 and long term data

ar0a This.reci rc"!atl°n exponent combined with a  normal SF bed should  need less land
for nJ&JEJtS  T^3" a SySt!m dSSl9ned for total  reliance °» Plant  amiable oJygen
h?L  ow t   I  ":    1S approach may be tne ««»t cost effective method  for ach eving
high levels of nitrogen removal 1n these constructed  wetland systems.      acrnevin9
Phosphorus Removal
     Phosphorus  removal was not consistent in the systems Included  in this studv  Tn

associated with  the gravel may have provided the adsorption sites needed for phosphorus



 Role of Vegetation
           °" *****  Jjvest1gat1ons described  in  this  paper  it appears the major

               r?h ?6  r9f ati?"  1n these SF systems 1s Serv1ce of the root/rhizome
               ?^strate for microbial  activity and as a  limited oxygen source for
               I   I suggests that  if the plant is expected to  play a major role the

           it 5?      ld  ?0t  6XCeed the  P°tential  ro°t development for the plant species
ma ntn'rnot  ™  f9?ests £hat a  management plan will be necessary to induce and
maintain  root  penetrations below the 0.3 m depth commonly found in most systems in the

in thIhtonth«rn°M  I?"*1™ harvesting is  somewhat  controversial in the U.S.  Some systems

       if  £ ?h2 U'f; °?ndUCt a  rout1ne  annua1  harvest regardless of the plant species
       ial r^l   ± L°Plni°n that SUch a program is not necessary when plants such
           Ai2S     ? Phragmites  are used,  but may be necessary when soft tissue
           plants are the dominant  species.
Preliminary Treatment Requirement
«„«+ S0m6 f°*m °f .Preljminary treatment  is necessary for these SF wetland systems  The
most common form  in the U.S.  is  facultative  lagoons since in many cases the wetland

XsTre sunablf for* IHK^^99^ ** *  P°11Shin3 ^ep  Se^HanK S'Soff
J™1 *    suitable for small  to  moderately sized systems. Larger size systems may need

SfTuInt?    m6ChaniCal Pre11min^y  treatment but only to the equivalent of pMmary
     Ihe C^ °J these systems are Presented in other papers at this conference   and

h ah!er? iR6ed> S>C" S- ?r°Wn'  1992)' The^°'- i^ue of- concern Is^SirolatlveW
high cost to procure and place the media  in the SF wetland bed. This one factor  III

s^TSc50 I? 62 PefCent °f the t0tal Construction costs. A cost "caparison between
mod3^   ,Hetcand SYStemS ^"^ Depend- on -the- cost -of the land-and on the cost of ?he
media for the SF concept. Even though the FWS system is likely to require a larger land

cos?                 wh1^ ^cept will be the more cost effective became of SIse two
     The SF concept offers other advantages over the FWS alternative in that the
subsurface flow provides positive control over odors and insec? vectors and  leslens
public access concerns. These factors are particularly important when systems  are  to be
located adjacent to habitations or at public facilities.              systems  ars  to be
                                     suited for smal1 to moderate
U S  lTrioia^TalJn m°St °^ th6 present seneration of operating SF systems in the
U.S. is deficient. The reason is believed to be the short detention time  and the lack
of oxygen in the bed profile to support nitrification.


_.H^!ggi^sggm to be avaiiabletojn^ucgjiand maintain root zone development and the
     Xy9e  source:~"™rs WTTT require •wa^erneWrmsflaggBiarc'^ the bed utffff^
 n            ,                                                      e  e  u
adjustable outlet mechanism, A system with "a fully developed root zone might stni
require up to six days to produce  low levels of ammonia.

     The surface flow observed on many systems in the U.S. is believed due  to
inadequate hydraulic design and not to clogging of the bed. The use of an adjustable
outlet mechanism should correct these problems.

     A recirculating nitrification filter in combination with an SF wetland seems to
offer promise as a cost effective method for achieving effective nitrogen removal in
these systems.

up JllvelTrfaf lEt'U'TSlSE'" d1SPUyS a I1near "'""-IP to  mass  ,oad.ng

     A first order plug flow kinetic model seems to provide a reasonably accurate
estimate of BOD removal capability in SF constructed wetlands.


 Bavor, H.J. et al. (1988). Joint  Study on Sewage Treatment Using Shallow
Lagoon - Aquatic Plant Systems, Water Research Laboratory, Hawkesbury
Agricultural College, Richmond, NSW, Australia.
                                          for yaste Ha"S9eKnt ' Treatment

Boon, A.6. (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 Center, Stevenage, England.

US EPA, (1988) Design Manual - Constructed Wetlands and Aquatic Plant
Systems for Municipal Wastewater  Treatment,, EPA 625/11-88/022, US EPA CERI
Cincinnati , OH.                                                           . '

WPCF, (1990) Natural Systems for  Wastewater Treatment, ! Manual  of Practice
FD-16, Water Pollution Control Federation, Alexandria, VA.

Conley, L.M., et al. (1991) An Assessment of the Root Zone Method of
Wastewater Treatment, Jour. WPCF  (63)3,239-247.

Neel  J.K., et al (1961). Experimental Lagooning of Raw Sewage, Jour.  Water
Pollution Control Fed., 33(6)603-641.
 •«-»  i'l' i1"°l %LS.1£" and Performance of the Constructed Wetland Wastewater
Treatment System at Phillips High School, Bear Creek, AL, TVA/WR/WQ-90/5 , TVA
Chattanooga, TN.                  •                                           *

Gersberg, R.M. ,  et al, (1985). Role of Aquatic Plants in Wastewater
Treatment by Artificial Wetlands, Water Research, 20:363-367.

Cooper, P.P.,. Find later, B.C., Editors, (1990) Constructed Wetlands in  •
Water Pollution Control, Pergamon Press, New York, NY.
Reed, S.C., D.  Brown (1992). Constructed Wetland Design - The First
Generation, Jour.  WEF, (in press).