&EPA
United States
Environmental Protection
Agency
                           SITETechnology Capsule
                           Anaerobic Compost Constructed
                           Wetlands Technology
Abstract

As part of the Superfund Innovative Technology
Evaluation (SITE) Program, the U.S. Environmental
Protection Agency (EPA) evaluated constructed
wetlands systems (CWS)  for removing high
concentrations of zinc from mine drainage at the
Burleigh Tunnel in Silver Plume, Colorado.

Exploration geologists have known for many years
that  metals, most  commonly  copper,  iron,
manganese, uranium,  and zinc,  frequently
accumulate in swamps and  bogs located in
mineralized areas.  This understanding forms the
basis for the design of CWS—essentially excavated
pits filled with organic matter—that have been
developed and constructed over the past 15 years
to treat drainage from  abandoned coal mines in the
eastern United States.  Mine drainage is routed
through the  organic  material, where metals are
removed through a combination  of physical,
chemical, and biological processes.

In fall 1994,  anaerobic compost wetlands in both
upflow  and  downflow  configurations were
constructed adjacent to and received drainage from
the Burleigh Tunnel, which forms part of the Clear
Creek/Central City Superfund site. The systems
were operated  over a  3-year  period.  The
effectiveness of treatment by the CWS was
               evaluated by comparing the concentration of zinc
               and other metals from corresponding influent and
               effluent analyses.  By far the dominant toxic  metal
               present in the drainage was zinc. The upflow CWS
               removed an average of 93 percent of the zinc during
               the first year of operation, and 49  and 43 percent
               during the second and third years. The downflow
               CWS removed an average of 77 percent of zinc during
               the first year and 70 percent during the second year.
               (Flow was discontinued to the downflow system in
               the third year.) Complete data were published in
               the innovative technology evaluation report (ITER)
               for the evaluation and are available from EPA.

               Introduction

               The SITE Program was established in 1986 to
               accelerate the development, evaluation, and use
               of innovative technologies  that offer permanent
               cleanup alternatives for hazardous waste sites. One
               component of the SITE Program is the Demonstration
               Program, that develops engineering, performance,
               and cost data for innovative treatment technologies.
               Data developed under the SITE  Demonstration
               Program enable potential users to evaluate each
               technology's applicability to  specific waste sites.
               The Colorado Department  of Public  Health and
               Environment (CDPHE) identified passive treatment
               by wetlands as the preferred remedial alternative
               for drainage from the Burleigh Tunnel. CDPHE is
                           SUPERFUND INNOVATIVE
                           TECHNOLOGY EVALUATION
                                                             * Printed on Recycled Paper

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responsible for remediating the site and worked with
EPA's National Risk Management Research Laboratory
(NRMRL) to construct the demonstration systems and
to design the evaluation.

The  primary objectives  of the SITE Program's
evaluation of the CWS were to (1)  measure the
reduction of zinc (the dominant toxic metal)  in
Burleigh Tunnel  drainage that resulted from CWS
treatment with  respect to cell configuration and
seasonal variation  (temperature);  (2) assess the
toxicity of the  Burleigh Tunnel  drainage; (3)
characterize the reduction in toxicity that resulted
from treatment of the drainage by the CWS; and (4)
estimate the reductions in toxicity in the stream
(Clear  Creek) that receives the Burleigh Tunnel
drainage. Reductions in the concentrations of other
metals were also measured as a secondary objective
and are reported in  the ITER.

Design of Constructed Wetlands System

For this evaluation, wetlands were designed and
constructed to treat mine  drainage through  a
combination of sorption,  precipitation,  and
biological sulfate reduction.  The evaluation was
conducted on both upflow and downflow CWS cells.

Both cells consisted of an 0.05-acre cell (pit) filled
4 feet  deep with  a  mixture of an  organic-rich
compost (96 percent) and alfalfa hay (4 percent).
The  cells were installed  below grade to reduce
freezing of the  cells during winter.  The earthen
sidewalls of both cells were bermed.  The base  of
each cell was made up of  a gravel subgrade, a 16-
ounce geofabric, a  sand layer,  a clay liner, and a
high-density polyethylene liner. The base was
separated from  the influent or effluent piping by a
geonet.  A 7-ounce geofabric separated the
perforated polyvinyl chloride (PVC)  piping from the
compost.  The compost was held  in place  with a
combination of 7-ounce geofabric and a geogrid  in
the upflow cell.  The perforated  effluent piping was
also  supported  by  the geogrid in the upflow cell.
Up to 6 inches of dry substrate material was located
above  the perforated piping.   The geonet and
perforated piping ensured even distribution of the
influent water  into the treatment  cells  and
prevented short-circuiting of water through the
cells.
Short-circuting causes a decrease in residence time
and often can impair performance.  Influent and
effluent distribution piping were also staggered
horizontally as an added precaution  against short
circuiting.

The flow to the CWS cells was regulated by a series
of concrete v-notch weirs, one for the influent and
one for the effluent of each cell. The effluent weir
controlled the flow and the hydraulic residence time
of the mine drainage through both CWS cells.  Mine
drainage  entered the upflow cell under pressure at
the base of the compost and discharged out the top,
whereas  flow entered the  downflow  cell  from  the
top and  flowed  by  gravity to the bottom  for
discharge.  Each cell was designed for a  flow of 7
gallons per minute (gpm), but loss of permeability
in the downflow cell blocked flow.  The remaining
flow from the drainage was diverted to Clear Creek
(untreated) via  the  influent weir.  A drainage
collection structure was constructed within  the
Burleigh  Tunnel to build sufficient hydraulic head
to drive the flow through the two CWS.

Results of Evaluation

This  section summarizes the laboratory analytical
data  from field sampling and in-field observations
as they  relate to the primary objectives of  the
evaluation.  Definitive  removal efficiencies and
other relevant data are published  in the  ITER,
available  from EPA.

Removal Efficiencies

Results from this SITE demonstration and additional
tests of the CWS technology suggest that it is capable
of reducing the toxicity of contaminated  mine
drainage by removing metals such as zinc, cadmium,
iron,  lead, nickel, and  silver.   Data  indicate that
both  systems  initially  removed  significant
percentages  of zinc and  other metals  and that
removal efficiency decreased over  time.  Trends in
the data and results for  efficiency are discussed in
this subsection by treatment cell.

Downflow Cell

In general, the  downflow cell was effective in
removing zinc during the first year of operation. Zinc
removal by this cell ranged from 69 to 96 percent

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with a mean removal  efficiency of 77 percent.
During the second year of operation, zinc removal
ranged from 62 to 79 percent with a mean removal
efficiency of 70 percent.
During the final 6 months of operation, loss of
permeability caused by  precipitation  of metal
oxides, hydroxides, and carbonates, and subsequent
settling of fine materials in the cell, combined with
compaction of the substrate material, reduced flow
through the  downflow cell, thereby increasing the
residence time of the mine drainage in the cell. The
increased  residence time improved zinc removal.
Zinc removal during this period ranged from 67 to
93 percent with  a mean of 82 percent.

Aqueous geochemical modeling, observations of cell
compost, results of the sulfate-reducing bacteria
count,  and  acid volatile sulfide data suggest that
biological sulfate  reduction is not the primary
removal mechanism for zinc within this cell. Instead,
the primary  removal  mechanism is thought to be
precipitation of zinc oxides,  hydroxides,  and
carbonates in aerobic sections of the downflow cell.

UpflowCell

During the first  6 months of operation, effluent
samples from the upflow cell contained  low (less
than 1 milligrams per liter [mg/L]) concentrations
of zinc.  However, during the later part of  1994 and
into 1995, zinc concentrations in effluent from the
upflow cell began to increase. The concentrations
of zinc ranged from 0.13 mg/L in early 1994 to 60.1
mg/L in May 1997.

In the spring of 1995, heavy runoff overwhelmed
the CWS, channeling 20 gpm of aerobic water (nearly
three times the design flow) through the upflow cell.
This high runoff also apparently mobilized more zinc
from  the  mine  workings or mine waters and
substantially increased the concentration  of zinc in
the mine drainage. The large flows created aerobic
conditions,  and  the increased zinc loading had a
detrimental effect on the  upflow cell.  These new
conditions apparently initiated a change in the cell's
microbial ecology.  After the high flow event, the
upflow cell  removed only 43 to  49 percent of the
zinc in the mine drainage.  Before the high flow
event, the upflow cell  removed more than 90
percent of the zinc (mean removal in year 1 was 93
percent).
The loss of hydraulic conductivity in the substrate
also affected  the upflow  CWS.   During the
demonstration, the height of  the influent weir was
periodically raised to increase the hydraulic pressure
to maintain flow through the upflow CWS. The water
level was raised approximately 1 foot over the 4-
year demonstration. In 1997,  this cell developed a
visibly obvious preferential pathway in the southeast
corner,  adjacent to the  bermed  sidewalk  This
preferential pathway was eliminated by terminating
flow to this section of the wetland by excavating
the wetland substrate to allow installation of  a cap
on the influent  line.

The high initial rates of zinc removal in the upflow
cell were likely the  result of adsorption and
absorption of metals along with biological sulfate
reduction.  The  decline  in metal removal by  the
upflow cell after the high flow event is likely related
to the decline in sulfate reducing bacteria in this
cell.  There are several  possible reasons for  the
decline of the sulfate-reducing bacteria, including
toxicity to the  bacteria  produced  by high zinc
concentrations, prolonged exposure to aerobic
conditions that  allowed other wetland bacteria to
outcompete the sulfate-reducing bacteria, or  the
consumption  of all the most readily metabolized
growth materials by the sulfate reducing  bacteria,
leading to lower activity and eventually  lower
populations.  Ultimately, the primary mechanism  for
metals removal  over the last several years of the
demonstration was likely  chemical precipitation.

Toxicity of the Burleigh Tunnel Mine Drainage

Water samples from  the Burleigh Tunnel  were
evaluated by  ERA'S National Exposure Research
Laboratory for aquatic toxicity to Ceriodaphnia dubia
(water fleas)  and Pimephalus  promelas (fathead
minnows). The water was found to be toxic to both
organisms at low concentrations. The concentrations
of mine drainage resulting in death of 50 percent of
the test organisms (LC50) ranged from 0.10  to 1.0
percent for the water fleas and from  0.62 percent
to 1.6 percent for the fathead minnows  over  the
course of the evaluation.

Reduction in Toxicity from CWS Treatment

Effluent waters  from the treatment cells were also
evaluated for aquatic toxicity using the same test

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organisms previously mentioned.  Effluents from
both treatment cells were not toxic to either test
organism during the  first  8 months  of the
demonstration. Nonetheless, although both systems
were able to significantly reduce toxicity to the test
organisms, this reduction declined over the first 2
years of operation.  The  reduction in  toxicity
correlated well with increasing zinc concentrations
observed during this time frame.
Reduction in Toxicity of Clear Creek

Stream samples were evaluated for aquatic toxicity
using the same test organisms previously mentioned.
None of the samples  were  toxic to the  test
organisms,  so toxicity  reduction could not be
ascertained using  this method.

Comparison to Superfund Feasibility Study
Evaluation Criteria

Table 1 summarizes the CWS performance compared
with the Superfund feasibility study (FS) evaluation
criteria.  This table is provided to assist Superfund
decision makers in considering these technologies
for remediation at  hazardous waste sites.

Status of Technology

Several  hundred constructed and  natural wetlands
are treating coal mine drainage in the eastern United
States.  In  addition, many  constructed wetlands
designed to treat metals-contaminated  mine
drainage have been  constructed and tested, or are
being tested, by EPA, various state agencies, and
industry. The references below can be used to obtain
more information about the technology.

   Hedin, R.S., Narin, R.W., and Kleinmann, R.LR
      1994. Passive Treatment of Coal Mine Drain-
      age.  United States Bureau of Mines Informa-
      tion Circular 9389.

   Kadlec, R.H., and Knight, R.L,  1996. Treatment
      Wetlands. CRC Press. Lewis Publishers. Boca
      Raton, Florida.

   Moshiri,  G.A.   1993.  Constructed Wetlands for
      Water Quality Improvement. Lewis  Publish-
      ers.  Boca Raton, Florida.

   United States Bureau  of Mines.  1994. Proceed-
      ings  of the  International Land Reclamation
      and Mine Drainage Conference on the Abate-
      ment of Acidic Drainage.  Pittsburgh, Penn-
      sylvania, April 24-29, 1994, Bureau of Mines
      Special Publication SP 066-4.

Technology Applicability

Constructed wetlands have been demonstrated to
be effective in removing organic, metal, and nutrient
elements including nitrogen and phosphorus from
municipal wastewater,  mine drainage, industrial
effluents  and agricultural runoff. The  technology is
waste-stream specific, and requires characterization
of all organic  and inorganic constituents. CWS
designs vary considerably, and can be simple, single-
cell   systems,   or   complex  multicell   or
multicomponent systems of varying depths.

The CWS designs used in this evaluation may be
applicable as a long-term remedial technology at
Superfund sites where  acidic mine drainage  is a
problem,  as the technology is capable of treating a
range of  contaminated waters that contain heavy
metals.  Influent  waters  must be characterized,
however,  because the effectiveness of a CWS can
be reduced in waters with high pH, as precipitates
form and  clog the system prematurely. Low pH mine
drainage  can  also be a problem because sulfate-
reducing bacteria cannot  survive in low pH
environments.

Limitations

Land required for  CWS is typically extensive
compared with conventional treatment systems.
Thus,  in areas with high land values, a CWS
treatment system may  not  be  appropriate.
Availability of land relatively close to  the source of
the contaminated water is preferred to avoid
extended transport.

Climate at potential CWS sites can also be a limiting
factor.  Extended  periods of severe cold, extreme
heat, arid conditions, and frequent severe storms
resulting in high  flows or flooding  can  result in
performance  problems.  Contaminant levels in
treated and discharged water can vary in response
to variations of influent volumes and chemistry. This
variation  may  also be a  limiting factor if there is no
tolerance in discharge requirements for contaminant
levels.

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Table 1. Evaluation of CWS Treatment Compared to Feasibility Study Criteria
   Criterion
Discussion
    1.  Overall  Protection  of  Human  Health  and  the
       Environment
As  tested, the  constructed  wetlands system  (CWS)
provides  only  short-term effectiveness.   In  different
circumstances, the CWS may provide short- and long-term
protection by removing mine drainage contaminants.

Substrate  is  a  recycled  product,  not  mined   or
manufactured.
   2.  Compliance  with  Applicable  or   Relevant  and
       Appropriate Requirements (ARAR)
   3.  Long-term Effectiveness and Permanence
   4.  Short-term Effectiveness
Wetland effluent discharge may require compliance with
Clean Water Act regulations.

Substrate disposal may require compliance with Resource
Conservation and Recovery Act regulations.

CWS  treatment   removes  contamination  from  mine
drainage but may not meet discharge requirements.

Use of CWS  treatment with other technologies may be
effective in meeting low-level discharge requirements.

Presents fewshort-term risks to workers, community, and
wildlife.
   5.  Reduction of Toxicity, Mobility, or Volume Through
       Treatment
Minimal  personal protective  equipment  required  for
operators.

CWS treatment reduces contaminant mobility, toxicity,
and volume.
   6.  Implementability
   7.  Cost
   8.  Community Acceptance
   9.  State Acceptance
Generally a passive treatment system, but can be active.

Construction uses standard materials and practices within
the industry.

Construction cost of a full-scale system (50 gallons per
minute) is estimated at approximately $290,000.

Operation  and  maintenance of  a  full-scale  CWS  is
estimated to be $57,000 per year.

The  public usually views the technology as a natural
approach to treatment;  therefore, the public generally
accepts this technology.

The Colorado Department of  Public Health and the
Environment (CDPHE) found the technology shows
promise for treating acid mine drainage.  Based  on the
cold climate and proximity to town, however, CDPHE
recommended not implementing a full-scale permanent
system at the Burleigh location.

The Colorado Division of Minerals has built several CWSs
to treat mine drainage.

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Sources of Further Information

Definitive data from the evaluation are published
in the ITER.

Further details regarding CWS are available in the
literature and from the EPA NRMRL work assignment
manager:

Edward Bates
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Office of Research and Development
26 West Martin Luther King Drive
Cincinnati, Ohio 45268
e-mail: bates.edward@epa.gov
Phone:(513)569-7774.

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