&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
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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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S.EPA
United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
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