Typical Constructed Wetlands System
Although the technology incorporates principal components of wetland
ecosystems; including organic soils, microbial fauna, algae, and vascular
microbial activity is responsible for most of the remediation.
Influent waters with
high metal concentrations and low pH flow through the aerobic and anaerobic zones of the
wetland ecosystem. Metals are removed through ion exchange, adsorption, absorption, and
precipitation with geochemical and microbial oxidation and reduction. Ion exchange occurs
as metals in the water contact humic or other organic substances in the wetland. Wetlands
constructed for this purpose often have little or no soil instead they have straw, manure
or compost. Oxidation and reduction reactions catalyzed by bacteria that occur in the
aerobic and anaerobic zones, respectively, play a major role in precipitating metals as
hydroxides and sulfides. Precipitated and adsorbed metals settle in quiescent ponds or are
filtered out as water percolates through the medium or the plants.
Influent water with explosive residues or other contaminants flows through and beneath
the gravel surface of a gravel-based wetland. The wetland, using emergent plants, is a
coupled anaerobic-aerobic system. The anaerobic cell uses plants in concert with natural
microbes to degrade the contaminant. The aerobic, also known as the
further improves water quality through continued exposure to the plants and the movement
of water between cell compartments.
Wetland treatment is a long-term technology intended to operate continously for years.
Constructed wetlands have most commonly been used in wastewater treatment
for controlling organic matter; nutrients, such as nitrogen and phosphorus; and suspended
sediments. The wetlands process is also suitable for controlling trace metals, and other
toxic materials. Additionally, the treatment has been used to treat acid mine drainage
generated by metal or coal mining activities. These wastes typically contain high metals
concentrations and are acidic. The process can be adapted to treat neutral and basic
The wetlands remediation technology must be adjusted to account for
differences in geology, terrain, trace metal composition, and climate in the metal mining
regions of the western United States.
The following factors may limit the applicability
and effectiveness of the process:
- The long-term effectiveness of constructed wetlands is not well known. Wetland aging may
be a problem which may contribute to a decrease in contaminant removal rates over time.
- The cost of building an artificial wetland varies considerably from project and may not
be financially viable for many sites.
- Temperature and fluctuations in flow affect wetland function and can
cause a wetland to display inconsistent contaminant removal rates.
- Colder conditions slow the rate at which the wetland is able break
- A heavy flow of incoming water can overload the removal mechanisms in
a wetland, while a dry spell can damage plants and severely limit
A detailed discussion of data elements is provided in Subsection 2.2.2 (Data Requirements for Ground Water,
Surface Water, and Leachate).
This technology was accepted into the Emerging Technology Program in 1988;
the project was completed in 1991. The purpose of the project was to build, operate,
monitor, and assess the effectiveness of a constructed wetlands in treating a portion of
acid mine drainage from the Big Five Tunnel site near Idaho Springs, Colorado. The Final
Report (EPA/540/R-93/523) is available from NTIS (Order No. PB93-233914). The Summary
(EPA/540/SR-93/523) and Bulletin (EPA/540/F-92/001) are available from EPA.
results indicated that heavy metal removal efficiency can approach the removal efficiency
of chemical precipitation treatment plants. Some of the optimum results from the 3 years
of operation are listed below.
- pH was raised from 2.9 to 6.5.
- Dissolved aluminum, cadmium, chromium, copper, and zinc concentrations were reduced by
99 percent or more.
- Iron was reduced by 99 percent.
- Lead was reduced by 94 percent or more.
- Nickel was reduced by 84 percent or more.
- Manganese removal was relatively low, with reduction between 9 and 44 percent.
- Biotoxicity to fathead minnows and water fleas was reduced by factors of 4 to 20.
Because wetland removal processes are primarily microbial, the technology can be
developed with traditional process engineering approaches. Laboratory studies can indicate
whether remediation is possible, while bench-scale experiments can determine the proper
loading and reactor design. Using this approach, five laboratory proof-of-principle
studies and three bench-scale studies have been performed, and at least four successful
demonstration reactors have been built to remove heavy metals from different types of
A final project goal was to develop a manual that discusses design and operating
criteria for constructing a full-scale wetland to treat acid mine discharges. The
"Wetland Designs for Mining Operations" manual is available from the National
Technical Information Service.
Based on the results from the SITE Emerging Technology Program, this technology was
selected to participate in the SITE Demonstration Technology Program. Under the
Demonstration Program, EPA is evaluating the effectiveness of biogeochemical processes at
the Burleigh Tunnel mine site, near Silver Plume, Colorado. Treatment of Burleigh Tunnel
discharge is part of the remedy for the Clear Creek/Central City Superfund site.
Construction of a pilot-scale system began in summer 1993 and was completed in October
1993. For more information on this project, refer to the Colorado Department of Health
profile in the Demonstration Program section (ongoing projects).
The USAEC is demonstrating a gravel based wetland system at Milan Army Ammunition Plant
through the ESTCP. The gravel-based system has been effective at degrading TNT and RDX,
with a total nitrobody concentration of 10,000 ppb. Analyses indicate degradation due to
the rise and fall of daughter products. TNT is reduced to less than 2 ppb. The
demonstration had been operational since June 1996.
Studies at Milan Army Ammunition Plant conducted by USAEC and Tennessee
Valley Authority (TVA) indicate that plants with nitroreductase, in concert with microbes,
can degrade explosive residues. It is estimated that amortizing the capital costs of
wetland treatment over a 10 year period results in a cost of $1.36/Kgal; over a 30 years
period, the cost is $0.45/Kgal.
Technologies: Field Scale Demonstration Project in North America,
of Remediation Case Studies, Volume 4, June, 2000, EPA
Guide to Documenting and Managing Cost and Performance Information for
Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007
Wetlands: Passive Systems for Wastewater Treatment, August
2001, Technology Status
Report prepared for the US EPA Technology Innovation Office
EPA, 1993. Constructed Wetlands-Based Treatment,
EPA, 1995. Colorado School of Mines profile in the
Emerging Technology Program.
Lefave, J.P., 1997. Constructed Wetlands for Treatment of NPS Pollution,
Points of Contact:
General FRTR Agency Contacts
Technology Specific Web Sites:
Government Web Sites
Non Government Web Sites
A list of vendors offering
Ex Situ Biological Water Treatment is available from EPA
REACH IT which combines information from three established EPA databases,
the Vendor Information System for Innovative Treatment Technologies (VISITT),
the Vendor Field Analytical and Characterization Technologies System (Vendor
FACTS), and the Innovative Treatment Technologies (ITT), to give users access to
comprehensive information about treatment and characterization technologies and
Health and Safety:
To be added