Description:

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 reciprocating cell, 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.

Synonyms:

Applicability:

Limitations:

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 down contaminants. 
• A heavy flow of incoming water can overload the removal mechanisms in a wetland, while a dry spell can damage plants and severely limit wetland function.

Data Needs:

Performance Data:

Study 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 water.

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.

Cost:

References:

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4,  June, 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

Constructed 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/540/R-93/523.

EPA, 1995. Colorado School of Mines profile in the Emerging Technology Program.

Lefave, J.P., 1997. Constructed Wetlands for Treatment of NPS Pollution, NFESC.

• Burleigh Tunnel Silver Plume, CO
• EPA Demo Burleigh Tunnel, CO
• Big Five Tunnel, CO
• Bear Creek, Oak Ridge, TN
• Naval Amphibious Base in Little Creek, Virginia

General FRTR Agency Contacts

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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 their applications.

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