(developed by COLORADO SCHOOL OF MINES)
(Constructed Wetlands-Based Treatment)
The constructed wetlands-based treatment technology uses natural geochemical and microbiological processes inherent in an artificial wetland ecosystem to accumulate and remove metals from influent waters. The treatment system incorporates principal ecosystem components found in wetlands, such as organic materials (substrate), microbial fauna, and algae.
Influent waters with high metal concentrations flow through the aerobic and anaerobic zones of the wetland ecosystem. Metals are removed by ion exchange, adsorption, absorption, and precipitation through geochemical and microbial oxidation and reduction. Ion exchange occurs as metals in the water contact humic or other organic substances in the soil medium. Oxidation and reduction reactions that occur in the aerobic and anaerobic zones, respectively, precipitate metals as hydroxides and sulfides. Precipitated and adsorbed metals settle in quiescent ponds or are filtered out as the water percolates through the soil or substrate.
The constructed wetlands-based treatment process is suitable for acid mine drainage from metal or coal mining activities. These wastes typically contain high metals concentrations and low pH. Wetlands treatment has been applied with some success to wastewater in the eastern United States. The process may have to be adjusted to account for differences in geology, terrain, trace metal composition, and climate in the metal mining regions of the western United States.
Based on the SITE Emerging Technology Program (ETP) results, the constructed wetlands-based treatment process was selected for the SITE Demonstration Program in 1991. For further information on the ETP evaluation, refer to the Emerging Technology Summary (EPA/540/SR-93/523), the Emerging Technology Report (EPA/540/R-93/523), or the Emerging Technology Bulletin (EPA/540/F-92/001), which are available from EPA.
Studies under the Demonstration Program are evaluating process effectiveness, toxicity reduction, and biogeochemical processes at the Burleigh Tunnel, near Silver Plume, Colorado. Treatment of mine discharge from the Burleigh Tunnel is part of the remedy for the Clear Creek/Central City Superfund site. Construction of a pilot-scale treatment system began in summer 1993 and was completed in November 1993.
The pilot-scale treatment system is about 4,200 square feet and consists of an upflow cell (see figure on previous page) and a downflow cell. Each cell treats about 7 gallons per minute of flow. Preliminary results indicate high removal efficiency (between 80 to 90 percent) for zinc, the primary contaminant in the discharge during summer operation. Zinc removal during the first winter of operation ranged from 60 to 80 percent. The demonstration is scheduled to run through the summer of 1997.
In 1994, removal efficiency of dissolved zinc for the upflow cell between March and September remained above 90 percent; however, the removal efficiency between September and December declined to 84 percent due to the reduction in microbial activity in the winter months. The downflow cell removal efficiency dropped to 68 percent in the winter months and was between 70 and 80 percent during the summer months. The 1995 removal efficiency of dissolved zinc for the upflow cell declined from 84 percent to below 50 percent due to substrate hydrologic problems originating from attempts to insulate this unit during the winter months. The 1995 removal efficiency of the downflow cell declined from 80 percent during the summer months to 63 percent during the winter again as a result of reduced microbial activity. The 1996 removal efficiency of dissolved zinc calculated for the downflow cell increased from a January low of 63 percent to over 95 percent from May through August. The reason for the increase in the downflow removal efficiency is related to reduced flow rates through the downflow substrate which translates to increased residence time.
EPA PROJECT MANAGER:
Edward Bates
U.S. EPA
National Risk Management Research
Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7774
Fax: 513-569-7676
TECHNOLOGY DEVELOPER CONTACT:
James Lewis
Colorado Department of Health
4300 Cherry Creek Drive South
HMWMD-RP-B2
Denver, CO 80220-1530
303-692-3383
Fax: 303-759-5355