WETLAND TREATMENT FOR TRACE-METAL REMOVAL FROM MINE DRAINAGE - THE IMPORTANCE OF AEROBIC AND ANAEROBIC PROCESSES

When designing wetland treatment systems for trace metal removal, both aerobic and anaerobic processes can be incorporated into the final design. Aerobic processes such as adsorption and ion exchange can successfully treat neutral drainage in overland flow systems. Acid drainage can be treated in anaerobic systems as a result of sulfate reduction processes which neutralize pH and precipitate metals. Test work on both aerobic and anaerobic systems has been conducted in Minnesota. For the past three years, overland flow test systems have successfully removed copper, cobalt, nickel and zinc from neutral mine drainage. Nickel, which is the major contaminant, has been reduced around 90 percent from 2 mg/L to 0.2 mg/L. A sulfate reduction system has successfully treated acid mine drainage for two years, increasing pH from 5 to over 7 and reducing concentrations of all metals by over 90 percent. Important factors to consider when designing wetlands to remove trace metals include not only the type of wetland required but also the size of the system and the residence time needed to achieve the water quality standards.