Sulfate and metal removal in bioreactors treating acid mine drainage dominated with iron and aluminum

Bioreactors represent an emerging technology for removing metals and sulfate commonly found in acid mine drainage (AMD). Six continuously fed anaerobic bioreactors employing organic and alkaline waste materials were operated to investigate relationships between metal and sulfate removal from AMD. Median AMD influent chemistry was 65.8 mg/L Fe (49.7-113 mg/L), 46.5 mg/L Al (33.5-72.4 mg/L) and 608 mg/L sulfate (493-1007 mg/L). Bioreactors containing mussel shells as an alkaline substrate amendment were more effective at removing metals and sulfate than those containing limestone. Experimental results indicated bioreactor design and operation should be dependent on treatment goals. These include 0.3 mol sulfate loading/m³/day for sulfate removal (mean of 94.1% (87.6-98.0%), 0.4 mol metals/m³/day for metal (mean of 99.0% (98.5-99.9%)) and partial sulfate (mean of 46.0% (39.6-57.8%)) removal and 0.8 mol metals/m³/day for metal (mean of 98.4% (98.2-98.6%) and minimal sulfate (mean of 16.6% (11.9-19.2%)) removal. Aluminum removal efficiency was on average 1.72% (0.04-3.42%) greater than Fe during stable operating conditions.