Bench-scale comparison of conventional and high rate clarification treatment processes for acid mine drainage

Acid mine drainage (AMD) is characterized as having low pH and high concentrations of sulfate and dissolved metals. This study compared treated water quality and sludge properties of three process technologies for AMD: conventional sedimentation, high density sludge (HDS), and ballasted flocculation. All three processes were found to be capable of removing regulated metals to concentrations below current Canadian discharge guidelines. However, ballasted flocculation was the only technology found to be able to meet the more stringent federal guidelines proposed for future implementation under the Fisheries' Act's Metal Mining Effluent Regulations . Specifically, arsenic and zinc concentrations in AMD treated by the conventional and HDS processes were above proposed future guidelines of 0.10 and 0.25 mg/L, respectively, while lead, copper, and nickel all met respective guidelines. Concentrations of all regulated contaminants were below proposed guidelines when treated by ballasted flocculation. The HDS process was found to produce a significantly more concentrated sludge than conventional sedimentation (i.e., higher solids content (19 ± 1% versus 7 ± 4% wet solids) and lower sludge volume index (SVI; 8.4 ± 0.8 versus 230 ± 20 mL/g)).