Passive co-treatment of acid mine drainage and sewage: Anaerobic incubation reveals a regeneration technique and further treatment possibilities

The passive co-treatment of municipal wastewater and acid mine drainage is an ecological engineering approach that has recently shown promise for the sustainable remediation of common liquid wastes. In order to investigate a possible regeneration technique and illuminate further treatment possibilities, four replicate columns containing limestone overlain with inert biofilm media that had been processing a passively pre-treated mixture of high-strength acid mine drainage and raw municipal wastewater were pulled offline, kept filled, sealed and incubated. The incubation period (91 d) was presumed to far exceed that required to reach a pseudo-steady state to drive reactions closer to completion than the standard residence time (42-h and 18-h in the inert media and limestone stages of the columns, respectively). Further nitrate reduction, alkalinity generation, and pH increase resulted. Further Al, Mn, and Zn removal was observed. Dissolved Fe increased from 45.3 to 147 mg/L, a likely result of Fe-reducing bacterial activity, indicating that accumulated Fe in solid phases could be transformed, remobilized, and flushed from the unit process. This phenomenon could serve to passively regenerate treatment cells by removing accumulated solids to avoid clogging and preferential flowpaths. Phosphate and other trace metals (e.g., As, Cd, Pb) associated with solid phases were not released into solution, indicating the stability of the phosphate-related solids formed during standard operation or the availability of further re-sorption sites. Overall, results revealed that most anaerobic treatment reactions were near completion under the previous treatment regime, that further Zn processing was likely practical, and a possible passive media regeneration technique, thus illuminating further options regarding high-strength acid mine drainage and raw municipal wastewater passive co-treatment.