Biological treatment of highly contaminated acid mine drainage in batch reactors: Long-term treatment and reactive mixture characterization.

Passive bioreactors involving sulphate-reducing bacteria (SRB) are a practical alternative technology to treat acid mine drainage (AMD). Careful selection of the organic carbon source is important to ensure performance and long-term efficiency of the treatment. However, a rigorous and methodical characterization to predict the biodegradability of organic substrates by SRB still needs to be investigated. In the present study, four natural organic materials were thoroughly characterized to assess their ability to serve as substrates and to find a parameter that links organic carbon sources with their biodegradability. Three reactive mixtures were then comparatively evaluated for their performance to treat a highly contaminated AMD in long-term (152 days) batch experiments. All three mixtures were successful for sulphate reduction and metal (Fe, Ni, Cd, Zn, and Mn) removal (91.8-99.8%). Higher efficiencies were observed in the reactors with 30% (w/w) cellulosic wastes (maple wood chips and sawdust) which decreased sulphate concentrations from 5500 mg/L to <1mg/L, than in reactors with 2-3% cellulosic wastes, where final sulphate concentrations were in the range 2000-2750 mg/L. Organic material characterization indicated that higher C/N ratios, chemical oxygen demand (COD)/SO(4)(2-) ratios and dissolved organic carbon (DOC)/SO(4)(2-) ratios were associated with better sulphate-reducing conditions and metal removal. This work suggests that C/N and DOC/SO(4)(2-) ratios considered together are key parameters to assess the biodegradability of natural organic wastes under sulphate-reducing conditions.