Complex biochemical reactors for selenium and sulphate reduction : organic material biodegradation and microbial community shifts

Biochemical reactors using complex organic materials for treatment of mine-affected waters are attractive low-cost solutions, but their widespread adoption is severely limited by poor reliability and limited longevity. This is in part due to lack of guidance on which organic materials to use, their degradation over time, and how this affects the microbial community composition, which in turn influences reactor performance. Continuous-flow column bioreactors containing differing ratios of a wood, hay and manure mixture were operated for 159 to 430 days including successful and decline phases of performance. Reactor performance, detailed organic matter composition and microbial community structure were measured for reactors with different wood to hay ratios and after different times of operation. Reactors with more hay than wood reduced sulphate from 500 mg/L to less than 100 mg/L and selenium from 20.3 µg/L to less than 0.2 µg/L with a retention time of 14 days for the whole period of operation. Whereas reactors with a high wood to hay ratio operated successfully for 100-200 days after which their performance fluctuated. Increase in more readily available organic compounds with decrease in recalcitrant fibrous materials was charted over time and correlated with changes in microbial community composition. More hemicellulose and α-cellulose were consumed in the bioreactors with more hay content. Lignin content remained the same for the wood rich bioreactors, and increased in the hay-rich columns. Ash content in bioreactors with either organic mixture increased over time. The labile components, determined as neutral detergent and water soluble compounds, fluctuated cyclically. The microbial communities that evolved in the bioreactors were distinctly different from those present initially. At the early stages, the communities were rich in organic matter degraders classified in the Bacteroides, Parabacteroides and Ruminococcaceae taxonomic groups. There was a shift towards Methanogens and Mollicutes and Spirochaetes classified groups for the longer running bioreactors. Sulphate reducing bacteria were mostly Desulfobulbus and Desulfovibrio related and they were more prevalent in the presence of high sulphate throughout the reactor history.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat