Inverse fluidised bed bioreactor enabled high-rate selenate reduction for wastewater treatment

Mine waters often contain selenate, which needs to be removed to avoid harmful human and environmental health impacts. This study evaluated the performance of a laboratory-scale inverse fluidised bed reactor (IFBR) for biological selenate reduction at 30°C. The IFBR with floating biomass carriers was fed with synthetic mine water (pH 6.0–7.0) containing ∼10 mM (1.4 g L−1) selenate, nutrients and 10 mM ethanol as electron donor and inoculated with selenate reducers enriched from anaerobic sludge and environmental samples. The bioreactor achieved a selenate reduction rate of 1.75 mM d−1 (250 mg L−1 d−1 selenate or 138 mg L−1 d−1 Se) representing 94% removal during stable performance at a hydraulic retention time of 5 d. The formation of a red precipitate indicated the formation of elemental selenium, and selenite concentration remained predominately at below 0.1 mM during stable operation. Alkalinity was generated during the biological selenate reduction, increasing the wastewater pH from 6.0 to 8.6. Redox potential gradually approached a value ranging from -500 mV to -600 mV vs. Ag/AgCl. The dominant prokaryotic phylum in the bioreactor was Proteobacteria, followed by Firmicutes and Tenericutes. This study affirmed that IFBR is a promising configuration for high-rate removal of selenate from wastewaters. It has the potential to minimise the liabilities associated with environmental impacts of selenium containing mine drainage.