Bioreduction of selenate using a hydrogen-based membrane biofilm reactor

A H-2-based, denitrifying and sulfate-reducing membrane-biofilm reactor (MM) was shown to be effective for removing selenate (Se(VI)) from water or wastewater by reducing it to insoluble Se-0.When Se(VI) was first added to the MR, Se(VI) reduction-first to selenite (Se(IV)) and then mostly to Se-0-took place immediately and then increased over three weeks, suggesting enrichment for dissimilatory selenium-reducing bacteria. Increasing the H-2 pressure improved the Se(VI) reduction rate and total-Se removal, and lowering the influent Se(VI) concentration from 1000 to 260 mu g Se/L increased the average Se removal to 94%, which corresponded to an effluent Se concentration of less than 12 mu g Se/L, a value well below the standard of 50 mu g Se/L. The fact that the effluent suspended solids contained reduced Se suggests that Se-0 was retained in the biofilm, which detached to form the effluent suspended solids. A series of short-term experiments elaborated on how decreased influent selenate loading and increased H-2 pressure could systematically improve the reduction of Se(VI) and removal of total Se. Short-term experiments also demonstrated that selenate reduction improved with lower influent nitrate concentration, suggesting that H-2 was more available for selenate reduction when the H-2 demand for denitrification was smaller. Complete sulfate reduction, which occurred in parallel to nitrate reduction, dominated the electron-equivalent flux. Like selenate reduction, but unlike nitrate reduction, sulfate reduction was sensitive to H-2 pressure and appeared to be inhibited by selenate. Finally, selenate reduction was relatively insensitive to pH in the range of 7.0 to 9.0. This research shows that the MW can be effective for removing Se(VI) in water or wastewater to below the 50 mu g Se/L standard.