BACKGROUNDSelenium (Se) discharged into natural waterbodies can accumulate over time and have negative impacts on the environment. Se-laden wastewater streams can be treated using biological processes. However, the presence of other electron acceptors in wastewater, such as nitrate (NO3-) and sulfate (SO42-), can influence selenate (SeO42-) reduction and impact the efficiency of biological treatment systems. RESULTSSeO42- removal by biofilms formed from an anaerobic sludge inoculum was investigated in the presence of NO3- and SO42- using drip flow reactors operated continuously for 10 days at pH 7.0 and 30 degrees C. The highest total Se (approximate to 60%) and SeO42- (approximate to 80%) removal efficiencies were observed when the artificial wastewater contained SO42-. A maximum amount of 68 mol Se cm(-2) was recovered from the biofilm matrix in SO42- + SeO42- exposed biofilms and biofilm mass was 2.7-fold increased for biofilms grown in the presence of SO42-. When SeO42- was the only electron acceptor, biofilms were thin and compact. In the simultaneous presence of NO3- or SO42-, biofilms were thicker (> 0.6 mm), less compact and exhibited gas pockets. CONCLUSIONThe presence of SO42- had a beneficial effect on biofilm growth and the SeO42- removal efficiency, while the presence of NO3- did not have a significant effect on SeO42- removal by the biofilms. (c) 2018 Society of Chemical Industry