Selenate removal in biofilm systems: effect of nitrate and sulfate on selenium removal efficiency, biofilm structure and microbial community

Abstract

BACKGROUND Selenium (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. RESULTS SeO42‐ 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 °C. The highest total Se (∼60%) and SeO42‐ (∼80%) removal efficiencies were observed when the artificial wastewater contained SO42‐. A maximum amount of 68 μmol 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. CONCLUSION The 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.