Comparative performance of activated sludge and aerobic granular sludge sequencing batch reactors for removing metalloid SeIV/VI oxyanions

Biotransformation of soluble selenite (SeIV) and selenate (SeVI) to biomass-associated Se0 (Bio-Se) provides a solution for mitigating Se pollution coupled to securing scarce element. Here, we demonstrated an efficient Se oxyanion removal process by converting to Bio-Se using aerobic granular sludge (AGS). SeIV and SeVI removal was dependent on lactate availability, and proportional to initial Se and biomass concentrations. Laboratory-scale sequencing batch reactors (SBR) were operated for 100 days by feeding an influent containing 100 μM SeIV, 100 μM SeVI, or 100 μM each of SeIV and SeVI to evaluate Se and nutrient removal performance. At 34 h hydraulic retention time, complete and sustained removal of influent SeIV, SeVI or SeIV-SeVI was achieved in 24 h cycles containing anaerobic (8 h) and aeration (15 h) phases. Removal of NH4+-N was sustained in AGS reactor through nitritation-denitritation under Se-loading conditions. AGS process demonstrated SeIV/VI, COD, ammonia, and total nitrogen removal efficiencies of 99 %, 99 %, 99 % and 98 %, respectively. Both batch and SBR experiments revealed that AGS is superior to activated sludge in achieving simultaneous SeIV/VI oxyanion and nutrient removal. This study shows that AGS process could be applied for treating acid mine and agricultural drainage that contain Se oxyanions and ammoniacal-nitrogen.