Biological treatment of selenite laden wastewater by a high-rate internal circulation (IC) reactor: Role of hydraulic retention time

Waste activated sludge (WAS) is promising for treating selenium laden wastewater by converting selenite to low-toxicity Se⁰. However, WAS reduction capacity is limited for practical application. This study for the first time introduced stepwise declining bioreactor hydraulic retention time (HRT) strategy to enhance WAS activity. A pilot-scale internal circulation (IC) reactor was selected to cultivate WAS due to its high biomass retention and mass transfer. Acetate was used as electron donor and carbon source for selenite reduction. At 168 h–12 h HRTs, IC removed 93.62 %–98.68 % of selenite, but the efficiency dropped to 81.65 %–10.53 % at 6 h–1 h HRTs. Stepwise declining HRT from 108 h to 3 h screened high-rate selenite-reducing consortia with stronger selenite tolerance (∼21 mM). The maximum specific activity reached 3.53 ± 0.21 m mol Se g⁻¹ VSS h⁻¹ at 12 h HRT. WAS had good electrochemical activity at HRTs ≥ 6h. The dominant selenite reducers were family Rhodobacteraceae, Rhodocyclacea, Comamonadaceae, and Chitinophagaceae at 168 h HRT, while shifting to Pseudomonadaceae, Comamonadaceae, and Rhizobiaceae at shorter HRTs of 12 h–1 h. Function predication suggested selenite reduction was contributed by electron transport-linked phosphorylation and detoxification pathways, which displayed different response to HRT changes. Overall, IC reactor is promising for remediating selenite laden wastewater and stepwise declining HRT can selectively cultivate high-rate selenite-reducing consortia from WAS.