Continuous sulfur biotransformation in an anaerobic-anoxic sequential batch reactor involving sulfate reduction and denitrifying sulfide oxidization

Abstract

The pathways and intermediates of continuous sulfur biotransformation in an anaerobic and anoxic sequential batch reactor (AA-SBR) involving sulfate reduction (SR) and denitrifying sulfide oxidization (DSO) were investigated. In the anoxic phase, DSO occurred in two sequential steps, the oxidation of sulfide (S2−) to elemental sulfur (S0) and the oxidation of S0 to sulfate (SO42−). The oxidation rate of S2− to S0 was 3.31 times faster than that of S0 to SO42−, resulting in the accumulation of S0 as a desired intermediate under S2−-S/NO3−-N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S0 suspended in the effluent, about 40% of S0 retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S0 was subsequently oxidized to SO42− under S2−-S/NO3−-N ratio of 0.5:1. In anaerobic, S0 coexist with SO42− (in fresh wastewater) were simultaneously reduced to S2−, and the reduction rate of SO42− to S2− was 3.17 times faster than that of S0 to S2−, resulting in a higher production of S0 in subsequent anoxic phase. Microbial community analysis indicated that SO42−/S0-reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S2−/S0-oxidizing bacteria (e.g. Paracoccus and Thermothrix) co-participated in continuous sulfur biotransformation in the AA-SBR. A conceptual model was established to describe these main processes and key intermediates. The research offers a new insight into the reaction processes optimization for S0 recovery and simultaneous removal of SO42− and NO3− in an AA-SBR.