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

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 (S ^2- ) to elemental sulfur (S ⁰ ) and the oxidation of S ⁰ to sulfate (SO ₄ ^2- ). The oxidation rate of S ^2- to S ⁰ was 3.31 times faster than that of S ⁰ to SO ₄ ^2- , resulting in the accumulation of S ⁰ as a desired intermediate under S ^2- -S/NO ₃ ^- -N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S ⁰ suspended in the effluent, about 40% of S ⁰ retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S ⁰ was subsequently oxidized to SO ₄ ^2- under S ^2- -S/NO ₃ ^- -N ratio of 0.5:1. In anaerobic, S ⁰ coexist with SO ₄ ^2- (in fresh wastewater) were simultaneously reduced to S ^2- , and the reduction rate of SO ₄ ^2- to S ^2- was 3.17 times faster than that of S ⁰ to S ^2- , resulting in a higher production of S ⁰ in subsequent anoxic phase. Microbial community analysis indicated that SO ₄ ^2- /S ⁰ -reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S ^2- /S ⁰ -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 S ⁰ recovery and simultaneous removal of SO ₄ ^2- and NO ₃ ^- in an AA-SBR.
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