Combined semi-continuous feeding and stripping: Improving volatile fatty acid production and sulfate reduction in a...

This study developed a two-phase anaerobic sulfate reduction system with semi-continuous feeding, integrated with vapor stripping, to assess its treatment performance at varying chemical oxygen demand (COD)/SO₄^2- ratios. At a COD/SO₄^2- ratio of 2, the system achieved high removal efficiencies of 91.85 % for COD and 92.70 % for sulfate, respectively. Vapor stripping effectively maintained low free H₂S concentrations, remaining below 50 mg/L in the first-phase reactor (Ra) and below 30 mg/L in the second-phase reactor (Rm). Additionally, the scouring effect of vapor stripping altered the functional group composition on the sludge surface, promoting the formation of sulfate-reducing granular sludge. Microbial analysis revealed a synchronized enrichment of Desulfovibrio and Enterococcus in Ra, as well as Methanosaeta and Enterococcus in Rm under a semi-continuous feeding regime combined with vapor stripping. Conductivity measurements and correlation analyses suggested that electroactive Methanosaeta and Enterococcus engaged in syntrophic metabolism via extracellular electron transfer, which was particularly beneficial for organic removal under low COD/SO₄^2- ratio conditions. At a COD/SO₄^2- ratio of 2, dissimilatory sulfate reduction (DSR) prevailed in Ra, whereas assimilatory sulfate reduction (ASR) dominated in Rm. This balance optimized sulfate removal efficiency while mitigating sulfide toxicity toward methanogens. Overall, this study presents a promising approach for the efficient treatment of high-sulfate organic wastewater with low COD/SO₄^2- ratio.