Is COD/SO42− ratio responsible for metabolic phase-separation shift in anaerobic baffled reactor treating sulfate-laden wastewater?

•Phase separation changes could reflect the stability of the anaerobic baffled reactor (ABR).•Phase separation changes not only attributed to COD/SO42− ratio variation, but also substrate concentration variation.•Phase separation in ABR could avoid the competition between or among SRB, APB, SOB and MPB.•The inhibition of sulfide oxidizing was resulted from extracellular S particles accumulation on SOB.

An anaerobic baffled reactor (ABR) with five compartments for treating sulfate-laden wastewater was started up and the multi-phase separation, i.e., acidogenesis, sulfidogenesis, sulfur-production and methanogenesis, was achieved in 130 days of this study. The present study assessed the variations in COD/SO42− ratios as an important factor to evaluate the competition between sulfate-reducing bacteria and the other anaerobic bacteria in the phase-separation system for reactor performance stability. The experiment was divided into Phase I, II and III. In Phase I, the influent COD/SO42− ratio decreased from 13.3 (4 g COD/0.3 g SO42−) to 2.0 (4 g COD/2 g SO42−) by increasing the sulfate concentration, the above phases shifted to subsequent compartments, the predominant sulfide oxidizing bacteria (SOB) and methane-producing archaea (MPA) shifted to subsequent compartments, the SOB and Methanothrix-like bacteria were coated with S particles and precipitated at a COD/SO42− ratio of 2.0, resulting in an inefficient removal/yield of COD and SO42−/methane, and showed the system to deteriorate. In Phase II, although the COD/SO42− ratio increased to 3.0 (6 g COD/2 g SO42−) with an increase in COD concentration, the phase-separation shift to subsequent compartments did not reverse and sulfidogenesis proceeded throughout all compartments, S particles and precipitates coating on SOB and Methanothrix-like bacteria more, sulfide oxidation was inhibited and the effluent quality further deteriorated. In period III, the COD/SO42− ratio further increased to 6.0 (6 g COD/1 g SO42−) with a decrease in sulfate concentration, sulfidogenesis and methanogenesis returned to the front compartments and the inhibition of sulfide oxidation was relieved, resulting in an increase in removal/yield of COD and SO42−/methane and the improvement of performance. These results indicate that the influent COD/SO42− ratio and substrate concentration affect the metabolic phase-separation and treatment system stability.