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

Abstract 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/SO 4 2− 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/SO 4 2− ratio decreased from 13.3 (4 g COD/0.3 g SO 4 2− ) to 2.0 (4 g COD/2 g SO 4 2− ) 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/SO 4 2− ratio of 2.0, resulting in an inefficient removal/yield of COD and SO 4 2− /methane, and showed the system to deteriorate. In Phase II, although the COD/SO 4 2− ratio increased to 3.0 (6 g COD/2 g SO 4 2− ) 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/SO 4 2− ratio further increased to 6.0 (6 g COD/1 g SO 4 2− ) 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 SO 4 2− /methane and the improvement of performance. These results indicate that the influent COD/SO 4 2− ratio and substrate concentration affect the metabolic phase-separation and treatment system stability.