Dissimilatory manganese reduction facilitates synergistic cooperation of hydrolysis, acidogenesis, acetogenesis and methanogenesis via promoting microbial interaction during anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is commonly limited to poor synergistic cooperation of four stages including hydrolysis, acidogenesis, acetogenesis and methanogenesis. Dissimilatory metal reduction that induced by metal-based conductive materials is promising strategy to regulate anaerobic metabolism with the higher metabolic driving force. In this study, MnO2 as inducer of dissimilatory manganese reduction (DMnR) was added into WAS-feeding AD system for mediating complicated anaerobic metabolism. The results demonstrated that main operational performances including volatile solid (VS) degradation efficiency and cumulative CH4 production with MnO2 dosage of 60 mg/g·VS reached up to maximum 53.6 ± 3.4% and 248.2 ± 10.1 mL/g·VS while the lowest operational performances in control group (38.5 ± 2.8% and 183.5 ± 8.5 mL/g·VS) was originated from abnormal operation of four stages. Furthermore, high-throughput 16 S rRNA pyrosequencing revealed that enrichment of dissimilatory manganese-reducing contributors and methanogens such as Thermovirga, Christensenellaceae_R_7_group and Methanosaeta performed the crucial role in short-chain fatty acids (SCFAs) oxidation and final methanogenesis, which greatly optimized operational environment of hydrolysis, acidogenesis and acetogenesis. More importantly, analysis of functional genes expression proved that abundances of genes encoding enzymes participated in acetate oxidation, direct interspecies electron transfer (DIET) and CO2 reduction pathway were simultaneously up-regulated with the optimum MnO2 dosage, suggesting that DMnR with SCFAs oxidation as electron sink could benefit stable operation of four stages via triggering effective DIET-based microbial interaction mode.