Enriching functional microbes with electrode to accelerate the decomposition of complex substrates during anaerobic digestion of municipal sludge

• MEC could accelerate the decomposition of proteins and carbohydrates in WAS.
• Methane production efficiency was significantly improved in MEC.
• Increased methane was equivalent to 13.4 times as the electric energy supply.
• Anaerobic fermentative bacteria were largely enriched in MEC.

Methane-production microbial electrolysis cells (MECs) have been widely reported as an efficient strategy to enhance anaerobic digestion of waste activated sludge (WAS). However, the primary mechanism for accelerating the decomposition of complex substrates contained in WAS remains unclear as so far. In this study anaerobic sludge digestion operated in a single-chamber methane-production MEC was investigated. It was found that the decomposition rate of proteins and carbohydrates were significantly accelerated in MEC, which resulted in the improvement of methane production as compared with the common anaerobic sludge digester. The energy income from the increased methane production was equivalent to 13.4 times as more as the electric energy supply. Further bacterial community analysis showed that anaerobic fermentative bacteria were largely enriched in MEC especially its anodic biofilm. Together with anodic exoelectrogenic bacteria (mainly Geobacter species) accounting for the dominant part of bacterial community in the anodic biofilm, it was suggested that the potential for syntrophic interaction between anaerobic fermentative bacteria and anodic exoelectrogenic bacteria enriched might be the important reason for accelerating the decomposition of complex substrates contained in WAS, which further resulted in the high-efficiency methane production as well as energy recovery.