An anaerobic digester with microbial electrolysis cell enhances relative abundance of methylotrophic methanogens in bulk solution

The microbial communities and operational performances of a conventional anaerobic digester (AD) and an AD combined with microbial electrolysis cells (ADMEC) were investigated. Primary sludge and waste-activated sludge were used as substrates, and next-generation sequencing (NGS) techniques were used to analyze the microbial characteristics. The results show that ADMEC can achieve a faster stabilization rate, higher organic decomposition, and methane production performance than AD. After both the ADMEC and AD reached a steady state, microbial results revealed that Methanobacterium beijingense and Methanosaeta concilii were the dominant methane-generating archaeal species in AD. In ADMEC, the relative abundance of methylotrophic methanogens (Thermoplasmata class), which has higher methane productivity than other methanogens, is significantly improved. For bacterial communities, an improved relative abundance of the Cloacamonas phylum, which is involved in amino acid fermentation, and in the Erysipelotrichi class, which grows well in environments with high organic concentrations, was observed in ADMEC. In summary, ADMEC is more efficient than AD because organic degradation and methanol production accelerated by bioelectrochemical reactions occur in ADMEC, leading to a favorable environment for the growth of methylotrophic methanogens in bulk solution.