Electrical stimulation enhancing anaerobic digestion under ammonia inhibition: A comprehensive investigation including proteomic analysis

Microbial electrolysis cell (MEC) coupled anaerobic digestion (AD), named as MEC-AD system, can effectively promote methane production under ammonia inhibition, but the inherent mechanism is still poorly understood. This study comprehensively explored the MEC-AD performance and mechanism under high-concentration ammonia stress including using proteomic analysis. It was found that the methane generation rates in MEC-AD systems were 2.0–2.7 times that of AD ones under 5.0 g/L ammonia stress. Additionally, the experimental conditions for methane generation in MEC-AD systems were optimized using response surface methodology. Further analysis indicates that the activities of acetate kinase and F420 were improved, and particularly the direct interspecies electron transfer (DIET) was promoted in MEC-AD systems, as indicated by increased electroactive extracellular polymeric substance, decreased charge transfer resistance, and enrichment of electroactive microbes such as Geobacter on the bioelectrodes. Moreover, proteomic analysis reveals that the DIET associated proteins such as Cytochrome C was up-regulated, and ammonia transfer-related proteins were down-regulated and ammonium detoxification-related proteins were up-regulated in MEC-AD systems. This work provides us a better understanding on the MEC-AD performance especially for the treatment of wastewater containing high-concentration ammonia.