Additional electric field alleviates acidity suppression in anaerobic digestion of kitchen wastes via enriching electro-active methanogens in cathodic biofilms

The aim of this study was to investigate the feasibility that the suppression of acidity in anaerobic digestion of kitchen wastes could be alleviated with additional electric field. The results showed that, the accumulation of acidity seriously suppressed methanogenesis, and no methane was detected in the electrode-supplemented digester without applied voltage. In contrast, with applied voltages of 0.6-1.2 V, the suppression of acidity was alleviated, and methane production rates reached 558-669 L/kg-volatile suspended solid that were higher than that previously reported with carbon-based conductive materials, such as granular active carbon and biochar. Although the reduced peak with a potential ranging from -0.3 V to -0.2 V close to the reduced potential of CO2/CH4 detected via cyclic voltammetry, the maximum methane yield derived from cathodic reduction of CO2 only accounted for 0.13-0.33% of total methane yield with applied voltages. Microbial community analysis showed that, Methanosarcina species were specially enriched with applied voltages, accounting for ca. 40-70% of the communities. Together with the increase in the relative abundance of Syntrophomonas species, Methanosarcina species directly accepting electrons from Syntrophomonas species via DIET might be the primary reason for alleviating the suppression of acidity. Further investigations via three-dimensional excitation emission matrix and in-situ electrochemical Fourier transform infrared spectroscopy found that additional electric field stimulated the secretion of EPS primarily comprised of protein-like substance, which might mediate the EET between Syntrophomonas and Methanosarcina species.