Microbial electrochemistry enhanced electron transfer in lactic acid anaerobic digestion for methane production

To address the severe inhibition effect of propionic acid derived from lactic acid degradation on methane production, microbial electrodes were used to enhance electron transfer in lactic acid anaerobic digestion system. Excitation-emission matrix fluorescence spectroscopy analysis revealed that electrochemical treatment reduced the content of humic acids, which implied that intercellular electron transfer pathway changed from extracellular polymer transfer to more efficient microbial electrode transfer. The electron-producing bacteria Firmicutes with increased abundance from 51.5% to 59.7% provided more electrons on anode by degrading acetates for methanogenesis, while hydrogenotrophic methanogens Methanoculleus with increased abundance from 66.5% to 77.2% produced more methane on cathode from H2 and CO2. The microorganisms formed stable biofilms on anode and cathode surfaces at 0.8 V, resulting in a 27.4% increase in methane yield to 477.1 mL CH4/g total volatile solids.