Electro-polarization of the sludge with dynamic magnetic field enhanced the interspecies electron transfer in ZVI-added anaerobic digesters

Zero-valent iron (ZVI) has been widely reported to strengthen anaerobic digestion; however, ZVI passivation prevents its continuous function. In this study, a dynamic magnetic field (DMF) was imposed on a ZVI-added anaerobic digester treating food waste to promote microbial interspecies electron transfer to enhance methane production. Results showed that methane production with DMF was 15.3% higher than that without DMF. Meanwhile, the Fe(II) concentration from ZVI increased by 39.46% with DMF, but the electron equivalent was far less than the increased methane production even if all electrons from the ZVI were used for hydrogenotrophic methanogenesis. In agreement, the relative abundances of hydrogenotrophic methanogens were less changed after DMF. Notably, an induced electric current ranging from −0.017–0.021 μA was detected with DMF. With the DMF-induced current as electrical stimulation, the sludge conductivity and capacitance increased by 242% and 18%, respectively. In-situ electrochemical tests on the functional groups of the sludge showed that the protein-like substances were polarized by the induced electromotive force to enhance the proton-coupled electron transfer in the sludge. Together with the enriched electrotrophic methanogens, the enhanced microbial interspecies electron exchange was the main reason for the improved performance of anaerobic digestion with DMF.