Performance and mechanism of conductive magnetite particle-enhanced excess sludge anaerobic digestion for biogas recovery

The aim of this study was to evaluate the effect of magnetite particles on the anaerobic digestion of excess sludge. The results showed that methane production increased with the increase in magnetite dosage in the range of 0–5 g L(−1), and the cumulative methane production increased by 50.1% at a magnetite dosage of 5 g L(−1) compared with the blank reactor after 20 days. Simultaneously, numerous volatile fatty acids (VFAs) were produced at high magnetite dosages, providing the required substrates for methanogenesis. The concentration of magnetite addition was positively correlated with methane production, which proved that magnetite was beneficial for the promotion of the conversion of VFAs to methane. Moreover, the degradation efficiencies of proteins and carbohydrates reached 64% and 52.6% at the magnetite dosage of 5 g L(−1), respectively, and corresponding activities of protease and coenzyme F420 were 9.03 IU L(−1) and 1.652 μmol L(−1). In addition, the Methanosaeta and Methanoregula genus were enriched by magnetite, which often participate in direct interspecies electron transfer as electron acceptors.