Adding conductive materials for enhancing methane production from biomass has caused widespread attention. However, the discharge of digestive residues inevitably causes the loss of conductive materials, which not only weakens the roles of conductive materials but also increases the operating cost. This study aims to reveal the potentials of recycled magnetic biochar (MBC) in sludge anaerobic digestion. Results suggested that the direct utilization was better than the indirect utilization. Compared to raw MBC group, methane production further increased by 6.5-7.0 % in multi-cycle MBC groups. The mechanism analysis illustrated that the recycled MBC accelerated the conversion of intermediates. Besides, the recycled MBC promoted the content of redox-active substances and the activity of intracellular electron transfer systems. In addition, the acetoclastic methanogenesis pathway was improved in recycled MBC biofilms while the hydrogenotrophic methanogenesis was reduced. More interestingly, the microbial communities in biofilms presented directional evolutions with the increased cycle numbers of MBC. Correspondingly, the dominant direct interspecies electron transfer patterns changed from Geobacter & Methanosaeta to Clostridium & Methanosarcina, and the electron mediators converted to MBC and c-type cytochrome. The findings provided new insights into reutilization of conductive materials in anaerobic digestion.
