Enhancing anaerobic digestion of sludge with dual-heteroatom-doped biochar derived from digested sludge: In-situ reuse of anaerobic residues for system self-circulation.

The effectiveness of biochar as a promoter in the anaerobic digestion (AD) of complex organic matter is constrained by its limited surface reactive sites. However, heteroatom doping can enhance the surface functional groups and defect structures of biochar, improving its electron exchange capacity and potential to promote AD. Dual-heteroatom-doped biochar derived from digested sludge (DH-DSBC) focused on stimulating the acetogenic-methanogenic stage of sludge AD. The addition of DH-DSBC accelerated the utilization and conversion of organic matter in the AD system and increased the concentration of methanogenesis-related enzymes (coenzyme F), ultimately boosting methane production by 52-60%. Additionally, the methane conversion rate increased by 60-69%. The enrichment of electroactive bacteria and Methanothrix (formerly Methanosaeta)/Methanosarcina suggests that DH-DSBC may facilitate direct interspecies electron transfer, as supported by the observed increase in sludge electrical conductivity and elevated electron transfer system activity. This study provides insights into the practical reuse of anaerobic residues in-situ and contributes to achieving self-circulation in AD systems.