Enhancement of methane production in anaerobic digestion via modified oil sludge-derived biochar additive: Mechanisms of electron transfer, proteins cleavage and microbial synergy

The escalating production of sewage sludge poses significant environmental and economic challenges, necessitating sustainable strategies for its management. This study investigated the enhancement of anaerobic digestion (AD) performance using oil sludge-derived biochar (OBC) and its modified forms—co-pyrolyzed biochar (COBC) and magnetic biochar (MOBC). Through comprehensive physicochemical characterization, MOBC exhibited superior electron donating capacity (EDC, 28.14 μmol e⁻ g⁻¹), electron accepting capacity (EAC, 66.25 μmol e⁻ g⁻¹), specific surface area (10.32 m² g⁻¹), and redox-active functional groups (e.g., C O, Fe₃O₄), attributing to the magnetization. Batch experiments demonstrated that MOBC achieved the highest cumulative methane yield (183.52 mL g⁻¹ VS), surpassing COBC (168.19 mL g⁻¹ VS), OBC (154.05 mL g⁻¹ VS), and the control (86.72 mL g⁻¹ VS). The addition of MOBC increased key enzyme activities, including protease, acetate kinase, phosphotransacetylase and coenzyme F420, compared to the control groups, facilitating hydrolysis, acidogenesis, and methanogenesis. Additionally, MOBC disrupted sludge protein secondary structures, and improved protein bioavailability, accelerating the degradation of tryptophan-like components. Furthermore, microbial community and KEGG analysis revealed that the additive of MOBC led to a significant enrichment of syntrophic bacteria (e.g., Chloroflexi) and methanogens ( Methanobacterium , Methanosaeta ), and upregulated genes associated with direct interspecies electron transfer (DIET) and methane metabolism pathways, further confirming its role in microbial synergy. These findings highlighted MOBC as a synergistic additive for optimizing sludge-to-energy conversion, offering a sustainable pathway for sludge valorization.