Sludge-derived biochar promoted the methane production in anaerobic digestion of thermo-alkaline pretreated waste-activated sludge: Performance and mechanisms

The addition of biochar is a promising strategy to improve CH₄ production by strengthening direct interspecies electron transfer (DIET) in anaerobic digestion (AD). In this study, three types of sludge-derived biochar prepared at 300/500/700 °C (designated BC300, BC500, and BC700) were evaluated for their effects in improving CH₄ production efficiency and system stability during AD of thermo-alkaline pretreated waste-activated sludge (WAS). The maximum CH₄ production for the control, BC300, BC500, and BC700 groups was 326.8 mL/g VS, 401.8 mL/g VS, 423.8 mL/g VS, and 420 mL/g VS, respectively. BC500 promoted more significantly than BC300 and BC700, with 35.4 % higher maximum CH₄ production than the control group. Electrochemical analysis demonstrated that BC500 possesses both surface redox functional groups and graphitic structure, endowing it with excellent capacitance (0.31 mF/g) and electrical conductivity (3.1 ×10⁻⁴ S/m), which facilitates DIET. Microbial community analysis revealed that the acetotrophic methanogens Methanosaeta, which are capable of participating in DIET, were enriched from 24.0 % to 32.3–34.0 % in the biochar-added groups. These findings suggest that DIET was preferentially stimulated to promote methanogenesis during WAS AD, replacing the thermodynamically unfavorable hydrogen-mediated interspecies electron transfer. This study provides a theoretical foundation for in-situ enhanced AD via sludge-derived biochar and proposes a closed-loop “treating waste with waste” strategy: improved energy recovery from WAS and sustainable sludge management in wastewater treatment plants (WWTPs).