Biochar-enhanced anaerobic digestion of sewage sludge: Comparative performance and mechanistic insights

Anaerobic digestion (AD) is a sustainable approach for sewage sludge stabilization and energy recovery, but its efficiency is often limited by microbial imbalance and intermediate accumulation. Biochar addition is a promising strategy for enhancing AD, however, the mechanisms underlying its effectiveness, particularly the impact of biochar origin, remain insufficiently understood. This study systematically compared corn stover-derived biochar (CSBC), mixed straw-derived biochar (MSBC), and digested sludge-derived biochar (SSBC) regarding their effects on methane production, microbial community structure, and electron transfer processes in sewage sludge AD. Results revealed that CSBC addition (10 g/L) yielded the most significant enhancement, increasing cumulative methane yield by 122.1 % to 153.7 mL/g VS compared to the control. This superior performance correlated strongly with CSBC's abundant oxygen-containing functional groups (OFGs), which facilitated higher redox activity (k_app 12-15 % greater than SSBC/MSBC) and electron transfer system activity (81.1 % increase over control). Microbial analysis revealed that CSBC was enriched with electron-transferring bacteria (e.g., Bacteroidetes_vadin HA17). Conversely, MSBC primarily facilitated hydrolysis, while SSBC preferentially stimulated hydrogenotrophic methanogens. These findings demonstrate that biochar surface chemistry and electrochemical properties are critical determinants of microbial interactions and methane yield, providing crucial insights for designing tailored biochars to optimize waste valorization and energy recovery in AD. This study offers a unique comparison of straw-versus sludge-derived biochars in sludge AD, highlighting the potential of SSBC in a circular economy context, and underscores the importance of linking biochar properties to functional microbial activity.