Redox-Active Biochar and Conductive Graphite Stimulate Methanogenic Metabolism in Anaerobic Digestion of Waste-Activated Sludge: Beyond Direct Interspecies Electron Transfer

Graphite and pyrolytic biochar are two electrically active but functionally distinct carbon materials that have been widely used to enhance methanogenic performance of anaerobic digestion (AD) of waste-activated sludge (WAS) for energy recovery. However, their stimulatory mechanism is not well understood. In this study, adding conductive graphite and corn stover biochar with high electron-donating capacity (EDC) remarkably boosted the methane produced from WAS by 38.3 and 46.9%, respectively. By investigating the role of graphite and biochar addition in AD of WAS during different stages (hydrolysis, acidogenesis, and methanogenesis), we intended to elucidate the complicated mechanism for the improved methane production. The biochar mildly improved hydrolysis and acidogenesis, while the graphite did not. During methanogenesis, both graphite and biochar increased methane production primarily by enhancing the acetoclastic pathway, while syntrophic acetate oxidation via direct interspecies electron transfer (DIET) was not thermodynamically favorable. In the CO2-reducing pathway, biochar was superior to graphite, mainly relying on the redox cycling of functional groups to deliver electrons for Methanosaeta. However, graphite outperformed biochar by 42.4% in terms of methane production under high H-2 partial pressure by facilitating DIET. Collectively, redox-active pyrolytic biochar would benefit the WAS methanogenic process, while graphite could be supplemented in high-rate AD systems, representing two economically feasible carbon additives to enhance AD performance under varied operational conditions.