Synergistic Effect and Mechanism of Biochar and Biogas Slurry Reflux in Anaerobic Digestion of Waste‐Activated Sludge

Biochar possesses abundant pore structures (e.g., micropores, mesopores, and macropores), providing ample attachment sites for microorganisms and facilitating the colonization of methanogenic archaea. Simultaneously, biochar contains rich functional groups (e.g., hydroxyl, carboxyl, and phenolic groups), which can regulate the system pH through mechanisms like ion exchange and adsorption, thus buffering potential acidification issues during anaerobic digestion (AD). Its surface minerals (e.g., K, Ca, and Mg) can also serve as sources of trace elements for microbial metabolism. Biogas slurry is rich in metal ions (e.g., Fe³⁺ and Mg²⁺) and nutrients (e.g., N and P), which can compensate for nutritional deficiencies in waste-activated sludge (WAS), optimizing the nutritional conditions for microbial growth. Both biochar and biochar coupled with biogas slurry can enhance the relative abundance of microorganisms involved in direct interspecies electron transfer (DIET), improving methane production efficiency during AD. However, whether the porosity or the electrical conductivity of biochar predominantly governs its influence on DIET in AD remains unexplored. This study investigated the mechanisms by which biochar coupled with biogas slurry affects the AD of WAS, using two types of bamboo-derived biochar with different specific surface areas (SSAs) and recirculated biogas slurry from a sludge treatment plant. Biogas slurry with a 60% recirculation ratio and biochar were added to AD reactors, and AD efficiency and microbial composition were compared over one reaction cycle. The influence of biofilms was isolated by using glass beads with the same SSA. The study found that biochar with high SSA provides a stable colonization environment for microorganisms due to its rich porosity, but possesses fewer surface functional groups. Conversely, biochar with low SSA exhibited more surface functional groups. The increase in system electrical conductivity was primarily attributed to these functional groups, while the influence of microbial biofilms on DIET was minimal. These results highlight the potential of leveraging biochar's electrical conductivity in AD processes to enhance renewable energy production and waste management, providing significant implications for future applications using biochar to promote the DIET process in the AD of WAS.