Application and Mechanisms of Biochar in Anaerobic Digestion: Towards Process Resilience and Waste Valorization

Anaerobic digestion (AD) is widely used for organic waste stabilization and bio-energy recovery, but its performance is often constrained by process instability, slow syntrophic metabolism, and sensitivity to acidification, ammonia, organic overloading, and inhibitory contaminants. Biochar has emerged as a promising strategy to enhance AD, with benefits extending beyond increased methane yield. This review examines how biochar properties, including pore structure, surface functional groups, alkalinity, and electrical conductivity, regulate different AD stages and improve process stability. Biochar provides microbial habitats, buffers pH, adsorbs inhibitors, accelerates volatile fatty acid conversion, and facilitates interspecies electron transfer. The effects of biochar dosage, feedstock type, reactor configuration, and operational conditions on digestion performance and resource recovery are critically evaluated. Broader contributions to organic waste valorization are also discussed. Future research should prioritize tailored biochar design, standardized characterization, long-term validation in continuous reactors, techno-economic analysis, and life-cycle assessment. These efforts are essential for translating laboratory findings into reliable and sustainable applications for organic solid waste treatment and resource recovery.