Adding biochar to anaerobic digestion (AD) system is an attractive regulation strategy to promote methane production efficiency from wastes/wastewater. The biochar has high porosity, large surface area, alkaline property, good conductivity, high ion exchange capacity, and abundant functional groups, among them, hierarchical porous structure is one key factor to affect AD. This review systematically summarized the recent progress for the roles of pore size of biochar in AD, including identification and regulation of pore size in biochar and potential improvement effects related to pore size of biochar in AD. Moreover, future perspectives and challenges have been concluded. The hierarchical pore structure and size of biochar play crucial roles in performance enhancement, environment improvement, substance transport promotion, microbial community regulation, and biogas purification. An effective synthesis strategy for achieving directed control of pore size is needed, for which, machine learning may be helpful. Then, the hierarchical porous channels of biochar can be monitored in situ while AD is in progress. It is important to understand the correlation between pore size in biochar and AD performance. Moreover, a comprehensive life cycle assessment focused on biochar preparation, AD efficiency, biochar recycling, final disposal of digestate is urgently needed to evaluate economic and environmental benefits of engineering applications. This review aims to provide some references for the latest developments in the use of hierarchical porous biochar in AD.
