Effects of biochar on anaerobic digestion: a review

In the context of climate change and the circular economy, most municipal wastewater treatment plants are not efficient because they generate huge amount of organic sludge, which in turn requires costly post-treatment by biological processes such as anaerobic digestion. An emerging solution is to add biochar to improve anaerobic digestion efficiency by enhancing microbial activity, aiding in the breakdown of complex organic compounds, producing more biogas, and promoting overall reactor stability. Here, we review the effects of adding biochar in anaerobic digestion, with emphasis on digester performance, process stability, biochar properties, and mechanisms. We discuss methane production, lag phase, electrical conductivity, volatile fatty acids, ammonia nitrogen, pH, and oxidation–reduction potential. We also review the process inhibition by biochar addition, with focus on phenols, heavy metals and microbial composition. Biochar properties are controlled by feedstock type, pyrolysis temperature, specific surface area, electrical conductivity, carbon and mineral content, electron exchange capacity, aromaticity, and particle size. We found that 6–16 g/L biochar supplementation consistently yielded higher cumulative specific methane compared to control without biochar, across diverse conditions and substrate types. Biochar’s role is explained by four mechanisms: enhancing functional microbes, facilitating direct interspecies electron transfer, improving the degradation of refractory compounds, and increasing reactor stability.