Syntrophic interactions in anaerobic digestion: how biochar properties affect them?

Biochar as a biomass derived, low cost, carbon conductive material is considered as an important supplement in the anaerobic digestion (AD) of organic matter. It functions as an electrical grid to allow direct electron transfer from fatty acid oxidizers to methanogenic archaea, thereby promoting syntophy between various microbial groups and leading to efficient methanogenesis. Specific properties of biochar play an important role in promoting syntrophic interactions in AD. As a physical indicator, surface area, porosity, particle size and surface texture of biochar play an important role in governing microbial attachment and enrichment on biochar. This influences the microbial degradation of fatty acids and their subsequent conversion to methane by methanogens. Chemical properties such as the presence of hydrophobic functional groups, molecular nature and redox active groups on biochar surface promote interaction between biochar and microorganisms and provide an increased degree of electron transfer between the attached microorganisms. The above characteristics depend on feedstock and pyrolysis conditions used for biochar production. Unlike previous reviews, herein the desired physical and chemical properties of biochar that promote syntrophy in AD and the factors that influence them have been discussed in detail. Furthermore, engineering of biochar properties by various activation methods to harness favourable characteristics of biochar as an effective AD additive is described. Such a comprehensive account would be useful for engineering efficient biochar-mediated digestions with enhanced syntrophy and overall AD performance.