Impact of biochar on anaerobic digestion process and microbiome composition; focusing on pyrolysis conditions for biochar formation

This study is a comprehensive investigation of the role of biochar in anaerobic digestion relating various physicochemical properties and the potential biodegradability of biochar to the digestion process. Seven biochars, produced from cedar wood, wheat straw, digestate, and municipal sewage sludge, at temperatures ranging from 400 to 950 °C, were evaluated during mesophilic digestion of cellulose as a model substrate. Pyrolysis conditions significantly affected biochar's properties, with high temperatures resulting in larger surface areas (up to 1262 m²/g) and pore sizes (up to 0.7 cm³/g), more organized graphene-like structures, and higher electrical conductivities. Irrespective of the concentration and diversity of their physicochemical properties, most tested biochars did not affect methane production, despite enriching the relative abundance of the methanogenic community, even up to 42.7 %. Interestingly, wood-based biochar, post-treated by gasification at a high temperature (800 °C), reduced methane yield by up to 52 %. Contrary, methane was increased by 40 % due to residual biodegradable organic matter in biochar produced at a low temperature (400 °C) under incomplete anoxic conditions. This indicates that once the stoichiometric methane potential is reached, no additional methane can be produced by adding biochar, unless it acts as a co-substrate.