Biogas Residue Biochar Enhances Thermophilic Anaerobic Co-Digestion of Sewage Sludge and Food Waste

Biochar addition in anaerobic digestion (AD) systems generally improves the transformation of organic waste and the production of methane, yet little is known on further recycling of the biogas residue into biochar for AD. Here we prepared biogas residue biochar by heating the residue of food waste after AD at 500°C. We comprehensively compared the performance of this biochar with coconut shell biochar and corn stalk biochar on anaerobic co-digestion of sewage sludge and food waste, and explored the corresponding mechanisms. Results show that the biogas residue biochar has the highest mean methane daily yield, of 432.16 mL per g of volatile solids, versus 377.66-386.30 mL for coconut and corn biochars. This finding is in agreement with the structure of biogas residue biochar, whose structure shows more abundant functional groups, especially basic functional groups, which can effectively neutralize fatty acids and alleviate systematic acidification. Results are also supported by characteristics of the biogas residue biochar, which enriched the abundance of electroactive Clostridia , Methanobacterium and Methan o brevibacter in sludge under thermophilic condition, thus shortening the process of methanation.