Rethinking the biochar impact on the anaerobic digestion of food waste in bench-scale digester: Spatial distribution and biogas production.

The improvement of biogas production in anaerobic digestion (AD) by biochar introduction has been demonstrated. However, the distribution of biochar in the digester and its effect on AD have been seldom explored. In this study, the distribution of biochar and their impact on AD were investigated in a 30 L semi-continuously operated bench-scale anaerobic digester. The results demonstrated that the biochar significantly increased biogas yields by 23.38 % under an organic loading rate (OLR) of 3.0 g VS/L·d. The stability of the AD under an OLR of 4.0 g VS/L·d was also improved by biochar introduction. The increased stirring speed of the digester enhanced the spatial distribution uniformity of biochar and increased biogas production by 5.89 %. Reducing the particle size of biochar improved its spatial distribution uniformity but did not significantly increase biogas production, likely due to excessive microbial accumulation on the biochar, which have caused substrate competition. Biochar aided AD by boosting microbial genera of Syntrophomonas, Bacteroidota, Cloacimonadot, and Methanosaeta, accelerating volatile fatty acids consumption, and improving microorganisms' spatial ecological niches. The economic analysis showed that applying residue-based biochar for biogas production presented superior benefits and greater development potential than residue incineration in the food waste AD process. Overall, this study presented a novel and comprehensive understanding of the biochar distribution and impact on food waste AD in digesters.