Enhancing humification and digestate maturity in high-solids anaerobic digestion of agricultural wastes using biochar

The formation of humic substances in anaerobic digestion is limited by the absence of an oxidative condition, which is required for humification. Recent studies have found that the formation of humic substances in anaerobic environments may rely on non-covalent interactions such as hydrogen bonding, π-π stacking, and hydrophobic interaction, but these have not been confirmed yet. The role of adding biochar in humification and methane production in the anaerobic digestion was studied in this work. The results showed that the addition of biochar increased methane production by 14.73 %, lower than some literature on biochar-enhanced anaerobic digestion. Importantly, the addition of biochar increased the humic acid content by 26.30 %, and the humic acid/fulvic acid ratio by 31.34 %. The accelerated humification might be attributed to non-covalent interactions between humification precursors (e.g., phenolic acids, amino acids, and sugars) and the aromatic or polar groups on the biochar surface, which facilitated molecular aggregation and structural stabilization. Solid-state ¹³C NMR and 2D FTIR-COS, as well as XPS analysis, indicated that the addition of biochar increased the relative abundances of aromatic carbon and oxidized structures of digestate. Microbial sequencing indicated that biochar enriched methanogenic archaea. The increase of humic substances with biochar might decrease the enzymatic activity related to hydrolytic acidogenesis, partially offsetting the positive effect of biochar as an electron shuttle in enhancing methanogenesis. This could be an important reason for the limited increase in methane production in this work. This research provided a new perspective on the influence of biochar addition on anaerobic digestion.