Cobalt-modified digestate-derived biochar enhances kitchen waste anaerobic dry digestion: Performance, microbial mechanisms, and metabolic pathways

To achieve simultaneous in-situ high-value utilization of digestate and efficient resource recovery of kitchen waste (KW). In this work, Co-modified digestate-derived biochar (DDB-Co) was prepared for the first time, the effects of DDB-Co on KW mesophilic dry anaerobic digestion were investigated, and the underlying mechanisms were elucidated. The results showed that the specific surface area of DDB-Co increased to 98.7 m(2)/g, which was 37.4 % higher than that of the unmodified group (DDB). DDB-Co exhibited a dose-dependent effect on biogas production from the KW dry digestion, with the optimal dose of 4.0 g/L (calculated on a solid content) DDB-Co yielding 205.3 mL/(g<middle dot>VS) of biogas, representing an 80.9 % increase compared to the CK group. However, higher doses, such as 8.0 g/L DDB-Co, reduced the cumulative biogas production. Mechanistic analysis indicated that 4.0 g/L DDB-Co facilitated the dissolution of particulate organic matter and accelerates the conversion process of volatile fatty acids (VFA) in KW, thereby preventing excessive acidification. Microbial community structure analysis revealed that DDB-Co reduced microbial richness and diversity but selectively enriched microorganisms such as Lactobacillus and Methanothrix, accelerating the direct interspecific electron transfer (DIET) reaction rate and increasing biogas production. Metagenomic analysis demonstrated that DDB-Co primarily upregulated the relative abundances of key enzymes in the acetate decarboxylation methanogenesis pathway, such as acetate kinase (ackA), phosphate acetyltransferase (pta), and acetyl-CoA synthetase (acs), enhancing biogas yield and maintaining system stability. The findings of this work expand the high-value utilization pathways for digestate and provide an alternative solution for the efficient resource recovery of KW.