High-load semi-continuous anaerobic digestion of kitchen waste: Regulation by iron oxide-modified digestate-derived biochar and metagenomic analysis of microbial functions

This study systematically investigated the effects of digestate-derived biochar (BC) and iron-oxide-modified digestate-derived biochar (BC-Fe2O3) on high-loading kitchen waste anaerobic digestion at organic loading rates (OLRs) of 5.0, 8.0, 11.0, 14.0, and 17.0 g VS/L/d. The results showed that BC-Fe2O3 had a specific surface area of 41.72 m2/g, approximately threefold higher than that of BC, and exhibited more developed pore structures and abundant surface functional groups. At an OLR of 5.0 g VS/L/d, methane yields across all reactors ranged from 550 to 588 mL/g VS and did not differ significantly (p > 0.05). When the OLR was increased to 8.0 g VS/L/d, the methane yield in the BC-Fe2O3 group reached 625.66 mL/g VS, whereas those in the contrast and BC groups decreased to 261.01 and 280.61 mL/g VS, respectively. At OLRs of 11.0 and 14.0 g VS/L/d, the BC-Fe2O3 maintained methane yields of 593.39 and 519.39 mL/g VS respectively while both the contrast and BC groups ceased methane production entirely. Metagenomic analyses revealed that the addition of BC-Fe2O3 significantly increased the abundance of genes and enzymes involved in methane metabolism. Under high OLR conditions, this effect was particularly pronounced as it enhanced expression key enzymes (e.g., phosphoacetyltransferase, isothiocyanate reductase) and promoted activity methylamine-to-methane pathway (M00563), thereby substantially improving system methane yields.