Biochar-Enhanced Anaerobic Co-digestion of Kitchen Waste and Excess Sludge: Performance, Stability, and Microbial Community Dynamics
Kitchen waste (KW) and excess sludge (ES) are urban biowastes with resource recovery potential, commonly treated via anaerobic digestion (AD) for methane production. However, KW mono-digestion suffers from acidification, while ES yields low methane. This study employed semi-continuous reactors to simulate practical AD, co-digesting KW and ES at a 4:1 volatile solids ratio with biochar addition (0.5, 1.0, 2.5, 5.0, 10.0 g/L). The optimal biochar dosage was 2.5 g/L, achieving cumulative biogas and methane volumes of 17.53 L and 11.63 L, respectively, representing 42.10% and 39.47% increases over the biochar-free control, and 34.45% and 43.30% enhancements relative to thermally hydrolyzed sludge. The methanogenic lag phase decreased from (5.65±0.11) d to (4.33±0.12) d. Process stability improved, with average volatile fatty acids (VFAs) during stable operation dropping from 1708 mg/L to 1033 mg/L. Microbial analysis revealed enhanced diversity and enrichment of Synergistetes and Syntrophomonas, indicating direct interspecies electron transfer (DIET) promotion. Biochar at low concentrations enhances AD by immobilizing microbes and facilitating electron transfer, while high concentrations (10 g/L) may inhibit methanogenesis due to fatty acid degradation blockage, yet total methane production remained above control. These findings demonstrate that biochar addition at 2.5 g/L effectively enhances methane production and process stability in KW-ES co-digestion.