• • Optimal biochar dosage of 2.5 g/L increased cumulative methane yield by 39.47% (to 11.63 L) compared to control, and by 43.30% relative to thermally hydrolyzed sludge, demonstrating significant enhancement in energy recovery.
• • Methanogenic lag phase was reduced from (5.65±0.11) d to (4.33±0.12) d, indicating faster start-up and improved kinetics, which is critical for reducing reactor idle time and operational costs.
• • Average VFAs during stable operation decreased from 1708 mg/L to 1033 mg/L with biochar addition, mitigating acidification risk and enhancing process stability, essential for continuous operation.
• • Microbial community analysis showed enrichment of Synergistetes and Syntrophomonas, key for DIET, and Methanosarcina dominance, suggesting biochar promotes syntrophic metabolism, leading to higher methane production rates.