Anaerobic Co-Digestion of Thermal Hydrolysis Pretreated Sludge and Food Waste: Insights of Different Solids Content on Methane Production, Substrate Metabolism, and Microbial Functional Profiles

This study assessed the anaerobic co-digestion (AcoD) potential of thermal hydrolysis (TH) pretreated waste activated sludge (WAS) and food waste (FW) across various solid contents (SC). Results demonstrated that along with the SC increased from 2 to 6%, the cumulative CH4 production was ascending from 85.0 to 96.3 mL/g VSS, and this value further improved from 99.2 to 122.0 mL/g VSS in the corresponding TH-treated reactors. TH improved soluble carbohydrate and protein concentrations from 34.2–80.7 and 51.1–380.1 mg/L in the untreated group to 59.3–172.1 and 129.3–731.9 mg/L in the corresponding TH-treated reactors. The surge in bioaccessible substrates contributed to the enrichment of hydrolysis acidogenic bacteria (e.g., Bacillota) and methanogens (e.g., Methanospirillum), and meanwhile stimulated their cooperative symbiosis (e.g., positive (93.14%)). Further explorations demonstrated that, accompanied with the more bioavailable organics induced by TH, the critical metabolic pathways (e.g., fatty acid biosynthesis and acetoclastic methanogenesis) and functional gene expression (e.g., mtrA, mtrB, and mtrC) were stimulated and upregulated, maintaining optimal metabolic activity for CH4 formation. This study investigated the response and mechanisms of CH4 production to SC during AcoD of TH pretreated-WAS and FW, which provided optimized strategy for energy recovery from organic wastes.