Enhancement of methane production from anaerobic co-digestion of food waste and dewatered sludge by thermal, ultrasonic and alkaline technologies integrated with protease pretreatment

[Display omitted] •Methane yield was increased 104.2% by thermal-netural protease pretreatment.•The release and degradation properties of organic matter were improved.•Functional microorganisms were enriched by thermal-netural protease pretreatment.•Thermal-netural protease pretreatment enhanced the activities of key enzymes. Pretreatments to improve the efficiency of anaerobic digestion (AD) have gained more attention. The efficiency and mechanism of neutral protease (NP) integrated with other methods remain unclear. This study investigated the efficacy of thermal, alkaline and ultrasonic technologies integrated with NP as the pre-treatments for AD of food waste and dewatered sludge. Results showed the thermal method integrated with NP (TH-NP) was the most effective, achieving a 104.2% improvement in methane production. In this case, TH-NP increased soluble chemical oxygen demand and protein concentrations by 8.6% and 39.8%, respectively. Microbial community analysis indicated that TH-NP promoted the symbiosis between Woesearchaeales and hydrogenotrophic methanogenesis. Furthermore, the PICRUSt2 analysis revealed that TH-NP increased the activities of most enzymes in the acetate and propionate metabolic pathways. In summary, TH-NP is more effective in increasing the AD efficiency compared to other combined pretreatments. This study provides theoretical support for protease-induced pretreatment technology.