Anaerobic co-digestion of corn straw, sewage sludge and fresh leachate: Focusing on synergistic/antagonistic effects and microbial mechanisms.

Effects of sewage sludge (SS) and fresh leachate (FL) addition on corn straw (CS) digestion and underlying mechanisms were investigated. Co-digestion of CS, SS and FL significantly increased cumulative methane production by 7.2-61.1%. Further analysis revealed that co-digestion acted mainly on slowly degradable substrates and exerted dual effects on methane production potential, which was closely related to the volatile solids (VS) content. Antagonistic effects of co-digestion resulted from the dominance of norank_c_Bathyarchaeia, a mixotrophic methanogen that may generate methane inefficiently and consume existing methane. The synergistic enhancement of methane production (0.7-12.7%) was achieved in co-digestion with 33.5-45.5% of total VS added as SS and FL. Co-digestion with more balanced nutrients and higher buffering capacity enriched Actinobacteriota, Firmicutes, and Synergistota, thereby facilitating the substrate degradation. Furthermore, the predominant acetoclastic methanogens, increased hydrogenotrophic methanogens, and decreased methylotrophic methanogens in the digester combined to prompt the synergy.