Exploring the biogas production and microbial community from co-digestion of sewage sludge with municipal solid waste incineration fresh leachate

Fresh leachate (FL) rich in organic matter is a potential co-substrate for anaerobic treatment of sewage sludge (SS); however, the understanding of how sludge–leachate matrix affects the biogas production and microbial community remains incomplete. This study investigated the mesophilic co-digestion of SS with different proportions of FL (0, 5%, 15%, and 30%). The results showed that the total biogas production from co-digestion was enhanced by 17.7–66.3%. Further analysis suggested that co-digestion with 5% (R2) and 15% (R3) FL had positive effects on organic removal and methane conversion, while R4 with 30% FL imposed negative effects. Kinetic analysis indicated that both FL dosage and digestion time should be considered to determine the maximum predicted biogas production. Moreover, the microbial community analysis confirmed that the bacteria related to the degradation of complex organic matter showed high relative abundance. Firmicutes were enriched from 6.44% to 22.06% as the FL dosage increased; Bacteroidetes and Chloroflexi presented high relative abundance of 20.69–25.69% and 11.77–21.21% in all bioreactors. The methanogens were influenced by free ammonia, and the dominant methanogens shifted from Methanosaeta (R1–R3) to Methanobacterium (R4). This study demonstrated that FL could serve as co-substrate to improve the biogas production from anaerobic co-digestion of SS.