Metagenomic reveals the methanogenesis metabolic mechanism of high-solids anaerobic digestion of human feces under gradient domestication.

[Display omitted] • Gradient domestication could achieve high-solids anaerobic digestion of human feces. • Methane production rate was the highest when TS was increased to 11% • Granular sludge as inoculum can efficiently remove E. coli and Salmonella. • Key methanogenesis enzyme-encoding genes were increased abruptly at TS 12% and 13% • The main methanogenesis types were diverse at different TS loadings. High-solids anaerobic digestion (HSAD) of low C/N ratio waste was difficult to long-term running. In this study, human feces (HF) HSAD were operated in semicontinuous culture with HRT of 20 days for 190 days under mesophilic condition (38 ± 1 °C), granular sludge as inoculum was gradient domesticated by gradually increasing total solid (TS) (8 % to 17 %). The results showed that the highest tolerated concentration of TS achieved 16 % and biogas was stopped producing at TS 17 %. Maximum methane production rates of TS and volatile solid (VS) were obtained at TS 11 %, 287.08 mL/g TS and 67.67 mL/g vS respectively. AD system was inhibited when thresholds of total volatile fatty acids and total ammonia nitrogen were exceeded 2414.68 ± 207.62 mg/L and 4361.33 ± 143.86 mg/L, respectively. It was worth noted that granular sludge as an inoculum could efficiently remove pathogens (E. coli , 99.80 % at TS 11 % and Salmonella , 88.98 % at TS 12 %). High concentrations of HF (TS 13 % to 17 %) suppressed the growth of dominant methanogens (Methanothrix soehngenii , Methanothrix sp., and Methanothrix harundinacea.) and facilitated the rapid proliferation of acidogenic bacteria (Defluviitoga tunisiensis , Methanoculleus bourgensis , and Tepidanaerobacter acetatoxydans). The absolute abundance of key methanogenesis enzyme-encoding genes (mcr, frh, and fwd) were suddenly increased significantly at TS 12 % and 13 %. The main types of methanogenesis (acetoclastic at 11 % TS, hydrogenotrophic at 12 % TS and methylotrophic at 13 % TS) would change with the variation of different TS. The above studies provide guidance for HF HSAD domestication in practical AD applications. [ABSTRACT FROM AUTHOR]