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

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 degrees 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.