Responses of anaerobic digestion of food waste to coupling effects of inoculum origins, organic loads and pH control under high load: Process performance and microbial characteristics

Abstract This study investigated responses of anaerobic digestion (AD) of food waste (FW) with different inocula to varying organic loads and to pH control under high load in terms of process performance and microbial characteristics. Without pH control, digester inoculated by thickened sludge obtained high methane yield of 547.8 ± 27.8 mL/g VS under organic load of 7.5 g VS/L but was inhibited by volatile fatty acids (VFAs) under higher loads (15 and 30 g VS/L). However, digesters inoculated by anaerobic sludge obtained high methane yields of 575.9 ± 34.2, 569.3 ± 24.8 and 531.9 ± 26.2 mL/g VS under organic loads of 7.5, 15 and 30 g VS/L and VFAs inhibition only appeared under extremely high load of 45 g VS/L. Digesters under VFA inhibition with high load were significantly enhanced by controlling single ecological factor pH at 6.5, 7.0 and 7.5, as indicated by shorter lag phases, higher peak values of methane production rate, greater methane yields and fast VFAs degradation. Maximum methane recovery was obtained with pH control at 7.5 under high load. VFA inhibition was accompanied by the degeneration of ecological functions of Syntrophomonadaceae and unidentified Bacteroidales and the dominant growth of unidentified Clostridiales. Under high load and pH control, high stability was strongly associated with obvious growth of Methanosarcina, which enriched methanogenic pathways thus improved system robustness and tolerance to VFAs. Moreover, pH control stimulated the growth of syntrophic Bacteria Syntrophomonadaceae while maintaining the high activity of hydrogenotrophic methanogens therefore sustained efficient syntrophic communities of Bacteria and methanogens and avoided over accumulation of VFAs. pH control promoted adaptive selection of methanogens, leading to obvious decline of archaeal community diversity. This study provided practical guidance on digester configurations of high-load AD of FW and expanded the understanding of responses to coupling effects of inoculum origins, organic loads and pH control under high load concerning process performance and microbial community dynamics.