Effects of reduced hydraulic retention time and self-induced mixing on two-stage anaerobic digestion utilizing acidogenic off-gas

The hydraulic retention time (HRT) and acidogenic off-gas (AOG) solubility in methanogenic digestate as the critical parameters are tightly correlate with the AOG utilization and the overall process, however, their potential impacts in the AOG coupled two-stage AD system remain unclear. In this study, the decreasing HRT and methanogenic self-induced mixing were conducted in a thermophilic two-stage reactor system that treats food waste, chicken manure and corn straw, and the acidogenic gas beyond atmospheric pressure was continually monitored and pumped into the bottom of the methanogenic reactor. The HRT of the acidification/methanogenesis stage was set at 96 h/12 d, 80 h/10 d, 64 h/8 d and 48 h/6 d, respectively. The highest hydrogen (H2) production of 96.4 mL/g VSin and H2 content of 50.8 % were obtained at an HRT of 48 h/6 d, though this also increased system instability, hindering organic matter removal and decreasing the specific methane yield (SMY) from 597.1 to 492.1 mL/g VS. Self-induced mixing positively impacted both SMY and volumetric methane yield, increasing them by 2.35 % and 9.16 %, respectively. As the HRT decreased, the dominance of Caproiciproducens appears to enhance both hydrogen production and its content. AOG utilization favored methanogenic microbes and increased agitation improved microbial community diversity. Compared to Methanosarcina, obligate hydrogenotrophic methanogens including Methanothermobacter and Methanoculleus showed stronger adaptability to reduced HRTs. Total VFAs provides the highest explanation for changes in microbial community structure. This study offers valuable insights for advancing two-stage AD technology.