Synergistic effects of enriched inoculum and headspace pressure on acidogenesis and its effect on methane yield in two-stage anaerobic digestion of food waste

Regulation of in situ hydrogen partial pressure enhances hydrolysis efficiency and reduces hydraulic retention time during the acidogenic phase of kitchen waste digestion. High-solids anaerobic digestion systems (> 20% total solids) require a comprehensive understanding of acidogenic microorganisms that can withstand dynamic headspace pressures and varying substrate compositions. This research explores a two-stage digestion process incorporating an enriched inoculum with pressure-assisted hydrolysis in a leach bed reactor (LBR) linked to an up-flow anaerobic sludge blanket (UASB) reactor to optimize energy recovery from kitchen waste (KW). The KW was mixed with the enriched inoculum and digested in LBRs under controlled headspace pressures of 3 psi (20.7 kPa) and 12 psi (82.7 kPa), using pre-mixed biogas (CH 4 :CO 2 = 60:40). The system operated at 12 psi exhibited elevated levels of soluble chemical oxygen demand (sCOD: 559.2 g/kg VS) and total soluble products (TSP: 362 g/kg VS) compared to the 3 psi condition (sCOD: 425.1 g/kg VS; TSP: 294 g/kg VS). Microbial community analysis revealed pressure-driven shifts favouring Pseudomonas , Escherichia , and Clostridium species. Metabolite profiling indicated butyrate dominance at 12 psi, while propionate concentrations were higher at 3 psi. Methane yield in the UASB reactor paired with the 12 psi LBR (421.8 L/kg VS) was higher relative to the 3-psi setup (310.1 L/kg VS). These findings underscore the effectiveness of combining tailored microbial consortia with pressure and gas composition control to enhance intermediate metabolite conversion and overall bioenergy output in high-solids anaerobic digestion systems.