Stabilizing High-Load Food Waste Digesters: Bioaugmentation versus pH Adjustment Effects on Microbial Consortium Dynamics and Methanogenesis

High organic loading of food waste digesters often causes volatile fatty acid (VFA) accumulation, ammonia inhibition, and process failure. This study compared pH adjustment with bioaugmentation using a rumen-derived syntrophic consortium (FUD-BAC1) for stabilizing high-load food waste digesters, emphasizing microbial community dynamics and methane production. Three identical mesophilic (37 °C) continuous stirred-tank reactors (4.0 L working volume) were run in parallel, one reactor per treatment: an unamended control (R_Ctrl), a pH-adjusted reactor (R_pH; NaHCO₃ or NaOH added when pH fell below 6.5), and a bioaugmented reactor (R_Bio; single 10% v/v dose of FUD-BAC1). Biogas, methane, VFAs, total ammonia nitrogen (TAN), and culturable microbial groups were monitored for 27 days. The control produced minimal biogas (2.7 ± 0.8 L) and no measurable methane. R_pH produced 9.4 ± 1.2 L of biogas (43.6 ± 2.1% CH₄; 8.5 ± 1.2 L CH₄/kg VS), whereas R_Bio produced 19.4 ± 1.5 L (60.8 ± 2.5% CH4; 23.1 ± 1.8 L CH4/kg VS) and achieved the highest substrate degradation (79.2 ± 2.8%, versus 49.0 ± 3.2% for R_pH and 6.4 ± 1.5% for the control). Relative to R_pH, the specific methane yield of R_Bio was about 2.7 times higher. Final total VFA was 3,900, 2,650, and 1,800 mg/L, and final TAN was 2,800, 2,100, and 1,950 mg/L, in the control, R_pH, and R_Bio, respectively. Bioaugmentation was accompanied by higher counts of culturable anaerobic bacteria and methanogens, and by the isolation of presumptive syntrophic acetogens. Under the conditions tested, rumen-derived bioaugmentation was associated with more effective restoration of methanogenesis than pH adjustment alone; replicated, longer-term trials are needed to confirm these findings. This work relates to SDG 7 (Affordable and Clean Energy) and SDG 12 (Responsible Consumption and Production).