The selective suppression of hydrogenotrophic methanogenesis remains a major challenge for stable CO2 bioacetogenesis in gas fermentation. Most studies rely on repeated additions of costly methanogenesis inhibitors. To overcome this challenge, we proposed a control strategy using endogenous microbial metabolites and controlled free volatile fatty acid (FVFA) accumulation to redirect CO2 utilization toward stable acetogenesis. A 15 L mesophilic anaerobic membrane bioreactor fed with H2 and CO2 was operated for 133 days, with an initial pH shock followed by FVFA regulation. The pH 3.5 shock stimulated acetogenesis, but methane production remained at 1.1 ± 0.3 L/L/d. Methanogenesis declined when FVFA reached 0.53 g-HAc/L, suggesting that FVFA accumulation was a key driver of niche selection and microbial succession. Pathway redirection was accompanied by a shift in community dominance from Methanobacterium (78.5%) to Clostridium sensu stricto 12, whose relative abundance increased from 1.4% to 45.3%. At a constant hydraulic retention time of 12 d, we adjusted the pH and carbon loading rate to maintain FVFA at 5.4 ± 0.7 g-HAc/L, which supported stable homoacetogenesis. The anaerobic membrane bioreactor achieved a maximum VFA concentration of 15.9 g-HAc/L, an average net VFA production rate of approximately 1.0 g-HAc/L/d, and an acetate selectivity of approximately 93%. This rate compares favorably with the 0.15–0.88 g-HAc/L/d reported for most inhibitor-assisted systems included in our comparison. Membrane separation retained biomass and produced acetate-rich permeate with negligible suspended solids, facilitating downstream recovery. These findings support controlled FVFA accumulation as a potential operational strategy for sustaining CO2 bioacetogenesis.
