Enrichment of homoacetogens converting H₂/CO₂ into acids and ethanol and simultaneous methane production

An anaerobic granular sludge was enriched to utilize H₂/CO₂ in a continuous gas-fed up-flow anaerobic sludge reactor by applying operating conditions expected to produce acetic acid, butyric acid, and ethanol. Three stages of fermentation were found: Stage I with acetic acid accumulation with the highest concentration of 35 mM along with a pH decrease from initial 6 to 4.5. In Stage II, H₂/CO₂ was replaced by 100% H₂ to induce solventogenesis, whereas butyric acid was produced with the highest concentration of 2.5 mM. At stage III with 10 µM tungsten (W) addition, iso-valeric acid, valeric acid, and caproic acid were produced at pH 4.5-5.0. In the batch tests inoculated with the enriched sludge taken from the bioreactor (day 70), however, methane production occurred at pH 6. Exogenous 15 mM acetate addition enhanced both the H₂ and CO₂ consumption rate compared to exogenous 10, 30, and 45 mM acetate by the enriched sludge. Exogenous acetate was failed to be converted to ethanol using H₂ as electron donor by the enriched acetogens.