Rate-limiting of hydrogen-producing acetogenesis to anaerobic digestion compared with methanogenesis

To identify the rate-limiting degree of hydrogen-producing acetogenesis and methanogenesis in anaerobic digestion, anaerobic activated sludge sampled from an UASB was cultivated at 37℃ with a series of initial pH ranged from 5.00 to 9.00. With butyric acid, acetic acid and H₂/CO₂ as substrate, respectively, the ecological amplitude of pH and the substrate conversion rate of syntrophic butyrate-oxidizing bacteria (SBOB), aceticlastic methanogens (ACM) and hydrogenotrophic methanogens (HTM) were evaluated according to the Shelford's tolerance law. The results showed that the ecological amplitudes of pH for SBOB, ACM and HTM were 6.19~8.59, 5.50~7.74 and 4.39~9.23, with an optimal pH of 7.39, 6.62 and 6.81 for their metabolism, respectively. With the optimal metabolic pH, the specific conversion rate of butyric acid, acetic acid and H₂/CO₂ by the sludge was 0.86, 1.04 and 1.09gCOD_equ/(gMLVSS·d), respectively. Obviously, the hydrogen-producing acetogens had narrower pH ecological amplitude and lower substrate conversion rate than the methanogens, indicating that hydrogen-producing acetogenesis in the anaerobic activated sludge had a stronger limitation on the anaerobic digestion.