Effects of organic loading rate and reflux ratio on two-phase anaerobic digestion of food waste

Objective  To examine the effects of different organic loading rate (OLR) and effluent recirculation rate on acid and gas productions during the two-phase anaerobic digestion of food waste.

Method  The effects of acid and gas productions of the two-phase anaerobic digestion of food waste were investigated by setting different organic loading rate and reflux ratio.

Result  Effluent recirculation significantly increased the pH of acidogenic reactor and accelerated the acidification of food waste. The concentrations of volatile fatty acid (VFA) in the acidogenic reactors were 11 598.48, 12 998.41 and 14 967.64 mg·L^–1, increased by 9.06%, 22.23% and 40.74% compared with CK when the recirculation rate was 10%, 30% and 50% at the 11.33 g·L^–1·d^–1 of OLR (based on volatile solid content, the same below), respectively. Under the condition of non-recirculation, the optimum OLR of methanogenic reactor was 6.38 g·L^–1·d^–1, but when it enhanced to 8.50 g·L^–1·d^–1, the system showed acid suppression phenomenon. The reflux ratio of 50% could help the system buffer and stabilize. The average methane production under the reflux ratio of 50% was 486.14 mL·g^–1 (based on volatile solid content), which was 29.84%, 20.80% and 9.13% higher than those under the reflux ratios of 0% (CK), 10% and 30%, respectively.

Conclusion  During the two-phase anaerobic digestion process of food waste, the optimum OLR of the methanogenic phase is 6.38 g·L^–1·d^–1 when the biogas slurry is not refluxed, and the system will appear acid inhibition when the OLR continuously increases. The optimum OLR of methanogenic phase could be increased to 8.50 g·L^–1·d^–1 when biogas reflux ratio is 50%.