Mechanisms of how exogenous CO affects methane production in an optimized high-solid anaerobic digester treating co-substrates of sewage sludge and food waste.

The CO addition could promote anaerobic digestion, but the exploration on bioconversion mechanisms of exogenous CO in high-solid anaerobic digestion (HSAD) system is still insufficient. This study investigated the performance of a CO-added HSAD treating co-substrates of sewage sludge and food waste (FW). The maximum methane yield of 623.4 mL CH/g-VS was obtained with FW proportion of 75 %, organic loading of 3.7 g-VS/L/d and intermittent stirring. The CO addition could improve the methane yield by 11.8 % under the optimized conditions. Thermodynamic analysis showed that the most energetically favorable reaction for CH production was acetoclastic methanogenesis (AM), and the main bioconversion pathway of exogenous CO was homoacetogenesis (HA). Significantly higher methanogenic activity was achieved with CO addition during acetate decomposition testing, suggesting enhanced AM pathway. The AM methanogens Methanosaeta were also enriched. Therefore, the main mechanism of the enhanced methane production by CO addition was the facilitation of coupled HA-AM pathway.