Pulsed CO₂ feeding promotes methane yield in anaerobic digester by diverting acidogenic pathways

CO₂ injection or biogas recirculation can affect the process of anaerobic digestion, but there is a lack of systematic study on the injection strategy in the continuously operated reactors. In this study, the performance of a continuous stirred anaerobic digester treating food waste were evaluated under different injection rates (RC1-1.32 L/L/d v.s. RC2-0.66 L/L/d) and different CO₂ injection modes (pulsed feeding v.s. continuous feeding). Under the pulsed CO₂ feeding mode, the methane yields of RC1 and RC2 were increased by 19.3% and 27.1%, respectively as compared to the control reactor, correspondingly 34.5% and 59.0% of injected CO₂ was reduced. When transferred to continuous feeding CO₂ mode, only 12.4% increment of methane yield was found in RC1 as compared to RB. The intermediate metabolites analyses indicated that CO₂ enrichment reinforced acetic acid and lactic acid production pathway and weakened the propionic acid pathway, which is favorable for elevating methane yield. Microbial community analysis revealed that the relative abundance of hydrogenotrophic methanogens increased accompanied with some acidogenic consortia and CO₂ fixation associating consortia. This work provides a new approach to promote methane generation from organic wastes with sequestered more CO₂ thus contributing to a circular economy.