The effect of CO2 sparging on high-solid acidogenic fermentation of food waste

Abstract Carbon dioxide (CO 2 ), a by-product of acidogenic fermentation, also acts as a reactant that affects carboxylic acid metabolism in reactors. However, the effect of headspace CO 2 in an acidogenic leach bed reactor (LBR) is unclear. In this study, high-solid acidogenic fermentation of food waste (FW) in LBRs with different fractions of CO 2 (CK: self-generated headspace, T1: 100% N 2 , T2: 30% CO 2 + 70% N 2 , and T3: 90% CO 2 + 10% N 2 ) was performed. CO 2 sparging significantly increased the acidogenic performance of FW, with T3 showing the highest soluble product yield of 0.81 g chemical oxygen demand per 1 g volatile solid removal (gCOD/gVS removal ). CO 2 sparging prolonged the fermentation of ethanol and lactic acid in T2 and T3 while it hindered the evolution of butyric and caproic acids. The relative abundance of the genus Limosilobacillus was enhanced by CO 2 sparging, reinforcing the ethanol and lactic acid metabolic pathways. However, sparging excessive CO 2 (≥30% CO 2 ) inhibited the colonization of the genus Clostridium sensu stricto 12, which suppressed the reverse β pathway with butyric and caproic acids as the metabolites. This work provides valuable insights into the targeted production of carboxylic acids from the acidogenic fermentation of FW via CO 2 regulation. Graphical abstract