Optimizing ethanol pre-fermentation of food waste to enhance batch anaerobic digestion: Regulation of the acidification pathway

In anaerobic digestion (AD) of food waste (FW), the acetogenesis stage is thermodynamically limited and often leads to acid inhibition. This study investigated the impact of optimizing ethanol pre-fermentation (EP) of FW through saccharification pretreatment (SP) on AD by regulating the acidification pathway of substrates. The results showed that the ethanol concentration of FW after SP-EP reached 50.81 g/L, which was 2.11-fold of that in EP. SP-EP enriched hydrolytic and acidogenic bacteria such as Alkaliphilus, Caldicoprobacter, Sphaerochaeta, and Fermentimonas. The analysis showed that the abundance of enzyme-encoding genes catalyzing the conversion of ethanol to acetaldehyde in SP-EP was 1.17-fold higher than control, which further indicated that SP-EP facilitated the conversion of substrate to glucose → ethanol → acetaldehyde → acetate. This process provided abundant acetate for AD, and enriched the acetoclastic methanogens Methanothrix. Consequently, the cumulative methane yield of SP-EP (557 mL/g vs) was 7.49 %–17.79 % higher than the other three treatments. Hydrogenotrophic methanogens such as Methanoculleus were also enriched in SP-EP due to the fact that acetogenesis was accompanied by hydrogen production, which enhanced the hydrogenotrophic methanogenic pathway. This study provides a potential strategy for balancing the processes of acetogenesis and methanogenesis, providing a practical basis for the industrial application of methane production from FW.