Effect and mechanism of lactic acid on anaerobic digestion of acidified food waste

Biodegradable food waste undergoes various levels of acidification before anaerobic digestion (AD). The effect of lactic acid (LA) as an important acidic substance on AD remains unclear. In this study, the effects of different concentrations of LA (0–15 g/L) on methane production and microbial community were investigated. The results showed that acidified food waste significantly enhanced methane production. The highest methane production in the reactor containing 5 g/L of LA was 280.13 ± 1.89 mL/g VSS, which was 68.05% higher than that of the control. However, the lag time of methanogenesis increased exponentially with the concentration of LA. The cyclic voltammetry test result revealed that LA decreased the electron transfer capacity between cells during the AD of food waste. The peak value of the electron transfer rate constant significantly decreased from 0.0643 ± 0.0022 s−1 (control) to 0.0297 ± 0.0035 s−1 (10 g/L), thus significantly hindering the AD efficiency. Additionally, LA reduced the diversity and richness of microbial communities, decreasing the abundance of functional bacteria. Enzyme activity prediction indicated that the lactate dehydrogenase abundance in the experimental group was higher, promoting the degradation of LA. These findings provide a theoretical basis for the resourceful utilization of acidified food waste.