Two-stage anaerobic digestion of food waste: Enhanced bioenergy production rate by steering lactate-type fermentation during hydrolysis-acidogenesis.

This study proposed a lactate-based two-stage anaerobic digestion (AD) process to enhance bioenergy production rate from food waste (FW) and investigated the effect of inoculum addition and culture pH on hydrolysis-acidogenesis and further methanization. A series of batch fermentations were performed with an enriched lactate-producing consortium and without inoculum addition under controlled (5.7) and uncontrolled pH (initial 6.7) conditions. The interplay between the studied factors dictated the fate of lactate, particularly if it is produced and accumulated in the fermentation broth or is consumed by butyrogenic bacteria. Only the self-fermentation of FW with uncontrolled pH resulted in lactate accumulation (0.2 g/g volatile solid (VS) fed) with limited off-gas production (0.32 NL/L) and VS losses (≈16%). Such lactate-rich broth was successfully digested through biochemical methane potential tests, resulting in a maximum bioenergy production rate of 2891 MJ/ton-VS fed per day, which was two-fold higher compared to that achieved by one-stage AD.