Biogas slurry recirculation regulates food waste fermentation: Effects and mechanisms.

Regularly adding biogas slurry into fermentation reactors is an effective way to enhance hydrogen or methane production. However, how this method affects the production of valuable organic acids and alcohols is still being determined. This study investigated the effects of different addition ratios on semi-continuous fermentation reactors using food waste as a substrate. The results showed that an addition ratio of 0.2 increased lactic acid production by 30% with a yield of 0.38 ± 0.01 g/g VS, while a ratio of 0.4 resulted in mixed acid fermentation dominated by n-butyric acid (0.07 ± 0.01 g/g VS) and n-caproic acid (0.06 ± 0.00 g/g VS). The introduction of Bifidobacteriaceae by biogas slurry played a crucial role in increasing lactic acid production. In contrast, exclusive medium-chain fatty acid producers enhanced the synthesis of caproic acid and heptanoic acid via the reverse β-oxidation pathway. Mechanism analyses suggested that microbial community structure and activity, substrate hydrolysis, and cell membrane transport system and structure changed to varying degrees after adding biogas slurry.