Salinity enhances high optically active L-lactate production from co-fermentation of food waste and waste activated sludge: Unveiling the response of microbial community shift and functional profiling.

Lactic acid (LA), a versatile platform molecule, can be fermented from organic wastes, such as food waste and waste activated sludge. In this study, an efficient approach using salt, a component of food waste as an additive, was proposed to increase LA production. The LA productivity was increased at 10 g NaCl/L and optical pure L-lactate was obtained at 30 g NaCl/L. The enhancement of LA was in accordance with the increased solubilization and the critical hydrolase activities under saline conditions. Moreover, high salinity (30-50 g NaCl/L) changed the common conversion of LA to volatile fatty acids. In addition, the key LA bacteria genera (Bacillus, Enterococcus, Lactobacillus) were selectively enriched under saline conditions. Strong correlations between salinity and functional genes for L-LA production were also observed. This study provides a practical way for the enrichment of L-LA with high optical activity from organic wastes.