Improved production of propionic acid driven by hydrolyzed liquid containing high concentration of l-lactic acid from co-fermentation of food waste and sludge

[Display omitted] •Two-stage propionic acid fermentation at high Cs was evaluated.•High concentration of l-lactic acid severely inhibited propionic acid production.•OA of l-lactic acid above 80% was favorable for P. acidipropionici.•FBB was utilized to improve propionic acid production by 3.33 folds. This study investigated the feasibility of improved production propionic acid-enriched volatile fatty acid (VFA) from high concentration (Cs) of food waste and waste activated sludge (WAS) via lactic acid pathway by using of Propionibacterium acidipropionici. It was observed that production of l-lactate overwhelmed to d-lactate at first stage, which improved from 3.21 to 35.45gCOD/L with increase of substrate Cs. However, kinetic model analysis indicated that P. acidipropionici growth rate μmax was decreased with increase of l-lactate concentration, which explained second stage free cell fermentation of propionic acid was inhibited when fed by first stage liquid from R-40, R-55 and R-70. Then, the fibrous bed bioreactor was employed to eliminate the feed inhibition. The maximal percentage of propionic acid (68.3%) and production (16.31gCOD/L) was obtained by feeding liquid of R-55, which was improved by 3.33 folds compared to the free cell fermentation.