Enhancement of propionic acid fraction in volatile fatty acids produced from sludge fermentation by the use of food waste and Propionibacterium acidipropionici

► A new method for propionic acid-enriched VFA fermentation was reported. ► The propionic acid percentage after optimization was much higher than that reported previously. ► The lactic acid from the first stage was found to be the substrate for propionic acid production in the second stage. ► Adding food waste to the pre-fermentation system of sludge improved lactic acid production due to the synergistic effect. ► The propionic acid-enriched VFA as supplementary carbon for BNR was observed to promote its performance.