Lactic acid accumulation from sludge and food waste to improve the yield of propionic acid-enriched VFA

• Lactic acid accumulation strategy improved the yield of propionic acid production.
• Consumption and production of lactic acid were favorable at 35 °C.
• Consumption of lactic acid was severely inhibited at 50 °C.
• Fermenting initially at 50 °C for 4 h enhanced lactic acid accumulation.
• Propionic acid from lactic acid was improved 47.1% than that of previous method.

According to our previous study, propionic acid-enriched volatile fatty acids (VFA), the preferred carbon source of biological wastewater nutrient removal, can be produced via the lactic acid pathway during the two-stage fermentation of organic wastes using Propionibacterium acidipropionici. In the current study a new strategy, based on lactic acid accumulation from sludge and food waste, was introduced to significantly increase the yield of propionic acid-enriched VFA. First, the effect of different temperature on the two-stage fermentation was fully discussed. Both advantages and disadvantages of the fermentation in different temperatures were elaborated. Furthermore, the proposed metabolic pathway at 35 °C and 50 °C was introduced. It was observed that by initially controlling temperature at 50 °C for 4 h and subsequently at 35 °C for 2 d, the steady and maximal accumulation of lactic acid was obtained in the first stage, which resulted in the yield of propionic acid-enriched VFA in the second stage being increased to 15.3 g COD/L and the percentage of propionic acid in VFA reaching 69.9%.

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