High-rate lactic acid production from food waste and waste activated sludge via interactive control of pH adjustment and fermentation temperature

- The highest lactic acid yield was obtained at 3 d under pH 9 in mesophilic and pH 7 in thermophilic.
- Factors of pH and temperature interactively affected total and l-lactic acid production.
- Optimal pH for lactic acid production was decreased with the increase of temperature.
- Solubilization and hydrolysis efficiency increased with the ascent of pH and temperature.
- Fermentative bacteria diversified at pH 9 in mesophilic condition.

Lactic acid, a versatile platform molecular, could be bio-produced by synergistic fermenting food waste (FW) with waste activated sludge (WAS), which was a base consuming process and required a relatively longer fermentation time of 5 d at ambient temperature. To achieve high-rate lactic acid fermentation in a short time with less alkaline consumption, a new strategy was developed via less intermittent base addition at higher temperature. Results indicated that high-rate lactic acid production were quickly observed at 3 d either under mesophilic fermentation of weak alkaline (25.51 g COD/L at pH 9) or thermophilic of neutral condition (21.01 g COD/L at pH 7), in which l-lactic acid was the most prevalent isomer in both cases. Response surface methodology (RSM) verified that interaction effect of alkaline adjustment and temperature was significant for production of total and l-lactic acid, but not for that of d-lactic acid. This was due to the inconformity influence of base addition and temperature on each metabolic fermentative step. Although solubilization and hydrolysis stage were increased with the ascent of pH and temperature, lactic acid production was only enhanced at weak alkaline of mesophilic fermentation, but contrarily suppressed at thermophilic condition. Meanwhile, microbial community structures varied significantly from each reactor, which confirmed the interaction effects of pH adjustment and fermentation temperature lactic acid production.

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