Lactic acid production from acidogenic fermentation of fruit and vegetable wastes

• Lactic acid with amount of 10–20 g/L was stably produced at pH 4.0.
• The fermentation type was finally converted into heterofermentation at pH 4.0.
• Hydrolysis was enhanced and lactic acid fermentation was improved at pH 5.0.
• Bifidobacterium played an important role for lactic acid production at pH 5.0.

This work focused on the lactic acid production from acidogenic fermentation of fruit and vegetable wastes treatment. A long term completely stirred tank reactor (CSTR) lasting for 50 days was operated at organic loading rate (OLR) of 11 gVS/(L d) and sludge retention time (SRT) of 3 days with pH controlled at 4.0 (1–24 day) and 5.0 (25–50 day). The results indicated that high amount of approximately 10–20 g/L lactic acid was produced at pH of 4.0 and the fermentation type converted from coexistence of homofermentation and heterofermentation into heterofermentation. At pH of 5.0, the hydrolysis reaction was improved and the total concentration of fermentation products increased up to 29.5 gCOD/L. The heterofermentation was maintained, however, bifidus pathway by Bifidobacterium played an important role.