Enhanced production of lactic acid with reducing excess sludge by lactate fermentation

The development of a facile technology for utilizing effectively and/or reducing excess sludge is one of the urgent problems since a large quantity of sewage sludge is formed by activated sludge processes. Excess sludge containing 50 mM sucrose was fermented at 50 °C using endogenous bacteria in excess sludge, resulting in a high lactic acid production (8.45 g/L) and in an increased sludge reduction (38.2%). Conversion rate to lactic acid was up to 106.0% by standard fermentation at 50 °C compared to 43.8% at 30 °C and this phenomenon that conversion rate was higher was observed only at 50 °C as the fermentation at less or more than 50 °C had lower conversion rate than that at 50 °C. Lactic acid bacteria increased at 50 °C during 1-d fermentation whereas the number of total viable bacteria only increased slightly, indicating that lactic acid bacteria in sludge at 50 °C were preferentially able to utilize the sucrose for producing lactic acid. Finally, pH-vibration fermentation at 50 °C enabled to completely consume residual sucrose in the normal fermentation, resulting in the maximum production of lactic acid. Lactate fermentation by a purely cultured lactic acid bacterium TS1 with autoclaved excess sludge containing 50 mM sucrose had more than 100% of conversion rate to lactic acid, indicating that a part of sludge was converted into lactic acid during the fermentation. Our technique is useful as a facile engineering for reducing excess sludge concomitantly with producing lactic acid by lactate fermentation.