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A process for the continuous production of high purity L-lactic acid in a membrane bioreactor at 65 degrees C has been developed. Two different Bacillus stearothermophilus strains have been tested in batch experiments. Lactic acid yields are between 60 and more than 95% of theoretical yields. The amounts of ethanol, acetate, and formate formed varied between 0 and 0.4, 0 and 0.1, and 0 and 0.5, respectively (mol/mol glucose). All byproducts are valuable and may be separated easily by rectification of the fermentation broth. Complete cell retention enables high volumetric productivity (5 g/Lh), and a minimum of growth supplements. The high temperature of 65 degrees C allows the autoselective fermentation without problems with contamination.

The thermophilic Bacillus strain BS119 was selected for this study to demonstrate the long term performance of lactic acid production and simultaneous pre-purification. Integrated continuous cell recycle cultivation using ultra-filtration membrane bioreactor (MBR) systems was investigated. The permeate from the MBR was routed to an on-line electrodialysis (ED) to recover, pre-purify and concentrate lactate. The cultivation and ED was operated at 60 degrees C for more than 1,000 h at a pH of 6.5. At lower dilution rate (0.02 h(-1)), lactate concentration reached a maximum of 55 g l(-1) with clearly lower residual glucose levels. At 0.04 h(-1), lactate concentration was significantly lower at 35 g l(-1). Maximal volumetric productivities of 1.38 g l(-1) h(-1) were achieved. Under stable conditions, lactic acid yield on consumed glucose appeared stable at around 80%. It could be demonstrated that the addition of supplements like yeast extract and peptone severely influences product formation. Integration of mono-polar ED with the MBR systems yields lactate solutions with concentrations of up to 115 g l(-1). Because of the low substrate feed concentrations (less than 50 g l(-1)), lactate flux was rather poor, reaching a low maximum of 140 g m(-2) h(-1); nevertheless, stack energy consumption was positive with an average of 0.49 kWh kg(-1) lactate.

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