Membrane bioreactor systems can enhance anaerobic lactic acid fermentation by reducing product inhibition, thus increasing productivity. In batch fermentations, the bioconversion of glucose is strongly inhibited in the presence of more than 100 g·L-1 lactic acid and is only possible when the product is simultaneously removed, which can be achieved by ceramic membrane filtration. The crossflow velocity is a more important determinant of flux than the transmembrane pressure. Therefore, to stabilize the performance of the membrane bioreactor system during continuous fermentation, the crossflow velocity was controlled by varying the biomass concentration, which was monitored in real-time using an optical sensor. Continuous fermentation under these conditions, thus, achieved a stable productivity of ~8 g·L-1·h-1 and the concentration of lactic acid was maintained at ~40 g·L-1 at a dilution rate of 0.2 h-1. No residual sugar was detected in the steady state with a feed concentration of 50 g·L-1.
Keywords: anaerobic membrane bioreactor; continuous fermentation; lactic acid; membrane filtration; membrane fouling; optical sensor.