Enhancing inhibited fermentations through a dynamic electro-membrane bioreactor

Reverse Electro-Enhanced Dialysis (REED) has been shown to intensify fermentations which are impaired by an ionic product inhibition. Productivity can be greatly enhanced by the in situ product removal from the cultivation broth. The REED process has been tested for different applications, where its strong potential for increasing productivity and product yield has been verified.
REED uses ion exchange membranes and electrical potential gradients to selectively separate the target ion. The main limitation of using membrane separation combined with bioreactors is membrane fouling. REED technology ensures long operation time by reversing periodically the polarity of the imposed electrical field to significantly reduce the influence of membrane fouling. The periodic nature of the electrically driven membrane separation process makes the membrane bioreactor operation non trivial. This challenging operation is associated with different dynamic behaviors of the individual units plus their interaction.
The purpose of this contribution is to show the results of experimental and model based efforts done in order to investigate the operation of a membrane bioreactor. From modeling point of view, it is interesting to reveal to which extend the REED module can facilitate the pH control in the fermenter. In this case, the membrane and reactor unit interactions are exploited to substantially increase the lactate productivity and substrate utilization compared to a conventional fermentation with a crude control of pH.
Experiments using multiple stacks with asynchronical current reversal intervals for improved pH stability were carried out in a bioreactor connected to a REED system. The REED was used for control of the pH process parameter of the bioreactor through exchanging the lactate ions (from lactic acid produced in the bioreactor) with hydroxide ions, which maintained a pH close to optimal growing conditions. The ion-exchange was in turn regulated by a PID control unit, which adjusted the electrical current output between the REED electrodes to match the growing production speed of lactic acid, which increased during the trials as the LAB culture grew in numbers simultaneously.
For the single stack trial, the fluctuations deviate significantly more from the setpoint, compared to the similar setup with multiple stacks operated with dispersed, asyncronical current reversal intervals. For the single stack, no other effect helps to stabilize the pH fluctuations until the current reversal transition has passed and process parameter control is reassessed to bring back the pH to its setpoint. For multiple stacks, the operating stacks immediately respond to the impact of one stack going into current reversal and increase their effect while the single stack recovers. Thereby the deviations are significantly lower, which is preferable, especially when operating with microbial process liquids or continuous process solutions.