COMBINING MEMBRANE SEPARATION AND FERMENTATION PROCESSES FOR IMPROVED PERFORMANCE
Integration of life sciences & engineering
Biochemical Engineering (T5-1)
Keywords: fermentation, membrane separation, integrated processes,
Introduction
Fermentation processes are often inhibited by either products or biproducts produced by the microorganisms thereby limiting the maximum product concentration which can be obtained during the fermentation. Also it will influence the maximum biomass concentration which can be obtained thereby decreasing the production rate. A typical batch fermentation consist of a lag phase where the microorganisms “wake up” followed by an exponential growth phase which goes into a stationary phase depending on the inhibiting effect of the different products. Removing these products directly from the fermentation by a membrane separation process makes it possible to operate at much higher biomass concentrations thereby increasing the production rate as well as the final product concentration which might have a beneficial influence on the further downstream processing. Depending on the type of products or inhibiting biproducts such membrane processes can be either ultra/microfiltration (size related separations), membrane distillation (volatile products like bioethanol) or electro membrane processes (charged products like organic acids or bases). Combining fermentation directly with membrane separation will further make it possible to operate at high cell density in even a continuous fermentation process which is today used on large scale in waste water treatment plants using membrane bioreactors (MBR) where the membrane plant can either be internal (submerged membranes) or external (recycle loop) to the biological waste water treatment plant.
REED Technology
The Reverse Electro-Enhanced Dialysis (REED) setup and design combines elements from electrodialysis reversal (EDR) and Donnan dialysis (DD) operations. The REED unit improves fermenter productivity by continuously removing inhibiting organic acids and replacing them by alkaline hydroxide ions. This removes the acids, which would otherwise inhibit the growth of the microorganisms, as well as provides a pH regulation of the fermenter. Fermentation broth is continuously taken from the fermenter and pumped into the REED unit and then back to the fermenter. The REED unit is a plate-and-frame module where ion-exchange membranes are placed in layers, separated by flow spacers, which facilitate the passage of the fermentation broth between each set of membranes. Inside the REED unit, the broth is diverted into every second flow spacer, where it streams along the surfaces of the two ion-exchange membranes on either side of the spacer. Inside the alternating flow spacers, an alkaline stream (the Dialysate) flows without direct contact to the broth. The ion-exchange membranes are positively charged, which allows the negatively charged ions to be exchanged through the membranes. Positively charged ions are excluded from passing through the membranes because they hold similar charges. The ion transport is carried by electrical current, which is added across the module from electrodes placed at each end of the stack. Since the concentration of organic acids is significantly higher than that of hydroxide ions in the broth, the current is mainly carried by the organics acids, which are transported to and through the ion-exchange membranes. In the alkaline solution, the concentration of alkaline hydroxide ions is much higher than that of the organic acids, which enters the solution through the membranes. Thus, in the alkaline solution the electrical current is mainly carried by hydroxide ions moving through the ion-exchange membranes and into the fermentation broth. This overall exchange of negative ions preserves electro-neutrality.
Bio-matter in the form of microorganisms, yeast extracts and other organic particular components as well as proteins and other high-molecular components are too bulky to pass through the dense ion-exchange membranes. But the bio-matter and proteins tend to foul the membrane surfaces, causing a build-up of organic matter on the membranes inside the flow spacers with fermentation broth. This membrane fouling causes a steady increase in the electrical resistance of the membrane stack. All the negative ions migrate in the same direction, towards the positive electrode (anode). Inside each flow spacer carrying fermentation broth, organic acids are leaving to one side while hydroxide ions are entering from the opposite site. The hydroxide ions entering the flow spacer through the ion exchange membranes destabilize any build-up of fouling on that side, which they enter. On the other side of the flow spacer, fouling builds up, only reduced by the shear stress from the bulk flow in the spacer along the membrane surface, which drags some of the top fouling layer off. The symmetrical setup of the REED system allows the direction of the electrical current to be completely reversed while the overall separation is continued. Thereby the fouling is effectively controlled and kept at a low value which makes it possible to operate in continuous mode for very long time.
Conclusion
The ability of the patented reverse electro-enhanced dialysis (REED) separation system to extract growth inhibiting lactic acid during live lactic acid bacteria fermentations has proven significant boosts in productivity and product yield and several case studies will be presented.
Presented Wednesday 19, 12:20 to 12:40, in session Biochemical Engineering (T5-1).
