Wong, M.; (2011) Application of electrodialysis in integrated microbial fermentation and enzymatic biotransformation processes. Doctoral thesis , UCL (University College London).

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Abstract

Electrodialysis (ED) is an established technology used to transport small ions from one solution to another through an ion exchange (IE) membrane under the influence of an applied electric potential difference. This project aimed to develop a novel integrated bioreactor-ED system and to explore its application to a variety of bioprocesses including microbial fermentation and enzymatic bioconversion. A custom ED module was first designed and constructed that enabled the flexible configuration of different IE membranes. In order to establish the performance of the ED module for extraction of charged organic molecules the mass transfer rate of acetate, lactate and malate were first quantified as a function of key operating parameters such as membrane area and current applied. For extraction of acetate mass transfer rates of up to 2.5 g.L-1.h-1 could be achieved. The primary application considered for ED was to overcome inhibition by metabolic acetate by-product formation in fed-batch Escherichia coli (E.coli) fermentation. Conventionally, a controlled substrate feeding strategy would be employed to repress acetate formation but at the expense of bioreactor productivity. With the application of the integrated bioreactor-ED system it was shown that acetate was removed instantly as it was formed during fermentation. For the heterologous expression of the model protein GFP this resulted in enhancement of protein production by up-to four fold. The level of enhancement depended upon the rate of acetate removal, residence time of feed in the ED module and reducing ammonium toxicity. Additionally a novel ED configuration incorporating a bipolar membrane was used to facilitate bipolar electrodialysis (BPED). In addition to normal ED function the BPED module generate hydroxide ions in situ to facilitate pH control without the requirement for extraneous acid/base addition. BPED was shown to achieve the same enhanced levels of GFP production but with a 50% reduction in base addition. The wider application of both ED and BPED technology to enzymatic bioconversions was also investigated for the lipase catalysed hydrolysis of ethyl acetate and fumaric acid. ED and BPED were used in both phases of the bioprocess, which included the initial pH adjustment and in situ removal of inhibitory molecule. In this case result showed a two-folded increase in product yield and base addition was reduced by 60% when ED was applied. Overall this work has shown the wide range of potential applications and benefits of a novel integrated bioreactor-ED technology. It illustrates some of the critical design aspects for larger scale application and also considers the regulatory and commercial potential of the technology.

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