Scale-down model to simulate spatial pH variations in large-scale bioreactors

For the first time a laboratory-scale two-compartment system was used to investigate the effects of pH fluctuations consequent to large scales of operation on microorganisms. pH fluctuations can develop in production-scale fermenters as a consequence of the combined effects of poor mixing and adding concentrated reagents at the liquid surface for control of the bulk pH. Bacillus subtilis was used as a model culture since in addition to its sensitivity to dissolved oxygen levels, the production of the metabolites, acetoin and 2,3-butanediol, is sensitive to pH values between 6.5 and 7.2. The scale-down model consisted of a stirred tank reactor (STR) and a recycle loop containing a plug flow reactor (PFR), with the pH in the stirred tank being maintained at 6.5 by addition of alkali in the loop. Different residence times in the loop simulated the exposure time of fluid elements to high values of pH in the vicinity of the addition point in large bioreactors and tracer experiments were performed to characterise the residence time distribution in it. Since the culture was sensitive to dissolved oxygen, for each experiment with pH control by adding base into the PFR, equivalent experiments were conducted with pH control by addition of base into the STR, thus ensuring that any dissolved oxygen effects were common to both types of experiments. The present study indicates that although biomass concentration remained unaffected by pH variations, product formation was influenced by residence times in the PFR of 60 sec or longer. These changes in metabolism are thought to be linked to both the sensitivity of the acetoin and 2,3-butanediol-forming enzymes to pH and to the inducing effects of dissociated acetate on the acetolactate synthase enzyme.