Influence of continuous membrane bioreactor operation on ferrichrome production using Ustilago maydis

Fermentations, utilizing a membrane bioreactor (MBR) can be advantageous as cells are retained in the vessel while soluble products are continuously withdrawn. It can be operated at high cell density, which allows increased productivity. Since hydraulic and solids residence times are independent of each other, MBRs offer an additional degree of freedom for process control. MBRs can be biologically distinguished from batch fermentors for various reasons: Like chemostats, by being operated in a distinct growth phase production of unwanted metabolites can be reduced. In addition, high cell densities may cause changes in metabolism. Although MBR fermentations have been studied frequently this influence of the operational mode, i.e. the membrane’s presence, has rarely been the main research focus. In this study, influences of the operational mode on kinetic parameters and productivity of Ustilago maydis are investigated by comparison with batch and chemostat cultures. In MBR fermentations, biomass concentrations rise up to 40 g·L–1. Possibly due to the effect of these high cell densities a low growth rate state is reached even at non-limiting substrate concentrations. MBR productivity reaches a maximum just after the onset of continuous fermentation and later decreases. Although the general possibility of MBR fermentation is shown, this highlights the need for optimization.

Yields, volumetric productivities, specific growth and production rates in batch and MBR fermentations have been compared for siderophores production using Ustilago maydis. Continuous fermentations both with and without biomass retention have successfully been carried out over a period of 2–3 weeks. Even at biomass concen- trations of 40 g·L–1 growth continued. In the MBR, two regions of different steady growth rates were observed. One corresponded to a steady S/X of around 1 where the specific growth rate was less than 0.005 h–1 although substrates concentration was well above a limiting value. If the presence of the membrane and therefore high cell densities caused the limited growth or if a vital nutrient was lacking will be the object of further investigations. Product yields were higher in batch fermentations. MBR productivity reached a maximum just after the onset of continuous fermentation and later decreased. The continuous process was shown to be generally possible but with regards to productivity, operation needs to be optimized. As MBRs offer the opportunity to independently control hydraulic and solids residence times, an optimal S/X can be adjusted. This way optimized process should yield higher volumetric productivity than batch fermentations.