Browsing by Author "Obondo, Tobias Omondi"

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    An investigation into process limitations in membrane bioreactor (MBR) systems used for lactic acid production
    (Stellenbosch : Stellenbosch University, 2022-04) Obondo, Tobias Omondi; Pillay, Visvanathan Lingamvrthi; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Lactic acid is conventionally produced through batch fermentation which suffers a major challenge of low lactic acid productivity due to end-product inhibition and low cell density in the fermenter. To overcome this challenge, several investigations have been focused on continuous lactic acid fermentation using membrane bioreactor (MBR) systems which have proved to be very promising in improving lactic acid productivity by alleviating the end-product inhibitory effects and increasing cell density in the fermenter. However, none of these MBR systems have been upscaled for industrial applications probably due to some process limitations associated with them. Therefore, the present study postulated and investigated some of these possible process limitations such as membrane fouling limitations, lactate inhibition to bacterial cells once the membrane has fouled limitations, nutrient limitations, and mass transfer limitations. A laboratory scale MBR system that consisted of an ultrafiltration (UF) membrane externally connected to a fermenter was used for continuous lactic acid fermentations at glucose concentrations of 60 g/L, 90 g/L and 120 g/L. The MBR system was operated at sub-critical flux conditions. Significant membrane fouling indicated by the permeate flux decline throughout the fermentation period was realized at all the glucose concentrations investigated, and it was most severe at glucose concentration of 120 g/L due to high biomass concentrations that blocked the membrane pores. Furthermore, the total volume of permeate at the end of the continuous fermentation runs at each glucose concentration investigated decreased in the order of 60 g/L > 90 g/L > 120 g/L, which confirmed the severe membrane fouling at higher glucose concentrations. For the investigation of lactate inhibition to bacterial cells once the membrane has fouled limitations, it was observed that lactate productivity decreased at all the glucose concentrations investigated. Similarly, there was noticeable decrease in biomass concentrations at glucose concentration of 120 g/L compared to 90 g/L and 60 g/L. Hence, this process limitation was found to have a significant impact on these MBR systems. The investigation on nutrient limitations was inconclusive since all the glucose concentrations investigated turned out to be above the threshold substrate concentration. On the other hand, mass transfer limitations were not found in these MBR systems. Since there was significant membrane fouling contrary to minimal fouling that was expected when the MBR system was operated at sub-critical flux conditions, further set of critical flux experiments using lactate fermentation broths were conducted whereby it was found out that the significant membrane fouling was probably due to the low cross-flow velocity that was used to avoid the possibility of bacterial cell damage. These experiments also established that operating below the critical flux conditions can significantly lower membrane fouling in MBR systems used for lactic acid fermentation, but this is only possible at high cross-flow velocities. The present study, therefore, identified membrane fouling and lactate inhibition to bacterial cells once the membrane has fouled as the main process limitations that have a significant impact on the MBR systems used for lactic acid fermentation. To improve these systems for industrial upscaling, further studies on suitable methods to minimize membrane fouling are necessary.