An investigation into process limitations in membrane bioreactor (MBR) systems used for lactic acid production

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dc.contributor.advisorPillay, Visvanathan Lingamvrthien_ZA
dc.contributor.authorObondo, Tobias Omondien_ZA
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Process Engineering.en_ZA
dc.date.accessioned2022-03-09T09:22:12Zen_ZA
dc.date.accessioned2022-04-29T09:25:00Zen_ZA
dc.date.available2022-03-09T09:22:12Zen_ZA
dc.date.available2022-04-29T09:25:00Zen_ZA
dc.date.issued2022-04en_ZA
dc.descriptionThesis (MA)--Stellenbosch University, 2022.en_ZA
dc.description.abstractENGLISH 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.en_ZA
dc.description.abstractAFRIKAANS OPSOMMING: Melksuur word konvensioneel geproduseer deur lotfermentasie wat die groot uitdaging duld van lae melksuurproduktiwiteit as gevolg van eindprodukinhibisie en lae seldigtheid in die fermenteerder. Om hierdie uitdaging te oorkom, het verskeie ondersoeke op kontinue melksuurfermentasie gefokus deur die gebruik van membraan bioreaktor (MBR) sisteme wat bewys is om ’n belowende verbetering van melksuurproduktiwiteit te hê deur die eindproduk se inhiberende effekte te verlig en seldigtheid in die fermenteerder te verhoog. Geen van hierdie MBR-sisteme is egter opgeskaal vir industriële toepassing nie, tien teen een as gevolg van sommige prosesbeperkinge geassosieer daarmee. Daarom het die huidige studie sommige van hierdie moontlike prosesbeperkinge soos membraanaanpakkingbeperkinge, laktaatinhibisie op bakteriële selle nadat die membraan aangepak het, nutriëntbeperkinge, en massa-oordragbeperkinge, gepostuleer en ondersoek. ’n Laboratoriumskaal MBR-sisteem wat bestaan uit ’n ultrafiltrasie (UF) membraan wat ekstern aan ’n fermenteerder gekonnekteer is, is gebruik vir kontinue melksuurfermentasies by glukosekonsentrasies van 60 g/L, 90 g/L en 120 g/L. Die MBR-sisteem is bedryf by subkritiese fluks kondisies. Beduidende membraanaanpakking aangedui deur die deurlaat fluks afname deur die fermentasieperiode is gerealiseer by al die glukosekonsentrasies ondersoek, en dit was ergste by glukosekonsentrasie van 120 g/L as gevolg van hoë biomassakonsentrasies wat die membraanporieë geblok het. Verder, die totale volume deurlaat aan die einde van die kontinue fermentasielopies by elke glukosekonsentrasie ondersoek, het afgeneem in die orde van 60 g/L > 90 g/L >120 g/L, wat die ergste membraanaanpakking by hoër glukosekonsentrasies bevestig. Vir die ondersoek van laktaatinhibisie op bakteriële selle nadat die membraan aangepak het, is dit waargeneem dat laktaatproduktiwiteit afgeneem het by al die glukosekonsentrasies ondersoek. Soortgelyk was daar opmerklike afname in biomassakonsentrasies by glukosekonsentrasie van 120 g/L in vergelyking met 90 g/L en 60 g/L. Daarom is hierdie prosesbeperking gevind om ’n beduidende impak op hierdie MBR-sisteme te hê. Die ondersoek op nutriëntbeperkinge was onbeslis aangesien al die glukosekonsentrasies ondersoek bo die drempel substraatkonsentrasie was. Aan die anderkant was massa-oordragbeperkinge nie gevind in hierdie MBR-sisteme nie. Aangesien daar beduidende membraanaanpakking was, anders as wat verwag is toe die MBR-sisteem bedryf is by subkritiese fluks kondisies, is ʼn verdere stel kritiese fluks eksperimente uitgevoer deur laktaatfermentasiesop te gebruik waar dit bevind is dat die beduidende membraanaanpakking moontlik as gevolg van die lae kruisvloeisnelheid is wat gebruik word om die moontlikheid van bakteriële selbeskadiging te vermy. Hierdie eksperimente het ook vasgestel dat bedryf onder die kritiese fluks kondisies membraanaanpakking in MBR-sisteme wat gebruik word vir melksuurfermentasie, beduidend kan verlaag, maar dis slegs moontlik by hoë kruisvloeisnelhede. Die huidige studie, daarom, het membraanaanpakking en laktaatinhibisie op bakteriële selle nadat die membraan aangepak is, geïdentifiseer as die hoof prosesbeperkinge wat ʼn beduidende impak het op die MBR-sisteme wat gebruik word vir melksuurfermentasie. Om hierdie sisteme te verbeter vir opskaal vir die industrie, is verdere studies op gepaste metodes om die membraanaanpakking te minimeer, noodsaaklik.af_ZA
dc.description.versionMastersen_ZA
dc.identifier.urihttp://hdl.handle.net/10019.1/124660en_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subjectLactic aciden_ZA
dc.subjectBioreactorsen_ZA
dc.subjectFermentationen_ZA
dc.subjectChemical inhibitorsen_ZA
dc.subjectUCTDen_ZA
dc.titleAn investigation into process limitations in membrane bioreactor (MBR) systems used for lactic acid productionen_ZA
dc.typeThesisen_ZA
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