Fouling mechanisms during depth and membrane filtration of yeast cell suspensions

Clarification of fermentation broths and cell cultures is a crucial step in the downstream purification of biotechnology products. Depth filtration and normal flow microfiltration are very attractive for initial clarification due to their ease of use, low cost, and relatively simple validation. However, filter fouling can severely limit the performance of these systems. The overall objectives of this thesis were: (1) to demonstrate the potential of using high throughput screening (HTS) methods as a process development tool for the design and optimization of membrane filtration systems, and (2) to develop an improved understanding of the underlying factors governing the performance of both depth and surface (membrane) filters for initial clarification.; High throughput screening studies were performed using baker's yeast suspensions as a model system. Data were obtained in 96-well plates, syringe filters, unstirred filtration cells, and small cartridge filters. The membrane resistance was very similar in the different formats, except for data obtained in the pleated filter which were influenced by large parasitic pressure losses in the cartridge. The specific resistance of Baker's yeast deposits was evaluated over a range of cake thickness, solution ionic strength, and in the presence or absence of a poly-cationic flocculant. The data obtained in 96-well filter plates were in good agreement with measurements in larger filters, demonstrating the scalability of the results. Effective conditions for yeast microfiltration were identified, suggesting that high throughput screening techniques can be a useful tool for process development and optimization.; The performance characteristics of a series of Ultipleat fibrous polypropylene depth filtration media were examined for the filtration of yeast cell suspensions. Data were analyzed using available fouling models to obtain insights into the flux decline mechanisms. Filters with small pore size provide high filtrate quality but at low capacity due to the formation of a yeast cell cake on the external surface of the filter. Media with large pore size have much higher capacity since the cells are captured throughout the porous structure, but filtrate clarity was typically poor. The multilayer structure of commercial depth filters leads to improved performance, although the choice of layer properties is critical. The highest capacity was achieved using a composite multilayer filter in which the upper layer allows significant yeast cell penetration into the filter matrix but still protects the retentive layer that is needed for a high quality filtrate.; The effects of membrane pore morphology on the fouling behavior were studied using novel micro-patterned membranes having well-defined slot-shaped or circular pores. The initial fouling was due to pore blockage, with the slotted pore membrane showing an accelerating rate of flux decline as the yeast cells progressively cover the area of the slot. The flux decline at long times was due to the formation of a yeast cell cake, with the specific resistance of the cake layer a function of both the pore geometry and the overall porosity of the membrane. Simple geometric models were developed to describe the increase in resistance as the cake grows specifically accounting for the underlying pore geometry.; These results provide important insights into the mechanisms governing both depth and surface filtration of cell suspensions, providing a framework for the development of improved filtration processes for the biotechnology industry.