Effect of membrane morphology and structure on protein fouling during microfiltration

Protein fouling remains a major problem in the use of microfiltration for many bioprocessing applications. Recent work has demonstrated that the initial fouling in these systems is typically caused by the deposition of aggregated and/or denatured protein on the membrane surface. These studies have also provided insights into the effects of solution environment on protein aggregation and the rate of flux decline. However, there is currently little understanding of the effects of the membrane pore morphology or structure on the nature or extent of protein fouling.; Experimental studies were performed using a variety of proteins and different membranes to explore the effects of pore interconnectivity on the rate arid extent of membrane fouling. Membranes with highly interconnected pores allow fluid to flow around and under any pore blockage on the membrane surface, significantly reducing the effect of this pore blockage on the filtrate flux. In order to quantify these effects, a new technique was developed to evaluate the extent of pore connectivity by measuring the hydraulic permeability and/or solute diffusivity in the radial and transverse directions of the membrane. Experiments were performed by blocking different regions of the upper and lower surfaces of the membrane to change the relative contributions of the radial and transverse flow. Data were analyzed using a theoretical model for two dimensional flow or transport in the membrane. Studies performed with polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membranes showed distinct differences in the extent of pore connectivity, with the PTFE membranes having a much smaller radial permeability. These results were confirmed by SEM images and are consistent with the different formation techniques used to prepare these membranes.; A new mathematical model was developed for the rate of flux decline which explicitly accounts for the effects of pore blockage, cake formation, and membrane pore connectivity. Fouling is assumed to occur first by pore blockage, with a cake then forming over these blocked areas. The model thus provides a smooth transition from the pore blockage to cake formation regimes, eliminating the need to use different mathematical formulations to describe these two phenomena. The model is in good agreement with flux decline data for the filtration of a variety of proteins including BSA, pepsin, lysozyme, and IgG. The model accurately predicts the different fouling behavior seen for membranes with straight through pores and for model membrane systems having more complex composite/asymmetric structures with different pore interconnectivity.; The experimental and theoretical results demonstrate that membrane pore morphology and structure can significantly alter the performance of membrane systems. The implications of these results for constant pressure and constant flux processes are discussed, and guidelines are presented for the development of new membrane morphologies with improved fouling characteristics.