Combined fouling of pressure-driven membranes treating feed waters of complex composition

Pressure-driven membrane filtration processes can serve as a reliable barrier for microorganisms, organics, precursors of disinfection by-products (DBPs), colloids, and other contaminants. However, membrane fouling impedes a wider application of membrane filtration technology. Because typical feed waters have complex composition and the pretreatment often can not guarantee a complete removal of foulants, different fouling mechanisms act simultaneously resulting in combined fouling. Although each of the individual fouling mechanisms has been extensively studied, there have been only very few studies examining the effect of combined fouling on membrane performance. The focus of this work is on identifying contributions of individual fouling mechanisms in the overall fouling process and on evaluating the synergistic effects stemming from interactions between individual mechanisms.;In Chapter 2, concentration of rejected salt at the membrane surface when colloidal particles were deposited on the membrane is determined experimentally based on measured salt permeability constant. This approach allowed for a clear identification of individual contributions of concentration polarization and colloidal fouling to the permeate flux decline.;In Chapter 3, a method based on a simple linear regression fitting is proposed and used to determine the type, the chronological sequence, and the relative importance of individual fouling mechanisms in the experiments on the dead-end membrane filtration of colloidal suspensions. For all membranes, flux decline was consistent with one or more pore blocking mechanisms during the earlier stages and with the cake filtration mechanism during the later stages of filtration.;In Chapter 4, a model is proposed for predicting the permeability of porous media consisting of fibers and spherical particles. The model combines Kozeny-Carman equation and Ethier's theoretical model to predict the permeability of the mixed media based on the permeability of the porous bed of particles alone and on the volume fraction of the fibers. The model prediction was found to be in accordance with the experimental observations for the fiber mass fractions of less than 50%.;Chapters 5 and 6 examine the effect of combined colloidal fouling and gypsum scaling on the performance of RO membranes. In Chapter 5, the effect of silica colloids on gypsum scaling is studied in a batch system. The results showed that silica colloids retarded the induction of gypsum crystallization and decreased the nucleation rate. It was also found that gypsum crystallization rate increased in the presence of silica colloids. Chapter 6 presents the first study on the combined colloidal fouling and gypsum scaling in RO membrane systems. A significant synergistic effect between gypsum scaling and colloidal fouling was observed.