Influence of crossflow membrane filter geometry and shear rate on colloidal fouling in reverse osmosis and nanofiltration separations

A laboratory-scale crossflow membrane filtration apparatus was designed to investigate the relative influence of filter geometry and shear rate on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes. An expression that allows clarification of the mechanisms of flux decline due to colloidal fouling in RO and NF separations was derived by combining the solution-diffusion model, film-theory, and a modified cake filtration model. With this new fouling model, the interplay between the salt concentration polarization layer and a growing colloid deposit layer may be quantified. The hydraulic pressure drop across a colloid deposit layer was shown to be negligible compared to cake-enhanced osmotic pressure. The difference in flux decline observed in filters with different channel heights resulted from different cake layer thickness, and thus, different cake-enhanced osmotic pressure. A moderate reduction in the initial concentration polarization and cake-enhanced osmotic pressure was obtained by operating at a higher shear rate within a given filter. However, thicker cakes were produced in the filter with greater channel height regardless of crossflow hydrodynamics, which resulted in greater loss of flux. In all modes of operation for either channel height, salt rejection decreased in proportion to the extent of flux decline. By decreasing channel height, both flux and salt rejection were enhanced by reducing all fouling mechanisms-salt concentration polarization, cake layer resistance, and the cake-enhanced osmotic pressure.