Cake-Enhanced Concentration Polarization:  A New Fouling Mechanism for Salt-Rejecting Membranes

Results from well-controlled colloidal fouling experiments with reverse osmosis (RO) and nanofiltration (NF) membranes suggest the existence of a new source of flux decline for salt-rejecting membranes-cake-enhanced osmotic pressure. The physical mechanisms leading to this enhanced osmotic pressure are a combination of hindered back-diffusion of salt ions and altered cross-flow hydrodynamics within colloidal deposit layers, which lead to an enhanced salt concentration polarization layer. A model that accounts for both hindered diffusion of salt ions and altered hydrodynamics within colloidal deposit ("cake") layers is presented. The model successfully links permeate flux and salt rejection to cake-enhanced…