COLLOIDAL FOULING OF REVERSE-OSMOSIS MEMBRANES - MEASUREMENTS AND FOULING MECHANISMS

The effect of chemical and physical interactions on the fouling rate o f cellulose acetate and aromatic polyamide composite reverse osmosis ( RO) membranes by silica colloids is investigated. Results of fouling e xperiments using a laboratory-scale unit demonstrate that colloidal fo uling rate increases with increasing solution ionic strength, feed col loid concentration, and permeate water flux through the membrane. It i s demonstrated that the rate of colloidal fouling is controlled by a u nique interplay between permeation drag and electric double layer repu lsion; that is, colloidal fouling of RD membranes involves interrelati onship (coupling) between physical and chemical interactions. For solu tion chemistries typical of natural source waters, permeation drag und er normal operating conditions plays a more significant role than chem ical interactions and may ultimately control the rate and extent of co lloidal fouling. In addition to permeation drag, it is shown that memb rane surface morphology has a marked effect an colloidal fouling. The higher fouling propensity of composite polyamide RO membranes compared to cellulose acetate RO membranes is attributed to the pronounced sur face roughness of the composite membranes. Implications of the results for developing means to reduce colloidal fouling of RO membranes are discussed.