Colloidal fouling of reverse osmosis membranes: Measurements and fouling mechanisms

The effect of chemical and physical interactions on the fouling rate of cellulose acetate and aromatic polyamide composite reverse osmosis (RO) membranes by silica colloids is investigated. Results of fouling experiments using a laboratory-scale unit demonstrate that colloidal fouling rate increases with increasing solution ionic strength, feed colloid concentration, and permeate water flux through the membrane. It is demonstrated that the rate of colloidal fouling is controlled by a unique interplay between permeation drag and electric double layer repulsion; that is, colloidal fouling of RD membranes involves interrelationship (coupling) between physical and chemical interactions. For solution chemistries typical of natural source waters, permeation drag under normal operating conditions plays a more significant role than chemical interactions and may ultimately control the rate and extent of colloidal fouling. In addition to permeation drag, it is shown that membrane 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 surface roughness of the composite membranes. Implications of the results for developing means to reduce colloidal fouling of RO membranes are discussed.