Colloidal fouling of thin film composite and cellulose acetate reverse osmosis membranes

This dissertation investigates colloidal fouling of reverse osmosis (RO) membranes by aluminum oxide and silica colloids. The membranes investigated include three types of thin film composite membranes and two types of cellulose acetate membranes. The important factors that affect colloidal fouling, namely, particle concentration, solution chemistry, electrokinetic charge of particles and membranes, permeation drag, and surface morphology, are studied in laboratory experiments.; Solution chemistries tested in this research include ionic strength, pH, divalent ions, and natural organic matter. Solution chemistry affects colloidal fouling of RO membranes by changing double layer forces between particles and membranes and between suspended and retained particles. Colloidal fouling occurs in two stages. At the initial fouling stage, particles interact with the clean membrane surface, whereas at the second stage particles deposit onto previously retained particles rather than onto the bare membrane surface. In the solution pH range investigated, deposition of aluminum oxide colloids onto the membrane surface is initially favorable and later becomes unfavorable as particles accumulate on the membrane surface. Because the silica colloids and membranes are negatively charged, deposition of silica is unfavorable throughout the entire fouling process.; A mechanistic explanation for colloidal fouling of RO membranes is proposed based on the ionic strength, permeation drag and electrokinetic charge of colloids and membranes. Several fouling scenarios are analyzed. The general conclusion is that, at high permeation drag, significant permeate flux decline will be observed regardless of solution chemistry, while at low permeation drag, significant permeate flux decline will be observed only when ionic strength is high. No permeate flux decline will be observed when both solution ionic strength and permeation drag are low.; Membrane surface roughness plays an important role in colloidal fouling. The difference in fouling behavior of thin film composite membranes from that of cellulose acetate membranes is primarily attributable to the different roughness of the membrane surfaces. The smooth cellulose acetate membranes exhibit less fouling tendency, whereas the rough thin film composite membranes are conducive to more severe colloidal fouling.