Implications of critical flux and cake enhanced osmotic pressure (CEOP) on colloidal fouling in reverse osmosis: Experimental observations

This paper describes the determination of critical flux and the cake enhanced osmotic pressure (CEOP) effect of colloidal silica fouling in reverse osmosis (RO) using a sodium chloride tracer response technique. The tracer technique gave the transient values of concentration polarization, CP, and cake resistance, R_f. From these values, the rate of accumulation of deposit, dm_f/dt, could be readily determined. The critical flux, J_crit, was defined as the flux at which the rate of deposition of colloidal silica on the membrane, became zero, i.e. dm_f/dt = 0. It was found that the critical flux concept was applicable to colloidal silica fouling in the RO process. The critical flux was strongly influenced by the crossflow velocity, such that J_crit ∼ v^0.4. In addition, the critical Peclet number, (J_v/k_m)_crit, which is a measure of the convective to diffusive forces, to trigger silica fouling was approximately 0.52. Furthermore, it was observed that the amount of particles convected to and finally deposited on the membrane surface, known as the fractional deposition constant, Φ, was strongly influenced by the crossflow velocity. At low crossflow (<0.1 m s^-1, shear rate <430 s^-1) Φ = 1.0, but at 0.22 m s^-1 (shear rate = 940 s^-1) Φ dropped to about 0.1. The CEOP effect, as deduced from the transient CP data, was found to be more severe at high flux and low crossflow velocity conditions, due to formation of a thicker cake layer and flux-induced polarization. Even for the small (20 nm) colloids used in this study, the CEOP effect can readily exceed the R_f effect due to fouling.