CFD modeling of gypsum scaling in cross-flow RO filters using moments of particle population balance

Abstract

Abstract Gypsum scaling over a RO membrane significantly reduces the permeate flux. Since, the phenomenon is linked to concentration polarization; the present study aims to understand the effect of high shear rates over the membrane in reducing gypsum scaling of cross-flow RO filters. We have proposed an integrated CFD and moment-based population balance (MPB) formulation which is less complex than tracking the growth of individual particles. Most of the previous studies on modeling of scaling and permeate flux decline employing MPB formulation were limited to dead-end filters. The present numerical model for cross-flow RO filter has been verified by comparing the precipitate mass and permeate flux decline with the published results for channel filter. This model has been employed for different cross-flow conditions and geometries. We have demonstrated that scaling can be mitigated by increasing the shear rate over the membrane surface. In roto-dynamic RO filters, a 50% increase in the initial permeate discharge can be achieved, and permeate flux decline in time can be restrained if the shear rate is increased by rotating the disk. For a channel of identical membrane area, a similar improvement of permeate discharge requires an approximately two orders of magnitude higher feed discharge. In a channel, gypsum scale formation is significant at larger distances from the inlet. While for roto-dynamic filters, scaling occurs at the stagnation zone near the axis of rotation, the scaling potential decreases with radius. Additionally, we have also discussed some striking differences between fouling and scaling patterns in cross-flow RO filters.