Chapter 7 - Reverse Osmosis

Abstract

This chapter presents elements of membrane separation processes and explains principles of membrane separation. The membrane-based processes include reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. There is an inherent difference in the separation mechanism in all filtration processes and the reverse osmosis process. In filtration, separation is made by a sieving mechanism, where the membrane passes smaller particles and retains larger ones. In osmosis or reverse osmosis processes the membrane permeates only the solvent and retains the solute. The microfiltration, ultrafiltration, and nanofiltration processes are used to separate the suspended material. On the other hand, the reverse osmosis process is used to separate dissolved solids. Nanofiltration is used for partial softening of brackish water. The direction of solvent flow is determined by its chemical potential, which is a function of pressure, temperature, and concentration of dissolved solids. Pure water in contact with both sides of an ideal semi-permeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides. The RO process is defined in terms of a number of variables, which includes: osmotic and operating pressure, salt rejection, and permeate recovery. Membrane manufacturing companies define system specifications in terms of the feed quality, which includes salinity and temperature.