ABSTRACT
Osmosis is a natural process involving fluid flow across a semipermeable membrane barrier. It is selective in the sense that the solvent passes through the membrane at a faster rate than the dissolved solids. The difference of passage rate results in solvent solids separation. 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 semipermeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides. If a soluble salt is added on one side, the chemical potential of this salt solution is reduced. Osmotic flow from the pure water side across the membrane to the salt solution side will occur until the equilibrium of chemical potential is restored. Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides is equal to the osmotic pressure. Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium. Additional pressure will raise the chemical potential of the water in the salt solution and cause water flow to the pure water side, because it now has a lower chemical potential. This phenomenon is called reverse osmosis (RO). The reverse osmosis technology developed about 50 yr ago, as a scientific experiment, is used extensively today to reduce salinity of various water sources and produce potable water in commercial systems. Other applications include production of low salinity water for industrial applications and reclamation of waste streams. The economics of RO technology is very competitive in comparison with other salt reduction processes and, in some cases, the cost of producing potable water using RO can be lower than water supplied from natural sources, if pumping water over long distances is required.
