HOW DOES A REVERSE OSMOSIS MEMBRANE WORK?
Reverse Osmosis (RO) is a modern process technology used to purify water by efficient removal of dissolved contaminants. Reverse Osmosis Systems utilize semi-permeable membranes to separate the dissolved contaminants from the processed water. Rather than retaining the separated contaminants, like a particulate filter collects sediment, the reverse osmosis system is designed to continuously flush the contaminants, still in a concentrated, solution state, to drain. “Semi-permeable” refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product. Osmosis is a natural process involving the fluid flow of across a semi-permeable membrane barrier. Consider a tank of pure water with a semi-permeable membrane dividing it into two sides. 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. 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 (Figure 1a). In scientific terms, the two sides of the tank have a difference in their “chemical potentials,” and the solution equalizes, by osmosis, its chemical potential throughout the system. Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides is equal to the osmotic pressure. The osmotic pressure is a solution property proportional to the salt concentration and independent of the membrane. With the tank in Figure 1a, the water moves to the salty side of the membrane until equilibrium is achieved. Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium (Figure 1b). Additional pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential. This phenomenon is called reverse osmosis.
The driving force of the reverse osmosis process is applied pressure. The amount of energy required for osmotic separation is directly related to the salinity of the solution. Thus, more energy is required to produce the same amount of water from solutions with higher concentrations of salt. In the real world, the “salt solution” is the source water to be purified. The HydroMax Reverse Osmosis System utilizes a pressure pump to apply pressure to this salt solution and drives purified water through the membrane while rejecting nearly all of the dissolved contaminants and flushing them to drain. The membrane barrier in a HydroMax system actually is configured into a space saving, convenient, tubular design as pictured below.