Semi permeable Membranes are the Heart of RO Systems The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use a screen to separate particles from water streams, an RO system employs a semi permeable membrane that separates an extremely high percentage of unwanted molecules. For example, the membrane may be permeable to water molecules, but not to molecules of dissolved salt. If this membrane is placed between two compartments in a container as shown in Figure 1, and a salt solution is placed in one half of the container and pure water in the other, water passes through the membrane while the salt cannot.

Pressure is applied to Reverse Natural Osmotic Flow Now a fundamental scientific principle comes into play. That is, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. The only way for this to happen in our example, is for pure water to pass through the membrane to the salt-water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis. But if the goal in our example water purification system is to remove the salt from water, it is necessary to reverse the natural osmotic flow by forcing the salt water through the membrane in the reverse direction. This can be accomplished by applying pressure to the salt water as it’s fed into the system, creating a condition know as “reverse osmosis.”( See Figure 1).

Cross-flow Filtration Permits Long-term Performance While the principals of reverse osmosis are simple, in practical terms, the RO process cannot go on indefinitely unless steps are taken to ensure that the membrane doesn’t become clogged by precipitated salts and other impurities forced against it by the pressurized stream of feed water. To significantly reduce the rate of membrane fouling, RO systems employ cross-flow filtration (shown in Figure 2), which allows water to pass through the membrane while the separate flow of concentrate sweeps rejected salts away from the membrane surface.