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Reverse Osmosis

Osmosis occurs when solutions of different concentrations are separated by a semipermeable membrane. The tendency to reach a state of equilibrium between the two solutions (the second law of thermodynamics) causes pressure to exist across the membrane, called osmotic pressure. For example, if salty water and fresh water are separated by a membrane, there is a pressure exerted by the dissolved salt to pass through to the less salty solution, the fresh water, and there is a pressure exerted by the fresh water to flow to the lower water concentration existing in the salty water. If the membrane is permeable to water molecules but not to salt, water will flow through to dilute the salt water. If sufficient external pressure is applied to the salty water solution, the flow of water will be reversed. This process, called "reverse osmosis" (RO), is slowly becoming technologically, commercially, and economically feasible for the production of high quality water from alkaline, brackish, or colored water.

The rate of water flow is proportional to the pressure applied to the higher concentration solution. This pressure is called the feed pressure and its normal range is 100 to 600 lbs per square inch (p.s.i.); however, some new home units run at 40 to 90 p.s.i. Since a semipermeable membrane acts in the system as a filter, its quality and properties are of major importance. The membrane should remove high percentages of dissolved solids, have good chemical and bacteriological resistance, and be able to operate under wide pH and temperature ranges. Most membranes are subject to fouling by hard water, making softening a required pretreatment. The two most commonly used membranes are cellulose acetate and nylon.

The quality and useful life of membranes are being constantly improved and this treatment may become cost-effective for individual houses in the near future.

Supplying good quality drinking water to some of the more rapidly growing coastal communities has become a major problem. In several areas, desalinization is a feasible way of using brackish ground water for potable supplies. The most common water treatment technique used for these conditions is reverse osmosis. Water treated by reverse osmosis may be desalinized to a degree that it can be blended with softened brackish water to lower the cost of treatment, while still meeting the standards for potable water.

One of the major problems with the reverse osmosis process is the disposal of the reject water, a high salt concentration solution. If this water contains high levels of toxic materials, special provisions for its disposal must be made. Most reverse osmosis systems operate at a 50 to 75% conversion rate for brackish water and a 20 to 30% conversion rate for seawater. As a result, the total use of water will be higher.