How Reverse Osmosis Systems WorkReverse osmosis is sometimes referred to as ultra-filtration because it involves the movement of water through a membrane. The membrane has microscopic openings that allow water molecules, but not larger compounds, to pass through. RO membranes also have an electrical charge that helps in rejecting some chemicals at the membrane surface. Proper maintenance is essential to retain effectiveness over time. Some units are equipped with automatic membrane flushing systems to clean the membrane.

The effectiveness of reverse osmosis units is characterized by the rejection rate or rejection percentage. The rejection rate is the percent of a contaminant that does not move through, or is rejected by, the membrane. These rejection rates are for single contaminants under design conditions.

Rejection rates need to be high enough to reduce the contaminant level in the untreated water to a safe level. To determine the needed rejection rate, it is necessary to consider the initial concentration. For example, if a water supply contains nitrates at a concentration of 20 milligrams per liter (mg/l), an RO unit rejecting at a rate of 85 percent, which means 15 percent remaining, would reduce the level to 3 mg/l (20 times 0.15 = 3). For more information about the expected rejection rates of Culligan Drinking Water Systems, please see our FAQs.