This is a process that is often described as filtration, but it is far more complex than that. We sometimes explain it as a filter because it is much easier to visualize using those terms. We should remember that osmosis is how we feed each cell in our bodies: As our blood is carried into the smallest of capillaries in our bodies, nutrients actually pass through the cell wall to sustain it's life. Reverse osmosis is just the opposite: We take water with "nutrients" (in this case, junk) in it, and apply pressure to it against a certain type of membrane, and, presto -- out comes "clean" water. Lets review the basics: If you take a jar of water and place a semi-permeable membrane (like a cell wall? or a piece of skin?) in it, dividing the jar into two sections, then place water in both sides to an equal level, nothing happens. But, if you place salt (or other such substance) into one side of the jar, you will notice that, after awhile, the water level in the salty side begins to rise higher as the unsalted side lowers. This is osmotic pressure at work: The two solutions will continue to try to reach the same level of salt in each side by the unsalted water passing through the membrane to dilute the salty water. This will continue until the "head" pressure of the salt water overcomes the osmotic pressure created by the differences in the two solutions. Researchers have discovered that if we take that membrane and feed water with sufficient pressure to overcome the osmotic pressure of the two waters, we can 'manufacture' clean water on the side of the membrane that has no pressure. We sometimes say we "filter" the water through the membrane. Depending on the membrane design, and the material it made from, the amount of TDS (total dissolved solids) reduction will range from 80 to over 99 per cent. Different minerals have different rejection rates, for instance, the removal rate for the membrane I am looking at now is 99.5% for Barium and Radium 226/228; but only 85.9% for Fluoride and 94.0% for Mercury. Removal rates are very dependent on feedwater pressures, and some membranes are not tolerant to high or low pH. For home use, it is important to make sure you get an RO *System*; i.e., a sediment pre-filter, a carbon pre-filter, membrane, storage tank and post carbon filter. Some of these filters may be combined into one, i.e., the pre-filter may be both a particulate and a carbon filter. A lot of comments have been made concerning the *wasting* of water by an RO. True, the old style units with the early type membranes were more prone to becoming plugged, or fouled by the "junk" they removed from the water. To help keep this from happening, a small amount of water was allowed to run across the membrane to help carry away those impurities to drain. Early designs only recovered 1 gallon of good water for every 4-8 gallons used to keep the membrane clean. And when your storage tank was full, water still ran to the drain because the early membranes were made of a material that the little bugs in your water supply (no, not pathogens, or dangerous to you in small numbers) loved to eat! So to prevent that, we just let the water run so they couldn't have time to stop and eat. :>) Now membranes are made that not only recover a much higher percentage of the feedwater, but the bugs don't eat them! Newer systems not only recover more water to begin with, they may also have a shut off device that stops all water flow when the storage tank is full. Actual recovery rate is dependent on several factors, including the TDS, and just what the TDS is composed of, in your feedwater. Temperature, pressure also have a big effect on the amount of product water you can make in a given period. Remember, all RO units are normally rated using a feedwater temperature of 77 degrees F -- is your feed water temperature that high?