Reverse Osmosis (RO) is a water treatment process that removes contaminants and impurities from water by forcing it through a semi-permeable membrane. The process is called “reverse” osmosis because it reverses the natural osmotic process, which occurs when water flows from a less concentrated solution to a more concentrated one through a semi-permeable membrane. In reverse osmosis, the water flows through a membrane in the opposite direction – from a more concentrated solution to a less concentrated one.