Reverse osmosis (RO) is a water treatment process that uses pressure to force water molecules through a semi-permeable membrane. It is called "reverse" osmosis because the pressure causes the water to flow in the opposite direction of what occur under natural osmotic pressure. A reverse osmosis membrane allow the water to pass, but traps any ionized salts, colloids, organic molecules or other impurities larger than 0.0001 microns. This eBook sheds some light on the science and process of reverse osmosis as it pertains to industrial water filtration.