Reverse Osmosis (RO) filtration is the tightest possible membrane process in liquid separation.  It works by using a high-pressure pump to increase the pressure on the contaminant side of the RO to force the water across a semi-permeable membrane.

Essentially, the RO membrane acts as a barrier to 95-99% dissolved salts and inorganic molecules, as well as organic molecules with a molecular weight of greater than 100. Pure water molecules pass freely through the membrane creating a purified product stream.