RO membranes are used to remove contaminants that are less than 1 nm nominal diameter. Reverse osmosis typically removes 90% to 99% of ionic contamination, most organic contamination, and nearly all particulate contamination from water. RO removal of non-ionic contaminants with molecular weights 300 Dalton and for particles, including colloids and microorganisms. Dissolved gases are not removed (eg. CO2). During reverse osmosis, pretreated water is pumped past the input surface of an RO membrane under pressure (typically 4–15 bar, 60–220 psi) in cross-flow fashion.
RO membranes are typically thin film composite (polyamide). They are stable over a wide pH range, but can be damaged by oxidizing agents such as chlorine, present in municipal water.
Pretreatment of the feed water with microporous depth filters, softener and activated carbon, Pre dosing systems for removal of oxidizing compound, Anti-scalent for lowering the scaling frequency of membrane & pH Correction chemical for lowering or increase the required pH values is usually required to protect the membrane from large particulates, hardness and free chlorine. Typically 75%-90% of the feed water passes through the membrane as permeate and the rest exits the membrane as concentrate that contains most of the salts, organics, and essentially all of the particulates.