Rejection rate is an important property of the reverse osmosis membrane. Due to the fact that RO membranes are designed to remove salts and other impurities, the rejection rate is the most meaningful way to measure performance. This is expressed by a simple formula: Rejection (%) = 1- Cp/Cf, where Cp is the concentration of salt in the permeate water and Cf is the log mean average salt concentration Cf = Feed Concentration * Ln[1/(1-Y)]/Y; Y – is the recovery rate.
The ro membrane rejection rate will be different for each type of ion. It depends on the molecular weight and charge of the individual ion. Usually, RO membranes reject divalent and multivalent ions such as calcium and magnesium much better than monovalent ions like sodium and chloride. This is why sodium chloride solutions have become the standard reference solution for testing the RO membrane performance.