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. In other words, if a membrane can effectively reject sodium chloride, then clearly it will reject other larger ions even better.
There are two different tests to establish the ro membrane rejection rate. One is the testing of a coupon (a small piece) of the ro membrane flat sheet in the laboratory. This allows the performance of the ro membrane flat sheet to be established under the almost ideal conditions.
The second approach is to perform the test on a ro membrane element while taking into consideration potential imperfections due to the production of the reverse osmosis membrane element. This method is more pertinent to the end users because it can determine the actual performance of the membrane element and the quality of the product water.
Each reverse osmosis membrane manufacturer follows its own testing conditions. There is no “standard” procedure or criteria for testing ro membrane housing or the reverse osmosis membrane. For example, the feed concentration, pH, water temperature, and pressure may vary for different membrane manufacturers. This makes it difficult to compare the performance of membranes from different companies “apples to apples.”