Thin Film Composite (TFC) RO membrane by CSM. The TFC semi permeable membrane rejects wide spectrum of impurities down to .0001 micron. An RO system, which has other components such as sediment and carbon filters is supposed to remove salt and many contaminants from water before reaching the membrane. It should be replaced every 2-3 years depending on total dissolved solids (TDS) in the supply water. If TDS is less than 180 mg/L, replace every 3 years. If TDS is above 180 mg/L, replace every 2 years. A TDS hand-held meter can be used to measure the TDS of water. Knowing the TDS of purified water and the supply water, you can calculate the rejection rate of the membrane. If pure water has 50 ppm TDS and the supply water has 300 ppm TDS, then the rejection rate is 83%. A new membrane should have a minimum rejection rate of 90%.