What Effect Does Residual Chlorine Have on Reverse Osmosis Membranes?

I. Residual Chlorine Generation: To prevent biofouling of reverse osmosis membrane elements, chlorine gas, sodium hypochlorite, calcium hypochlorite, and other chlorine-containing substances are commonly used for disinfection. These substances rapidly hydrolyze in water to form hypochlorous acid: Cl₂ + H₂O ⇌ HOCl + HCl; NaOCl + H₂O ⇌ HOCl + NaOH; Ca(OCl)₂ + 2H₂O ⇌ 2HOCl + Ca(OH)₂. Hypochlorous acid decomposes in water into hydrogen ions and hypochlorous acid ions: HOCl ⇌ H⁺ + OClCl₂; NaOCl + Ca(OCl)₂; HOCl + OCl⁻. The sum of these is called free chlorine (FAC), and the substances remaining after disinfection are called residual chlorine (FRC), expressed in mg/L (Cl₂). Some of the residual chlorine after disinfection reacts with ammonia nitrogen in the water to form chloramines. Since chloramines have a very small damaging effect on the membrane, this can be disregarded.

II. Effects of Residual Chlorine on Reverse Osmosis Membranes: Residual chlorine oxidizes the reverse osmosis membrane, resulting in a significant decrease in desalination rate. Approximately 1 PPM of residual chlorine can degrade membrane elements within 200-1000 hours. The rate of chlorine attack on the membrane depends on the water quality environment, and is much faster under alkaline conditions than under neutral or acidic conditions. Therefore, to prevent the degradation of reverse osmosis membranes by the oxidizing effect of residual chlorine, dechlorination is necessary. Activated carbon dechlorination has been discussed in the previous section; here, we mainly introduce the removal of residual chlorine using sodium bisulfite. Sodium bisulfite (SBS) ⇌ NaHSO3 + HOCl → HCl + NaHSO4.

III. Precautions for Chemical Use

1. When both acid and SBS are added to the system, the SBS injection point should be after the acid addition point. This is because adding acid after adding sodium bisulfite will cause SBS to form sodium sulfite.

2. In the pretreatment system, it is crucial to prevent residual chlorine from entering the reverse osmosis and nanofiltration systems. If residual chlorine levels are found to be excessive, it must be reduced with SBS. The general dosage is 1.8-3 times the residual chlorine (it must be food grade).