Impact of Residual Chlorine on Reverse Osmosis Membrane Elements

Forming of Residual Chlorine

To prevent membrane contamination of biofouling, chemical substances such as chlorine, sodium hypochlorite, calcium hypochlorite are usually used for disinfection. They rapidly hydrolyzed into hypochlorous in water.

Cl2 + H2O ← → HOCl + HCl NaOCl + H2O ← → HOCl + NaOH 
Ca (OCl) 2+ 2H2O ← → 2HOCl + Ca (OH) 2 in the water as hypochlorous acid, and hypochlorous acid is decomposed into hydrogen ions with the ions. These substances sum of HOCl ← → H ++ OCl- Cl2 NaOCl Ca (OCl) 2 HOCl OCl- is called free chlorine (FAC), after disinfection, they are called Free Residual Chlorine (FRC mg/l CL2). Partial Chlorine and Ammonia in water forms the chloramines after disinfection.

The Influence of Residual Chlorine

The residual chlorine and reverse osmosis membrane will form oxidation, its salt rejection rate will decline. Approximately 1PPM of Chlorine will lead to membrane element degradation within 200 - 1000 hours and the speed of this occurring depends on the quality of water. When the membrane is under alkaline conditions, the speed of degradation will be faster than membrane that is slower and medium under acidic conditions. To prevent residual chlorine damage to the membrane film, dichlorination should be implemented. Activated carbon dechlorination has been a standard way of reducing residual chlorine in feed waters, but it may also be done by using Sodium Bisulfite (SBS): NaHSO3+ HOCl→HCl+ NaHSO4

Caution with Chemicals Application

  1. When both the acid and SBS are added to the system, the acid should be added first because the SBS will form Sodium Sulfite in acid conditions.
  2. Among pretreatment systems, must make sure no residual chlorine flows into the Reverse osmosis or nanofiltration system. If excessive chlorine is found, the SBS should be applied. The amount of is typically 1.8 to 3.0 times of residual chlorine (food grade)

This information is offered for educational purposes only and no warranty is express or implied. The user is fully liable for any outcome, and it is recommended that only an experienced water treatment professional should operate, maintain and install the above referenced equipment and products at any time.