Reverse osmosis feed water chlorine residual removal method

When reverse osmosis membrane elements are used, the RO feed water should be dechlorinated in order to prevent the membrane from being oxidized. RO membranes have a degree of resistance to chlorine until a significant drop in desalination occurs, which is approximately 200-1000 hours of exposure to 1 ppm residual chlorine, after which membrane degradation may occur.

The rate of chlorine attack on the membrane depends on the feed water characteristics and is faster under alkaline conditions than under neutral or acidic conditions. In any case, acidic pH conditions are more suitable if chlorination sterilization process is used. Chlorine attack is also accelerated when heavy metal content (e.g. iron) and high water temperature conditions are present, and they catalyze the membrane degradation reaction.

The residual chlorine can be reduced to harmless chloride ions by activated carbon or chemical reducers, and the activated carbon column is very effective for dechlorination of RO feed water as shown by the following reaction.

C+2Cl2+ 2H2O → 4HCl + CO2

Sodium metabisulfite (SMBS) is the most commonly used chemical to remove residual chlorine and inhibit microbial activity, while other reducing agents such as sulfur dioxide have not proven to be cost effective over SMBS.

When it is dissolved in water, SMBS forms sodium bisulfite (SBS)

Na

2S2O5+ H2O → 2NaHSO3

SBS then reduces hypochlorite to

NaHSO

3+ HClO → HCl + NaHSO4

According to theoretical calculation, 1.34mg of SMBS can remove 1.0mg of residual chlorine, but in engineering practice, 3.0mg of SMBS is required for every 1.0mg of residual chlorine.

SMBS must be food-grade, free of impurities, and also needs to be a product that has not been activated by cobalt. Although the dechlorination itself is fast, it should still have complete mixing with the water to be treated, and the recommended dosing point is set up with a static mixer.

In order to ensure that the security filter cartridge is still under the protection of residual chlorine sterilization, the injection point can be set at the outlet of the security filter. At this point, the SMBS solution to be added should be filtered, and the dechlorinated water should not be allowed to be stored in the tank for too long.

In the mixing point of the downstream line, you can install the oxidation - reduction electrode, monitoring whether the chlorine is removed, when the residual chlorine is found, the detection electrode should be able to send a signal to stop the high-pressure pump.