The ion exchange resin used in the treatment of softened water and desalinated water is a high-molecular organic compound. If it is oxidized, it will destroy the cross-linked bond of the resin, which will chemically degrade the resin and reduce the exchange capacity. The chlorine added during the pretreatment is a strong oxidant. Therefore, the excess residual chlorine must be removed before the water in the cation exchange tower for desalination treatment (or the effluent from the carbon filter).
However, if the residual chlorine in the inlet water of the cation exchange tower is removed, although the oxidation of the resin can be controlled, the water quality at this time loses its continuous sterilization ability and is easily polluted, and it may grow on the surface of the cation exchange resin inlet water. Microorganisms cause the resin to be invaded by organic matter. We need to maintain a certain amount of residual chlorine to weigh the gains and losses. Generally keep the residual oxygen at 0.02-0.1mg/L.
Moreover, in the pure water system, if the reverse osmosis process is used as a means of desalination in the subsequent stage, the reverse osmosis membrane will not have enough resistance to strong oxidizing substances such as residual chlorine, which will affect the physical structure of the reverse osmosis membrane and cause reverse osmosis. The membrane is irreparably damaged, and the reverse osmosis influent requires residual chlorine to be less than 0.1mg/L.
Most of the methods to remove residual chlorine use activated carbon adsorption. After the free residual chlorine (HClO, ClO-) in the water enters the activated carbon device, it chemically reacts with activated carbon C:
HClO + C(activated carbon) → C(activated carbon) O + H+ +Cl
CIO- + C(activated carbon) → C(activated carbon) O + Cl