This process is applied to soften the water by filtering it through the Na-cation exchange resin layer. During this process Ca2+ and Mg2+ ions causing the hardness of incoming water (hereinafter referred to as the hardness ions) are kept by cation-exchange resin and replaced with the equivalent number of Na2+ ions.

The softening process by means of Na-cation exchange is finished when the hardness ion breakthrough is occurred, after it exhausted cation-exchange resin in the filter is subject for regeneration, i.e. regeneration its ion exchange capacity. Regeneration of the exhausted cation-exchange resin is performed by 6-10% NaCl solution supplied through the resin. Due to the relatively significant concentration of Na+  ions, these ions exchange previously absorbed Ca2+ and Mg2+ cations in the regeneration solution.

The ion-exchange resin in filters could be regenerated by several ways that differ from each other by the flow direction of processed water and regeneration solution. When these flows are supplied in the same direction, usually downward, this regeneration is called direct-flow regeneration; such regeneration creates unfavourable conditions for the softening process due to the distribution of Na+ and hardness ions in the layer of regenerated filtering bed. At the outlet of cation-exchange resin layer the softened water where the hardness ions concentration is significantly decreased, passes through the areas of poorly regenerated cation-exchange resin and that's why could not be softened enough. Besides, due to this factor some hardness ions may get into the water from the cation-exchange resin.

During the counter-flow regeneration the regeneration solution is supplied in the direction opposite to the processed water flow, in this case outlet layers (taking into account the water flow) of the cation-exchange resin are regenerated more completely. This helps not only to keep the constant value of the ions residual concentration in the permeate, but to obtain the permeate of the higher quality with the lower surplus of reagent, and as a consequence with the lower volumes of the waste water. Effective counter-flow method allows to reduce the number of water-treatment stages by means of the permeate quality improvement. The disadvantage of the counter-flow regeneration is a more complicated filter's construction that serves to prevent mixing of ion-exchange resin layers when the solution or water is supplied to the lower part of the permeate.

Besides, the removal of absorbed impurities and crushed resin particles is more complicate in the up-to-date counter-flow filters, especially when such filters are almost completely filled by the ion exchange resin. It requires the preliminary water treatment of higher quality before the water is supplied to the counter-flow filters.

To reduce reagents consumption when the direct-flow process is used, two stages Na-cation exchange is applied, i.e. two filters installed in series. The primary filter is regenerated with the largest surplus of NaCl (n=1.8-2.4), that's why residual hardness of the permeate occurs to be high (about 0.1 mg-eq/dm3). The secondary filter is regenerated with 6.5-7.5-fold of salt surplus, that's why residual hardness of water drops from 100 down to 5 mg-eq/dm3. Feasibility studies showed that in spite of filters number increase in two-stage configuration, this configuration has certain advantages over one-stage configuration due to the reduction of NaCl consumption with quality of softened water being equal.

______________
* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)

---