In this type of process, it is sought to remove the hardness of a water, product of its calcium and magnesium concentrations. The exchange of these ions is done by sodium ions, through the use of a strong cationic type resin (it is not the only type of resin that can be used for this process, but it is the most used). In this way, only sodium, calcium and magnesium ions intervene in the softening of hard water.
From the point of view of drinking water for human consumption, “hard water” is understood to be that which exceeds 125-150 ppm of calcium carbonate (CaCO3) (OMS), being able to generate precipitations of CaCO3, forming incrustations in the systems of distribution, especially if it is at an elevated temperature. At the other extreme, waters with very low hardness can be corrosive to the facilities, depending on their alkalinity, pH and dissolved oxygen.
The generic reaction that governs the process of “softening” is the following:
CaSO4 + 2Na+R– Ca2+R– + Na2SO4 where R is the exchange resin.
Usually, common brine is used as a regenerant for this type of resin. However, there are other sources of sodium ions as are some salts and seawater.
To carry out this process, you must have a vertical column with the resin inside, interconnections of pipes and valves. Usually these columns work under pressure, but there are facilities at the municipal level that have been installed to gravity.
Systems that are operated against the current can reach leakage values in the order of <1 ppm CaCO3, which is why it is the most used mode (versus co-current). Then you can have a sense of upward flow and a regenerating downward flow. Once the softening cycle has been completed, a wash is carried out to expand the resin bed, remove the particles that have been retained in it and avoid the channeling of the mantle during the regenerative phase. This wash cycle is usually short (<5 min).
In the regeneration, the softener receives the 10% brine solution through a distributor system that allows controlling the speed, in order to achieve a uniform passage through the mantle, to obtain a good level of regeneration. Once the brine is applied, there will be an “exhausted” flow of this solution that must be eliminated before the unit goes back into service, for which a rinse is performed, which allows the chloride concentration to be lowered to acceptable levels. Then the unit can re-enter service.
To ensure a uniform performance, the raw water to be treated by the softener must be, as in other ion exchange processes, clean in order to avoid fouling in the mantle and equipment. If the iron concentration of the raw water is high, it must be treated before entering the exchanger. Below is a diagram of a softener operated against the current: