Water Softening & Ion Exchange

Now when this calcium bicarbonate in solution flows through the exchange material in the softener, the chemical change which occurs is diagrammed below.


After a vast number of hardness ions in the water has become affixed to the softening material through the attraction of positive and negative charges, and most of the sodium ions have been released, the unit can no longer soften the water. It has become temporarily exhausted.

In actual practice, a small number of sodium ions remains on the softening material after the unit is exhausted. If no new chemical reaction is set into operation at this point, the incoming calcium bicarbonate ions flow untouched through the unit.

Just one of the ions causing hardness is shown for sake of clarity in diagramming. Actually, most water supplies contain a number of various hardness ions. The same process in each case applies equally in their removal from the water.

Water Entering Softner:

(1) The calcium ions in the water enter the ion exchange column. Here the waters pass through the bed of the softening material.

(2) The softening material consists of fixed irreplaceable anions. Affixed, that is, chemically bonded to them are mobile, replaceable cations of sodium.

(3) As the softening material anions have a greater affinity for the calcium ions than for sodium ions, it attracts them. In the process the calcium ions "knock" the sodium ions off the exchange material. As this continues, the exchange or softening material becomes loaded with calcium ions. Note that two sodium ions are released for each of the calcium ions absorbed by the softener.

Water Leaving Softner:

Water that contains calcium ions as it enters the softener will have a chemically equivalent amount of sodium ions in it on leaving the softener.

(4) After a certain prescribed amount of water has gone through the unit, the calcium ions will replace all but a small percentage of the sodium ions in the softener At this point, the softener is considered (Unit is now exhausted and requires regeneration).

(5) Now a rich brine solution is introduced into the softener by backwashing.

Note: to recharge a softener a concentrated solution of the regenerant (sodium chloride) is accumulated calcium ions free of the softening material.

(6) The calcium ions and excess brine solution are rinsed away.

(7) When this process is completed, the unit is again charged with sodium ions and is ready to continue the process of softening the water.

RECHARGING OR REGENERATION

Recharging or regeneration is necessary at this point. To do this a reverse ion exchange operation is now put into motion. In this reverse process, it is necessary to bombard the exchange material with the original type of cations in a concentrated solution. The affinity of the exchanger for the hardness ions is overcome by the use of a relatively strong solution of sodium ions. Generally, sodium chloride in a concentrated solution is used for this purpose. What occurs in all examples of ion exchange is a "swap" or balanced exchange of ions.

The calcium ions in the softening process are not destroyed. They have merely been replaced in the water by a chemically equivalent amount of sodium ions. The same type of balanced exchange occurs with whatever other hardness minerals that are removed from water.