How Water is Softened

Hard water caused by the presence of calcium and magnesium ions (which form insoluble precipitates with soaps) is softened by exchanging its calcium and magnesium ions with sodium ions. To accomplish this, the hard water is passed through a column of cation exchanger containing sodium ions.

After the column has been in use for some time, calcium and magnesium begin to appear in the water leaving the column. Then the column must be regenerated by passing a concentrated solution of common salt slowly through the column; the excess sodium ions displace the ions that produce the hardness so that, after flushing with water, the bed of exchanger is ready to be used again. At first, the exchangers used for this purpose were natural aluminosilicates; but later, synthetic resins came to be used instead.