Hard water is water that contains dissolved chalk, lime, and other minerals. Hardness in water can create scale and clog the equipments. The common unit of expression of hardness is mg/L as CaCO3.
Water softening is the process of removing calcium and magnesium in hard water. Those ions are exchanged for sodium ions. When raw water flows through cation exchange resins, the dissolved calcium and magnesium ions in the water are caught by the resin and are exchanged with the sodium ion. When there is no sodium ion left, it needs to be recharged by passing concentrated NaCl solution back through the bed. This regeneration takes about 2 hours every cycle.