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Water Softening Resin Selection

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Water Softening Resin Selection

A variety of ion exchange resins can be used for water softening. They are the heart of the Water Softener. When hard water that contains calcium and magnesium (the primary hard water constituents) passes through a bed of resin, calcium and magnesium are removed from the water. Together these impurities are referred to as the total hardness (TH) of the water.

Depending on the accuracy needed, the water hardness can be tested by trillion or by the simpler dropper bottle test. The result for the total hardness test can be as either parts per million (PPM) of hardness, or grains per gallon (GPG) or even milligrams per litre (MD/L), and usually expressed as Calcium Carbonate (CaCO3). (Grains per US Gallon x 17.118 = parts per million).

Household water containing less than 1 grain per gallon, or (17.1ppm total hardness as CaAo3) is generally considered to be soft water.

However, water for industrial or commercial use (e.g. boiler feed water or other more demanding applications) may require the water hardness to be reduced to less than 1ppm total hardness.

In both cases, similar softening processes can be used. To achieve the lower lever of total hardness required for the industrial application, the design for the industrial and commercial units is likely to be more stringent.

In addition to removing hardness from water, ion exchange resins will also remove soluble iron from the water. It is therefore important to test water for presence and quality of soluble iron.

Standard resins are generally limited to a maximum of 3ppm of soluble iron. The resin can be applied to remove a higher amount of iron providing steps are taken to ensure iron fouling does not occur. Iron fouling could occur by removal of the iron before the water has been fed through the resin or by applying a resin-cleaning chemical during the regeneration process.

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