Ion Exchange (Water Softening)

Ion Exchange (Water Softening)

Problems associated with hard water can be minimized by using a water softener. Many automatic water softeners utilize a top mount valve which features a co-current regeneration sequence based upon elementary principles of ion exchange.

Initially, strong acid cation resin in the sodium (Na+) form is placed into service. During the sodium cation exchange water softening process, sodium ions are exchanged for undesirable quantities of calcium (Ca++), magnesium (Mg++), and iron (Fe++). Sodium ions already present in the water being softened pass through the process unchanged. Upon exhaustion of the resin (as indicated by unacceptable hardness leakage), a regeneration sequence involving sodium chloride (NaCl) is utilized.

The regeneration sequence reverses the above process and converts the strong acid cation resin back to the sodium form for the subsequent water softening cycle.
Capacities range from 20,000 to 30,000 grains per cubic foot depending upon the total dissolved solids content of the water, the effluent water quality desired, and the amount of sodium chloride regenerant used. It should be noted that although the water is softened, the total dissolved solids content remain unchanged. Further, the effluent contains the same amount of anions as the raw water.

“Soft Water” is generally defined as having less than one grain per gallon (17.1 mg/l) of dissolved calcium and magnesium ions. The quality of the softened water refers to the amount of hardness still remaining after passage through the ion exchange resin.

The amount of salt used to regenerate the exchange material governs both its hardness removal capacity and water quality. A lower salt dosage of 6 lbs will yield a softening capacity of 20,000 grains per cubic foot. The lower salt dosage is more efficient, but requires more frequent regenerations.

The total dissolved salts (TDS) content of the water also influences the effective softening capacity of a water softener. TDS content is the sum of all the ions present in the water. It varies for each water supply. Highly mineralized waters tends to reduce the efficiency of a softener and therefore should be considered when selecting the salt setting. If the TDS limitations are not observed, some passage of hardness into the product water can occur.

In situations where the conditioned water is used for general purposes, soft water is not critical. Slight traces of hardness don’t influence the overall operation and are ignored. When the TDS level is low, lower salt dosages are recommended for general use to provide the greatest operating economy. Although more frequent regenerations would be required, the operation is normally fully automatic.
Reduction of scale build-up from hard water (4 gpg – ASPE Handbook) for: boiler and cooling tower pretreatment, laundry operations, ware washing in foodservice, vehicle wash, manufacturing processes, and pretreatment for other water treatment applications such as reverse osmosis and deionization.

Untreated water can cause: increased utility bills, higher operating costs, decreased equipment efficiency and life; increased use of detergents and chemicals, reduction in linen life, dingy laundry and increased boiler blow-downs and downtime.