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Not one drop of the water we consume every day is comprised exclusively of hydrogen and oxygen molecules. It contains some amount of minerals, impurities, microorganisms and other contaminants. These substances may be present in only trace amounts, and don’t necessarily have negative health effects. 
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What is water quality improvement? 

Water treatment, or conditioning, is the processing of water, by any means, to modify, enhance, or improve its quality or to meet a specific water quality need, desire, or set of standards. Water treatment involves disinfecting and purifying untreated ground and surface water. Water quality is important for health, and it is good for appliances, too!

Is softened water corrosive?
No. The suggestion that ion exchange water softeners promote corrosion is a misperception and the suggestion to use scale build up as a corrosion control method is inappropriate as proven by the EPA.  The conclusions were drawn from an EPA pilot study conducted by Thomas J. Sorg and Michael R. Schock of the EPA's Drinking Water Research Division as project manager and principal investigator. Simply put, the removal of hardness with an ion exchange water softener does not affect the factors which cause or even accelerate corrosion.  Corrosion is caused by a change in the pH or carbon dioxide concentration, the dissolved oxygen concentration, or the total chemical concentration of minerals. None of these factors is affected by water softeners.

What is hard water?
Hard water is a common quality of water which contains dissolved compounds of calcium and magnesium and, sometimes, other divalent and trivalent metallic elements. The term hardness was originally applied to waters that were hard to wash in, referring to the soap wasting properties of hard water. Hardness prevents soap from lathering by causing the development of an insoluble curdy precipitate in the water; hardness typically causes the buildup of hardness scale (such as seen in cooking pans). Dissolved calcium and magnesium salts are primarily responsible for most scaling in pipes and water heaters and cause numerous problems in laundry, kitchen, and bath. Hardness is usually expressed in grains per gallon (or ppm) as calcium carbonate equivalent.

 

What causes hard water?
Hardness minerals – calcium and magnesium – are in plentiful supply. While they are not found in their elemental form in the earth, they occur in combination with other elements in an abundance of forms. Common calcium minerals include chalk, limestone, and marble. These substances are chiefly calcium carbonate (CaCO3) or mixtures of calcium and magnesium carbonates and other impurities. The hardness in water that is caused by calcium, magnesium, and other cations is usually described in terms of the calcium carbonate equivalent.