“Hardness” in water is the sum of all ions that react with soap to produce soap scum or “bathtub ring,” and also inhibit lathering. The problem ions are all metals with more than one “+” charge-mostly calcium (Ca+2) and magnesium (Mg+2) in most water supplies, but also including zinc (Zn+2), copper (Cu+2), manganese (Mn+2), and others if present. In addition to interfering with cleaning, hardness also combines with alkalinity to form “lime scale,” which is calcium carbonate, CaCO3. Scale is a problem because it forms hard deposits in water lines and water-using equipment which can scratch valves, insulate the equipment and interfere with heating or chilling operations, and cause clogging. The standard treatment for hard water is by “softening,” or “water conditioning” by “sodium-cycle ion exchange.” Hard water is directed through a bed of cationic ion exchange resin in “sodium form” (with Na+ ions attached to the resin), which exchanges the Na– ions for hardness ions in the water.
However, softening is not always desirable, because, the Na” ions put into the water may be unwanted. Even if the possible negative health effects of Na+ on blood pressure are avoided by using K+ as the exchanged ion instead of Na”, both the taste and the TDS effect of either one can make the treated water taste salty and be unsuitable for brewing coffee or making soft drinks. The taste threshold for Na+ is in the 100 – 150 ppm range for most people, and softening-water with a hardness of 217 ppm as CaCO3 will result in the addition of 100 ppm Na+ to the water. Thus, about 15 grains per gallon or 257 ppm as CaCO3 should be considered the maximum hardness that can be treated by softening without producing a salty taste. Sometimes reverse osmosis is used to remove excess sodium and other dissolved ‘minerals with health effects.