When a household requires soft water, it is out of the question to operate a small installation for the lime-soda processes. Instead, vendors supply ion exchange resin in tanks, deliver them to the home, and periodically replace them with regenerated resin. An expense that would seem high for a community dealing with large volumes seems not so great for a homeowner with small volumes who is influenced by television advertising.
Water may be termed hard because of dissolved salts, but some ions are much more objectionable than others. Ordinary soap is composed of fatty acids that have insoluble salts with calcium or magnesium ions. Washing with soap in hard water produces a dingy precipitate that gives clothing the tattle tale grey look. Modern detergents are formulated to minimize precipitation with ions in water, but the feel and cleaning action of hard water are not as good as with soft water. Advertisements by companies that provide water softening service are common.
Hardness is expressed in parts per million of dissolved solids. This is almost the same as milligrams per liter. Someone accustomed to soft water with only 35 ppm dissolved solids can tell from taste, feel, and washing action that water with 125 ppm is hard. Most people would say that water at 150 ppm seems hard, and 175 ppm would trigger sales of water softeners.
Hard water shortens the service life of pipes and chemical equipment. It is also bad for boilers, and a power plant that generates steam will always treat its water. When possible, condensate that should be essentially distilled water is fed back to the boiler. Simple softening is not enough for boiler water, and deionization by ion exchange may be used.
If everyone in a community wants softened water, chemical treatment at the water works is only about one-sixth the cost of treatment at each individual household. Of course, a complicated chemical process is impractical for each home, and ion exchange is preferred. Fresh resin in the sodium form replaces spent resin. The cylinders of resin are heavy and cumbersome but can be handled by a muscular person. Back at the company, the cylinders of resin are flushed with water to remove dirt and then with brine to elute the Ca++ and Mg++ while replacing them with Na+. The high concentration of brine that would replace almost all of the divalent ions would constitute a serious disposal problem. More dilute brine is only 45 to 70 per cent efficient in regenerating the resin, but the economics including cost of brine and disposal are better.
The "Lime -Soda Process" is a chemical method that is less costly than ion exchange. Furthermore, ion exchange substitutes ions for other ions with no effect on the total chemicals equivalents (unless the ions that are released react as in the case of H+ and OH- to form water). The Lime-Soda process removes ions and precipitates excess reactants so that the treated water has less ions than the input water. Lime contains both Ca++ and Mg++. The mineral dolomite is dug from the ground and roasted in a lime kiln to produce dolomitic lime that has a typical composition of:
CaO 0.475
MgO 0.343
CaCO3 0.018
Soda is sodium carbonate, Na2CO3. The process
aims at precipitating the salts with the lowest solubility products; these
are:
Mg(OH)2 Ksp = 5.5 x 10-12
CaCO3 Ksp = 4.82 x 10-9
Deionized water is less expensive than distilled water and is usually acceptable for boilers and for industrial processes. The ions in water can be reduced to very low concentrations by exchange with two different ion exchange resins - one charged with hydrogen ions and the other charged with hydroxyl ions. The exchange results in the formation of water. While the resins could be used in series, efficiency is greatly enhanced by mixing the resins together. As a hydrogen ion or hydroxyl ion is released, it helps drive the exchange of a nearby site to get the ion for the reaction to water. Using two resins of opposite charge together is called mixed-bed ion exchange.