Abstract
Chapter 17 introduces the key principles of water softening. Hardness creates nuisance scums, wastes soap when used for laundry or ablution, and deposits scale in hot-water services. Hardness is caused by calcium and/or magnesium ions in the water. Softening is practiced when the hardness of the outflow water is not acceptable. Lime-soda, lime, and excess-lime softening methods are explained. Lime is a cheap form of alkali, which will raise the pH to between 9.5 and 10.0 so that bicarbonates will be converted to carbonates and the calcium precipitates as calcium carbonate. Lime softening is used for calcium hardness of the carbonate form. The addition of lime will form the insoluble calcium carbonate. Excess lime softening is used for the removal of magnesium carbonate hardness. Finally, lime recovery process principles are outlined. Recovery is possible by calcining calcium carbonate sludge and subsequently slaking calcium oxide with water. After reading this chapter, the audience should appreciate different softening processes and related resource recovery strategies.
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