Quality of Lime Desired for Water Treatment

Lime has numerous applications in water treatment. It is widely used as a softener for the precipitation of magnesium compounds and for the removal of carbonate hardness. For the removal of turbidity, it may be applied to hard water as a coagulant, and it is used extensively to produce the proper pH zone for maximum coagulation results. It is also added as a secondary treatment to adjust the pH value and reduce corrosion. In conjunction with lime treatment, it has been shown that lime has a high bactericidal action, reduces organic matter and thus assists to some slight extent in the removal of tastes and odors from water. Either quicklime or hydrated lime of high calcium content can be used for water treatment. Quicklime if made from pure calcium carbonate is calcium oxide. It slakes and produces considerable heat upon the addition of water. Lime is usually produced however from commercial limestone and may be high-calcium lime, calcium lime, magnesium lime, high-magnesium lime or hydraulic lime, depending upon the quality of the stone burned. For water treatment purposes, high-calcium or calcium lime only is used. Hydrated lime, Ca(OH)2, is a dry powder and is produced by adding sufficient water to quicklime to combine chemically with its calcium oxide content to produce calcium hydroxide: CaO + H2O = Ca(OH)2. In other words, hydrated lime is the product obtained when from 22 to 27 per cent of water have been added to quicklime. High-calcium quicklime usually contains from 88 to 90 per cent watersoluble calcium oxide, and calcium lime 80 to 85 per cent. High-calcium hydrated lime usually contains from 67 to 70 per cent calcium oxide, equivalent to about 90 to 95 per cent calcium hydroxide. Calcium hydrated limes usually contain from 62 to 65 per cent calcium oxide, equivalent to about 84 to 87 per cent calcium hydroxide.