What causes hardness in water? Explain how hard water can be softened using the

Question

What causes hardness in water? Explain how hard water can be softened using the lime and soda ash technique in a split treatment mode? The practical limits of hardness removal is 30 mg/L of CaCO_3 and 10 mg/L of Mg (OH)_2. Calculate the quantity of softening chemicals required in pounds per million gallons of water to soften water having the composition shown below. Ca^2+ = 185 mg/l alkalinity = 200 mg/l CO_2 = 8.8 mg/1 as CO_2 Mg^2+ = 50 mg/l SO_4^2- = 58 mg/l Na^+ = 20 mg/l Cl = 36 mg/l pH = 7.7

Explanation / Answer

The terms hardness or softness are expressions of the amount of calcium and magnesium salts dissolved in the water. Calcium and magnesium react with the fatty acids in soap to produce a scum and if there is a high amount of calcium it makes it difficult to wash yourself and your bedding and clothes.

When water contains high amounts of magnesium hardness, split treatment may be used. Approximately 80 percent of the water is treated with excess lime to remove magnesium at a pH above 11, after which it is blended with 20 percent of the source water. Split treatment can reduce the amount of carbon dioxide required to re-carbonate the water as well as offer a savings in lime feed. Since the fraction of the water that is treated contains an excess lime dose, magnesium is almost completely removed from this portion. When this water is mixed with the water that does not undergo softening, the carbon dioxide and bicarbonate in that water re-carbonates the final blend. Split treatment reduces the amount of chemical needed to remove hardness from water by 20 to 25 percent.

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