Chapter 20 - Water softening

Abstract

This chapter focuses on water softening process used to reduce the hardness of the water. Hardness is caused by calcium and magnesium ions in the water and is classified in two ways, namely, calcium or magnesium hardness and carbonate or noncarbonate hardness. Carbonate hardness is caused by calcium or magnesium ions associated with bicarbonate ions and noncarbonate hardness is caused by calcium or magnesium ions associated with chloride, sulphate, or other ions. For general treatment of municipal supplies, a process of precipitating the calcium and magnesium hardness by adding lime (Ca(OH)2) and/or soda (caustic soda (NaOH) or soda ash (Na2CO3)) is usually adequate. The quantities of soda ash (Na2CO3) and lime (Ca(OH)2) are estimated from a chemical analysis of the water, and are subsequently added to the influent stream. Sometimes, a small amount of iron salts or alum is added as a flocculation aid. Lime softening is used for calcium hardness of the carbonate form, where the addition of lime forms the insoluble calcium carbonate. In cases where the target hardness and raw water quality are suitable, split treatment can eliminate the need for recarbonation and results in the saving of chemicals. Excess lime softening is used for the removal of magnesium carbonate hardness. Lime recovery is possible by calcining CaCO3 sludge and subsequently slaking CaO with water. In this way, more lime than what is required in the plant is produced, and the surplus can be sold.