Water Softening with lime-soda process

The old lime-soda process is now obsolete but was very useful for the treatment of large volumes of hard water. Addition of lime (CaO) and soda (Na2CO3) to the hard water precipitates calcium as the carbonate, and magnesium as its hydroxide. The amounts of the two chemicals required are easily calculated from the analysis of the water and stoichiometry of the reactions (Scheme 8.9). Since calcium

carbonate and magnesium hydroxide are not completely insoluble, the water obtained has a residual hardness of not less than 20 ppm CaCO3. A major problem of this type of process is the disposal of the sludge of precipitated calcium carbonate and magnesium hydroxide.