Water Treatment-Water softening

Hardness in water is the result of concentrations of calcium and magnesium ions measured in mg/l as calcium carbonate (CaCO₃).  Water hardness can cause scaling problems in water heaters and soap doesn’t lather well in hard water. Also, many industrial uses need soft water.

Different processes have been developed to remove hardness from water:

• Chemical precipitation,
• Ion exchange resins,
• Reverse osmosis.

Caustic soda can be used to remove both carbonate (temporary) and non-carbonate (permanent) hardness.  Calcium and magnesium hydrogen carbonates react with caustic soda to produce insoluble calcium carbonate and magnesium hydroxide.

       [Ca²⁺ + 2HCO₃^-] + 2NaOH  ® CaCO₃¯ + 2Na⁺ + CO₃^- + H₂O

       [Mg²⁺ + 2HCO₃^-] + 4NaOH ®  Mg(OH)₂¯+ 4Na⁺ + 2CO₃^- + 2H₂O

       Mg²⁺ + 2NaOH ® Mg(OH)₂¯ + 2Na⁺

Caustic soda softening is more efficient at pH of 9.0 to 9.5 for calcium and at PH of 11.0 for magnesium ; therefore, extra caustic soda is added in this latter case to raise the pH.  The pH must be neutralised at the end in order to obtain the desired pH range.

It is usually not desirable (or possible) to remove all the hardness by precipitation. Typically, the minimum possible calcium hardness is about 20 mg/L and the minimum magnesium hardness is about 10 mg/L, both as CaCO₃.  Therefore, about 30 mg/L total hardness as CaCO₃ can remain.

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