Total number of methods which can remove permanent hardness of water :
(a) Clark's method
(b) Ion-exchange method
(c) synthetic resin method

(d) Calgon method

(e) treatment with sodium carbonate

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answer is 4.

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Detailed Solution

Clark's method is used for removal of temporary hardness.

Clark's method is one of the ways to cure hard water. The amount of dissolved divalent metal cations in water is is known as its hardness. Ca(OH)₂ is used in Clark's method to soften hard water.

Hence all the other given methods are used to remove the permanent hardness.

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