Question

in ion exchange method water is first passed through cation exchanger and then through anion exchanger give reason

Answer 1

If water first pass through an anion exchanger all the cations will convert to Hydroxides e.g Ca(OH)2, Mg(OH)2,NaOH etc. Because of Hydroxide the water pH will be high. Hence , calcium Hydroxide will be precipitated and block the exchanger.

Answer 2

Thank you for asking this question , let me help you in finding the answer . 
Let's suppose water pass through the anion exchanger then in that case all the cations present there will be converted in to the hydroxides just like calcium hydroxide and sodium hydroxide. This phenomenon would result in higher pH as the  result hydroxide will also get precipitated and will  result in the blocking of the exchanger. This is why water is passed through cation exchanger first.