Why do municipal water treatment plants not use distillation?

Costly and distilled water is not potable water

Explanation:

Distillation is not considered a practical approach to treat the municipal waste water because of the following two reasons -  

a) The municipal water is drinkable i.e potable water and hence it should have certain amount of necessary ingredients which are considered essential for the human body which a distilled water lacks.

b) Distilled water has application in industries, factories etc.  

c) Distillation is costly method of water treatment. Thus, until and unless there is requirement of distilled water , there is no need to use distillation method of municipal waste water.