what will we do if a day come, when water is completely poisioned by pollution.

Well lucky for me where I live in the US we don't have to boil water or filter it ourselves to prevent disease, but if water was poisoned I'd probably try to set up some kind of steam distillation rig in my home. The way that works is that you raise water to boiling temperature (just when it starts to boil), and then you keep it that way and collect the steam in another vessel, allowing it to condense into perfectly pure and sterile water. It seems to me that's the best way to really purify water in the home, and it works on salt water/sea water as well.