If you need to get rid of everything in the water (ie. toxins; heavy metals, etc.), distillation is the way to go.
There is a nifty one out there called "Survival Still" (Google is your friend).
I liked the idea a lot, the accompaning price tag not so much.
I instead studied the concept and I'm in the process of making my own DIY version from a large SS dogfood bowl.
It actually will closely resemble the original when done.
I will post pics when it is finished.
ATB, Pharaoh.