How to Purify Water?

There's a mass of basif industry that would need to be recreated on mars. If you are missing any, any element of that, then if the ships from earth stopped coming, it would die out. I think there's a critical threshold is, how many people? How many people i need on mars for mars to be self-sustaining? It'uld be like if you had everything except figtem and sea. Im so im see tu space that go eager after go mind some dusty ice and and then a melt it and sent it througha bunch of water purifiers, and you cand drink it. A crazy wher do you expect the population to be in mars