You'd have to distinguish between potable water and just any water too. https://en.wikipedia.org/wiki/Drinking_water https://en.wikipedia.org/wiki/Drinking_wat

You'd have to distinguish between potable water and just any water too. https://en.wikipedia.org/wiki/Drinking_water https://en.wikipedia.org/wiki/Drinking_water mentions that Christchurch, NZ has a sufficient supply of pure enough water that no treatment is required. So you could probably build a passive filtration system for a one time cost. It just may not be fast enough to treat enough water if your population is too big.