It's not that hard... Maybe it's because I grew up in a family that builds water and sewage treatment plants for over 100 years, but we have local surface water, a water table that in the rainy season comes up to 2 feet, and we get about 30" a year here.
And it's not hard to make it safe.
Remove the stuff from it. Whatever it takes. Filter through gravel and sand to remove big chunks, drip it through charcoal to remove the bad flavors, add chlorine or idodine or boil it to kill the bitty nasties that are left.
Add air to it to make it palatable.
/johnny
But now you got women who want a nice hot bath! THAT’s when the SHTF!