Already a good part of the world faces water issues. Here in the tropics, water on this island comes from a hellishly expensive reverse osmosis system that uses a lot of energy, or it is barged in from natural lenses of fresh water from another island. This is unsustainable, especially with the population growth of people migrating to the capital city to look for jobs and improve their way of life.
So how are we going to resolve this water issue? Simple. Using technology. The way that the astronauts do. The only answer is to recycle water.
I would imagine that each house would have a water recycler just like a furnace. All of the drains lead to the recycler. When you boil your noodles, and dump the water down to the recycler, it comes back pure and clean without the salt. When you have a pee, it comes out a pure drinking water. The astronauts now drink the water from their pee after it has been purified.
It will even go further than that. There will be machines to take the humidity out of the air and turn it into drinking water. Water will be caught and recycled from clothes dryers. Water will become so valuable, that it will be worthwhile to process it from wherever we find it. This will be good news for the Scotsman who will go green by drinking his Scotch straight, instead of adding water to it.