What should comprise the water add-on?
Since there is unlikely to be a water grid, I would envisage the water add-on would comprise rain-water harvesting. This would include collection (i.e. from the roof of structures) -> filtering macro-particles -> micro-particles -> UV/chemical to remove bacteria/viruses/protozoa -> storage.
Q. Does each home need to process gray water? E.g. from washing dishes, showers.
Q. Does each home need to process wastewater from toilets?