Water is cycled through a system of tanks that purify it using a kind of artificial wetlands, producing plants and fish. Bathrooms and kitchens work the same: faucets with hot and cold potable water, flush toilets, etc. but the water is reused locally and all "waste" reclaimed into the system.
E.g.: https://www.youtube.com/watch?v=f-sRcVkZ9yg