There are many reasons we might be interested in ultra-low water use.
To begin a list, we might live in an area which has low rainfall and limited water abundance. We may want to reduce expenses from high water use, wherever we live. We might seek to stop unsustainable draws on local groundwater, and thus perhaps ensure adequate spring and surface water for natural wildlife and the carbon-sequestering ecosystems around us. Or either practically or philosophically, we may wish to build off-grid in as many ways as possible, be free of centralized utilities and their bills, and live with a higher degree of natural autonomy, freedom, and resilience than is typical today.
Whatever our motivations for examining and pursuing this goal, let me say upfront that genuinely radical reductions in water use are normally possible in much of the industrially developed world, without significant reductions in our material quality of life. As we will discuss, thanks to modern technology, and in most areas – and almost always in ones with above 30 cm (12 inches) of annual rainfall – it is possible to live a fully modern life with on-site captured rain and other precipitation as our sole source of water.
Importantly, while our discussion will focus on residential or domestic water use, all of its its lessons are directly applicable to commercial and institutional buildings. On the other hand, water use in industrial manufacturing is clearly a separate and more ranging topic, with different issues and differing opportunities across various industrial sectors.
However, while we will only briefly touch on this area here, the case of both industrial and domestic food production is worth highlighting as part of our core discussion. Simply put, with careful water consumption, the use of modern permaculture techniques, and movement to more natural and naturally water-conserving perennial food systems (a topic I have summarized here), the above rule of deriving all needed water from on-site precipitation also broadly applies to agriculture as well.
Lastly for this introduction, our discussion notably will assume the presence of abundant low-cost electricity, a proposal that seems reasonable, across the developed world at least, in our era of increasingly low-cost solar collectors and batteries (a trend I have explored here).