&gt;Our first problem is lack of cheap energy. Energy purifies water, moves it to where its needed.<p>You&#x27;re probably picturing desalination&#x2F;filtration plants and pipelines here, but for millions of years this task has been amply and efficiently performed by plants, fungi, and soil organisms.  This explains why mountain streams are so clean... unless clear-cutting begins.<p>Natural systems prove it&#x27;s <i>possible</i>. The trick is to use those same principles to design and organize a human-serving ecosystems for us to inhabit (as opposed to the human-destroying[1] systems we currently employ to grow food, source water, dispose of waste, etc). Just the techniques of rainwater harvesting, contour earthworks, and guild planting are sufficiently powerful and flexible to create a zero-input food system (ie no inputs except for sunlight&#x2F;air&#x2F;bedrock).<p>[1] You can substitute the fashionable euphemism &quot;unsustainable&quot; here, but I feel this term is more honest.