Retrofit your residence with a second set of pipes (use Pex) that all return water to a centrally located submerged tank of gray water, treated with UV to keep it clean. A switch would be installed on all the fixtures that would send that current flow to the tank rather than to the sewage line. Better yet, have the system analyze the water just before it goes out the sewer. Scratch the re-plumbing idea in fact, just have the central sewage line leading out of the residence analyze the water for grey or black and act accordingly. My shower water/dishwashing water really shouldn't be classified the same way the water from my toilet is. Gray water return isn't really my idea, but I'd like to see it done uniformly and safely.
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