Most of the smaller 1.5" pipe drains (sinks, tubs, washers) make their way under the house via PVC networks until they connect with larger 3" and 4" drains used by toilets. You could reroute just the shower drain on millions of homes to a tank and a small put could feed it back to toilets. It would never mix with raw sewage. It wouldn't be reasonable with homes built on a slab, you could still do it pretty easy on millions and millions of homes, apartments and such. That would save millions and million of gallons of water.