I'd actually prefer the gray water be used to heat the floor in the bathroom. If you've ever stepped out of a shower onto a tile floor you know how cold it can

I'd actually prefer the gray water be used to heat the floor in the bathroom. If you've ever stepped out of a shower onto a tile floor you know how cold it can be sometimes. And if you've ever stepped onto a (electrically) heated floor, you'll know how awesome it is. Running the warm water through a series of pipes below the bathroom floor would seem like a pretty good use of it to me, when compared to just dumping it into the sewer as-is. Then of course, in a green house, the gray water would be collected and used to flush the toilets.