When I visited Japan, I saw many toilets that had a little faucet&#x2F;sink built into the toilet lid, so you could wash your hands with it before it was used to fill the tank.<p>However, I&#x27;m not sure how practical that kind of thing would be to do at house-scale.  You&#x27;d need to have extra set of plumbing, plus storage tanks for the greywater (and probably pumps to get it there), among other extra things.