I live in a tall building (20 floors) on a mountain. Because the water pressure from the water company is not enough, there is a water pump at the last floor which is activated each time someone is using fresh water in his apartment.
To me, that's a big waste of energy, and I think that in theory it would be possible to reuse the used water which is going down to pump some fresh water up.
I wonder if such device already exist on the market, and I would like to know how such device would be called. Would there be a problem to operate with such a height difference?