Tom Broadbent, a student at De Montfort University in Leicester, UK, built a machine that harvests the kinetic energy of water that is flushed down a toilet. It's called HighDro Power and can save about £926 in a seven-storey building:
He added: “HighDro Power works by using the water discharged from appliances such as showers, toilets and sinks in high-rise apartments. The water goes down the pipe and hits four turbine blades that drive one generator.
“The whole thing was influenced by traditional waterwheels to ensure that any solids passing through had limited effects on whether they could function.”
Link via Popular Science | Photo: Creative Boom
if it were there probably won't be enough head(pressure) for some of the higher apartments.
Overall, I applaud this type of thinking. This kind of reverse engineering is the best way to solve long term problems and a novel idea.
Or £1487 if the building is next door to a Taco Bell.
really really sharp blades
or even better: cutting lasers (I know they would waste much more energy then it brings but it solves the problem)