When we flush, it goes down by force of gravity. When it rains or we wash our cars, it goes down the drains by force of gravity.
When all of this goes from place to place, that momentum can be used by a turbine. That's how the turbines generate electricity from Niagara falls, the momentum of the water caused by gravity forces the turbines to turn.
If the drains are strategically constructed (which I assume most of them are underground to lead to main terminals), electricity can be pulled from those sources.
So every time you flush, wash your car, or a rain storm comes in, you can wink at your generated electricity.
I have more ideas for electricity generation to be posted on here soon. Hopefully, many of which have already been thought of and are in action.
If anyone wants to partner with me, I'll further explain the logistics in private. You can just ask me questions here too, because I'm willing to share the details with anyone so long as our energy situation in the world benefits. Either way, some questions will take some research to answer.