A (Very) Brief Summary:
In tall, multi-floor buildings:
Have a ‘micro-hydro-electric-turbine’ inserted into the ‘SEWAGE OUT’ Pipe/Conduit just before this Pipe Meets the underground Municipal Sewage System.
(Exact execution/installation details later).
The turbine generates electricity which can then be stored in ‘TESLA PowerWall’ Style High Capacity Lithium-Ion Batteries - for use when needed/appropriate.
The above methodology captures both:
(1) A large (but always lower than 100%) portion of the Gravitational Potential Energy of the Clean Water in the Water Tank at/near the top of the building
(2) Plus some portion of the the Gravitational Potential Energy of some of the soaps / shampoos / clothes-washing-liquids-or-powders / food-and-beverage / other grocery-items / etc. brought in and up (whether by elevator / ‘lift’ and/or stair-cases) and consumed or used in other ways by the people living and/or working in the building - including the ‘human waste’ that gets discharged into the ‘common’ sewage piping.
This methodology scales with buildings of any height / any number of floors (except - maybe - some short buildings with only around 1 or 2 floors).
Rain-Water Harvesting, too:
This methodology can also be used to capture some of the Gravitational Potential (and, if appropriately modified / designed, ALSO some of the KINETIC) Energy of each of the many, many Rain-Drops being captured by ‘RAIN-WATER HARVESTING’ systems.