Reverse osmosis is very effective at turning contaminated water into potable water. The main downside of this filter technology is the significant energy needed to pump the water through the small filter openings. This concept uses gravity rather than electric pumps to produce the required osmotic pressure. For reference, water coming down from 30 stories up has a head pressure of over 125 psi, more than enough to accomplish reverse osmosis.

In practice, the greywater waste stacks of new or existing buildings would feed into a commercial reverse osmosis system in the basement (see picture). This pressure and RO system creates a building-level water treatment plant.  Buildings with rainwater catchment, seawater access, and greywater collection systems would be most compatible with implementing this concept.

When installed in tall buildings (over 30 floors) the force of gravity on the greywater in the waste stack would be sufficient to overcome the osmotic pressure for purification. In shorter buildings, the system would be plumbed into the hydraulic elevators so that every time an elevator comes down, water is purified.  A preliminary circuit schematic diagram is included showing one possible embodiment of the hydraulic elevator application. The latest diagram shows the same rotary actuator being used for the hydraulic pump and the motor to drive the pump in the R.O. inlet water system.

Note that all of the underlying technologies required to implement this are already developed and proven. The prefilters, filter sizing, post sterilization devices are already available,  It is simply a a matter that no one has yet had the insight to to process rain and greywater with gravity already within the building itself.

Most importantly, the global trend is the need to find additional fresh water sources.  Reverse osmosis is also effective at turning saltwater into potable water. Therefore, with this system in place, buildings along the coastlines could actually produce their own fresh water from the sea. I see all the high rise building along the shoreline with great views and think there is sure a lot of gravitational energy that might be used to desalinate seawater for the resident's use.

Investment would include the Reverse osmosis plant, plumbing changes, and backflow preventers at each main water connection feeding the system (see final diagram).

Cost savings would be obtained by reducing the building's municipal water bills, lower (to no) sewage fees, and reduced electrical energy usage if tied to the elevator(s).

It makes sense to start using this system to treat the water coming into the building and then move on to treating the greywater as it comes down to the basement.  Eventually, enough knowledge base will be in place to start the difficult task of treating human and industrial wastes, on site.  It can be done, but one step at a time.