Accept it!    The water you drink every day has likely gone through someone else's body before you. Given this uncomfortable, yet proven, fact, does it really matter if that prior use happened decades, or days ago, as long as the water is clean?

Taken to the extreme in the urban setting, I suggest that we repeatedly reuse and recycle the same water as it flows down through a building. Basically, the waste water from the roof (water catchment) or top floor would be purified and reused to supply all the water to the next floor down. This would be repeated all the way down to the basement drawn through the system by gravity. A holding tank and pump in the basement would provide water back to the top floor. No privileges for those in the penthouse! ;-)

One way to accomplish this building-level purification at minimal cost and minimum energy input is to create the new bio-filter panels as pictured and discussed briefly here. Note that much more (but rather boring and informational) description of some of the details is included below, if desired.

These bio-filter panels are factory-assembled from shatter-proof glass, plastic, and/or stainless steel. In the first embodiment, they are basically double paned glass windows sized to replace conventional windows on the south side of buildings (in the northern hemisphere). Most importantly, these panels incorporate a slow sand filtering system in combination with solar water disinfection to purify water to potable standards as it slowly passes through. The panels are thin enough (no more than 2 inches thick) to allow a warm glow into the room while also having enough thermal mass to allow for passive solar heat as night temperatures drop.  Of course, a third glass panel and Xenon gas layer, could be added to decrease the U value in more Northern climates. (see below for further discussion on low light applications).

A second embodiment has the inner panel opaque and mounted to the side of the building. Again, solar U.V. and TiO2 (see below) to clean the water to potable standards. If not exposed to sunlight, the graphene version provides the sterilization. Simple, in line testing can be done for pathogens to ensure that nothing enters the building if dangerous.

So how many of these bio-filter panels are needed? My calculations show that between 35 and 86 square feet per occupant, depending upon a lot of variables. For example, there would be less greywater filtering needed for office buildings (where people are not showering or washing) and more needed on the side of apartment and condominium buildings (if all those water-wasting Americans are producing 36 gallons a day).
35 to 86 square feet equates to approximately 1 to 3 of these biofilter panels per occupant..... Again, based upon a lot of assumptions.
Viewed with a broadbrush, this seems very practical.

So this concept provides a practical way to simultaneously obtain passive water purification, passive solar heat, softening of intense sunlight, and maybe even produce some electricity.

Some boring technical stuff...

1. The internal geometry of the panels pass the water through evenly the slow sand filter elements and simultaneously expose it to sufficient solar U.V. for disinfection. Automatic solar monitoring devices would control the variable orifices to slow the flow through the panels under cloudy sky conditions. Remember, the water needs to be exposed to U.V. for hours to be rendered clean.

2. The pressure of the water in the bottom-most panels could get very high. Pressure regulators would keep water pressures within limits so as not to pop out the panels on the lower levels.

3. The sand layers contained in these panels would trap the larger particles. However, the sand itself could be lightly coated with titanium dioxide (Ti02) (this is the active ingredient in many sunscreens) to enhance the action of the solar radiation. When exposed to sunlight, Ti02 releases free electrons and ions which react with contaminants in the water in a process called photocatalysis. With this addition, bacterial cells and Algae blooms are eliminated as these bioagents do not survive through the photocatalysis process. Intentionally, the large surface area of the panels allows for all contaminants to be exposed to the UV and free electrons produced by the TiO2.

4. It may also be possible to use opaque materials for these panels and coat the insides that come in contact with the water with sheets of graphene (nanometer thick layers of carbon). When the water flows over the graphene, chemical ions present in the water stick to its surface. These ions provide similar disinfection action as TiO2, without the need for solar U.V.. Therefore, graphene-treated panels could be used in northern climates or northern elevations.

5. Here is something really cool. In addition, the friction force between the water flow and the layer of adsorbed ions in the graphene causes the ions to drift along in the flow direction. The motion of these ions drags the free charges present in graphene along with them, resulting in an internal electrical current. Although this induced current is very small when generated over small surfaces, the massive surface of a building with glass curtain walls could achieve harvesting of useful quantities of electricity.