John notes in an earlier post that gray water re-use is, well, a gray area. However in fact it has been studied and documented, and is accepted in the IPC, or International Plumbing Code. Most municipalities use this or the Universal Plumbing Code, (UPC) as their standards, and neither is international or universal, but that is an aside. According to Ecospace:
The details: Basically what the IPC is now saying is that water coming from bathtubs, showers, lavatories (read sinks), and clothes washers are no longer required to discharge into the sewer main. This gray water is now considered collectable for the use of flushing toilets, (and subsurface landscape irrigation) if the proper procedure is followed.
In essence, you would be required to have a sensible storage tank (at least 50gal) that won’t leak connected up with appropriate piping. Additionally, they stipulate that the water must be disinfected, be stored no longer than 72 hours, and be died either blue or green with vegetable dye. (code notes here)
So it's not such a big deal; properly installed, nobody is going to be drinking green water from the tap, and it is unlikely kids are going to be playing in gray water sprinklers. Our big step in building: Put a gray water recovery system in every new house and retrofit any house where the owner wishes to have a lawn irrigation system. ::Ecospace via ::After Gutenberg