The Paris water system contains pairs of pipes -- potable for drinking, and non-potable for cleaning. (I'm not sure if the non-potable is just for street cleaning, or if it's used for toilets too.)
It seems so much more obviously cost-effective, I wonder why that never took off anywhere in the US, as far as I know?
http://parisianfields.wordpress.com/2012/03/11/a-most-unusua...