Many green design enthusiasts are aware of greywater systems (using non-potable water where filtered drinking water is not required, for example in irrigation or flushing of toilets), but have considered the design and construction to be complicated and expensive. Sloan Valve company has developed a system that captures lavatory water and diverts it into the toilet tank for flushing, saving as much as 5000 gallons of water per year.
http://www.sloanvalve.com/index_AQUS_Greywater_System_ENU_HTML.htm
Residentiality - An Architect's view on Residential Design 