The concept you are asking about is the reuse of what is called "grey water". This includes used water from household activities - sinks, dishwashers, washing machines, and showers. While this water cannot be consumed, it is possible to install a system that stores the water for reuse in a toilet or for other applications such as irrigation.

For more information, see these links:

http://greenupgrader.com/2347/flush-your-toilet-with-recycled-gray-water/ http://en.wikipedia.org/wiki/Gray_Water

For a company that may be able to help you install a grey water system in your own home, see Saniflo:

http://www.saniflo.com/

I hope this helps. Good luck!