If it is legal in your location, which is unlikely, you would have to install a separate drain system for gray water. It would involve extensive plumbing and would be expensive.

Removed duplicate

This post was edited by aliceinwonderland_id on Tue, Jan 8, 13 at 15:19

removed duplicate

This post was edited by aliceinwonderland_id on Tue, Jan 8, 13 at 15:18

In most locations in the East, it's prohibited. In the West, where water is scarcer, it is sometimes permitted, but only under a strict set of plumbing guidelines. You first need to contact your local codes office to determine if this would be legally possibly for you to do. And if so, then it would involve replumbing the drains from your tubs and washer, which will not be cheap.