Yes you can. You can use a device like the Aqus grey-water recycler to conserve and reuse! The Aqus, from Water Saver Technologies, and other systems like it can reuse the water you swish down the drain and use it to flush your toilet.
It sounds gross, I know. But it really works. First, a brief history of gray (or grey? who knows anymore…) water: Gray water is simply water that is dirty and used, but non-toxic. Most sinks and shower drains produce gray water. If the water contains toxic or decidedly un-nice things, like a toilet, it is NOT gray water. Gray water can be cleaned and filtered using relatively low-tech means, so it can be done in your house. In this case, right under your sink.
The Aqus starts by collecting your bathroom sink water into a 5.5 gallon tank under the sink. It’s then filtered first by the water flowing down the drain and over three disinfecting tablets in a chamber. The tablets need to be replaced once a year. Next, and last, the water flows through a screen that catches all the gunk that goes down the drain (like hair, toothpaste, your wife’s engagement ring…) This filter needs to be cleaned once a year, also.
A small 12 volt pump pumps the water from the under-sink tank to the toilet tank. Once in there, the toilet works normally. If the Aqus doesn’t have enough water to fill the tank, it will open a valve to fill the tank from the house supply (the way the toilet tank normally fills)
There are all sorts of safety things going on there, as well. The Aqus has a backflow preventer to keep the gray water in the tank, and not getting back into the house water supply. It also runs on 12 volts, so it’s safe under water.
Using a gray water filter system to fill your toilet tank can save you 600-700 gallons of fresh water a year!