The Aqua catches the water flowing down your sink drain, filters and disinfects it, and stores it ready for the next toilet flush. It is designed to be easily retrofit into existing fixtures and boasts low maintenance. Gravity and a small electric pump transport the water from the 5.5 gallon (21 L) holding tank installed under the sink. A device in the toilet tank prevents fresh water inflow as long as sufficient water is available in the Aqus to do the job. Awesome!