The traditional gravity toilet now only uses 1.28 gallons per flush (gpf), reducing water use by 20 percent compared to the older requirement of 1.6 gpf. This technology uses a siphon to "pull" water through the trapway.
The dual-flush toilet is designed with two options for flushing volume; one button is for a full flush (1.6 gpf), which is meant for solid waste, and another button for a partial flush, 1.0 gpf for liquid waste. The operator needs to understand when a partial flush will get the job done, since if the user always uses the full flush there is no water savings. This, too, uses siphon technology.
An alternative system is a 1.0 gpf pressure-assist toilet. This system is used the same way as conventional systems; however, there is a vessel inside the tank designed to use water line pressure to "push" water through the trapway. This source of energy (the water line pressure) is delivered to your home from your local water municipality or well pump system. This pressure-flush device delivers the water to the bowl at three times the standard flow rate of the siphon-driven technologies, enhancing flush performance. Although it is a very quick flush, its performance may be perceived as louder.