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A low flow toilet is a toilet designed to remove waste using a reduced amount of water compared to a traditional toilet. They are also commonly referred to as low flush toilets and modern toilets, and they've been in use in the U.S. since 1994 when a federal regulation mandated that only 1.6 gallons (6 liters) of water be used per flush. This change was enacted in response to environmental concerns. Although the first models had issues, the technology has improved considerably.
Older toilet designs prior to 1994 used large volumes of water to help gravity propel waste down the drain. Typically 5 to 7 gallons (19 to 26 liters) of water was used with each flush. The release of this amount of water rushing from the tank generated adequate force to move any waste that was present in the bowl through the trapway, or the hole and passage at the bottom of the bowl, the pipes, and out of the building. The change to a low flow toilet design using only 1.6 gallons of water represented a huge shift.
The primary reason for the change to low flow toilets in 1994 was water conservation; reducing the amount of water used to 1.6 gallons per flush is estimated to save thousands of gallons per person each year. The U.S. regulation means that all new toilets sold must meet this standard, and other countries may have similar regulations. Low flow toilet designs must be used in all new construction in order for the project to meet building codes. They are also estimated to save homeowners significant amounts of money on their water bills. Some localities even offer rebates to homeowners who upgrade older toilets to more efficient low flow models.
The first low flow toilet designs simply changed the tank size, thereby reducing the amount of water used without making any other modifications. These early models had many problems and often became clogged or required two flushes to adequately remove waste. These issues frustrated homeowners, making them reluctant to purchase the new toilets. They repaired their old ones or purchased used models instead.
These complications prompted manufacturers to make modifications and improve their low flow toilet models. Most currently available models work in a comparable fashion to older pre-1994 designs. Some of the changes that have helped include widening and straightening the trapway, and finishing the passage in a manner that reduces friction. Other models use air pressure or pumps to help the water move with added force. Innovative designs that originated in Australia save even more water; they flush two ways, using only 0.8 gallons (3 liters) of water for liquid waste and 1.6 gallons (6 liters) for solids.