Modern air travel would not be possible without the advent of the jet propulsion engine. A jet is a high-velocity stream of fluid that is used for a variety of engineering applications. In the case of a jet engine, incoming air is compressed to pressures over 100 psi, mixed with fuel and combusted to create up to 9,000 pounds of thrust. Similarly, many modern conveniences would not be enjoyed without the advent of the siphon-jet toilet. Using similar technology, a high-velocity stream of water is directed near the base of a porcelain-coated bowl to initiate the evacuation of a solid/liquid mixture through an engineered trapway with a final siphonic phase action.

A siphon is defined as a “tube bent to form two legs of unequal length by which a liquid can be transferred to a lower level over an intermediate elevation by the pressure of the atmosphere in forcing the liquid up the shorter branch of the tube immersed in it while the excess of weight of the liquid in the longer branch when once filled causes a continuous flow.” In the case of a toilet, it consists of a trapway designed into the molded clay, vitreous coated casting. The trap is designed to hold a specific volume of water and needs to be big enough to pass an expected amount of solid/liquid biomass. The high water mark, based on third-party testing, is 1,000 grams for commercial toilets. A natural question is: “Why not just make the trapway as big as possible, then toilets would never clog?” The bigger the trapway, the more water it takes to keep it full and maintain a siphon during flushing.

Other flushing mechanisms include the “washdown” and the “pressure-assist.” While the washdown may utilize a bigger trapway and evacuate the toilet bowl in a more brute way, pressure assist uses an air chamber in the water tank to pressurize the water and add extra velocity to the flushing action. Pressure-assist is a variation of the siphon-jet and claims to need less water because the added velocity can break up solids and utilize a smaller trapway, hence a smaller volume of water is needed to induce siphonic action. Pressure-assist could be compared to an afterburner in a low bypass jet engine, where the added pressure gives the system some extra “oomph.” So, if you happen to use a pressure-assist water closet, rejoice in the idea that you are using the same technology that is used in fighter jets; at least the same engineering concept that maximizes efficiency at the cost of something else; in this case, lower noise.

Anyone of us involved in plumbing engineering knows the maximum volume of water used per flush is 1.6 gallons. When a siphon jet water closet is flushed, some of the water is directed through small holes around the rim while the remainder of the 1.6 gallons is directed through the siphon jet. If it weren’t for the messy nature of, well, Mother Nature, I imagine that a greater portion of the flush would be directed to the jet, and not as much washdown would be required.

There are guidelines and standards that provide minimum performance metrics in order to bring water closets to market. Those standards include WaterSense and the Code of Federal Regulations, 10 CFR 429.30, which addresses water closets. The WaterSense specification provides a method for testing with a pass or fail criteria of clearing 350 grams, while the Code of Federal Regulation outlines a standard method of sampling and reporting. Here are some of the protocols outlined in the WaterSense specification.

1.3 Test media is comprised of the following:

1.3.1 Seven (7) test specimens at 50 ± 4 grams per test specimen (“test specimen”) consisting of soybean paste forming a “sausage” approximately 4 ± 0.5 inch (100 ± 13 mm) in length and 1 ± 0.25 inch (25 ± 6 mm) in diameter. The total mass of test media used for each test shall be 350 ± 10 grams.

1.3.2 Four (4) loosely crumpled balls of toilet paper (“paper”).

And, the waste extraction test is outlined below.

2.3 Waste Extraction Test:

2.3.1 Test specimens shall be formed such that they are roughly cylindrical in shape and uniform in diameter.

2.3.3 Seven (7) test specimens (350 grams) shall be freely dropped in a vertical orientation through opening through a drop guide into a bowl.

2.3.4 Immediately remove drop guide and freely and randomly drop four balls of crumpled toilet paper over center of bowl sump, wait 10 seconds & flush sample.

Just like the standards and regulations that make sure we can travel safely in the air, there are standards and regulations that ensure the plumbing products we use are safe, hygienic and efficient. Some of these standards encourage development in areas that put new emphasis on important priorities. Even though the siphon-jet design had been around since the 1800s, improvements in toilet bowl and flushing technology is what led manufacturers to meet the requirements of the 1.6 gpf benchmark of the 1992 Epact law.     

Engineering has a long history of discovery and innovation, fueled by our curiosity and desire to make things better. The way we shape the physical world and create new devices is fascinating and rewarding. Harnessing the power of water goes back millennia to the times of the Greeks and Romans. Harnessing the power of electricity and carbon-based fuel combustion allowed us to create devices like the jet-propelled aircraft. Flying around this round globe of ours brings such marvel that we can say it’s like “flying the friendly skies.” Thankfully, there are also plumbing systems on aircraft that can contain our human waste until landing — or else, the skies would not be so friendly after all.