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Modern low-flush toilet with white porcelain body and white plastic seat standing on wooden floor

How toilets work

Toilet, lavatory, loo, water closet, WC, John, crapper, can—it's amazing we have so many names for something we care to talk about so little. Toilets are hardly the most glamorous of inventions, but imagine trying to live without them. A fifth of the world's people (some 1.7 billion of us) are in that unhappy position, lacking even basic sanitation. [1] At the opposite end of the scale, in Japan, people have amazing electronic toilets that do everything from opening and closing the lid automatically to playing music while you use them. Most of the world's toilets are more modest than this, but they're still pretty ingenious "machines." Let's take a closer look!

Photo: Poop-tastic! Like most new toilets, this low-flush model is designed to save water; the two buttons on top let you choose whether to flush with a large or a small amount. Exactly how much difference that will make to your water consumption varies from one household to another. An old-style flush toilet typically uses 13 liters (3.4 US gallons), where a low-flush model will use only 6 liters (1.6 US gallons) and some models use only 4.8 liters (1.3 US gallons). If you save 7 liters (1.8 US gallons) per flush and people in your home flush 10 times a day, you'll save at least 25,500 liters (6700 US gallons) per year. You'll save more or less depending on how many people there are in your household.

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  1. Flush toilets
  2. Vacuum toilets
  3. Composting toilets
  4. Find out more

Flush toilets

At first sight, toilets seem quite simple: you have a waste pipe going through the floor and a tank of water up above (called a cistern) waiting to flush into it when someone pushes a button or pulls a lever or a chain. Most flush toilets are purely mechanical: pull the chain and the cistern empties through the force of gravity, washing the bowl clean for use again. They are literally mechanical because they flush and refill using levers inside—and levers are examples of what scientists call simple machines.

Artwork of old-fashioned high-cistern toilet

Artwork: Old-fashioned toilets gained extra flushing power from high cisterns like this, which fed the bowl through narrow pipes (along the blue-dotted path). Artwork from Text-Book of Hygiene: A Comprehensive Treatise on the Principles and Practice of Preventive Medicine from an American Stand-point by George H. Roheé, 1890, p.170. [2]

There's a little bit more to toilets than this. When you flush, the cistern has to refill automatically from a kind of faucet on the side and the refilling operation has to last just long enough to fill the tank without making it overflow. The "hole in the ground" is more sophisticated than it looks as well. You may have noticed that toilets always have a little water in the bottom of them; even when you flush them, they never empty completely. Some water is always trapped in a big curved pipe at the base of the toilet known as the S-bend (or S-trap). This little bit of water effectively seals off the sewage pipe beneath it, stopping germs and bad smells from coming up into your bathroom. The S-bend also means that the pipe running out from the toilet bowl curves upward, before curving back down again. That means when water flows into the bowl from the cistern, and drains out through that pipe, it has enough momentum to produce a siphon (sucking) effect, which properly empties the bowl.

Flush mechanism inside the cistern of a toilet

Photo: Lift the cistern on a toilet and this is what you'll find inside. The cistern (upper tank of water) drains through a valve in the center through the force of gravity. The valve and flushing mechanism in the middle is called the siphon. The blue, balloon-like object on the left is a plastic float that drops when the water level falls. This tilts the white plastic lever, opening a ball valve (sometimes called a ball cock) and allows the cistern to refill. As the water rises, the float rises with it, tilting the lever and slowly shutting off the ball valve.

What happens when you flush?

Animation showing the different parts of a toilet and how they work during a flush

  1. Press the handle to flush the toilet and you operate a lever (dotted line) inside the cistern.
  2. The lever opens a valve called the flapper (green) that allows the cistern to empty into the toilet bowl beneath through a mechanism called a siphon.
  3. Water flows from the cistern through holes in the rim so it washes the bowl as well as flushing the contents away.
  4. There's enough water flowing down from the cistern to flush the toilet around the S-bend (S-trap). This produces a siphon effect that sucks the bowl clean. It also ensures some water remains at the bottom of the bowl, which improves hygiene.
  5. The contents of the toilet are flushed down the main drain.
  6. As the cistern empties, the plastic float (red) falls downward, tilting a lever.
  7. The tilting lever opens the ball valve (ball cock) (green) at the base of the cistern (or on one side of it), which works a bit like a faucet (tap). Pressurized water flows in, refilling the cistern, and pushing the float back up again. When the float reaches the correct level, the ball valve switches off the water supply and the toilet is ready to flush again.
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Macerating toilets

Flush toilets come in many shapes and sizes, but one thing they all have in common is a relatively large and ugly outlet for the waste, sometimes known as a soil pipe. These pipes are so large and wide that they limit the places where normal toilets can be fitted. If you need a bathroom in the middle of a building where it's impossible to route a large soil pipe, what can you do? Most toilets also work through the gravity-siphon effect, but what if you need to put one in a basement and the drain you want flush into is up above it?

One solution to problems like this is to fit a macerating toilet (sometimes called an upflush toilet), which is a bit like a cross between a conventional toilet and a waste disposal unit. The waste passes into a kind of blender that mashes it up into a liquid, before pumping it up through a thin pipe connected, eventually, to the large, main soil pipe. The big advantage of toilets like this is that you can fit them almost anywhere in a building where there's both water (for the flush) and electricity (to power the macerator and pump). Unfortunately, they're often bigger and bulkier than conventional flush toilets, noisier and less discreet, and (since they have more moving parts) less reliable. Although they're more expensive than normal toilets, they can still work out cheaper than replumbing your house for a conventional toilet. According to Saniflo, one of the leading manufacturers, a typical macerating toilet can pump waste an impressive distance: with an upward rise of 5m (16ft), a horizontal distance of 20m (66ft); or with an upward rise of 1m (3.3ft), a horizontal distance of 100m (328ft). That's more than enough for most homes and many commercial buildings.

Simple line artwork showing the main parts of a macerating toilet and how they work.

Artwork: How a macerating toilet works. The toilet itself is much like any other, but it has a macerating and pumping unit (red) behind and underneath its cistern. The electrically powered pump (black) sucks waste in from the toilet's large soil pipe (substituting for the suction effect of a normal toilet), chops it into a liquid, and then pumps it out through a small pipe on top. This type of macerator is separate from the main toilet unit and can be placed in a different room; it's effectively an adapter that makes a conventional toilet work with a small-bore (small diameter) waste outlet pipe.

Who invented the flush toilet?

Although it's popularly believed that flush toilets were invented by an English plumber called Thomas Crapper (c.1836–1910), it's an unhelpful myth, for two reasons: flushing toilets are an ancient technology, and no single person can really claim to have invented them (dozens of different inventors have been involved in their development over the years, especially since Crapper's lifetime). Archaeological evidence shows that primitive toilets using river water to flush wash away waste are over 5000 years old and date back to ancient Mesopotamia in the fourth millennium BCE. The two inventors who have the best claim to our modern toilet-flushing system were born hundreds of years before Crapper. Among his many other achievements, prolific Arabic inventor and engineer Al-Jazari developed a flushing hand-washing device in 1206, while English writer and courtier Sir John Harington (1561–1612) described a method for flushing a toilet in 1596 in his article A New Discourse of a Stale Subject, Called the Metamorphosis of Ajax.

Search through the invention records at the US Patent and Trademark Office and you'll find literally hundreds that relate to toilets and their flushing mechanisms. I've chosen two examples from 1874 to give you a flavor of what you can find. On the left (and drawn in plan view, from above), we have the self-disinfecting water-closet basin developed by Jabez Burns, Charles Higgins, and William Higgins ("Improvement in Water-Closet Basins", US Patent#149,195). Their simple innovation was to make the pipe that fills the toilet basin squirt sideways over a bar of soap, thus disinfecting the basin and stopping any smell. On the right, you can see Archibald McGilchrist's trap-less water closet ("Improvement in Water-Closet Apparatus", US Patent#157,211). Unlike with an S-bend closet, there is no water trap to stop odors. Instead, the flush mechanism raises and lowers a ball-shaped valve that seals the waste pipe. A rising and falling float (I've colored it green in the artwork) operates a valve mechanism (colored yellow) to refill the basin in the usual way. You can explore lots more similar inventions with a search for "water closet" on Google Patents (it just gave me 13,000 results!).

Water closet with built-in disinfectant dispenser Water closet with trapless flush mechanism

Artwork: Two examples of 19th-century improvements in water closets (toilets) by American inventors.
Images courtesy of US Patent and Trademark Office with added coloring and annotations by

Vacuum toilets

One of the biggest drawbacks of conventional flush toilets is the sheer amount of clean water they squander. Even the leanest, greenest, most efficient toilets waste several liters (over one US gallon) of water per flush, which poses a real problem for vehicles like trains, planes, and long-distance buses: do you really want to carry a giant, heavy tank of water just for flushing the toilet? That's why mobile public toilets typically now flush with air instead of water; they're the toilet equivalents of vacuum cleaners: vacuum toilets, in other words. Vacuum toilets might sound like a strange idea until you remember that a conventional toilet flushes not just by washing the waste down but by sucking it away with the siphon effect. A vacuum toilet does exactly the same, but the sucking is created by low air pressure instead of the dragging power of draining water.

A blue and silver vacuum toilet on a train.

Photo: A typical vacuum toilet on a train. Toilets like this are designed to use minimal amounts of water but tend to be more mechanically complex (and therefore less reliable) than ordinary flush toilets.

How does a vacuum toilet work? There's a sliding valve in the floor of the toilet bowl sealing it off completely from the rest of the mechanism. When you flush the toilet, a small amount of water rinses out and partly fills the bowl (typically no more than about 0.4–0.5 liters, or about 10 percent as much as in a low-flow, conventional flush toilet). Meanwhile, out of sight, behind the wall, a small, plastic, intermediate tank has the air rapidly pumped out of it to create, in effect, a vacuum. The valve under the toilet bowl slides open, creating a huge difference in air pressure between the room and the intermediate tank. This sucks the waste from the toilet into the tank and the sliding valve closes again. Air is now blown into the intermediate tank, flushing its contents into a much larger waste tank that can be emptied periodically (for example, when a train arrives at a station or a plane lands on the ground).

Animation showing how a vacuum toilet works.

Animation: How a vacuum toilet works (simplified). The main parts of the toilet are a toilet bowl (blue), an intermediate tank (gray), a vacuum pump (red), a pressurizer (green), and a main waste tank (orange), all separated by valves (black). 1) To begin with, there is waste in the bowl (blue) and the intermediate tank is full of air. 2) When you flush, the vacuum pump sucks the air from the intermediate tank, leaving a partial vacuum. 3) The valve under the toilet bowl opens and the intermediate tank sucks the waste out of it. 4) The pressurizer blows the waste from the intermediate tank into the main waste tank. Water plays a relatively small part in a toilet like this so I've missed it off this diagram for clarity.

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Composting toilets

Flush—and it's gone. Toilets are one of those inventions we really take for granted. Until you stop to think about the two billion or so people in developing countries who don't enjoy the same luxury, you might not realize just how lucky you are to be able to solve such a horrible little problem with a quick press of a switch. There's just one slight difficulty: your toilet doesn't actually dispose of sewage: it just washes the problem down a long smelly pipe so it ends up somewhere else—and it uses lots of water in the process. Even in the world's richest and most sophisticated countries, sewage disposal is a major issue. We still have dirty beaches, algal blooms on rivers, and major health issues like shellfish poisoning caused by sewage pollution. Wouldn't it be better if toilets could actually convert sewage into a form we could dispose of safely and simply? That's the basic idea behind composting toilets, which turn the stuff we don't like to talk about into compost we can use to fertilize our land. How do they work?

A composting toilet in Israel.

Photo: Despite what you might think, composting toilets are just as convenient as flush toilets. They're also more civilized, since you're not dumping your waste elsewhere and expecting someone else to deal with it. This is a composting toilet at Kibbutz Lotan, Arava Valley, Israel (note the bag of sawdust and straw at the back to help the aeration process). Photo by Hanan Cohen published on Flickr in 2007 under a Creative Commons Licence.

What's the problem with ordinary toilets?

In nature, there's no such thing as waste. Leaves fall to the ground, rot down, and fertilize the trees that dropped them. Long ago in history, humans would have been just as clever without even thinking: all our "soil" would have disappeared harmlessly into the soil and made things grow again in future. Unfortunately, when the Industrial Revolution kicked off, and masses of people started living very close together in towns and cities, sanitation became a major issue and a massive public health problem. That's how we came to have toilets, sewers, and sewage treatment plants. Now, sewage is still a problem but for different reasons. Water is much scarcer than it used to be and climate change will make it even more precious in future. Do we really want to use something so valuable for something as crude and basic as flushing away our waste? Probably not. One solution to the problem is for homes to have a separate greywater system, where relatively clean wastewater from things like baths and showers is stored temporarily and used to flush toilets. Composting toilets are a different solution.

What is a composting toilet?

A composting toilet at an exhbition in South Africa.

Photo: A sophisticated composting toilet and the system it feeds. The toilet part is the white bit at the top. You can also see the large black waste tank and a silver ventilator on top. Most of this would normally be hidden inside a building, but it's on show here in an exhibition. Photo by Sustainable Sanitation Alliance published on Flickr in 2005 under a Creative Commons Licence.

The idea is simple. Instead of flushing your waste down a pipe, from where it could travel maybe several miles to a treatment plant, a composting toilet turns sewage simply and safely to compost in your own home. Although there are many different types of composting toilet, the principle is the same in all cases: the waste falls into a well-ventilated container where, over a period of time, aerobic bacteria (supplied with lots of oxygen) greatly reduce its volume (much like kitchen waste on a compost heap) and destroy harmful pathogens (the bacteria, viruses, and so on that cause diseases). The end-product looks a bit like rich soil. Some composting toilets separate out the liquid and solid wastes, both of which may be suitable for use as "humanure" garden compost (though not for growing food). Generally, composting toilets can also be used to dispose of food waste and other materials you might put on your compost heap.

Composting toilets vary greatly in sophistication. At the simplest end, it's perfectly possible (subject to all the usual planning rules and regulations) to build your own composting toilet using a few bits of wood and a shop-bought seat (the excellent Humanure Handbook is a good starting point). Next up come ready-made, self-contained composting toilets that look a bit like traditional toilets. Instead of a flush handle, you'll typically find a little bowl positioned near the toilet filled with sawdust or similar material. You sprinkle some of this down the bowl to help separate the waste and build up air pockets inside it to encourage rapid digestion. More sophisticated models are electrically powered, with macerating cutter blades to chop up the waste, fans to aerate it, and heating elements to maintain reasonably high temperatures and promote aerobic digestion. (Roof-mounted solar cells are sometimes used to power fans, so minimizing environmental impact.) Other models have rotating drums you turn to tumble the waste and help it compost. Bigger buildings and public toilets use "fall-through" toilets where the waste drops down, out of sight, into a much larger receptacle that can be emptied after weeks, months, or even years.

How does a composting toilet work?

Real composting toilets are nothing like as earthy and hippie as you might imagine; the most sophisticated ones look as sleek and neat as ordinary flush toilets and sit just as happily in a contemporary bathroom. They're also convenient and easy to use, with handles to rotate the waste drum and neat little drawers for removing the composted waste.

Here's an example of a typical, modern composting toilet developed by Henric Sundberg for Sun-Mar in the early 1990s. The diagram is a cutaway taken from one of the original patent drawings, but I've simplified the numbering quite a lot and colored the main features to make it quick and easy to understand.

A cutaway diagram of a Sun-Mar composting toilet with the main parts numbered and labeled.

Artwork: US Patent #5,345,620: Composting Toilet by Henric Sundberg (Sun-Mar), courtesy of US Patent and Trademark Office.

  1. Outer toilet housing.
  2. Rotating waste drum (blue) with a gear mechanism (red) attached to the back rim to make it rotate.
  3. Mesh at the base of the drum so liquid waste drops through.
  4. False floor of the toilet where liquid waste collects and evaporates.
  5. Heating element to warm the toilet, encouraging liquid waste to evaporate and solid waste to compost.
  6. Composted waste falls through from the drum to the drawer at the front.
  7. Sliding drawer can be removed and emptied every 2–3 weeks.
  8. Crank handle rotates the waste drum to encourage aeration and composting.
  9. Opening from toilet bowl into waste drum.
  10. Toilet bowl mounted on top of opening.
  11. Small gear connects crank handle to waste drum so the drum rotates when you turn the handle.
  12. Outlet pipe allows waste gases to escape.
  13. Perforations in waste drum allow air to get in to encourage aeration.

For much more detail, take a look at US Patent #5,345,620: Composting Toilet.

Advantages and disadvantages of composting toilets


Although there are savings to be made on your water bill (a composting toilet could save as much as 50,000 gallons of water a year), having a composting toilet fitted isn't about selfishness, but exactly the opposite. It's a great example of environmentalism: by disposing of your own waste, you're taking a more responsible attitude and living in a more sustainable way for the long-term benefit of humankind and the planet. You won't be worsening problems like sewage washing up on beaches or nutrients overloading rivers, and you'll be recycling a handy amount of nutritious compost for your garden! Another great advantage of composting toilets is that they can be used in remote places where mains sewers are not fitted.

Cows grazing near a pile of cow manure.

Photo: Despite millions of years of civilization, humans are still far worse at disposing of their sewage than animals like these cows, who recycle their waste effortlessly and with no fuss whatsoever. Cows are natural masters of composting toilets.


The main drawback of composting toilets is that they need more thoughtful use in everyday operation. Just like a traditional garden compost heap, you have to be careful you don't let the composting mixture get too wet or too dry; if the waste doesn't mix up and compost properly, it will start to smell and could even become a health hazard (a problem that doesn't affect properly installed composting toilets operating normally). Also, you have to empty the container, periodically, though in a properly operating toilet the waste you'll be removing will be dry and odorless, like garden compost, and shouldn't be a bother. Smaller toilets without a fall-through system and hidden container may leave waste on display, which can be troublesome to some people. If that's a worry, opt for a different, more sophisticated system.

If you are planning your own composting toilet, be sure to check with your state or local authority exactly what the regulations are on sewage waste disposal before you start. You may be required to bury the compost in a certain way or have it removed by a licensed septic hauler.

A pair of gray portaloos in Langton Matravers, England

Photo: Portable toilets are a godsend for outdoor events like rock festivals, where traditional plumbed toilets aren't available, but almost none of them are composting; most have to be emptied by hand after use. This pair is stationed in a remote carpark near a clifftop beauty spot.

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For much more technical detail, try these:


  1.    Sanitation, World Health Organization Fact Sheet, 21 March 2022.
  2.    Why don't modern toilets have high cisterns? Clearly they don't need them: they can flush perfectly well without. High-level cisterns need a lot more space and are difficult to maintain without the help of a ladder. Putting the cistern higher up creates a bit more flushing power, but not enough to justify the inconvenience. Better pipe and siphon design gave us more compact toilets that flush just as satisfactorily with much less water.

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Text copyright © Chris Woodford 2008, 2023. All rights reserved. Full copyright notice and terms of use.

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