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A composting toilet in Israel.

Composting toilets

by Chris Woodford. Last updated: September 30, 2013.

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? Let's take a closer look!

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?

Modern low-flush toilet

It's wintertime where I live and Tom, the farmer I went to school with, is driving back and forth endlessly with a gigantic muck-spreader hooked to the back of his tractor. Now I always fancied driving a tractor when I was young, but I don't envy him this job: he's depositing mountains of steaming hot cow manure on his fields. Farmers, you see, know a thing or two about recycling. Nature does too. In nature, there's really 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.

Photo: Ordinary toilets waste huge amounts of water. This is a low-flush model but, even so, it's flushing waste with sparkling water that's good enough to drink!

What is a composting toilet?

A composting toilet at an exhbition in South Africa.

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.

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.

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 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.

  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.

Artwork: Patent drawing by Henric Sundberg (Sun-Mar) courtesy of US Patent and Trademark Office.

Advantages and disadvantages of composting toilets

Advantages

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.

Disadvantages

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.

Important note!

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.

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Text copyright © Chris Woodford 2010, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2010) Composting toilets. Retrieved from http://www.explainthatstuff.com/composting-toilets.html. [Accessed (Insert date here)]

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