How Does a Composting Toilet Work

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Composting toilets are a great option for anyone looking to live off the grid, in a tiny house, RV, boat, or cabin. This is also true for those living in any place where city hookups or septic systems are not an option or cost effective.

These systems allow you to safely deal with human waste, without relying on something very expensive, like a septic system, or being hooked up to the city sewer.

But, how do these eco-friendly toilets work?

The Essential Functions of a Composting Toilet

There are three functions a compost toilet has to perform in order to process human waste quickly and safely.
  • Evaporate the moisture

Typically, 90% of the waste will be water and will evaporate quickly. Some units have a separate waterless urinal. This allows much of the liquid to skip the composting chamber and evaporate more quickly, or be removed manually when full.

A vent system allows the evaporated liquid to escape into the air outside.

Compost Toilet Evaporation

Modern systems often have a small heating element to aid in the evaporation process and speed things up. It’s important to note that while evaporation of the liquid waste is important, you do not want to dry out the compost completely.

A healthy, active compost pile must remain moist, not wet, in order for the waste to break down quickly.

  •  Compost the solid waste without odor

While the waste itself would eventually break down if left on its own, the process would take a long time and produce foul odors. This could make your house smell like an outhouse.

A properly operating compost system requires the addition of an added material such as peat mix, sawdust, popcorn, or some other additive

These materials are often referred to as bulking material, and they help break down the remaining waste more quickly. They also ensure that the resulting product is safe to handle and use.

Bulking material is used to ensure the proper aeration of the compost pile, allowing oxygen in to assist the bacteria in breaking down the solid waste.

composting toilet Finished compost

Some creative, compost toilet veterans even add used coffee grounds to their systems after making coffee each morning. This not only acts as a good bulking material, but adds a bit of good smell.

The main function of these systems is to create the best possible environment to promote quick, efficient break down of organic waste.

Modern compost toilets have certainly accomplished this.

  • Produce compost that is safe and easy to handle

Once a unit is full, it will need to be emptied. This is the part that makes most people a bit squeamish.

Understandably, handling human waste is not something we all get up in the morning looking forward to. But if your system is working properly, you should be handling a safe, nutrient rich soil, like garden compost, with no real signs of human waste to be seen nor smelled.

Buying a system that is sized properly to your needs will help ensure very infrequent empties (like once or twice a year in some cases). This gives the system plenty of time to do its job and create a great additive for your flower garden or shrubs.

Meeting the Three Requirements

Different manufacturers use different methods to accomplish all three of these tasks. For example…

Sun-Mar systems use a bio-drum to separate solids from liquids and to help mix solid bulking material into the waste.

The Sun-Mar composting toilets also use a three chamber system that allows finished compost to be removed from a finishing drawer, while unfinished compost stays in the bio-drum to be broken down further.

Sun Mar Composting Bio-Drum

Nature’s Head composting toilets on the other hand, use a liquid separation BEFORE mixing with the compost. All liquids drain directly into a front mounted urine bottle to be emptied when full.

All the solid waste is deposited into the main composting chamber where it is mixed with bulking material by turning a mixing handle after each use.

natures head


Basic Design

Most modern compost toilets use a chamber design. Some use a three chamber design where each of the three main functions take place in its own chamber. There are also several one and two chamber designs.

The basic design allows the waste to enter through the toilet into a composting chamber.
Multi-chamber designs divert urine into a separate evaporation chamber. The waste is then mixed with one of the additives discussed above. In most systems, the additives are added through the toilet.

Like your garden compost bin, someone will need to occasionally turn the composting chamber to mix the contents. Oxygen is also introduced into the chamber when it is mixed in this way.

This quickens the composting process. Oxygen feeds the bacteria that are turning the waste into compost.

finish product

In multi-chamber units, excess moisture from the composting chamber is usually diverted into the evaporation chamber through a screen.

In a single chamber design, the waste may need to be transferred to another location to dry.

In most multi-chamber toilets there is a finishing chamber or finishing drawer where the compost completes the process of breaking down and drying. This prevents compost that is further along in the breaking down process from being contaminated with fresh waste.

The dry compost can be safely removed from the chamber and used as needed.
These toilets solve one of the most difficult and unpleasant problems of living off the grid. You can avoid using any precious water resources to divert the waste. You can safely store and break down the waste.

Without a proper system, diseases like dysentery, and hepatitis A can lead to potential health risks.

Homemade or poorly maintained outhouse type systems can also expose people to bacteria such as e-coli and salmonella. A compost toilet also helps eliminate the smell of an outhouse.

A composting system lets you use the waste by turning it into composting material that can be safely used in your garden. This furthers your independence from outside resources.

Breaking It Down Even Further

Another important distinction between the types of composting systems is the self-contained compost toilet and the central compost toilet system.


 system types

Self-contained units act as the toilet as well as the storage and breaking down chamber for the system. Central systems use a remote storage container that processes and composts the waste.

When using a central compost system, you have the ability to keep your bathroom looking a bit more traditional with a smaller more standard looking toilet.

You also gain the benefit of having more storage allowing for longer times between clean-outs.

Self-contained units are great for locations without a crawlspace or basement and are in most cases, much easier to install.

What Do I Get Out of This?

Great news, there are lots of reasons why a composting toilet is better than a regular toilet. Some of the most notable benefits to you are the following...
  • Smells less than a regular toilet

That's right, composting toilets actually create less smell than a regular toilet. By separating the liquid waste from the solid waste, you are preventing the typical sewage smell you get with water flush toilets and outhouses.

Most composting toilets also have a built-in fan that carries any bad odors outside instead of letting them linger in the house.

The typical compost toilet should smell something like fresh dirt if anything at all.

  • Saves a lot of water and energy
No more flushing thousands of gallons of fresh water down the crapper each year! You are also not adding waste to the local waste plant that spends untold amounts of money to break down human waste each year.
  • If you are in an RV, No more black tank to empty
Compost toilets installed in RVs allow you to skip the dump station completely. If you spend much time in an RV or camper, you know what kind of pain it can be to tear down in the middle of a camping trip to go empty the black tank.
  • Great plant food for free
People pay a lot of money each year to buy nutrient rich soil for their gardens and landscaping. You are literally crapping money and you never have to buy plant food again!
  • Simple to install

Most modern compost toilets are made with the DIYer in mind and are pretty straight forward to install. My wife and I installed our first compost toilet in under 3 hours, and half of that was arguing over why I wasn't reading the instructions!


  • More Information

 Still hungry for more information? have a look at what Wikipedia has to say about this topic.

Also, have a look at the Sustainable Sanitation and Waste Management site for their take on this topic.

Nancy Harrington from Lets Go Green has a great article on the Sun-Mar toilets HERE.


Shop Tiny Houses is an authorized dealer for the following brands.


Nature's Head



Dry Flush

Want More Info on Composting Toilets?


Simply the most comprehensive resource on composting toilets today, learn the ins and outs of Composting Toilets in this eBook.

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