This process of composting, in which waste is converted into a nutrient-rich soil amendment, makes composting toilets a more sustainable and environmentally friendly alternative to conventional sanitation.
But how does a composting toilet work? Do they actually compost? Do they smell?
In this article, we're going to explain how composting toilets work in general, and how the different styles and components work.
Table of content
TL;DR: A composting toilet works by using aerobic decomposition to start the process of transforming human waste into a humus-like material suitable for use as fertilizer. Unlike a traditional toilet which flushes waste away into a septic tank or sewage system, these toilets treat waste locally to produce a solid organic substance while liquids are separated to ensure an aerobic environment for the microbial activity.
Now, let's dive into the details of how composting toilets work!
What is Composting?
First, let’s start with the basics. Before we can understand how a composting toilet works, we need to understand what composting is.
Composting is a controlled, aerobic (requiring oxygen) process that converts organic materials into a nutrient-rich, stable soil amendment through natural decomposition process by microorganisms and aerobic bacteria.
The microorganisms feed on the carbon and nitrogen in organic materials to grow and reproduce. These microorganisms require oxygen to break down the organic materials into carbon dioxide, water vapor and a dark brown nutrient rich soil amendment we call compost.
Four important things to manage when composting
While composting uses natural microorganisms, it is a controlled process and requires specific conditions to be effective. The four things you need to watch out for are:
- Moisture
- Carbon Nitrogen Balance
- Oxygen
- Temperature
Well designed composting toilets provide the right conditions for the composting process to begin and can handle both solid and liquid waste. This ensures they remain odor free and provide a better, lower maintenance solution.
Let's explore these conditions more.
Moisture control
Maintaining the right moisture level is incredibly important. Too little moisture and the composting process won't happen. Too much moisture and the compost will become saturated so the microbes doing the hard work become starved of oxygen. When the compost pile becomes anaerobic, things get smelly as methane is produced instead of carbon dioxide.
The #1 question we get asked is: do composting toilets smell? We’re happy to report that no, they don’t. And that’s all because of moisture control. And how do we control moisture? With urine diversion!
Urine Diversion
Urine diversion helps reduce od for two reasons. Firstly, it prevents the solids from becoming saturated and turning anaerobic. Secondly, it ensures liquid is kept separate so it can be dealt with or used before the urea turns to smelly ammonia. Urine won't smell initially but will when it is left to stand.
How does a urine diverter work and what makes a good one?
A urine diverting toilet has a specially designed bowl or urine diverter that collects the urine in the front portion of the toilet. The back part of the toilet features a hole for solids.
Because separation of liquids and solids is so effective in reducing odors, the most advanced urine diverters often feature a moving cover in the bowl that prevents urine from entering the solids chamber.
These urine diverting covers also serve a very important dual function of preventing a direct view into the solids bin and helping to prevent smells from warm deposits rising back out of the toilet bowl.
Due to the shape of the bowl, it is recommended that everyone sits to pee. This is a surprisingly easy change and I personally now sit to pee even when using regular flush toilets!
No aiming, no splashing - sprinkles are for cupcakes not for toilet seats!
Benefits of Urine Diversion
Urine diversion has become a popular choice for several reasons. Most of the nutrients are contained in the urine and will self-sterilise over several weeks if left to stand. The source-separated urine can then be diluted and used as a fertilizer or used directly on a mature tree (too much of a good thing may kill a smaller tree or bushes!).
Most of the human waste by volume is urine. We produce between 0.8 and 2 liters of urine per day but only about 1lb of poop. By diverting the urine, less material is needed. This saves space in the bathroom and also means less emptying and fewer trips a central composting pile. For larger families additional liquid chambers can be stored nearby.
Moisture Control Options Without Urine Diversion
There are some composting toilets that work without diverting urine. You might have seen one at a trail head or deep in the backcountry, and some built-in household model options. There are a few ways to control moisture, although they do add a level of complexity and additional costs. It’s also very difficult to manage in a portable composting toilet since you’re limited on overall capacity.
Leachate Collection
Leachate is the term used for liquids that come from compost. If urine diversion is not employed, additional cover material must be added to absorb the moisture, or the liquid allowed to drain away from the solids chamber.
Evaporation
Excess moisture can also be controlled through evaporation. Evaporation is aided by ventilation fans or heating elements. However, this adds cost and complexity to the design of the composting toilet and diversion is often seen as an easier option.
Why Carbon:Nitrogen Ratio is Important
Carbon nitrogen balance is important for composting, and requires a ratio in the region 30:1 carbon to nitrogen. Poop is relatively high in nitrogen so addition of a carbon-rich cover material is needed to maintain the right carbon nitrogen balance. Urine is very high in nitrogen (and very wet) so a large volume of material is required to maintain the right ratio. Due to the practicalities of sourcing, storing and emptying this material, urine diversion is a more popular solution.
How Choosing the Right Covering Material Can Help You Achieve Balance.
When choosing the right medium for your composting toilet, coarse sawdust or fine wood shavings, coconut fiber (coco coir), hemp, straw are all suitable cover material. For compact spaces like RVs and boats or on camping trips, we recommend coconut fiber bricks for their compact size.
What about sand or soil as a cover material?
If you're not sure if you can use something as a cover material, ask yourself "if it was dry, would it burn?" Carbon rich materials suitable for composting will burn when dry. Other materials like sand and soil don't contain carbon and so can't be used as a cover material.
Oxygen Management
Aeration and Agitators
The next condition is ensuring you have an oxygen rich environment in the solids chamber. In simpler, portable composting toilets, adding a layer of cover material between deposits helps trap air in the solid waste. They are great for short trips, or when you have the ability to empty them more often.
Self contained toilets feature an agitator for the solid waste to help mix the materials and provide an oxygen rich environment for the microbes to work. This needs to be stainless steel however to prevent corrosion in the solids bin. After using, you simply 'flush' the toilet by agitating the solids chamber to mix the latest addition with the cover material.
Pros of Having an Agitator in a Composting Toilet
There are multiple benefits to having an agitator. One is the space you save by not needing to add cover material after each use. Instead, the cover material is added at the start and agitated after each use. You also don’t have to empty the solids bin as often, since solids start breaking down when mixed with organic matter. When layering, everything just “piles up” quickly.
Agitators also provide more effective mixing with the cover material and ensure oxygen is available throughout the solids chamber.
I personally find that having an agitator reduces the chance of the toilet smelling because of the better aeration and mixing. This combined with the reduced space requirements (I have a campervan) make it my personal preference.
Cons of Having an Agitator
Having an agitator in a composting toilet means the solids chamber cannot be lined with a bag. Instead, the solids chamber is removed and the contents either emptied onto a compost pile or added to a bag and disposed of responsibly.
Having an agitator in a DIY composting toilet adds complexity (although I've seen some ingenious designs!) and cost. Many units with an agitator in the base require disassembly and removal to empty which can require multiple people. Having a removeable solids bin was a key requirement when I was designing Cuddy for this reason.
If you’ve ever wondered if you can use toilet paper in a composting toilet, this leads us to the final disadvantage of having an agitator. Toilet paper can become wrapped around the rotating shaft and will fill up your solids bin quicker. This is why it is common practice to bag toilet paper separately.
To Agitate or not to agitate - you can have both!
Some composting toilet users informed us that they prefer not to use an agitator for shorter trips or when they're staying at campsites. Instead, they swap out the solids bin with an agitator for one without. Whether you prefer to have an agitator therefore also depends on the length and type of trip you're planning.
How Ventilation Fans Work in Composting Toilets
Ventilation is another important consideration. Simpler, smaller toilets like Cuddy Lite may not need ventilation if they are only containing the material while on a camping trip, or for a short time before it is transferred to a separate compost pile. For toilets where the composting process has time to start, ventilation is suggested to reduce humidity and prevent too much moisture in the solids chamber.
When venting your composting toilet, ensure you use a fly screen to reduce the chance of insects getting into the toilet. The addition of a solar-powered fan or cowling is a good ventilation choice for off-grid situations. And cowlings look pretty too!
Temperature management in composting toilets
The final condition is temperature. Maintaining an appropriate temperature within the solids chamber is not difficult, although some designs are better than others. The ambient temperature of an indoor toilet that is in use will typically be comfortable for both people and microbes to begin their work. The microbial activity within the solids chamber will begin generating its own heat. By using a separate solids chamber within the toilet, it gives extra insulation from the external temperature.
How Composting Kills Pathogens
Maintaining and ensuring balance for the four conditions leads to ideal composting conditions which not only help to stop your poo from smelling but also starts to kill pathogens.
If you continue the process in a composting toilet system, it kills pathogens by increasing the temperature during the composting process. This is a well studied phenomena and scientists define a time-temperature curve for each bacteria, virus or protozoa. For compost piles where a high temperature is reached toilet waste can be sanitized within an hour. If the compost pile does not achieve high temperatures the materials need to be left to sanitize and mature for 12 months to two years depending on the climate before they are safe to be used.
It is important to note that an entire composting system is required to completely treat human waste safely. A self contained toilet may start the composting process to reduce odour, but it is central compost systems that treat the waste so that it is safe to use once the process is complete.
Adding urine to your compost pile to help boost the temperature.
Just as farmers add fertilizers to their fields to help crops grow, urine can be added to a compost pile to help boost the temperature by providing additional nitrogen.
Odor Management
A properly functioning composting toilet won't smell. Once the solids begin to break down they no longer smell like poo, but instead smell loamy like a damp forest floor. It is a common comment from people experienced with these toilets that dealing with the liquid is more smelly than the solids! Here are some tips to ensure you have a smell-free experience:
Be lazy - DON'T sanitize the solids bin
I've heard many stories of people bleaching the solids bin after emptying. This is not only a real chore and bad for the environment, it can cause the toilet to smell as the composting process needs to start again from scratch. While pooping into a nice clean bucket may be appealing - you only get to do it once and after that it’s dirty again! Keeping the bacteria in the solids bin allows them to establish the composting process more quickly and reduces odor.
If the contents of your solids bin have the right amounts of moisture and carbon to be functioning correctly then you can leave the solids bin partially full for short breaks from home provided the temperature won't become too hot or so cold that the contents of the bin freeze.
When travelling round the US one year, Erica and I left a used Cuddy in our van Rodger until we returned several months later! You can sometimes find a build-up of fine white mold on top of the solids but this is completely fine so just mix it in with the agitator and continue to use it.
Put a one-way valve in your urine bottle
I designed our toilets with a sealing, one-way valve to reduce odour from my previous experience with a smelly toilet
These fantastic devices keep ammonia odors from the urine bottle inside the bottle and prevent splashing on bumpy roads in a campervan or choppy seas on a boat!
I've since discovered this is a known solution (despite nobody using these at the time when I added them to our toilets). In fact, they were recommended in publications by the WHO on separating toilets - you can even use a condom with the end cut off. This universal and readily available solution would be a relatively simple installation to retrofit into a DIY composting toilet anywhere in the world!
If installing a one-way valve isn't possible, an alternative solution is a ping-pong ball in the urine diverter. This will float up when the toilet is used, but re-seal the hole once the liquid has drained away
Keep it clean
The urine diverter will begin to smell unless it is cleaned. It is recommended to spray the bowl with an acidic mixture of vinegar and water in a ratio of 1:10. This acidic mixture helps flush urine out of the bowl and lowers the PH to prevent the urea hydrolysing to ammonia.
Use an activated carbon filter
Activated carbon is a natural material that has countless pores to absorb smells and chemicals. Carbon filters have started to be adopted, as they eliminate the need to vent outside of a building. All Cuddy Composting Toilets ship with a carbon filter as standard to recirculate air through the filter within the toilet
Vent Externally
For a completely odor-free setup, venting your composting toilet to the outside is also an option. The constant flow of air outside will keep malodours from the toilet flowing into your living space.
Tip: If you have an extractor fan in your bathroom, be sure it won't overpower the toilet ventilation fan. This can cause air to flow back through the toilet!
Common Questions About How Composting Toilets Work
We get asked a lot of questions about how composting toilets work, best practices on using them, how to clean them, moisture management, and more. So we’ve put together an extensive FAQ to (hopefully) answer all of your questions!
