The best composting toilet mediums are rich in carbon, cheap, readily available and sustainable. Sawdust, pet bedding, coco coir, hemp are all suitable. There are other factors that can also influence the best choice such as local availability, ease of storage and the ability to absorb excess liquid that can depends on your style of toilet and location.
In this guide we'll discuss the pros and cons of each option to help you make the best choice for your composting toilet.
A composting toilet medium is also known as a bulking agent, composting material, cover material, litter or substrate. In all of these cases, it is a substance added to a composting toilet to aid in the breakdown of waste and to prevent odors.
The toilet medium can be any organic carbon rich material such as sawdust, animal litter, coconut coir, hemp, animal bedding, or sphagnum peat moss (although we don’t recommend peat moss because of it’s sustainability questions- see below). These help to create air pockets in the compost pile, which is necessary for aerobic decomposition.
The composting medium does many important jobs.
The extra carbon balances out the nitrogen in your bodily waste and provides food for the microbes to decompose it.
It traps air so the decomposition remains aerobic (with oxygen) and doesn’t create bad smells.
It covers the poop to reduce odour and stops you looking at poop
It maintains the right moisture level by absorbing excess liquid
All of these things add up to making your toilet smell less and just make it a happier place for both people and those good bacteria.
We’ll be returning to these factors as we look in more detail at the different options.
The most common composting toilet cover materials you can use are:
Coco coir, is natural coconut fiber extracted from the husk of a ripe coconut. It is brown in color and should not be confused with white coir made from unripe coconut. Make sure to look for coco coir manufactured using freshwater over salt water, which can cause high salt levels in your resulting compost. Coco Coir is a waste material from the 60 million tonnes of coconuts produced for food each year.
One of the biggest benefits of coco coir is that is typically sold in compacted bricks, making it easy to store extra material. This makes it a good choice where you have limited space such as RVs, narrowboats, sailboats, trailers or tiny houses.
To use a coco coir brick as a cover material it needs to be re-hydrated with water. A top tip is to hydrate the brick in a 2.5 gallon zip-lock bag to allow the liquid to access all of the brick by rotating it without getting your hands dirty!
Coco coir you can help control the amount of moisture in the solids waste container of urine diverting toilets or the bucket for all-in-one composting toilets by using more or less water when rehydrating it.
It's readily available online, but can also be found in pet stores (look for the terrarium section) or garden centers at reasonable prices.
For me, the biggest advantage of Coco coir however, is that it is dark brown in colour. This helps disguise the contents and reduces the overall amount light in the solids container.
Unless you live near a coconut farm, you are unlikely to have access to cheap coconut coir. Its use for horticulture and pets can mean it is more expensive than other cover materials so may not be affordabel for full-time use. However, for intermittent use or for small spaces this may be outweighed by the need or convenience of its compact form.
Coconuts are not grown everywhere in the world so while it is a waste product, there is an environmental impact to transporting it.
Derived from the Cannabis sativa stem, Hemp Shiv (the inner part of the stem), is cut like short wood chips. It is a waste product from the hemp fabric and marijuana industries.
Hemp degrades faster than traditional wood shavings.
You should be able to find hemp bedding readily available to purchase at builders' merchants, hardware stores, garden centers, or pet stores. It can also be purchased in bulk for full time composting toilet users.
The short strands of hemp are not as fine as sawdust or coco coir this would mean that when adding hemp after using a layering toilet, a little more may be required to cover a deposit.
Hemp also isn't as absorbent as coco coir or sawdust so additional material will be required for all-in-one composting toilets.
A byproduct of sawmills and woodworking operations, "sawdust" comprises small particles of wood created from sawing. Wood shavings are slivers of wood from planing of wood or wood working.
With sawdust, a coarser sawdust is better than fine sawdust as fine sawdust doesn't trap as much air so can become waterlogged. Conversely, fine wood shavings used in pet bedding are better than coarse wood shavings. coarse wood shavings will be slow to break down, won't absorb as much moisture or be as effective at blocking odors in all-in-one toilets.
It is important to use sawdust from un-treated timber as the chemicals and glue can be harmful to soil organisms.
Damp wood shavings are better than dry wood shavings at beginning the composting process as they become biologically active. If you are using a urine diverting toilet like cuddy or cuddy lite, consider storing your shavings outside in the rain or spraying them with water before storing it near the toilet for use.
Wood is produced and used all over the world, making this one of the most common and available composting toilet mediums. It is likely there's a business nearby who has unwanted sawdust you may be able to get for free.
If you can't get a free supply, it is readily available at pet stores as pet bedding and can be bought in bulk.
It is a waste material from a renewable source that can be sourced locally all over the world so is one of the best choices for a composting toilet cover material.
The fine nature of sawdust make it an excellent option for all-in one toilets due to its ability to block odor and being highly absorbent.
Wood is a hardy material thanks to the lignin within it so can be slightly slower to break down. ensuring you moisten your sawdust before using will give it time to become biologically active.
Sawdust is very light so it can be messy transferring it into the toilet after use. Keep a small dustpan and brush handy to clean up any spills.
Compressed wood pellets, used for small animal litter or heating, are compressed sawdust, so much like coir, they are easier to store because they take up less bulk and are an industrial by product. Unlike sawdust they won't be available free as they need to be manufactured and transported.
You can readily buy compressed wood pellets from pet stores, which is excellent if you are on the road.
They're more compact than sawdust so are better suited to small spaces.
In their raw form, compressed wood pellets won't form an effective odor barrier to cover deposits for both separating and non-separating toilets. Their round shape mean they're prone to rolling off from what they're intended to cover.
While they absorb moisture effectively, they are likely too dry to use immediately for a cover material
We're still experimenting and are currently trying out partially hydrating them. If you have advice or experience using them as a cover material we'd love to chat!
Sphagnum Peat Moss is formed from the partial decomposition of sphagnum moss. It is widely used in horticulture and gardening as a soil conditioner or a component in potting mixes.
However, functioning peatland ecosystems are some of the most carbon rich terrestrial ecosystems on earth, storing up to 30 times more carbon per hectare than a healthy tropical rainforest. Globally peatlands lock-up an estimated 550 billion tonnes of CO2e.
I tried to determine if it was sustainable while researching this article but couldn’t get a satisfactory answer. There is a lot of debate on the internet about the sustainability of Peat Moss with information from producers that it is renewable at our current rate of consumption. In the UK, compost is now peat free for consumer use by law and will be for commercial use in due course.
Because peat bogs are so valuable for the climate crisis, I’d prefer not to encourage people to consume more of a natural resource when there’s alternative waste streams we can use for the same purpose..
Sugarcane bagasse is the term to describe the crushed stems of sugarcane or sorghum. Dry and fibrous in appearance, it has been used successfully as a cover material for composting toilets. Importantly, it is the material of choice for Soil in Haiti on their toilet collection service EkoLakay.
This material is not readily available in the USA or Europe so I haven't had a chance to personally comment on its performance.
There’s a simple way to work out if you can use something as a cover material and that is to ask if I dried it, would it burn? All of the above materials will burn on a fire.
Sand and Soil don’t burn so don’t contain carbon. The microbes can’t feed on them and so they won’t help the composting process.
Not everything is suitable for use in a compost toilet. Here's a list of what you may have read about that we don't recommend using and why.
Either non-clumping or clumping cat litter are a no-go for composting toilets. They don't generally hide the smell over time or contain carbon to start the breakdown process. You end up with a dusty and dirty solids bin, meaning regular clean-up.
Hay and straw typically come in long lengths so aren't easy to put into the solids container and would interfere with an agitator. However, you could use them if the pieces were short enough. They would act similarly to hemp fiber but take longer to break down and are less absorbent. Hay can also have an unpleasant whiff if it gets too wet, so for that reason, it sits on our "would rather not recommend" list.
However, hay and straw are great as an insulator for compost piles!
Larger pieces of wood don't degrade quickly and won't absorb as much liquid due to the low surface area. The larger pieces also mean a poor odor barrier or biofilter for smells.
Some people have reported using shredded junk mail and paper as a cover material. As paper gets moist, it starts to form clumps. Over time, you'll end up with a paper mâché artwork you won't want to show off! In addition, the inks used in printing may not be biodegradable and could lead to unhealthy soil.
Dried coffee grounds are great for covering up toilet smells and, in small amounts, can be suitable for compost piles. However, coffee grounds are high in nitrogen so don't help to increase the carbon-nitrogen balance. They're also quite acidic so having larger quantities of coffee grounds in your compost can kill the beneficial microbes. The other issue is the amount of coffee grounds you have to discard daily. It's probably not enough to cover your poo with each visit unless you own a coffee shop
Sand, Soil and terra preta are mainly minerals and don't contain enough carbon for the microbes to eat. The other issue for those of you on the road with your portable composter is that you can't just go digging anywhere you want.
Synthetic pet bedding and other materials don't break down in compost or landfill and should not be used.
If you've read the Humanure book, you will have read Joseph Jenkins saying that you must use plant cellulose over other materials. Ashes, lime, and sand can be toxic to plants or degrade the soil acidity you'll later need for plant growth. However, a little ash can be helpful if you are not separating liquid waste from your solid chamber. Wood ash from your campfire is very alkaline, which raises the PH and can help prevent the formation of ammonia (a common reason for a smelly toilet).
Which medium is best will depend on your style of toilet and your location. We prefer using by products of manufacturing and advocate sourcing it locally if you can. Here's our recommendations for each type of toilet but we suggest choosing what makes sense to your situation.
Due to mixing solids and liquids, additional cover material will be required. This will mean the bucket will fill more quickly so a compact absorbent material like sawdust is perferable. This also forms a good odor barrier and is most likely to be locally available.
For urine diverting toilets without an agitator you have less need to absorb excess moisture so any of the above mediums are suitable and whichever is most convenient for you will be ok.
If the soilds bin on your composting toilet has an agitator like Cuddy, we have found that a more crumbly substrate like damp coco coir is our preference. It is effective at covering the solids and is easily mixed.
Coco coir is my personal preference as my composting toilet is in my campervan. Because of this being able to store a spare brick easily without taking up much space is a big factor. I also like that it's brown!
For smaller units, this will depend on whether you have an agitator or not.
"I read two scoops somewhere; is that right?"
Honestly, you'll add unnecessary volume to your solid waste container, meaning you'll be emptying it more quickly (and spending more money). Here’s what we recommend instead.
For composting toilets without an agitator, like Cuddy Lite start by adding a 3/4" layer of material to the empty bin. Then, after each poo break - add a sprinkle to hide the evidence and stop odors for the next visitor.
For compost toilets with an agitator like Cuddy, you will start off with enough bulking agent already in the solids bin and turn it after each poo - this drys out your poop and breaks things down, meaning you don’t need to empty it as often or add more scoops of bulking material. The other great thing about using a unit with an agitator is that you also don’t need a separate container in the loo area for your bulking agent.
How much to add will depend on your manufacturer. For Cuddy, we recommend starting with the cover material 1" above the end of the agitator prongs.
Whether you have an agitator or not, if the contents are ever too moist due to diarrhoea or other issues, simply adding more composting agent is recommended. The aim is to always maintain a moist but not wet consistency to avoid unpleasant odors.
One of the things you can find with any decomposing matter is the occasional fly strike. We aren't talking big blue bottles, but the tiny and harmless earth flies - Fungus gnats, also known as sciarid flies.
Diatomaceous earth is a great tool to help you combat that and maintain a happy composting toilet. You can get a small pot at a garden center or online.
Occasionally, use a fine sprinkle on top of your solid and bulking agent mix; it will deter flies and won't kill or harm earthworms and beneficial insects when it's later composting in the garden.
We typically don't recommend adding toilet paper to composting toilets with an agitator as it can interfere and wrap around the agitator. Instead it is common practice to keep it in a separate trash bin.
For composting toilets without an agitator, putting toilet paper in the solid bin is a personal choice. Toilet paper is very bulky so can quickly fill the solids bin leading to more emptying. It also does not break down quickly so should not be considered as a source of carbon in smaller systems.
For larger split chamber or moldering systems, toilet paper is fine to add as it will compress under the additional weight and break down over time.
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