Understand more about composting toilets

This information on this page relates to composting the solid ‘waste’ from domestic urine-diverting toilets (such as Separett and Air Head) is provided in ‘good faith.

Composting is a completely natural process that is part of nature’s great recycling scheme and has been going on for millions of years. Left long enough, anything that was once living, will compost and add nutrients and structure back to the soil. Done right, composting will happen fairly quickly (around 12 months), won’t smell during the process, and will create a safe, pleasant smelling compost that you can use in your garden.

The process of composting ‘humanure’ within a composting bin has been shown scientifically, to render harmless any human disease pathogens, so that the end result poses no greater threat to health than any regular compost. However, if your compost toilet has been used by a variety of people, whose health and background you know little of, it’s wise to exercise caution in terms of what you do with the final compost (such as just using it as a mulch around trees etc and/or by composting it for longer).

If you don’t have access to land, so can’t compost, then you can still have a compost toilet, but your options in dealing with the solids are slightly different. If you have small amounts, and your use is infrequent, one option is to double bag the dry solids (using approved, appropriately labelled bags) and dispose of in appropriate waste bins, but there are many narrowboaters for example, who do manage the whole composting process onboard using a variety of containers and methods – if you have the will, there is usually a way!

Whatever method you choose, always do it responsibly with the utmost consideration for the environment, your health and the health of those (human and non-human) around you.


Composting needs some moisture, oxygen and a blend of nitrogen and carbon source materials.

A bit of warmth will help. Then the magic just happens! 

Which compost bin?

220 litre compost bin

Compost bins are broadly classed as either ‘cold’ or ‘hot’.

Cold (or moldering) bins can be made from plastic (such as the classic ‘dalek’ or Johanna bins available at garden centres and DIY stores), metal or wood.

Hot compost bins are made using highly insulating expanded plastic (or similar products) and are designed to keep the heat in (the heat is generated by the bacteria feeding off the contents) to accelerate the composting process and potentially reduce it down to 90 days in ideal conditions.

Hot compost bins will need regular feeding to keep the process going, so require a little more hands-on management than regular bins, but they produce the end results much faster.

How many bins?

If you have a cabin in the woods, that gets just weekend or irregular use, then just one compost bin will usually be fine. You’ll be surprised at how much the contents reduce in volume as composting takes place – it’s not unusual to see a 75% or more reduction!

Empty the contents of your compost toilet (the solids container) and if necessary add some grass clippings, wood shavings, wood chips, sawdust or another carbon source. Give the heap a bit of mix if you can, to make sure air is getting through. After 12 months or so (longer if you want), you can start harvesting the compost from the bottom of the heap, whilst continuing to add fresh material to the top.

If you’re using the toilet more full time, or there are more people, then 2 or 3 compost bins/bays could be used in a classic rotation system. In year one, start filling bin 1 and after 12 months, or when the bin is full, put the lid on and let it ‘sit’. Now start filling bin 2. After another 12 months or when bin 2 is full, you cap it and empty bin 1, which should now be full of beautiful compost! Now start filling bin 1 again and continue the cycle… If you are filling a bin in less than 12 months, then simply increase the number of bins in the rotation system to suit your needs.

Can I add garden and kitchen waste to the compost bin?

It’s entirely up to you. There’s no reason why not, but some people may want to keep the ‘humanure’ compost separate from compost made from general garden and kitchen waste, in terms of how it is used subsequently.

Kitchen and garden waste will add to the variety and probably the overall quality and structure of the final compost. Composting properly is all about getting the balance right – not too wet, not too dry, not too much carbon, not too much nitrogen. However, ultimately, everything will compost, given time…

Can I really grow vegetables and fruit in what was once my poo?

In the UK, some sewage treatment plants sell ‘treated sewage sludge’ to farms as a nutrient source to spread on fields. The key thing is that the treatment process renders the sludge safe in terms of ensuring no pathogens end up in the soil. Arguably, this professionally treated ‘sludge’ might contain heavy metals and other industrial waste (which ends up in the sewer system), so it’s not ‘perfect’, but it does pass pathogen safety tests.

It’s understandable that some people just don’t like the overall concept of growing food in what most people class as a waste product, so if growing vegetables in this compost doesn’t appeal, just use it as mulch for trees, fruit bushes or flowers – whichever way you use it, your garden will love the nutrients and structure it adds to the soil!

The one exception is that you mustn’t grow commercially using humanure compost without getting approval from the Environment Agency (in England and Wales, or SEPA in Scotland). In the interests of environmental safety and human safety, they’ll need to know that the composting process you are using is ticking all the right boxes, however, provided it’s fully composted, there’s no reason not to use it.

Will the compost bin smell like a sewer?

No! There will be a lovely ‘earthy’ smell, just like you get from any other compost pile/bin. Your neighbours won’t know what’s going on unless you want to tell them!

If you have an open top bin, just make sure the fresh materials are covered with a layer of straw or grass or even an old piece of carpet – this will help keep the heat in.

The only time a compost bin would smell is if the contents get compacted and/or waterlogged – under these conditions, composting will change from aerobic (with oxygen) to anaerobic (without oxygen) and that might smell – but that could happen with kitchen/garden waste too. Remember that one of the key ingredients is air (oxygen).

Other safety considerations

When handling dry waste from a urine-diverting toilet, always wear appropriate PPE and wash your hands thoroughly afterwards.

Ensure that the compost bin(s) is located in a safe or secure place, taking reasonable steps to ensure other people and domestic animals don’t have access to it.

Site your compost bins in a place where flooding is unlikely.

Protect your compost bins from getting too wet which might cause leaching (ie use a cover, but equally, don’t let them dry out or the composting process will stop – remember that composting needs some moisture).