The simplest DIY compost toilet

Low Impact – by Lesley Anderson

For several years I lived in a tent in woodland and I never really readjusted to this business of flushing poo away with clean drinking water. Besides, I have always liked to feel I am dealing with my own… stuff, both figuratively and literally. So when we moved into our current house a DIY compost toilet was first project on the list. 

Initially I imagined installing an underfloor twin vault system with urine separation. Even going DIY, installing this system was going to be expensive, and either way would involve disruptive house alterations. Then I read The Humanure Handbook by Joseph Jenkins. Jenkins advocates a very simple low-cost system which requires no house alterations and takes up relatively little space. He also strongly, and convincingly, argues that the compost produced in a thermophilic (hot) compost heap is safe enough to use on your vegetable patch. Here’s how it works: 

The toilet

A 20 litre bucket sits inside a plywood box with a toilet seat on it. Next to it is a bin filled with sawdust, ideally hardwood, this is your ‘soak’. The sawdust should be a little damp as this makes it more effective at filtering odours. Do your normal thing, no urine separation, add the sawdust and that’s it.

Believe it or not, it doesn’t smell, provided you add a good layer of damp sawdust. Once the bucket is full, replace the tight-fitting lid and add a new bucket. You need at least three identical buckets, so two can be full and one can be the current receptacle.

Here’s the catch: you need to empty the bucket. I realise for some people this is a pretty big catch, but it’s probably not as bad as you think. The bucket full of sawdust doesn’t smell bad and your well maintained composting heap doesn’t smell bad. You can quickly rinse the bucket using a hose with high pressure attachment and tip the water onto the heap. I then dry my bucket with a rag, which goes straight in the washing machine, then spray the bucket with vinegar. As a family of three we empty two buckets at a time once a week and it only takes 10 minutes.

The heap

This is the important bit, your compost heap is the key to success. Jenkins recommends a container 1.6m² and 1.3m high. I have found 1.1m²  is big enough, you should be able to go even smaller if you insulate your heap. You need at least two containers, one is resting while the other is filling up. Jenkins also recommends a third central bay, for storing soak and covering material. We don’t have that much space so we make do with two.

Once you have constructed your bins, place a thick bed of organic matter, such as straw or cardboard, in the bottom of your bin and begin to fill it. Keep the heap covered with a layer of straw or cardboard. Every time you add a bucket of waste make a hole in the middle of the heap with the fork (used only for that purpose), add the waste, pull the older waste back over and cover the heap with straw.

There are a number of things you do to keep your heap hot and healthy:

1. Flatten the top of your heap, think ‘cube’ rather than ‘pyramid’. The reduced surface area minimises heat loss, and the shape prevents fresh material from rolling down to the base of the heap, where it won’t become hot.
2. Bury new material in the centre of the heap where it is hottest and do not mix or turn the heap. The fresh material heats up quickly, then gradually cools as it becomes the lower layers. The cooler material at the base is maturing: becoming more stable and being colonised by a wide range of insects, worms, fungi and microorganisms. If we mix the heap we disturb this process and we cause the fresh material -the stuff we really want to heat up- to cool down.
3. Add both urine and faeces. In vault systems there are good arguments for separating urine but hot compost heaps are thirsty. Without the urine they will become too dry and the process will slow down. You also need the high nitrogen content of the urine to balance the carbon of the wood.
4. Cover the heap with straw or cardboard, this provides some insulation, prevents smells and can help soak up excess liquid, should your heap become too wet. Your heap should never smell: if it smells, add some cover material, if it still smells, add some more.

You can add all your kitchen and soft garden waste to the heap as well. After a year start a new heap, leaving the old one to mature for a year. Once the first heap has rested a year it will be beautiful compost, ready to add to your garden.

Soak and cover material

The ideal combination is damp hardwood sawdust for soak and straw for covering. The cover material is less critical, ideally it will be biodegradable, capable of absorbing excess moisture and able to let rain through, but its most important job is to block any smell. If I can’t get straw I use cardboard or dead leaves, if I am out of cardboard and leaves I might use weeds and grass clippings, or an old rug or black plastic as a last resort.

The ideal soak is damp hardwood sawdust. Soft wood sawdust will be slower to break down but in my experience still works ok. If you have a local sawmill or firewood supplier this is the place to ask. I have also used hardwood shavings from a wood-turner; they come in beautiful curls and spirals and are pleasant to handle. Unfortunately they do not block smells as well as the denser sawdust. They do cause the heap to heat up very quickly, as they are less dense and trap more air. In some cases you may be able to obtain only kiln dried sawdust, this will still work, but won’t be as effective at blocking smells. In this case, you can leave the sawdust out in the rain to at least get wet and, even better, begin to break down a little.

If you cannot obtain sawdust it may be worth experimenting with shredded paper, straw or leaf litter. These may not filter odours as effectively so an extra tight lid on the toilet may be required. In the case of paper or leaves, you may find it hard to maintain a hot heap, you could try insulating your container and finding ways to introduce more air into your heap.

James Chapman of Willowburn Lea uses spruce needles, of which there are a plentiful supply on his land, as both a soak and a cover material. These do work well and are effective at filtering smells, although, again, make it difficult to maintain a hot heap.

Health and hygiene

This is a huge topic and one I can only skim over here. If you are particularly interested, or particularly concerned, I suggest you read the Humanure Handbook, which is packed with data and references. The important points are:

• You are unlikely to become ill through handling your own fresh toilet waste; if it’s in there it came out of you, so you have it already! Most of us are pretty confident we can change a toddler’s nappy without dire health consequences and personally I find this a rather messier problem than emptying my toilet bucket.
• Heat kills bacteria and parasites. Many people who plan to use their compost for food growing use a thermometer to monitor the temperature of their heap to make sure it heats up sufficiently to destroy disease-causing organisms. Your heap doesn’t even need to be that hot, just hot enough, for long enough. See the graph below.
• But not just heat. We cannot guarantee that all parts of the waste are heated sufficiently to destroy all the bacteria and parasites, but it doesn’t need to be. The composting process, time, exposure to sunlight and competition with other microorganisms all take their toll.
• It works and soil isn’t sterile. The response to all of the above is often, ‘but how can you be sure it’s all gone?’ We are not looking for the compost to be sterile, just for it to have the same level of pathogens as your normal garden soil. Jenkins has been growing food in his garden using his humanure compost for over 30 years with no health problems. He has even had his own faeces tested on a couple of occasions to prove the point.

Most importantly you choose how to use the resulting compost. You may not be convinced by Jenkins’s argument, or perhaps your heap did not get hot enough, or you have a lot of visitors and are not comfortable using compost from the waste of the wider population. You might choose to leave your heap an extra year to mature, then you might use it only to mulch around your fruit trees, or even only around ornamental plants.

I don’t want to empty a bucket!

I realise this system isn’t for everyone. You have to love your compost toilet, if you hate the whole process it’s never going to be sustainable. The most tried and tested DIY alternative is the twin vault with urine separation. These systems usually involve a cool composting process, which may not as reliably destroy bacteria, but it is generally considered safe to use the resulting compost on fruit trees, for example. I recommend the book ‘Lifting the Lid’ by Harper and Halestrap if this is something you are interested in.

If a vault is not an option, you might want to explore the possibility of hot composting in insulated containers which can act both as a receptacle and composting vat, at least for the initial part of the process. I would love to hear from anyone who has experimented with this. There are also commercially available compact systems, which I believe are becoming more reliable. Again, please share your experience of these in the comments.

Whatever you choose to do I hope you gain the satisfaction of knowing you are dealing with your own…stuff.

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See pics here: Low Impact

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