What’s the best cover material for a compost toilet?

What’s the best cover material for a compost toilet?

Some compost toilets, like the Separett Villa 9000 / 9010 and the Separett Weekend, don’t need or use any cover material at all. Even so, they can benefit from an organic soak material in the bottom of the solids container to absorb any excess liquid that might get in.

Others, like the Air Head, have a coir medium placed in the base at the start and the turning action will mix solids with this, meaning you don’t usually need to add anything else after the initial setup.

For other compost toilets and for many self-build units, you will need to use a cover material, so read on to explore our findings.


A question we’re often asked is “what’s the best cover material for use on a compost toilet?”


Even if your compost toilet has a fan for odour control, a cover material is often used as a ‘soak’ at the bottom of the solids container (a 2.5 – 5 cm or so layer is ideal), and the occasional sprinkling of carbon-rich material will help with the final composting process (if you don’t add much to the bucket whilst in use, you should add a lot of carbon-rich material and mix it with the contents of the toilet in your compost heap to get a good compost).

In many ways, the best cover material for a composting toilet is whatever you can get for free or cheap, and if it works, stick with it!

Coir

Coir or Coconut Coir, as a gardening product has gained a lot of attention in recent years as an alternative to peat (which is an environmental no-no). It’s a by-product of the coconut industry, and it’s convenient to buy it as highly compressed blocks which are then reconstituted with water (warm water does the trick even quicker). The distance travelled (air miles) can be an ecological issue to consider, but there are a few other practical issues that have come to light in terms of its suitability as a compost toilet cover material.

Firstly, in reconstituting it with water, you are clearly making it quite moist, which goes against the principle of cover materials in that they should be fairly dry. So you need to prepare coir in advance and leave it a fair period of time to dry out again sufficiently.

Coir is available in some garden centres, bargain stores, online and at many ‘hydroponics’ shops.

Chopped Straw

Finely chopped straw, often sold as animal bedding can be used. It’s not that absorbent, but it’s OK. People using it have had mixed success with some saying it’s worked well, and others saying that it’s not so good at odour control. It might be an idea to try alternating covers, so you could try some straw and every other use, try sawdust or shavings.

Wood ash

Wood ash (never use coal ash as it’s highly toxic) is often available for free during the cooler months and has excellent odour control properties. However, it can be a bit messy and dusty in use and a composting expert colleague of ours makes the following observations on wood ash

“I choose not to use wood ash on compost heaps as when it meets up with high nitrogen materials such as manures or food waste, it makes ammonia which evaporates and thus you lose the nitrogen to the air, rather than having it as nitrates in the soil, where they are a fertiliser. Also, the nutrients in wood ash are extremely soluble so they wash through the compost if it’s open to the rain, as most of mine are. Wood ash is best used around fruit trees in the spring before they come into leaf, helps prevent against bitter pit in apples.”

John ‘Compost’ Cosham

Our personal take on wood ash is to use it occasionally and in conjunction with other covers, especially when you need a bit more odour control for whatever reason 😉

Sawdust

Sawdust as a term can mean different things to different people. The very fine ‘dust’ from electric saws and sanders used in woodworking is too fine – it’s great at being absorbent but rubbish at odour control (although if you have access to some, you could mix it with other cover materials), so we don’t recommend this alone.

However, the chunkier bits or ‘nibs’ that might come off electric planers, routers and the like are better.

One of the best ‘sawdusts’ is the ‘nibs’ of wood you get from using a chainsaw – they are often slightly damp when fresh, but perform extremely well.

Sawdust performs best as a cover material when it’s been allowed to mature. Leave it somewhere, open to the elements (but don’t let it get too wet), and good bacteria will start to colonise it, making it even better as a cover as the biological actions of composting will have already started.

Wood Shavings

The easiest option for most people is fine wood shavings, often sold as pet bedding in various size compressed blocks and readily available on the high street at low cost. As you can see from the photo below, the shavings are fairly fine, but not a true ‘sawdust’. They are usually sourced from timber processing companies, so are technically a local waste product and are very dry and easy to store.

Wood shavings (pet bedding) used as a cover material for a compost toilet.
Wood shavings (pet bedding) as a cover material.

We prefer to wet then slightly or it’s too ‘light and fluffy’ and if you have a chance to let it mature outside, even better (let the bacteria get to start on it). The slight downside to shavings is that they will take longer to compost down so if you want fast compost, maybe consider something else.

Compressed Wood Pellets

Compressed wood pellets, sold as cat litter, stove pellets or horse bedding are one of our favourites and give a reliable ‘all round’ performance. Sprinkle some water over them to ‘fluff’ them up and they’ll cover well. Use them uncompressed as the soak layer at the bottom of your solids container.

They’re not the cheapest cover material, but are a great option that works and readily available from the high street or supermarket.

Wood based cat litter used as a cover for compost toilets.

The Rest

As we mentioned above, if you can get something cheap or for free, and it’s fairly fine and organic, then give it a try! We’ve had customers use spent coffee grounds obtained for free from high street coffee shops (the only issue with coffee grounds is that they can be too damp when fresh, so need a bit of drying out), but if you like coffee, your toilet will smell great! Other people have tried dried leaves either exclusively or mixed in with other materials. Another customer has mentioned that ‘shredded hemp stalk’ sold as horse bedding in equine supply shops “rots down quickly, is highly absorbent and reduces smells and is cheap in square bales”.

Keep it dry – for the moment!

As you might gather, the key aim in a urine-separating compost toilet is to keep the solids bucket fairly dry whilst it’s in situ – the contents (faeces is around 80% water when ‘fresh’) will start to break down and compost, but not rapidly. When the solids bucket is full and you transfer it onto your secondary compost pile/bin, then this is the time to reintroduce some moisture (urine?) as you accelerate the composting process, but that’s another topic for another day!

And the winner is….

If you haven’t already sussed it out, we recommend you start with either fine wood shavings (pet bedding) or compressed wood pellets both as a soak in the base of the solids bucket and as a cover material. They’re fairly inexpensive, readily available, easy to store and very dry on purchase. Above all else, they simply works in most cases.

Of course, if your compost toilet manufacturer specifically recommends something else, then you should follow their recommendations until you are familiar with the product and whole emptying and composting process.

Another issue to consider with any compost toilet (whether you use a ventilation fan or not) is ventilation – the toilet box itself should be ventilated and the room must have some natural ventilation too.

The final issue is whether to put toilet paper in the loo or not. If your loo is being used in a glamping site or holiday let, then it can get complicated giving people too many instructions, so the best approach here is let people put paper in the solids bucket, let them use as much cover material as they need, and ensure adequate ventilation (or use a fan to vent odours outside for the ultimate in odour control with minimal user intervention). If it’s your own loo, you might want to consider having a separate bin for loo paper which can be composted or burned, but it’s all up to you…

Charging your cover material with bacteria

Very dry cover, such as wood shavings or wood pellets, will be devoid of any natural bacteria. If possible, leave your cover material outside and slightly exposed to the elements for as long as possible – this will mean bacteria will start to inhabit the cover, and once sprinkled in your loo, they can kick-start the composting process, reducing odours etc.

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