How-to

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 any cover material at all. This is because they use a fan to actively remove foul odours and help regulate the moisture content of the solids container.

However, they might still benefit from an organic soak material in the bottom of the solids container to absorb any excess liquid that might get in – perfect in glamping or shared facilities where guests aren’t used to urine diverting compost toilets


In this article, we’ll cover the most commonly used cover materials and give our view, and the views of others. Remember that every one is different, and what works for you with your compost toilet, might not necessarily work as well for someone else.

When you’re starting out, don’t invest in too much of one material – keep an open mind and experiment to see what works best for you. Ask around to see what’s working for other people.

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 an occasional sprinkling of carbon-rich material over the ‘solids’ will help with the final composting process and provide a way for people to disguise their deposits if they wish to.

Coir bricks

Compressed coir brick

Coir or Coconut Coir 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 in highly compressed blocks (about the size of a house brick) which are then reconstituted with water (warm water does the trick even quicker).

A typical coir block will expand to around 8 litres.

Although it’s a waste product, 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 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 might have to leave it a period of time to dry out again sufficiently.

When first using coir, people often add too much water (manufacturers instructions often relate to using it as a garden product), so we advise starting with less than half the recommended amount of water and add more if necessary.

Coir is available in some garden centres, online and at many ‘hydroponics’ shops. It makes sense to buy in bulk as it’s reasonably easy to store in the compressed ‘brick’ form and will keep (as long as it doesn’t get wet) indefinitely.

Chopped straw

Chopped straw

Often sold as animal bedding, Chopped Straw can be used as a cover material and soak. It’s not that absorbent, but it’s just about OK.

Small bags (compressed) are often available in supermarkets and pet shops, and if you buy larger quantities from farm or equine supplies, it becomes very inexpensive.

People using it have had mixed success with some saying it’s worked ok, 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.

If you have access to some, or can’t get your usual cover, then it’s worth a try, but generally we wouldn’t put high up the list of ideal cover products.

Wood ash

Wood ash (never use coal ash as it’s highly toxic) is often available for free during the cooler months from your wood burner, and has excellent odour control properties.

However, it can be a bit messy and dusty in use and a composting expert friend 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 is a very broad term and 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. Don’t ever use sawdust from treated wood, plywood or MDF.

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. Left somewhere, slightly 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. Sold as pet bedding in various sized 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’. Shavings are usually a waste product from timber processing companies, so they’re 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 they can be too ‘light and fluffy’. Like sawdust, it’s even better to let it mature outside for a while (let the bacteria get to start on it). The downside to using shavings is that they will take longer to compost so if you want fast compost, maybe consider something else.

If you’re after larger quantities of wood shavings, try farm supply or equine supply stores – they usually sell large bales at a very low price compared to buying the small bags from the high street.

Compressed wood pellets

Compressed wood pellets, often 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 and they’ll expand and soak up excess liquids.

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

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

And the rest…

If you can get something cheap or for free, and it’s fairly fine and organic, then give it a try! We’ve heard of people using 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 person 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”.


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 both 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 fan or not) is ventilation – the toilet box itself should be ventilated and the room must have some natural ventilation too, otherwise you’ll get a lot of condensation in the toilet and possibly the room too.

Keep it dry – for the moment!

As you might gather, a key aim in a urine-separating compost toilet is to keep the solids bucket fairly dry whilst it’s in situ as this helps with odour control. The contents (faeces is around 80% water when fresh) will start to break down with naturally occurring airborne bacteria, and primary composting might start, 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!

What about toilet paper?

The final issue is whether to put toilet paper in the loo (solids area) or not?

In a glamping/camping site or holiday let, the best option is to let people put loo paper in the solids bucket. To many dos and don’ts will just confuse people, so keep it simple.

If it’s your own loo, you might want to consider having a separate bin for loo paper which can then be composted or burned.

Adding used toilet paper into the solids container will mean it fills up faster, but the flip side is simplicity. It’s up to you which you consider the best option.


What’s been your experience?

Why not let us know what you’ve been using and how well it’s worked for you? Tell us in the comment box below, and don’t forget to mention what make of compost toilet or separator you are using.

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