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How Does a Composting Toilet Work? Waterless Toilets Explained
No water. No sewer. No septic tank. Here is what actually happens inside a waterless toilet — and why a well-built one does not smell.
That is the two-minute version. The rest of this page is the same thing in more detail, including the one design decision that separates a waterless toilet you would happily have indoors from one you would not.
Start with what a flush toilet actually needs
Every flush sends clean drinking water down a pipe, and that pipe has to go somewhere — a municipal sewer, or a septic system on your own land. On a serviced lot you never think about it. On a cabin lot, a steep parcel, a tiny home or an ADU, that second half is the whole problem. A septic system has to be permitted, excavated and maintained, and on difficult ground it is the single most expensive thing about adding a bathroom. Sometimes the soil or the setbacks mean it simply cannot be done.
A waterless toilet removes the pipe from the equation. Nothing leaves the room in a drain, so there is nothing to connect to.
What happens when you use one
You use it like any other toilet. There is no flush and no water, so waste and toilet paper fall straight down into a chamber below the seat.
Inside that chamber, the same microorganisms that break down material in a garden compost heap get to work. They need three things to do it well: air, the right moisture level, and time. Almost every design decision in a good waterless toilet exists to supply one of those three.
Covering material
After use, many systems ask for a cupful of covering material — hemp, peat or a similar dry carbon source — sprinkled over the top. It is not there to mask a smell. It adds carbon to balance the nitrogen in the waste, and it keeps the surface of the pile open so air can get in. You can also add a larger amount less often, weekly rather than each time.
The three things that make it work: liquid is drained away from the solids, air is pulled through the chamber and out a vent pipe, and the material is given time. Get those three right and the room smells of nothing at all.
Liquid separation — the part that decides whether it smells
This is the step people miss, and it is the one that matters most.
Solid waste on its own composts quietly. Solids sitting in liquid go anaerobic — the oxygen-breathing bacteria die off, the ones that replace them produce ammonia and hydrogen sulphide, and that is the smell people associate with a bad outhouse. It is not an inevitable feature of a waterless toilet. It is what happens when liquid is allowed to pool.
So the chamber is built to get liquid away from the solids, and there is more than one way to do it. Many of our systems use a double-base design: urine and solids go into the bin together, most of the liquid is absorbed and evaporates away through the ventilation, and only the excess the solids cannot hold drains through the double base and out of a liquid outlet underneath — to a gravel soakaway or a separate container, depending on what your site allows. Others separate at the source, with a urine-diverting pedestal that sends liquid down its own outlet before it ever reaches the solids.
Either way the principle is the same: keep the pile damp, never wet.
Ventilation — why the room stays fresh
A vent pipe runs from the chamber up through the roof, and a low-energy fan moves air through it continuously.
That airflow does two jobs at once. It carries moisture out, which keeps the pile from getting too wet, and it puts the chamber under slight negative pressure so air is always travelling down through the seat and out through the roof — never back up into the bathroom. It is the same reason a chimney draws.
This is why a correctly installed and vented waterless toilet is odor-free indoors, and why the vent pipe is not an optional extra.
Two ways to build it: below-floor and above-floor
Every waterless toilet has to put the chamber somewhere. There are only two answers, and which one suits you is decided by your building, not by preference.
Below-floor, also called a split system
The pedestal sits in the bathroom and the chamber sits under the floor, out of sight, connected by a short chute. Waste drops straight down. Because the chamber is not limited by what fits in a bathroom, it can be much larger — which means more capacity and longer gaps between servicing. All the servicing happens outdoors, underneath the building, so nothing is carried through the house.
The requirement is clearance. You need usable space below the floor and a way to get to it.
Above-floor, also called floor-standing
The chamber is inside the toilet unit itself, so the whole system sits on the bathroom floor. Nothing is needed below the floor at all, which makes it the answer for a slab, a ground-floor room, or any building with no crawl space. The trade is capacity: the chamber can only be as big as the unit, so it is serviced more often.
| Below-floor (split system) | Above-floor (floor-standing) | |
|---|---|---|
| Where the chamber sits | Under the floor, out of sight | Inside the toilet unit |
| Needs underfloor space | Yes — and access to it | No |
| Capacity | Higher | Limited by the size of the unit |
| Servicing interval | Longer | Shorter |
| Where servicing happens | Outdoors, under the building | At the unit |
| Typical fit | Cabins and homes on piers or a crawl space | Slab floors, ground-floor rooms, tiny homes |
If you are not sure which one your building can take, the underfloor clearance is the question to answer first. Everything else follows from it.
Below-floor systems
Above-floor systems
What you end up with
Given air, the right moisture and time, the material in the chamber turns into compost — dark, crumbly and a fraction of the volume that went in. That reduction is why a chamber lasts as long as it does.
How servicing actually works
Most of our systems use batch composting with two containers. When one fills up, you seal it with a closed lid and leave it outside to finish composting, and you put the spare one into use. The toilet is back in service immediately. By the time the second container is full, the first has finished.
It takes minutes, it needs no tools, and it means nobody ever handles raw waste. If your household needs more capacity, the answer is usually another spare container rather than a bigger system.
Set up correctly, no — and usually far less than the conventional flush toilet you’re used to. Two things do the heavy lifting. The first is keeping the solids dry enough to compost: odor comes from solid waste that is too wet to break down properly, which happens in systems that do not separate liquid from the solids effectively. Many of our systems use a double-base design — urine and solids go into the composting bin together, most of the liquid is absorbed and evaporates away through the ventilation, and any excess drains through the double base and out of a liquid outlet underneath. If you would rather keep liquid out of the solids from the start, we also offer urine-diverting options. The second is continuous airflow, which draws air out of the composting bin through a vent, so any smell is pulled away from the unit.
It is not magic: a fan that is switched off, a blocked vent or a blocked liquid hose will cause smell — all easily avoided and easily fixed. Full answer in the FAQ.
Liquid separation keeps composting toilets odor-free and efficient by preventing excess moisture in the compost. It can be done directly at the toilet seat (urine-diverting toilets), where urine is diverted into a separate container, or within the composting container, where most liquid is absorbed or evaporates and only excess is managed.
Effective separation ensures proper composting conditions, controls odors, and allows the nutrient-rich urine or excess liquid to be repurposed as fertilizer for non-edible plants. Full answer in the FAQ.
First, the one people ask about most: yes, regular toilet paper is fine. Almost all commercial toilet paper is fully organic, so you can add it with confidence. Beyond that: urine and solid waste, a modest amount of water if you are cleaning, organic cleaning products, and bulking material.
Bulking material should be fully organic and compostable, dry, and a bit coarse rather than fine — anything that packs down too densely works against you. Wood shavings are a good example, and every one of our products ships with hemp shavings to get you started.
What does not belong in there: wet wipes, diapers, chemicals, cat litter, cigarette butts and feminine hygiene products. None of these compost, so they need a regular trash bin instead. Full answer in the FAQ.
Some do, some don’t — and where they do, it is a very small amount. Most of our composting models run a small 12V extractor fan that keeps air moving and waste drying. It draws roughly 3W — little enough to run continuously off a modest solar setup or leisure battery without you ever noticing it on your power budget.
Because the fan runs on low-voltage DC, it pairs naturally with our Solar Power Bank Kit. And if you want zero power, our Whirlybird wind-driven ventilator vents passively and needs no electricity at all — ideal for remote spots with no power source whatsoever. Full answer in the FAQ.
This depends entirely on your toilet model and how it is used — how many people, how often, and in what kind of setting. As a general rule, service intervals on our waterless toilets are measured in months, not weeks, for the vast majority of typical households. Larger households with daily, year-round use, or public and commercial settings, will see that interval shrink.
As a rule of thumb, the larger the compost bin and the bigger the model, the longer you will go between services — if you have the space to fit a genuinely large bin, that is not the place to cut costs. Past around ten daily users, the natural move is a second toilet rather than pushing a single unit beyond what it is built for. Full answer in the FAQ.
That depends far more on your climate than on the toilet. Heat speeds the process up and cold slows it right down. In a hot southern climate a batch can finish in 2–4 months over the summer, or 6–9 months over the winter. In the northern US, Canada or Alaska it can take 12–18 months. The month you service it in matters too: a container sealed in March has a whole summer to work on it, one sealed in October does not.
Plan on five to six months as an absolute minimum wherever you are. You will know it is finished because it stops looking and smelling like waste and starts looking like ordinary soil — trust that rather than the calendar. Even when it is finished, use it only on ornamental or non-food plants, never on vegetable beds.
This is exactly why the two-container method matters: the full container gets months to finish while the spare one is in use, so the process is never rushed. Full detail is in our FAQ on composting time.
Our FAQ Center answers around fifty questions in detail, from permits and installation to day-to-day cleaning and servicing.
Still deciding?
The fastest way to narrow it down is to answer a few questions about your building and how it will be used. Our waterless toilet finder does exactly that, and it will tell you honestly when none of our systems is the right fit.
If you would rather just ask a person, get in touch — tell us about your floor, your clearance and how many people will use it, and we will tell you what works.
