How Urine Separation Works (and Why It Solves the Composting Toilet Smell Problem)

If you've ever walked into an outhouse or a poorly designed composting toilet and been hit by that distinctive ammonia fog, you'll know the smell we're talking about. The good news is that it's almost entirely avoidable — and the solution is simpler than you might think. It's called urine separation, and once you understand why it works, you'll wonder why every composting toilet doesn't come with it as standard.

Why the Smell Happens in the First Place

The culprit isn't actually faeces on its own. Fresh solid waste, while not exactly fragrant, doesn't produce the eye-watering ammonia smell most people associate with outdoor toilets. The problem is what happens when urine and faeces mix together.

Urine contains urea, which bacteria break down rapidly into ammonia — particularly in warm, wet conditions. When urine pools in with solid waste, you create the perfect environment for this process: a warm, nitrogen-rich, moist soup that off-gases ammonia continuously. The more urine mixes in, the wetter and smellier the whole mass becomes, and the slower it composts.

Keep the two streams separate, and you eliminate the problem almost entirely.

How a Urine Separator Actually Works

A urine separator — sometimes called a urine diverter — is a specially shaped insert that fits inside your toilet seat. The bowl is divided into two zones. At the front, a smaller, angled funnel collects urine and channels it away through a 32mm outlet pipe. At the back, a larger opening allows solid waste to fall straight down into the composting chamber below.

The shape is everything. The front funnel is positioned and angled so that when you sit normally, urine flows naturally into it without any particular effort or adjustment needed. A well-designed separator works for most body types and sitting positions without fuss.

The 32mm outlet pipe connects to whatever drainage arrangement you've set up — a soakaway, a collection container, or a pipe run to a suitable discharge point. The pipe run should have a gentle fall (around 1:40 is ideal) to prevent pooling and blockages.

What Happens to Solid Waste When It Stays Dry

This is where things get interesting. When solid waste isn't being constantly wetted by urine, several good things happen at once:

First, the volume reduces dramatically. Dry material composts much more efficiently and shrinks to a fraction of its wet equivalent. Second, the smell virtually disappears — dry faeces doesn't off-gas the way wet faeces does. Third, the composting process speeds up considerably, because the bacteria and fungi doing the work prefer a moist-but-not-wet environment rather than a soggy one.

A light covering of a carbon material — wood shavings, sawdust, dried leaves, or a proprietary composting medium — after each use helps absorb any residual moisture and further suppresses odour. Some people also add a small handful after each use as a matter of habit; others find it barely necessary once the system is running well.

What to Do with the Urine

Human urine is actually a remarkably useful substance, which is worth knowing if you're dealing with it in any volume. It's sterile when fresh (contrary to popular belief, healthy urine contains very few harmful pathogens), and it's rich in nitrogen, phosphorus, and potassium — exactly the nutrients plants need.

The most common options are:

Soakaway. Pipe the urine away to a soakaway — a buried pit filled with gravel — where it disperses gradually into the surrounding soil. This is the lowest-maintenance option and works well in gardens with reasonable drainage. Keep it at least 10 metres from any watercourse and 50 metres from a well or borehole.

Diluted fertiliser. Dilute fresh urine at roughly 10 parts water to 1 part urine and apply it directly to plants as a liquid feed. It's particularly good for hungry crops like brassicas, squash, and fruit trees. Avoid using it on seedlings or directly on leaves. Don't use urine that's been sitting for more than a day or two — once it starts to break down, the ammonia smell returns with a vengeance.

Collection tank. For off-grid situations where you want maximum flexibility, a sealed collection tank gives you options. You can empty it periodically, use it diluted on the garden, or arrange for collection. A 25-litre container typically handles several days of use for one person.

Installing a Urine Separator: A Brief Overview

Fitting a urine separator is a straightforward DIY job for most people. The separator comes with a paper template that you use to mark the cut-out on your toilet seat or bench lid. A jigsaw or a hole saw makes quick work of the opening.

The separator drops into the cut-out and is typically secured with a couple of screws or a mounting bracket depending on the design. The 32mm outlet pipe connects to the back of the separator and runs down through the floor or the side of the composting chamber to your drainage arrangement.

A trap or S-bend in the pipe run isn't usually necessary because the separator itself creates enough of a seal, but it's worth thinking about your pipe run layout before you start — a straight run with a consistent fall is easier to maintain than one with awkward bends.

Is It Worth It?

Almost universally, yes. The difference a urine separator makes to a composting toilet is night and day. Systems that previously struggled with smell and slow composting become genuinely pleasant to use. The investment is modest, installation takes an afternoon, and the ongoing maintenance is minimal.

If you're building a composting toilet from scratch, we'd strongly recommend designing the urine separation in from the start rather than retrofitting it later — it's much easier to route the pipe correctly when you're building the structure.

Take a look at our urine separators if you're ready to make the upgrade — we stock a range of designs to suit different installations, from compact units for tiny structures to larger-format separators for full-size composting toilets.