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

If you've ever encountered a composting toilet that smells, there's a very good chance the culprit isn't the composting process itself — it's urine mixing with solid waste. Understanding why this happens, and how a simple urine separator fixes it, is the key to a composting toilet that actually works well.

Why the Smell Happens in the First Place

When urine and faeces combine — which is exactly what happens in a conventional toilet — the nitrogen in urine reacts with bacteria in the solid waste to produce ammonia. Ammonia is that sharp, eye-watering smell you associate with poorly maintained portable loos. It's also what makes a wet composting chamber slow, sludgy, and unpleasant to manage.

The science is straightforward: faeces on its own is largely solid organic matter. It composts relatively quickly and, once dry, produces very little odour. Urine on its own is mostly water with dissolved salts and urea. The trouble begins the moment the two meet. Keep them apart, and you've removed the primary source of smell from the equation entirely.

How a Urine Separator Actually Works

A urine separator — sometimes called a urine diverter — is a specially shaped toilet insert that sits where the seat meets the bowl. The front section of the insert is angled and curved to catch urine as it flows forward naturally during use, directing it through a 32mm outlet pipe to wherever you want the liquid to go. Solid waste falls straight through the rear opening into the composting chamber below.

The shape is everything. A well-designed separator doesn't require any special technique from the user — the natural anatomy of sitting down does the work. The liquid channels away cleanly, the solids land dry, and the two never meet. It sounds almost too simple, but in practice it works remarkably well.

The 32mm outlet pipe can run to a soakaway, a collection tank, or even directly to a patch of ground away from the toilet structure. As long as there's a gentle fall on the pipe — even just a few degrees of slope — liquid drains freely without blocking.

What Happens to the Solid Waste When It Stays Dry

This is where the real magic is. Dry solid waste composts faster, produces far less volume, and generates almost no smell. Without the constant addition of liquid, the composting chamber stays aerobic — meaning it has enough oxygen for the beneficial bacteria to do their work efficiently. Wet, anaerobic conditions are what cause the sulphurous, rotten smells associated with poorly functioning composting toilets.

A dry composting chamber also means you're emptying it far less often. Many people find that a well-separated system serving two adults produces a finished, soil-like compost that needs removing only once or twice a year. Volume reduction through composting can be dramatic — up to 90% in some cases.

Adding a carbon cover material after each use — wood shavings, sawdust, or dried leaves — accelerates this process further. The carbon balances the nitrogen in the solid waste and helps maintain the right conditions for fast, clean composting.

What to Do With the Urine

This is the question we get asked most often, and the good news is there are several sensible options depending on your setup.

Soakaway: The simplest approach is a shallow soakaway — a pit filled with gravel or rubble, located away from any water sources. Urine is roughly 95% water and is sterile when fresh, so it disperses into the ground harmlessly. Keep the soakaway at least 10 metres from any watercourse, borehole, or well, and you're unlikely to have any problems.

Diluted fertiliser: Urine is genuinely excellent liquid fertiliser — it's high in nitrogen, phosphorus, and potassium, and plants love it. Dilute it at roughly 10:1 with water and apply it to the base of plants, avoiding leaves. Avoid using it on edible crops in their final weeks before harvest, but for trees, shrubs, lawns, and vegetable beds earlier in the season, it's a free, effective feed.

Collection tank: If you're in a situation where you can't use a soakaway or garden application, a collection tank or IBC container gives you a buffer while you work out what to do with the liquid long-term. Some people have their collection tank emptied periodically, in the same way a holding tank on a boat or motorhome might be.

Installing a Urine Separator — What's Involved

Installation is straightforward for anyone comfortable with basic DIY. Most urine separators come with a paper cutting template that you use to mark the hole in the toilet seat or bench top. You cut the hole with a jigsaw, drop in the separator, and connect the 32mm outlet pipe to wherever you want the urine to drain.

The outlet pipe should fall away from the separator at a consistent angle — even a shallow slope of around 5° is enough to keep things draining cleanly. If your pipe run is long, it's worth checking the fall along its full length and avoiding any dips where liquid could pool.

Sealing around the separator with silicone ensures a clean, hygienic finish and prevents any liquid from getting into the bench structure. Most installations take an hour or two, and the results — a dry, odour-free composting chamber — are immediately noticeable.

The Simple Solution to a Common Problem

Composting toilets have a reputation they don't always deserve, often because they're installed without a urine separator. Add a good separator and you've addressed the single biggest cause of odour and maintenance problems at a stroke. It's one of those small changes that makes a disproportionately large difference.

At Free Range Designs, we've been fitting urine separators in our composting toilet builds for years — they're a standard part of almost every system we put together, from woodland tree bogs to permanent off-grid bathroom installations.

Browse our urine separators and composting toilet products →