Building a composting toilet — closing the loop

A flush toilet flushes several litres of fine drinking water away with every visit — to dilute a valuable resource and pipe it expensively elsewhere. The composting toilet reverses this: no water, no connection, no smell — and at the end you have Humus instead of sewage. For a garden house, tiny house, builder's wagon or the self-sufficiency garden, it's the consistent permaculture answer to the question of where to put your own nutrients.

The most common objection — "but that stinks" — is justified for badly built models and plainly wrong for well-built ones. Odour arises when urine and solids mix and putrefy. The solution is the basic principle of every good composting toilet: separate.

Why separation decides everything

Fresh urine is nearly sterile and low in odour. Solids with the right bulking material are too. Only together does the anaerobic putrefaction begin that creates the typical pit-latrine stench. A urine-diverting toilet therefore has a small spout at the front that leads urine into a canister, and an opening at the back through which the solids fall into the collection container. Two streams, two routes — no smell.

The two designs

• Urine-diverting toilet (most widespread): a diverter insert + two containers. Simple, cheap, low-odour, ideal for home use. The solids are collected with bulking material and composted separately.
• Composting toilet with in-vessel decomposition: a larger chamber in which composting happens directly (often with a drum/stirring mechanism). More technology, more volume, better suited to permanently inhabited places.

For most gardens the urine-diverting toilet is the right choice — the rest of this article describes it.

Building a urine-diverting toilet

1. Housing & seat: a sturdy box (wood) at seat height, with a seat plate and opening on top.
2. Diverter insert: a standard separator (plastic or ceramic) is set into the opening — urine channel at the front, solids hole at the back.
3. Urine drain: a hose leads from the front spout into a canister (5–10 l). Sealable, easily removable.
4. Solids container: a bucket with a compostable bag or placed directly underneath. Put a starting layer of bulking material in.
5. Ventilation: a vent pipe from the container leading outside above the roof. The chimney effect (boosted by a small 12 V fan) draws off moisture and any last odours — the decisive comfort factor.
6. Insect mesh at the pipe outlet to keep insects out.

The bulking material — the "flush water" of the composting toilet

After every "number two", a handful of dry, carbon-rich material is added on top. It binds moisture, covers, and supplies the carbon for the later composting — exactly the brown-material principle from the compost pile. Suitable:

• Sawdust and wood shavings (untreated wood)
• Bark mulch, wood chips (fine)
• Dry, shredded leaves
• Coir or hemp bedding, a little rock dust against odour

Using it — and the legal reality

Urine is a high-quality, nitrogen-rich liquid fertiliser. Diluted 1:8 to 1:10 with water it's ideal for Heavy feeder — but only on plant parts that aren't eaten raw, and not on root vegetables shortly before harvest.

Solids must compost fully before they go anywhere. The professional standard: two years of hot composting in a separate, dedicated container so that pathogens die off safely. The finished material is then used only for ornamental plants and woody plants, not in the vegetable bed.

Important: the legal situation around using human waste is strict in Germany and regulated differently from one municipality to the next. Check locally before you spread the material — build and use the toilet within the safe limits.

Avoiding odour — the checklist

• Strictly divert the urine, empty the canister regularly (fresh is low-odour, old is not).
• After every visit, cover generously with bulking material — better too much than too little.
• Keep the ventilation running permanently (a small solar/12 V fan).
• Keep the solids dry — don't add any liquid.

The composting toilet closes the loop that the garden begins: what goes into the soil through targeted planting and compost comes back as food — and at the end returns once more to the cycle. Together with the compost heater it's the clearest example that in a well-thought-out system there's no "waste", only raw materials in the wrong place.