A composting toilet uses no water. Solid waste drops into a chamber where you cover it with a carbon material — sawdust, coconut coir or peat — and aerobic bacteria break it down into a soil-like material over months. Liquid is diverted into a separate container, because the single biggest determinant of whether the system smells is whether the two stay apart. Emptying the solids chamber is a job every one to four weeks depending on use.
The water saving is genuinely large. A conventional toilet at the US federal maximum of 1.6 gallons per flush, used five times a day, gets through roughly 2,336 gallons a year for one person, and older pre-1994 toilets use several times that per flush. Estimates for a composting toilet put the saving at over 6,600 gallons per person annually once you account for typical household patterns. Toilets are the largest single indoor water use in most homes.
The two things doing the actual work
Separation. Urine is roughly 90 percent water and makes up the great majority of what a toilet handles by volume. Let it soak into the solids chamber and you get an anaerobic, waterlogged mess that smells strongly of ammonia. Divert it and the solids chamber stays at the moisture level aerobic bacteria want. This is why almost every modern design is urine-diverting: a moulded bowl channels liquid forward into a jug or a drain while solids fall straight through the back.
Carbon. Human waste has a carbon-to-nitrogen ratio around 20:1, which is far too nitrogen-rich to compost cleanly on its own. Sawdust sits nearer 400:1. Adding a scoop after each use pulls the mix toward the roughly 30:1 that aerobic decomposition wants, absorbs moisture, and creates air pockets so oxygen can reach the bacteria. It also physically covers the waste, which is most of the odour control.
The main types
| Type | How it works | Best for |
|---|---|---|
| Self-contained urine-diverting | Bowl separates liquid; solids compost in a bin below the seat | Tiny homes, vans, boats, cabins |
| Centralised / remote | Toilet drops to a large composting chamber in a basement or below the floor | Houses with space beneath |
| Simple bucket (“sawdust toilet”) | Bucket plus carbon, emptied to an outdoor compost bay | Off-grid, lowest cost |
| Electric / dehydrating | Heats and evaporates moisture, sometimes with a mixing auger | Where power is available |
Self-contained urine-diverting units are what most people mean by a composting toilet today. They need no plumbing, and typically need only a small fan.
What is the maintenance actually like?
This is the part sales pages skip, and it is the part that decides whether people keep using one.
- Liquid container: emptied every one to three days for a couple in full-time use. This is the frequent job. It can be diluted heavily and used on non-edible plants, or emptied into a normal drain or sewer where local rules permit.
- Solids chamber: every one to four weeks depending on capacity and use. The material at this stage is not finished compost — it is partly broken down and still needs to be composted further, usually a year or more in a dedicated outdoor bay before it is safe to handle as soil.
- Carbon supply: you need a steady source of sawdust or coir on hand. Running out is the most common cause of a smelly system.
- Fan: most units have a small extraction fan running continuously to maintain airflow and carry moisture out. It draws a few watts.
Do composting toilets smell?
A correctly run one smells of the carbon medium — earthy, like a sack of coir. When they do smell, it is nearly always one of three causes: urine reaching the solids chamber, not enough carbon added after use, or a fan that has failed or was never fitted. Fix those and the smell goes.
Is the compost safe to use on a garden?
Not straight out of the toilet, and this is where caution genuinely matters. Material from the solids chamber is partly decomposed and can still contain pathogens. It needs an extended secondary composting period — commonly cited as a year or more, longer in cold climates — in a dedicated bin before it is handled as soil.
Even then, standard guidance is to use it only on ornamental plants, trees and shrubs rather than on food crops, unless it has been through a thermophilic process with verified temperatures. If you want compost for a vegetable bed, make it from kitchen and garden waste instead — the material is easier, faster, and carries none of the same risk.
Are they legal where you live?
This is the first thing to check, and it varies enormously. Many jurisdictions require a permitted, certified unit rather than a homemade one; some require a connection to an approved system for the liquid fraction; some only allow them in buildings without sewer access. Local building and health departments are the authority, and getting this wrong is expensive to unwind.
Who they actually suit
Composting toilets make obvious sense off-grid, in vans and boats, in cabins without a water supply, and in places where a septic system would be prohibitively expensive. In an ordinary suburban house on mains water and sewer, the honest comparison is against a modern dual-flush toilet, which uses little water and requires no maintenance at all — and against simply fixing the toilet you have, since a single running toilet wastes more water in a week than most households would save in months of composting.