A composting (or dry) toilet doesn't use any water. The waste is composted so that you can use it on the garden. Most types, commercial or DIY, need a fair bit of room to allow composting to occur at a steady pace, as it is best left for up to two years to decompose thoroughly.
Addition of the right amount of ‘soak’ gives good decomposition. A ‘soak’ is a source of carbon - typical materials include sawdust, straw & earth.
Keeping urine separate is usually the key to a successful composting toilet, otherwise they can become anaerobic and smelly. As urine is high in nitrogen, potassium and phosphorus, if collected separately it can be used as a fertiliser (on non-food plants) when mixed about 1:10 in water.
Another simple option is a 'straw bale urinal' - a bale of straw (ends uppermost), for men to urinate on. The liquid soaks in and composts the middle of the bale in a few months. Technical approaches include flat panels in the front of compost toilets to capture and divert urine.
Bucket toilets are the cheapest and most basic kind of dry toilet. The waste needs to be emptied into a suitable container for composting. There are now more ‘civilised’ versions of these, and as they are fairly compact and portable they can be put in almost any location. If you can keep the urine separate, then in an average house these toilets might only need emptying once every few months.
De-watering toilets are more expensive, and use fans or electric elements to dry the waste. They use lots of energy and don’t compost the waste, so generally we do not recommend them.
To avoid handling raw sewage, you can buy or build a dual-chamber compost toilet. Each chamber is sized to fill up over a year. The seat can be switched between chambers, so that the second is filled whilst the first composts down completely. The soil produced is more pleasant to remove, and can be put straight on the garden (although preferably on non-food plants).
See our Composting Toilet Design sheet for some more advice on twin-chamber designs and on managing a composting toilet.
You can spend anything from a few hundred to several thousand pounds on a composting toilet; those at the lower end of this scale require a more ‘hands-on’ attitude to maintenance.
For those put off by the front end of compost toilets, a device called an Aquatron (about £500) can be fitted to the outlet of a flush toilet, to separate the solids and liquids. The former drop into a composting chamber; the latter are treated, for example, in a leachfield or reed bed.
If you are off mains sewerage and install a composting toilet, you will still need to deal with grey water. This may just involve a simple soak-away pit, or diversion for garden irrigation. Other approaches will be similar to those listed above - perhaps a small septic tank & leachfield system, or a vertical flow reed bed.