A composting toilet treats wastewater on-site, recycling the nutrients into fertilizer you can use in the garden. There are many types of composting toilets, from expensive proprietary systems to simple build-your-own designs. keeping urine separate is often the key to successful composting; otherwise it can become anaerobic and smelly.
A composting toilet is a good option when the water supply is limited, because it doesn’t need water to flush. in a place completely free of water, you can use alcohol gel to wash your hands, but otherwise you will need facilities to wash your hands.
An important thing for a low-impact toilet is to keep electricity consumption low. it takes a small amount of power just to run a fan and is fine for more compact models. however, we would not recommend dry toilets that use electric elements to heat the compost.
In general, successful composting toilets need enough space to allow the compost to sit for at least a year, maybe more. this will be in chambers under the seat or in bins that can be removed and left somewhere else to be fully composted.
the composting process
excrement falls into a composting chamber directly below the toilet pedestal. bacteria, fungi, worms, and other natural organisms will thrive on this organic matter and break it down into humus. use of an appropriate “soaking” material (see below for more information) is required, as is moisture control through urine separation and ventilation.
Human excreta may contain pathogenic bacteria, viruses, and protozoa, and may be hazardous to human health. composting kills these human pathogens and the finished compost is safe and odor free. The compost, which is usually removed once a year, is an excellent soil conditioner, ideal for use on flowers and ornamental shrubs.
double chamber composting
A reliable and robust approach to UK weather is to switch between two separate cameras annually. the active chamber fills with droppings and becomes waterlogged over the course of the first year. when it reaches its capacity, the pedestal is moved to the resting chamber for the second year. the contents of the first chamber have a full year to become a subscription with no new additions.
By the end of the second year, the contents of the original chamber have been fully composted and can be safely disposed of. to continue the cycle, the pedestal moves back over the empty chamber while the other matures. this batching means no contamination of mature compost with fresh faeces.
starting a new camera
With a closed toilet chamber or container, it’s a good idea to start a new empty chamber by adding a shovelful of compost and perhaps a few extra worms. these should be brand worms, which are common in gardens (also sold as fishing tackle).
for a batch composting system (when you change chambers every year or so) you can “seed” a new chamber using a small amount of the compost from the chamber that is left to sit.
on a continuous system (without any changes to new chambers or containers), then once this is up and running the ecosystem should be established. then no intervention should be necessary, in terms of adding worms or other decomposers. a system like this should be established within a few months.
If the collected debris doesn’t seem to break down well, try adding a shovelful of well-composted garden compost or some brand-name worms. many decomposing organisms, such as bacteria and fungi, should go in anyway.
Proper ventilation is essential for effective composting and to avoid bad odours. design the vent to draw air through the top of the chamber, to promote the composting process. it should also draw air from inside the toilet cubicle, towards the toilet pedestal. passive ventilation is possible with careful design. Alternatively, a low voltage fan gives excellent results and costs only a few pounds a year to run.
To prevent bad odours, it is essential to prevent air from entering the cubicle from the chamber. this problem can occur in poorly designed bathrooms on windy days. You will need a properly sealed compost chamber, so it may be worthwhile to seek detailed advice from a competent installer.
Usually you will need to add a “soaking” material to the composting chamber to aid in the decomposition process. in our experience, wood chips work very well. conventional wisdom is to add a soak after each use of the toilet (as an alternative to ritual flushing). you should add only as much soaking as is necessary to keep the chamber size as small as possible.
In a public restroom, people who are not used to composting toilets may put too much liquid down the toilet. this affects the composting process, while the bin can also become messy, with scattered sawdust. our experience at cat is that a daily or even weekly addition of soak by the “operator” may be better than letting users add it indiscriminately.
keeping the compost fairly dry, for example by urine separation, will minimize the risk of a fly infestation. a well-sealed chamber and a protected vent should prevent flies from entering. Flies inside the chamber can be attracted by a dedicated fly trap or by light shining down a ventilation tube, becoming trapped in the fly screen at the top. see the related question below for more information on fly control.
For efficient and odour-free composting in the UK climate, diverting urine away from the composting chamber is essential. Urine-diverting toilet pedestals are available that trap urine on a metal plate and divert it into a drain. separation only works when users are seated. Since men often won’t sit down to urinate unless forced to, it may be worth installing a separate urinal (which can be waterless). this could lead to drainage itself, or collection for reuse as fertilizer.
Urine diversion systems can be built themselves, but proprietary urine diversion seats or pedestals are available. the urine is sterile, so it’s safe to handle, or you can funnel the urine to a sink outside the building. a typical water hole might be 4 meters long, 500mm wide, 500mm deep, and filled with debris or large stones (40-50mm).
maintenance and emptying
Once a week, the toilet operator should open the access hatch and inspect the composting chamber. they should add an appropriate amount of soak if necessary. if a large “spike” builds up under the plinth, they should level it by raking it forward.
emptying the compost can be an annual procedure. For a completely odour-free toilet, the access hatch can be located inside the toilet cubicles because this protects against odors from drafts. however, this can make removing the compost a little more difficult than access hatches on the outside of the building.
see the related questions below for more details. For in-depth advice, we run regular courses on composting toilets.
The Cat Store Sells The Low Impact Living Book Compost Toilets: A Practical DIY Guide, which has lots of tips on installing and using composting toilets.
This page was written by Cat Information Officer Joel Rawson. you can contact me if you have more questions (choose “free information service” in the form).