A composting toilet design is, in essence, quite simply a mouldering pile in a bucket.
If you replicate that as the basis for your design, you are halfway there.
The fine details then revolve around how many people are going to use it, what space you have available to place it, urine separating or not, whether you are going to vent it and how to treat the greywater..
We take on these considerations one at a time:
The Container
The container contains the composting pile. So it must be strong, impervious to water and easy to build or transport.
Composting toilets have been made from various types of plastic, stainless steel, sheet metal, block and brick, cast concrete and fibreglass.
The container needs an access at the top for inspection and maintenance, and an access at the bottom (usually) where the finished compost is. Systems where the toilet deposits are composted away from the toilet have both at the top (the lid!).
There is usually a venting system into the top of the container for extracting iar from the pile and helping a continual flow of air for composting.
There is usually somewhere at the bottom of the container for a pipe to take away liquid end product.
Toilet use and Space
The amount of users dictates the size of the container and the space you have available dictates the location of the composting chamber.
If you have low use such as a seasonal cabin use or a small house with only 1 or 2 persons using the system then your composting chamber can be small and placed on top of the floor.
If you have a family or more use (up to heavily used public facilities) then you have the options of small containers replaced regularly and composting in a separate area from the toilet, or a large container. The large container can be either under the floor (a direct system) or remote (outside tank). The outside tanks systems usually requires full or minimal flushing with water to get the deposits out to the container.
Urine separating?
Separating the urine from the solids enables a smaller composting pile and is generally more successful to maintain, so is useful for small systems. There is now a large range of urine separating pedestals available in plastic, fibreglass and porcelain which can be put on top of any composting chamber.
But you then of course have the urine to deal with. This can be used as a liquid fertiliser, or transferred into a suitable, small treatment system.
Venting
Venting of air through the systems helps maintain air flow to the pile and takes odours away directly from the composting chamber. A positive venting system can make composting toilets have less odour than conventional toilets as conventional toilets have a water trap that seals the odours in the room until vented out.
Venting systems can be mains electric fan, solar powered fan or passive system such as wind extraction or black painted pipe to encourage solar heating and thermal rising.
Liquids end product
Some systems have sufficient venting to allow liquids to dry up and then no liquids can accumulate, but for large use systems that are not able to dry out all liquids, the liquids are run off from the bottom of the container and treated in a suitable greywater treatment system.
A suitable screen is needed to filter out the liquid as well.
Greywater
An important concept for those wanting to have a house system that uses exclusively composting toilets is the need for a greywater system for treating sink, washing machine, dishwasher and bath liquids called greywater.
In conventional systems this liquid would just be mixed with the wasters from the toilet and treated by sewage system or septic system.
Now that we have separated the “blackwater” from the greywater we have to have a system for treating the greywater.
While greywater is much less of a nutrient loading that solid toilet deposits, it still needs a good system and there are a range of systems available now for treating greywater.
Summary
We very much prescribe to the KISS design system. Keep It Simple Stupid!
The more moving parts and engineering bits attached to systems, the more we have seen failures. Failures in a composting toilets are not pretty, so we encourage simple systems, the larger the better, for fail safe use.
The major component in successful composting toilet system is actually not the design, but the operator.
So a mindful persons who is treating the composting pile like the living system that it is, will result in a system that takes little time to maintain and is successful in treating toilet deposits and producing healthy, safe, composted end product.