Another name for a composting toilet is "a waterless toilet" or dry toilet.
There are many commercial systems and home built systems around the world.
Besides the advantage of requiring no water the composting toilet system can be isolated from the ground water system easily and cheaply.
Here lies one of its main advantages. The composting toilet will provide a human waste processing system that will not pollute ground water systems. It is being used in many 3rd world countries in this vein. In India it is asserted that a rural family can build their own toilet for around 100USD. If e.g. a whole village were worried about the pollution of their underground well water by wet toilets then they could all convert to composting toilets and save their ground water for safe consumption.
The composting toilet works by drying the faeces and either diverting the urine or providing a system to evaporate the water from the urine. In this way the volume of the waste is reduced by an order of magnitude e.g. 1/10 of the size.
When the waste is dried and processed by the microbes in the system it becomes valuable fertilizer.
Internet searches will reveal information using the keywords e.g: "clivus multrum", rotaloo, eco-solutions.org etc.