Great! You should be commended for considering installing or creating your own composting toilet. Amongst the many reasons for their benefits to the environment are that they reduce domestic water use, cut down on chemicals and pollutants going into the septic system and then eventually the ground water, and finally demonstrate that human waste actually does have a benefit, and can be reused, rather than just casually flushed away, out of site, out of mind.
Recent developments in agriculture show that human urine can easily be used as fertiliser, saving money in researching and developing chemical options, and being so much better for the plants and soil – and ultimately us, the food or plant consumer! It is a joyful and natural process to either pee directly onto a home compost system, or to store it to pour into the rotting vegetable matter: the acids and enzymes in urine break down living organic matter quickly.
You are right however that the Environment Agency wouldn’t be happy if anything from a homemade system got into the groundwater system – some breakdown and decomposition process of human waste needs to happen safely to destroy any bugs, parasites or potentially damaging viruses that can pass through our bodies.
Most of the written material I have read about this process recommend separating dry and wet waste within the home composting toilet. This might involve 2 toilets, or 2 collection buckets, or separating the toilet seat into 2. What goes into the buckets and how they are stored needs to be researched thoroughly, as different experts recommend different systems. Certainly for issues of smell, most home compost toilet users buy bags of sawdust, and throw this in after use. This also helps stick to the waste, much like cat litter has been developed. Other home compost toilet users collect local wood shavings, or shredded paper, leaves….something like this which maintain some decorum in the bucket!
I would recommend thorough research through finding books at your local library, and visit any local ecological and environmental education centre, which may well have a home composting toilet system in operation. Ask around – you never know which friends or family may have one, and be very happy to talk you through the process, and particularly if they mention any pros or cons about their system. It is far better with a subject like this to talk to those who have built one, and see the system in operation. Good luck, and congratulations again for choosing to use this more natural system!