Certain insect larvae can live solely on feces. They produce valuable proteins.
Dry composting toilets use bacteria to decompose feces into CO2 and a biological compost usable for plants. Using it directly for edible vegetable production is an open issue.
The Bill Gates foundation is working on a similar system using worms to decompose the feces.
It is possible to burn feces in a thermal toilet. Products of combustion are mainly CO2 and some volatile organic compounds, plus some ash. The CO2 and the gases can be collected by vents are treated irn an air cleaning system, the ash can be collected and used as fertilizer.
Technical waste water recycling is used on the ISS. However feces are not included in the system. It filters some chemicals out and provides fresh water as a result. Urine contains high levels of nitrogen and phosphorus and can be used to make fertilizers.
Waste water treatment is a well developed technology that is commonly used on Earth in local or regional type systems. Packages systems exist for locations ranging from a construction camp to a small village. Larger systems are usually build per specifications and entire cities can be handled by a single centralized system.