They are an excellent way to utilize human excrement and make it safe for fertilizer. They have to be maintained at a certain temperature to keep the aerobic methane producing bacteria happy; this works (as i understand it) much like composting to destroy any pathogens and parasites in the excrement.
Running several in rotation so as they fill they can work out, cool and be pumped either to compost piles for a second ‘cooking’ or spread as fertilizer (typically on orchards, or fallow fields, again to minimize any risks) is a sensible management system for a small town or community. The digesters do require a certain volume of the appropriate feedstock when active in order to maintain their temperatures.
I’ve not made one, but have plans for small scale unit printed for when the time comes.
Imagine it; fertilizer, light, cooking, sanitation and heat, all essentially from s**t!