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This same approach is used in fully developed areas for things like outdoor schools for kids. Another variant in the more luxury area is boat rentals, using the heads here can result in unpleasant smells for later users even if holding tanks are pumped - you basically put a special bag with a cat litter like substance into the actual toilet, then when you hit the dock you take the bigger bag out (if more than one use).

Bottom line, less is more and leverage solutions that have demonstrated actual market traction. A lot of stuff geared towards the poor has no market in the paying world for a reason - it's crap (no pun intended).

Perhaps there is a human manure angle that gets missed with these other approaches, and they may be more resource intensive in terms of plastic?