This may be a stupid question, but isn't there some kind of a one-way valve that could be put at the junction where pipes go into the tank? That way if it filled up you might end up with your sink backed up with grey water that couldn't proceed into the tank, but all the really nasty black water would stay in the tank. Seems like a system that results in poop in the showerpan is less than optimal, and seeing as how we can put robots on Mars and all...
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Stephanie
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