One solution is the composting toilet. They have been around for a while; the Clivus Multrum was invented in 1939 by Rikard Lindström, and can hold enough poop for a year. It is big, and is commonly used in larger installations. (This may change, more on this later). But it works on the same principle as almost every composting toilet: Give it time, warmth and let nature take its course in breaking down the poop.