I've read several articles about methane digesters. Once met a guy who installed this sort of thing on trash dumps. They line the dump with plastic sheeting, insert wells and burn off the gas produced by the decomposing garbage.
For small setups, though, the general consensus is that you need a large amount of excrement, and perhaps some greenery thrown in as well, to produce a usable amount of gas. You also need to make sure the amount of solids is high in relation to the amount of liquids. The output of a typical toilet, for instance, contains much more water than could be productively digested. Maintaining one seems like a full time job.
I've been thinking about putting together something like this as a small septic system for a while now, using a sealed plastic 55 gallon drum. Since your post, I've given it some more thought. The design I have in mind would basically separate the liquids and solids under pressure with some type of filter. The solids would be digested. The liquids would be bled off by a trap valve into a grey water disposal pit. It would probably also include some sort of relief valve. I don't see any reason the entire thing couldn't be made of plastic.
Hopefully it would produce enough gas to maintain a small pilot light. The drum could be insulated or buried fairly easily.
And actually I could find a couple of uses for one of these at the moment. So there's a decent chance it will get built. If anyone is interested in collaborating, let me know.
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