I've been a little obsessed with the idea of "anaerobic digestion" lately and how it can help a farm become self-sufficient. All the non-scientific writing on the subject is a bit preoccupied with poo. I have to admit that it is kind of fascinating that the back end of a cow can ultimately provide enough power to service all of a farm's electrical needs. The problem is, it is a pretty complicated process, and the methane gas that is produced is highly flammable. Luckily, the good people at Penn State's Department of Agricultural and Biological Engineering have a great website to help those of us so interested to learn more about how to turn poo into power. Using words like biogas, slurry and influent and mesophilic, they explain (and remind us) that the poo doesn't tend to go in solid, but is mixed with water to a maximum of 15% solids. Eeew! With a little heat, microbes in the air-tight tank break down the "slurry" or "influent" into biogas (methane and carbon dioxide) and nutrient rich "efflluent" that can ultimately be used in fertilizer. The biogas can then be sent to run generators, and the heat energy can be used to heat the "digester" or the farmhouse. For a great case study, check out the Hillcrest Dairy Farm. It is a really well written piece with some great photos to help you understand what's going on.
There are a variety of "digesters," but they are all sealed to keep gases in and have a way to move "slurry" through the system. None of them can be built or run in a standard suburban backyard. Of course, most suburban backyards don't have enough cows, sheep, goats or pigs to provide enough "influent" to make it work!
Even if you can't build one yourself, I hope that you can be just a little impressed with the ingenuity of some very modern farmers!