Back in college I saw a few on some large dairies, I assume we're talking about the same thing, they called them methane digesters. They seemed like a really good idea. There are some old ones out there that have failed, but it appears the kinks have been worked out. The slurry flows through a series of concrete baffles and a plastic cover traps the methane. This methane is pumped into a converted generator, any excess is burned off. This was like 7 years ago when I saw one the last time, I asked about using the methane in a veichle, considering you can switch to propane and that methane is a more simple hydro-carbon. I was told the methane is fairly dirty and wasn't really a consideration. Maybe with high fuel prices it is more feasible to clean it up now?
Here's the other thing, the manure has less odor after going through the digestor.That helps with the neigbors. This farm then put the slurry through a solid seperator, solids were composted and the liquid went to a lagoon and then pumped out to the irrigation system. As fuel goes higher these things should make more and more sense. I have since gotten away from the CAFO type of mindset and am much more interested in grazing, now if only there was a way to capture methane from grazing.
Most of these dairies that have installed the digesters have probably dones so through some kind of cost sharing or a grant, if not you can probably bet they've crunched the numbers. There is info out there on the return on investment. The question is how many head does it take to justify the construction cost?