Generating electricity from biogas is not difficult, judging by the experience in China and even Brazil, keeping in mind that one cannot get anything from nothing.  If one has a 100kw generator, then we should have the right number of a particular livestock and the right size of digester with the right ambient temperature. In China, there are many big farms and industries self-sufficient in fertilizers and/or energy, with one or more digesters of up to 2,000m3 capacity.  The biggest one so far is 8,000m3.