There are places where animal manure is being used for the biogas production. The scale of the cesspool in the link is way above any small farm biogas tank, but

There are places where animal manure is being used for the biogas production. The scale of the cesspool in the link is way above any small farm biogas tank, but then there must be better ways to speed up the biological decomposition of the manure. Mixing it with plant material, increasing surface and pumping air i.e.