I have had an interest in bio-gas generation for a long time. While I have done my own small scale experiments in using rabbit manure. The design and operation of a full-scale anaerobic dairy manure digester is already pretty established.
This particular PDF shows a large design for a dairy farm. The conversion of waste is just as important as the production of biogas.
If you have a large amount of daily inputs then this is a great Methane digester design. However, if you are smaller scale, then you may need to tweak the perimeters. This will ensure you have the right sized plant.