Dear Humphrey, I am also totally interested in this problem and I am looking about it, I will ask to our coffee growers. If I have some information about it gladly I will share with you.
Dear Humphrey Lumadede Mudoga, in the manual (Biogas guide - From production to use) is presented a method for the production of biogas calculating from the analysis of organic samples. In chapter 2 (Fundamentals of anaerobic digestion) the authors present a method to measure the productivity of methane in a particular case.