The biochemical reactions which take place during anaerobic digestion of waste are terribly complex and, to be honest, there really is no single "best way" to simulate as far as I know.

   The models that are used are, for the most part, applicable only to a small sub-set of reactors/systems. Of the more complex models, the ADM1 is perhaps the most comprehensive but even it does not really address issues of spatial variability that affect biogas production.

  Depending on just how accurate your simulation(s) needs to be, you could rely on one of the simpler models (of which there are many - all dependent upon the specifics of the reactor you have in mind); or, if you require a model that takes into account actual biochemical reactions and can be used as a general modeling application for biogas production, then you would probably be best served by the ADM1 model.

  If you happen to be using or considering the use of a plug-flow digester, however, you may want to take a look at the 3-D numerical model developed in 2009.  I have attached publications concerning both the ADM1 and the 2009 model (Wu, B., et al.). The 2009 paper has some good information about some competing models as well.

  I hope it helps!

We have a report that has some experimentally derived factors:

https://www.researchgate.net/publication/269874743_Anaerobic_Digestion_Assessment_for_Contingency_Base_Waste?ev=prf_pub

Technical Report Anaerobic Digestion Assessment for Contingency Base Waste

 Thank you gentlemen for your contributions

You can analyze the contents of C, H, O, and N of the sludge, and then caculate the theory methane production. Or you can measure the BMP of the sludge by anaerobic digestion experiment. And the papers will be useful for the experiment design.