Revision history [back]
Hi Binaya / Richard,
I do not have first-hand experience I'm afraid, so it would be better if an actual expert could advise. Unfortunately in this case, many designs are for municipal solid waste and less for agricultural + human waste. You should get expert advice before embarking on a biogas idea anyway as these are notoriously complex to manage in order to get the right result.
For sizing, you can use Practical Action's own basic guide (with part 2 here). Part 2, from page 40 onwards, has details on sizing, but this needs more details than what you have asked here. The retention time will be your main factor governing size, and that would be at least 20-30 days for cow and pig manure (if you don't add plant materials), but usually upwards of 40. However temperature also has a big effect (where are you planning this?), as well as the carbon / nitrogen balance.
I suggest you also look at the SuSanA case studies on biogas, some of them have similar inputs as yours.
In terms of emissions, note that you are not reducing carbon emitted: what you are doing is capturing the methane that develops from anaerobic digestion, and instead of letting it go into the air, burning it. This burning releases CO2, but methane is apparently 21 to 34 times worse in terms of greenhouse effects (estimates vary), so this is an improvement. Appendix 1 of this guide shows how to account in terms of carbon management. Biogas plants may reduce nitrous oxide too but this is harder to measure and account for.
Best wishes, Rémi