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You have not given enough info. Somewhere in the problem you should have been given the products and means of use.
Eg how much methane is evolved per ton?...multiply that figure by the heat of combustion of CH4. Was the residue burned?...how much heat is derived from that?
Add more detail and I'll guide you through the problem.
Well, we are getting there but still do not know how much CH4 is produced. This is how to approach your problem:
The reaction, CH4 + 2O2 ---> CO2 + 2H2O
produces 50MJ/kg (megajoules/kilogram of methane) of heat.
One MJ of heat can be converted to 0.28 KWH (kilowatthours) of electricity. (This is theoretical, there is some energy loss in the generator, etc.)
So, 50MJ/kg x 0.28KWH/MJ = 14 KWH per kg of methane...or you must burn 1/14 kg of methane to get one KWH of power.
Find out how many tons of waste is needed to give off one kg of methane, then divide that figure by 14 to get tons waste needed to generate one KWH of electricity.
Is this clear? Are you given the methane produced? Questions? More info please, and back to the drawing board :-))