I have found that since the electric heat generator had its eu cost increased (i assume so you cant chain eu heat gen > boiler > turbines for infinite eu), that the EU used to produce the biogas from 1 fermenter + electric heat generator (10 coils) does not produce a viable energy profit when the biogas produced is burned in semifluid gens, i propose that the fermenter converts more biomass > biogas per HU used, preferably by the same increase that the electric heat generator got its EU cost increased, it was just about balanced before.
Biogas production eu cost
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As far as I can tell, the electric heat generator has not been nerfed. The infinite energy exploit with the steam kinetic generator was fixed by nerfing the kinetic output. If anything, the electric heat generator has seen a buff since it was first implemented, when it accidentally cost double the EU.
What has changed lately, however, is the fermenter. It's balancing values are now part of the IC2.ini.
It currently it takes 8000HU to produce a batch of 200mB of biogas. Burning 1000mB of biogas yields in a semifluid yields 32000EU. Yes, this is a deficit if you're using EU to produce HU.
However using a fluid fueled heat generator will produce 64000HU per 1000mB of biogas. In a sense, the semifluid generator has an efficiency of 0.5EU per HU, same as the stirling generator.
If you want to maximize biogas output, use a fluid heat generator to power the fermenter. You'll end up with a little excess biogas, which you can convert to power however you please, and you should be overall gaining about 12000EU per 1000mB of biogas.
If that sounds somewhat underwhelming to you, you can always adjust the ini file. I would like to point out that using hotcoolant from a reactor is a nice way to heat your fermenter, since 40000HU -> 32000EU is actually better efficiency than a steam turbine right now.
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Biogas is an amazing power source as long as you have a way to keep it stocked on bio-chaff, but I always used lava to heat it, since the electric one only used to make 60 hU while the liquid heat exchanger made 200.
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This is my biogas setup. It is pretty much the same as what kaldskryke said.
There is a pump under the caning machine that feeds it water to make biomass.
A hopper clock times the introduction of biochaff to stop the machine form getting clogged up.
The canning machine fluid ejects to the right side into some buildcraft pipe that leads into three fermenters.
The fermenters fluid eject down into some fluid heat generators and to the right into some more pipe for storage.This setup seems to be able to run about 8 semi fluid generators constantly.
Renewable energy is my favourite!