Does it smell?

Funny you should say that.  I mean really funny, as in it makes me laugh.  I used to think biogas would have to be stinky too, because it was my job as a kid to take out the garbage and it made me want to vomit.  Bleccccchh!  Now I recoil when I am in households that still throw their food scraps away, with all the odors and the inevitable flies, and long to be home or in a sensible place where all the food waste is sent to a biogas system.

Why is a biogas system cleaner?

Think of your own stomach and intestines.  They are filled with rotting food stuff most of the time.  In effect, to be indelicate about it, you are "full of sh!t"! 

The reason you don't smell is because you mostly keep it contained in your body (I say mostlly because everybody farts, except those who are liars and say they don't).  Biogas systems can actually be less smelly than people because we capture the biogas (which is, effectively, fart gas) in an airtight tank, chamber or balloon, and when we burn it it has no smell at all. 

Like with our answer to the maintainance question this one can also be answered "it depends on the design"; our IBC systems are completely sealed so there is no smell possibility at all. That is why we have built them in basements of residential homes in New York.  But even the open systems, like the ARTI India and other floating drum digesters, don't have an objectional smell.  On rooftops in Cairo where we have built them, even on the hottest stillest day you can't smell much of anything unless you go and stick your nose right up at the gap between the gas holder and the digester tank.  And the smell you get there isn't unpleasant.  It doesn't stimulate your gag reflex, it doesn't have the objectionable smell of carnivore or omnivore crap.  It smells, if anything, like a barn.  Like horses and cows. And only when you are sticking your nose in it really close.  This is because the digesting material is under water at all times and the odors don't rise. 

There are times when it can smell a bit -- if you overfeed the system and it goes acid, the slurry can smell sour.  Again you wouldn't really notice unless you were right next to the system, but if you put your nose to the fertilizer effluent when the tank has gone acid you can tell it has an acrid odor to it and you know you should stop feeding and  it needs to be pH balanced to work well again.

The liquid fertilizer, when you use it on your garden, does smell like fertilizer -- again, like dung.  If that bothers you there is a simple solution -- aerate it.  We use biogas slurry in my mother's tower garden in her apartment in New York.  When we pour it in directly the area around the tower smells a bit like dung. But because the tower garden is aeroponic and rains the slurry through every fifteen minutes, picking up lots of oxygen, the smell disappears completely in just a few hours.  Some people run the output of their digester through a bucket that has an aquarium air stone bubbling through it (I do this too).  Within 4 to 6 hours all smells are pretty much gone and the liquid has a slightly musty neutral soil smell.

I am talking here about indoor biogas systems.  Out in the garden or on the roof or porch you would hardly know the system is there, open or sealed.  Certainly the sealed systems are undetectable by the human nose (and must be by other noses too because we rarely if ever see insects around them -- they don't seem to attract flies and they certainly don't get noticed by rats, mice, dogs, cats, possums, birds or any other wildlife or possible "vermin".  So in that sense they are much much much superior to compost bins -- in composting we are told not to throw in fats and oils and meat and such because they would create odors and attract vermin. With a biogas system you can throw EVERYTHING in!! 

Look ma, no smell!

If there is an objectionable odor it means you didn't design your system right. And even then, as I've mentioned, the smell isn't a bad one, just the same smell of fertilizer you get when you put fertilizer on your plants that you erroneously spent your hard earned money on from a home and garden shop, not knowing you could make your own from your very own garbage!