I have experimented for about 6 months with a home built methane digester using the batch method that operated on vegetable based inputs. I used cow manure in the original starter mixture along with grass clippings and
newspaper because animal manure contains the appropriate methane producing bacteria which I wanted to inoculate the digester with. I used a 60 liter (approximately 15 U.S gallons) sealed plastic
water barrel with the gas coming out and passing through a smaller container of iron oxide that I purchased from a pottery store. The hose went into the bottom of the iron oxide container so the gas permeated under its own rising pressure up through the powdered iron oxide powder. The iron oxide reacts with hydrogen sulfide in the gas to produce solid iron sulfide and this scrubs the hydrogen sulfide (rotten egg gas) from the biogas. From that container the gas flowed into another container with pickling lime (calcium hydroxide) which takes a lot of the carbon dioxide out of it (the carbon dioxide is converted into water and calcium carbonate, it forms a slurry at the bottom of the container due to the water), And then had the gas flow into a truck inner tube for collection.
Problems : (1) : Well the truck inner tube inflates nicely and the great thing is that it provides gas pressure which is useful when burning (gets to about three atmosphere in pressure). However the pressure inside the inner tube meant that the whole system became pressurized from the digester itself through all the tubes and the hydrogen sulfide scrubber and the carbon dioxide scrubber. This increased leaks issues as the pressure tests the system really well. But it is doable. It is important to flush the first lot of gas build up as the first lot can contain residual air and air and methane mixed inside an inner tube that you connect to an ignition source is always a bad idea.
(2) : The amount of iron oxide (what is essentially just rust) required is small. The amount of hydrogen sulfide produced per liter of gas is only milligrams so 1 kilogram of iron oxide is able to process a massive amount of gas. But the amount of pickling salt required to process the carbon dioxide was a lot due to the large quantities of carbon dioxide in the gas (If I calculated it correctly I was using about 750 grams per 1,000 liters of gas). I did collect the gas at first without the carbon dioxide scrubbers but the collection volume was greater and I am looking at using it for inside cooking gas at a later date so I didn`t want to introduce all that carbon dioxide into the house.
Another process of removing the carbon dioxide and the hydrogen sulfide is to absorb them in water. Hydrogen sulfide and carbon dioxide both adsorb into water quite well but methane doesn`t. However I read that for best results it is best to pressurize the processing system to 10 atmospheres and that level of mechanization for a system introduces too many issues. So I preferred the chemical way of scrubbing the gasses. If you are only ever going to cook outside on it then I guess the carbon monoxide scrubbing isn`t required. Although as a personal preference I would still do the hydrogen sulfide scrubbing as hydrogen sulfide is poisonous.
I was getting around 70 liters of gas (after scrubbing most of the carbon dioxide out of it, so I guess that is mostly methane with some water residue) for every 1 kilogram (2.2 pounds) of grass clippings I put into it. The water from the digester made great fertilizer after I diluted it down for the kitchen garden. I put grass clippings and things like newspapers etc in the digester. I don`t keep my own animals and personally if I did would probably prefer to
compost it as the gas digester doesn`t kill all the pathogens in the manure however proper composting should kill most things due to the heat. For the same reason I definitely would never methane digest human manure, but of course in certain countries doing so is much more preferable than having it deposited where it can get into the water system.
I operated it in a Japanese Spring and Summer and early Autumn (the temperature where I live in summer often stays around 30 degree`s Celsius even at night in summer). So heating it wasn`t an issue. But I`m making plans for a bigger system at a later date and would prefer to use a
solar water heater system for it in the cooler months as opposed to burning some of the methane to heat it. Burning the methane involves a flame and mixing a flame and a methane gas producing unit together … I`m sure it could be made safe but personally not my cup of tea.
One thing to note is that methane is about 50% lighter than air so at least if there are leaks the gas mostly dissipates upwards and away which is great (unlike say propane which accumulates in the lowest place). A lot of the collection systems seem to be quite high such as up by the barn ceiling so that would really help with gas dispersal in the case of leaks. I wouldn`t try anything with the gas unless you have the space where it can be away from the house.
I got the idea for using inner tubes as a collection source from this site;
http://echonet.org/repositories#138:d:Biogas.08 When I first started playing around I used plastic softdrink bottles with different mixtures in them with aquarium tubing going into a large party balloon. A T-junction in the aquarium tubing between the bottle top and the balloon meant that I could draw of the gas as it built up and test it.