Methane Digester Experiments

Experiment by Tony T (from WoodGas)

A small methane digester that we ran off of chicken manure. It was a very small digester built out of a one gallon pickle jar (glass). As a storage device I used a 12 inch bicycle inner tube.

The fist couple of days the digester is getting rid of the air in the digester and producing CO2, alot of it. After about 3 days CO2 production stops as the bacteria deplete the oxygen and die. After a lull of a couple of days the anaerobic bacteria kick in and start producing methane. I have allowed the system to fill my storage device with CO2 and then I empty it every day (and squeeze out every bit of CO2 by rolling the tube up). By the time it produces methane there is only a very tiny bit of CO2 left in the tube. As I burned the gas for the first few days the fuel quality starts to come up as the rest of the CO2 is "flushed" out of the system with the gas.

This little inner tube would hold enough gas to run my 8hp lawn tractor for about 7 minutes at full throttle. I would hold the tube and squeeze it with my hand to keep the pressure up.

I know that during WWII they would use gas bags and apply a weight to it to keep the pressure up so I assume we can do this with other gases. The only reason I see for compressing the gas is to put more in less space and get longer run times which is more practical than a gas bag/inner tube. If you need a quick easy way to store gas the bag/inner tube would work (IMHO).

The manure I used was just scooped up from the bottom of my chicken house. The litter would get packed down and mixed with all the manure, I'd just clean out the house and use whatever I scooped up. I would fill the gallon jar about a quarter of the way up with manure/litter and add enough water to make a thin slurry --- that's it. I'm sorry I really didn't messure anything I just guessed and mixed it until it looked right.

I'd bring the digester into the utility closet where my water heater sits because it's warmer in there than anywhere else in my house (somewhere around 75 to 80 degrees). By the next morning the inner tube would be filled with CO2 to the point where it was about to pop. I would take the unit outside and open the valve to relieve the pressure. I'd sqeeze the tube until all the gas was out and then close the valve and put it back into the closet, I repeated this everyday. After 4 or 5 days it would stop making any kind of gas and it would sit there for a couple of days.

Slowly the tube would start to fill again and it would take a few days to fill the tube with gas (methane and CO2) very weak gas. I had a little burner I made out of copper tubing and steel wool so I'd take the unit outside and try to light the gas. It would make a flame and go out several times then it would make a very tiny flame for a few minutes. The quality of gas would get much better over the next couple of days and the volume would come up also. The inner tube would start filling up once a day for a month or so and then it would slack off and stop. You could shake the jar a bit to mix the slurry and it would make a little more gas and then die yet again.

To run my yard tractor I removed the air filter and stuck a small piece of aquarium tubing (the small clear stuff)down into the carburetor and use a small spring clip to hold it in. I'd start the engine on gasoline and let it warm a bit. When the engine was running well I'd shut the gasoline off at the fuel tank valve and wait for it to run out of fuel. As the engine started to starve for fuel I'd slowly turn the valve on to the methane. The engine would pick back up and level out. I'd continue adding methane until it sounded like it was flooding and then back the methane down until it ran well.

We (my daughter and I) would ride around the yard for 5 to 7 minutes in first gear until the tube ran out of gas -- it was a blast!!

Mary Beth took the digester to school for a show and tell when she was in the sixth grade, it was better than anything the other kids brought.

For about a month the tube would fill once a day. After that time it would start to take a few days to fill the tube again. Keep in mind though that this is only a tiny little gallon digester less than a quarter full with slurry.

I built a digester out of 55 gallon barrel and a big tractor inner tube. It made a lot of gas when the weather was warm and near nothing when it cooled off outside. In the hot part of the summer it worked great but only when the night time temps stayed above 65 or 70 degrees. I needed some way of heating the digester to keep it at a more even temprature. It was also a pain in the aXX to clean the 55 gallon drum out. I needed a drum with a removable top but I just used the one I had ---- you guys know how that goes.

I would fill the digester and then put the top on. The glass jar I used had a metal top. I drilled a hole in the top and bolted a closet pole flange (the kind that has pipe threads in it for a 1/2 inch pipe) to it with four bolts and lots of sealer. I would screw a pipe nipple into the flange and a 1/2 inch tee into the nipple. Out the top of the nipple I adapted it down to a 3/8 hose barb. The side opening of the tee was adapted down to the 1/4 tubing I was using to run to my burner or carburetor. I had a small brass cut-off valve in the clear tubing (the kind that controls aquarium accessories). I attached a 3/8 fuel hose to the 3/8 barb in the top of the digester. I removed the Schroeder valve from the valve stem of the inner tube and pushed the other end of the 3/8 fuel hose over the valve stem and clamped it on both ends. That's about it.

Memories from John (WoodGas)

Interesting the temperatures you found necessary for your mini digester as they are more or less the same as we held the rather bigger digesters as the sewage treatment plant I worked at in the 1960s. Exact sizes now are but a memory but figures like 50 feet diameter come to mind and we had 8 in use. The temperature was kept at 70 deg F by circulating the hot water from the diesel engine cooling system through pipes laid inside each digester. The digestion period in these primary digesters was about 3 months with activated sludge being added and removed continuously and was then followed by a secondary digestion but in open tanks for another 6 months after which the sludge was run out onto drying beds or sprayed onto fields as fertilizer. Dried sludge was put into paper sacks for sale and was or still is a very good natural fertilizer but has one drawback.

Tomato seeds do not get digested either by animals or science and a good crop of tomatoes could be expected a few months after spreading the fertilizer on the garden