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Biogas Plant Science Fair Project

There are plenty of options for anaerobic digesters as all you need is a gas tight container and some organic waste. I think the simplest method is if you can find two buckets/containers such that one will fit upside down inside the other with some clearance (not that easy to do in spite of the proliferation of plastic containers). The bottom container holds the digesting liquid and the inverted one becomes the gas holder - it may need some guides so it does not topple over as it rises with gas. To avoid the possibility of a gas leak I would use waterproof tape/glue/silicon sealant to hold a plastic tube from the top of the gas space running out through the side of the digester bucket (you will notice a liquid leak!) with enough slack to allow the gas holder to rise (See sketch below, 2004 version, guides not shown - a challenge for you!).

I hope by this stage you have read and understood the safety page http://www.adelaide.edu.au/biogas/safety/index.html although it is written with larger digesters in mind - for example you do not have to have the flame 20 m from the digester or have a flame trap, as in the most unlikely event of the flame burning back to the digester you will not get much more of a bang than when lighting a gas stove with such a small digester!
Can you get some cow manure (make sure NO antibiotics are given to the animals) - enough to quarter fill the digester container? I have found that dog manure does not work on its own, like poultry manure or food waste you may still need some cow manure as a starter. Fill the rest of the digester with water and mix it up, then place the gas holder on top and let all the air out (some weight on the gas holder will help). The digester needs to be placed somewhere warm (15-20 C or 60-70 F) and gas production should start in a day or so. Put the free end of the plastic tube in a jar of water to seal it. If you are in a hurry 35-40 C (95-105 F) will give quicker response. Let the gas holder fill up (the volume will probably be a bit less than the volume of liquid, I hope) and discard this gas, as the first gas will be mainly carbon dioxide (CO2) and won't burn (it will actually extinguish a match, but don't allow smoking as there may be some methane present - all being well).
To make a burner you need a small hole for the jet into a larger tube as a mixer, a bit like a bunsen burner as used in chemistry labs, but I do not worry about the air holes. I use a 1 mm hole into 12 mm tube, so a 1/16" jet opening into 1/2" tube (this should be metal) would work well with enough weight to make about 25 mm (1") water pressure - more weight may blow out the flame and less may not give enough gas. About 20 litres (I think that's 5 gallons) of gas will normally boil a cup of water.
You may find that you get a bluish flame when the match is held there, but the flame goes out without the match - that means you have less than 50% methane in the biogas or the pressure is too high, just remove some weight and if the flame still goes out burn off the gas and try again in a few days. As methane is a greenhouse gas try to burn off the excess gas rather than just letting it escape to atmosphere.
You could measure how high the gas container rises each day to record the volume of gas generated and once everything is going well (maybe a week or so) add some more waste (no more than 1/20 of the liquid volume each day) to feed the digester - the overflow is good organic manure.
Here is my 2005 version, made out of soft drink bottles and containing the smell better!

It is a bit hard to see, but the smaller plastic jar inverted in the right hand container is for gas storage and the water level in the inner jar is slightly below that in the outer container. If you look carefully you will see the tube connecting the digester head space (left container) to the gas storage headspace (right container). The tube coming off to the front is the gas outlet. As gas is generated the inverted jar will rise.
The Syringe Protocol provides a cheap method of finding the biogas composition. More than 50% Methane will burn with a stable flame.



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