Biogas is the natural product of bacterial digestion of organic wastes. It is 50 to 65 per cent or more methane gas, which is a powerful greenhouse gas when released into the atmosphere, but burnt as a fuel for cooking, heating or even running a generator, it is cleaner than gasoline or wood. Biogas can be made either aerobically (in contact with the air) or anaerobically in a sealed chamber, which is the more common approach as it makes collecting the gas simpler and avoids the unpleasant odour. A small-scale batch digester can be constructed for home use.
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Things you need
- 250 litre (55 gallon) steel or plastic drum (tightly sealed, with two threaded holes in the top -- one large, one small)
- Threaded cap for large hole
- 30 cm (12 inch) metal or PCV pipe
- 20 cm (8 inch) metal or PCV pipe
- Pipe sealing compound
- Brass shut-off valve
- Two lengths of plastic tubing
- Large inner tube with an air valve
- Bacterial starter (such as Rid-X or other septic tank conditioner)
- Broom handle
Insert the 30 cm (12 inch) pipe approximately halfway into the drum through the small hole and seal it tight. Spread sealing compound around the hole until it is airtight.
Attach the T-connector to the top of the 30 cm (12 inch) pipe, with the 20 cm (8 inch) pipe attached to the top end. Screw the shut-off valve onto the top of the 20 cm (8 inch) pipe, making sure all the connections are tightly sealed.
Attach a 1.2 to 1.5 m (4 to 5 foot) length of plastic tubing to the side opening in the T-connector. Attach a length of plastic tubing long enough to reach the gas stove, heater or other appliance.
Remove the inner valve from the stem and connect the end of the plastic tube to it, sealing that connection with sealant.
Fill the drum with a slurry of two parts water to one part cow dung until the mixture is 20 to 25 cm (8 to 10 inches) from the top rim. Add the bacterial starter and stir the mixture with the broom handle. Cap the large hole tightly but do not use sealant, so it can be reopened when necessary.
Tips and warnings
- The system should produce gas for two to three months before it needs to be emptied and refilled with fresh slurry. Stirring the slurry with the broom handle and recapping it every few days will improve the efficiency of the digester. The used slurry can be used as an organic fertiliser without further treatment.
- Tap the gas for a few days to a week before attempting to use it for the first time to purge the system of air -- the mixture of air and biogas can be dangerous.
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