Home-made biogas anaerobic digester

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Posted Mar 8 2016 by Dave Darby of Lowimpact.org

Thought you might like this. Just came across an old photo of a simple biogas digester built by Lowimpact.org co-founder Phil when we were based at Redfield Community in Buckinghamshire.

It was a little experiment to find how much biogas could be produced from a tiny system with an oil drum for a digestion chamber. It’s ideal for a smallholder with livestock or somewhere with lots of cooked food waste. We put animal manure and cooked food waste into the drum, and made the top airtight with a neoprene skin, tightened with a ratchet strap.

Then there’s a  ‘gas container’ consisting of two large plastic orange juice barrels with the tops cut off, one slightly larger than the other. The large barrel was filled with water, and the second one was upturned, and a pipe with a tap fixed to it. It was then introduced into the larger barrel (with the tap open) until it was filled with water. A flexible hose connected the pipe from the upturned barrel to a pipe from the top of the oil drum.

As the waste decomposed anaerobically it gave off gas which made the neoprene ‘lid’ bulge. When this happened, both taps were opened and the pressure forced the gas through the pipe into the gas container. The water in the large barrel acted as a seal to stop the gas escaping, and the small barrel rose out of the water. When the small barrel had risen almost a metre from the surface of the water, both taps were closed. We now had a store of biogas, and the bulge in the neoprene lid had gone.

Then the flexible hose was removed from the pipe from the oil drum and attached to a small camping gas hob, and several meals could be cooked until the gas had all gone and the small barrel had again fallen to the level shown in the photo. But then more waste had been added to the digester and the process was repeated to store more gas.

After a few cycles the oil drum was emptied and the contents put onto the compost heap. This digester could be positioned outside the kitchen and the gas piped through the wall to the hob. With a few animals and/or a supply of food waste, it could be possible to provide enough gas for cooking for a family. In fact, in India, it’s estimated that over 2 million families do just that – see image below.