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Easy instructions to build your own biodigester
The purpose of this post is to provide instructions for how you can build your own biodigester just as we did. Biodigesters sound like they should be complex and difficult to build. They’re not. Two large buckets or water tanks, some plumbing tubes and a couple of taps and you’re well on your way. That said, they can be explosive so it’s important to be aware of the risks.
The principle is simple: collect the methane that is given off by naturally organic matter as it decomposes.
At the beginning of September our group of six got together and built a small prototype following instructions sent by Martin Funk from Tamera. We made sure to only use materials you can by at your nearest DIY shop and tools that you probably already have in the shed.
Below is a list of what we used:
- 2 x Plastic buckets or water storage tanks – one needs to fit tightly inside the other
- 1 x Plastic tubing to carry the methane
- 3 x brass screw fitting to tube
- 1 x T fitting
- 1 x Gas tap with tube fitting
- 1 x Brass bolt for fixing the tap
- 4 x 1m PVC Piping
- 5 x Caps for the piping
- Bulkheads for the PVC piping: 2 males (F1); 1 female (F2); 1 90 corner (F3); 1 cap piece (F4)
- 1 x PVC corner (F5)
- 1 x Tap (F6)
- 1 x Strap and buckle or winch
- 1 x teflon tape for waterprooffing the bulkheads
- PVC glue to seal all fittings
- Drill for making holes (large and small)
- Scissors for cutting thick plastic
- Screw driver and screws
- bolts and hexagonal spanner
- The digester needs to be fed with kitchen and garden waste that has been blended and mixed with enough water to pour it into the inlet pipe.
Step by step instructions to build a biodigester
- Start by making an tight hole to attach the first bulkhead to the top of the outer or base bucket. To do this measure your bulkhead (How 1 a) and make a hole (How b, & c) that is slightly smaller this way no water (or worse!) can leak through.
- Saw the part of the bulkhead (How 1 e) that will be on the inside of the bucket so that it doesn’t get in the way of the rising and falling inner or top bucket (How 1 j).
- Use and “O” ring and sealing tape to make the fitting water tight (How 1 f, g, h & i).
- Attach the outer tap (How 1 j & k). Then attach the 90 corner (How 1 l). This will be used to collect the fertiliser
- Attach another bulkhead to the bottom of the lower bucket (How 2 a, b & c). This will be used to attach the feeding pipe.
- Prepare the top bucket by removing the rim and handles so that it can slide as snuggly as possible within the lower bucket (How 3 a, b, c, d & e).
- Cut one of the plastic lids to size (How 4 a).
- Make holes ready for 5 bolts (How 4 b, c, d, e).
- Place screws in the holes: one at the centre and the others top bottom, left and right (How 4 f.
- Make holes in the caps ready for the nuts and bolts (How 4 g).
- Prepare fixings for the “microbe motel” towers by bolting the pipe caps to a plastic lid (How 4 h, i & j).
- Drill holes in the pipes (How 4 m) and cut them to a length that doesn’t interfere with the top bucket as it slides up and down (How 4 k & l).
- Fix the pipes to the base (How 4 n).
- Please the lid with the pipes into the lower bucket and add stones (How 4 o). The pebbles aren’t pictures but they should be about 5cm deep to provide service areas for the bacteria at the bottom of the bucket.
- Attach the feeding pipe to the lower bucket (How 5 a, b & c). We used a metal strip to make sure that the feeding pipe wouldn’t wiggle lose over time.
- Attach the top to the bottom of the top bucket (How 6 a, b, c & d)
- Attach the plastic tube to the tap (How 6 e)
- Prepare the T fitting (How 7 a, b & c)
- Attach it to the inlet pipe (How 7 d)
- Connect the tap to the T (How 7 e) making sure that the tube running between the tap and the T is long enough to allow for the top bucket to rise as it fills with methane.
- Attach a water container that is about 30cm high to the bottom of the T (How 7 f). This will act as a condensation collector to ensure that water does not collect at a low point in the tube and block the gas from travelling from the biodigester to the outlet stove. A short tube should come down from the stem of the T. No gas can escape here because the container’s water pressure blocks it.
- Test all the fittings and connections by filling the bottom tank with water through the inlet pipe. The top bucket should slide up. It may be necessary to add some guides or rails depending on how tight the fit between the buckets is. In any case make sure that the top bucket is free to rise and fall as gas is produced and emptied.
- You are now ready to add your “starter mix”. This is a slurry of fresh manure and water (How 9) with a ratio of 1:1.
- The volume of this mixture should be around 200 litres for a 3000 litre digester or roughly 30-40 kg of animal
- manures per cubic meter of digester tank space. Our 120 litre buckets needed 8 litres of slurry (4kg of manure mixed with 4 litres of water).
- Leave this in a warm place so the bacteria can reproduce. They should be ready after about 3 weeks. At this point you are ready to start adding your ground organic slurry to the inlet pipe to feed the bacteria so they can start producing methane.
- The maximum ratio is about 25 litres of feedstock slurry for every 1000 litres of digester space. In our 120 litre tank this means around 3 kg of feedstock slurry. It’s best to start feeding slowly before the digester is at full production. For instance 1kg per day the first week, 2 kg the second week and finally 3kg after that.