For my chemistry assignment, I have to build a biogas generator. I found the following diagram, enter image description here

This is the website that the image is from: http://www.motherearthnews.com/renewable-energy/other-renewables/biogas-generator-zm0z14aszrob (Manure goes through the feeding tube, then anaerobic bacteria produces methane. The methane then goes to the second tub and is collected)

On the right side of the diagram,

  1. why does the barrel need to be immersed in water? Does it do something to the methane?

  2. why does the methane not immediately go from the first tank through the tube out to the burner but instead goes into the second tank?

  3. Once the methane is inside the second tank, what's stopping it from rising and going out?

  • 1
    $\begingroup$ Could you provide some additional information. For example, nothing in the diagram is clear about the reaction being done to form the methane. Also, where did this image come from? If you could link to the site, that may help someone else help you interpret it. $\endgroup$
    – Ben Norris
    Apr 2, 2017 at 18:21
  • $\begingroup$ Nothing mysterious, just anaerobic digestion en.wikipedia.org/wiki/Anaerobic_digestion // The anaerobic digestion creates methane which builds up pressure in first container and then flows into the second container. The rise of the second container is just an indicator that you've actually made some methane. $\endgroup$
    – MaxW
    Apr 2, 2017 at 21:49

1 Answer 1


The reasons are as follows:

  1. The inverted barrel must be full of water. Then, when methane is formed, it will displace the water and the barrel will start floating. Methane does not do anything with water. It is almost insoluble (solubility is around 40 mg/L) and therefore does this setup work.
  2. The second tank is storage. Without storage you have to burn the methane when it is formed. With the second tank you can use it when you need it and you can a higher flow rate.
  3. If to much methane is produced and not consumed at the burner, it will come out of the second tank. But you can control that by stopping the feed in the first tank.

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