A critical subject for any biogas digester is pressurizing the gas. To be able to use the gas, pressure must be applied in one or another way.

Usually, ballast is applied on the digester tank or separate gasbag to push the gas out. However, you need quite a lot of ballast to apply enough force and you need this ballast to be spread out nicely, otherwise you will end up with pockets of gas not being pressurized.



Ballast on a gasbag.


One of the directions we’ve been exploring is the behavior of a flexible tank on flat ground. Main question here would be: can a flexible tank placed on flat ground supply enough pressure on the gas inside?

We expect the shape of the tank and the pressure inside are subject to the gravitational force of the ‘slurry’ inside being lifted. When the tank blows up and gets rounder, the slurry inside is lifted which means its stores potential energy.



A 9,5L tank for testing


Shape and volumes measured


And then we started testing. We made a small tank (about 9,5L) and filled it up with different amounts of water. Then we added air until full and during this process we measured pressure and the shape of the tank. Results are below. The lines represent different water volumes in the tank.


test graph.jpg


We think the steep (darkblue) curve of the tank without water represents the elastic stress of the tank. Interesting is to see the curve shifting to the left as the tank is filled up with more water at the start. The tank with 8.2/9.5 L has a pressure of 4 mbar as soon as it contains a little bit of gas.

The image below are stills of the tank with 6.2 L water; clearly visible is the lifting of the waterlevel.


video stills 6-2 L.jpg

Stills of the video made from the 6.2L filled tank.


This first test brings up a lot of questions:

  • How is this effect scalable? (does it translate to a 4000-liter tank?) and;
  • Are pressure curves this steep allowable?

Please comment! Any ideas or suggestions are welcome.