Biogas project, prevention of concrete cracking by working the concrete whilst wet.

Biogas project Update

Biogas project Update

In the May Biogas course, we built some pillars in the biodigester with leftover bricks, on which plywood was placed, cut to size to fit perfectly the diameter of the pit. This would then support soil to form the base on which to pour concrete for the dome.

Bricks to support the dome for the biodigester
Building the bricks to rest the plywood base onto to build the soil dome for the biodigester.

Tom, students and helpers ensured the plywood was stable and then they proceeded to pile soil on top of it, keeping it slightly wet and shaping it as a dome. On top of the soil, sand was patted down so the cement wouldn’t stick to it. Once that was done, they mixed the cement and gently shaped and padded the cement in place, smoothing it out as they went. A good 10 days of cool and damp weather was perfect to cure the concrete without cracks. On the top of the dome Tom needed to build a brick tower to house the gas pipe, which he did after the concrete dome had set.

Soil is put on top of the plywood base to start the dome for the biodigester
Starting to put soil on top of the ply base
Soil is shaped as a dome to form the base of the biodigester's roof
Soil is whetted down and shaped as a dome
Concrete biogas dome with gas pipe, covered with sand to prevent cement sticking
The dome exterior is covered with sand to stop cement sticking to the soil, note the gas pipe sticking out.
Concrete is poured and worked on top of the biodigester's soil dome base.
Bit by bit concrete is poured and worked on top of the soil dome. It is a slow and laborious process to ensure the concrete stays smooth all around.
Biogas biodigester cement dome
The finished cement dome for the roof of the biogas biodigester

Once the cement had dried, Tom had to remove the plywood to drop the soil into the pit, and shovel it out of the confined space. This took most of one day (and a lot of sweat!!).

The next stage of the biodigester is building the inlet and outlet, and if time permits starting on the toilet which will feed into the biodigester as well. This will be done as part of the next biogas course and runs from 19 to 23 August. Please email info (at) permaculturesunshinecoast.org for more information or to book.

5 Comments


  1. Have you produced a how to DVD for this?

    1. No Patrick, unfortunately not, but we will attempt to get all documentation together and publish it. Time, as usual is an issue for us in these things…

  2. Do you have the plans available?
    Or Can you connect me with Bill Mullison who gave you the Biogas plan?
    Or is there someone else you recommend who has plans?

    Lastly, I live in varied temperatures: 100+ in summer and 30/40 below in winter (farenheit)
    Any suggestions for the biogas system?

    Do you use it for heating anything else such as water etc.?

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