How to Build a Biogas Plant in your Backyard

Anybody who generates a significant amount of bio-waste, and especially those among us with country farms, often have one single dream- and that is to have their own DIY biogas plant.

The reason why not so many have succeeded in building it, however, is because it is way too easy to overestimate the dimensions, and consequently fail to make a functioning one.

This is why we felt it is important for us to give this simple step-by-step tutorial and help many realize that ultimate dream of theirs. The instructions are kindly provided by the Nepal Biogas Support Programme. If followed correctly, the result is a 2047 design GGC model biogas plant, which can easily appear in your back yard. So, let’s begin.

1. Sizing the plant

As it is with any DIY project, before acquiring materials and beginning construction, one should make sure they know exactly how big the biogas plant should be in accordance with the amount of dung that can be placed in daily. Of course this depends on the number of cattle you have, but it is safe to say that if you have more than 2 animals, then you will have a sufficient amount to input.

To help you make this decision, you need to divide the amount of dung that you can feed into the plant every day by six. The result would be the capacity of your plant in cubic meters. To have a nicely functioning GGC model plant, it is recommended that you try to make something in the range between 4 and 20 cubic meters. It is easy to assume that 2-3 animals can run a 4 cubic meters plant, while more than 14 are needed for the 20 cubic meters plant. However, to avoid any miscalculations, it might not be a bad idea for you to collect some dung for a week and see what amount your cattle actually produces.

The exact dimensions of your DIY biogas plant and each of its components are summarized in the table on the left hand side. Each component is represented by a capital letter, which is then shown on the figure below (step 3 to be exact) displaying the plan for the general biogas plant.

Now, once you have calculated and estimated everything, it is time to review what is needed.

2. Materials

Again, as it is with everything, a huge recommendation is to get the best quality materials you can find (and afford, of course). The better the materials, the better the quality of your biogas plant, and therefore, the better the efficiency.

You will need fresh cement, clean sand, medium size clean gravel (not larger than 2 cm in diameter), clean water, bricks, and stones. You will also need gas pipes (the shorter the better to reduce costs and gas leakage), a shovel and the necessary spatulas for mixing and spreading the cement, a peg, a cord (the length should be the radius of the digester), and emulsion paint coating. The exact dimensions and a complete list of all materials are shown on the image on the left hand side.

Do verify the quality and cleanness of your materials, and if there are impurities or imperfections, remove them. Make sure your cement has been properly stored, the dirt from the sand and the stones is carefully wash off, and the bricks have been soaked for a few minutes before usage so that they do not soak moisture later on.

3. Site selection

OK, yes, we said “the backyard”, but there is a bit more to this. A few considerations should be kept in mind if the project is to succeed in being efficient, safe and of course non-polluting.

First, pick a sunny site, so that you ensure easy maintenance of the right temperature. Try to locate it as close to the source of dung as possible and make sure it is also in a close proximity to a water source, to avoid any waste or unnecessary long transportation of water and input material. Ideally, the plant should also be located near the point of use, so that it is easily reached and operated.

In addition, make sure that it is located at least 10 meters away from any wells or drinking water sources, as a precaution against possible water pollution. To avoid damage, try to construct the plant at least 2 meters away from the closest house or building.

4. Digging

This is where the fun part begins and we can finally start getting our hands dirty. Stick the peg in the center spot of where the digester will be. Attach the cord to it and carefully stretch it, outlining the dimensions of the pit.  Start digging, and try to keep the walls as straight as possible, and place the removed soil at least a foot away from the layout.

5. Build the Round-wall and the floor

Place the 0.5″ gas pipe vertically in the center of the newly dug pit, and secure it with a horizontal pipe, or a pole. Attach a wire to the vertical pipe to measure the exact dimensions of the digester. Then you can begin laying the bricks, making sure that the bricks in the first row are positioned on their sides, while all the rest are positioned on their lengths. Once the round-wall is complete, plaster it with a layer of cement mortar (1:3 cement to sand). The floor can be made of bricks, or stones, also plastered on top with cement mortar. Last step here is to fill the gap between the backside of the wall and the earth, to prevent cracks due to pressure of earth.

6. Build the Dome

First, make a mould for the dome by filling the pit with earth. Check the dimensions by marking the vertical center pipe, and fill with soil up to that mark. Once this is done, remove the long pipe and replace it with a shorter 0.5″ one. Use the template to construct the dome ensuring that it is placed properly and it touches the round wall evenly everywhere. In addition, make sure that the earth inside the mould is well compacted and damp. Then, gather all necessary materials, and start casting using fresh concrete. Do this quickly and efficiently, wasting as little time as possible.

Keep a close look on the thickness of the dome, and leave the small pipe in the top until you install the main gas pipe. Along the process of casting, protect the concrete from strong sunlight using bags or straw mats, and keep these there for at least a week. Once this time has passed, remove the mould through the manhole and them remove all earth from inside. Wash the surface and apply plaster coats on the clean surface, allowing a day in between coatings.

7. The outlet tank

The most important part here are the dimensions. Check carefully both the table and the plan above and ensure that you have everything accurately measured, because this step determines the capacity of the gasholder.
Excavate behind the manhole and making sure that the earth behind it and under the outlet floor are very well compacted to prevent cracks. Level the outlet floor and walls, and finish the walls by applying a smooth layer of cement plaster. Support the wall on the outside with sufficient amount of soil.

The biogas tank should be constructed a bit higher than the surrounding. This is also the time when the dome casting and the concrete slabs should be finalized. Make sure that all slabs have the right size and thickness, the surface is flat and clean, and the concrete is properly compacted. All these can be crucial and prevent an unwanted accidents and collapses.

8. The Inlet Pit

This is the part where water is mixed with the dung, and it is entirely up to you whether you would like to include a mixing device. Although it is recommended to include one, it is definitely not essential.

A few things to consider. The top of the structure should not exceed a meter from the ground in height, and the inside and outside of the pit should be carefully covered with plaster. The bottom of the tank should not be placed lower than 5 cm over the outlet overflow level. The inlet pit should be round, regardless of whether there is a mixing device or not. Here you can also choose to attach a toilet to the plant for giving that little bit of extra boost to your biogas generation.

9. Pipeline lay-out

Here it is very crucial that you only use the highest quality possible light galvanized iron pipe so that you minimize any possible damage that could be caused by people or animals. If possible, bury the pipe at least a foot below ground level for extra security. Seal all fittings with zinc putty or taflon tape, but avoid fittings all together whenever possible.

Along the process of biogas production, condensation forms on the walls of the pipeline, which should be properly taken care of to prevent clogging, Therefore, make sure you place the appropriate drain for the pipeline vertically below the lowest point.

Make sure that you check all joints, connections and taps as soon as your plant begins production. This is best done using soap solution.

10. Compost pits

This is the final component of the plant. You would need at least two compost pits dug near the outlet overflow so that all slurry flows right into them. The volume of the compost pits should be equal or larger than the volume of your plant.

To ensure safety and proper insulation, place at least 40cm thick layer of compacted earth on top of the dome.

If you followed all steps correctly, you should be able to enjoy a fully functioning, high quality biogas plant for many many years to come.

Images (c) Sundar Bajgain