Biogas :: The making of ...

A few weeks ago I went to a Biogas workshop which was held in an area not far from us. Here we learnt how to make a bio digester using commonly available materials.

For those of you who may not be aware, a Biogas digester is a device that is used to generate biogas (a mixture of methane, CO2 and hydrogen sulphide) from organic waste material. Biogas can be used for cooking, heating, lighting or even running a heat engine to generate mechanical or electrical power. About 2kgs of organic waste material is required to create about 1m3 of biogas. This could run one average size cooking burner for about 3 hours. It depends on various factors - some waste streams can generate far more gas than others, and some anaerobic bacteria better at producing methane than others. For the sake of this article, the above approximations hold roughly accurate.

The digestor has two main components, the digesting compartment (bottom tank) and the gas collector (top tank). In some cases these two tanks are combined into a single tank with a seperate gas collection chamber.

The gas is generated in the process of breaking down the organic material. It  bubbles up in the digestor tank and is collected in the top tank. As the gas collects it displaces the water out of the top tank.  This forces it either to rise up or (if it is fixed in place) will force the water down. The maximum pressure achievable under this design is the height of the water displaced. Once all the water is displaced, the biogas generated will just bubble out the sides of the top tank and escape into the environment.

Although methane is many times worse a greenhouse gas than CO2, it is important to note that decaying organic material in low oxygen concentrations already generate this gas. The process of harnessing this gas and burning it changes this methane into carbon neutral CO2.

The steps to making a biogas digester is as follows:

First get 2 plastic water tanks, one larger than the other. In this case one was a 1000L and the other was a 2000L tank.

Cut the top off the smaller tank first, this will be the gas collector of the bio digester.

Invert the smaller tank over the larger one and mark out the perimeter with a marker. Try to get this as central as possible so it doesn't look weird.

Lay the larger tank on its side and cut out the hole that has been marked. You can utilise power tools or manual alternatives - plastic isn't difficult to work with. In this case pilot holes were first drilled and then a jigsaw used to cut along the line.

Now that the hole has been cut in the larger tank, a hole saw is used to create two large holes at the bottom of the large tank. Standard PVC drain pipes are used to create the inlet and outlet points as shown below. The hole saw size is chosen to a specific diameter that is required for the rubber seal to keep things tight and water proof. The workshop attempted to utilise commonly available materials and fittings to keep costs down.

Note the use of a plug-in powered drill (rather than a cordless one) - this is because the hole saw is sizable and requires a significant amount of grunt to work. Careful however when the saw first catches on the plastic as it can twist the drill significantly and may injure your arm if you're not expecting it.

An elbow with a screw on side cover outlet is used in the event that a blockage might need to be cleared.

An additional valve is added in the event that lower tank needs to be drained for whatever reason.

Here you can see the inlet pipe complete. The top of the pipe is fitted with a standard drain funnel that helps when adding new waste material in. The waste is mixed with water in a 1:1 proportion, so it flows well and maintains a good mix of water in the system.

The process is repeated on the opposite side of the tank to create the outlet pipe, shown here. The liquid that exits the tank (called the digestate) is highly nutritious for plants, having ideal NPK (nitrogen, phosphorus and potassium) ratio mix that is very compatible for direct absorption by the plant's roots.

The smaller tank is also prepared with two small outlets at the top, again using standard PVC (irrigation) fittings. The gas that exits the top of the digestor is at very low pressure (no more than 2-3m of head of water), the pipe can easily do 30x that pressure. The reason why there are two outlets in this case is to stop the smaller tank falling into the larger tank, which apparently is very difficult to get out again - the elbow fittings stop this from happening.
The larger tank is then carried to the site where it will be placed. It is then filled with garden waste and inoculated with anaerobic bacteria to help get the process started. This tends to occur naturally given enough time, however seeding the tank with this bacteria means the tank can quickly startup operation and become usable. Water is filled into the tank and the smaller gas collector tank is placed into the larger tank.

A great group effort!

The digestor gets its first feed and seeded with anaerobic bacteria. The inoculant was brought along by the guy running the workshop. It basically contained the digestate material from one of his other digesters, plus some secret herbs and spices?? He did suggest that bacteria used in sour dough could also be used.

Just add water ...

And finally, the smaller tank used as a gas collector goes on the top. Water is added via the inlet pipe until the tank is full. Note how the plastic elbows stop the top tank dropping into the bottom tank.
You will note that in this case, the bottom digestor has a diameter that's quite a bit larger than the top gas collector. This is not ideal as there will be side leakage of the generated gas. It is the price one has to pay for using standard available materials to do the job - rather than custom making the tanks. The gases escaping this particular digestor is however a small percentage of the overall gas collected. This setup uses a floating tank system, so the top tank will rise up as more gas is collected - and drop down again as the gas is used - so it is a bit like fuel gauge. Placing some weight on the top of the tank will pressurise the system if the gas needs to be delivered a longer distance.

There will be a follow up workshop at some stage in the future where we learn about how this digestor is performing, and perhaps even cook something on the BBQ from it.

I look forward to it ... :)

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