How does a biodigester work?

Biodigesters are the environmental technology that everybody wants to know about, but few know how to. Certainly. the implementation of a biodigester requires more knowledge and money than composting and to operate it efficiently you need to know a bit more than the basics. Therefore in this blog, I'll teach you how to build a small one where it will be easy for you to learn the basics and gain the confidence to build a bigger one.

The methane generated in a biodigester concentrates a lot of energy which can even be used to generate electricity

A biodigester is an anaerobic chamber where several groups of bacteria oxidise the organic matter into carbon dioxide and methane. This means that inside a biodigester, there are different types of bacteria waiting for you to feed them with manure, food waste, fats, wastewater or even cannabis (for real!, soon I'll show that research here) to produce biogas which is a flammable gas that contains methane.  As you might be guessing, or already know, the coolest thing of this process is that this biogas can be used in your kitchen to substitute the natural gas you normally use. One key aspect of this process is that oxygen should not enter the biodigester. Well, actually, there is always a tiny small amount of oxygen inside but it does not interfere with the whole process. Just remember, no oxygen allowed.

Most publications describe two system designs for biodigesters:

1 - Batch: This is basically, just feeding once and leave it for fermentation. This is only useful to generate the digestate which can be used to fertilise plants.

2 - Continuous: Under this system a biodigester is fed constantly and a continuous production of biogas and digestate is possible.

So, let's start with the batch type. Here is a picture of the simplest biodigester ever:

Batch system

So, as you can see in the picture, a batch system can be made by joining two water drums with a gas hose. The water drum to the left is filled up to 3/4 of its volume with manure and water. In the image I used one part of cow manure and two parts of water. Then, this water drum is sealed with a stopper that has a small tube attached to hose whose extreme is inserted into water. So, instead of the water drum to the right you can just use a bucket with water. The fact that the hose is inserted into water is to prevent the air from entering into the hose and reach the water drum to the left. One day after the system is set up, you will be able to see bubbles coming from the water. Those bubbles are the biogas which is making pressure through the water column to escape; however, this biogas is mainly carbon dioxide which is not flammable. More time is needed for the methane to appear, and burn.

Biogas escaping from water

I suggest to keep running this system up to one month, after which, most of the biogas will already be released. Of course you can also capture the biogas in life guard rings, but the amount will be so small that it is not worth to use it. So, if the biogas is lost, what is this system for?

1 - To produce digestate: After one month you will have a digestate that can be used to fertilise the plants in the garden or any crop you have (except crops whose leafs are eaten). Do not worry, it does not have a bad smell. You can dilute it with water (one part of digestate and two parts of water).

2 - To produce inoculate: The digestate generated is also a good inoculate because it is rich in methanogenic bacteria.

As you can see this system is very simple, but do not look at it over the shoulder, you may want to try this system first because you will need the inoculate to produce biogas in your future biodigester. You can give it a try using dog poop. In the next post we will see how to make a continuous system.