This week in class we looked at a video produced by EARTH University about biodigesters. We will be helping to install biodigesters on area farms as part of the service learning component of the Costa Rica trip, so I was very curious to see what they looked like.

To my surprise, the way they make biodigesters in Costa Rica is by using large spools of heavy plastic. The plastic is double wrapped, with a release valve installed on top. All the photos of biodigesters I had seen in the United States show hard plastic chambers or even wood, and they can be quite expensive. But these biodigesters will be much cheaper, easier to make on site, and more flexible.

Basically what a biodigester does is act like a stomach to digest waste, creating natural gas that people can use for cooking or heating. This is useful in developing countries for two reasons: one, they have a place to put waste such as animal manure besides rivers and streams, and two, the gas produced on site means they don’t have to buy and bring in propane from the outside.

Anaerobic digestion, or digestion in the absence of oxygen, happens in four stages of chemical reactions:

• Hydrolysis
• Acidogenesis
• Acetogenesis
• Methanogenesis


In hydrolosis, bacteria break down large organic molecules from manure or other waste into soluble solutions that other bacteria can use. Those bacteria then convert the sugar and amino acids into carbon dioxide, hydrogen, ammonia, and other organic acids. Next, still more bacteria convert these substances into acetic acid along with more ammonia, hydrogen and carbon dioxide. Finally, it is converted into methane and carbon dioxide, which can be used as a form of energy for cooking and heating.

Here’s a YouTube video that shows what the biodigesters that we will be making look like. I am looking forward to it!