Anaerobic digestion (AD) is a natural biological process carried out by bacteria in the absence of air, by which organic material is broken down into stable fertiliser and useful biogas. These anaerobic bacteria are an integral component of nature’s waste management and are commonly found in soils and deep waters, as well as in landfill sites.

The science of anaerobic digestion

The break down of organic compounds is achieved in a soup of many types of bacteria including those that generate carbon dioxide and methane (acetogenic and methanogenic bacteria respectively, if you really wanted to know). These bacteria function at two optimum temperature ranges, 35-39°C (mesophilic) and 55-60°C (thermophilic).

The organic waste is broken down into sugars and amino acids by enzymes similar to those found in our mouths that help digest our food. These are then fermented to produce volatile fatty acids and then converted by acetogenic bacteria in to hydrogen, carbon dioxide and acetate. Finally methanogenic bacteria produce biogas, a mixture of carbon dioxide (40%) and methane (60%) and other trace elements.