Anaerobic Digestion (AD) is a natural process where plant and animal materials are broken down by micro-organisms in an air-tight tank, or digester.
This releases a biogas that can be used to generate renewable heat, power or transport fuel. Biogas is a mixture of around 60% methane, 40% carbon dioxide and traces of other contaminant gases. The Biogas will be upgraded to grid quality gas and injected into the local medium pressure national gas network, where it can be used for heating or as a transport fuel.
One cubic metre of biogas at 60% methane content converts to 6.7 kWh energy. AD also produces a valuable organic bio-fertilizer which can displace expensive mineral fertilizers whilst improving the soil and protecting the environment.
The process of AD occurs in several steps and requires a community of microorganisms. 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). These bacteria function at two optimum temperature ranges, 35-39°C (mesophilic) and 55-60°C (thermophilic).
The organic material 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.