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.

How does Anaerobic digestion work?

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.

While there are many ways of building anaerobic digesters (AD) the basic principle takes a feedstock that is fed into a completely enclosed tank, which is heated and regularly mixed

FEEDSTOCKS. Organic feedstocks are very flexible, ranging from farm manures and sewage sludge to catering wastes and food wastes comprising uncooked and cooked food, meat and bone. Feedstocks usually have a high moisture content making them more suited to the AD process. These waste also contain high levels of pathogens such as Salmonella and E.coli. which are dealt with during the process.

PRETREATMENT. Both the law and the AD process require the feedstock to be liquefied - reducing its particle size to below 12mm. This step will also remove unwanted materials such as metals, plastics or stone, and protects downstream treatment processes. Where appropriate pre-treatment also includes pasteurisation at 70°C for 1 hour to irradiate the pathogens.

ANAEROBIC DIGESTION. This is the simple process of creating optimum conditions for bacteria growth. As time goes by the feedstock is digested (eaten) by these bacteria that in turn generate two byproducts - digestate and biogas.

POST-TREATMENT is completed by the stabilisation and sanitation of the solid and liquid digestate through storage before its application to farmland. Further treatment of the solid and liquid fertilisers can produce products such as compost and soil conditioners.

BIOGAS & CHP complete the flow sheet. A combined heat and power unit (CHP) is used to convert the biogas in to electricity and heat. Normally a biogas engine can gain an electrical conversion efficiency of up to 35% with the remainder being available as heat.