Anaerobic digestion (AD) is a natural biological process carried out by bacteria in the absence of air, whereby organic material is broken down into stable fertiliser and useful biogas. DGH is currently advising on facilities ranging from 8m3 to 27m3 biogas plants, fed by the manure 4-100 cows in Kenya, to regional facilities taking a variety of waste streams in the UK.
The breakdown of organic compounds is achieved via a mix of many types of bacteria, including those that generate carbon dioxide and methane. The waste is broken down into sugars and amino acids by enzymes similar to those found in our mouths that help digest food. These are fermented to produce volatile fatty acids and then converted by acetogenic bacteria into hydrogen, carbon dioxide and acetate. Finally methanogenic bacteria produce biogas, a mixture of carbon dioxide (40%), methane (60%) and other trace elements.
While there are many ways of building anaerobic digesters, the basic principle takes a feedstock that is fed into a completely enclosed tank and regularly mixed.
Feedstocks: the type of organic waste used as AD feedstock can vary greatly, ranging from farm manures, human waste and sewage sludge, to catering wastes, vegetation and food wastes. Typically, the feedstocks usually have high moisture contents, making them more suited to the AD process. These wastes also contain high levels of pathogens, such as salmonella and e-coli, which are dealt with during the process.
Pre-treatment: both the law and the AD process require the feedstock to be liquefied to reduce its particle size to below 12mm diameter. This also removes unwanted materials, such as metals, plastics or stone and protects the downstream treatment processes. Where appropriate, pre-treatment also includes pasteurisation at 70°C for 1 hour to irradiate the pathogens. In many ways, this is often the most difficult part of the process, i.e. to produce a homogenous material suitable for the digesters.
Digestion: this is the simple process of creating optimum conditions for bacteria growth. As time goes by, the feedstock is eaten by the bacteria, which then generates two by-products – digestate and biogas.
Post treatment: this 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. In the UK, this material is usually sent to farmland or used in restoration. In Kenya, AD plants produce a fertiliser that is almost pathogen-free and is a much better than the raw manure traditionally used.
In addition, there are many health and social benefits to the villagers, with a safer fertiliser, greatly reduced numbers of flies and a healthier cooking fuel. The gas produced from the AD plant burns more efficiently than the smoky, three stone open fires commonly-used for cooking and, using gas greatly reduces the amount of local deforestation.
The larger AD facilities use the biogas either in a boiler to produce heat, or in an engine to generate both electricity and heat. The electricity produced is classed as renewable energy and receives significant financial incentives if produced.
For further information on AD uses and projects in either the UK or overseas, contact David Hooper