Anaerobic digestion (AD) involves the break down of biodegradable material in the absence of oxygen by micro-organisms called methanogens. It is already widely used to treat wastewater in the UK and can also be used to treat other organic wastes, including domestic and commercial food waste, manures and biofuel crops
Types of AD
There are two main types of anaerobic digestion called thermophilic and mesophilic – the primary difference between them is the temperatures reached in the process. Thermophilic processes reach temperatures of up to 60 degrees centegrade and mesophilic normally runs at about 35-40 degrees centegrade.
AD sites also have to comply with the Animal By Products regulations so a mesophilic site would also have a pasteurisation unit to make sure the end product is safe.
Which method to choose
The system chosen will largely depend on the feedstock to be processed. For example, 'high solid materials', such as a garden and food waste mixture, tend to be processed at a thermophilic temperature using the batch system, while 'low solid materials', such as animal slurry mixed with industrial and municipal food wastes, are more likely to be processed at a lower temperature using a continuous flow system.
The process of anaerobic digestion provides a source of renewable energy, since the food waste is broken down to produce biogas (a mixture of methane and carbon dioxide), which is suitable for energy production. The biogas can be used to generate electricity and heat to power on-site equipment and the excess electricity can be exported to the National Grid. Other possible uses for the biogas currently being explored in the UK include injection to the gas grid and using it as a vehicle fuel.
A further by-product of the process is biofertiliser, which is rich in nutrients such as nitrogen, phosphorus and other elements required for healthy plant growth and fertile soil.