Anaerobic digestion (AD) is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen and is widely used to treat wastewater. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich digestate can be used as fertiliser.
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens are able to convert these products to methane and carbon dioxide.
The methane rich gas, commonly known as biogas, can then be combusted in a gas reciprocating engine to power a generator for supplying electricity to the electricity network. In addition to the electricity, a large amount of heat energy is also produced which can be utilised for a multitude of uses. The nutrient-rich digestate which remains after the digestion process is used as fertilizer to grow crops.
It is used as part of the process to treat biodegradable waste and slurry. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.
Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.