The process of Anaerobic Digestion (AD) uses naturally occurring bacteria to break down a wide range of organic matter in the absence of oxygen, hence “anaerobic”. The feedstock breaks down, or is “digested”, in very large sealed tanks. The bacteria give off biogas, which is rich in methane, leaving a fibrous residue called digestate. The biogas fuels a generator producing saleable electricity and heat. Unlike incineration, often referred to as Energy from Waste (EfW), the AD process returns most of the nutrients and carbon in the feedstock to the soil through the soil conditioner, which is the dried digestate. By contrast incineration either destroys nutrients or vents them up the chimney.
The AD process is subject to strict regulation and inspection by the Environment Agency and Animal Health agency, in order to ensure safe operation and no odours. The feedstocks will arrive in sealed vehicles, will be unloaded in a sealed building and will be processed in sealed tanks, kept out of contact with the air until the process is complete. The digestate end product is an inert material similar to peat in texture and smell.
Although the AD process is fairly new to the UK, it is in widespread use on the continent, with more than 3,000 plants operational in Germany alone making a material contribution to national energy supply. The previous government instigated an action plan to raise the useage of AD in the UK. The current coalition government has stated the aim (para 11 of the “Agreements Reached” document of 11th May) of promoting a “huge increase” in the use of AD to generate energy from food waste.
