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Anaerobic Digestion - A Rough Guide


Anaerobic Digestion is not a new technology.  It has been in widespread use in other countries (in particular Germany) for many years.  It is, however, relatively new to the UK, and there are a lot of misconceptions about the process.  


AD consists of a process by which organic matter (which might be food waste, animal waste, or vegetable matter including crops of all types) is broken down by enzymes and bacteria in a process involving no oxygen. During this process, which has several stages, one of the major products is biogas, which consists approximately 55% of methane, and most of the remainder is carbon dioxide.  Methane is one of the single largest contributors to our greenhouse gas emissions, and the AD process prevents it from being released into the atmosphere.




















For the avoidance of doubt, the plant at Icknield Farm does not use any food, garden, domestic or industrial waste, ever (these require an entirely different and more complex treatment process including pasteurisation).  The only feedstocks for the plant are slurries and crop exclusively from local farms.


A small amount of the biogas produced by the anaerobic digestion process at Icknield is used in an untreated form to run a combined heat and power engine ("CHP"), which provides heat and power for the plant itself.  The majority of the gas goes through a process of purification, extracting the CO2 and any traces of sulphur and other impurities, leaving just biomethane, to which can then be added a small amount of odorant before it is injected directly into our gas grid, where it is used in just the same way as all of our regular natural gas, for cooking, heating etc.


After digestion, the residue organic material, known as digestate, can be used in both solid and liquid form as a highly nutritious organic fertiliser, thereby reducing the need for expensive and potentially harmful non-organic chemical fertilisers.  The digestate has the added advantage that for input materials such as animal slurry, that would otherwise be spread in their untreated (and potentially very smelly) form onto a farmer's field, the liquid digestate has been effectively de-odoured, meaning that when spreading does occur, there is far less smell in the local environment.


One of the myths about AD plants is that they are both noisy and smelly.  Neither is true in any meaningful sense for the Icknield plant. Whilst it is true that the plant operates 24 hours a day, the 'noisy' parts of it (the CHP engine and the compressors for the gas clean-up equipment) are housed within heavily insulated containers, meaning that the level of external noise is minimal.  As to odours, the entire system is sealed, including for the residue storage, and so there is very little odour other than the standard farmyard smell of silages (maize and hybrid rye) which are stored on site in silage clamps.  By way of affirmation, the nearest dwelling to the plant at Icknield is that of Guy Hildred himself!


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