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 proposed site at Icknield Farm will not be using any food, garden, domestic or industrial waste, ever (these require an entirely different and more complex treatment process including pasteurisation).  The only feedstock for the plant will be slurry and crop exclusively from farms.


The biogas produced by the anaerobic digestion process can either be used in an untreated form to run a combined heat and power engine ("CHP"), or it can go 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 propane before the mix is ready to be injected directly into our gas grid, where it will be used in just the same way as all of our regular natural gas, for cooking, heating etc.


The first of these two processes is the most common in AD plants, resulting in the generation of renewable electricity which can be exported into our electricity grid.  The CHP engine also produces a large amount of heat, and in many cases this heat is simply wasted.  Consequently, up to 50% of the energy in the biogas may be lost.


In 'Gas to Grid' plants such as the one contemplated at Icknield Farm, the full energy value of the biogas is maintained, making it the most efficient form of usage for an AD plant.


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. Whilst it is true that the plant will be operating 24 hours a day, the 'noisy' part of it will be the CHP engine (and, in the case of a gas to grid plant, the compressors for the gas clean-up equipment), and all of these will be 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 silage when maize, grass or whole crop silage is one of the feedstocks for the digester, necessitating silage clamps to be constructed.  By way of affirmation, the nearest dwelling to the proposed site is that of Guy Hildred himself!




Visual Impact


AD Plants do not have to be an eyesore.  Most of the equipment has the look of agricultural buildings, and if sympathetically sited, and properly landscaped, they can even enhance the visual aspect of a location.