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Anaerobic Digestion
This method of waste disposal is still a popular choice where the waste to be processed is of a moist and organic nature. The process is quite simple. The waste is loaded into a vessel within a controlled environment. This is known as a digester. The process involve two main stages. The first of these occurs within the air tight digester. After loading the waste microscopic enzymes are used to convert the organic waste in fatty acids, hydrogen and acetic acid. After the process methanogenesis converts the organic acids to methane and carbon dioxide, two of the by products which are produced during an anaerobic digestion process. The gas that is produced during the initial stage of the process is then cleaned to a level where it can be used to generate electricity, while the liquid and solid digestates are used as a form of fertiliser.
Anaerobic Digestion will result in a number of different by-products. The Biogases which are produced are typically around 60% methane and 40% carbon dioxide. These are used for power generation by means of burning them. The bio liquid or liquor digestate are often used as a fertiliser but their suitability for use in this method will depend heavily on the quality of them. The final by product, fibre digestate, is also used as a form of compost. However you look at it, although AD produces some useful by products you will always end up with a by-product which can be un-suitable for use anywhere, leaving the issue, and cost, of how to dispose of it.
Although a good means of waste disposal AD has a number of clear shortfalls. Firstly, you can only process organic waste by this method. Packaging waste cannot be processed meaning the waste producer still has to dispose of this in another way, meaning additional costs to a process like Inetecs abrasive drying. This is not to say it doesn’t have its place in the recycling and renewable energy sector, but it is more suited to applications such as farming where the waste will be solely organic. The enzymes which are used to break down the waste operate at their best when the waste stream is kept both constant and consistent in terms of make up. Something which cannot always be guaranteed. There is also the possibility that if the feedstock is poor quality the enzymes cannot process the waste into soil enhancers and additional composting techniques may be required to complete the process, again adding further costs. A key disadvantage to this method of waste disposal is that in some cases the digestate may not meet with government standards which poses obvious problems. Further more the water which is produced during the process will contain a high level of nitrate which will require specialist water treatment before it can be drained away to foul sewer as an acceptable effluent.
This does not mean that AD plants do not have a puprpose as part of the recycling chain. It is just a limited option and requires other methods to work in conjunction with it to deal with mixed waste streams.