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What is the Anaerobic Digestion Process?
Watch our video slide show below and we think that you will in less than 4 minutes have learnt the answer to the question of “What is Anaerobic Digestion?” Just click on the arrow in the centre of the screen to start the video, and give it a moment to start.
Is this a new process?
When the term “anaerobic” is used it may seem to the non-technical among us that science has conjured up a new form of digestion. Of course, this is not the case.
Methane bacteria are everywhere, and always have been. They not only occur widely in the bottom sediments of ponds, marshes, lakes and swamps, but much closer indeed in the intestinal tracts of man and many animals (especially cattle and other herbivores). Methane bacteria are usually found in association with sulphate-reducing bacteria.
Why are they all about us, you might ask? The reason is that they are scavengers, removing the decomposition products of other anaerobic bacteria and converting them to a mixture of carbon dioxide and methane gas. This mixture is that we call "biogas".
What happens in an MSW Anaerobic Digestion Plant
Refuse collection vehicles (RCVs) deliver the collected waste to the plant and the degree of sorting then applied varies. Source separated garden and food waste often can go straight into the process, but mixed residual (‘black bag’) waste needs sophisticated sorting mechanically to remove the non-biodegradable contaminants. The plant in which this sorting is done is called a Materials Recycling Facility (MRF).
Sorting may involve screens, rotating drums for segregation, air classifiers, and powerful magnets. The organic waste fraction is then shredded and usually mixed with water. The waste and water slurry is then pumped into a sealed vessel where it is heated, and stirred where the it stays for up to about 3 weeks. This is known as the digestion or fermentation stage.
During this period the bacteria digest the waste and create a gas comprising of about sixty percent methane with the remainder being mostly carbon dioxide. This can be used as the source of the heat energy to warm the digestor(s), and there is usually sufficient methane left over to power an electricity generation set.
The process is normally continuous and filling and removal of the treated material takes place simultaneously. The output takes two forms. There is a solid digested material (digestate) which is often pressed to reduce the water content. The solid digestate is fibrous and can be used as a soil improver once it has been further matured usually by being placed in piles to aerobically compost, further reducing its weight, for about two weeks.
The digestate is very similar to compost once it has stood in the air for this period.
Unfortunately, even for most source segregated wastes there will be foreign matter, especially plastics etc, in the matured digestate. So, additional sorting is usually be required to remove contaminates before it can be used, and the most common is the use of a small mesh size screen.
The liquid fraction can be re-circulated back into the process, but in almost all process designs some excess water is generated. And depending on the removal of, or avoidance of, the presence of possible infectious agents from the feedstock, this can be used as a fertiliser. If the waste source was classed as contaminated (eg food waste) and the waste is not then pasteurised within, or after, the digestion stage the resulting liquid product cannot be used on the land and has to be disposed of to sewer.
What is the Chemical Equation?
The reactions involved can be represented as follows:-
CO2 + 4H2 > CH4 + 2H2O (e.g. Methanobacterium bryantii)
CH3COOH > CH4 + CO2 (e.g.Methanosarcina barkeri)
Syntrophic bacteria such as Syntrophobacter wolinii and Syntrophomonas wolfei provide the methane bacteria with their food supply via the reactions which convert propionate and other volatile acids into acetic acid and hydrogen gas.
Other supplies of food to the methanogens are other anaerobic (food decomposing) bacteria which decompose complex organic substances into volatile fatty acids.
Thus, methane bacteria provide the ultimate waste disposal system for a whole community of bacteria that grow together to provide the biological basis for what has become known as the "anaerobic digestion process", which occurs naturally in sludge digesters at sewage works and in damp putrescible deposits in landfill sites.
Methane production also takes place in the bottom sediments of ponds, marshes, lakes and swamps, and phenomena referred to in old accounts such as the “will-o-the-wisp”, no doubt originate from methane from such sources.
Effectively the anaerobic digestion process has been devised simply to "engineer the inevitable" and allow this natural fermentation to proceed to completion before discharging the fibrous "digestate" to landfill or to a "composting plant" further mixing and decomposition can take place to make the digestate suitable for agricultural, and horticultural applications.
