The four stages of anaerobic digestion
Knowing the four stages of anaerobic digestion will allow you to explain
why biodigesters fail when they are overloaded,
how to fix this issue, why biogas do not
burn in the beginning and many other things. So let's start:
In the early years of anaerobic digestion research, Omelianskii (1906)
described the Methanobacillus omelianskii as
the responsible bacteria for converting organic matter into methane. By this
date, methane was a well-known gas
because people related it with the awareness of
not lighting a fire in swamps. Anyway, later in 1967, Bryant found out that
the Methanobacillus omelianskii was
not a single bacteria, but an association of two microorganisms. Indeed, two types of bacteria participate in
the conversion of organic matter into methane and carbon dioxide: acidic and
methanogenic bacteria.
The following stages occur all at the same time; however, they are
called stages because the products
released by bacteria in one phase is substrate for the bacteria participating
in the next one.
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| Source: Van Lier (2014) |
Hydrolysis: This phase is slow and
consists in the breakdown of all the big and complex molecules (polymers) into
smaller units (monomers and oligomers). This breakdown is done by enzymes released by hydrolytic
bacteria. Proteins are broken down into amino acids,
fats into long chain fatty acids, and sugars into monosaccharides.
Acidogenesis: This is the quickest stage. Here, acidogenic bacteria
convert all the monomers and oligomers into volatile fatty acids (VFA). During the
process carbon dioxide is released. Because carbon dioxide does not burn, a
non-flammable biogas means this stage is prevailing in the biodigester.
Acetogenesis: Some authors combine this stage with Acidogenesis. In this
phase, volatile fatty acids are converted
into acetate. Carbon dioxide and hydrogen are
also released during the process.
Methanogenesis: Methanogenic bacteria are very slow to duplicate which
is a disadvantage compared with acidogenic bacteria which need few hours to replicate,
methanogenic require days.There are many ways in which methane is formed, but two routes prevail. The biggest
part of the methane comes from the acetate (70%), and the rest of the carbon dioxide and hydrogen conversion
(30 %).
The main aspects from the fours stages of anaerobic
digestion are:
- All these phases occur at the same time in the biodigester
- Acidogenic bacteria duplicate quicker than methanogenic bacteria
So, we can now easily explain what happens in a Batch System. Once the substrate is mixed with the water inside the biodigester,
the different types of bacteria that the substrate contains start degrading the
organic matter. In the beginning, hydrolysis will prevail because complex molecules mostly compose substrates like
manure. However, there are also simpler molecules which are substrate
for acidogenic bacteria. The release of carbon dioxide from this stage is the
main constituent of the biogas after the first day of digestion. As days pass
by, most of the complex molecules of the manure are already broken into simpler
molecules, and acidogenic bacteria start
processing them. Methanogenic bacteria are also working since the very beginning
because there is also acetate in the manure, but as days passes by, more
acetate, carbon dioxide, and hydrogen are
formed, and more methanogenic bacteria
are produced to deal with them to generate methane.
How do you think the biodigester will work if you feed it daily? Don't
you think a non-flammable biogas is more likely to be produced if acidogenic
bacteria have an advantage over methanogenic bacteria because of the duplication rate) Well,
we can avoid this by regulating the amount of feed we put inside the biodigester.
If we feed the biodigester with moderate
amounts of substrate, then methanogenic bacteria can reach an equilibrium with acidogenic bacteria and a flammable
biogas (rich in methane) will continuously
be produced. Now, you may be thinking how much is low and how much is high? The anaerobic digestion parameters (HRT and OLR) will
help us with that, but we will see this in another post.
References
Van Lier, J. 2014. Anaerobic Fundamentals and COD Balance. TU Delft. [Retrieved from https://www.youtube.com/watch?v=Yr6LIu379Yw&list=PLrwuNGSwGLHcrfxlTiJ2zTGWi6-fEZV8m&index=5]
