Biogas is produced by anaerobic bacteria that degrade organic matter in four general stages:
1. hydrolysis,
2. acidification,
3. acetic acid production, and
4. methane production
The gas phase product of anaerobic digestion is named biogas and its yield depends significantly
on the substrate or whatever raw material you used.
Unlike in cow where it has stomach divided physically into few compartments, we build a one
stage anaerobic digester (4 in 1) which has invisible compartments as you can image at the
lowest stage is where hydrolysis takes place then the next upper level is where acidification is
going on and so forth until to the top level on the surface where the biogas is produced and
released.
You can imagine in reverse as below:
4. Methanogenesis - acetic acid and methanoic acid turn into
methane and carbon dioxide
3. Acetogenesis - volatile fatty acids turn into acetic acid and
methanoic acid
2. Acidogenesis - organic molecules turn into volatile fatty acids
1. Hydrolisis - complex organic matter (eg. food waste) turn into
simpler organic molecules
So, at initial stage when you add food waste as raw material at pH of 3, 4 and 4.5 or so there
has been no biogas produced yet. The best range to observing methane production is in the
range of pH 6-8.
The lack of gas production at low pH was attributed by total inhibition of methanogenic
activity. While excessively alkaline conditions might trigger the disintegration
of microbial granules and the consequent collapse of the digesting process.
Usually the pH of a stable digester is self-adjusting in which you do not have to intervene.
Digester will become unstable if you feed over its capacity to handle the digestion process.
There is a parameter ratio to consider while feeding between the temperature you are
operating, size of digester and volume of raw material you feed into it.
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