The following extracts resulted from a query about the biology behind biogas, thanks for your contributions Gasan.

On 23/03/2011 07:49, Gasan Osojnik wrote:

I do not wish to engage nor in a lengthy philosophical or physiological debate, but I do have one or two points to make:

1. Methanogenic archaea do not degrade sugar or even complex substances, they use either acetate or hydrogen + carbon dioxide to survive. They are old an primitive organisms, that originate back to the beginning of life, even before glucose was formed by other organisms, therefore they can feast on very basic energy sources. There are not any other "methanogens" in other branches of the evolution tree.
2. The stechiometric ratio of methane/carbon dioxide fromation from carbohydrates is CH4/CO2 = 50/50, from fats = 62.5/37.5 and from proteins 71/29 (due to absorptive properties of the sediment), so the number mentioned is presumably based on anaerobic microbial protein degradation?
3. The chain of microorganisms is not only highly likely, but is confirmed by the means of certified analytical techniques, such as the techniques of molecular biology and can be even seen under the electron microscope. The sole biochemistry and the termoenergetics of the methane formation process from polymers reveal, that it is impossible for the process to start and finish in only one type of unicellular procaryotic microorganisms (or any other). We have pictures of microorganisms of species that are literary "glued one another" for better substrate / intermediate exchange, and this is no exception but a necessity for their survival. Currently it is believed that around 800 species are involved in the biogas formation community (not all at the same time) but this number is increasing rapidly (e.g. 2008 this number was around 400). Personally I believe this number to be much greater, as methanogenic microbiota is found on very diverse parts of the planet and is a common way of surviving in areas with no / low oxygen concentrations.
4. The issue of CO2 which has ben adressed needs some basic insight in the process. The dissolved co2 that is produced intermediately in the proceses of acetogenesis (some also in the hydrolysis ans acetogenesis) is, as said, a substrate for the production of methane, and is taken up very rapidly by the archaea. Therefore, you should not look at the intermediate CO2 as a product but as a reactant. As most of the biogas (at least up to 70%) is formed via acetate decarboxylation to methane and CO2. The partial pressures of surplus CO2 equilibrate in the headspace of the reactor and the liquid, so the CO2 that you get in biogas is actualy mostly the product of acetoclastic methanogenesis.

BR, Gasan

He then provided the following References

Here is the full sources list for the basic anaerobic degradation described on 23 Mar 2011 in the topic "The biology of Biogas Production".

(i) As already mentioned, basic biology for engineering purposes (encluding the biochemisty of the biogas formation from various sources) is explained in:

Deublein & Steinhauser's Biogas form waste and renewable resources (Willey, 2009)) and many other pollution engineering handbooks.

(ii) The microbial syntrophism explained (interspecies hydrogen transfer), essential for biogas production, is explained in e.g.:

Schink, B (1997) Energetics of syntrophic cooperation in methanogenic degradation. Microbiology and Molecular Biology Reviews, 61 (2), 262-280,

(an example was explained already in 1967 by Bryant et al., Methanobacillus omelianskii, a symbiotic association of two species of bacteria.

Arch. Microbiol. 59:20.)

(iii) Regarding the biodiversity in the reactor, the number 800 species was given recently at a lecture from prof. Romana Marinsek-Logar from UL. In a recent publication, Jaenicke et al (2011), Comparative and Joint Analysis of Two Metagenomic Datasets from a Biogas Fermenter Obtained by 454-Pyrosequencing, the taxonomic profile of the biogas producing community is updated with several new genera (s.a. Streptococus and genera in the Firmicutes phylum), altogether 40 genera was identified. Furthermore, I'm am anxios to see the outcome of the biogas-producing microbial community sequencing (Martin Wu, UC Davis & DOE JGI), a project which is bound to produce interesting results.

(iv) General facts about Archaea etc. can be found in Brock's Biology of Microorganisms and in Bergey's Manual .

(a.d.) Also, i have come across an interesting educational cartoon describing the biological process of biogas formation . It is informative and has one or two slips (e.g. in the animation water is added after the hydrolysis, where in fact it has to be present before the process for the decay of complex molecules), but still very nice to see.

BR, Gasan Osojnik

and on 30/6/2011

The biochemistry of biogas production dictates that CO2 and CH4 must be produced simultaneously to achieve stable operation. The product CO2/CH4 ratio is governed by the type of substrates used. There are two main pathways (also some others that are currently regarded as exceptions & less important) of biochemical pathways, that result in biogas:

- acetoclastic pathway (acetic acid is decarboylized to methane and carbon dioxide)
- hydrogenotrophic pathway (hydrogen and carbon dioxide are utilized to form methane alone)

As both these processes are are mutually dependable on the other one, CH4 and CO2 must be formed together, or none at all. An disturbance in the either one of these processes results in the condition which I'm sure we are all familiar with, t.i. the acidification of the reaction. The amount of H+ in the broth becomes easily toxic to acetoclastic methanogens, therefore in a stable process, all the H+ must react with the dissolved carbonate, and the carbonate that remains is therefore the source of final CO2 in the biogas.

Further on, regarding the CO2 fixation from the gas phase above the fermentation broth (e.g. in digesters equipped with floating drum, inflatable cover and similar); the methane ration in the "headspace" does not affect the level of methane production, as the amount of the methane that is made is governed by the biochemical properties of the substrate. Also, methane is poorly soluble in water , so there is no physiochemical mechanisma that would neable the microorganisms to "know" what the methane content in the produced biogas is.

Interesting mechanism described by Khrishna, are you refering to the CO2 fixation from the gas phase (biogas) or from the liquid phase (fermentation broth itself)? [in another post, ed]

I remember some systems were proposed, where the CO2 formed due to methane combustion in the cogeneration plant would be purged through the fermnetation broth to achieve higher methane yields, but I have no knowledge if any commercial applications of this exist.

Also, I think, there is the general problem of carbon dioxide that must be addressed, as we must start to view the produced CO2 as a product and not as an impurity. A vast amount of biochemical energy is used to produce carbon dioxide in the biological process, so by utilizing the produced CO2 rather than discarding it, the energy efficiency of such processes could be considerably increased. E.g. the biogas plants can be could be coupled to greenhouses to enhance the productivity of plants. Similarly, there are attempts being made, to flue gases from biogas co-generation units to feed the algae plantations. Further on, a wast scientific community is quite successful in researching new ways to catalytically transform ch4 and co2 to synthesis gas or similar intermediates, that can be further transformed to various high-value chemicals.

BR from SI, Gasan

Some More Information from LinkedIn posts

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I have found the following very helpful:1. Anaerobic Biotechnology for Bioenergy Production by Samir Kumar Khanal (2009).2. Biogas from Waste & Renewable Resources by Dieter Deublein et al (2011).Both are current & easy to absorb. Posted by Kev Hughes

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There is also the UK's Official Information Portal on AD (, hosted by us at NNFCC - it carries a wealth of information on all aspects of AD; including an up to date map of UK activity, an interactive calculator, a resource library and a series of case studies. Posted by Lucy Hopwood

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I would like to contribute with links to a few free online resources also worth reading:Biogas – Green Energy Process, Design, Energy Supply, Environment BioExell training manual Good Practice in Quality Management of AD Residues Microbial Handbook for Biogas Plants Posted by Michael Madsen