[Digestion] The biology of biogas production

Thu Jul 26 19:51:11 CDT 2012

Dear Gasan,
I apoligise for the delay in reacting to your communication. Firstly I
thank you for the information provided by you that carbon dioxide can be
converted into methane by the methanogens. However, I spent the last months
meditating on the following question. I think that I have found the answer
to it.
A biogas plant is an open system, into which we feed dung and also all
kinds of organic garbage. A large number of micro-organisms get introduced
into the biogas plant along with the feedstock. The feedstock is thus being
consumed by all of them, and yet we can say with certainty that a certain
type of feedstock would yield so much methane. How is that possible?
Having chains of organisms helping each other to digest the organic
material appears to be the currently accepted opinion, but I feel that
there is no need to explain methanogenesis from different categories
of organic substances by assuming the presence of such chains of organisms,
each one bringing the process one step forward. The fact is that while
producing energy from food the penultimate product in the case of almost
all organisms is acetic acid. In the presence of oxygen, the acetic acid
gets oxidised into carbon dioxide and water, but under anaerobic
conditions, this process stops at the acetic acid stage, and
the organisms which can no longer use the acetic acid, excrete it into the
medium. The methanogens pick it up and convert it into methane and carbon
dioxide. Thus, according to my opinion, any organic substance, consumed by
any microbe in a biogas plant, gets ultimately converted into acetic acid,
and then into methane.  In the case of our urban domestic biogas plant,
which converts food waste into biogas almost at the rateof 1:1, overfeeding
leads to lowering of the pH of the system, which in turn stops methane
production. This can be explained by the argument that under abundant
supply of food, the non-methanogenic microflora in the biogas plant produce
so much acetic acid, that the methanogens are unable to process it into
methane and carbon dioxide.  The acetic acid, accumulating in the medium,
turns the medium acidic, which inhibits the methanogens in their function
of producing methane. Had there been conveyer belts in the form of chains
of micro-organisms, this type of acidification of the medium would not have
occured.
Yours
A.D.Karve

On Wed, Mar 23, 2011 at 3:49 PM, Gasan Osojnik <gasan.osojnik at gmail.com>wrote:

> Dear dr. Karve
>
> 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
>
>
>
> Dear Mr. Afilal,
> if you used any substance that is digested by humans,(sugar, starch,
> digestible protein or fat), it gets completely converted into biogas by
> the methanogens. 1 kg of any of these substances would yield about 1 kg
> biogas, containing the theoretically calculated proportion of roughly 25 to
> 30% methane and 70 to 75% carbon dioxide. The presence of a chain of
> micro-organisms, with each one producing a product that serves as food for
> the next one in the chain is not believable. If it were really so, one
> would
> get a much higher concentration of carbon dioxide in the resultant biogas,
> because the intermediate organisms produce only carbon dioxide and not
> methane.
> Yours
> A.D.Karve
>
>
>
> On 22 March 2011 20:00, <digestion-request at lists.bioenergylists.org>wrote:
>
>> Re: The biology of biogas production
>
>
>
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> Beginner's Guide to Biogas
> http://www.adelaide.edu.au/biogas/
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>
>
>

-- 
***
Dr. A.D. Karve
Trustee & Founder President, Appropriate Rural Technology Institute (ARTI)
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