<div><div>I have shared my thoughts on the matter earlyer with dr. Karve, and as this debate has gotten some mettel on this list, I am posting my reply here, likewise.<br><br>T<span class="il">he</span>
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:<br>
<br>
- acetoclastic pathway (acetic acid is decarboylized to methane and carbon dioxide)<br>
- hydrogenotrophic pathway (hydrogen and carbon dioxide are utilized to form methane alone)<br>
<br>
As both these processes are are mutually dependable on <span class="il">the</span> other one, CH4 and CO2 must be formed together, or none at all. An disturbance in <span class="il">the</span> either one of these processes results in <span class="il">the</span> condition which I'm sure we are all familiar with, t.i. <span class="il">the</span> acidification of <span class="il">the</span> reaction. <span class="il">The</span> amount of H+ in <span class="il">the</span> broth becomes easily toxic to acetoclastic methanogens, therefore in a stable process, all <span class="il">the</span> H+ must react with <span class="il">the</span> dissolved carbonate, and <span class="il">the</span> carbonate that remains is therefore <span class="il">the</span> source of final CO2 in <span class="il">the</span> biogas.<br>
<br>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); <span class="il">the</span> methane ration in <span class="il">the</span> "headspace" does not affect <span class="il">the</span> level of methane production, as <span class="il">the</span> amount of <span class="il">the</span> methane that is made is governed by <span class="il">the</span> biochemical properties of <span class="il">the</span> substrate. Also, methane is poorly soluble in water , so there is no physiochemical mechanisma that would neable <span class="il">the</span> microorganisms to "know" what <span class="il">the</span> methane content in the produced biogas is.<br>
<br>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)?<br><br>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.<br>
<br>Also, I think, there is <span class="il">the</span> general problem of carbon dioxide that must be addressed, as we must start to view <span class="il">the</span> produced CO2 as a product and not as an impurity. A vast amount of biochemical energy is used to produce carbon dioxide in <span class="il">the</span> biological process, so by utilizing the produced CO2 rather than discarding it, the energy efficiency of such processes could be considerably increased. E.g. <span class="il">the</span> biogas plants can be could be coupled to greenhouses to enhance <span class="il">the</span> productivity of plants. Similarly, there are attempts being made, to flue gases from biogas co-generation units to feed <span class="il">the</span>
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>
<br>
BR from SI, Gasan<br>
<br>
------------------------------<br>From: <b class="gmail_sendername">Gasan Osojnik</b> <span dir="ltr"><<a href="mailto:gasan.osojnik@gmail.com">gasan.osojnik@gmail.com</a>></span><br>
Date: 16 May 2011 09:17<br>
Subject: Re: Digestion Digest, Vol 7, Issue 13<br>
To: <a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a><br>
<br>
<br>
<div>Dear David and Melvin, <br><br>I apologize for not responding sooner, a lot of things were prioritized.
Here is the full sources list for the basic anaerobic degradation
described on 23 Mar 2011 in the topic "The biology of Biogas
Production".<br><br>(i) As already mentioned, basic biology for engineering
purposes (encluding the biochemisty of the biogas formation from various
sources) is explained in:<i><br>
Deublein & Steinhauser's Biogas form waste and renewable resources (Willey, 2009))</i> and many other pollution engineering handbooks.<br>
<br>(ii) The microbial syntrophism explained (interspecies hydrogen
transfer), essential for biogas production, is explained in e.g.:<br>
<i>Schink, B (1997) Energetics of syntrophic cooperation in methanogenic
degradation. Microbiology and Molecular Biology Reviews, 61 (2), 262-280</i>,<br>(an example was explained already in 1967 by <i>Bryant et al., Methanobacillus omelianskii, a symbiotic association of two species of bacteria.<br>
Arch. Microbiol. 59:20.</i>) <br><br>(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, <i>Jaenicke
et al (2011), Comparative and Joint Analysis of Two Metagenomic
Datasets from a Biogas Fermenter Obtained by 454-Pyrosequencing,</i> the taxonomic profile of the biogas producing community is updated with several new genera (s.a. <i>Streptococus</i> and genera in the<i> Firmicutes</i>
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. <br>
<br>(iv) General facts about <i>Archaea</i> etc. can be found in <i>Brock's Biology <span></span>of Microorganisms</i><i> </i>and in <i>Bergey's Manual</i> .<br>
<br>(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><br><a href="http://www.youtube.com/watch?v=bUBAndojs50&feature=player_embedded" target="_blank">http://www.youtube.com/watch?v=bUBAndojs50&feature=player_embedded</a><br><br>BR, Gasan Osojnik</div>
</div>
<div> <br></div><br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
------------------------------<br>
<br>
Message: 3<br>
Date: Wed, 29 Jun 2011 12:14:39 +0930<br>
From: "Paul Harris" <<a href="mailto:paul.harris@adelaide.edu.au">paul.harris@adelaide.edu.au</a>><br>
To: "'For Discussion of Anaerobic Digestion'"<br>
<<a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a>><br>
Subject: Re: [Digestion] Fwd: The biology of biogas production<br>
Message-ID: <00fb01cc3606$7e81c800$7b855800$@<a href="mailto:harris@adelaide.edu.au">harris@adelaide.edu.au</a>><br>
Content-Type: text/plain; charset="iso-8859-2"<br>
<br>
G'day A.D. Karve et al,<br>
<br>
This post from a while back prompted an interesting thought - if you are<br>
right we may be able to acclimate a digester to using just CO2 as the carbon<br>
source, so we could convert CO2 back to methane and solve both CO2 emissions<br>
and "peak oil".<br>
<br>
The quality ratio of CH4 to CO2 does vary a bit, sometimes being less than<br>
50% methane and sometimes claims of over 90% methane, but David Fulford has<br>
shown that sugar and starch give 50% methane so any improvement is due to<br>
some methanogens using CO2 produced earlier in the sequence and H+ from<br>
water and acids to make CH4 and H2O.<br>
<br>
Happy digesting,<br>
<br>
HOOROO<br>
<br>
Mr. Paul Harris, Room 202 Charles Hawker Building, Faculty of Sciences, The<br>
University of Adelaide, Waite Campus, PMB 1, Glen Osmond SA 5064 Ph : +61<br>
8 8303 7880 Fax : +61 8 8303 4386<br>
<mailto:<a href="mailto:paul.harris@adelaide.edu.au">paul.harris@adelaide.edu.au</a>> mailto:<a href="mailto:paul.harris@adelaide.edu.au">paul.harris@adelaide.edu.au</a><br>
<<a href="http://www.adelaide.edu.au/directory/paul.harris" target="_blank">http://www.adelaide.edu.au/directory/paul.harris</a>><br>
<a href="http://www.adelaide.edu.au/directory/paul.harris" target="_blank">http://www.adelaide.edu.au/directory/paul.harris</a><br>
<br>
From: <a href="mailto:digestion-bounces@lists.bioenergylists.org">digestion-bounces@lists.bioenergylists.org</a><br>
[mailto:<a href="mailto:digestion-bounces@lists.bioenergylists.org">digestion-bounces@lists.bioenergylists.org</a>] On Behalf Of Anand Karve<br>
Sent: Thursday, 2 June 2011 12:50 PM<br>
To: <a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a><br>
Subject: [Digestion] Fwd: The biology of biogas production<br>
<br>
<br>
---------- Forwarded message ----------<br>
From: Anand Karve <<a href="mailto:adkarve@gmail.com">adkarve@gmail.com</a>><br>
Date: 2011/6/2<br>
Subject: Re: [Digestion] The biology of biogas production<br>
To: Gasan Osojnik <<a href="mailto:gasan.osojnik@gmail.com">gasan.osojnik@gmail.com</a>><br>
<br>
Dear List,<br>
<br>
Irrespective of the substrate, design of the system or the temperature of<br>
the reaction, the proportion of methane to carbon dioxide in the biogas<br>
seems to be constant all over the world. Is there an equilibrium between<br>
these two gases that keeps this proportion constant? Because in that case,<br>
one might introduce carbon dioxide from outside into the biogas plant and<br>
see if the organisms in the digester produce more methane to reach the<br>
euilibrium constant. I shall be grateful to get your reaction on this.<br>
<br>
Yours<br>
<br>
A.D.Karve<br>
<br>
------------------------------<br>
<br>
Message: 9<br>
Date: Wed, 29 Jun 2011 22:23:27 +0800<br>
From: Anand Karve <<a href="mailto:adkarve@gmail.com">adkarve@gmail.com</a>><br>
To: <a href="mailto:paul.harris@adelaide.edu.au">paul.harris@adelaide.edu.au</a>, For Discussion of Anaerobic Digestion<br>
<<a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a>><br>
Subject: Re: [Digestion] Fwd: The biology of biogas production<br>
Message-ID: <BANLkTi=iCPU9n=<a href="mailto:ULeu9W-5JM6%2BMNCXWAMQ@mail.gmail.com">ULeu9W-5JM6+MNCXWAMQ@mail.gmail.com</a>><br>
Content-Type: text/plain; charset="iso-8859-2"<br>
<br>
Dear Paul,<br>
The text book opinion is that methanogens are helped by a large number of<br>
other bacteria in the process of methanogenesis. Some time ago I had posed a<br>
question to Gasan that if there are so many different organisms involved in<br>
this process, why we did not get a lot more carbon dioxide in our biogas<br>
than the 40% that everybody reports. He wrote that the organisms that<br>
associated themselves with the methanogens used the carbon dioxide as the<br>
source of carbon. Gasan may elaborate this point.<br>
Yours<br>
A.D.Karve<br>
<br>
------------------------------<br>
<br>
Message: 11<br>
Date: Wed, 29 Jun 2011 21:55:08 +0530<br>
From: Murali Krishna <<a href="mailto:bmkrishna6@gmail.com">bmkrishna6@gmail.com</a>><br>
To: For Discussion of Anaerobic Digestion<br>
<<a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a>><br>
Subject: Re: [Digestion] Fwd: The biology of biogas production<br>
Message-ID: <BANLkTimK=<a href="mailto:jpAkgRjbfxa%2BHJbdpnncQV2RA@mail.gmail.com">jpAkgRjbfxa+HJbdpnncQV2RA@mail.gmail.com</a>><br>
Content-Type: text/plain; charset="iso-8859-2"<br>
<br>
Good day Paul,<br>
<br>
CO2 is isolated and streamed into a separate chamber. Carbonic Anhydrase<br>
enzymes are used to isolate CO2 from the digester. Carbonic anhydrase,<br>
rapidly converts carbon dioxide to bicarbonate. Thus CO2 is efficiently<br>
removed from the solution in the digester. The product of this reaction,<br>
bicarbonate, can then be reacted with sodium to form NaHCO2, baking soda.<br>
By this process CO2 emission into atomosphere is reduced substantially and<br>
there is more space in the digester. There is a patent on this.<br>
<br>
The isolated CO2 after modifications is sent back into the digester and<br>
Methane is generated. (This second part I need to yet confirm). I shall<br>
come back to you soon on this.<br>
<br>
Krishna<br></blockquote></div><br>