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<DIV dir=ltr align=left><SPAN class=062064108-15102010><FONT face=Arial size=2>I
agree completely with Duncan, we use VS mostly because it is so simple
to measure and when combined with a little experience we can
often make a reasonable guess at the biogas yield. I and many
others have tried to make accurate predictions of biogas yield with a large
number of parameters, including in vitro digestibility but the results are not
particularly good, at least not yet.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=062064108-15102010><FONT face=Arial
size=2>Regarding the statement that manure cannot serve as food for methanogens,
this is completely untrue, as any manure-only digester can prove.
</FONT></SPAN><SPAN class=062064108-15102010><FONT face=Arial size=2>The reason
there is interest in manure digestion is simple. We know that manure is a fairly
poor substrate for biogas, but anaerobic digestion is primarily a method of
treating manure and recycling nutrients whilst simultaneously
giving a useful bi-product called biogas. We are well aware that food wastes,
i.e. feedstocks from which animals have not taken their share, would
significantly increase biogas production when compared to second-hand feedstocks
like manure. The problem is that once there is a demand for "wastes" they start
to command a price.</FONT></SPAN><FONT face=verdana size=2><BR><BR>Med venlig
hilsen / Regards <BR><FIRSTNAME><LASTNAME>Alastair James
Ward<BR><BR></DIV></FONT><!--eMS7.0.4.D8M.7Y.2010-->
<DIV> </DIV><BR>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B> digestion-bounces@lists.bioenergylists.org
[mailto:digestion-bounces@lists.bioenergylists.org] <B>On Behalf Of </B>Duncan
Martin<BR><B>Sent:</B> 15. oktober 2010 10:28<BR><B>To:</B> For Discussion of
Anaerobic Digestion<BR><B>Subject:</B> Re: [Digestion] Attachment to previous
Article - More scientific based research and questions<BR></FONT><BR></DIV>
<DIV></DIV>Much of what Dr Karve writes here would, in broad terms, be accepted
by many biogas workers.<BR><BR>VS is a crude (but easy) measure of total organic
content - whether or not biodegradable, aerobically or anaerobically.<BR><BR>BOD
is (by definition!) a measure of the biodegradable content amenable to fairly
quick aerobic breakdown. <BR><BR>So neither is a good measure of anaerobic
biodegradability. However, for a given kind of feedstock, the correlation with
anaerobic biodegradability will be fairly constant, so they can be convenient
ways of assaying feed concentration. The mistake is to expect the same biogas
yields per kg of VS or of BOD from a<I> different</I> feedstock.<BR><BR>I also
agree that the use of an aerobic step predecing an anaerobic one makes very
little sense. <BR><BR>Yes, the gut methanogens do, in a sense, eat what the
animal eats. However, it would be more accurate to say that their diet is
derived from what the animal eats. The methanogens in the gut of a cow are
surrounded by celluose and other biopolymers but they cannot digest them. They
live on the waste products of other microbial processes. The web of metabolic
interactions is well known. <BR><BR>Where I would "hoot out" Dr Karve is his
belief that dung cannot serve as food for the methanogens because they are
"thrown out" of the body along with the dung. I don't understand the logic
here.<BR><BR>Firstly, the words "thrown out" are misleading. They imply a
'deliberate' rejection, in the sense that it is somehow beneficial. But it
isn't. Retaining the bugs would save the nutrients need to keep replacing those
lost in the dung. In fact, the methanogens are inevitably lost with the dung
because they are intimately mixed with the gut content. <BR><BR>If evolution
could figure out a separation process or suitable biomass support, no doubt we
would have ruminants with fixed-bed digesters in their guts, rather than CSTRs
and plug flow digester hybrids we see in reality. Such "high-tech" animals would
produce much less dung, with very low energy content. However, evolution has
favoured a simpler and more 'generous' path: eat more, shit more - and leave a
bit for the next guy in the food chain! <BR><BR>Secondly, the retention time in
the gut of an animal is limited. Monogastric herbivores eat large volumes of
vegetation and digest it inefficiently, so their dung has a high level of
residual energy content - hence high biogas yields. Ruminants digest less good
but more efficiently, with a longer retention time, so their dung has a lower
level of residual energy content - hence lower biogas yields. But far from zero.
<BR><BR>The methanogens in dung have not finished 'eating' - they have just had
their dinner rudely interrupted by the act of defaecation - and the consequent
exposure to oxygen. Put them in AD plant and they will continue their dinner for
several weeks.<BR><BR>Duncan Martin<BR>Cloughjordan Ecovillage<BR>Ireland
<BR><BR><BR>
<DIV class=gmail_quote>On 15 October 2010 05:37, Anand Karve <SPAN
dir=ltr><<A href="mailto:adkarve@gmail.com">adkarve@gmail.com</A>></SPAN>
wrote:<BR>
<BLOCKQUOTE class=gmail_quote
style="PADDING-LEFT: 1ex; MARGIN: 0pt 0pt 0pt 0.8ex; BORDER-LEFT: rgb(204,204,204) 1px solid">
<DIV>Dear Alex,</DIV>
<DIV>I give below the philosophy of our biogas work. Our first assumption
is that because the methanogenic archaea reside in the guts of animals, they
eat what the animals eat. Our second assumption is that these organisms are
universally found in the fecal matter of animals because they are thrown out
of the body along with the dung. Therefore we do not accept that dung
serves as food for the methanogenic archaea. In fact, it is mentioned in the
textbooks on biogas technology that several species of bacteria are involved
in reducing the dung to acetic acid and that the methanogens turn the acetic
acid into methane. Our third assumption is that using the terms VS and BOD to
describe the feedstock are wrong. Neither of these parameters is
correlated with the quantity of the biogas generated. The use of these
parameters in biogas work is comparable to using the phlogiston theory in
chemistry. We therefore propose that digestibility of the feedstock be
considered as the correct parameter to describe the feedstock. Methods are
available to determine the in vitro dry matter digestibility (IVDMD) and these
values are available in books on cattle fodder. We ask the users of our biogas
plants to ask themselves the question if the feedstock would be digested by
animals. If the answer is yes, it is the right feedstock. We also feel
that the fetish of C/N ratio should be discarded. We have operated our biogas
system for months on end, using only green leaves, or oilseed cake, which
have a C/N value of less than 10, some time as low as 5. <BR>We make
only sparing use of a biphasic system. In fact, my advice is to avoid the use
of a biphasic system. In a biphasic system, in order to break down the
difficult to digest material, one makes use of an aerobic fermenter. In this
phase, a lot of the easy to digest material, which would have yielded methane
in the anaerobic phase, is lost, being converted into carbon
dioxide.</DIV>
<DIV>You can now understand, why the biogas workers hoot me out and don't
believe in me. </DIV>
<DIV>Yours</DIV>
<DIV>A.D.Karve <BR></DIV>
<DIV class=gmail_quote>
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<DIV class=gmail_quote>
<BLOCKQUOTE class=gmail_quote
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<DIV class=im>On 07/10/2010 01:39, Alexander Eaton wrote:
<BLOCKQUOTE type="cite">Dr Karve, <BR><BR>Your innovation and work
in the field is quite appreciated, and your system really opens doors
for us who are also not technically focused in the biology of biogas,
but rather its application to families and communities. That is
why it seems your use of food waste and loading rates based on gas
production for a family really widens the populations we may be able to
work with globally. Do you have a paper or document that has this
data and other user data available?
<BR><BR>Best,<BR><BR>Alex<BR><BR></BLOCKQUOTE></DIV></DIV></BLOCKQUOTE></DIV></BLOCKQUOTE></DIV><BR>_______________________________________________<BR>Digestion
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more information about digestion, see<BR>Beginner's Guide to Biogas<BR><A
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