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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Paul<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>There is agreement that exothermic energy from aerobic processes
exceeds anything that might theoretically be gained anaerobically by orders of
magnitude. However, there are many aerobic effluent treatment systems in
existence which demonstrate no appreciable temperature increase, i.e. sewage
treatment works (approx 300mg/l COD), industrial effluent treatment plants etc.
The only time an aerobic temperature rise is effectively detectable is when the
energy density (COD or TVS) is great enough, i.e. compost systems. Some of the
first autothermal aerobic systems were pioneered at Cornell University in the
late 1980s as TAD processes and later developed further in the UK
(Thermophyllic Aerobic Digestion) for very high strength liquid effluents. These
could indeed maintain 55 deg C under aerobic conditions and specific
operational circumstances.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I work daily with anaerobic treatment of waste streams at
50,000-250,000mg/l COD ( very large scale: POME, Ethanol, Cassava; small scale:
Catering Waste) and there is no observable temperature rise in anaerobic systems.
In addition, this is at high ambient temperatures, so losses are at a minimum;
further many are flexible liner reactors with low surface area to volume ratios
(30,000 – 100,000m3), so if there was any observable impact this would be
noted. These systems are also intensive monitored with daily profiles etc.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Whatever the theoretical considerations, for design purposes we
would be best advised to consider the process as neither positive or negative
in terms of metabolic heat loss or gain.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Regards<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Dr Stephen Etheridge<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>PS an interesting contribution to the debate which might be explored
further:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>“Long chain fatty acids degradation in anaerobic digester:
Thermodynamic equilibrium consideration”, Oh, S.T. , Martin, A.D., Process
Biochemistry, Volume 45, Issue 3, March 2010, Pages 335-345<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Abstract<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The biological oxidations and reductions occurring in batch
anaerobic digesters may approach equilibrium. The approach depends strongly on
the activities of the micro-organisms present. The thermodynamics of linear
long chain fatty acid degradation in a batch reactor was modeled. The
substrates considered are the saturated fatty acids from acetic acid to stearic
acid. From the thermodynamic perspective, the fermentation (acidogenesis and
acetogenesis), decomposing the long chain saturated fatty acids to acetic acid
though shorter chain acids, could not proceed spontaneously (ΔH </span><span
style='font-size:11.0pt;font-family:"Cambria Math","serif";color:#1F497D'>≫</span><span
style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> 0
and ΔG </span><span style='font-size:11.0pt;font-family:"Cambria Math","serif";
color:#1F497D'>≫</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> 0). However the model suggests that the major driving force for
the fermentation may be found in the methanogenesis. The model results show two
distinct cases: (ΔS > 0 an</span><span style='font-size:11.0pt;font-family:
"Calibri","sans-serif";color:#1F497D'>d ΔH > 0) and (ΔS < 0 and ΔH <
0), relating to spontaneous but endothermic and non-spontaneous but exothermic
processes respectively. Where, spontaneous digestion is associated with high
initial concentrations of LCFA and endotherm. This implies that the digestion
of this type of substrate might be better facilitated by the supply of
supplemental heat. The digestion of very low concentrations of LCFA is found to
be non-spontaneous due in large part to the solubility of carbon dioxide. This
implies that the digestion of this type of substrate might be enabled by
selectively removing carbon dioxide.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";
color:windowtext'>From:</span></b><span style='font-size:10.0pt;font-family:
"Tahoma","sans-serif";color:windowtext'> Digestion
[mailto:digestion-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>David<br>
<b>Sent:</b> 21 November 2012 06:21<br>
<b>To:</b> For Discussion of Anaerobic Digestion<br>
<b>Subject:</b> Re: [Digestion] Reaction Kinetics for AD<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
Paul,<o:p></o:p></p>
<div>
<p class=MsoNormal>On 11/20/2012 12:43 AM, Paul Muthui wrote:<o:p></o:p></p>
</div>
<blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'>
<p class=MsoNormal><br clear=all>
<o:p></o:p></p>
<div>
<p class=MsoNormal>G'day All,<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>How much energy is given out in the digesters as a result of
anaerobic reaction? I can't seem to get any publication with enough details on
this. I would like to know the temperature rise that would be caused by
the reaction. <o:p></o:p></p>
</div>
</blockquote>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
Dr. Fulford is correct, in essence, in that according to ordinary experience,
no functional metabolic heat is available from anaerobic digestion, as
contrasted to the very considerable metabolic heat available from aerobic
digestion (at about 40% of the energy provided as methane in the former case).
However:<o:p></o:p></p>
<blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'>
<h3><a
href="http://ebookbrowse.com/self-heating-of-anaerobic-digesters-using-energy-crops-pdf-d61331597">Self-heating
of anaerobic digesters using energy crops</a><o:p></o:p></h3>
<p class=MsoNormal>H. Lindorfer, R. Kirchmayr, R. Braun<br>
Water Sci Technol. 2006;53(8):159-66. <o:p></o:p></p>
<h4>Abstract<o:p></o:p></h4>
<p class=MsoNormal>With the increasing application of energy crops in
agricultural biogas plants and increasing digester volumes, the phenomenon of
self-heating in anaerobic digesters appeared in some cases. Until now this development
was just known from aerobic systems. To get an idea of the thermodynamics
inside an anaerobic digester, a detailed analysis of all heat fluxes in a full
scale agricultural biogas plant was carried out. Several experiments were
realised to quantify the influences of different internal and external energy
sources. To estimate the impact of self-heating in anaerobic systems, data of
other full scale agricultural biogas plants in Austria were collected.
Alternatives to the cooling of the digesters are discussed basing on individual
experiences of several plants. A connection between carbohydrate rich
substrates, especially with high starch contents, and the self-heating could be
shown. But from the results it can be assumed that heat enthalpy due to anaerobic
microbial metabolism plays a key role in self-heating, which is in contrast to
the current thermodynamic knowledge.<o:p></o:p></p>
</blockquote>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
And <a
href="http://www.matheon.de/about_us/people/list_members.asp?action=detail&id=812">Prof.
Dr. Christof Schütte</a>-- a mathematician, to be sure, but as applied to
biological systems-- goes so far as to <a
href="http://www.matheon.de/research/show_project.asp?id=134">claim</a> that<o:p></o:p></p>
<blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'>
<p class=MsoNormal>"Knowledge of the net energy production of anaerobic
fermenters is important for reliable modelling of the e<span style='font-family:
"Cambria Math","serif"'>ffi</span>ciency of anaerobic digestion processes.
Furthermore energy balancing is crucial, because present biogas plants still
suffer from severe instabilities caused by self-heating and full-scale plants
typically have to be cooled down."<o:p></o:p></p>
</blockquote>
<p class=MsoNormal><br>
<br>
These studies and statements, however, have a context, which is the
Austrian/German biogas industry, where digestion is a high-tech (some might say
gold-plated) affair in large part because of the previous regime of
above-market price guarantees for electricity from biogas. That is to say, the
digesters being analyzed and discussed above are very likely extremely well
insulated.<br>
<br>
But as far as I know for the kinds of digesters with which you are working--
underground masonry, if I'm not mistaken-- I believe you can assume that the
contents of the digester will be at ambient temperature six feet deep.<br>
<br>
<br>
d.<o:p></o:p></p>
<div>
<p class=MsoNormal>-- <o:p></o:p></p>
<div>
<p class=MsoNormal><span style='font-size:13.0pt'>David William House</span><o:p></o:p></p>
<div>
<p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-size:9.5pt'>"The
Complete Biogas Handbook" </span><code><span style='font-size:10.0pt'><a
href="www.completebiogas.com">www.completebiogas.com</a></span></code><span
style='font-size:9.5pt'><br>
<em>Vahid Biogas</em>, an alternative energy consultancy </span><code><span
style='font-size:10.0pt'><a href="www.vahidbiogas.com">www.vahidbiogas.com</a></span></code><span
style='font-size:9.5pt'><o:p></o:p></span></p>
</div>
<div>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>"Make
no search for water. But find thirst,<br>
And water from the very ground will burst." <o:p></o:p></span></p>
<div>
<p class=MsoNormal><span style='font-size:8.0pt;font-family:"Courier New"'>(Rumi,
a Persian mystic poet, quoted in <em><span style='font-family:"Courier New"'>Delight
of Hearts</span></em>, p. 77) <br>
<br>
<a href="http://bahai.us/">http://bahai.us/</a><o:p></o:p></span></p>
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