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Alex,<br>
<br>
<div class="moz-cite-prefix">On 9/25/2012 1:53 PM,
<a class="moz-txt-link-abbreviated" href="mailto:alexanderb.eaton@gmail.com">alexanderb.eaton@gmail.com</a> wrote:<br>
</div>
<blockquote
cite="mid:13842572-1348606399-cardhu_decombobulator_blackberry.rim.net-481102534-@b14.c13.bise6.blackberry"
type="cite"> David,<br>
<br>
Some really interesting points here. For example, any idea how
much ferric oxide would make sense to add to a digester? How would
that effect the effluent as a fertilizer? </blockquote>
<br>
Well of course the question before your question has been mentioned
already, which is whether one needs to do anything at all. I don't
think
it makes sense to do a preemptive strike on H<sub>2</sub>S, but
rather
find out if it will be a problem, then if it is, to deal with it.
And of
course whether it may be a problem depends on a number of factors.
And
remembering that the original question was about small-scale, farm
or
household digestion, of the millions of Chinese- and Indian-style
digesters extant, I don't know of any significant use of scrubbing H<sub>2</sub>S
that is used. Your thought appears to be the same since you say "In
the
SNV program they essentially use no filtration, and they really do
not
have issues."<br>
<br>
That said, using one of the methods mentioned by Henry of adding a
source of the ferric ion, based on the figures he presents-- and
they
were minimal in his paper-- one would need 2 g Fe<sub>2</sub>O<sub>3</sub>/l
of
slurry. That can end up being quite a lot of material.<br>
<br>
But it is important to note that Henry does not assert that this is
a
well-tested dosage. In fact, according to his paper, he tried this
technique only twice-- not with a range of dosages, in a series of
digesters with varying amounts of sulfides, but only two times, once
each in two test digesters. He says that <br>
<blockquote>
<blockquote>"It can be seen that the amount of sulfide in the
sludge in a digester is not a measure of the hydrogen sulfide in
the digester gases. The sulfide may be tied, as in this case,
with iron [DWH: i.e. often by adding it]. The iron content, of
course, is not an exact figure. Any small bit of rust would have
shown up too." </blockquote>
</blockquote>
<br>
So even the 2g/l figure is "more or less", and really this
comes down to a sort of general guideline: if one has "too much" H2S
in
the biogas, then try adding "some" rust (or perhaps a bit of
iron-containing soil, such as hematite) to the slurry.<br>
<br>
<br>
<blockquote
cite="mid:13842572-1348606399-cardhu_decombobulator_blackberry.rim.net-481102534-@b14.c13.bise6.blackberry"
type="cite"> My sense is that the sulpher and iron would actually
work well as micro nutrients, as both would be present in high
concentrations. We could make a powdered mixture with cal (lyme?)
To both raise pH and add the iron. Both materials are really cheap
and effective in small volumes. </blockquote>
<br>
S (more likely) and Fe (less likely) might be micro-nutrients, but
that
would depend on the soil to which the effluent is being added. And
even
cheap materials, given that one would have to mix and add them, have
a
marginal cost in labor and complexity, so again either additive
should
earn its place following some degree of proof that it was actually
needed, or sufficiently beneficial to warrant inclusion. <br>
<br>
<br>
<blockquote
cite="mid:13842572-1348606399-cardhu_decombobulator_blackberry.rim.net-481102534-@b14.c13.bise6.blackberry"
type="cite">Regarding the biological filtration, I think the
thesis you posted talks about running the gas through a drier mix
of cow manure. </blockquote>
<br>
The cow manure filtration actually is mentioned in a different
paper,
although at the moment I don't have the time to track it down. The
thesis I mentioned offers a lot of information that might be adapted
to
lower-tech biofiltration of H<sub>2</sub>S.<br>
<br>
<br>
<blockquote
cite="mid:13842572-1348606399-cardhu_decombobulator_blackberry.rim.net-481102534-@b14.c13.bise6.blackberry"
type="cite"> I have seen the industrial systems (RCM International
a really impressive model), but I have not been able work through
the pumping of substrate and air at a small scale. <br>
</blockquote>
<br>
Any digester evolving biogas will provide enough pressure to cause
the
biogas to flow. The problem, as you indicate, would be getting
enough
but not too much air into the gas stream just before the point of
exposure to whatever media was supporting the sulfur-loving
bacteria.<br>
<br>
<br>
<blockquote
cite="mid:13842572-1348606399-cardhu_decombobulator_blackberry.rim.net-481102534-@b14.c13.bise6.blackberry"
type="cite">Aside from engines, the question that SNV made me ask
was: why filter for small systems? If the combustion is complete
the sulphur becomes elemental, with a slight odor but not
dangerous, or at least how I understand it. We have never
registered any dangerous sulphur compositions in a kitchen.
Thoughts?
</blockquote>
<br>
Actually when H<sub>2</sub>S is burned, it produces SO<sub>2</sub>,
which is quite irritating (that concentrations of three to five PPM
are
readily detectable). Further, in the presence of water (and when the
exhaust gases cool, water will be present), SO<sub>2</sub> combines
to
produce sulfurous acid (H<sub>2</sub>SO<sub>3</sub>), which can
corrode
appliances, flues, and the like. I use sulfur to control gophers on
my
farm, for example, by putting some elemental sulfur in a run that
I've
dug up, then burning it into the burrow with a propane torch.
Believe
me, it's sufficiently deadly.<br>
<br>
I think, in sum, that the reason so little scrubbing is done is
because
few substrates contain sufficient sulfur to be of much concern, and
those which do contain sulfur in some form (pig manure, for example)
are
generally digested at a pH sufficient to reduce the production of H<sub>2</sub>S
(and/or there is sufficient ferric ion present-- widely available in
soil and water-- to suppress the production of H<sub>2</sub>S) that
it
simply is not a problem in most situations.<br>
<br>
<br>
<br>
d.<br>
<div class="moz-signature">-- <br>
<div style="font:Georgia" ;=""><span style="font-size:110%;">David
William House<br>
</span>
<div style="padding-left:3em;font-size:80%;">"The Complete
Biogas Handbook" <code><a href="www.completebiogas.com">www.completebiogas.com</a></code><br>
<em>Vahid Biogas</em>, an alternative energy consultancy <code><a
href="www.vahidbiogas.com">www.vahidbiogas.com</a><br>
<br>
</code></div>
<code>
<div style="padding-left:2em;">"Make no search for water.
But find thirst,<br>
And water from the very ground will burst."
<div style="padding-left:2em;font-size:80%;">(Rumi, a
Persian mystic poet, quoted in <em>Delight of Hearts</em>,
p. 77) <br>
<br>
<a class="moz-txt-link-freetext" href="http://bahai.us/">http://bahai.us/</a></div>
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