[Digestion] Two-stage stuff
david at h4c.org
Tue Nov 18 19:30:20 CST 2008
I have looked in vain for a precursor message to this one. Are you
responding to someone or a statement made by someone? If so, I must have
In any case:
Warren Weisman wrote:
> Is it necessary to also send the carbon stuff through a 1st stage? This doesn't seem to build up the acid the fats and proteins do?
> Or are there benefits to mixing them together and grow the bacteria together before sending them into a 2nd stage?
I believe you are referring to two-stage digestion, where one stage
produces the volatile organic/fatty acids (VOAs or VFAs) and the next
Note that these are biological reactions. Thus both stages follow
certain principles and have certain parameters which must be satisfied,
and these are the conditions that support life; anaerobic life,
microbial life, but life none the less. Except in special circumstances,
I am not sure in a practical sense how one would separate "carbon stuff"
from fats and proteins, but even if circumstances make that possible,
one would want to have the proper carbon-nitrogen ratio (C/N) or the
microbes responsible for generating the volatile acids will not be happy.
The most important of these acids are the saturated monocarboxylic
acids: formic (largely responsible for the smell of crushed ants),
acetic (prominent in vinegar), propanoic, butanoic, pentanoic, and so
on. In general, as Buswell and Boruff pointed out in 1932 ("The Relation
Between the Chemical Compositions of Organic Matter and the Quality and
Quantity of Gas Produced During Sludge Digestion." Sewage Works J.
4:454.), the chemical formula for these acids is such that for every
atom of carbon, there is one atom of hydrogen and two of oxygen. For
this reason if for no other, if the first stage of digestion is starved
of carbon, it will be far harder for the acidogens, the microbes that
produce the fatty acids, to do that.
In other words, in answer to your questions: 1) yes it is necessary; 2)
regardless that fats and proteins produce more acids/methane than other
components, the C/N must be held in the proper range; and 3) in most
circumstances yes, one should usually mix and use the substrates one
has, given that the proper parameters (C/N, temperature, HRT, etc. etc.)
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|
"Make no search for water. But find thirst,
And water from the very ground will burst."
(Rumi, a Persian mystic poet, quoted in /Delight of Hearts/, p. 77)
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