[Digestion] Headspace gases from Acetogenesisprocess

Wed Feb 27 09:14:22 CST 2008

Dear Asim

I have never heard of "dark fermentation" - but if you mean anaerobic 
digestion, read on.

You are right that some hydrogen is produced in the first stage but there 
might not be very much -- and there is usually quite a lot of methane, with 
carbon dioxide as by far the biggest component. In practical applications of 
the process, the gases from the two stages are normally mixed, as there is 
unlikely to be an economic justification * for keeping them separated, so, 
as you say, it can be hard to find data for the first stage taken alone. (* 
See footnotes)

I attended a conference last week in Berlin on " Dry Fermentation" * which 
included several papers on two-stage digestion processes. However, there was 
at least one paper which did give such data.  I am away from home at the 
moment, so do not have access to the proceedings -- but I will check them 
out when I return home. My rough notes say that there was more hydrogen than 
they expected from stage 1 -- rather more than 1%.  However, such a small 
amount is unlikely to be commercially useful -- the methane content from 
Stage 1 was 14-20%, (with 74-80% from stage 2). Bear in mind also that these 
concentrations don't mean much in the absence of volumetric data.

There has also been a lot of research work done on two-stage fermentation, 
including both fixed bed (as above) and conventional (suspended-culture) 
digestion, so I would be surprised if there was no data on the composition 
of the gas produced from the first stage. You might need to look a little 
further back in the literature, because the subject of two-stage digestion 
seems to have fallen out of favour somewhat in recent years.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NOTES

1. "Dry Fermentation" is the rather irrational term that the Germans like to 
use for what my own research publications refer to as "Solid-State 
Digestion".  There was some informal discussion of the terminology during 
the breaks in the symposium and I would say that the most commonly preferred 
term would be "Fixed-Bed Anaerobic Digestion". "Dry" is hardly an accurate 
description of a process taking place at a moisture content of, typically, 
60-80% -- and perhaps also subjected to regular irrigation. (I am tempted to 
advise the Irish Tourist Board to start advertising the Irish climate as 
"dry" in their German advertising!)

Anyway, whatever they are called, these processes work -- and there are now 
quite a number of commercial installations, some of them at quite a 
substantial scale.
Some comprise a single stage bed of suitable feedstock, with appropriate 
seeding -- which has been the subject of the majority of my relevant 
publications between 1999 and 2004. The simplicity of these seems a very 
attractive option for installation on farms.

However, there are also some successful two-stage processes, in which a 
fixed bed of feedstock is irrigated with recycled leachate, with the liquid 
produced being pumped into a second (methanogenic) stage. Both stages 
produced some methane but by far the bulk of it is produced in the second - 
as in the example I quote above.

2. Reading between the lines of your enquiry, would I be right to infer that 
you are thinking of the process as a commercial source of hydrogen?

My advice is to forget it.  Microbial processes produce an awful lot of 
different products in small amounts and many researchers get excited by the 
idea that yields could readily be increased to commercial levels.  This is 
occasionally feasible when the product is a secondary metabolite, such as an 
antibiotic -- and especially so when the product is of high value.  It is 
far more difficult to make fundamental changes to primary metabolism -- and 
far more likely to cripple the microorganism responsible.

That said, you are not alone.  There seems to be a lot of current research 
into microbial sources of hydrogen. However, I suspect that much of it is 
undertaken by researchers with very little grip on the realities of process 
engineering and of real-world economics. The prospect of a "hydrogen 
economy" as the (latest) quick fix for the world's environmental problems 
tends to result in the suspension of rational judgement. And some very silly 
research gets funded as a result!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Duncan J Martin

Centre Councillor
Republic of Ireland Centre
Chartered Institution of Wastes Management

================================
CONTACT DETAILS
Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
24 Townsfield, Cloughjordan, N Tipperary, Ireland
Mobile: +353 86 8377 906
Home: +353 505 42087
Email: duncanjmartin at eircom.net
================================

----- Original Message ----- 
From: "Asim Khan" <asimlaeeq at hotmail.com>
To: <digestion at listserv.repp.org>
Sent: Wednesday, February 27, 2008 12:22 PM
Subject: [Digestion] Concentration of headspaces gases from 
Acetogenesisprocess

Dear all,
I am working on hydrogen production from dark fermentation of sewage. I am 
looking for the composition of headspace gases generated from the 
acetogenesis process. All the literature that i have searched give the 
composition of biogas generated from methanogenesis process. Nobody talks 
about the composition of acetogenesis products. Literature says that its 
hydrogen and carbon dioxide. But are there any other trace gases? If yes 
what is their percentage?

Asim
They are not dead , who live in the hearts   , they leave behind.
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