Environmental Engineering Reference
In-Depth Information
Table 11.2  Advantages and disadvantages of dark fermentation processes
No
The process
Organisms
Key enzyme
Advantages
Disadvantages
1
Dark anaerobic
fermentation
Wide range of
anaerobic
bacteria
Hydrogenase
Substrates:
wide range
of organics
including
wastes; high-
est rates of the
process
Low yield of
the process
(not more
than 4 moles
H 2 per 1 mol
of glucose);
organic acids,
alcohols as
by-products;
high H 2
concentration
inhibits the
process
2
Dark aerobic or
anaerobic
All chemot-
rophic
diazotrophic
bacteria dur-
ing nitrogen
fixation
Nitrogenase
All organic
substrates
available for
aerobic or
anaerobic
decompo-
sition by
diazotrophic
bacteria under
nitrogen fix-
ing conditions
Low efficiency
of the bio-
conversion,
especially
under aerobic
conditions
3
Dark anaerobic
CO decompo-
sition (water-
shift reaction)
Some purple
bacteria
Hydrogenase
in conjunc-
tion with
CO-dehydro-
genase
The substrate:
toxic gas
which repre-
sents big part
of syngas
High sensitivity
to inactivation
by oxygen;
organic sub-
strates and H 2
can inhibit the
process
and acetate during facultative anaerobic pathway and ethanol, butyrate, butanol,
acetone during strict anaerobic pathway).
Pyruvate, which is the key intermediate, is catabolized in two different ways.
Facultative anaerobes degrade pyruvate to formate and acetyl-CoA through the ac-
tion of pyruvate formate lyase (PFL). Strict anaerobes produce acetyl-CoA, CO 2 ,
and reduced ferredoxin with the help of pyruvate ferredoxin oxidoreductase enzyme
(PFOR) (Hallenbeck 2009 ). Thus, facultative anaerobes produce hydrogen from
formate by the formate hydrogen lyase (  fhl) complex which possesses an ener-
gy-converting Ni-Fe hydrogenase, a member of the Ech family of hydrogenases
(Vignais and Billoud 2007 ; Vignais 2008 ) and in the other case, hydrogen produc-
tion is derived by reduced ferredoxin by a Fe-Fe hydrogenase (Hallenbeck 2009 ;
Hallenbeck and Ghosh 2009 ). Reducing ferredoxin can lead to additional hydrogen
production by re-oxidizing NADH under very low hydrogen partial pressures which
is not common in typical hydrogen fermentations. In both facultative and strict an-
aerobic hydrogen production pathways, some of the acetyl-CoA is used for the
 
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