Environmental Engineering Reference
In-Depth Information
Fig. 11.2
Hydrogen generation via photofermentation. (Koku et al.
2002
)
requires anaerobic conditions under illumination (Bolton
1996
). Even though these
organisms prefer organic acids as carbon source, other industrial effluents are ame-
nable to H
2
production (Koku et al.
2002
).
H
2
production rates by photo-heterotrophic bacteria are mainly affected by the
factors such as light intensity, carbon source, enzymes involved and the type of mi-
crobial culture. Nitrogenase, however, is the key enzyme catalyzing H
2
production
by these bacteria. The presence of oxygen, ammonia and high N/C ratios inhibit
the activity of nitrogenase. For example, H
2
production by
R. sphaeroides
is com-
pletely inhibited at ammonia concentrations above 2 mM (Koku et al.
2002
). Under
the presence of high nitrogen, metabolic pathway shifts from hydrogen production
to the utilization of organic substrates for cell growth which in return prevents the
light penetration. For this reason, ammonium concentration in the reactor has to be
limited and oxygen should be eliminated. For nitrogen source proteins such as al-
bumin, glutamate, and yeast extract are generally preferred. Uptake of hydrogenase
enzymes in photo-fermentative bacteria, oxidizes H
2
and are antagonistic to nitro-
genase activity, therefore uptake hydrogenase activity should be eliminated for en-
hanced H
2
production. Two to three times more H
2
production has been achieved by
using hydrogenase deficient mutant cultures of photo-fermentative bacteria (Kars
et al.
2008
).
Light intensity is another important parameter affecting the performance of pho-
to-fermentations. Increasing light intensity has a stimulatory affect on H
2
yields
and production rates, but has an adverse effect on the light conversion efficiencies.
It was reported that the reduced antenna mutant of
R. sphaeroides
MTP4 produces
