Environmental Engineering Reference
In-Depth Information
on light transmitting matrices with high surface area/volume ratios. In this case, up
to 12 g of cells (dry weight) can be immobilized in 1 L of matrix. Therewith, the
rates of H
2
evolution per unit volume increase considerably. Thus,
R. sphaeroides
immobilized on a porous glass steadily evolves H
2
at a rate of 1.1 L L
−1
h
−1
for
more than 1,000 h. The maximum volumetric H
2
rate attained is 3.8 L L
−1
h
−1
, with
an 80 % conversion of the organic acid substrate (Tsygankov et al.
1998
). Use of
mutant photosynthetic bacteria has also been considered by many researchers to
enhance the light conversion efficiency, and hence H
2
production rate. Although an
improvement has been observed by mutant type, the light conversion efficiency was
around 6 % (El-Shishtawy et al.
1997
), which is still less than theoretical efficiency.
The light penetration length is important for the hydrogen productivity. In rela-
tion to solar energy driven H
2
production, the light conversion efficiency has been
reported to be less during mid-day because of high light intensity (1.0 kW m
−2
).
In addition, a delay of 2-4 h has been observed in maximum hydrogen production
rate (3.4 L H
2
(m
2
h
−1
) after the highest light intensity at noon with an average light
conversion efficiency of 1.4 % (El-Shishtawy et al.
1997
). A 3.5 % light conversion
efficiency with an over 0.8 kW m
−2
light intensity at midday has been obtained
using a photo-bioreactor system with light shade bands, whereas photo-inhibition
has been observed at 0.4 kW m
−2
in photo-bioreactors without shade bands Miyake
et al.
1999
).
Mixing of reactor content is the other important factor affecting H
2
production.
Some literature reports suggest gas injection using argon gas for mixing, although
not cost-effective. On the other hand, it is also known that continuous argon sparg-
ing may inhibit the growth of
Rhodopseudomonas
in a pneumatically agitated pho-
to-bioreactor. Re-circulation of reactor content was also a choice for mixing which
provides better growth of the culture. A novel flat-panel airlift photo-bioreactor with
baffles has provided a significant increase in the biomass productivity and therefore
it could also be used for H
2
production (Wakayama et al.
2000
).
Multi-tubular photo-bioreactors made up of parallel transparent tubes are another
preferred reactor configuration, generally used for the cultivation of
Spirulina
. The
system is inclined with a 10-30 % slope to allow gas bubbles to rise. The hydrogen
production rate from lactate using a modified tubular reactor reaches 2 L m
−2
h
−1
with
light conversion efficiency of 2 % in outdoor experiments (Modigell and Holle
1998
).
4.4
Biohydrogen Production by Hybrid Systems
Although the theoretical maximum yield of H
2
from a single dark fermentation
reaction is limited to 4 mol H
2
mol
−1
glucose, yields higher than 4 mol H
2
mol
−1
glucose can be achieved through hybrid systems. Hybrid hydrogen production sys-
tem is composed of two sequential reactors in which dark fermentation is carried
out in the first reactor and then a second photosynthetic reactor is integrated where
hydrogen atoms sequestered in low molecular weight VFAs are converted to H
2
via
photosynthetic organisms. This is required since hydrogen production is possible up
to a certain extent in dark fermentation and significant amount of H
2
is sequestered
