Environmental Engineering Reference
In-Depth Information
substrates compared with the soluble cellulodextran cellobiose, with an average yield
of 1.6 mol H
2
mol
−1
glucose equivalent (Islam et al.
2009
). The major soluble fermen-
tation byproducts include ethanol, acetate, and formate, with lactate being produced
when the pH drops below 6.3. Hydrogen production by
C. thermocellum
27405 was
also investigated using dried distillers grain (DDGS), barley hulls (BH), or
Fusarium
head blight contaminated barley hulls (CBH) as the carbon source in batch fermen-
tation experiments (Magnusson et al.
2008
). Overall, DDGS produced the highest
concentration of H
2
gas at 1.27 mmol H
2
mol
−1
glucose equivalent, while CBH and
BH produced 1.18 and 1.24 mmol H
2
mol
−1
glucose equivalent, respectively.
Hydrogen production in a continuous pure culture of
C. thermocellum
27405
was established in a 5 L working volume fermentor, and growth experiments were
maintained for over 3,000 h (Magnusson et al.
2009
), substrate concentrations var-
ied from 1 to 4 g L
−1
and the feed was introduced with continuous N
2
gas sparg-
ing to prevent clogging of the feed-line; pH and temperature of the reactor were
maintained at 7.0 and 60 °C, respectively, throughout the study. At concentrations
above 4 g L
−1
, the delivery of α-cellulose was impaired due to feed-line clogging
and it became difficult to maintain a homogenous suspension. At a dilution rate
of 0.042 h
−1
and substrate concentration of 4 g L
−1
, the H
2
production rate was
5.06 mmol L
−1
h
−1
. Acetate and ethanol were the major soluble end-products, while
lactate and formate were greatly reduced compared to production in batch cultures.
Concentrations of all metabolites increased with increasing substrate concentration,
with the exception of lactate. Despite a number of short-term electrical and me-
chanical failures during the testing period, the system recovered quickly, exhibiting
substantial robustness. A carbon balance indicated near 100 % carbon recovery. This
study shows that long-term, stable H
2
production can be achieved during direct fer-
mentation of an insoluble cellulosic substrate under continuous culture conditions.
4
Light—Driven Biohydrogen Production
Light energy is essential to hydrogen evolution by photosynthetic cells. Table
11.4
compares various light driven bio-hydrogen processes including enzymes involved.
Photoautotrophic green microalgae and cyanobacteria use sunlight and CO
2
as the
sole sources for energy and carbon. The reducing power for cellular photosynthesis
and/or biophotolysis comes from water oxidation under light irradiation.
4.1
Bio-Photolysis Based Hydrogen Production
Bio-photolysis is classified in to two groups;
direct
and
indirect
. Direct photolysis
refers to sustained hydrogen evolution under light irradiation. The light energy is
absorbed by the pigments at PSII, or PSI or both, which raises the energy level of
electrons from water oxidation when they are transferred from PSII via PSI to fer-
rodoxin. A portion of the light energy is directly stored in hydrogen gas.
