Environmental Engineering Reference
In-Depth Information
H
2
more efficiently under high light intensity as compared to the wild type strain.
Reduced antenna mutants have been studied for many biotechnological applications,
since the benefits deriving from a reduced absorption of light may affect a number
of physiological pathways, in different microorganisms. Torzillo et al. (
2009
) has
reported that a great benefit can be derived from the mutants of green algae. Light
intensity also affects the consumption rates of organic acids. For example, butyr-
ate consumption requires higher light intensities (4,000 lx) as compared to acetate
and propionate. Exposure time to light also affects H
2
production. Alternating 14 h
light/10 h dark cycles yielded slightly higher H
2
production rates and cell concen-
trations compared to continuous illumination. More frequent exposure to dark/light
cycle has a better effect on H
2
production (Kapdan and Kargi
2006
).
Industrial effluents that do not cause any inhibition on light penetration (colored
wastewater) are also amenable for H
2
production by photosynthetic organisms. Am-
monia content of industrial effluents may also inhibit the nitrogenase enzyme and
reduce the H
2
productivity. Therefore, pretreatment to remove ammonia and toxic
compounds (heavy metals, phenols, etc.) and dilution of high organic matter content
(COD) in industrial effluents may be required before using such industrial wastewa-
ter during biohydrogen production.
An extensive summary of H
2
production studies from some food industry waste-
waters has been given by Kapdan and Kargi (
2006
). Glucose, sucrose, starch, wheat
starch, lactose, food waste, potato processing waste, apple, domestic sludge, molas-
ses, rice winery, biosolids, filtrate, sweet potato starch residues, and organic frac-
tion of municipal solid wastes have been used as substrates for H
2
production. Tofu
wastewater, which is a carbohydrate and protein rich effluent, has also been used
for H
2
production. Hydrogen yield from tofu wastewater (1.9 L H
2
L
−1
wastewater
at 30 °C) has been reported to be comparable to H
2
yield from glucose (3.6 L H
2
L
−1
wastewater) using
R. sphaeroides
RV immobilized in agar gel (Zhu et al.
1999
). No
ammonia inhibition (2 mM) was observed and 41 % of total organic carbon (TOC)
was removed. Similarly, the dilution of the wastewater at a ratio of 50 % resulted in
an increase in H
2
yield of up to 4.32 L H
2
L
−1
wastewater and 66 % TOC removal
(Zhu et al.
1999
).
Other agro-based waste materials such as potato starch, sugar cane, juice and
whey have also been investigated in terms of H
2
production using
Rhodopseudomo-
nas
sp. Sugar cane juice yielded the maximum level of H
2
production (45 mL [mg
DW h
−1
basis]) as compared to potato waste (30 mL [mg DW h
−1
basis]) and whey
(25 mL [mg DW h
−1
basis]). There was no H
2
production by the photosynthetic bac-
terium,
Rhodobium marinum
using raw starch as the substrate (Singh et al.
1994
).
Use of photo-bioreactors is also critical factor for efficient photobiological H
2
production. Most common photo-bioreactors configurations used for H
2
production
in literature are tubular, flat panel, and bubble column reactors. One of the attempts
to increase volumetric hydrogen production in suspended cell bioreactor systems is
to keep high biomass inventory in the reactor, but it fails due to the exponential de-
cay of light intensity with increasing density of the cell culture. Therefore, suspen-
sion layer thicknesses of 1-5 cm have to be provided in photo-bioreactors. Another
attempt to provide higher biomass inventory in the reactor is by cell immobilization
