Environmental Engineering Reference
In-Depth Information
in the form of volatile fatty acids (VFAs) such as acetic acid, butyric acid, propionic
acid etc. This type of hydrogen production system strategy includes dark fermenta-
tion followed by photo-fermentation or dark fermentation followed by a microbial
electrohydrolysis cell (MEC), which is also referred to as 'electrohydrogenesis'.
Thermodynamic constraints limit the release of the hydrogen atoms bound up in
fermentation end-products by dark fermentation, so integration of dark fermenta-
tion with photosynthetic bacteria is needed for the maximization of H
2
yield.
The combined use of anaerobic bacteria and purple non-sulfur photosynthetic
bacteria for efficient conversion of wastewater into H
2
using effluents from three
different carbohydrate-fed reactors (CSTR, ASBR, and UASB) has been reported
by Lee et al. (
2002
). The authors report that CSTR effluent is the most suitable for
photohydrogen production. Azbar and Dokgoz (
2010
) have reported the use of a
two-stage reactor to maximize the H
2
yield from cheese whey wastewater. For this
purpose, effluent from a thermophilic anaerobic digester fed with cheese whey has
been used in photo-fermentation reactors using
Rhodopseudomonas palustris
strain
DSM 127. In this study, overall H
2
production yield (for dark + photo fermentation)
has been found to vary between 2 and 10 mol H
2
mol
−1
lactose. It is suggested that
cheese whey effluent with a co-substrate containing L-malic acid, such as apple
juice processing effluents could provide successful hydrogen production.
A hybrid hydrogen production system employing dark-fermentation process
followed by a photo-fermentation process has been used by Lo et al. (
2008
) for
hydrogen production from acid-hydrolyzed wheat starch. The effluent from dark
fermentation reactor in which hydrolyzed starch was continuously converted to
H
2
by
Clostridium butyricum
CGS2, was fed into photo H
2
production process
inoculated with
Rhodopseudomonas palustris
WP3-5 (ToC = 35 °C, pH 7.0, light
100 W m
−2
irradiation). Combining enzymatic hydrolysis, dark fermentation and
photo fermentation has led to a marked improvement of overall H
2
yield, up to
16.1 mmol H
2
g
−1
COD or 3.09 mol H
2
mol
−1
glucose, and COD removal efficiency
(ca. 54.3 %), suggesting the potential of using the proposed integrated process for
efficient and high-yield bio-H
2
production from starch feedstock. Similar experi-
ments have been conducted using
Enterobacter cloacae
DM11 in the first stage,
followed by photo-fermentation by
Rhodobacter sphaeroides
strain OU001 (Nath
et al.
2008
). The yield of H
2
in the first stage has been approx. 3.3 mol H
2
mol
−1
glucose (approx. 82 % of theoretical), while the yield of H
2
in the second stage is
between 1.5-1.7 mol H
2
mol
−1
acetic acid (37-43 % of theoretical). The combined
yield of H
2
in the two-stage process is 4.8-5.0 mol H
2
mol
−1
substrate, significantly
higher than the 3.3 mol H
2
mol
−1
glucose obtained in the dark fermentation alone.
5 Conclusions
H
2
fuel is clearly a promising solution for energy security as a sustainable alterna-
tive energy carrier and also a reliable choice against climate change. Biotechnology
seems to provide much more environmentally friendly alternative H
2
production in
