Environmental Engineering Reference
In-Depth Information
[23] to produce additional H 2 . Photosynthetic bacteria can produce H 2 by consum-
ing organic acids which are abundant in the effluents generated from acidogenic
H 2 fermentation processes [4, 6, 110]. Theoretically, the maximum H 2 yield may
be obtained when glucose is converted to acetate as the terminal product through
dark fermentation, then subsequently converted into H 2 through photo-fermentation
[113]. Integrated systems showed higher H 2 yields compared to single-step fermen-
tation [6, 13, 23, 73]. A two-stage process has been envisioned to obtain yields
closer to the theoretical stoichiometric yield of 12 mol H 2 /mole glucose [86, 113].
However, the efficiency of both H 2 production and substrate degradation were found
to depend on the process used in the first stage along with the composition of
the substrate [23]. The effluent from the first stage of operation generally contains
ammonia, which inhibits the second stage process. This can be restricted by dilu-
tion and neutralization (to adjust the pH to 7) prior to feeding [10]. Integration of an
acidogenic H 2 production process followed by a methanogenic anaerobic digestion
for CH 4 production facilitated an enhanced energy yield along with higher substrate
removal efficiency [23, 75, 114, 115]. Integration of the acidogenic process with a
photo-fermentation process showed a more positive influence over the correspond-
ing methanogenic process integration (Table 4). This might be due to the presence
of a relatively higher concentration of VFA bound residual carbon corresponding
to the methanogenic process. Multi-stage process was often used to maximize H 2
production. Initially, the process consisted of two stages, dark fermentation followed
by photo fermentation [10] but three or even four stages have since been proposed
in different configurations [109]. The acid-rich organic effluent generated from the
initial process of dark fermentation was sent to photo-fermentative process followed
by direct photolysis finally using microbial electrolysis cells to produce H 2 at fourth
7.2 Microbial Electrolysis
Microbial aided electrolysis cells (MEC), also called bio-electrochemically
assisted microbial reactor (BEAMR), use electro-hydrogenesis to directly con-
vert biodegradable material into H 2 by applying external voltages in fuel cells in
an anaerobic microenvironment [116, 117]. The supplemented voltage helps to
decompose acetate spontaneously under standard conditions [116, 118]. Based on
a thermodynamic analysis the addition of greater than 0.11 V to that generated by
bacteria (-0.3 V) will yield H 2 gas at the cathode, but voltages of -0.2 V are needed
because of electrode over-potentials [116]. This process, referred to as electro-
hydrogenesis, provides a route for extending H 2 production past the endothermic
barrier imposed by the microbial formation of fermentation end products, such as
acetic acid [116]. Microbial electrolysis makes it possible to generate H 2 utilizing
effluents generated from acidogenic fermentation and opens the possibility of using
diluted organic matter varying in composition, such as wastewater, for H 2 produc-
tion [119]. Membrane-less continuous flow microbial electrolysis cell (MEC) with
a gas-phase cathode was also used to produce H 2 [119].
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