Environmental Engineering Reference
In-Depth Information
iii.
Photo fermentation:
The molecular hydrogen is evolved by purple non sulfur bacteria using nitro-
genase, in nitrogen deficient conditions, in the presence of energy and reduced
compounds. The main reaction is as follows (Kotay and Das
2008
):
C
Η + Η → Η+
O
12
O
12
6CO
6
12
6
2
2
2
Photoheterotrophic bacteria have the potential of converting light energy to hydro-
gen gas by using waste organic compounds in batch processes, continuous cultures
or cultures of bacteria immobilized on several materials but the hydrogen manufac-
ture using these bacteria is advanced when cells are restrained in or on a solid matrix
related to the cells that are free-living (Nath and Das
2003
).
iv. Hydrogen production by water gas shift process:
Some photoheterotrophic bacteria have the ability to grow in dark by using CO
as a source of carbon in order to generate ATP releasing hydrogen gas and CO
2
.
The process of oxidation of CO to CO
2
with the release of hydrogen gas takes
place by water gas shift reaction, which occurs at low pressure and temperature.
The process is explained below via equation (Nath and Das
2003
).
CO(g)
+Η →
O (l)
CO (g)
+Η
(g)
2
2
2
v. Dark fermentation:
Dark fermentation produces hydrogen gas with no light energy requirements.
Variation of carbon sources are used for this purpose. This is an anaerobic pro-
cedure as there is no oxygen limitation problem. The hydrogen producing anaer-
obic bacteria are developed in dark on high sugar substrates. The process of
fermentation can be done at different temperature ranges (Kotay and Das
2008
).
In the above methods pure hydrogen gas is produced but biogas containing mainly
hydrogen gas and CO
2
in this process but less amount of methane, CO and H
2
S
are present. The bacteria used for the production of hydrogen gas via this process
include
Enterobacter, Bacillus, Clostridium
etc (Kotay and Das
2008
).
Different amount of hydrogen gas can be produced liable on the fermentation
pathway used and products formed (Azbar
in press
). For example when acetic acid
is the main product four moles of hydrogen gas are produced. The reaction is given
below (Kotay and Das
2008
):
C
Η +Η → Η Η+Η+
O
2
O
2C
COO
4
2CO
6
12
6
2
3
2
2
vi.
Two-stage hybrid system
The two stage hybrid system uses the combination of photo and dark fermenta-
tion for better production of hydrogen gas. This process utilizes the substrate
to the maximum limit which else fails to reach complete transformation due to
thermodynamic restriction (Nath and Das
2003
).
