Environmental Engineering Reference
In-Depth Information
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Fig. 8.4
Main parameters affecting the anaerobic (dark) hydrogen fermentation process
over axenic (mono) cultures since this mode can utilize a wider range of feedstocks
without sterilization, and can be easier to operate and control. However several
problems arise when using mixed cultures; one of these being the coexistence in na-
ture of H
2
-producing and H
2
-consuming bacteria (e.g. methanogens) which oxidize
H
2
and reduce CO
2
.
8.3.3
Parameters Affecting Dark Hydrogen Fermentation
Dark hydrogen fermentation is affected by temperature, pH, inorganic content and
inocula (Fig.
8.4
). Temperature is the most important factor for bacterial growth.
The relationship between temperature and chemical reaction rates can be adapted to
microbiological processes within limited temperature ranges, and can be modeled
using the Arrhenius equation (specific thermodynamic data for hydrogen fermenta-
tion using mixed microflora are rarely available). In general, dark hydrogen fermen-
tation reactions can be operated at the following temperature ranges: mesophilic
(25-40 ºC), thermophilic (40-65 ºC), hyper-thermophilic (>65 ºC).
Most experiments on continuous dark hydrogen fermentation have been con-
ducted at 35-55 ºC and not under hyper-thermophilic conditions. However, hyper-
thermophilic hydrogen fermentation has the advantages of:
1.
A high hydrogen production yield and rate is achieved
. It has been reported that
extreme-thermophilic anaerobic hydrogen fermentation can result in the produc-
tion of more hydrogen and higher hydrogen production rates than mesophilic
hydrogen fermentation. It also has been reported that at 70 ºC, the hydrogen
