Environmental Engineering Reference
In-Depth Information
aim of optimizing the process [71]. This helped to identify the influence and con-
tribution of individual selected factors on the process and to derive the relationship
between variables and operational conditions. By adopting the derived optimum
conditions, the performance with respect to H
2
production and substrate degradation
could be improved significantly.
6
Limitations in Fermentative H
2
Production
In spite of striking advantages, the main challenge encountered with fermentative H
2
production processes are low substrate conversion efficiency and residual substrate
present in acid-rich wastewater generated from the acidogenic process. Anaerobic
bacteria have a theoretical maximum yield of 4 mol H
2
/mole glucose [3]. In prac-
tice, yields are lower, as the NADH oxidation by NFOR is inhibited under standard
conditions and only proceeds at very low partial pressures of H
2
[11].Upto4
molesofH
2
can theoretically be produced per mole of glucose through the known
fermentative pathways [109]. However, various biological limitations such as H2-
end-product inhibition and waste-acid and solvent accumulation limit the molar
yield to around 2 moles per mole glucose consumed. Typical H
2
yields range from
1to2molH
2
/mol glucose and result in 80-90% of the initial carbon remaining in
the wastewater [7, 23, 25, 51, 109, 76, 110, 111]. Even under optimum conditions
about 60-70% of the original organic matter remains as residue in the wastew-
ater. Also a maximum yield of 4 mol H
2
/mole glucose is still low for practical
applications [3].
The generation and accumulation of soluble acid metabolites causes a sharp drop
in the system pH and inhibits H
2
production. H
2
yield is lower when more reduced
organic compounds, such as lactic acid, propionic acid, and ethanol, are produced
as fermentation products, because these represent end products of metabolic path-
ways that bypass the major H
2
-producing reaction [11]. The undissociated soluble
metabolites can permeate the cell membrane of H
2-
producing bacteria and then
dissociate in the cell leading to physiological balance disruption [91]. Thus, some
maintenance energy should be used to restore the physiological balance in the cell,
which reduces the energy used for bacteria growth and inhibit the bacterial growth
on the other hand. If the dissociated soluble metabolites is present in the system at
a high concentration, the ionic strength will increase, which may result in cell lysis
[91]. High concentrations of soluble metabolites can inhibit H
2
-producing bacterial
growth thereby reducing H
2
production [91, 78, 112]. The fermentation metabolic
end-products and the resultant H
2
yields vary based on the environmental conditions
even within the same bacterium [3, 86].
H
2
production is limited by the thermodynamics of the hydrogenase reaction,
which involves the enzyme-catalyzed transfer of e
-
from an intracellular electron
carrier molecule to H
+
[11]. The partial pressure of H
2
is one of the important fac-
tors, as the pressure increases, H
2
production decreases [7]. H
2
production becomes
thermodynamically unfavourable at H
2
partial pressures greater than 60 Pa [11].
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