Environmental Engineering Reference
In-Depth Information
organic wastes are availability, low cost, carbohydrate content, and biodegradabil-
ity. In this chapter, each potential biohydrogen production method is discussed.
3
Light Independent Hydrogen Production—Dark
Fermentation
Dark fermentation, which is a naturally occurring process for a variety of microbes,
can convert organic material (especially carbohydrate rich ones) into H
2
, CO
2
, and
organic acids. This is a promising alternative to light dependent processes, particu-
larly when waste biomass is used as a feedstock for the generation of H
2
. The main
advantages of this method over light dependent biohydrogen method is that fer-
mentation does not require a constant light supply, it can be run continuously using
inexpensive and commercially available systems and H
2
production rates are much
higher than photosynthesis-based systems (Levin et al.
2004
). On the other hand,
there are also some disadvantages as summarized in Table
11.1
.
Although fermentative hydrogen production has many advantages as mentioned
above, it is necessary to note that there are also some constraints such as thermody-
namic limitations, product inhibition, the presence of branched catabolic pathways,
media composition, and the nature of substrate, which all have an impact on hydro-
gen yields. These constraints are discussed below.
3.1
Basics of Dark Fermentative H
2
Production
Fermentative H
2
production yields only about 10-20 % of the hydrogen potentially
available in the substrate (theoretical upper limit: 12 mol H
2
mol
−1
hexose) (Hawkes
et al.
2007
; Kraemer and Bagley
2007
; Hallenbeck and Ghosh
2009
). Somewhat
higher yields, up to 25 % (3 mol H
2
mol
−1
hexose) can be achieved with thermo-
philic fermentations using either pure cultures or co-cultures at the expense of volu-
metric productivities (Panagiotopoulos
2010
; Zeidan and van Niel
2009
).
Fermentative hydrogen production, where hydrogenase enzymes are involved, is
carried out via two metabolic types; facultative anaerobes, such as
Escherichia coli
,
and strict anaerobes, like
Clostridia
as shown in Fig.
11.1
.
Heterotrophic organisms (bacteria growing on organic substrates) have special
problems with respect to the disposition of electrons from energy-yielding oxida-
tion reaction during the anaerobic mode of growth. Various kinds of specific con-
trols are required to regulate electron flow in the metabolism of strict and facultative
anaerobes. The ability of “disposing off” excess electrons (e
−
) in the form of H
2
through the activity of hydrogenase is among them.
As shown in Fig.
11.1
, substrate is first broken down to pyruvate by Embden-
Meyerhof-Parnas pathway (glycolysis) which results in ATP production and reduc-
tion of NAD to NADH. Oxidation of NADH is necessary for glycolysis to continue
and this is achieved via the production of a variety of reduced products (e.g. ethanol
