Biohydrogen Production from Anaerobic Fermentation - Biotechnology in China: Biofuels and Bioenergy - page 145

depending on the species and the operating environments, prevalent end products
can be significantly different. Generally, there are two widely known fermentation
types, butyric-type fermentation and propionic-type fermentation, according to the
main end products [ 11 ]. The butyric-type fermentation results in the formation of
H 2 ,CO 2 , butyrate, and acetate as the dominant products, whereas the propionic-
type fermentation produces mainly propionate, acetate, and some valerate, with
little hydrogen being produced, and this should be controlled from the viewpoint
of hydrogen production. In 1994, a third type, ethanol-type fermentation, was
discovered by Ren et al. [ 12 ], with ethanol, acetic acid, H 2 , and CO 2 as major end
products. The maximum theoretical yields of hydrogen production are 2-3 mol/
mol glucose in butyric-type fermentation and ethanol-type fermentation.
For practical applications, dark-fermentative hydrogen production has more
advantages than photofermentation, such as significantly high hydrogen produc-
tion rate, energy savings, broad spectrum of feedstocks, simple process control,
and lower maintenance costs.
2 Biohydrogen Production from Wastewater
In China, Ren et al. [ 12 ] initiated hydrogen production with dark fermentation
from wastewater rich in carbohydrates in the 1990s, and this has attracted great
interest since then.
2.1 Hydrogen-Producing Bacteria
The success of hydrogen production depends on the choice of the host microor-
ganisms. During recent decades, intensive research has been conducted to isolate
high-performance hydrogen-producing microorganisms [ 13 - 23 ], which are sum-
marized in Table 1 .
Among them, Ethanoligenens harbinense B49 isolated from a continuous
stirred tank bioreactor (CSTR) is worthy of note, because the ''ethanol-type''
fermentation was identified with this bacterium for the first time on the basis of
its main soluble end products acetate and ethanol [ 17 ]. Afterward, another
strain, E. harbinense YUAN-3, was isolated and nominated as the model strain
of the genus Ethanoligenens [ 18 ]. The maximum hydrogen yield and produc-
tion of E. harbinense YUAN-3 T are 2.81 mol H 2 /mol glucose and 27.6 mmol
H 2 /g dry cell weight (DCW)/h, respectively. Additionally, the strain exhibits
self-aggregation ability during shake cultivation, which is an advantage for its
future application.
To improve hydrogen production, genetic engineering approaches have been
applied for elucidating functional genes and their structures and regulation
mechanisms. The iron hydrogenase gene (hydA), which is the principal gene