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18 Advancements in Bio-hydrogen Production from Waste Biomass

by photo-fermentation [35]. Although the literature includes a large number of

studies on the batch mode of H2 production, but it has some disadvantages over

other modes of operations. The culture conditions are highly unstable due to the

periodic removal of the sample. Moreover, due to the closed system, the removal of

extracellular products is not possible, which results in product inhibition by liquid

and gaseous metabolites.

18.4.5.2

Continuous Mode

The continuous mode of operation is often favored for biohydrogen production

from various strains. Chemostat is characterized by a constant supply of cul-

ture medium or substrate to the reactor, which maintains the microbes in the

exponential growth phase for prolonged periods and regular removal of product

streams. A continuous stirred tank reactor (CSTR) is widely used for both photo

and dark fermentation. Rhodopseudomonas and Clostridium are cultivated in CSTR

for photo- and dark-fermentation, respectively [36]. Kim and Lee (2010) have

used a microbial consortium in CSTR to obtain 2.3 mol-H2/mol-glucose, whereas

3.4 mol-H2/mol-glucose was obtained by using anaerobic sludge [37, 38]. The

chemostat mode of fermentation is also used to enhance the yield of biohydrogen

from 5.8 to 11.61 mol-H2/mol-sucrose by varying the pH of the media and cultiva-

tion temperature. To further increase the biohydrogen production using CSTR, the

use of biofilms or immobilized organisms has been reported previously [39].

18.4.5.3

Fed Batch

The fed-batch mode of fermentation involves the periodic addition of substrate

to the reactor, followed by the removal of the product only after completion of a

reaction cycle. This mode of fermentation ensures that there is no saturation or

substrate inhibition. The literature survey suggested that only a few reports on

fed-batch dark fermentative hydrogen production as compared to other modes of H2

production. This is mainly due to the massive formation of organic acids, alcohols,

and other metabolites in the reactor, which lowers the kinetics of the process and

also inhibits the growth of the microorganism. The fed-batch fermentation has

reported up to 3.1 mol-H2/mol-glucose using microbial consortium and a yield of

2.15 mol-H2/mol-substrate using a recombinant strain of Clostridium [32]. The

potential of the fed-batch process for efficient production of fermentative H2 needs

further detailed investigations in terms of quantity and quality.

18.5

Strategies to Enhance Microbial Hydrogen

Production

The earlier studies on the improvement of biohydrogen production have suggested

several approaches, such as statistical optimization of process parameters and

fermentation medium, reconstruction of a metabolic network by metabolic flux

analysis (MFA), genetic engineering of microorganisms, and improved fermenta-

tion kinetics by application of ultrasound. This section brings forward the strategies

in detail and describes their potential to enhance microbial H2 production.