18.4 Factors Affecting Biohydrogen

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18.4.4

Substrates

Like other biofuels, biohydrogen generated by the biological route is also highly

influenced by the type of substrate, availability, and cost of substrate. Among all

fermentation routes, dark fermentation has received enormous popularity due to

its ability to utilize a wide range of feedstock. Hydrogen-producing microbes can

metabolize both simple carbons like reducing sugars (glucose/xylose) obtained by

pretreatment and saccharification of lignocellulosic biomass and complex substrates

like starch, sucrose, and wastewater through dark fermentation. Hydrogen is clean

energy, and its production from waste further makes this process an economically

attractive concept for large-scale production. Table 18.2 lists various substrates

(simple and complex) used by different microbes for hydrogen production. Several

studies have demonstrated that it is essential to determine the optimum range of

a substrate for fermentative H2 production, as increasing substrate concentration

beyond the optimum range may inhibit the growth of H2-producing bacteria, which

would eventually result in a decrease in H2 yield. This is because the active sites

of enzymes catalyzing H2 production are saturated with a particular concentration

of substrate, beyond which may result in enzyme inhibition or substrate inhibition

kinetics.

18.4.5

Type of Reactor

The yield and production rate of H2 by biological routes highly depend on the vessel

used for fermentation. In order to attain the required rate of production, it is essential

to operate under controlled ambient conditions, which can be achieved by using an

automated bioreactor. Apart from this, reactor configuration and mode of operation

also play an essential role in maximizing the production rates. Biohydrogen reactors

can be operated under the following modes of operations, viz. batch, fed-batch, and

continuous.

18.4.5.1

Batch Mode

The batch process is the most widely used mode of operation for biohydrogen

production from various feedstocks operated under a wide range of operating condi-

tions. In the batch process, strain cultivation is carried out in closed vessels, which

offers many advantages such as simple configuration, ease in monitoring substrate

utilization, and effect of physical factors on H2 production. Several studies have been

reported on batch fermentation for H2 production by different species of Escherichia,

Clostridium, Enterobacter, Archaea, Rhodopseudomonas, and Rhodobacter. Under

batch operation, E. cloacae DM 11 and Caldicellulosiruptor owensensis have reported

the highest hydrogen yield (YH2∕S) of 3.9 and 4.0 mol-H2/mol-substrate, respectively.

Clostridium is the most studied organism for biohydrogen production via dark

fermentation, showing a hydrogen evolution rate of 27 mmol/l/h [32, 33]. Batch

fermentation with other species of Clostridium reported a maximum H2 yield of 3.35

and 2.3 mol-H2/mol-glucose [34]. Under a similar mode of operation, a maximum

hydrogen yield of 6.63 mol-H2/mol-sucrose was achieved by using Rhodobacter