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18 Advancements in Bio-hydrogen Production from Waste Biomass

18.4

Factors Affecting Biohydrogen

18.4.1

Influence of pH

The pH is a major contributing factor of fermentative hydrogen production as it

affects the growth and enzyme activities of hydrogen-producing microbes. Hydrogen

is a growth-associated product and is produced during acidogenesis phase, which

is marked by the co-production of volatile fatty acids. The pH range of 4–7 is the

most preferable for biohydrogen production. However, the optimum values depend

upon substrate and inoculum used. Hydrogen-producing microbes generally require

acidic pH for their growth and metabolism as the hydrogenase enzyme has higher

catalytic activity in a pH range of 5–6. Most of the studies on biohydrogen are carried

out in batch mode, where only the initial pH is monitored and optimized. However,

the optimum value obtained for each study varies with other parameters used in

the fermentation process. For example, the optimal pH for H2 production reported

by using organic waste as inoculum such as compost, anaerobic sludge, municipal

sewage, and activated sludge is in the range of 4.5–6.5. Whereas cracked cereals or

anaerobic digested sludge is used as inoculum, the reported optimum pH for H2 pro-

duction is 8 and 9, respectively [11]. It is also noted that the use of pure cultures such

as Enterobacter cloacae IIT-BT 08, Clostridium butyricum CGS5, Thermoanaerobac-

teriumthermosaccharolyticum PSU-2 as inoculum for H2 production requires slightly

optimum pH of the fermentation medium in the range of 5.5–6.0. In contrast, the

use of mixed cultures as inoculum favored an optimal pH of 7 [12]. This is due to the

diversity of H2-producing microbes present in the mixed cultures, which requires

different optimal pH for their growth and metabolism; hence, neutral pH is favored

for optimum growth of all microbes.

18.4.2

System Temperature

Temperature is another crucial factor that influences fermentative hydrogen

production. As the enzyme kinetics depends on the system temperature, an

optimum range is essential for higher activities of the enzymes that catalyze the

metabolic pathways of H2-producing bacteria. The optimum range of temperature

for H2 production varies again with the type of inoculum and substrate used for

fermentation. The temperature range of 35–40 ^∘C is mostly favorable for microbial

H2 production. The decrease in temperature below 35 ^∘C favors the production

of other metabolites such as ethanol, acetic acid, butyric acid, and propionic acid

due to metabolic pathway shift, thus favoring the production of co-products of the

pathway. However, the optimum temperature range of mesophilic bacteria and

thermophilic bacteria was higher and varied between 40 and 60 ^∘C. Batch mode

of H2 production by a thermophilic bacteria, Thermoanaerobacteriumthermosac-

charolyticum PSU-2, produced 2.53 mol/mol hexose at 60 ^∘C [13]. Yokoyama et al.

(2007) also reported 60 ^∘C as the optimum temperature for H2 production by using

cow dung as inoculum, which resulted in a maximum H2 yield of 743 ml/kg cow

dung [14].