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18 Advancements in Bio-hydrogen Production from Waste Biomass

18.2.2

Dark Fermentation

The dark fermentation is advantageous over light-dependent routes in terms of

a high rate of hydrogen production, utilization of various organic substrates, and

the presence of a diverse microbial catalyst. It is a complex process carried out

in an anaerobic environment manifested by a series of enzymatic reactions. The

key enzymes that drive this process in diverse groups of bacteria are mostly Fe

only and Fe–Fe hydrogenases. Both facultative and obligate anaerobes produce H2

through dark fermentation, but the mechanism driving the H2 producing pathway

is different. Both types of anaerobes consume organic carbon redirecting carbon

flow toward pyruvate. In facultative anaerobes, pyruvate is transformed to acetyl

CoA and formate catalyzed by pyruvate formate lyase followed by reduction of

formate by formate hydrogen lyase to produce H2. On the other hand, obligate

anaerobes transform pyruvate to acetyl-CoA and carbon dioxide, catalyzed by

pyruvate ferredoxin oxidoreductase enzyme as depicted in Figure 18.1b [5]. Sol-

ventogenesis and acidogenesis are two major phases of product formation during

anaerobic fermentation, and H2 is a primary product of acidogenesis resulting in

the formation of volatile fatty acids as a by-product. The yield of H2 varies based

on the type of acids produced during the phase. Acetate as the by-product releases

maximum H2 as 4 mol of H2/mol of acetate, while the butyrate pathway generates

2 mol of H2/mol of glucose [6].

18.2.3

Photo-Fermentation

The photo-fermentation route of H2 production is distinguished by higher hydrogen

yield, the use of organic acids as substrates, and sunlight as a source of energy.

During the photo-fermentation, organic acids such as acetic acid and succinic acid

are metabolized by photosynthetic bacteria to produce nicotinamide adenine di

nucleotide (NADH), which reduces nitrogenase via reverse electron transport [7].

The reduced nitrogenase, in turn, uses photosynthetically produced adenosine

triphosphate (ATP) to reduce the protons to molecular H2, as shown in Figure 18.1c.

2H⁺ + 2e⁻+ 4ATP →H2 + 4ADP + 4Pi

(18.1)

Although this process has some distinct advantages, it shows poor light conversion

efficiency due to the high energy (ATP) requirements of the nitrogenase enzyme.

Moreover, photo-fermentative bacteria cannot directly feed on sugar hydrolysates;

instead, they feed on volatile fatty acids, which are major by-products of dark fer-

mentation. Thus, an integrative process of dark and photo-fermentation can be used

to produce a generous amount of H2 from waste biomass.

18.3

Biomass as Feedstock for Biohydrogen

Biomass has recently received much attention as a suitable feedstock for waste to

energy conversion, and it can be categorized as: