Waste to Renewable Energy: A Sustainable and Green Approach Towards Production of Biohydrogen by Acidogenic Fermentation - Sustainable Biotechnology: Sources of Renewable Energy - page 134

the reduction of molecular nitrogen to ammonium by consuming reducing power
(e - mediated by ferredoxin, NAD + etc.) and ATP. Nitrogenase catalyzes H + reduc-
tion in the absence of nitrogen gas. Even in nitrogen atmosphere, H 2 production is
catalyzed by nitrogenase as a side reaction at a rate of one-third to one-fourth that
of nitrogen-fixation. Nitrogen-fixing cyanobacteria are potential candidates for H 2
production by nitrogenase but it is an energy-consuming process due to breakdown
of many ATP molecules.

2.2 Soluble Metabolic Acid Intermediates

The soluble acid metabolites or volatile fatty acids (VFA) formed during the aci-
dogenic process help in understanding the metabolic pathway [18]. The following
equations show variable soluble acid metabolites generation during acidogenic
fermentation.

C 6 H 12 O 6 +2H 2 0 → 2CH 3 · COOH + 2CO 2 +4H 2 [Acetic acid]

C 6 H 12 O 6 → CH 3 · CH 2 · CH 2 · COOH + 2CO 2 +2H 2 [Butyric acid]

C 6 H 12 O 6 +2H 2 → 2CH 3 · CH 2 · COOH + 2H 2 O [Propoinic acid]

C 6 H 12 O 6 +2H 2 → COOH · CH 2 · CH 2 O · COOH + CO 2 [Malic acid]

→

CH 3 ·

C 6 H 12 O 6 CH 2 OH + CO 2 [Ethanol]

Depending on the pathway used by the microorganism and the corresponding
end-products, H 2 yields are variable. Products formed from pyruvate such as acetate,
butyrate, butanol, acetone, lactate or ethanol determine the theoretical yield of H 2
[3]. In obligate anaerobes, pyruvate is converted to H 2 from the reduced Fd by the
action of hydrogenase resulting in maximum yield of 2 mol H 2 /mole glucose. Two
additional moles of H 2 can be produced from NADH produced during glycoly-
sis, where NADH is oxidized by Fd reduction by NADH:ferredoxin oxidoreductase
(NFOR) [3]. Further, H 2 can be produced from the reduced Fd by hydrogenase. The
highest theoretical yield of 4 mol H 2 /mole glucose can be obtained when acetate
or acetone is the fermentation end-product. Two molecules of formate are produced
from two pyruvate molecules where a theoretical maximum yield of 2 mol H 2 /mole
of glucose can be obtained. In the case of butyrate as the fermentation end-product,
the maximum theoretical yield is 2 mol H 2 /mole glucose. When alcohols are the
end-products, lower yields of H 2 are obtained as alcohols contain additional H 2
atoms that have not been converted to H 2 gas [3]. The presence of higher concentra-
tions of propionic acid or solventogenesis is generally not considered to be feasible
for H 2 production.

3 Waste and Wastewater as Substrates for H 2 Production

One of the sustainable ways to reduce the cost of waste or wastewater treatment is to
generate bio-energy, such as H 2 gas, from the organic matter present. Waste biomass
contains enough energy to meet a significant fraction of the world's entire energy
demand, if it could be efficiently converted to useful energy forms [19]. According