Page 266 - Industrial Wastewater Treatment, Recycling and Reuse

240 Industrial Wastewater Treatment, Recycling, and Reuse

The proton-reducing reactions aid in the formation of H 2 , a common
fermentation by-product when electron-acceptor is limited (Madsen,
2008). Both obligate and facultative AB can catalyze H 2 production from
organic substrates (Hallenbeck and Benemann, 2002; Vardar-Schara et al.,
2008; Venkata Mohan, 2009, 2010; Venkata Mohan and Pandey, 2013).
Pyruvate enters the acidogenic pathway and generates H 2 along with
VFA (Equations 6.1–6.4). Obligate anaerobes convert pyruvate to acetyl
coenzyme A (acetyl-CoA) and CO 2 through pyruvate ferredoxin oxidore-
ductase by the reduction of ferredoxin (Fd) (Kraemer and Bagley, 2007;
Vardar-Schara et al., 2008). Pyruvate is converted to acetyl-CoA and for-
mate by the action of pyruvate formate lyase by facultative anaerobes that
produce H 2 by formate hydrogen lyase (Vardar-Schara et al., 2008).
Hydrogenase and nitrogenase are the two most important enzymes involved
in fermentative H 2 production. They catalyze the reversible reduction of H +
to H 2 (Hallenbeck and Benemann, 2002) while [Fe-Fe]-hydrogenase
removes the excess reducing equivalents.

6.3.1.1 Selective Enrichment of Biocatalyst
Diverse groups of microorganisms—anaerobic, photosynthetic (heterotro-
phic and autotrophic), and microalgae—are capable of producing H 2 by
taking advantage of their specific metabolic route under defined con-
ditions. Obligate anaerobes, thermophiles, methanogens, and a few faculta-
tive anaerobes can produce H 2 through the dark-fermentation mechanism.
After wastewater started to be used as a feedstock, particularly in the last
decade, the application of mixed consortia as a biocatalyst received a great
deal of attention and is considered to be a practical options for scaling up.
Mixed cultures facilitate operational flexibility, restrict the requirement of
sterile conditions, can use a broad range of substrates, have good stability,
and infuse diverse biochemical functions (Angenent et al., 2004; Venkata
Mohan, 2008, 2010; Venkata Mohan et al., 2013b; Wang and Wan,
2009). Therefore, producing H 2 with mixed consortia offers lower opera-
tional costs and an ease of control in concurrence with the possibility of using
waste as a feedstock.
Mixed microbes encompass various physiological groups of bacteria have
diverse metabolic functions that are not necessarily specific to H 2 produc-
tion. The mixed consortia support proton reduction during methanogenesis
rather than its shuttling between intermediates during the interconversion of
metabolites, which is presumed to be necessary for H 2 to form as an end-
product (Venkata Mohan and Goud, 2012; Venkata Mohan et al.,