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Reorienting Waste Remediation Towards Harnessing Bioenergy 239

of substrates ranging from inorganic to organic acids in the presence of light
(Allakhverdiev et al., 2010; Beer et al., 2009; Ntaikou et al., 2010; Venkata
Mohan, 2008, 2009, 2010; Venkata Mohan and Pandey, 2013). The dark-
fermentation process proceeds to the anaerobic process, where acidogenic
bacteria (AB) metabolically generate H 2 along with VFA and CO 2 through
acetogenesis. Synthetic enzymes mediate in vitro H 2 production, which is
one of the most fascinating routes envisaged by scientists, albeit still at the
laboratory scale. Microbial electrolysis is a hybrid strategy wherein external
potential is applied to the microbial fuel cell (MFC) to enhance biological H 2
production. At present, H 2 is being produced mainly from fossil sources and
the electrolysis of water.

6.3.1 Dark-Fermentation

Fermentative conversion of organics to their end products involves a series
of biochemical reactions, such as hydrolysis, acidogenesis, acetogenesis, and
methanogenesis manifested by five physiologically distinct groups of micro-
organisms. The complex organic compounds get degraded to monomers
during hydrolysis by hydrolytic microorganisms. Further, these monomers
will be fermented by AB in order to generate a mixture of low molecular
weight volatile organic acids (e.g., acetic acid, propionic acid, butyric acid,
malic acid) associated with H 2 and CO 2 production (Equations 6.1–6.5).
The reversible interconversion of acetate from H 2 and CO 2 by acetogens
and homoacetogens can also be considered for H 2 production. Acetoclastic
methanogens convert organic acids to CH 4 and CO 2 through methanogen-
esis. Dark-fermentation by anaerobic (acidogenic) bacteria is the most
widely understood process for biohydrogen production. AB grow in syn-
trophic association with the hydrogenotrophic methanogens (H 2 consum-
ing MB) and keep H 2 partial pressure low enough to allow acidogenesis so
that the reaction thermodynamically favorable by interspecies H 2 transfer.
Methanogenic activity needs to be restricted to make H 2 a metabolic
end-product.

C 6 H 12 O 6 +2H 2 O!2CH 3 COOH+2CO 2 +4H 2 (acetic acid) (6.1)
C 6 H 12 O 6 !CH 3 CH 2 CH 2 COOH+2CO 2 +2H 2 (butyric acid) (6.2)
C 6 H 12 O 6 +2H 2 !2CH 3 CH 2 COOH+2H 2 O (propionic acid) (6.3)
C 6 H 12 O 6 +2H 2 !COOH CH 2 CH 2 COOH+CO 2
(malic acid) (6.4)
C 6 H 12 O 6 !CH 3 CH 2 OH+CO 2 (ethanol) (6.5)