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Reorienting Waste Remediation Towards Harnessing Bioenergy 237

Waste® Waste®
Carbohydrate Butyrate
®Ethanol ®Butanol

Biohydro- Solventogenesis
genesis (Complete) Waste® Simpler
Waste® Algal sugar + H 2 ®
Biomass® Lipid Methane+ H 2 O
Methano-
Lipogenesis genesis

Anoxic Bioelectro-
respiration genesis
Waste® VFA® Waste ® Simpler
PHA Solventogenesis sugar® CO 2 +H +
(Incomplete) −
+e
Waste®
Carbohydrate®
Ethanol
Figure 6.1 Various possible routes of bioenergy generation from waste through
simultaneous remediation.

After glycolysis, the aerobic metabolism proceeds with the tricarboxylic acid
cycle and oxidative phosphorylation, whereas the anaerobic process con-
tinues with interconversion (dehydrogenation), decarboxylation, solvento-
genesis, methanogenesis, and other mechanisms. During glycolysis, glucose
molecule is converted to pyruvate, which is the key molecule of microbial
fermentation. During the aerobic process, pyruvate transforms to CO 2 and
H 2 O. Pyruvate has a different fate during anaerobic fermentation under dif-
ferent environmental conditions. If it enters the acidogenic pathway, it gen-
erates volatile fatty acids (VFAs) in association with the generation of H 2 and
CO 2 . During aerobic respiration, the electron molecules pass through a
redox cascade of the respiratory/electron-transport chain where their energy
is gradually transformed to ATP through oxidative phosphorylation and get
reduced in the presence of an externally available TEA (oxygen). However,
ATP generation is not assured during these processes because the energy
from the reducing equivalents (protons and electrons) will be used to com-
plete the terminal reduction reaction with TEA but not necessarily trans-
ferred to the bonding between ADP and inorganic phosphate (Pi) to
generate ATP at the ATPase complex. On the contrary, oxygen does not
act as a TEA in an anaerobic metabolism and hence the electrons flow occurs
via a series of interconversion reactions that can lead to the formation of