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energy-rich reduced end products. During anaerobic respiration, bacteria

have the ability to utilize a wide range of compounds, such as NO 3 ,
2
SO 4 , organic and inorganic compounds as electron acceptors for ATP
generation with their simultaneous reduction.

6.3 BIOHYDROGEN PRODUCTION FROM WASTE
REMEDIATION
H 2 has long been recognized as a promising, green, and ideal energy carrier
of the future due to its cleaner efficiency, high energy yield (122 kJ/g), and
renewability. The H 2 gas generated either by biological machinery or
thermo-chemical treatment of biomass is normally termed biohydrogen
production (Figure 6.2). The research fraternity has shown immense interest
in biological routes of H 2 production. The past decade has witnessed signif-
icant research on biohydrogen. Biological H 2 production processes can
broadly be classified into light-independent (dark)-fermentation and light-
dependent photosynthetic processes. The photo-biological process can
again be classified either into photosynthetic or fermentation processes
depending on the carbon source and the biocatalyst used. Biophotolysis
of water using green algae and cyanobacteria or photo-fermentation
mediated by photosynthetic bacteria (PSB) are light-dependent processes.
Cyanobacteria and microalgae undergo direct and indirect biophotolysis
to produce H 2 by utilizing inorganic CO 2 in the presence of sunlight and
water, while PSB manifest H 2 production by consuming a wide variety

Biohydrogen

Biological Thermochemical
Anaerobic Microbial electrolysis

Dark-fermentation
Photobiological Enzymatic
Light- Direct Indirect
fermentation photolysis photolysis
(anoxic) (oxygenic) (oxygenic)
Figure 6.2 Schematic illustration showing various possible routes of biohydrogen
production.