1.2.3 Acetogenesis
Acetogenesis is the conversion of VFAs such as propionic, butyric, and valeric acids
as well as alcohols to acetate, H 2 , and CO 2 (Table 8.2: Eqs. 10 to 13). Acetogenesis
is carried out by a specific group of bacteria called proton-reducing acetogenic
bacteria. These bacteria produce the substrates (acetate and H 2 ) utilized by
methanogens [49]. Acetogens suffer from product inhibition under elevated H 2
levels [52]. Generally, acetogenesis from propionic and butyric acids is desired
because it releases H 2 and contributes to increasing its yield.
1.2.4 Homoacetogenesis
Homoacetogens are fast-growing anaerobic bacteria that grow on H 2 and CO 2 and/or
on the fermentation of organic compounds to produce acetate [53]. They oxidize or
synthesize acetate depending on the external H 2 concentration [54]. At high P H 2
(>500 Pa), acetogenesis is favored (from H 2 and CO 2 ), and at low P H 2 (<40 Pa),
acetate oxidation occurs [55]. Homoacetogenesis (Table 8.2, Eq. 14) is undesirable
during H 2 fermentation because it decreases the H 2 yield.
1.2.5 Methanogenesis
Methanogenesis is the final stage of anaerobic digestion. Methanogens produce CH 4
either by cleavage of acetic acid molecules to CO 2 and CH 4 or by reduction of CO 2
with H 2 (Table 8.2; Eqs. 15 and 16). They grow directly on acetate and H 2 /CO 2 and
other one-carbon compounds such as formate, methanol, and methylamine
[56]. Acetoclastic methanogens can utilize only acetate, while hydrogenotrophic
methanogens use H 2 /CO 2 . Therefore, they live in syntrophic association with
acetogens because the latter produce the substrate for the former. Hydrogenotrophic
methanogenesis regulates the anaerobic process by maintaining low P H 2 , affects the
syntrophic acetogens, and influences the whole fermentation process [56]. In anaerobic digestion, up to 70% of CH 4 is produced from acetate, while 30% is derived
from H 2 /CO 2 [57]. Methanogens are sensitive to oxygen concentration and to
changes in operational parameters and environmental conditions such as pH, temperature, hydraulic loading rate, organic loading rate (OLR), and feed
composition [58].
Hydrogenotrophic methanogens are more resistant to environmental changes and
grow faster than acetoclastic methanogens [49]. Inhibition of methanogens by low
pH or toxic chemicals results in the accumulation of H 2 and VFAs; such conditions
increase concentrations of reduced products [54].
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331
Acetogenesis is the conversion of VFAs such as propionic, butyric, and valeric acids
as well as alcohols to acetate, H 2 , and CO 2 (Table 8.2: Eqs. 10 to 13). Acetogenesis
is carried out by a specific group of bacteria called proton-reducing acetogenic
bacteria. These bacteria produce the substrates (acetate and H 2 ) utilized by
methanogens [49]. Acetogens suffer from product inhibition under elevated H 2
levels [52]. Generally, acetogenesis from propionic and butyric acids is desired
because it releases H 2 and contributes to increasing its yield.
1.2.4 Homoacetogenesis
Homoacetogens are fast-growing anaerobic bacteria that grow on H 2 and CO 2 and/or
on the fermentation of organic compounds to produce acetate [53]. They oxidize or
synthesize acetate depending on the external H 2 concentration [54]. At high P H 2
(>500 Pa), acetogenesis is favored (from H 2 and CO 2 ), and at low P H 2 (<40 Pa),
acetate oxidation occurs [55]. Homoacetogenesis (Table 8.2, Eq. 14) is undesirable
during H 2 fermentation because it decreases the H 2 yield.
1.2.5 Methanogenesis
Methanogenesis is the final stage of anaerobic digestion. Methanogens produce CH 4
either by cleavage of acetic acid molecules to CO 2 and CH 4 or by reduction of CO 2
with H 2 (Table 8.2; Eqs. 15 and 16). They grow directly on acetate and H 2 /CO 2 and
other one-carbon compounds such as formate, methanol, and methylamine
[56]. Acetoclastic methanogens can utilize only acetate, while hydrogenotrophic
methanogens use H 2 /CO 2 . Therefore, they live in syntrophic association with
acetogens because the latter produce the substrate for the former. Hydrogenotrophic
methanogenesis regulates the anaerobic process by maintaining low P H 2 , affects the
syntrophic acetogens, and influences the whole fermentation process [56]. In anaerobic digestion, up to 70% of CH 4 is produced from acetate, while 30% is derived
from H 2 /CO 2 [57]. Methanogens are sensitive to oxygen concentration and to
changes in operational parameters and environmental conditions such as pH, temperature, hydraulic loading rate, organic loading rate (OLR), and feed
composition [58].
Hydrogenotrophic methanogens are more resistant to environmental changes and
grow faster than acetoclastic methanogens [49]. Inhibition of methanogens by low
pH or toxic chemicals results in the accumulation of H 2 and VFAs; such conditions
increase concentrations of reduced products [54].
8 Biohydrogen of Industrial Waste
331