6.1.1 Biogas Production Process
The four fundamental steps of anaerobic digestion include hydrolysis, acidogenesis,
acetogenesis, and methanogenesis (Fig. 6.1). Throughout the entire process, large
organic polymers that make up biomass are broken down into smaller molecules by
chemicals and microorganisms. Upon completion of the anaerobic digestion process,
the biomass is converted into biogas consisting of carbon dioxide and methane, and
digestate (Dioha et al. 2013).
Table 6.1 shows the biochemical reactions taking place in each step during biogas
production, and the overall reaction is shown below:
6.1.1.1 Hydrolysis
In general, hydrolysis is a chemical reaction in which the breakdown of water occurs
to form H
+ cations and OH
À anions (Sivamani et al. 2018). Hydrolysis is often used
to break down larger polymers, often in the presence of an acidic catalyst. In
anaerobic digestion, hydrolysis is the essential first step, as biomass is normally
comprised of very large organic polymers. Through hydrolysis, these large
polymers, namely, carbohydrates, Lipids, and proteins, are broken down into smaller
molecules such as simple sugars, fatty acids, and amino acids, respectively
(Sivamani et al. 2018).
Biomass
Hydrol
ysis
Acidog
enesis
Acetog
enesis
Metha
nogene
sis
Biogas
Fig. 6.1 Biogas production process
Table 6.1 Biochemistry of biogas production (Bajpai 2017)
Step
Reaction
Hydrolysis
Carbohydrates + H 2 O ! Glucose
Lipids + H 2 O ! Fatty acids
Proteins + H 2 O ! Amino acids
Acidogenesis
C 6 H 12 O 6 + 2 H 2 ! 2 C 2 H 5 COOH + 2 H 2 O
C 6 H 12 O 6 ! 2 C 2 H 5 OH + 2 CO 2
Acetogenesis
C 2 H 5 COOH + 2 H 2 O ! CH 3 COOH + CO 2 + 3 H 2
C 6 H 12 O 6 + 2 H 2 O ! 2 CH 3 COOH + 2 CO 2 + 4 H 2
C 2 H 5 OH + H 2 O ! CH 3 COOH + 2 H 2
C 6 H 12 O 6 ! 3 CH 3 COOH
Methanogenesis
2 C 2 H 5 OH + CO 2 ! 2 CH 3 COOH + CH 4
CH 3 COOH ! CH 4 + CO 2
CO 2 + 4 H 2 ! CH 4 + 2 H 2 O
CH 3 COOH + H 2 SO 4 ! 2 H 2 O + 2 CO 2 + H 2 S
7 CH 3 COOH + 8 NO 2 ! 2 H 2 O + 14 CO 2 + 8 NH 3
Overall
C a H b O c N d S e + f H 2 O ! p CH 4 + q CO 2 + r NH 3 + s H 2 S
142
S. Sivamani et al.
The four fundamental steps of anaerobic digestion include hydrolysis, acidogenesis,
acetogenesis, and methanogenesis (Fig. 6.1). Throughout the entire process, large
organic polymers that make up biomass are broken down into smaller molecules by
chemicals and microorganisms. Upon completion of the anaerobic digestion process,
the biomass is converted into biogas consisting of carbon dioxide and methane, and
digestate (Dioha et al. 2013).
Table 6.1 shows the biochemical reactions taking place in each step during biogas
production, and the overall reaction is shown below:
6.1.1.1 Hydrolysis
In general, hydrolysis is a chemical reaction in which the breakdown of water occurs
to form H
+ cations and OH
À anions (Sivamani et al. 2018). Hydrolysis is often used
to break down larger polymers, often in the presence of an acidic catalyst. In
anaerobic digestion, hydrolysis is the essential first step, as biomass is normally
comprised of very large organic polymers. Through hydrolysis, these large
polymers, namely, carbohydrates, Lipids, and proteins, are broken down into smaller
molecules such as simple sugars, fatty acids, and amino acids, respectively
(Sivamani et al. 2018).
Biomass
Hydrol
ysis
Acidog
enesis
Acetog
enesis
Metha
nogene
sis
Biogas
Fig. 6.1 Biogas production process
Table 6.1 Biochemistry of biogas production (Bajpai 2017)
Step
Reaction
Hydrolysis
Carbohydrates + H 2 O ! Glucose
Lipids + H 2 O ! Fatty acids
Proteins + H 2 O ! Amino acids
Acidogenesis
C 6 H 12 O 6 + 2 H 2 ! 2 C 2 H 5 COOH + 2 H 2 O
C 6 H 12 O 6 ! 2 C 2 H 5 OH + 2 CO 2
Acetogenesis
C 2 H 5 COOH + 2 H 2 O ! CH 3 COOH + CO 2 + 3 H 2
C 6 H 12 O 6 + 2 H 2 O ! 2 CH 3 COOH + 2 CO 2 + 4 H 2
C 2 H 5 OH + H 2 O ! CH 3 COOH + 2 H 2
C 6 H 12 O 6 ! 3 CH 3 COOH
Methanogenesis
2 C 2 H 5 OH + CO 2 ! 2 CH 3 COOH + CH 4
CH 3 COOH ! CH 4 + CO 2
CO 2 + 4 H 2 ! CH 4 + 2 H 2 O
CH 3 COOH + H 2 SO 4 ! 2 H 2 O + 2 CO 2 + H 2 S
7 CH 3 COOH + 8 NO 2 ! 2 H 2 O + 14 CO 2 + 8 NH 3
Overall
C a H b O c N d S e + f H 2 O ! p CH 4 + q CO 2 + r NH 3 + s H 2 S
142
S. Sivamani et al.