2.2
Biogas
Anaerobic digestion of organic materials produces biogas which is mainly composed
of methane and carbon dioxide, with other trace gases such as hydrogen sulfide
(McKendry 2002; Hiremath et al. 2009). The process takes place in the absence of
oxygen and bacteria degrade organic materials (Incinerators 1977). Biogas composition varies with operating conditions of the digester and nature of the feedstock.
The major portion of biogas is composed of 50–75% methane (v/v) and 25–50%
carbon dioxide (v/v). Other trace components (v/v) such as water vapor (1–5%),
nitrogen (0–5%), ammonia (0–500 ppm) and hydrogen sulfide (0–5000 ppm) are
also present (Braun 2007).
2.2.1 Biogas Production Process
Biogas production is a multi-stage and complex process that involves a consortium
of bacteria (Fig. 2.2). The followings are the steps involved in the production of
biogas.
2.2.1.1 Hydrolysis
In the first step, the complex components of waste or organic matter (carbohydrates,
lipids, and proteins) are degraded into simpler compounds (sugars, amino acids,
alcohols, and fatty acids) by cellulolytic, lipolytic, and proteolytic bacteria, respectively. Both facultative and obligatory anaerobes such as Bifidobacterium,
Megasphaera, Sporobacterium, Sphingomonas, Propionibacterium, Lactobacillus
are commonly used in hydrolysis (Khanal 2011).
adipic acid þ water ! glucose þ hydrogen
ð2:1Þ
(Korres et al. 2013)
2.2.1.2 Acidogenesis
Acidogenesis or acid production is the second stage in biogas production. The
products of the hydrolysis stage are further broken down in the second step by
acidogenic bacteria producing volatile fatty acids (short-chain organic acids),
e.g. propionic acid, lactic acid, and butyric acid. Meanwhile, carbon dioxide and
methane are also produced. Like hydrolysis, both facultative and obligatory
anaerobes such as Escherichia coli, Staphylococcus spp., Lactobacillus spp., Corynebacterium spp., Desulfovibrio spp., Bifidobacterium spp., Peptococcus
anaerobius, Clostridium spp. take part in this phase (Metcalf 2003; Khanal 2011).
2 Microbial and Biotechnological Advancement in Biogas Production
35
Biogas
Anaerobic digestion of organic materials produces biogas which is mainly composed
of methane and carbon dioxide, with other trace gases such as hydrogen sulfide
(McKendry 2002; Hiremath et al. 2009). The process takes place in the absence of
oxygen and bacteria degrade organic materials (Incinerators 1977). Biogas composition varies with operating conditions of the digester and nature of the feedstock.
The major portion of biogas is composed of 50–75% methane (v/v) and 25–50%
carbon dioxide (v/v). Other trace components (v/v) such as water vapor (1–5%),
nitrogen (0–5%), ammonia (0–500 ppm) and hydrogen sulfide (0–5000 ppm) are
also present (Braun 2007).
2.2.1 Biogas Production Process
Biogas production is a multi-stage and complex process that involves a consortium
of bacteria (Fig. 2.2). The followings are the steps involved in the production of
biogas.
2.2.1.1 Hydrolysis
In the first step, the complex components of waste or organic matter (carbohydrates,
lipids, and proteins) are degraded into simpler compounds (sugars, amino acids,
alcohols, and fatty acids) by cellulolytic, lipolytic, and proteolytic bacteria, respectively. Both facultative and obligatory anaerobes such as Bifidobacterium,
Megasphaera, Sporobacterium, Sphingomonas, Propionibacterium, Lactobacillus
are commonly used in hydrolysis (Khanal 2011).
adipic acid þ water ! glucose þ hydrogen
ð2:1Þ
(Korres et al. 2013)
2.2.1.2 Acidogenesis
Acidogenesis or acid production is the second stage in biogas production. The
products of the hydrolysis stage are further broken down in the second step by
acidogenic bacteria producing volatile fatty acids (short-chain organic acids),
e.g. propionic acid, lactic acid, and butyric acid. Meanwhile, carbon dioxide and
methane are also produced. Like hydrolysis, both facultative and obligatory
anaerobes such as Escherichia coli, Staphylococcus spp., Lactobacillus spp., Corynebacterium spp., Desulfovibrio spp., Bifidobacterium spp., Peptococcus
anaerobius, Clostridium spp. take part in this phase (Metcalf 2003; Khanal 2011).
2 Microbial and Biotechnological Advancement in Biogas Production
35