4.4.1.4 Methanogenesis or Methane Production
Methanogenic bacteria belonging to the genus Methanosarcina and Methanosaeta
carry out acetoclastic methanogenesis wherein acetic acid is changed into carbon
dioxide and methane which ultimately reduce oxygen in the system. Some microbial
species, called hydrogenotrophic methanogens, may produce methane from the CO 2
and H 2 produced as products in prior stages (Alexander 1961).
In addition to energy generation, degradation of organic wastes also suggests
some other benefits, including decrease in odor discharge and decrease in pathogens
levels. Apart from all these, nutrient-rich digested residues can be used as organic
fertilizers rather than mineral fertilizers and a biological substrate for the cultivation
in greenhouse (de Vries et al. 2012, Abdeshahian et al. 2016). Among the raw
materials for biogas production, waste streams of the farm and animals and
biological materials obtained from domestic and industrial activities are important
sources for biogas production.
4.4.2 Substrates Traditionally Used
Generally lignocellulosic waste obtained from municipal, agricultural, and other
routine may be used as feedstock for biogas production. Usually, slurry and animal
manure, sewage sludge, food waste, and municipal solid waste are used as feedstock
comprising of hemicelluloses, proteins, carbohydrates, fats, and celluloses. Recent
trend for enhancing biogas production involves addition of co-substrates like organic
wastes from collected municipal biowaste and/or agriculture-related industries and
food waste from households. The yield and composition of biogas are determined by
the type of co-substrate composition and feedstock. Even though proteins and
carbohydrates demonstrate earlier conversion rates than fats, it is reported that the
latter provide a higher biogas yield (Braun 1982, Braun 2007; Zubr 1986).
Large quantities of organic solid waste are produced through human activity, which
can be discussed in the form of feedstock for the production of biogas as before. On the
basis of utility, agricultural waste, various waste streams, and municipal solid waste
(MSW) can be classified as waste derived from industrial activities and urban
activities. Three billion urban residents were generated by 1.3 billion tons of MSW
per year according to a 2012 World Bank report, which would enhance to 2.2 billion
tons by 2025 (Hoornweg and Bhada-Tata 2012). MSW mainly consists of paper,
glass, plastic, wood, metal, yard trimmings, paper, paperboard, and food waste.
However, its structure varies depending on those countries and regions in which it is
collected. In order to be able to use this fraction for biogas production, inert material
including all metal, glass, and plastic should be removed before marketing. In addition,
about 15 billion tons of waste, animal manure, and crop residues are generated
annually from the agricultural sector worldwide (Donkin et al. 2013).
The food processing industry also produces waste, although its estimation is
extremely difficult, because it relies greatly on the industry and useful technology.
As an example, in the juice-producing industry, about 50% of processed fruit will
finish up as waste. In addition, 30% of the weight of chicken is not appropriate for
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Methanogenic bacteria belonging to the genus Methanosarcina and Methanosaeta
carry out acetoclastic methanogenesis wherein acetic acid is changed into carbon
dioxide and methane which ultimately reduce oxygen in the system. Some microbial
species, called hydrogenotrophic methanogens, may produce methane from the CO 2
and H 2 produced as products in prior stages (Alexander 1961).
In addition to energy generation, degradation of organic wastes also suggests
some other benefits, including decrease in odor discharge and decrease in pathogens
levels. Apart from all these, nutrient-rich digested residues can be used as organic
fertilizers rather than mineral fertilizers and a biological substrate for the cultivation
in greenhouse (de Vries et al. 2012, Abdeshahian et al. 2016). Among the raw
materials for biogas production, waste streams of the farm and animals and
biological materials obtained from domestic and industrial activities are important
sources for biogas production.
4.4.2 Substrates Traditionally Used
Generally lignocellulosic waste obtained from municipal, agricultural, and other
routine may be used as feedstock for biogas production. Usually, slurry and animal
manure, sewage sludge, food waste, and municipal solid waste are used as feedstock
comprising of hemicelluloses, proteins, carbohydrates, fats, and celluloses. Recent
trend for enhancing biogas production involves addition of co-substrates like organic
wastes from collected municipal biowaste and/or agriculture-related industries and
food waste from households. The yield and composition of biogas are determined by
the type of co-substrate composition and feedstock. Even though proteins and
carbohydrates demonstrate earlier conversion rates than fats, it is reported that the
latter provide a higher biogas yield (Braun 1982, Braun 2007; Zubr 1986).
Large quantities of organic solid waste are produced through human activity, which
can be discussed in the form of feedstock for the production of biogas as before. On the
basis of utility, agricultural waste, various waste streams, and municipal solid waste
(MSW) can be classified as waste derived from industrial activities and urban
activities. Three billion urban residents were generated by 1.3 billion tons of MSW
per year according to a 2012 World Bank report, which would enhance to 2.2 billion
tons by 2025 (Hoornweg and Bhada-Tata 2012). MSW mainly consists of paper,
glass, plastic, wood, metal, yard trimmings, paper, paperboard, and food waste.
However, its structure varies depending on those countries and regions in which it is
collected. In order to be able to use this fraction for biogas production, inert material
including all metal, glass, and plastic should be removed before marketing. In addition,
about 15 billion tons of waste, animal manure, and crop residues are generated
annually from the agricultural sector worldwide (Donkin et al. 2013).
The food processing industry also produces waste, although its estimation is
extremely difficult, because it relies greatly on the industry and useful technology.
As an example, in the juice-producing industry, about 50% of processed fruit will
finish up as waste. In addition, 30% of the weight of chicken is not appropriate for
4 Biogas: An Effective and Common Energy Tool – Part II
111