2. Ex Situ Chemoautotrophic Method
Using external sources, the carbon dioxide is provided, and the hydrogen in the
anaerobic reactor containing hydrogenotrophic culture converts hydrogen to methane (Kougias et al. 2017). Simpler biochemical process, biomass-dependent process,
maintenance of the biogas process stability, usage of external sources, feasibility of
generating current to remote areas and increased stirring speed (Luo and Angelidaki
2012) are some of the advantages of ex situ chemoautotrophic method (Bassani et al.
2017; Angelidaki et al. 2018).
3. Microbial Communities in Biological Biogas Upgrading Systems
In this method, the biogas upgrading is done in two different ways. One is by
using hydrogenotrophic methanogenic archaea that converts the carbon dioxide to
methane with the hydrogen donated from external sources, and this process is known
as hydrogenotrophic methanogenesis (Stams and Plugge 2009). The other way is by
using homoacetogenic bacteria for converting carbon dioxide to acetate. Widely
used hydrogenotrophic methanogenic genera includes Methanobacterium,
Methanothermobacter, Methanoculleus and Methanomicrobium (Agneessens et al.
2017; Bassani et al. 2017; Luo and Angelidaki 2013; Mulat et al. 2017) and rarely
used genera include Methanosarcina (Agneessens et al. 2017; Mulat et al. 2017).
(b) Photoautotrophic Methods
To obtain a gas rich in methane, the photoautotrophic method is the most suitable
method with maximum carbon dioxide sequestration. In addition, the impurity-hydrogen
sulphide can be removed by using this method. About 97% methane is recovered in this
method, and the recovery depends on the selected species of algae and the type of the
reactor. Algae is widely used in the conversion process, and it is mass cultured in a
closed- or open-type ponds. The photosynthetic efficiency is high when the process is
undertaken in a closed-type pond. During the process of upgradation, the biogas is
allowed to pass through the photobioreactor for efficient conversion of the gas to
methane. The major drawback lies in the high investment cost (Angelidaki et al. 2018).
(c) Fermentation Method
In fermentation method, the carbon dioxide is converted to valuable products
such as ethanol, acetate and butyrate (Agler et al. 2011; Kennes et al. 2016). The
synthesised fatty acids like butyrate and acetate can be used for the production of
biofuels (Agler et al. 2011; Martin et al. 2016; Zhang et al. 2013). Microbes such as
Butyribacterium methylotrophicumn, Acetobacterium woodii and Clostridium
scatologenes can convert carbon dioxide and hydrogen to liquid products (SchielBengelsdorf and Durre 2012) by undergoing Wood-Ljungdahl pathway/reductive
acetyl-CoA pathway (Latif et al. 2014; Angelidaki et al. 2018).
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B. S. Dhanya et al.
Using external sources, the carbon dioxide is provided, and the hydrogen in the
anaerobic reactor containing hydrogenotrophic culture converts hydrogen to methane (Kougias et al. 2017). Simpler biochemical process, biomass-dependent process,
maintenance of the biogas process stability, usage of external sources, feasibility of
generating current to remote areas and increased stirring speed (Luo and Angelidaki
2012) are some of the advantages of ex situ chemoautotrophic method (Bassani et al.
2017; Angelidaki et al. 2018).
3. Microbial Communities in Biological Biogas Upgrading Systems
In this method, the biogas upgrading is done in two different ways. One is by
using hydrogenotrophic methanogenic archaea that converts the carbon dioxide to
methane with the hydrogen donated from external sources, and this process is known
as hydrogenotrophic methanogenesis (Stams and Plugge 2009). The other way is by
using homoacetogenic bacteria for converting carbon dioxide to acetate. Widely
used hydrogenotrophic methanogenic genera includes Methanobacterium,
Methanothermobacter, Methanoculleus and Methanomicrobium (Agneessens et al.
2017; Bassani et al. 2017; Luo and Angelidaki 2013; Mulat et al. 2017) and rarely
used genera include Methanosarcina (Agneessens et al. 2017; Mulat et al. 2017).
(b) Photoautotrophic Methods
To obtain a gas rich in methane, the photoautotrophic method is the most suitable
method with maximum carbon dioxide sequestration. In addition, the impurity-hydrogen
sulphide can be removed by using this method. About 97% methane is recovered in this
method, and the recovery depends on the selected species of algae and the type of the
reactor. Algae is widely used in the conversion process, and it is mass cultured in a
closed- or open-type ponds. The photosynthetic efficiency is high when the process is
undertaken in a closed-type pond. During the process of upgradation, the biogas is
allowed to pass through the photobioreactor for efficient conversion of the gas to
methane. The major drawback lies in the high investment cost (Angelidaki et al. 2018).
(c) Fermentation Method
In fermentation method, the carbon dioxide is converted to valuable products
such as ethanol, acetate and butyrate (Agler et al. 2011; Kennes et al. 2016). The
synthesised fatty acids like butyrate and acetate can be used for the production of
biofuels (Agler et al. 2011; Martin et al. 2016; Zhang et al. 2013). Microbes such as
Butyribacterium methylotrophicumn, Acetobacterium woodii and Clostridium
scatologenes can convert carbon dioxide and hydrogen to liquid products (SchielBengelsdorf and Durre 2012) by undergoing Wood-Ljungdahl pathway/reductive
acetyl-CoA pathway (Latif et al. 2014; Angelidaki et al. 2018).
258
B. S. Dhanya et al.