condensation and separation of hydrocarbons and water from the biogas (Zhang
et al. 2006). The main disadvantage of this method is the expensive cost (Sahota
et al. 2018).
(A) Biological Methods
The biological biogas upgrading technologies are categorised into four types:
(a) Chemoautotrophic method
(b) Photoautotrophic method
(c) Fermentation method
(d) Microbial electrochemical method
(a) Chemoautotrophic Method
In chemoautotrophic method, the hydrogenotrophic methanogens utilise hydrogen and convert carbon dioxide to methane gas. Since it is a biological method, the
hydrogen to be utilised is generated from the renewable resources, and so, the water
is hydrolysed using renewable electricity to produce hydrogen. Converting electricity to chemical energy is highly promising. For methane, the energy required is about
36 MJ/m
3 and for hydrogen it is about 10.88 MJ/m
3 . Also, the investment cost is less
when compared to other upgrading techniques. The major advantage of this method
is that carbon dioxide is not separated; instead, it is directly converted to methane
(Kougias et al. 2017).
It can be again classified into two types:
1. In situ chemoautotrophic method
2. Ex situ chemoautotrophic method
3. Microbial communities
1. In Situ Chemoautotrophic Method
In in situ chemoautotrophic method, the hydrogen is allowed to couple with the
carbon dioxide that is generated in the anaerobic digester, and it is directly converted
into methane by the involvement of autochthonous methanogenic archaea (Kougias
et al. 2017). The methanogenesis process tends to be inhibited, when the pH is about
8.5. Increase in pH leads to the bicarbonate ion removal. Inhibition of
methanogenesis is a challenge to be tackled, and to overcome this, codigestion
process was performed, thereby stopping increase in pH (Luo and Angelidaki
2013) or the pH level is controlled completely (Luo et al. 2014). Alcohols and
fatty acids undergo oxidation which leads to decreased concentration of hydrogen
(Batstone et al. 2002).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
257
et al. 2006). The main disadvantage of this method is the expensive cost (Sahota
et al. 2018).
(A) Biological Methods
The biological biogas upgrading technologies are categorised into four types:
(a) Chemoautotrophic method
(b) Photoautotrophic method
(c) Fermentation method
(d) Microbial electrochemical method
(a) Chemoautotrophic Method
In chemoautotrophic method, the hydrogenotrophic methanogens utilise hydrogen and convert carbon dioxide to methane gas. Since it is a biological method, the
hydrogen to be utilised is generated from the renewable resources, and so, the water
is hydrolysed using renewable electricity to produce hydrogen. Converting electricity to chemical energy is highly promising. For methane, the energy required is about
36 MJ/m
3 and for hydrogen it is about 10.88 MJ/m
3 . Also, the investment cost is less
when compared to other upgrading techniques. The major advantage of this method
is that carbon dioxide is not separated; instead, it is directly converted to methane
(Kougias et al. 2017).
It can be again classified into two types:
1. In situ chemoautotrophic method
2. Ex situ chemoautotrophic method
3. Microbial communities
1. In Situ Chemoautotrophic Method
In in situ chemoautotrophic method, the hydrogen is allowed to couple with the
carbon dioxide that is generated in the anaerobic digester, and it is directly converted
into methane by the involvement of autochthonous methanogenic archaea (Kougias
et al. 2017). The methanogenesis process tends to be inhibited, when the pH is about
8.5. Increase in pH leads to the bicarbonate ion removal. Inhibition of
methanogenesis is a challenge to be tackled, and to overcome this, codigestion
process was performed, thereby stopping increase in pH (Luo and Angelidaki
2013) or the pH level is controlled completely (Luo et al. 2014). Alcohols and
fatty acids undergo oxidation which leads to decreased concentration of hydrogen
(Batstone et al. 2002).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
257