(d) Microbial Electrochemical Method
Microbial electrochemical method is considered as one of the most environmentally sustainable and cost-effective biogas upgradation methods to produce methane
and to remove carbon dioxide (Lovley and Nevin 2013; Van Eerten-Jansen et al.
2012). An example for microbial electrochemical method is microbial electrolysis
cell. In microbial electrolysis cell, the oxidation of organic compounds by bacteria
releases electrons in the anode chamber and is combined with the protons in the
cathode chamber to synthesise hydrogen that can be used for upgradation of biogas
(Lu and Ren 2016; Zhang and Angelidaki 2014). Utilising the biocathode in
microbial electrolysis cell, methane can be produced by undergoing reduction of
carbon dioxide, attaining 80% energy efficiency (Cheng et al. 2009). The reduction
of carbon dioxide to methane was based on the electron transfer and the hydrogen
produced. Depending upon the cathode potential, the reduction process occurs
(Villano et al. 2010). In situ (the bioreactor is microbial electrolysis cell’s cathode)
and ex situ methods (introduction of biogas to the cathode) of biogas upgradation
technology using microbial electrolysis cell are experimentally tested to prove the
efficiency. The end result proves that the efficiency of in situ biogas upgrading
method is better with greater carbon dioxide removing capacity. Moreover, it was
found that removal of carbon dioxide is associated with both generation of methane
and ionisation of carbon dioxide. The ionisation is because of the generation of
alkalinity in the cathode (Xu et al. 2014).
Carbon dioxide can also be removed to another chamber for separation. Interestingly, it was found that the CO 2 removal was attributed to not only the production of
methane but also the CO 2 ionisation due to alkalinity generated in the cathode.
Recently, another method was presented, in which the CO 2 was removed from the
gas to a separate chamber. Microbial electrolysis cell consists of two membranes:
anion exchange membrane and proton exchange membrane. Comparing both
membranes for the removal of carbon dioxide, the proton exchange membrane
shows better removal of carbon dioxide of about 78 Æ 7% and with 83 Æ 24 meq/
Ld methane production. But this attributes to high energy requirement (Zeppilli et al.
2016). The removed carbon dioxide is converted to bicarbonate in the presence of an
alkaline environment, and the generation of this bicarbonate leads to less production
of methane content from the biogas. The electrons are provided by the water during
current generation accompanied with the reduction of carbon dioxide (Van Eerten
Jansen et al. 2012). The efficiency of biocathode is questionable if the oxygen that is
formed during oxidation gets diffused into the cathode, but it does not affect the rate
of methane production (Sadhukhan et al. 2016; Wang and Ren 2013). The usage of
cobalt tetra-amino phthalocyanine and carbon nanotubes as cathode enhances the
conversion of carbon dioxide to formic acid (Zhao et al. 2012). In the same way, the
formic acid can be synthesised by immobilising the Methylobacterium extorquens
AM1 (Hwang et al. 2015). Also, from carbon dioxide, 2-oxybutyrate and acetate can
be synthesised in microbial electrolysis cell containing Sporomusa ovata (Nevin
et al. 2010). As a whole, microbial electrochemical method is an environmentally
pleasing method for biogas upgrading (Angelidaki et al. 2018).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
259
Microbial electrochemical method is considered as one of the most environmentally sustainable and cost-effective biogas upgradation methods to produce methane
and to remove carbon dioxide (Lovley and Nevin 2013; Van Eerten-Jansen et al.
2012). An example for microbial electrochemical method is microbial electrolysis
cell. In microbial electrolysis cell, the oxidation of organic compounds by bacteria
releases electrons in the anode chamber and is combined with the protons in the
cathode chamber to synthesise hydrogen that can be used for upgradation of biogas
(Lu and Ren 2016; Zhang and Angelidaki 2014). Utilising the biocathode in
microbial electrolysis cell, methane can be produced by undergoing reduction of
carbon dioxide, attaining 80% energy efficiency (Cheng et al. 2009). The reduction
of carbon dioxide to methane was based on the electron transfer and the hydrogen
produced. Depending upon the cathode potential, the reduction process occurs
(Villano et al. 2010). In situ (the bioreactor is microbial electrolysis cell’s cathode)
and ex situ methods (introduction of biogas to the cathode) of biogas upgradation
technology using microbial electrolysis cell are experimentally tested to prove the
efficiency. The end result proves that the efficiency of in situ biogas upgrading
method is better with greater carbon dioxide removing capacity. Moreover, it was
found that removal of carbon dioxide is associated with both generation of methane
and ionisation of carbon dioxide. The ionisation is because of the generation of
alkalinity in the cathode (Xu et al. 2014).
Carbon dioxide can also be removed to another chamber for separation. Interestingly, it was found that the CO 2 removal was attributed to not only the production of
methane but also the CO 2 ionisation due to alkalinity generated in the cathode.
Recently, another method was presented, in which the CO 2 was removed from the
gas to a separate chamber. Microbial electrolysis cell consists of two membranes:
anion exchange membrane and proton exchange membrane. Comparing both
membranes for the removal of carbon dioxide, the proton exchange membrane
shows better removal of carbon dioxide of about 78 Æ 7% and with 83 Æ 24 meq/
Ld methane production. But this attributes to high energy requirement (Zeppilli et al.
2016). The removed carbon dioxide is converted to bicarbonate in the presence of an
alkaline environment, and the generation of this bicarbonate leads to less production
of methane content from the biogas. The electrons are provided by the water during
current generation accompanied with the reduction of carbon dioxide (Van Eerten
Jansen et al. 2012). The efficiency of biocathode is questionable if the oxygen that is
formed during oxidation gets diffused into the cathode, but it does not affect the rate
of methane production (Sadhukhan et al. 2016; Wang and Ren 2013). The usage of
cobalt tetra-amino phthalocyanine and carbon nanotubes as cathode enhances the
conversion of carbon dioxide to formic acid (Zhao et al. 2012). In the same way, the
formic acid can be synthesised by immobilising the Methylobacterium extorquens
AM1 (Hwang et al. 2015). Also, from carbon dioxide, 2-oxybutyrate and acetate can
be synthesised in microbial electrolysis cell containing Sporomusa ovata (Nevin
et al. 2010). As a whole, microbial electrochemical method is an environmentally
pleasing method for biogas upgrading (Angelidaki et al. 2018).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
259