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The Use of Bioelectrochemical System (BES) to Upgrade Biogas into Biomethane under Thermophilic Conditions

Liu, Siying (2017) The Use of Bioelectrochemical System (BES) to Upgrade Biogas into Biomethane under Thermophilic Conditions. Masters by Research thesis, Murdoch University.

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Anaerobic digestion (AD) is used increasingly worldwide to convert organic waste materials into the renewable energy of methane gas. However, the produced biogas consists of a mixture of typically 50% methane CO2, respectively. The presence of CO2 in biogas affects engine performance; therefore, removing CO2 content will significantly improve the usability of biogas. Recently, integrating bioelectrochemical systems (BESs) with AD processes have been considered as a way to increase methane content.

The principle of the BES in enriching biogas in methane content is thought to be in the transfer of electrons from an active cathode either directly to methanogens or via reduction of protons to H2 and subsequent H2 transfer to hydrogenotrophic methanogens that reduce CO2 to CH4. Two different types of BES have been described in this thesis: one is membrane-free single-chamber reactor which contains both anode and cathode; the other reactor consists of two-chambers separated by an ion-exchange membrane in order to avoid interference of the anodic reaction with the cathode.

This study combined BES with AD (thermophilic anaerobic sludge used as an inoculum) and investigated the capability of upgrading biogas to methane under thermophilic conditions. For the first time, we showed a direct comparison between single and two-chamber systems. Based on the results, the following observations were made:

1. The application of the BES to a glucose fed AD resulted in an immediate increase in biogas production (approximately from 1 L/L/d to 1.5 L/L/d) and methane content (approximately from 50% to about 65%). This applied for both reactor configurations (single- and two-chamber for the first 4 days. However, after one week the methane content of the single chamber decreased to that of the control AD without BES, while the two-chamber reactor increased its methane level to > 90%. The reason for the diminished longer term performance of the single chamber reactor is presumably the production of oxygen by the anode as indicated by increased redox potential.

2. pH adjustment was needed during the operation of the two-chamber reactor as its cathodic chamber tended to be alkaline (>9); while the pH of the single-chamber reactor was in the range of 6-7.

3. After 3 weeks experiment, both single- and two-chamber reactor showed signs of failure by an accumulation of VFA and diminished methane production. The build-up of VFA was attributed to the H2 produced by the BES inside the AD which was known to result in VFA accumulation.

To avoid the volatile fatty acids (VFAs) accumulation, the BES was removed from the AD and placed in-line with the AD, receiving the biogas by functioning as a biogas-filter. This novel concept of using a BES as a biogas-filter was evaluated as an alternative way of upgrading biogas to methane. And the microbial community of a mixed-culture methane producing biocathode was investigated to illuminate the possible role of microbial in methane production. Results revealed that:

1. When applying a WE potential at -1.1 V vs. Ag/AgCl the biofilter increased the methane content of inflowing biogas from 50% to 85%.

2. The microbial community at the start of the experiment was dominated by two phylotypes of Archea- Methanosarcina and Methanobacteria. However, at the end of experiment hydrogenotrophic Methanothermobacter was the majority population of Archea (95.4%).

3. Based on the analysis of energy efficiency, it was showed that final energy output as extra methane was 57.1 kJ and the overall energy efficiency was 56%.

4. The use of BES as an inline biofilter to remove CO2 from biogas represents an alternative, not yet published approach that is not likely to cause digester failure as observed by BES imbedded in the AD but showing similar performance.

Overall, this thesis showed the capacity of BES on biogas upgrading and the selection of electromethanogens on the biocathode during the BES operation in the two-chamber reactor. Thus, further studies on the optimisation of BES process and the mechanisms of catalysis of biocathode will provide essential knowledge for improving BES performance for the practical application.

Publication Type: Thesis (Masters by Research)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Cord-Ruwisch, Ralf, Charles, Wipa and Cheng, Kayu
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