Abstract
Microbial fuel cell (MFC) is a device that can transform organic substrates to electricity and treat wastewater at the same time. But it is limited to its low power density and have not been applied with a commercial scale. The purpose of this study is to explore the possibilities of using Azotobacter vinelandii in MFCs. Different MFC designs and cathodes, including a microalgae bio-cathode, are applied to increase the power density. First, the influences of A. vinelandii, either in anode or in cathode, on the power density is investigated. The optimal setup obtained in the above tests is applied to construct the MFC with the microalgae bio-cathode to investigate if microalgae can help increase power density or can produce sufficient biomass. Results show that when two-chamber MFCs, with known electrogenic Proteus hauseri ZMd44 in the anode and A. vinelandii in the cathode, operated under a 1kΩ external load, output voltage decreased from 60 mV to 20 mV and average power density decreased from 0.5 mW/m2 to almost 0 mW/m2. This shows that A. vinelandii was not suitable for applications on the cathode. When A. vinelandii was applied in the anode with different cathode designs (PTFE air cathode, PVDF air cathode, PTFE air cathode with PEM), the power density increased by 3.6 %, 31.2 %, 19.6 %, respectively, compared with those using P. hauseri ZMd44 in the anode. This suggests that A. vinelandii has the potential in serving as an electricity generating microorganisms in MFCs. Finally, applying the optimal parameters obtained in the above tests (A. vinelandii as the anode microbes and PVDF air cathode as the cathode electrode), a two-chamber MFC with microalgae bio-cathode has been constructed to investigate the effects of Scenedesmus abundans GH-D11 on the MFC performance. Results show that the MFC with S. abundans GH-D11 in the cathode had 5 % lower dissolved oxygen compared with the control experiment (BG-11 medium in the cathode) but had 39.3 % higher power density, owing to the merit of the lower internal resistance of 1499 Ω (6423 Ω for the control experiment). However, the optical density (OD) of S. abundans GH-D11 only increased to 1.5 and it was much lower than the OD value in the original 1L culture (OD= 5~10). Optimization of culture of S. abundans GH-D11 is necessary in the future to improve the biomass productivity. In conclusion, A. vinelandii has great potential in producing electricity in MFCs and S. abundans GH-D11 in this study increased power density not by increasing the dissolved oxygen but by decreasing the internal resistance.