Biological wastewater treatment is typically aerobic and an energy intensive process, mainly due to the required aeration. Alternative sustainable processes are sought, such as Microbial fuel cells (MFC) where electrogenic bacteria can degrade organic matter present in the waste stream while simultaneously generating electricity.
MFCs represent an emerging technology which may deliver the capability to reduce the pollution potential of low strength wastewaters (< 1500 mg COD l-1) while generating electricity which could be used to self-power the process. Waste streams high in volatile fatty acids (VFAs) with high conductivity are particularly preferred substrate streams. These may include the effluent from two stage bio-hydrogen and bio-methane systems, which in this study were treated in a four-module tubular MFC (V=1 l) to reduce the chemical oxygen demand (COD) and recover further energy from the substrate. It was shown that the power increased with increasing organic loading rate (0.036-0.572 g sCOD l-1 d-1), but COD removal efficiency decreased. The Coulombic Efficiency (CE) was found to decrease significantly at OLR ˃ 0.6 g sCOD l-1 d-1 and the energy recovery was 92.95 J l-1 (OLR=0.572 g sCOD l-1 d-1). Also, wash-down waters from a chilled food producing company were treated in the same tubular MFC, reducing the soluble COD content by 84.8%.
The low power (≈ 30 W m-3) and cell potential (≈ 0.5 V) makes it necessary to investigate methods such as external capacitors, DC/DC converters or serial and parallel connection to improve the power quality. In this thesis, the use of the intrinsic capacitance was tested by switched mode, open and closed circuit (OC/CC) operation of a 2-module tubular MFC with high surface area carbon veil anode. The charge accumulated during OC and released when switched to CC was dependent on the external resistor (R = 100-3 kΩ) and duty cycle. Short period OC/CC switching further increased potential due to the pseudo-capacitance of the reactor, but only at the expense of energy efficiency, compared to continuous operation (CC) under constant load.
Another approach to enhance the practical implementation of MFCs is integration with other processes such as reverse electrodialysis to increase MFC’s cell potential or e.g. desalination. In this study a MFC was integrated with supported liquid membrane technology (SLM) for the first time, for the removal of metal ions of wastewater. A three chamber reactor, with a common cathode/feed phase containing 400 mg Zn2+ l-1, enabled
the simultaneous treatment of organic- and heavy metal containing wastewaters. The MFC/SLM combination produces a synergistic effect which enhances the power performance of the MFC significantly; 0.233 mW compared to 0.094 mW in the control. It is shown that the 165±7 mV difference between the MFC/SLM system and the MFC control is partially attributable to the lower cathode pH in the integrated system experiment, the consequent lower activation overpotential and higher oxygen reduction potential. The system demonstrates that within 72 h, 93±4% of the zinc ions are removed from the feed phase. A further study, with continuously operated cathode/feed chamber (100 mg Zn2+ l-1), showed that an enhanced effect on increasing cell potential was possible and could also be maintained in continuous operation.
|Date of Award
|Giuliano Premier (Supervisor), Richard Dinsdale (Supervisor) & Alan Guwy (Supervisor)
- Biological treatment