3D-Printed Air-Cathodes for Microbial Fuel Cells: Advancing Pollutant Removal and Energy Recovery from Wastewater

Abstract

Worldwide, 300 billion m3 of domestic wastewater was produced every year, which contained around 600 billlion kWh of energy. Current wastewater treatment methods, which rely heavily on aeration, are highly energy-intensive, consuming up to 3% of global electricity. This creates a paradox where energy is needed to treat wastewater, which itself contains potential energy. Microbial fuel cells (MFCs) present a promising solution by simultaneously treating wastewater and recovering energy. Air-cathode MFCs have attracted considerable attention because they do not require aeration. However, there is still no any real world commercial MFC application, primarily due to the high cost and the manufacturing challenges associated with scaling up air- cathodes. The rapid advancement of 3D printing technology in recent years offers a potential solution to these challenges. 3D printing allows for the rapid and precise fabrication of complex structures with minimal human intervention. Leveraging these benefits, this study utilized 3D printing technology to address the challenges associated with scaling up air-cathodes for MFCs. The research began with 3D printed flat air-cathodes with Fused Filament Fabrication (FFF) 3D printer and commercially available conductive filament. However, MFCs with this cathode design showed limited performance in electricity generation and wastewater treatment. To improve the performance, new flat air-cathodes were printed using virgin PLA as a substrate and coated with carbon compounds. MFCs with such cathodes demonstrated effective wastewater treatment capabilities, including the removal of 83.06% of ammonia, 94.56% of nitrate, and 90.05% of total organic carbon (TOC). Building on this success, cylindrical air-cathodes were subsequently 3D printed and tested, which can efficiently enlarge MFCs to industrial scale. This air-cathode achieved a high power density of 145.55 mW/m² and demonstrated efficient wastewater treatment, removing 85.31% of TOC and 63.03% of total nitrogen over 24 hours. III Building on all the above modifications and mechanism investigation, a pilot-scale MFC equipped with 1.1-meter air-cathode was constructed for real-world application. An 820-liter MFC reactor was tested at the Limerick Bunlicky wastewater treatment plant in Ireland. The 3D-printed air-cathode MFC effectively treated municipal wastewater, particularly in removing organic pollutants, achieving an average TOC removal rate of 57% and TN removal rate of 34% with the hydraulic retention time (HRT) of 12-hour. The design balanced oxygen permeability, waterproofing and resistance to water pressure, while significantly reducing costs by eliminating expensive membrane materials, resulting in a total cost of €3,706.03. The air cathode cost was €165.23/m². In this study, the nitrogen removal mechanisms in the 3D printed air-cathode MFCs was investigated. The results indicated that ammonia is primarily removed through simultaneous nitrification and denitrification processes, with ammonia volatilization occurring but not being essential to the process. 70%-77% of the total nitrogen was eliminated via simultaneous nitrification and denitrification. These findings were subsequently validated through microbial community analysis and consistent with previous studies. In conclusion, this thesis provides valuable insights into the 3D-printed air-cathode MFCs for sustainable wastewater treatment and energy recovery, and demonstrated the application of 3D-printed air-cathode MFCs for real-world wastewater treatment.