In-Situ Electrochemical Synthesis of a Potential Bacterial Killing Catholyte from Ceramic Microbial Fuel Cells Treating Urine
Authors: Merino Jimenez Irene , Pasternak Grzegorz , Gajda Iwona , Greenman John , Ieropoulos Ioannis 
Published: February 2020

Current disinfection methods employed for treating wastewater are expensive and use chemicals harmful to the environment. Microbial fuel cells (MFCs) offer an excellent solution to tackle some of the major challenges currently faced by mankind: sustainable energy sources, waste management and water stress. Besides producing useful electricity from urine (1), MFCs can also generate catholyte in-situ, which can be used as disinfectant for practical applications (2). Anodic bio-electrochemical reactions oxidise the organic matter from the urine releasing electrons, which are captured by the anode and transferred to the cathode via the external circuit, resulting in the generation of electricity. The separation between anodic and cathodic chambers by ceramic clay offers a low cost alternative to the ionic exchange membrane. Cylindrical ceramic MFCs have been reported to produce useful catholyte in the cathode chamber, formed by a combination of factors, including the: i) ORR taking place in the cathode electrode, ii) electro-osmotic drag, iii) diffusion due to a concentration gradient on both sides of the ceramic membrane and iv) hydraulic pressure affected by porosity of the material and the MFC design. In order to obtain a useful catholyte, an optimization of the different parameters affecting its quality, include: i) ceramic thickness, ii) electricity production and iii) time during which the catholyte was accumulated in the cathode chamber. The first two parameters have been previously studied showing a correlation between the thickness of the ceramic membrane, the amount of catholyte collected and its properties. The current generated also showed a considerable effect as a consequence of the electro-osmotic drag, increasing the pH of the catholyte with the current generated. However, the accumulation time also needs optimisation, since a more concentrated product with a higher pH will be generated, increasing the possibility of generating a highly alkaline solution, which might act as a bacterial killing agent.
Fine fire clay with three different thicknesses, 2.5, 5 and 10 mm, were tested under the optimum external load, 60 Ω, and under open circuit, and the catholyte generated was collected every 7 days. The daily production of catholyte was evaluated and the change in the catholyte properties was analysed including pH, conductivity, total solids, anion concentrations, COD reduction and cathode electrode redox potential. The effect of the catholyte properties on the MFC power was also monitored. Microbial analysis was also performed using plate count method and flow cytometry (FCM) for an accurate determination of live, dead, and total bacteria in the catholyte samples, and to determine whether the number of alive bacteria decreased with time as the solution becomes more concentrated and highly alkaline. The number of viable bacteria in the catholyte samples was also estimated.
The results show a correlation between the catholyte properties and the thicknesses of the ceramic membrane, the electricity generated from the MFCs and the operation time. The MFCs generated a constant power throughout the duration of the experiment, producing an average of 1.1, 1.4 and 1.9 mW per MFC of 2.5, 5 and 10 mm thickness, respectively, showing also a correlation with the membrane thickness, as previously suggested. The catholyte pH showed a clear dependency on the thickness, starting from 9, 9.4 and 9.7, from the MFCs with ceramic thickness of 2.5, 5 and 10 mm, respectively after one day of operation. A pH increase with time was also observed, which was more pronounced as the ceramic thickness increased, reaching 9.2, 10.3 and 11.5, respectively after 42 days of operation under the optimum load. The catholyte generated revealed killing potential against bacterial cells, which was dependent on the membrane thickness, showing that the highest killing potential was observed from the catholyte generated in the thickest FFC MFCs (10 mm). The viable counting also showed a correlation between the time the catholyte was accumulated and the number of living bacteria in the catholyte.
In this work a bio-electrochemical system, capable to decontaminate urine, generate electricity and produce catholyte with bacterial killing potential, is presented for the first time. The optimization of the FFC ceramic thickness and the catholyte accumulation time revealed that a 10 mm ceramic is required to produce good quality of catholyte after 42 days of operation, as well as generate useful constant power output. The possibility to electrochemically generate in-situ a bacterial killing agent from urine offers great opportunity for water reuse and resource recovery for practical implementations.

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