The cleaning and treatment of wastewater is a necessary, yet energy intensive practice. One of the greatest challenges society faces is therefore how to reduce the energy consumed in this process. A promising pathway to tackle this problem is by using innovative, low or zero energy consuming, technologies such as microbial fuel cells (MFCs), which harness the natural metabolism of bacteria and generate power. These microorganisms breakdown the organic content in wastewater i.e. treat/clean the liquid, and release electrons as part of their anaerobic respiration. Previous studies using MFC technology have been able to reduce the COD of the influent by up to 95% (1). Although MFCs do not generate large amounts of power, by incorporating them into existing treatment systems, the amount of energy needed to further clean wastewater is reduced. In addition, they continuously produce electricity, which can be fed back into the system and used to run low power devices (e.g. pumps for wastewater circulation).

The configuration of MFCs greatly affects their performance, with the structure and materials that they are made out of being the governing factors for energy production, catholyte synthesis and wastewater treatment. In response to the need for optimum configuration, this study explores whether the positioning of the anode and cathode (either internally or externally on a cylindrical MFC) has a significant effect on the overall performance of an MFC.

Two identical MFCs were tested, with the only variable being the anode and cathode configuration (internal anode with external cathode, and vice versa). The dimensions of the outer electrode were 40mm x 90mm, and 50mm x 70mm for the inner electrode, giving similar surface areas of 36cm² and 35cm², respectively. Results have so far shown that MFCs with the anode inside the cylinder and the cathode outside produce power outputs which are three times greater than those with internal cathodes and external anodes, with peak power outputs of 655µW and 219µW respectively.

Practically, the benefits of having an internal anode include the advantage of using the MFC directly as part of the hydraulic (pipe) network, with the substrate fed through directly. This kind of design reduces the amount of external plumbing that is needed throughout the structure, and therefore reduces the risk of leakage. In addition, the external cathode can be partially covered or contained, which allows the half-cell to remain hydrated, and for any excess liquid (catholyte) to be easily collected. This is highly desirable due to the disinfectant properties of this liquid (2), which once again can be fed back into a wastewater treatment system and reduce overall energy demand.

The acquisition of this knowledge is essential as MFCs need to be designed to suit target environments and applications, for both industrial and societal means. It is anticipated that the findings of this study will provide the MFC community with a decisive answer to how best design a cylindrical cell, which can be used as benchmark for future research.

References:

(1) Ieropoulos, I.A., Stinchcombe, A., Gajda, I., Forbes, S., Merino-Jimenez, I., Pasternak, G., Sanchez-Herranz, D. and Greenman, J., 2016. Pee power urinal–microbial fuel cell technology field trials in the context of sanitation. Environmental Science: Water Research & Technology, 2(2), pp.336-343.

(2) Gajda, I., Greenman, J., Melhuish, C. and Ieropoulos, I.A., 2016. Electricity and disinfectant production from wastewater: Microbial Fuel Cell as a self-powered electrolyser. Scientific reports, 6, p.25571.