Field Trial of Self-Stratifying Membrane-Less Microbial Fuel Cells Stacks in an Autonomous and Self-Powered Urinal

Abstract

The main competitive advantage of the microbial fuel cell (MFC) technology is to generate electricity from organic waste, which is otherwise considered expensive to treat; as such, interest in this field has intensified over the two last decades. Efforts have mainly focussed on improving the materials employed, the design configuration and the energy-harvesting peripherals. Many improvement solutions have been proposed within the last decade, more and more pilot-scale systems feeding on various kinds of feedstock have been tested (e.g. domestic wastewater, brewery waste, marine sediments, urine). A previous field trial at Glastonbury Music Festival has already demonstrated the feasibility of directly using urine from festival goers to light-up a urinal, thereby setting a benchmark in urine fuelled-MFCs for self-sustainable applications [1]. Furthermore, the concept of self-stratifying membraneless MFC (SSM-MFC) has also been reported. This approach allows scaling-up units’ sizes without significant power density losses, within the tested range (from 900 mL to 5000mL), a factor that has proven an obstacle in previous studies. This design had only been tested under controlled laboratory conditions and never trialled under real usage conditions [2, 3]. In the current study the results of an autonomous system, which was tested at the Glastonbury Music Festival 2016 are presented. To perform this trial, large MFC modules were built, tested and integrated in a setup that comprised a urinal, a stack of 12 SSM-MFCs modules, and an energy management control system harvesting the generated energy to power the lighting of the urinal. The urinal was large enough to accommodate 12 users at any given time and consisted of troughs directing urine to a buffer tank. This tank, equipped with an over flow redirecting excess urine, was connected to a passive feeding mechanism that was supplying urine to a MFC stack of 12 modules every time a volume of 9 L was reached. The stack was set with 6 independent cascades, each having 2 MFC modules electrically connected in parallel. Each module of the cascade comprised 38 MFCs submerged in the same electrolyte and electrically connected in parallel. All six cascades were electrically connected in series. The energy was then harvested and stored in a battery bank. At night (≈9h30 duty-cycles), the control board was redirecting the energy towards 6 LED strips (2.862W) lighting the urinal. Results from laboratory conditions have shown that the power density of a single module was ~2.75 W.m-3, whereas under real conditions the power density ranged from ~1.70 to ~2.36 W.m-3 (total volumetric footprint). The energy harvested from the undiluted urine was sufficient to power the PEE POWER® lights for 9h30 every day. Under laboratory conditions, at 44h hydraulic retention time (HRT) the COD was reduced from 5.586 mg COD.L-1 to 0.625 mg COD.L-1 (88%); the nitrogen was also reduced by 29%. In the field, with a HRT of 11h40, the COD decreased by 48% and the total nitrogen content by 13%. When plotting data from the laboratory tests together with the ones of the field trial, the fitted Michaelis-Menten curve (r2=0.960) indicates that with a HRT of ≈64h, the COD could be reduced to the European Union standard for discharge (0.125 mg COD.L-1). Compared to the 2015 field trial benchmark [1], the present system demonstrates a 37 % higher COD removal with a 50% shorter HRT, and produced ≈30% more energy in a third of the total volumetric footprint. Overall, these results correspond to an over 7-fold technological improvement. [1] Ieropoulos IA, Stinchcombe A, Gajda I, Forbes S, Merino-Jimenez I, Pasternak G, Sanchez-Herranz D and Greenman J. Pee power urinal - microbial fuel cell technology field trials in the context of sanitation. Environ. Sci.-Wat. Res. Technol. 2016;2:336-343 [2] Walter XA, Gajda I, Forbes S, Winfield J, Greenman J and Ieropoulos I. Scaling-up of a novel, simplified MFC stack based on a self-stratifying urine column. Biotechnology for Biofuel 2016;9:93 [3] Walter XA, Stinchcombe A, Greenman J and Ieropoulos I. Urine transduction to usable energy: a modular MFC approach for smartphone and remote system charging. Applied Energy 2017;192:575-581