Scientists at the University of the West of England in Bristol have carried out research testing the viability of urine as a potential fuel for Microbial Fuel Cells in order to directly produce electricity.

MFCs consist of two half-cells - an anode and a cathode - that are separated by an ion selective membrane. Commonly, bacteria are in the anode side and chemicals or oxygen are in the cathode side, which complete the reactions (i.e. close the circuit) to generate power.

So far the use of urine as a biomass that can be converted to power via MFCs has been neglected by scientists, despite the fact that urine is an abundant waste product. Each human produces approximately 2.5 litres of urine a day, amounting to around 6.4 trillion litres globally each year.

"Urine is chemically rich in substances favourable to the MFCs. Our research found that the output of electricity was consistent and measurable depending on the volume of urine and the timing of the doses. At the moment the output from one MFC is small," study author Ioannis Ieropoulos said in a statement.

He asserted that the study shows that by miniaturisation and multiplication of the number of MFCs into a stack and regulating the flow of urine, it may be possible to look at scales of use that have the potential to produce useful levels of power, for example, in a domestic or small village setting.

"A stack consists of a number of MFCs, each just a few millilitres of volume, connected together so that the stream of urine runs through the MFCs and produces power as the microbes inside the MFCs get to work on this abundant fuel, which is rich in (amongst other things) carbohydrates, nitrogen, potassium, phosphorous and other organic compounds that collectively make it very good for the microbial fuel cells," Ieropoulos added.

The study outlines urine production from humans; farm animals worldwide produce two to three times as much urine as humans, adding up to approximately 38 billion litres a day. The scientists suggest that by scaling up MFCs into stacks, further research might bring the levels of electricity production closer to those currently produced by biofuels. Urine produced by farm animals can also have benefits for the environment.

The advantage of an MFC system that uses urine - whether human or animal - is that the process within the MFCs effectively "cleans" the urine so that it can safely be discharged into the environment, removing the need for conventional treatment by wastewater companies.

Authors Ioannis Ieropoulos, John Greenman and Chris Melhuish have published "Urine Utilisation by Microbial Fuel Cells; Energy Fuel for the Future" in the journal Physical Chemistry Chemical Physics.

"The impact of this could be huge, since it enables us to think of 'waste' in a new way, and offers great potential for the future and we are grateful to the EPSRC* for funding this work," Ieropoulos concludes.