Why let your waste go to waste when it could be powering your mobile phone - or even your car?
It is a bright spring morning here at Heriot-Watt University in Edinburgh, UK, where I have come to meet my interviewee for this article, Shanwen Tao. Normally when I interview someone, I give them a business card and maybe the latest issue of New Scientist. Today, I give Tao a bottle of my own pee.
Chemist Tao doesn't find this odd. Urine, he believes, could help solve the world's energy problems, powering farms and even office buildings. And he has agreed to use my offering to show me how.
Urine might not pack the punch of rocket fuel, but what it lacks in energy density it makes up for in sheer quantity. It is one of the most abundant waste materials on Earth, with nearly seven billion people producing roughly 10 billion litres of it every day. Add animals into the mix and this quantity is multiplied several times over.
As things stand, this flood of waste poses a problem. Let it run into the water system and it would wipe out entire ecosystems; yet scrubbing it out of waste water costs money and energy. In the US, for instance, waste water treatment plants consume 1.5 per cent of all the electricity the country generates. So wouldn't it be nice if, instead of being a vast energy consumer, urine could be put to use.
That thought occurred to Gerardine Botte, a chemical engineer at Ohio University in Athens, during a discussion in 2002 with colleagues about possible sources of hydrogen for use in fuel cells.
Hydrogen can be produced from fossil fuels in large quantities, but it is difficult to store and distribute. Another option is to split water on the spot, releasing hydrogen directly into a fuel cell - but here as much energy is needed to split the water as is released by the hydrogen.
Botte's brainwave was to use urine instead of water. By weight, urine contains roughly 2 per cent urea, and each urea molecule contains four hydrogen atoms, which, crucially, are less tightly bound to the molecule than the hydrogen in water. Splitting these bonds would require less energy, making hydrogen production more efficient.
Last year, Botte's team reported that they had been able to generate hydrogen from urine using an electrolytic cell with cheap nickel-based electrodes running at only 0.37 volts- much less than the 1.23 volts it takes to split water (Chemical Communications, 2009, p 4859). Pure hydrogen bubbled off at the cathode, while nitrogen and carbon dioxide formed at the anode.
Botte calculates that with more efficient electrodes, hydrogen could be produced from urine at a cost of less than $US1 per kilogram. She thinks the technology could be useful wherever large numbers of people congregate and enough urine can be collected to make the process worthwhile. "An office building where 200 or 300 people work could produce about two kilowatts of power," she says.
Another approach is to forget about hydrogen and use urine directly as a fuel. This is the approach being taken by Tao and his colleague Rong Lan, along with John Irvine from the University of St Andrews, also in the UK. Since 2007, the team have been developing a fuel cell that can produce electricity directly from urine (see diagram). No voltage needs to be applied to break down the urea; instead, a low-cost electrode makes the reaction happen spontaneously. The details of the electrode are still secret.
Inside the fuel cell, water and air close to the 1cm sq cathode generate hydroxide ions, which are attracted to the anode. There they react with urea to form water, nitrogen and carbon dioxide. This reaction also generates electrons, which flow back to the cathode through an external circuit, forming a current that the team hope will one day be large enough to power electrical devices (Energy and Environmental Science, vol 3, p 438).
To show me the process in action, Tao and Lan add my urine to the fuel cell. As it flows into the cell, a screen shows the output voltage rising to about 0.6 volts. While this prototype is too small to power a light bulb - its output is about half that of an AA battery - scaling up the cell and connecting several cells together should produce practical amounts of power.
Tao hopes that even small urine fuel cells will one day become useful, if the right electrode materials can be found to boost their power output. They could be used to power radios or phones in remote locations, for example. "You could carry a small fuel cell for low-power mobile communications without having to carry the fuel," he says.
A larger-scale application could be found in farms. As the urine from all mammals contains urea, that from cattle, say, could be used to generate electricity to run farm buildings - assuming the cows' urine could be kept separate from other waste.
This, like all the applications mentioned so far, will only work with relatively concentrated urine. That rules out the most urine produced in people's homes, which goes into the sewerage system along much larger quantities of waste water - but even this resource need not go to waste.
By the time the urine reaches a sewage treatment plant it is not only dilute, but also contaminated with a cocktail of chemicals. What's more, most of the precious urea it contains has broken down into ammonia. Nevertheless, Botte says that her technology should be able to deal with this. She plans to adapt it to split ammonia into hydrogen and nitrogen, and she hopes to secure funding within a year to test the technology at a treatment plant.
Another promising option would be to use microbial fuel cells to generate electricity from all kinds of compounds in mucky waste water, not just urea and ammonia. These devices can break down all the organic matter the water contains, cleaning it at the same time, says Bruce Logan, who develops microbial fuel cells at Pennsylvania State University in University Park.
They take advantage of the fact that waste water naturally contains bacteria and organic matter. When bacteria "consume" this food, they produce electrons that would normally combine with oxygen. But if kept in an oxygen-free chamber they can feed those electrons to an electrode and from there into an external circuit. Protons, meanwhile, pass through a membrane that divides the cell, to reach another electrode - the cathode - where they combine with the incoming electrons from the external circuit, and oxygen, to form pure water.
Experimental microbial fuel cells have generated power densities of up to 6.9 watts per square metre of electrode surface (Environmental Science and Technology, vol 42, p 8101). "Maybe 6.9 watts doesn't sound like a lot, but we have very large reactors in waste water treatment plants, and if you have tens of thousands of square metres, that's going to be a lot of power," says Logan. The technology is being tested at pilot plant scale.
Alternatively, these cells can be modified to produce hydrogen fuel instead of electricity by keeping the cathode as well as the anode oxygen-free. Logan's team recently completed field trials of a 1000-litre version of a hydrogen-producing microbial cell at a winery, where the waste water contained leftovers from grape crushing and fermentation, such as sugars and ethanol. Logan says the cells coped well with the real-world conditions, such as varying composition of waste water, but won't discuss the details until the work is published.
Logan is focusing on scaling up the microbial cells and finding the materials for electrodes that make them work most efficiently. "We expend a lot of energy on waste water treatment right now, and these technologies hold the promise to convert this process from an energy consumer to a net energy producer," he says.
No one claims that urine will ever be the complete answer to our energy needs, but Botte argues that the more sources we have for our energy, the better. "We have gigantic energy needs. We are talking billions of megawatt-hours each year in the US alone," she says. "Trying to find one solution is not the answer. There is room for many technologies with different market shares."
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Image Credit: Ajay Tallam