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Believe it or not, "urban mining" is an emerging academic interest.

There will always be people who canvas the city for scrap metal. Like whoever stole that entire bridge in western Pennsylvania a couple years back, or lifted the bronze grave markers and pieces of lead roof and metallic flush valves on public toilets that have gone missing in recent years.

But it's a bit of a twist to learn that many would-be urban prospectors hold PhDs, as Kate Ravilious reports in a recent issue of New Scientist (subscription only). Ravilious writes of an emerging academic interest in mining the many layers of city infrastructure for precious metals and other materials. An inaugural three-day Symposium on Urban Mining was even held in Europe last year.

"Their target is 'urban ore' — forgotten supplies of metals that lie in and under the city streets," writes Ravilious.

Her story centers on an environmental engineer from Linkoping University in Sweden named Mats Eklund. For several years now, Eklund and colleagues have been searching for vast deposits of base metals like copper, iron, and aluminum beneath the streets of Gothenburg, Linkoping, and Norrkoping. They use everything from utility company data to Industrial Revolution-era maps to locate these so-called "hibernating stocks."

Recently Eklund and company have focused their attention on copper stocks embedded in municipal power grids. They estimate that there's upwards of 90,000 tons of the stuff buried beneath Swedish cities, though recovering it may not be "economically justified" at the present time. If they did find a cost-efficient way to harvest and recycle the copper, however, the environmental savings compared to traditional mining would be substantial — roughly 360,000 tons of carbon dioxide, according to Ravilious.

While studying to become a chemical engineer, Angela Murray of the University of Birmingham in England established a company called Road to Riches [PDF] to recover the precious metals that might be trapped in a city street gutter near you. Every day cars cruising the urban network eject particles of platinum, palladium, and rhodium from their catalytic converters. Murray and company sift through the road dust with a patented separation technique.

A few years ago Murray told the Times Higher Education that people laugh and say what she finds can't be worth the effort. Yet rhodium, for instance, evidently goes for £90 (roughly $138) a gram, and Ravilious reports that £64 million ($98 million) worth of platinum metals accumulate on British roads every year. Murray has compared her enterprise to a "low-grade mine":

And in mining, you have to go deep underground, it's intensive and it's environmentally damaging. Our process means that it is sitting on the surface, just waiting to be collected.

Then there's Bernd Lottermoser of the University of Exeter in England, who think it's worthwhile to expand the search into sewers. Lottermoser has been studying gold concentrations in municipal treatment plants for more than a decade. (Little flakes of gold from jewelry can enter wastewater when a person does the dishes or takes a shower.) Ravilious reports that concentrations about 50 times greater than those at conventional gold ore mining sites have been found at one facility in Nagano, Japan.

Of course when it comes to "urban mining" there's the small matter of the disruption caused by tearing up city streets. So several years back some interested parties joined up to start a project called Mapping the Underworld, whose mission is to locate buried urban assets without excavation. "Such technology raises the prospect that urban mining will soon be viable," writes Ravilious. The rush is on.

Top image: DigitalMagus/Shutterstock.com

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