Engineers have analysed the contents of sewage sludge to discover that in a city of a million people, there's as much as $13 million worth of valuable metals, including gold and silver.

A new study has estimated that if you take all the sewage sludge produced by a population of 1 million - so, what's left behind when all the toilet water, storm run-off, and industrial waste that ends up in our drains is treated - you'll find over $2.5 million worth of gold and silver, plus other metals worth millions more.

It's been known for years that sewage sludge contains a bunch of metal. In fact, it's considered a nuisance because if any of these metals have reached toxic levels, this sludge can't be deposited into streams or used as industrial fertiliser (60 percent of sludge produced in America ends up feeding its farms), and has to be deposited as landfill, which can be pricey. But until now, no one had really considered that this metal could actually be worth something.

A team at Arizona State University (ASU) in the US decided to investigate, gathering sludge samples from all over the country, and identifying all the different kinds of rare-earth elements and minor metals within, using a mass spectrometer. 

"For a community of 1 million people, metals in biosolids were valued at up to US$13 million annually," they conclude in a paper published in Environmental Science & Technology. "A model incorporating a parameter to capture the relative potential for economic value from biosolids revealed the identity of the 13 most lucrative elements with a combined value of US $280/ton [907 kg] of sludge." That equates to about $8 million in a hypothetical city of 1 million people.

Those 13 most lucrative elements are silver, copper, gold, prosperous, iron, palladium, manganese, zinc, Lawrencium, aluminium, cadmium, titanium, gallium, and chromium. Warren Cornwall reports at Science Magazine that, rather than anyone pooping them out like a grotesque spin on Aesop's famous goose, these metals wind up in our sewers thanks to "mining, electroplating, electronics and jewellery manufacturing, or industrial and automotive catalysts".

Now, before anyone gets too excited - seriously, contain yourselves! - there's likely never going to be a practical method for extracting every last speck of those valuable metals, but engineer Jordan Peccia from Yale University in the US, who was not involved in the study, told Cornwall the results could convince the waste treatment industry to adjust their disposal methods to reap some of the benefits from the runoff of society. "We're not going to get rid of this sewage sludge. We need to make this push where we stop thinking about it as a liability and instead we think about it as a resource. And anything we can find in sewage sludge that's valuable, it's good."

How to extract more value out of a city's waste is something that scientists around the world have been increasingly interested in. Earlier this month, it was announced that the Bill and Melinda Gates Foundation has financed the development of technology that turns untreated human waste into electricity, drinking water and ash. And a Kenyan teenager has also recently figured out how to derive value out of his school's waste - turning it into energy to fuel his school kitchen.

Cornwall reports at Science Magazine that in Suwa, a city in central Japan, a treatment plant that receives a lot of industrial runoff has already figured out that if they burn the sludge, they can extract nearly 2 kilograms of gold in every metric tonne of the resulting ash. That makes this ash "more gold-rich than the ore in many mines", he says.

Whether treatment plants around the world will find this process, or ones yet to be developed, economically viable it the question, but "We think it is," Peccia says. 

Source: Science Magazine