There are millions of dollars in gold and other metals in the sewage sludge in major cities. A new study has found that in a city with 1 million inhabitats, there’s as much as $13 million worth of valuable metals, including gold and silver.

Image via Water Desalination Plants.

It’s been known for quite a while that sewage sludge contains significant quantities of valuable metals, but this is the first study I could find which quantifies that amount. For every 1 million people, on average, you’ll find over $2.5 million worth of gold and silver, plus other metals worth millions more.

“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.

Furthermore, these metals are actually costing governments good money; from a point of view, they’re a pollutant. If they reach a high enough quantity, then the sludge can’t be used as a fertilizer and instead has to be deposited as landfill – turning it into a cost, from an asset (60 percent of sludge produced in America ends up feeding its farms).

The amount won’t shake the world markets, but it can be a way for cities to get some extra value. The city of Suwa in Japan is already working on extracting the gold. They installed a treatment plant near a large number of precision equipment manufacturers reportedly collected nearly 2 kilograms of gold in every metric ton of ash left from burning sludge, making it more gold-rich than the ore in many mines.

Image via Discover Magazine.

Still, before we get to excited, it has to be said that there is no practical way of recovering every bit of gold, but still, scientists argue that the extraction of gold and silver from sludge can be quite profitable. Jordan Peccia from Yale University in the US, who was not involved in the study agrees.

“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.”

But gold and silver are not the only things of value from the sludge. A small number of sewage plants are already removing phosphorous and nitrogen, which can be sold as fertilizer. Sweden, which recycles most of its waste is testing the feasibility of making bioplastics from wastewater. A model sewage incinerator that generates electricity and drinking water was just promoted by the Bill & Melinda Gates Foundation, which helped fund its construction.

All in all, there’s big money in sewage sludge – we just have to find a way to get it.

Scientific Reference: Science| DOI: 10.1126/science.aaa6359