More than 150 years after gold miners first filled the Wild West, people are still looking for precious metals in essentially the same way. Later this year, a mining company plans to open a new gold mine carved into a mountain outside Butte, Montana. But are there better ways to mine in 2015?
One unlikely potential source of gold, silver, platinum, and other metals: Sewage sludge. A new study estimates that in a city of a million people, $13 million of metals could be collecting in sewage every year, or $280 per ton of sludge. There’s gold (and silver, copper, and platinum) in them thar poop.
Funded in part by a grant for “nano-prospecting,” the researchers looked at a huge sample of sewage from cities across the U.S., and then studied several specific waste treatment plants. “Initially we thought gold was at just one or two hotspots, but we find it even in smaller wastewater treatment plants,” says Paul Westerhoff, an engineering professor at Arizona State University, who led the new study.
Some of the metals likely come from a variety of sources–we may ingest tiny particles of silver, for example, when we eat with silverware or when we drink water from pipes that have silver alloys. Medical diagnostic tools often use gold or silver. Other particles sometimes come from waste that flows down drains in manufacturing.
“One direction we’re going in the future is to figure out where the metals are coming from,” says Westerhoff. “And if it’s there, could we economically get it out? We want to investigate transforming the way we think about these solids from waste into more of a resource.”
Though extracting the metals may be difficult, Westerhoff believes it’s possible. “There’s nothing you can buy off the shelf today to do it,” he says. “But are there strategies? Yeah, we think there are. One of the keys is understanding the structure of the metals–these things are very small particles–because there are different approaches to getting them out of the water.”
The process may make most sense in big cities that currently pay to haul sewage hundreds of miles away. “We think the opportunities are in places like Los Angeles, Chicago, New York City,” Westerhoff says. “We’ll start thinking about those larger communities first, because getting rid of biosolids in the middle of the city is actually a fairly high economic burden.”
It also makes sense in cities with large manufacturing plants–in 2009, a Tokyo suburb filled with factories discovered more gold in its sewage than in a nearby gold mine.
Someday, you may buy a phone or another gadget made with sewage-sourced metals. For now, there are also easier ways to supply the metals we need–like just recycling the billions of smartphones we buy each year. By some estimates, there’s as much gold in a few dozen smartphones as in a ton of ore.