Gold Rush! Could Biosolids Contain Millions in Precious Metals?

Every year, tons of metals enter the wastewater system. A new study asks whether treatment plants can recover those lost riches.
Gold Rush! Could Biosolids Contain Millions in Precious Metals?

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In the classic 1969 Western musical, Paint Your Wagon, Ben Rumson, played by Lee Marvin, and Pardner, played by Clint Eastwood, discover there’s more gold filtering through the floorboards of No Name City’s many saloons than from the nearby hills.

Tunneling under city streets to reclaim the fallen glitter, the two soon create a thriving business. Yet even as the mythical boomtown turns to bust, it appears Ben and Pardner might have been on to something — had they only set up shop in a wastewater treatment plant.

Lost treasures
According to a new scientific study published in Environmental Science & Technology, mining, electroplating, electronics, jewelry manufacturing and similar industries discharge tons of metals into sewers each year. Upon treatment, about 60 percent of the remaining municipal biosolids are spread across fields as fertilizer with 40 percent sent to landfills or incinerated.

The metal content in biosolids has the potential to be an environmental risk, promoting a modern-day rush to extract gold, silver and other remaining minerals from the residual waste.

In 2009, a waste treatment plant in Japan extracted about 2 kilograms (4.4 pounds) of gold from a ton of incinerated sludge – 50 times more than Japan’s top-producing Hishikari Mine extracted from each ton of ore.

It’s estimated that a ton of treated sludge could contain several hundred dollars of metals. In a city of 1 million residents, about the size of San Jose, Calif., that could add up to $13 million a year, including $2.6 million in reclaimed gold and silver, according to Environmental Science & Technology.

Pipe dreams
Each year, treatment plants in the United States generate about 8 tons of biosolids, but much of the metals left behind are microscopic — so diluted they rival amounts found in everyday soil.

Taking a closer look, scientists at Arizona State University-Tempe focused on 13 of the most concentrated minerals with the highest value in samples from across the country stored at the U.S. National Biosolids Repository. The researchers estimated treatment plants could mine $280 in platinum, gold, silver, copper, iron and zinc from each ton of biosolids.

Although theoretically possible, the technology does not yet exist to extract large amounts of metals from biosolids without the need for enormous amounts of energy and harsh chemicals. Call it fool’s gold, but until the recovery process becomes more feasible, striking the mother lode in municipal waste could be a case of gold fever.

Still as interest levels increase, the opportunity exists to turn a potentially toxic and costly disposal problem into untold riches for millions of treatment plants worldwide. Perhaps municipalities need to begin staking a claim.


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