Researchers at Arizona State University measured levels of metals in sewage sludge, also known as biosolids, collected across the United States using a mass spectrometer and an electron microscope. They found that amounts of the 13 most valuable elements, including silver, copper, gold and platinum, were worth about $ 280 US ($ 350 Cdn) per tonne of sludge.
That means about $ 13 million worth of metals a year could theoretically be mined from the sewage produced by a city of one million people, said Paul Westerhoff, a professor at the School of Sustainable Engineering and the Built Environment at Arizona State University in an interview with CBC’s Quirks & Quarks.
Westerhoff added that cities currently pay about $ 300 to $ 400 per tonne to get rid of their sewage sludge or biosolids.
“But yet you’re throwing away or giving away two or three hundred dollars worth of these metals for that same tonne of metal,” he said in an interview that airs Saturday.
However, the metal found in sewage typically comes in the form of very small particles.
“You can’t go out and pick out a gold nugget,” Westerhoff said.
Westerhoff and his colleagues aren’t sure exactly where all that metal in sewage is coming from, but he suggested some might be from drug therapies, dentist’s offices, foil coatings on food, or companies that make semiconductors or different coating materials.