The sewage being treated by your city could be a veritable gold mine. That’s because all kinds of metals end up in sewage sludge, which is the leftovers from treated sewage, Science reports.
Researchers in Arizona recently found that a city of a million people could generate up to $13 million a year in such metals, with $2.6 million of that coming in the form of gold and silver.
They studied the samples, which came from 32 states, with a mass spectrometer, Discover reports; the tool uses extreme heat to reveal elements. On a global scale, they found, sludge probably contains about 360 tons of gold each year.
“While we expected that the metals were present at low concentration, the fact that the small amounts represent such a significant economic value was definitely surprising,” a study co-author tells Time.