In the least glamorous corner of modern cities—beneath our feet, inside sewers, and through treatment plants—lies an unexpected treasure.
Literally.
Engineers from Arizona State University have discovered that in a city of one million people, sewage sludge holds up to $13 million worth of valuable metals like gold, silver, copper, and palladium.
That’s not a typo.
What we flush and rinse away could, in theory, bankroll entire municipal programs—or at least pay for a few streetlights.
It turns out, waste is more than just waste.
The contents of your city’s sewage may be quietly richer than some gold mines.
The Dirty Goldmine Beneath Our Cities
Let’s get one thing straight—this isn’t about mining poop (although headlines might cheekily suggest that).
This is about biosolids: the gunk left behind after wastewater—think toilet flushes, storm runoff, and industrial drainage—is treated and cleaned.
This biosolid is where metals concentrate.
The ASU team collected samples from across the U.S. and used high-resolution mass spectrometry to identify elements hiding in the sludge.
They found not just the usual suspects like iron and zinc, but also traces of gold, silver, palladium, gallium, and even rare-earth elements.
In total, the team calculated the metal value of biosolids from one million people could be as much as $13 million annually.
That’s not just pocket change for municipalities.
That’s real, extractable value from material that would otherwise be buried in a landfill—or spread on farms as fertilizer (as happens to about 60% of it in the U.S.).
Let’s Do the Math
The researchers outlined 13 of the most lucrative elements they found:
- Silver
- Copper
- Gold
- Phosphorus
- Iron
- Palladium
- Manganese
- Zinc
- Lawrencium
- Aluminum
- Cadmium
- Titanium
- Gallium
- Chromium
The combined value?
About $280 per metric ton of sludge.
To put that in perspective, that’s more value than some lower-grade mining operations yield from actual ore.
For cities dealing with thousands of tons per year, that adds up fast.
And it’s not just about cashing in.
These metals are critical to everything from smartphones to solar panels.
As global demand grows, so does the pressure to source them sustainably.
What if the answer has been circling our sewer systems all along?
Wait—Sewage Isn’t Supposed to Be Valuable, Right?
This is where the narrative gets flipped.
We’re conditioned to see sewage as the ultimate waste—something to be gotten rid of quickly, efficiently, and preferably without thinking too hard about what’s in it.
But that assumption may be costing us.
“We need to stop thinking about sewage sludge as a liability,” says Yale engineer Jordan Peccia, “and instead view it as a resource.”
He wasn’t part of the ASU study, but he’s one of a growing number of researchers who believe the waste treatment industry is ripe for reinvention.
In fact, one Japanese city has already proven this theory.
Suwa, in central Japan, processes sewage with a high concentration of industrial runoff—and by burning the biosolids into ash, they’re extracting almost 2 kilograms of gold per ton.
That’s not a quirky side project.
That’s more gold per ton than many active gold mines.
Why This Matters Now
Think about it: as the demand for precious and rare metals surges in the tech and renewable energy sectors, the price of these elements keeps climbing.
And traditional mining?
It’s environmentally brutal—destructive, energy-intensive, and often plagued by labor and ethical concerns.
If our cities already contain these metals in a retrievable form, shouldn’t we be tapping into that supply?
This also comes at a time when urban infrastructure is hungry for innovation. Waste treatment plants are costly to operate and maintain.
Finding a way to offset those costs—even partially—by selling recovered metals could change how cities think about waste management entirely.
A Global Trend is Flushing Forward
The concept of turning waste into wealth isn’t isolated to this study.
There’s a rising wave of interest worldwide:
- The Bill & Melinda Gates Foundation has funded tech that transforms raw sewage into drinking water, electricity, and ash.
- A teenager in Kenya developed a method to convert human waste into cooking gas for his school.
- Dutch scientists have even turned wastewater into bioplastics.
This shift reflects a broader philosophy—circular economy thinking, where waste is redefined as a resource.
In essence, we’re sitting on top of a goldmine—literally—and we’ve been dumping it into landfills.
But Can We Actually Extract It?
Here’s where things get murky.
Recovering metals from sewage is tricky.
The concentrations are small, the sludge is complex, and traditional mining methods don’t translate well into wastewater treatment plants.
But that’s not to say it’s impossible.
Technologies already exist to extract specific metals from industrial waste.
Electrochemical processes, thermal treatments, and even bio-mining using specially engineered bacteria are all being explored as ways to pull metals from complex matrices like sludge.
The real question is economic feasibility.
Can the value of the extracted metals offset the cost of setting up and running extraction facilities?
Early indicators suggest that it might actually be cost-effective, especially if cities already handle large volumes of industrial waste.
And when you factor in the environmental savings—less landfill use, lower mining demands, reduced greenhouse gases—the equation looks even better.
Where This Could Go
Let’s imagine a future city.
One where wastewater plants double as resource hubs, extracting not just clean water but also precious metals, clean energy, and industrial chemicals.
Picture urban mines—not of concrete and steel, but of organic refuse, sludgy runoff, and yes, human waste.
Cities could sell these recovered elements to tech companies, construction firms, and energy manufacturers.
What was once an operational expense becomes a revenue stream.
And instead of trying to bury or neutralize the problem, we’re turning it into a solution.
So… Are We Really Pooping Gold?
Not quite. The gold isn’t coming from us, per se—it’s being washed in from the industries that surround us: electronics manufacturing, auto shops, jewelry makers, electroplating facilities.
But the sewage system is where it all ends up.
And now, thanks to advanced technology and a fresh perspective, we’re realizing that what we’ve been calling “waste” is actually laced with value.
What This Means for the Rest of Us
No one’s saying you should start panning for gold in your bathroom pipes.
But this research does invite us to reframe how we think about waste.
Not just in cities, but in our own homes, schools, and communities.
If sewage sludge can hold $13 million worth of metals, what else are we throwing away without a second thought?
Cities of the future won’t just be smart—they’ll be savvy, circular, and surprisingly rich in unexpected places.
And it all starts with a simple, strange truth: there’s treasure in the toilet.
Sources: Environmental Science & Technology, Science Magazine, National Geographic, Yale University, Arizona State University
