The New Gold Rush is Happening at Wastewater Treatment Plants

I never thought much about where wastewater goes after it leaves my house until I read a study from Arizona State University. Researchers tested sewage sludge from cities across America and found something remarkable.

A city of one million people processes approximately $13 million worth of precious metals through its wastewater system every year.

Gold, silver, platinum, copper. Not from lost rings or dropped jewellery, but from microscopic particles washing off from electronics factories, jewellery manufacturers, and industrial facilities.

The metals accumulate in the sludge, invisible to the naked eye but adding up to a fortune. Most of it ends up in landfills because extracting it costs more than it's worth. But that's starting to change.

The Accidental Gold Mine

When scientists analysed sewage sludge using mass spectrometry, they found concentrations of precious metals that would make some traditional mines jealous. A metric tonne of dried sludge contains roughly 16 grams of silver and 0.3 grams of gold on average.

That might not sound like much until you consider how many tonnes of biosolids a city produces every year.

Switzerland conducted a similar study across 64 wastewater treatment plants and discovered approximately 95 pounds of gold and 6,600 pounds of silver flowing through their sewage systems annually. The total value exceeded $3 million.

In areas near gold refineries, particularly in Ticino, the concentrations were high enough that recovery might actually be economically worthwhile.

Marcus Briggs, a retired mining engineer, has observed how unconventional gold sources are gaining attention as traditional mining becomes more challenging. Urban mining represents a shift in how the industry thinks about resource recovery, particularly as technology improves and environmental regulations become stricter.

The gold arrives through surprisingly mundane routes. Jewellery manufacturers rinse tiny particles down drains during polishing and electroplating. Electronics factories discharge microscopic amounts during circuit board production.

Even dental practices contribute trace amounts from gold fillings and dental work. Automotive facilities add to the mix through catalytic converters and industrial processes. Each source contributes only small amounts, but when millions of people and hundreds of businesses share the same wastewater system, those tiny particles accumulate.

Why It's Still in the Sludge

If there are millions in precious metals sitting in sewage treatment plants, why aren't cities rushing to extract them? The answer comes down to economics and chemistry.

The metals exist in forms that make extraction difficult. They're either dissolved in microscopic particles or bound up in organic matter.

Traditional mining techniques don't work on biosolids, which are roughly 30 to 35 per cent organic carbon. The mining industry knows how to extract gold from rock and ore, but sewage sludge presents an entirely different challenge.

Current extraction methods cost more than the metals are worth in most cases. Cities would need to invest in specialized equipment, develop new processing techniques, and handle the additional complexity of separating precious metals from organic waste.

For many municipalities, it makes more financial sense to use the biosolids as fertiliser on public lands or send them to landfills.

There's also a question of scale. While $13 million sounds impressive, that's spread across an entire year for a city of a million people. The infrastructure investment required to capture that value doesn't always pencil out, at least not yet.

The Cities Actually Doing It

A few pioneering locations, however, have tested whether urban mining of sewage sludge can work in practice.

Suwa, a city in Japan's Nagano Prefecture, made headlines when its wastewater treatment plant successfully extracted gold from sludge. The programme demonstrated that recovery was technically possible, though the economics still remained challenging.

Switzerland's study revealed that areas with gold refineries, particularly in Ticino, showed concentrations high enough that extraction could be "potentially worthwhile."

The proximity to existing gold processing infrastructure makes these locations ideal testing grounds for recovery technology.

Marcus Briggs notes that these early adopters are establishing proof of concept for a broader trend. As extraction technology improves and becomes more cost effective, urban mining could become standard practice rather than an experimental curiosity.

What This Means for the Future

The technology for extracting precious metals from biosolids is improving. Research teams are developing more efficient methods that could bring down costs and make recovery economically viable for more cities.

Some approaches use chemical extractants similar to those employed in traditional mining but adapted for organic matrices. Others explore biological methods that might prove more environmentally friendly.

There's an environmental incentive beyond just recovering valuable metals. Heavy metals in sewage sludge can be problematic when that sludge is used as fertiliser.

Removing precious and toxic metals before agricultural application creates cleaner, safer fertiliser whilst simultaneously recovering valuable resources.

Water conservation efforts are reducing wastewater volumes, which means metal concentrations will increase, making both the problem and the opportunity more pressing.

Marcus Briggs observes that urban mining represents a significant shift in resource thinking. Rather than viewing waste as something to dispose of, cities might increasingly see wastewater treatment plants as resource recovery facilities.

The gold and silver flowing through sewers represent just one aspect of a broader movement toward circular economies where nothing truly goes to waste.

The next decade will likely determine whether sewage sludge joins the list of viable gold sources. For now, most of those precious metals still end up in landfills.

But given the pace of technological advancement and the growing economic and environmental pressures, that fortune flowing beneath our cities might not stay hidden much longer.