Sewage sludge could contain millions of dollars worth of gold

Sewage sludge could contain millions of dollars worth of gold

Scientists strike gold in the sewer

f the holy grail of medieval alchemists was turning lead into gold, how much more magical would it be to draw gold from, well, poop? It turns out that a ton of sludge, the goo left behind when treating sewage, could contain several hundred dollars’ worth of metals—potentially enough to generate millions of dollars worth of gold, silver, and other minerals each year for a city of a million people.

Metals have long been known to concentrate in sewage, which mixes toilet water with effluent from industrial manufacturing, storm runoff, and anything else flushed down the drain. It’s a headache for sewage utilities that must cope with toxic metals lacing wastewater headed for streams or sludge that might otherwise be spread on farm fields.

But what if those metals had value? In a new study, scientists at Arizona State University (ASU), Tempe, quantified the different metals in sewage sludge and estimated what it all might be worth. They took sludge samples gathered from around the country and measured the metal content using a mass spectrometer that can discern different elements as they are ionized in a superhot plasma. The upshot: There's as much as $13 million worth of metals in the sludge produced every year by a million-person city, including $2.6 million in gold and silver, they report online this week in Environmental Science & Technology.

That amount won't be rattling the world gold market, nor would it be feasible to extract every last bit. But the study’s lead author, environmental engineer Paul Westerhoff, says it could prove worthwhile for cities looking for ways to gain value from something that can be a costly disposal problem. One city in Japan has already tried extracting gold from its sludge. In Suwa in Nagano Prefecture, a treatment plant near a large number of precision equipment manufacturers reportedly collected nearly 2 kilograms of gold in every metric ton of ash left from burning sludge, making it more gold-rich than the ore in many mines.

Although no U.S. sewage plants have followed suit, the new study adds to a growing push to rethink sewage as a valuable commodity, says Jordan Peccia, a Yale University engineer who was not involved in the work. Approximately 8 million tons of biosolids—a dried derivative of sludge—are generated every year in the United States.

Today, about 60% of the sewage sludge in the United States is already spread on fields and forests as fertilizer. But there are concerns it poses contamination risks from toxic chemicals and pathogens, a subject Peccia’s lab studies. The remaining sludge is burned in incinerators or dumped in landfills.