Every year, Americans are flushing a fortune down the toilet. Literally. More than 7 million tons of biosolids—treated sewage sludge—pass through US wastewater facilities annually. Contained within our shit are surprisingly large quantities of silver, gold, and platinum.
At treatment plants, raw sewage is processed by a series of physical, biological and chemical processes and transformed into treated water and biosolids. Roughly 60 percent of biosolids are applied as fertilizer to fields and forests. The rest are either incinerated or buried. While biosolids are routinely screened for hazardous heavy metals including lead, arsenic, and cadmium, few studies have tested our waste for anything as valuable as, say, gold or platinum.
But that’s starting to change. Earlier this year, a study led by Paul Westerhoff at Arizona State University profiled over 50 metals in biosolid samples from 94 wastewater treatment plants across the US. Most samples were substantially enriched in rare and precious platinum-group metals, silver, and gold. Extrapolating from their data, the authors worked out that the waste produced annually by a million Americans could contain as much as 13 million dollars worth of metals. That’s over four billion dollars worth of gold coming out of our collective arses every year.
Researchers are also planning to test biosolids across the country for precious metals, to search for any geographic or demographic patterns in their distribution. So far, the group has collected waste from several small towns in the Rocky Mountains, rural areas, and big cities. Astonishingly, in nearly all the samples they’ve examined, the team has found commercially mineable concentrations of gold.
It’s not entirely clear how these precious metals are getting into our waste, Smith says. Potential culprits include hair products, cosmetics, and detergents. But humans could also be playing a more direct role, by concentrating the trace metals we eat during digestion, and sending gold-and-silver-fortified defecations down the tube.
Whatever the reason, one thing’s clear: our sewers are a lot prettier than we realized.
**photo credit: Microscopic gold-rich and lead-rich particles in a municipal biosolids sample. Image: Heather Lowers, USGS Denver Microbeam Laboratory