But our days of wasting human waste may be numbered, if Kathleen Smith of the US Geological Survey has anything to say about it. She’s leading a new research program that’s examining the feasibility of extracting precious metals from sewage. As Smith will explain Tuesday at a press conference at the annual meeting of the American Chemical Society, recovering metals from waste could reduce the need for environmentally-destructive mining programs, and make biosolids a safer source of fertilizer to boot.
“There are metals everywhere,” Smith said in a statement. “If you can get rid of some of the nuisance metals that currently limit how much of these biosolids we can use on fields and forests, and at the same time recover valuable metals and other elements, that’s a win-win.”
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.
Microscopic gold-rich and lead-rich particles in a municipal biosolids sample. Image: Heather Lowers, USGS Denver Microbeam Laboratory
Smith’s team is now on a mission to figure out which metals are the most economically viable to recover, and how we can extract them.
“We have a two-pronged approach,” she said. “In one part of the study, we are looking at removing some regulated metals from the biosolids that limit their use for land application. In the other part of the project, we’re interested in collecting valuable metals that could be sold, including some of the more technologically important metals, such as vanadium and copper that are in cell phones, computers and alloys.”
To do so, the team is working to modify extraction procedures used in industrial mining to leach metals out of minerals. “Traditional extractants will behave differently with the organic matter [in biosolids],” Smith told me over the phone. “But if we can find an extractant that does a good job, this procedure may be incorporated into current biosolid treatments.”
Smith and her fellow 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.
“What’s interesting is that we’re seeing nearly the same amount of gold in all of these samples,” Smith told me. “It seems like there’s some source of gold that’s prevalent across the board.”
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.