Feces to fortune: US sewage may contain billions in precious metals
More than seven million dry tons biosolids (read: poop) are generated in the US annually by more than 16,500 municipal wastewater treatment facilities. And that sewage contains metals that people ingest and otherwise flush down the toilet, or rinse out in the laundry and shower.
"There are metals everywhere," Dr. Kathleen Smith of the US Geological Survey (USGS) said in a statement, noting that they are "in your hair care products, detergents, even nanoparticles that are put in socks to prevent bad odors."
Smith leads a team of researchers looking to get metals out of biosolids because about half of all human waste ‒ about 3.5 million tons in the US ‒ is used as fertilizer on farms and in forests, while the other half is incinerated or sent to landfills.
“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," Smith said.
It may be odd thinking of precious metals like silver and gold as nuisances, but they impede the usefulness of fertilizers.
"We have a two-pronged approach," Smith 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," she added.
Smith’s team has collected samples from small towns in the Rocky Mountains, rural communities and big cities, but will also combine their findings with years of existing data collected by the US Environmental Protection Agency (EPA) and the USGS. They will present their research at 249th National Meeting & Exposition of the American Chemical Society on Tuesday.
The EPA analyzed 28 metals for their 2009 Targeted National Sewage Sludge Survey, using samples randomly selected from 3,337 facilities that treat more than one million gallons of sewage per day, Smith wrote in her paper’s abstract. The agency discovered that the samples averaged 30 mg of silver per kilogram, 563 mg of copper per kilogram and 36 mg of vanadium per kilogram of waste.
A similar eight-year study by the USGS involved monthly sampling and analysis of biosolids from a municipal wastewater treatment plant. Those samples contained an average 28 mg of silver, 638 mg of copper, 49 mg of vanadium and less than one milligram of gold per kilogram of waste.
About 80 percent of vanadium is used in producing rust resistant and high-speed tool steels, according to Los Alamos National Laboratory.
"The gold we found was at the level of a minimal mineral deposit," Smith said, meaning that if that amount were in rock, it might be commercially viable to mine it. She added that "the economic and technical feasibility of metal recovery from biosolids needs to be evaluated on a case-by-case basis."
In a January Environmental Science & Technology paper, scientists at Arizona State University (ASU) in Tempe calculated that the waste from one million Americans could contain as much as $13 million worth of metals, including $2.6 million in gold and silver.
The group analyzed sewage sludge for 58 regulated and non-regulated elements, and used electron microscopy to explore opportunities for removal and recovery. Based on their model to capture the relative potential for economic value from biosolids, the 13 most lucrative elements (silver, copper, gold, phosphorus, iron, palladium, manganese, zinc, iridium, aluminum, cadmium, thallium, gallium and chromium) present had a combined value of US $280 per ton of sludge.
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 place has already figured out how to profit from poop: A sewage treatment facility in Japan has been mining sludge for gold since 2009, Reuters reported at the time. The Nagano prefecture, northwest of Tokyo, once recorded finding 1,890 grams of gold per ton of ash from incinerated sludge. The district expected to earn about 15 million yen (currently US$125,000) for that fiscal year, depending on the price of gold.