Gold and other valuable metals may be harvested from sewage

There's gold – and lead, apparently – in them thar biosolids (Photo: Heather Lowers, USGS Denver Microbeam Laboratory)

Mining operations tend not be very good for the environment, nor does the disposal of treated solid waste that still contains potentially-toxic metals. Now, however, scientists are looking into taking that waste and harvesting its trace amounts of metals such as gold, silver and platinum. Doing so could ultimately reduce the need for mining and decrease the amount of metals entering the environment, while also turning sewage into a source of revenue.

So, what are those metals doing in our poop in the first place? Well, we're not necessarily ingesting them. According to lead scientist Kathleen Smith of the US Geological Survey, metal nanoparticles from sources such as detergents, hair care products and antimicrobial clothing enter the waste stream in wash water, and end up getting combined with fecal matter at wastewater treatment plants.

After the treatment process is complete, approximately 50 percent of the resulting biosolids are used as fertilizer, while the other half are put in landfills or incinerated (in the US, at least). The presence of metals in the biosolids is one of the reasons that a greater amount of them can't become fertilizer.

To retrieve those metals, Smith and her team are considering utilizing chemicals that would cause the metals to leach out of the biosolids. The same type of chemicals are currently used in the mining industry for extracting metals from rock, and their use in that application is controversial – despite the presence of safeguards such as liners and drainage systems, they can make their way into the groundwater and pollute it. In the case of biosolids treatment, however, Smith believes that the chemicals could easily be contained.

In order to evaluate the economic feasibility of the system, the researchers have used a scanning electron microscope to analyze biosolid samples from small towns, rural communities and big cities. According to Smith, they found gold "at the level of a minimal mineral deposit" – in mining terms, this means that it might be financially worthwhile to extract.

Overall, though, she stated in a paper that "the economic and technical feasibility of metal recovery from biosolids needs to be evaluated on a case-by-case basis."

The research will be presented this Thursday in Denver, Colorado, at the 249th National Meeting & Exposition of the American Chemical Society.

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