We’ve all flushed something down the toilet we wish we hadn’t, but apparently there’s more of a goldmine down there than you might have suspected – quite literally.
Scientists have determined that, in Switzerland at least, around US$1.5 million each worth of gold and silver are ending up in the sewer system every year. That’s a cool US$3 million annually ending up down the loo.
But before you get your waders and noseplugs ready, they’ve also determined that going in after it is not really worth the trouble – even including all the other metals they’ve found down there.
The team from the Swiss Federal Institute of Aquatic Science and Technology (Eawag) under commission by the Swiss Office for the Environment analysed 64 wastewater treatment plants around the country.
They found, per year, entering the sewer nationwide: 43 kilograms (95 pounds) of gold; 3,000 kilograms (6,614 pounds) of silver; 1,070 kilograms (2,359 pounds) of gadolinium; 1,500 kilograms (3,307 pounds) of neodymium; and 150 kilograms (330 pounds) of ytterbium.
The presence of metals in wastewater has been known for a very long time, because often it ends up being used for crop irrigation, and it’s important to know how the composition of the water affects the soil it’s being used on. Some elements are toxic, and using them on food crops is a monumentally bad idea.
It wasn’t until a few years ago that researchers in the US thought to look for valuable metals. They found US$13 million worth of gold in American waste sludge.
According to the Eawag researchers, the metals found in Switzerland’s sewer do not pose a risk to the environment. Their research, rather, shows just how much precious metal gets wasted.
Gadolinium, neodymium and yttrium are all rare earth metals with applications that range from neutron radiography for treating tumours, to magnets, to alloys.
How do they all get down there, you might be wondering. Some of it, obviously, is from households.
A 2002 study found that a lot of the mercury detected in the wastewater in Stockholm, Sweden came from the amalgam used to fill tooth cavities, for instance, while lead and chromium came from car washes.
The 2015 US study also noted that a lot of the metals they found were probably the result of industrial activity. The Eawag team found that the concentrations of metals in the wastewater varied from region to region, which fits in with that hypothesis.
“For example, elevated concentrations of ruthenium, rhodium and gold were found in the Jura (presumably from the watchmaking industry), and of arsenic (presumably geogenic) in parts of Graubünden and Valais,” they wrote in their announcement of the findings.
“At certain sites in Ticino, concentrations of gold in sewage sludge are sufficiently high for recovery to be potentially worthwhile. This can be attributed to the presence of several gold refineries in the region.”
Overall, though, trying to recover the metal simply isn’t worth the effort involved, either financially or for the amount of metal that could be retrieved.
The total amount of aluminium and copper estimated to be in the wastewater, for example, would amount to just 0.2 percent and 4 percent of annual Swiss imports, respectively.
But the research still has significant value.
“This study shows that wastewater treatment plants can be used to identify elements for which wastewater treatment plants effluent fluxes may have significant environmental impacts,” the researchers wrote in the paper.
“Local and federal governments may therefore use continued elemental screening of sewage sludge and treated wastewater as a way to monitor the (long-term) temporal trends of selected elements of interest after optimisation of analytical methods.”
The paper has been published in the journal Environmental Science and Technology.