Gold in the Swiss Sewers
About 95 pounds of gold and 6,600 pounds of silver
pass through Switzerland's wastewater stream every year.
Swiss refineries handle 70% of the world's gold production,
A fraction of that makes its way into the sewers.
The gold and silver certainly get our attention as they are
classic precious metals.
The amounts in national sewage add up to values of about
1.5 million Swiss Francs worth of each (or about 2 million US$ each).
Several valuable rare earth metals and other trace elements appear
in significant amounts, by-products of various fields of manufacturing,
metallurgy, and medicine.
Other sources are pharmaceuticals and personal care products,
surfactants in detergents and other cleaning products,
and engineered nanoparticles now used for a wide variety of purposes.
As I explained in an
earlier article,
valuable metals are found in sewage most anywhere we look closely.
Swiss scientists have been able to take a closer look than would
be practical in many countries.
Swiss Water Flows
Switzerland is well suited for this type of study.
About 98% of the population is connected to wastewater treatment systems.
The four major Swiss rivers — Rhône, Rhine,
Inn, and Ticino — all have their origins within the country,
and all four are monitored where they cross the border into
neighboring countries.
All the water flowing through those border monitoring stations
originates
within Switzerland.
The study, reported in the paper
"Quantification
of Element Fluxes and Wastewaters: A Nationwide Survey in
Switzerland", collected samples of both discharged water and
sludge at 64 wastewater treatment plants in both urban and rural areas.
Those 64 plants handle the wastewater of 54% of the national population.
They also collected samples of water from the
Rhône, Rhine, Inn, and Ticino rivers where they cross into
France, Germany, Austria, and Italy, respectively.
[Environmental Science and Technology Letters,
2017, pp 10943-10953]
The element concentrations vary widely from plant to plant.
Concentrations of any element vary by factors up to 100.
For example, gold, ruthenium, and rhodium appear at higher concentrations
in the Jura canton in the northwest,
where the watchmaking industry is concentrated.
Arsenic is at much higher concentration in Graubünden and Valais,
where it comes out of the local rock and soil — the brown limestones
of the Dogger formations, and the granites and shales of the
Schwarzwald Massif.
Italian-speaking Ticino, the southernmost canton of Switzerland,
is named for the Ticino river.
It is home to the city of Lugano and to three of the world's
largest gold refineries.
The Pamp refinery in Castel San Pietro is
the world's leading manufacturer of minted gold bars.
Some of that gold escapes down drains or into the air or ground,
where runoff carries it into the wastewater treatment system.
Ticino has enough gold in its sewage sludge
to make recovery potentially worthwhile.
The gold content of the sludge in Ticino is similar to what you
look for in profitable mining ore.
The study dismissed the metal content of sludge and discharged water
at most plants as far from enough to consider recovery operations.
However, to cite just one example, over one year the total copper output
is almost 4% of the amount imported.
Sludge digesters at the Wards Island Wastewater Treatment Plant
in New York City.
Silver sulfide nanoparticles (α-Ag2S) became common
in the 2000s.
The Project on Emerging Nanotechnologies at the
Woodrow Wilson International Center
for Scholars inventoried over 1,000 consumer products using
nanoparticles in 2009.
These included food storage containers, liquid detergents and fabric
softeners, clothing and fabric, personal-care products and cosmetics,
and even dietary supplements.
Titanium dioxide (TiO2) nanoparticles are also widely used
as a whitening agent in a wide range of substances
including paint and toothpaste.
The Elemental Scoreboard
Iron rules the sludge, with concentrations from 6 to 124 grams
per kilogram.
That's because treatment plants add iron to sludge
in order to precipitate and remove phosphorus.
Close behind iron in descending order of concentration are
phosphorus and calcium.
Phosphorus is high because of its importance as a fertilizer
and the wide use of organophosphorous compounds in both
detergents and insecticides.
Then it's a big jump down below 10 grams per kilogram
to the nearly equal cluster of potassium,
silicon, sulfur, and magnesium.
Then it's a bigger jump down to sodium,
still well above 1 gram per kilogram in sludge.
And then there's the "long tail" of trace elements found below
1 gram per kilogram, all the way to indium, lutetium, thulium, and gold,
and beyond, down below 1 microgram per kilogram in sludge.
Those are the numbers per kilogram, but you have to add this up across
all the sludge in Switzerland.
It's a relatively small country, but the total includes
1500 kilograms of neodymium per year (used to make glass and powerful magnets),
1070 kilograms of gadolinium (used in metallurgy, nuclear shielding,
and, most importantly, as contrast media for magnetic resonance imaging),
and 150 kilograms of ytterbium (used in metallurgy,
in lasers and atomic clocks, and as a gamma ray source).
Supplementary
material
Overall, the numbers are similar to those found in
China, France, Luxembourg, Japan, Slovenia, Sweden, and the U.s.
Want to dig deeper?
The paper's
supplementary material
has several tables and visualizations of the data.
Next article:
Discovering the Roman Military Diet through Biochemistry
