Sweetheart in the sewage

Thousands of kilos of valuable metals end up in sewage treatment plants every year

The wastewater also gets valuable metals into the sewage treatment plants. How much have Swiss researchers determined? © Eawag / Elke Suess
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Silver, gold and rare earths: The sewage ends up with real treasures in our wastewater treatment plants, as Swiss researchers have discovered. Thus, in the sewage and sewage sludge of Switzerland, about 3, 000 kilograms of silver, 43 kilograms of gold and hundreds of kilograms of various rare earth metals accumulate each year. A recovery of these raw materials, however, would be worthwhile only in a few places, as the researchers report.

Above all, the electronics industry, but also other high-tech industries require numerous different precious and rare earth metals. Because demand is rising sharply, some of these raw materials could even become scarce in the future. At the same time, however, the ubiquity of these metals ensures that traces of them are also found in the wastewater. In the US, researchers have even found more gold, silver and other valuable metals in sewage sludge than in some reservoirs.

As we look in Central Europe with the "treasure" in the wastewater, Bas Vriens have now examined by the EAWAG and his colleagues. For this purpose, they analyzed samples from 64 wastewater treatment plants throughout Switzerland for their content of 69 different elements. The researchers sampled both the treated wastewater and the sewage sludge and the water of large rivers. Your study is the first to systematically record this for a whole industrialized country.

3, 000 kilos of silver

The result: Extrapolated to Switzerland, considerable quantities of valuable metals end up in sewage treatment plants. After all, this is about 3, 000 kilograms of silver and 43 kilograms of gold per year. Also, copper, zinc, titanium and manganese and iron were common, as the researchers determined. Among the rare earth metals are gadolinium at 1, 070 kilograms, neodymium at 1, 500 kilograms and ytterbium at 150 kilograms.

Average quantities of 62 elements in the wastewater per person per day in Switzerland © Eawag

If this is converted into per capita values, then the span ranges from a few micrograms per day per head to a few grams per day per head. Among the rarer finds are gold, indium or lutetium, in the milligram range are zinc, scandium, yttrium, niobium and gadolinium. With more than a gram per day and head pretty much represented are the elements phosphorus, iron and sulfur. display

Ruthenium in the Jura, gold in Ticino

However, metal grades vary significantly, depending on the region and wastewater treatment plants - sometimes by a factor of 100. For example, analyzes in the Jurassic revealed elevated levels of ruthenium, rhodium and gold, which are believed to have come from the watch industry. In parts of Graub nden and the Valais, the arsenic levels were probably higher, because the rock naturally contains a lot of arsenic.

On the other hand, the scientists found particular gold in some places in Ticino. There the gold concentration in the sewage sludge is so high that even a recovery could be worthwhile. The explanation: There are several gold refineries in this area. With their wastewater, remnants of gold dust also reach the sewage treatment plants.

Recuperation is worthwhile only occasionally

However, with the exception of these gold-rich locations, the recovery of raw materials from sewage or sewage sludge is hardly worthwhile, as Vries and his colleagues explain. In addition the expenditure would be too large and the yield too small. The amount of aluminum found, for example, is only 0.2 percent of Switzerland's annual imports, with copper just under 4 percent.

As disappointing as this may be for "treasure hunters, " this is rather good news for the environment and our health. As the analyzes have shown, only the values ​​for copper and zinc in the effluents or sludges are occasionally too high. In most places, however, no kotoxikologisch relevant or legally defined limits are exceeded. This is different in Germany: a few years ago researchers in the Rhine have measured potentially toxic concentrations of gadolinium, lanthanum and other heavy metals. (Environmental Science & Technology, 2017; doi: 10.1021 / acs.est.7b01731)

(EAWAG: Swiss Federal Institute of Aquatic Science and Technology, 11.10.2017 - NPO)