Precious metals in wastewater

Precious metals in wastewater? Trace elements are constantly used in the high-tech industry and in medicine: Transition metal tantalum or the semimetal germanium, for example, in electronic components, gadolinium in X-ray contrast media and luminous paints, niobium and titanium in alloys and coatings. But once these elements have done their job, they go virtually nowhere.

Many - but not all - elements end up in wastewater. A group of researchers led by Eawag environmental chemists Bas Vriens and Michael Berg has therefore carried out the first systematic study of 64 Swiss wastewater treatment plants to determine which elements, and in what quantities, "go swimming" with treated wastewater or are disposed of with sewage sludge. The study was commissioned by the Federal Office for the Environment, FOEN.

1070 kilogram gadolinium

Interesting is the conversion of the concentrations to the daily per capita turnover of the respective elements in the Swiss population. This ranges from a few micrograms (e.g. gold, indium, lutetium) to a few milligrams (e.g. zinc, scandium, yttrium, niobium, gadolinium) to several grams (e.g. phosphorus, iron, sulfur). This seems to be little at first sight, but extrapolated to Switzerland and the whole year, the researchers then come to considerable amounts, about 3000 kg silver, 43 kg gold, 1070 kg gadolinium, 1500 kg neodymium or 150 kg ytterbium (see table 9 in the supplementary information of the original article).

Recycling worthwhile in places

The mean values and extrapolated tonnages say little about the actual concentrations of the elements found. They vary greatly from one wastewater treatment plant to another, sometimes by a factor of 100. For example, elevated levels of ruthenium, rhodium and gold were found in the Jura (presumably from the watchmaking industry) or elevated arsenic levels in parts of Graubünden and Valais (presumably of geological origin).

At individual locations in Ticino, the gold concentration in sewage sludge is so high that it could even be worth recovering. The explanation might be gold refineries in the region. Overall, however, the recovery of elements from wastewater or sewage sludge is currently hardly worthwhile from the researchers' point of view, neither financially nor in terms of quantity. For example, the amount of aluminum found corresponds to only 0.2 percent of annual imports, while the figure for copper is just under 4 percent.

Wastewater treatment plants only "hotspots" to a limited extent

The researchers were more interested in the basic material flows and mass balances than in the monetary value of the trace elements. This is because the study is the first to systematically record these for the wastewater of an industrialized country. To this end, they investigated what proportions of each element the treated wastewater downstream of sewage treatment plants contributes to the total pollution of streams and rivers. While 83 percent of the input of gadolinium enters waterways via wastewater treatment plants, the proportion for zinc is only 24%, for lithium 7% and for arsenic only 1%.

For the important nutrient phosphorus, the measurements of the study confirmed earlier calculations: 50% of the phosphorus load in the major Swiss rivers enters the rivers via wastewater treatment plants, i.e. originates from wastewater.

Finally, scientists have also looked at the significance of element concentrations for the environment. Studies in Germany have reported locally critical levels of the rare earths lanthanum and samarium in the Rhine. This does not seem to be an issue in Switzerland: at the vast majority of locations, no ecotoxicologically relevant or legally defined limits have been exceeded. Only the heavy metals copper and zinc are too high in the effluents or sludge at some locations. However, the authors of the study emphasize that little is known about the possible toxicological effects of many of the "new" elements.

Original publication: Quantification of Element Fluxes in Wastewaters: A Nationwide Survey in Switzerland; Bas Vriens, Andreas Voegelin, Stephan J. Hug, Ralf Kaegi, Lenny H. E. Winkel, Andreas M. Buser, and Michael Berg; Environmental Science & Technology 2017 51 (19), 10943-10953 (cover story of October issue):

You can find more details under this Link