Wastewater treatment plants retain 96 percent of mercury

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Master’s degree student Lara Cayo takes samples of sewage sludge at Werdhö
Master’s degree student Lara Cayo takes samples of sewage sludge at Werdhölzli sewage treatment plant. (Photo: Elke Suess, Eawag)

Each year, around 130 kilograms of mercury flow into Swiss wastewater treatment plants, which filter out the lion’s share from the waste water. This is shown by a survey of almost thirty wastewater treatment plants conducted by the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and the Federal Office for the Environment (FOEN).

Mercury is extremely toxic for humans and ecosystems. It evaporates very quickly, is transported in the air over long distances and accumulates in organisms. The heavy metal has therefore been known for decades as a pollutant of global significance and is banned in Switzerland for almost all applications. However, if old thermometers break, for example, mercury still ends up in wastewater. There have been no representative studies to date on what quantities actually flow into wastewater treatment plants (WWTPs) and how effectively they filter out the toxic substance.

In a study just published in the journal "Aqua&Gas", researchers from Eawag in collaboration with the FOEN have now shown that Swiss WWTPs filter out 96 percent of mercury. Expressed in figures: Every year, 130 kilograms of mercury enter the WWTPs with the wastewater, of which 5 kilograms are discharged into streams and rivers. "Compared to the total amount of mercury in Swiss surface waters, however, the inflow from wastewater treatment plants only accounts for about 1.5 to 3 percent," says Michael Berg, geochemist at Eawag and co-author of the study. The remaining mercury is discharged into the aquatic environment from diffuse, as yet unidentified sources, for example by atmospheric transport from the oceans or soil, or leaching from historically contaminated sites or sediments.

High values in French-speaking Switzerland

Compared to other countries, relatively little mercury enters waste water treatment plants here and the figures are similar to those in Canada or England. The highest mercury levels are found in WWTPs in western Switzerland, especially in the regions where the watch and jewellery industry is located. On the other hand: "Whether the mercury actually originates from these regions or was transported through the atmosphere from far away would have to be investigated more closely," says Michael Berg. The concentrations measured in the water are, however, harmless to health.

Basis for the Minamata Convention

The study comes at an important time. For the Minamata Convention, which came into force in 2017, requires all countries to draw up an inventory of mercury concentrations in their waters. The aim of the convention is to contain and monitor mercury emissions worldwide. Because the study by Eawag researchers is representative of other developed countries around the world, it now provides methods and figures for the inventory.

Suess, E.; Berg, M.; Bouchet, S.; Cayo, L.; Hug, S. J.; Kaegi, R.; Voegelin, A.; Winkel, L. H. E.; Tessier, E.; Amouroux, D.; Buser, A. M. (2020) Mercury loads and fluxes from wastewater: a nationwide survey in Switzerland, Water Research, 175, 115708 (10 pp.), doi: 10.1016/j.watres.2020.115708 , Institutional Repository

  • Berg M., et al. Quecksilber in Schweizer Abwasser - Konzentrationen, Massenflüsse, Speziierung und Rückhalt. Aqua & Gas, 1/2021, pp (2021) [2.23MB]