The term micropollutants or trace substances covers a wide range of substances, for example medicines, pesticides, food additives, ingredients of cosmetics or cleaning agents. Many of these substances enter the WWTPs via domestic and industrial wastewater and are discharged into rivers and streams if they are not treated with an EMC stage. There they influence the aquatic ecology and can endanger aquatic organisms. Micropollutants not only have a negative impact on water quality, they are also found in groundwater, which is the most important drinking water resource in Switzerland.
The new stage, which went into operation at the Oberglatt WWTP in autumn 2021, uses powdered activated carbon. In this process, micropollutants bind to carbon particles, which are then separated from the wastewater. The Herisau WWTP had already chosen this process for its additional stage, which has been in successful operation since June 2015. Shortly after commissioning, thanks to this improvement, significantly fewer micropollutants entered the Glatt with the treated wastewater - and as environmental monitoring showed, this also reduced stress factors for fish, among other things. However, the improvement achieved in the upper reaches of the Glatt was practically cancelled out by the wastewater from the Oberglatt WWTP, which had not yet been built at the time.
Risk from drug residues has decreased
This is no longer the case today. Various studies conducted before and after the commissioning of the stage against micropollutants in Flawil by the Office for Water and Energy (AWE) of the Canton of St. Gallen and by Eawag, the water research institute of the ETH Domain, show a positive development in the quality of the water that is discharged into the Glatt after purification, both in terms of chemical and biological effects. The risk to aquatic organisms from the active ingredient diclofenac, which is contained in the painkiller Voltaren, for example, has halved compared to 2020. Overall brown trout showed a reduction in stress factors after the additional stage was put into operation.
"It is still too early to know the positive consequences of the new purification stage in detail," says Vera Leib, head of the Water Quality Department of the St.Gallen Office for Water and Energy, "but already one year after commissioning it can be said that the stage has led to a considerable reduction in the load of micropollutants now downstream of the Oberglatt WWTP."