Drought raises wastewater share in Swiss streams

Drought and low water levels are increasing the proportion of treated wastewater in some Swiss streams, creating challenges for wastewater treatment plants, SRF reported on Friday. Switzerland received only 30 to 70 percent of its long-term average rainfall in July, according to MeteoSwiss.

Households continue to produce roughly the same amount of wastewater, Michael Mattle, a wastewater treatment expert at SVA, told SRF. As rivers and streams carry less water, treated wastewater makes up a larger share of their flow. Even after treatment, it still contains pollutants and nutrients.

The problem is expected to affect waterways across Switzerland, said Martin Märki, a water protection expert with the Aargau environmental department. The increase is less noticeable in large rivers and lakes fed by meltwater. In smaller streams connected to one or more wastewater treatment plants, however, treatment plant discharge can account for up to 80 percent of the water.

With less water available to dilute the discharge, pollutants can exceed ecotoxicologically problematic thresholds, Mattle said. Spot checks in Aargau have found that certain limits in waterways were exceeded, Märki added. Combined with high temperatures and slow flow, the additional substances can promote algae growth and put further pressure on aquatic animals and plants.

Rain can create new problems

Rainwater often enters the sewage system together with wastewater. During dry periods, dirt accumulates on roads, said Roberto Frei, head of technology at the Pro Rheno wastewater treatment plant, which treats household, commercial and industrial wastewater from the Basel region. When rain arrives, the accumulated dirt makes treatment more demanding.

Heavy rainfall can also temporarily overload treatment plants, causing wastewater to be discharged directly into rivers. This can increase pollutant levels in waterways as well.

Plants set to undergo upgrades

Felix Kreidler, president of the Association of Wastewater Treatment Plants in Aargau, said treatment plants must be upgraded to carry out a fourth purification stage that removes even the smallest contaminants. The first three stages remove solid materials, organic substances and nutrients. The fourth targets trace substances from medicines, hormones, cosmetics and plant protection products using activated carbon or ozone.

A national expansion programme for this technology has been under way for about 10 years. Federal figures showed that almost 40 plants had been upgraded by the end of 2024. By 2040, a total of 140 plants in Switzerland are expected to be able to remove trace substances.

Because the upgrades require major investment, the Aargau government wants to encourage treatment plants to merge. The aim is eventually to replace the canton’s 41 smaller plants with 20 larger facilities.