That is the finding of biologists at Goethe University Frankfurt, who say effluents from wastewater treatment plants have a dual effect: some species disappear, while others increase in number.

The impact of effluent is especially noticeable with certain insect species, such as stonefly and caddisfly larvae, which are decimated. By contrast, certain worms and crustaceans, can increase in number.

The research team examined 170 wastewater treatment plants in the state of Hesse in western Germany, in relation to the composition of species of invertebrates. This showed that human-induced stressors do reduce the number and diversity of species in a habitat, but that a shift in species composition can also be observed.

The researchers say wastewater treatment plants are an indispensable part of our modern infrastructure and have made a significant contribution to improving the quality of surface waters. However, their ability to completely remove micropollutants from wastewater has been found to be limited.

These substances include active ingredients from pharmaceuticals and personal care products, pesticides and other synthetic substances. These compounds enter waterbodies via treated wastewater, placing an additional burden on rivers and streams and exacerbating the challenges faced by vulnerable insect communities and aquatic fauna.