Effluents from wastewater treatment plants can benefit some species.

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.

Previous studies – which have primarily focused on single wastewater treatment plants – have already shown that invertebrate communities downstream of such effluents are generally dominated by pollution-tolerant species. Until now, say the researchers, it was unclear how ubiquitous these changes are.

The researchers were able to identify significant shifts in the composition of the species community between sites located upstream and downstream of wastewater treatment plants. This change is particularly noticeable in streams and smaller rivers.

Overall, wastewater treatment plants alter conditions downstream to the advantage of pollution-tolerant taxa and to the disadvantage of sensitive ones.

How can we reduce water pollution?
Modern treatment techniques such as ozonation and activated charcoal filtering can make wastewater treatment plants more efficient, allowing a wider range of pollutants to be removed before it is released into the environment. Merging smaller wastewater treatment plants can also help reduce the burden on the environment.

Whatever measures are taken, it is important to make sure that upstream sections are not already degraded and are in a good chemical and structural condition.

The study, led by Daniel Enns and Dr Jonas Jourdan, is published in the journal Water Research.