Even treated wastewater may affect rivers

Some species vanish due to wastewater treatment plant effluents, whereas other species gain. Larvae of some insect orders, such as the stonefly and caddisfly orders, are particularly badly affected. In contrast, some worms and crustaceans have the ability to proliferate. This was supported by a thorough study that was just released in the journal Water Research by a group from Goethe University Frankfurt led by Daniel Enns and Dr Jonas Jourdan. They examined 170 wastewater treatment plants in Hesse in relation to species composition.

Wastewater treatment facilities are an essential component of our contemporary infrastructure and have significantly improved the quality of our surface waters. However, their capacity to entirely remove wastewater's so-called micropollutants is largely constrained. These compounds, which add to the burden on rivers and streams, include, for instance, active components from pharmaceuticals and personal care items, pesticides and other synthetic substances.

The difficulties experienced by already endangered insect groups and aquatic fauna are exacerbated by this. Previous research, which mostly concentrated on individual wastewater treatment facilities, has already demonstrated that invertebrate populations downstream of such effluents are typically dominated by species that are tolerant of pollution.

Until now, however, it was unclear how ubiquitous these changes are. That is why a team of biologists from Goethe University Frankfurt has now studied extensively how wastewater from 170 wastewater treatment plants in Hesse has an impact on the species composition of invertebrates. This has prompted a change in the common conception that human-induced stressors reduce the number of species in a habitat and thus their diversity: Rather, the findings indicate that a shift in species composition can be observed.

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. Some species were particularly affected by effluents from wastewater treatment plants – such as stonefly and caddisfly larvae, which disappear entirely in some places. Other taxa, such as certain worms and crustaceans, by contrast, benefit and are found in greater numbers.

This change can be observed especially 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 or activated charcoal filtering can make water treatment in wastewater treatment plants more efficient, allowing a wider range of pollutants, including many trace substances, to be removed from the wastewater before it is released into the environment. Merging smaller wastewater treatment plants can also contribute to reducing 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 good chemical and structural condition.