Cleared water also disrupts river life

The treated water from sewage treatment plants is often discharged into nearby streams and rivers. In this way, critical problem substances still get into the water. © Jonas Jourdan

Intensively cleaned - but that does not mean that the water from sewage treatment plants is clean, as a study in Hesse shows: The inflows from sewage treatment plants, which are still contaminated with certain anthropogenic problematic substances, can therefore significantly change the composition of organisms in water bodies. There is a shift in conditions in favor of tolerant species groups and to the detriment of more sensitive species groups. This in turn can affect nature in complex ways. The cleaning power of the sewage treatment plants against trace substances should therefore be further improved, say the researchers.

They are undoubtedly a great achievement in the history of environmental protection: sewage treatment plants have made a significant contribution to improving the quality of water bodies. As a result, many ecosystems that were once extremely stressed have recovered and aquatic organisms have been able to resettle in them. According to studies, however, the aquatic environment does not return to its original state. This has to do, among other things, with the water quality, which is still being significantly altered by humans. An important aspect is that the cleaning processes of the sewage treatment plants have so far hardly been able to remove so-called trace substances from the sewage from civilization. These are a variety of residues from medicines, personal care products, pesticides and other synthetic substances that can pollute organisms.

Species comparison upstream and downstream

The study by the research team led by Daniel Enns from the Goethe University in Frankfurt expands the information available so far on how this elusive cocktail of substances in the wastewater can affect aquatic organisms. So far, studies have mostly focused on individual plants and bodies of water. In order to provide more comprehensive data, the scientists examined how the wastewater from 170 sewage treatment plants in Hesse affects the species composition of invertebrates. To do this, they recorded these creatures in the stream and foot sections above and below the respective tributaries from the facilities at a total of 366 sampling points.

As the scientists report, the results of the surveys revealed a clear change in the composition of the species communities. As they point out, the result contradicts the popular notion that human-caused stress simply reduces species numbers and hence diversity. Instead, an exchange can be observed: some species are even completely lost through the discharges from sewage treatment plants, while others can benefit from the changes and settle or spread. An average species turnover of 61 percent is emerging, the researchers write. In concrete terms, the numerous losers include aquatic snail species and, above all, the aquatic larvae of stoneflies and caddis flies. The winners, on the other hand, are certain representatives of worms and crustaceans. The scientists found that streams and smaller rivers are particularly affected by this change in species composition.

Critical Species Shift

"Our results show that wastewater treatment plants change the conditions downstream in favor of more environmentally friendly species groups and to the detriment of more sensitive ones," the team summarizes. Even if the bottom line is that the diversity of species can remain similar, this must be seen as a clear sign of deteriorating ecological health. Because in the complex systems of nature, certain changes in the well-established species communities can lead to further complex effects. It is particularly critical when aquatic larval stages are affected by insect species. This could contribute to the worrying decline of these ecologically important organisms. "Accordingly, it is important to find out how and why communities change," the researchers write.

In order to counteract the problematic effect of exposure to trace substances, they campaigned for a further improvement in the cleaning performance of sewage treatment plants. Because there are possibilities: Modern purification techniques such as ozone treatment or activated carbon filtration can remove a broader range of pollutants from wastewater. The University of Frankfurt writes that the merging of the many small sewage treatment plants into more effective large-scale plants could also help to relieve the burden on the environment.

Source: Goethe University Frankfurt, specialist article: Water Research, doi: 10.1016/j.watres.2023.120388

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