According to the North Rhine Medical Association (Ärztekammer) 400 active pharmaceutical ingredients can be detected in the water, with "massive" consequences for the environment. For example, the active ingredients can influence the mating behaviour of animals or they can cause organ damage. They are particularly dangerous for fish, says the State Nature Agency (Landesamt für Natur). The substances contained in the treated wastewater may also increase the challenges involved in purifying drinking water.

The Federal Environment Agency (Umweltbundesamt) also points out that there have been indications since the 1990s that diclofenac is a danger to animals. The active ingredient is contained in gels or tablet including products intended to help with inflammation or pain. It ends up in the sewage because it is excreted in urine or because we wash the gels off when we take a shower.

Antibiotics also end up in our wastewater, where they are not filtered out by sewage treatment plants and end up back in the environment. A team of researchers led by Martin Exner has proven this. Exner was Director of the Institute of Hygiene at the University of Bonn and is a member of the Drinking Water Commission at the Federal Ministry of Health. "If antibiotic residues get into the environment, they can contribute to the further spread of resistant bacteria in the environment," says Exner. Antibiotics are extremely important in modern medicine, but they become ineffective as a result.

Many drugs, including antibiotics, are largely excreted by the body unchanged. In the case of diclofenac, this figure is 70 per cent. According to the German Medical Journal Ärzteblatt, Germans consume around 90 tonnes of the active ingredient every year, which means that 63 tonnes of diclofenac end up back in the water cycle via the toilet. Modern wastewater treatment plants can only filter out a third of the active ingredient from wastewater.

This also applies to the wastewater treatment plants in Bonn. Active pharmaceutical ingredients are released into the environment and into drinking water, writes the Ärzteblatt. The concentration of diclofenac, at which "no effects on the ecosystem" occur, was exceeded at 21 out of 24 monitoring sites in Germany.

In Duisdorf, too, the water leaving the sewage treatment plant still contains diclofenac - as well as residues of other active substances including diazepam or ibuprofen. This is all in a list compiled by the State Agency for Nature Conservation. The list is several pages long and is lying on the table in front of Achim Höcherl. In recent years, samples of treated wastewater from Bonn's sewage treatment plants have shown peak values of three micrograms per litre for diclofenac, meaning it can be assumed that there is an effect on the environment.

Höcherl is responsible for the city's four wastewater treatment plants in Bonn. This morning, he is sitting in the entrance lobby of the Duisdorf plant, the list in his hand. But it doesn't worry him much. "We treat the water in accordance with the law," says Höcherl. This does not stipulate any limit values for how high the concentration of active substances in the treated wastewater may be.

Sewage is treated in three stages at the Bonn wastewater treatment plants: During mechanical cleaning, stones, leaves and toilet paper get caught in a metal screen as the water runs through the sewer into the plant. It is then purified biologically. Bacteria remove the organic compounds from the water. Finally, the last stage is chemical purification which takes place in another tank and ensures that the water no longer contains any phosphorus. Because this leads to negative environmental processes in bodies of water. Before the water leaves the treatment plant again, a final tank filters it through sand.

There are around 9500 public sewage treatment plants in Germany, most of which have three purification stages. However, according to the German Association for Water, Wastewater and Waste (DWA), there are now 51 plants that use a fourth stage - mainly in North Rhine-Westphalia and Baden-Württemberg. They use activated carbon or ozone to remove drug residues from the water.

The fourth stage could soon become mandatory for more wastewater treatment plants, as the European Union is currently revising its Urban Wastewater Directive. The final version is expected in spring 2024. It is still unclear what size of wastewater treatment plant the directive will apply to and by when it must be implemented; 2035 has been mentioned.

Peter Esch, Head of the Bonn Civil Engineering Office (Tiefbauamt), says in writing that the city is "not prioritising" the use of a fourth stage. Esch also emphasises the possible negative consequences of residues in treated wastewater, which are "not fully" known. "For this reason alone, sewage treatment plants should be equipped with a trace substance elimination system. The Civil Engineering Office expressly supports this," writes Esch.

Esch says it is not possible to say for certain how much it would cost to set up and operate the fourth stage as long as there are no plans to do so. A study by the German Association of Energy and Water Management concluded that it would increase the wastewater charges for a four-person household by an average of 60 euros. The DWA considers this figure to be very high, especially as the EU directive stipulates that manufacturers of pharmaceuticals should contribute to the costs of the fourth treatment stage.

Scientist Exner does not believe that a fourth stage is absolutely necessary, but is in favour of making a decision based on the level of contamination in the water. This can be measured precisely and is high where there is a lot of sewage, for example from hospitals.

Höcherl then gives a short tour from the entrance area of the sewage treatment plant to the basin where the water is finally filtered through the sand. "It works like a coffee filter," says Höcherl. This would be the point at which the fourth stage would start, using activated carbon instead of sand, for example. This would also be the end of the line for drug residues.

Original text: Dennis Scherer

Translation: Jean Lennox