With the imminent introduction of the
fourth purification stage in the EU and Austria, wastewater treatment plants are facing major challenges. As a pioneer in chemical water treatment, Donau Chemie began preparing for this development years ago. It is important to learn from practical experience: to this end, we are going on an information trip to Baden-Württemberg - a region that has been gaining experience with the fourth purification stage for around 20 years.
KARL - The new municipal wastewater guideline
The European Union's Urban Wastewater Directive (
KARL) came into force on January 1, 2025 and must be transposed into national law by the member states by July 2027. The introduction of a fourth treatment stage in wastewater treatment plants to remove micropollutants such as pharmaceuticals, cosmetics, pesticides and other chemicals is central to this. The implementation particularly affects large wastewater treatment plants with a population equivalent (PE) of 150,000 or more as well as medium-sized plants with a PE of 10,000 or more if they are located in areas that are classified as sensitive to trace substances.
Read also: KARL, who? The revision of the Urban Waste Water Directive
On the trail of modern sewage treatment technology - and a Gothic masterpiece
Our journey to Baden-Württemberg took us through the Katschberg and Tauern tunnels to Salzburg, then via Munich and Augsburg to Ulm. From our hotel, we started our excursions to learn about the common processes of the fourth purification stage - including filters with granulated activated carbon, ozone systems and powdered activated carbon contact basins.
Of course, a visit to Ulm city center was not to be missed - after all, the famous Ulm Minster with the highest church tower in the world is located here. At 161.53 meters, it not only towers above Cologne Cathedral, but also all other church towers worldwide. The foundation stone for the late Gothic masterpiece was laid in 1377, but it was only completed 513 years later, in 1890.
The imposing church tower fascinates around one million visitors every year. Those who climb the 768 steps to the platform at a height of 143 meters are not only rewarded with a breathtaking panoramic view of Ulm, Neu-Ulm and the Danube, but also gain exciting insights into the work of the tower keepers and the cathedral's eventful history.
Steinhäule sewage treatment plant on the Danube - cradle of the Ulm process
A particular highlight of our trip was the visit to Ulm's main sewage treatment plant, the birthplace of the well-known “Ulm process”. The impressive plant treats the wastewater of around 450,000 inhabitants and uses a proven and efficient process to remove trace substances in the fourth treatment stage.
The Ulm process in detail
The classic mechanical, biological and chemical purification is followed by the fourth purification stage, which proceeds as follows:
- activated carbon addition: powdered activated carbon is added to the wastewater in the contact reactor and the two are mixed together. This causes the trace substances to attach to the activated carbon particles.
- sedimentation: The water-activated carbon mixture is then fed into the sedimentation tank, where the activated carbon is separated from the water again.
- optimization: The addition of aluminium salts as precipitants and polymers as flocculants improves separation and increases the efficiency of the process.
Decades of experience and continuous optimization impressively demonstrate how efficiently and reliably the fourth purification stage can function in practice.
DONAU PAC® AQUACLEAR as an alternative to existing processes
During our excursions, we were able to get a comprehensive picture of how various processes work, including ozonation with a biological filter stage, adsorption on powdered activated carbon (PAC) with various separation techniques - including the tried-and-tested Ulm process - and the use of granulated activated carbon filters (GAC).
These technologies are established solutions for the removal of trace substances, but often involve high investment costs and infrastructural adjustments. In view of the increasing demands on wastewater treatment, the question therefore arises of a more economical alternative, especially for small and medium-sized wastewater treatment plants.
Based on this, it can be said that DONAU PAC® AQUACLEAR can close precisely this gap. The innovative combination product from Donau Chemie Wassertechnik enables a cost-efficient implementation of the fourth treatment stage - without operational changes, multi-million dollar conversions or additional safety measures.
The procedure can be easily integrated into existing processes, requires no additional personnel and no costly adjustments to the infrastructure. In this way, local authorities can achieve considerable savings, which ultimately also reduce the burden on taxpayers.
All-in-one purification for micropollutants and phosphate
DONAU PAC® AQUACLEAR is an innovative combination of powdered activated carbon and a precipitant that enables simultaneous micropollutant removal and phosphate precipitation. The high effectiveness of the product has been scientifically tested and confirmed by renowned, independent institutions such as the Vienna University of Technology.
“Donau PAC® AQUACLEAR is less of a liquid like water, but behaves more like a toothpaste - it is slightly thicker than our usual products,” says Wolfgang Binder, describing the innovative solution.
The product can be easily dosed into the biological treatment stage without any structural or mechanical modifications. All that is needed is to replace the conventional precipitant, which every wastewater treatment plant needs anyway, with Donau PAC® AQUACLEAR. The tried and tested processes for delivery, storage and dosing remain unchanged.
Read also: Wastewater treatment explained simply: the 4 treatment stages
Conclusion: Well prepared for the future of wastewater treatment
Our trip to Baden-Württemberg gave us valuable insights into the fourth treatment stage. It became clear which requirements are placed on modern processes and which solutions have proven themselves in practice. With our portfolio of granulated activated carbon, powdered activated carbon and Donau PAC® AQUACLEAR, we are ideally prepared for this. We will continue to focus on continuous further development in the future in order to be able to support users - especially when using AQUACLEAR in the fourth purification stage.
We will continue our learning journey with further visits and look forward to breaking new ground in wastewater treatment together with our partners.
Read also: DONAU PAC® AQUACLEAR: Successful practical test for the removal of trace substances from wastewater