How can we clean the Spree
The filter system and how it works
The hydrologist Heiko Sieker explains, how the filter system of Flussbad works and how the Spree at Kupfergraben could turn into river with a good water quality.
THE NECESSITY OF A FILTER SYSTEM FOR THE FLUSSBAD BERLIN
The water quality in the Spree River has improved significantly in recent decades. An evaluation of our regular measurements has proven this to be true. Upstream, that is, before the river reaches the centre of Berlin, there are even several swimming areas that are highly popular among Berliners. For example, the lake known as the Große Müggelsee, through which the Spree flows, has been classified according to the EU Directive as having an "excellent quality". In the subsequent inner-city area, however, a number of elements are introduced into the river – both from the rainwater sewer system and from the combined wastewater system. Add to that the Münchehofe wastewater treatment plant, which also pumps its treated wastewater via the Neuenhagener Mühlenfließ (Erpe) into the Spree. A significant level of water pollution comes especially from the combined wastewater overflows, which pump a mixture of wastewater and rainwater after strong rainfall into the Spree. Storm water runoff from city streets – especially dog droppings and cigarette butts – also contributes to water pollution. This is why it's necessary to clean the Spree water before it reaches the future Flussbad swimming area. A plant filter installed in the Spree Canal in the section just above the swimming area is therefore an essential component of the Flussbad project.
FILTER SYSTEM REQUIREMENTS
The filter is designed to treat the inflowing water in such a way that the Flussbad always has a good water quality. In order to achieve this, we have to reduce bacteria and algae in particular. According to the EU Bathing Water Directive, a low bacterial count is essential in order to avoid health risks. While algae are not harmful in and of themselves, they can still affect visibility and thus also safety. They also have an effect on our subjective feelings: for example, cloudy water is often considered "dirty". Excessive algae growth can also lead to the "collapse" of the water.
At first, we considered disconnecting the Spree Canal from the main Spree and circulating the water for the Flussbad by means of a filter instead of allowing it to continue to flow through the canal. But the character of a Flussbad – a flowing stream – would be lost in such a setup. Plus the circulation pumps would require the use of a lot of energy. Instead, we opted for a natural plant filter that keeps operating costs low compared to a mechanical filter system. The design of the filter system must fulfil yet another requirement, that is, the guaranteeing of flood protection. Seeing as the flow through the main Spree is limited due to the site of the Mühlendammschleuse, a part of the runoff (50 m³/s) in the case of flooding must be able to be diverted via the Spree Canal.
The Spree water will be purified in a reed-gravel filter through which it will flow vertically. In order to keep the required volume small at high capacity, the filter can be artificially ventilated with compressed air. These kinds of plant filtration systems have been used successfully for years to treat wastewater and for swimming ponds. The image above shows the setup of the filter. In principle, it would be possible to install a further treatment stage (e.g. a UV disinfection system), even though it's not considered necessary at this point. The space required for this would be available in the area between the filter and the beginning of the swimming area. A new weir would be built between the filter and the swimming area (roughly at the height of the Schleusenbrücke) as a replacement for the existing weir; the new weir would regulate the water level in the system's upstream area to the same as current levels. This would help up us avoid affecting the groundwater. Rectangular profiles would be installed under the filter to remove flood water. When the swimming pool is in operation, these rectangular profiles over the weir would be closed; they can then be opened in the case of floodwater. Hydraulic calculations have shown that these rectangular profiles could guarantee the transport of the required 50 m³/s runoff in the case of floodwater. A new and adjustable weir would be installed at the end of the swimming area, right before the confluence of the Spree Canal into the main Spree. This would prevent the reflux of Spree water back into the Flussbad. It would also regulate the water level in the swimming area to a height that lies a few centimetres above the downstream level. The weir would be able to be lowered in the case of flood runoff or to facilitate the entry of ships in the lower Kupfergraben. It would also take on the function of a "skimmer" that generates a surface current in the bathing area – much like in a swimming pool – and thus also remove floating matter.
CONSTRUCTION OF A PARALLEL COMBINED WATER PIPE
One problem is the fact that combined sewage currently discharges directly into the forthcoming Flussbad area at several points. The largest combined sewage discharge is found directly at the Schleusenbrücke underneath the weir, which means that it cannot be cleaned by the filter. The plan is therefore to install a so-called "management structure" at the lead-in; this would clearly reduce the input quality by means of roughly 4,300 m³ of storage volume. We are also examining to what extent we could reduce the volume and frequency of overflows by means of the decentralised management of rainwater in the area at the discharge point. These measures would provide a noticeable improvement not only in the swimming area, but also in the Spree overall. If necessary, it would also be possible to install a new pipe within the bathing area that takes up the combined wastewater and passes it on into the Spree beyond the Flussbad.
The filter system presented here makes it technically possible to generate high-quality bathing water in the future Flussbad. Such filtering would also act as a model project for the further cleaning of the Spree that would fulfil the EU water framework directive.
1 www.berlin.de/badegewaesser/ badegewaesserprofil/strandbad-mueggelsee.html
2 Coliform bacteria, escherichia coli
3 The cyano bacteria commonly known as "blue-green algae" are not algae in the biological sense. The Flussbad system would also prevent them from forming en masse.
The design of the filter system took place as part of a framework study called the "Analysis, Change and Development of Select Aspects of the Urban Concept of the Flussbad Berlin". The study was funded by the foundation known as the Stiftung Deutsche Klassenlotterie Berlin. The following individuals participated in the design of the filter system: Prof. Dr. Heiko Sieker, Mike Post, Nicolas Neidhart / Kai Schroeder, Katharina Teuber and Klaus-Jochen Sympher at Dr.-Ing. Pecher und Partner Ingenieurgesellschaft mbH, Prof. Dr. Jens Nowak, Heribert Rustige at AKUT Umweltschutz Ingenieure Burkard und Partner,