The reed-filled canal section at Friedrichsgracht
A natural reed-filled canal section alsongside Friedrichsgracht helps to clean the dirty water in the Spreekanal.
The Flussbad project foresees cleansing the water in the Spree Canal effectively and ecologically using a natural plant filter. This is the purpose of the second section of the Flussbad, i.e. the roughly 300-metre canal strip between Gertraudenbrücke and Schleusenbrücke where the water will be cleaned using natural methods. The water meanders down through an 80-centimetre gravel sediment into a drainage layer lying underneath. After that, the purified water flows on in the direction of Museum Island. An external energy source is not necessary to push the water through the gravel filter. Instead, the filter system simply makes use of the ideal local conditions: the slope of an already existing, one-and-a-half metre high dam at the level of the former Staatsratsgebäude means that the water needs gravity alone to pump it through the filter system at 500 litres per second. This rate is sufficient to exchange all of the water in the subsequent swimming area located between the Humboldt Forum and Bode Museum in the space of one day.
In the filter area, the gravel layer will be covered with reeds, as their roots guarantee the sustained permeability of the filter surface. A gap is foreseen at the historic Jungfernbrücke so as to be able to retain that bridge's unique appearance and reflection in the water. Not all of the wastewater currently being funnelled into the Spree Canal will be treated by this filter system. In fact, today, six additional overflow pipes belonging to the sewage system flow into the Flussbad area further north between the Humboldt-Forum and Bode Museum. In other words, when heavy rainfall overloads the capacity of the existing sewage system – which occurs roughly ten times per year – the result is that unfiltered sewage water is introduced into these areas as well. In order to gain the upper hand over such events, the Flussbad project envisages installing a control structure above the Schleusenbrücke in the largest of these overflow pipes. The accumulated waste water would thus be retained and then, as soon as the rain has passed, pumped successively to sewage treatment plants. The small amount of remaining overflow will then be pumped into the Main Spree beyond the Bode Museum by means of a long pipe, to which the five other overflow pipes will also be connected. This will prevent sewage water from making its way into the Kupfergraben.