The Federal Intelligence Service is getting a new neighbor who knows how to disguise himself: next to the BND headquarters on Chausseestrasse in Mitte, the Berliner Wasserbetriebe are currently constructing by far the largest single project in their sewage storage program. These reservoirs are intended to prevent the inner-city combined sewage system from overflowing in heavy rain, i.e. a diluted mixture of house and street sewage rushing into the waterways.
Day in, day out, a green and a yellow excavator splash their shovels into a huge, round basin, half-filled with groundwater, that was erected on the site of the former BND construction site. More specifically, it was built under this square in the form of a ring-shaped concrete wall and is now being excavated. According to site manager Jens Richter, 20 of the 23 meters have been completed. But now, having cleared the layer of sand and rubble, the excavators are struggling through marl, which is almost as solid as cement. In heavy chunks he claps from the shovels to the edge of the pool.
It will be another four years before the reservoir is put into operation – then with a concrete floor secured with steel anchors against the buoyancy of the groundwater. And with a thick concrete cover over which grass is to grow and the Mitte district is planning to build a playground. The children swing, slide and then climb around on a vessel that, at 16,750 cubic meters, holds as much water as six to seven Olympic swimming pools. Except that in this pool near the Schwartzkopffstraße underground station there is a broth that nobody should bathe in. In the densely built-up area there is no space to allow all the rainwater to seep away during storms. But thanks to the reservoir, it should only rush untreated into the Spree in exceptional cases, but be held back until the sewage treatment plants have capacity again. It usually only takes a few hours to smooth the tsunami into a steady stream.
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When the tank has been pumped out after the rain and the sludge has made its way to the sewage treatment plant, the tank is flushed and is ready for the next downpour. At 750 hectares, the catchment area of ​​the new reservoir is twice the size of Tempelhofer Feld and, with a degree of sealing of 78 percent, is a classic problem. This is because water cannot seep into sealed surfaces, it can only flow off. According to the plans of the city planner James Hobrecht, it does this together with the house sewage.
This type of sewage system was revolutionary 150 years ago, but is proving to be more and more of a problem in the climate crisis, because the already scarce water supply does not reach the groundwater, but is channeled out of the city via the water – depending on the type of rain with an intermediate station in the Sewage treatment plant or as a dirty torrent that additionally consumes the scarce oxygen supplies in the ever warmer waters of the Spree and pollutes the bathing areas on the Havel with algae and germs.
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There will be a total of 300,000 cubic meters of storage space that the water company will create for the waste water. Most of it is finished, far more than 100 million euros have already been invested in underground retention basins, chokes and weirs in the past 20 years.
Andreas Irmer, head of sewage disposal at the water company, sees “the largest rain barrel in Western Europe” next to the BND as the finale of the program, which also includes a huge basin under the Mauerpark. Other large storage facilities are hardly feasible in the city center – on which the problem is concentrated.
Instead of creating smaller and smaller reservoirs for more and more money, there should be a paradigm shift: “We have to concentrate on the sponge city with all our strength,” says Irmer. So make sure that as much rainwater as possible is used on the spot – for groundwater formation or for watering the nearest trees and green spaces. Green roofs are also considered effective because they also store a lot of water for a short time and can improve the microclimate through evaporation.
As clear as the goal of the absorbent “sponge city” is, the way to get there is proving difficult: The decoupling of one percent of the area associated with the mixed sewerage system agreed by the previous red-red-green coalition has failed. The goal of the current coalition is: 20 percent fewer discharges into the particularly polluted Landwehr Canal.
Irmer hardly sees any chances for additional memory there. So all that remains is becoming a sponge. This can be promoted here and there by simple means, for example by no longer sloping a renewed sidewalk towards the gullies, but towards the tree grates. But in order to really make progress, a massive area has to be unsealed – for example from parking lots. Irmer expects hard fights that are actually long overdue.
