Can Berlin turn into a sponge city amid rising temperatures and drought?

First came the construction of several enormous underground overflow basins. They function like giant wastewater parking lots. When it rains, water from the surrounding area is funneled into the basin and then pumped to a treatment plant.

Nine such facilities are already in place, among them one under Mauerpark, a popular hangout spot in the district of Prenzlauer Berg, where parts of the Berlin Wall once stood.

What is being constructed in Berlin is still the largest inner-city wastewater basin. It will be over twice the size of the facility in the Mauerpark. The round concrete basin will stretch 30 meters below ground and have almost 17,000 cubic meters of rainwater when it is completed in 2026. That is about the size of nearly seven Olympic pools.

Reducing sewage overflows

In cases of heavy rain, this surplus water is stored in the basins because Berlin’s sewage system overflows. Afterward, it is cleaned in a purification plant to be led back into Berlin’s canals and rivers once the rain has stopped.

This will prevent feces and wastewater from being flushed into the Spree River during heavy rain, said Astrid Hackenesch-Rump of Berlin’s water works BWB. BWB is responsible for the drinking water supply in the city and for collecting and treating wastewater throughout the city.

“The driving force behind this program was resource conservation, drought, and the prevention of combined sewer overflows”, said Hackenesch-Rump.

Turning Berlin into a sponge city

Much of Berlin’s open spaces where water could percolate into the ground have long since been built on. So when there’s a lot of rain, instead of being sucked up by soil and plants, it runs off the concrete or asphalt and can end up mixing with sewage.

“One percent more sealing results in a 3% increase in overflows,” Hackenesch-Rump added.

That’s why the Berlin Senate, together with the water utility BWB, founded a “rainwater agency.” It advises urban planners on ways of designing green roofs and buildings and comes up with innovative ideas for collecting and storing rainwater so it doesn’t mix with sewage.

The city of Berlin has then legislated that only a little rainwater from a new building property can flow into the sewage system. The rest has to evaporate or seep into the ground.

For example, a newly built apartment block had a large artificial pond collecting rainwater. Plants nearby help clean up the water for eventual reuse in watering plants.

This also keeps temperatures at bay and protects against flash floods.

“To come up with a solution to the water crisis, it would require people’s willingness to think beyond boundaries, even if that means only thinking beyond their property line,” she explained.