How Berlin’s giant tunnels help fight water shortages – DW – 08/10/2024

Berlin is in the driest part of Germany, and water supply is a hot topic every summer. That is why the city takes steps to absorb and store rainwater like a sponge and release it when water is needed.

But how exactly does that work?

Building underground wastewater storage facilities

The first step was to build many underground basins filled with large amounts of water, which act as large parking lots for garbage. When it rains, water from the surrounding area is collected in a basin and pumped to a treatment plant.

Nine of these facilities have been completed, including one under Mauerpark, a popular hangout area in the Prenzlauer Berg district, where parts of the Berlin Wall once stood.

The largest wastewater treatment plant in the city is under construction. It will be more than twice the size of Mauerpark. At a depth of 30 meters (98.4 feet) into the ground, the circular concrete basin will hold about 17,000 cubic meters of rainwater once it is completed in 2026. This is equivalent to about seven Olympic-sized swimming pools.

To reduce sewage flow

When it rains heavily and Berlin’s sewage system is in danger of breaking down, the remaining water is stored in basins. It is then pumped to a treatment plant before being returned to Berlin’s canals and rivers once the rain stops.

This will prevent sewage and waste water from being discharged into the River Spree during heavy rains, said Astrid Hackenesch-Rump, spokeswoman for the Berlin BWB water works. BWB is responsible for the city’s drinking water supply, as well as the management and treatment of wastewater throughout the city.

“The driving force behind this program was not only resource conservation and drought, but also preventing combined sewers,” Hackenesch-Rump said.

Such flows occur in combined sewer systems, where storm water and domestic sewage are collected through the same network of pipes. These systems were originally designed to transport wastewater to a treatment plant before being discharged into natural waters.

However, during heavy rains, the volume of water entering the system can exceed its capacity. When this happens, excess water – including stormwater and untreated sewage – flows directly into nearby rivers.

About 2,000 out of 10,000 square kilometers (6,214 miles) of the city’s sewers are combined sewer systems with additions at 180 locations. In fact, these are the sewers leading to the River of the Spree, Hackenesch-Rump explained.

Berlin’s waterways are also small and slow compared to other cities. Take a large river like the Rhine, which flows through many urban areas, including Cologne. It has an average flow rate of 2,200 cubic meters per second and can therefore clean itself quickly.

“We have flow rates in Berlin of less than 10 cubic meters per second, so anything that flows naturally will stay there for a long time,” Hackenesch-Rump said.

“That’s why these sewage overflows often lead to fish kills and oxygen depletion in the water,” he added.

However, water designers quickly realized that basins could solve part of the problem because most of them were the city is sealed with concrete and a waterproof surface.

“That means that we were no longer able to meet the goal of reducing the flow. Instead, we maintained the status quo, which means that if we didn’t build​​​​ [the basins]it could have been worse,” he said.

Turning Berlin into a sponge city

Berlin has already built most of its open spaces where water could seep into the ground. So, when it rains a lot, instead of being absorbed by the soil and plants, the water runs off the concrete or asphalt and can end up mixing with sewage.

“One percent more tightening results in a 3% increase in flow,” Hackenesch-Rump said.

That is why the Berlin Senate and the water utility BWB have created a “rainwater center.” The center advises urban planners on how to design green roofs and buildings and come up with new ideas to collect and store rainwater so it doesn’t mix with sewage.

The city of Berlin has passed a law which means that only a small amount of rainwater in a new building can flow into the sewage system. The rest must evaporate or go into the ground.

For example, a new building has recently been built with a large reservoir that collects rainwater. Nearby plants help purify the water, which can be used for irrigation.

Green measures like this also help keep temperatures down and protect against flooding.

“To solve the water problem, we need people to be willing to think outside the box, even if it’s just to think beyond your property line,” said Hackenesch-Rump.

Edited by: Anke Rasper