Berlin is located in a relatively dry area of ​​Germany – hence in the summers problems with water supplies are likely to arise. The city decided to take various measures in order to be able to “absorb” and store rainwater like a sponge.

But how exactly will this be done?

The underground storages for waste water

The first step was to build several huge underground basins, which act as large-scale storage areas for waste water. When it rains, water from the surrounding area is collected in the respective basin and then channeled to treatment plants.

Nine such facilities have been completed so far, but the largest is still under construction and is expected to be ready in 2026. It will be 30 meters deep and have a capacity of around 17,000 cubic meters of rainwater – the size of a Olympic size swimming pool.

Reducing overflow waste

Whenever there is heavy rain and Berlin’s sewer system is in danger of overflowing, the excess water is stored in the basins. It is then channeled elsewhere to be cleaned and released back into Berlin’s canals and rivers after the rain stops.

This also prevents the river Spree from filling up with excrement and sewage, says Astrid Hackenes-Rubb, spokeswoman for the Berlin water company BWB. BWB is responsible for the supply of drinking water, as well as for the management and treatment of wastewater.

Waste overflow occurs in combined sewer systems, where stormwater runoff and domestic waste are collected in the same network of pipes – the purpose of these systems was to transport all the water to a treatment facility before it returns to natural water bodies.

Nevertheless, in a heavy rainfall, the volume of water entering the system may exceed its capacity. When this happens, the excess water overflows directly into nearby rivers.

About 2,000 of the city’s total 10,000 kilometers of pipes are combined sewer systems – and overflows can occur in 180 different locations. These sites are essentially openings in the drainage system that lead to the River Spree, as Hakenes-Rubb explains.

In addition, Berlin’s canals are quite small and slow-flowing compared to other cities. The Rhine, for example, has an average flow of 2,200 cubic meters per second and can thus “self-clean”.

“In Berlin the flow is even below 10 cubic meters per second, so whatever ends up in the canals stays there for a long time,” says Hakenes-Rubb. “Hence, waste overflows often result in fish dying and greatly reducing the oxygen in the water,” adds the expert.

However, the authorities quickly realized that basins can only partially solve the problem, because too much of the city is sealed with cement or other similar materials.

“Therefore, we could not reduce the overflows any further. So we kept the situation as it was, except that if we hadn’t built the basins, then things would have been even worse,” says Hakenes-Rubb.

Turning Berlin into a ‘sponge’ city

Berlin has already built roads and buildings across most of the city’s open spaces, where water could escape underground. Therefore, when the rain is heavy, the water cannot be absorbed and may end up mixing with the waste.

That is why it has now been decided to establish a special service responsible for rainwater management, which provides advice on the creation of green roofs and buildings and promotes innovations for the collection and storage of rainwater.

In addition, Berlin passed a law stipulating that new buildings being built will only be allowed to channel a small portion of the site’s rainwater into the sewer system. The rest should, for example, be vented or collected, cleaned and then used for water supply.

All these measures naturally also help to keep temperatures low and prevent flooding. “Finding solutions to the water crisis requires people’s willingness to think outside the box, even if that means thinking outside the boundaries of their homestead,” says Hakenes-Rubb.

Edited by: Giorgos Passas