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Paris unveils massive underground water storage basin to clean up Seine River ahead of Olympics

  • Paris unveils massive underground water storage basin to clean up Seine River ahead of Olympics
  • Paris has completed a giant underground basin to store wastewater during extreme precipitation events and prevent discharges into the Seine River.

  • The newly built asset is part of the City’s plans to make the Seine swimmable by 2025, accelerated by the need to host swimming events during the 2024 Olympics.

In a monumental stride towards environmental sustainability, Paris has inaugurated the Austerlitz water storage basin, a subterranean marvel – dubbed Paris’ “underground cathedral” – designed to improve the water quality of the Seine River. This giant reservoir, boasting a capacity of 50,000 cubic metres, will store wastewater and stormwater during heavy precipitation, with the objective of rendering the Seine swimmable for the 2024 Olympic and Paralympic Games, and for the public by summer 2025.

The completion of this ambitious project, spearheaded by the City of Paris over three years, marks a significant milestone in the city's commitment to river restoration and ecological stewardship.

Situated beneath the Austerlitz train station, the vast storage basin will prevent the discharge of untreated water into the Seine and its tributary, the Marne, during torrential rains. With the capacity of 20 Olympic swimming pools, the basin comprises a cylindrical structure 50 metres in diameter and 30 metres deep, supported by 16 pillars descending up to 80 metres underground. This engineering marvel is poised to revolutionize Paris' wastewater management by storing excess water and gradually channelling it into the sewer network for treatment.

Credit: Ville de Paris

Funded collaboratively by the City of Paris, the Seine Normandy Water Agency, and SIAAP, the wastewater utility for the Paris metropolitan area, the project underscores the collective effort to safeguard the Seine's ecological integrity. Over the past three decades, wastewater discharge into the river has been reduced tenfold through automation and network monitoring, with the Austerlitz project poised to further decrease discharge days by a factor of ten.

Paris’ combined sewer network was built in the mid-nineteenth century, with 44 overflow outfalls into the river in case of excess capacity. Last summer, swimming events during the Open Water Swimming World Cup in Paris had to be cancelled due to poor water quality in the Seine after heavy rainfall.

The newly built underground water holding structure will contribute to bringing Parisians closer to their river, starting in the summer of 2025, with the opening of three swimming sites in the city. The Austerlitz storage basin is a major asset of the Swimming Plan (Plan Baignade), with an investment of 1.4 billion euros to make the Seine and Marne rivers swimmable for the general public by 2025, according to a statement from the Île-de-France Region. Marc Guillaume, Prefect of the Île-de-France region and Prefect of Paris, said the Games have been a tremendous accelerator to enable the allocate human and financial resources: "In thirty years, we will remember the opening ceremony, but we will also remember that it is thanks to the Olympics that we can swim in the Seine", he said.

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