Paris aims for a (safely) swimmable Seine by 2025

The dream of public swimming in the Seine by 2025 is alive and well, with three sites due to open in Paris – at Port de Grenelle, bras Marie and near Bercy.

The city’s mairie launched a €1.6billion clean-up plan – dubbed the plan baignade – in 2016, following decades of promises from politicians.

It has been working alongside the Syndicat interdépartemental pour l’assainissement de l’agglomération parisienne (SIAAP), the organisation responsible for both the Seine and Marne rivers’ overall cleanliness.

SIAAP opened a 9km-long tunnel in July to transport dirty water to a treatment facility in Valenton (Val-de-Marne), located upstream from the three chosen swimming sites. SIAAP was created in 1970, some 30 years after Paris opened its first – and at the time only – water waste treatment facility in Saint-Germain-en-Laye.

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Seine clean-up

It has since built a comprehensive roadmap to turn rivers from effectively open sewers into waterways that can be bathed in several days per month.

Its facilities treat 2.5million square metres of waste water per day, produced by the nine million people living in the Ile-de-France region.

The waters of Ile-de-France’s two main rivers are funnelled into six clean-up factories with five being for pre-treatment. There are also eight storage basins and four reservoir tunnels spread over a 472km network. In addition, 26 floating weirs capture larger items of rubbish.

The water is collected in Paris’s sewage systems and funnelled into pipes buried 100m below sea level to be transported into the treatment facilities. 

Water purification process

They go through five processes: dégrillage, dessablage and dégraissage, décantation, traitement biologique and clarification. 

Dégrillage sees the water pass through smaller and smaller grills to capture larger solids. 

Dessablage and dégraissage remove sand and fat, while décantation ensures remaining fine particles (called matières en suspension) are captured when they reach the bottom.

The biological treatment sees air being blown into the water to increase bacteria that consume invisible pollutants such as carbon, azote and phosphates. The clarification process removes the bacterias before the water is released into the Seine and Marne.

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Raw sewage discharge

Studies have shown a huge improvement in the quality of river water over the years. From just three varieties of fish in 1970, the Seine and Marne now boast 33 and 36 different types respectively.

Likewise, levels of ammonium and faecal residue have decreased tenfold and oxygen dissolved in the water has nearly doubled.

Dealing with heavy rainfall remains the final challenge for SIAAP, when water treatment facilities are permitted to discharge raw sewage into rivers if there is a risk that the system may be overwhelmed.

The postponement of the Olympic triathlon races was for this very reason. Indeed, several athletes claimed to have gastroenteritis symptoms after the race, casting further doubts on the Seine’s swimmability.