How Paris fixed Seine's ‘poop problem', made river fit for swimmers

Indian Express

05-07-2025

For over a century, the River Seine in Paris had a 'poop problem' — untreated sewage frequently made it to the river, making it a breeding ground for E coli and thus unfit for swimmers. But not any more.
Parisians on Saturday (July 5) rushed to take a dip in the Seine, with authorities opening up the river to public swimming for the very first time since 1923. This came after an extensive clean-up operation spurred by the Paris Olympics last year, in which the river was used as a venue for certain swimming events.
Here's what ailed the Seine, and how authorities cleaned the river up.
The Seine has always been a romantic river. It has been the subject of many paintings, sepia-tinted photographs, and ballads. And of course, people love to fall in love on the banks of the Seine.
Yet, for more than a century, the Seine was notorious for being filthy. Very filthy. So much so that it was too toxic for most fishes, and swimming in the river was banned since 1923.
This was courtesy Paris' aged sewage system in which rainwater and wastewater used to flow in the same pipes. During heavy spells of rain, these pipes reached their capacity and start overflowing, which led to untreated sewage being directly discharged into the Seine, instead of first going to a treatment plant.
This contamination sent the concentration of E coli in the river skyrocketing. A bacteria found in the faecal waste of humans and other animals, some strains of E Coli can cause severe illness, including gastrointestinal and urinary tract infections (UTIs).
It is the presence of this bacteria, in many times the safe limit, that made the Seine unsafe for humans and aquatic life alike.
To fix the Seine, Paris spent well over a billion dollars to improve its infrastructure: this was a key promise made by organisers after the city won the bid for the 2024 Olympics in 2016.
At the heart of Paris' solution for fixing the Seine has been the creation of a giant underground rainwater storage tank in southeastern Paris, capable of holding enough water to fill up 20 Olympic-size swimming pools. This tank is meant to hold most runoff water during heavy rains, and prevent overwhelming the city's sewege systems.
Apart from this, the city made major investments to connect tens of thousands of homes to the sewer system, and upgrade water treatment facilities.
While heavy rains nonetheless affected a few Olympic training sessions and the men's triathlon event, competitions eventually went ahead in the Seine last year. Now the river has been opened to the public.
Authorities say that daily water quality tests will be conducted during the swimming season at designated swimming areas. At the moment, there are three sites along the banks of the Seine in Paris which can accomodate more than 1,000 swimmers daily in the summer.
This is an updated version of an explainer first published last year.