Trying to showcase a cleaner, greener Paris, the French government has undertaken an ambitious project to make the Seine river swimmable in time for the Olympic open water swimming events and triathlon on July 26th. However, with costs ballooning, engineering challenges mounting, and a viral Tiktok trend threatening to undermine the effort, the question remains - will the French be able to un-stain the Seine?
The Problem
The Seine has long been plagued by pollution. Multiple cholera outbreaks, typhoid fever, and even malaria were traced back to the river’s waters.
Today, the seine sees over 360,000 m^3 per day of raw sewage discharged into the river from Paris' aging combined sewer system. This is the equivalent of 120 Olympic swimming pools!
Because the sewers are combined, the sewage is a mixture of wastewater and stormwater, which means that during heavy rains, the system gets overwhelmed, resulting in untreated waste flowing directly into the river.
The Project
To combat this, the city has embarked on a €1.4 billion project. One cornerstone of the project is the Austerlitz basin - a cavernous structure 50 meters below ground capable of holding 50,000 cubic meters of water - the equivalent of 20 Olympic swimming pools. During storms, wastewater will be diverted to the basin instead of flowing into the Seine.
A useful way to think about the basin is in the context of flow equalization. During periods of high flow, an equalization basin can store wastewater, and keep discharge rates steady. This protects all the downstream infrastructure at the treatment plant from being overloaded. Then, the stored wastewater can be discharged smoothly over time, keeping the reduction in flow after a rain event to a smooth taper. The ability to smooth out the peak flow allows us to build less capacity, and keep the capacity we have built running more consistently. For context, here is a infographic from the city of Springfield, MO showcasing some of the most common steps in a typical WWTP.
Another key component to Paris’ megaproject is the €500 million upgrade to the Seine Aval wastewater treatment plant, to which the water captured by the Austerlitz basin will be pumped. This plant processes about 80% of the wastewater from Paris. The planned improvements are slated to boost treatment capacity by 30%. But the plant has a checkered environmental record, often discharging excess flows to the river, and locals worry it won't be able to handle the increased load. As one resident put it, "they're spending all this money, but at the end of the day, it's still going into the same old antiquated system."
The plant’s spotty record may be caused in part by the combined sewer system. Beyond the problem of large volumetric fluctuations, combined systems cause a lot of variance in waste concentration. The microbes we use to process our waste (see step 4 in the infographic) need a steady supply of food, and are sensitive to the concentrative fluctuations that can result from large volume wet weather events in combined sewer systems. Too little food, (which might be caused by too much rainwater diluting the nutrients in the wastewater) and the microbes will starve, meaning they won’t be able to handle the next peak in concentration, and waste organics will their way through the process.
Paris' propensity for summer thunderstorms adds another layer of uncertainty to the project. Officials acknowledge that even with the upgrades, significant rainfall could still lead to overflows. As one engineer candidly stated, "We're doing everything humanly possible, but we can't control the skies." Sounds like a job for Augustus Doricko of Rainmaker.
The PR Nightmare
Hoping to instill confidence, Paris Mayor Anne Hidalgo and President Emmanuel Macron have vowed to take a celebratory swim in the Seine this Sunday (June 23rd). But their PR stunt may be torpedoed by a tongue-in-cheek Tiktok trend called "Je Chie Dans La Seine Le 23 Juin" (I Shit in the Seine on June 23). Hundreds of thousands have watched videos of people pledging to take a "crap for freedom" to protest what they see as political greenwashing and government over-spending. If even a tiny fraction follow through, it could make for quite a messy situation.
In the end, Paris may pull off the engineering feat and avoid a watery fiasco come late July. But with so many variables in play it’s a sink or swim situation. As the Olympics draw nearer, all eyes will be on the Seine - and Parisians will be holding their noses.
Thank you for reading.
-Connor, OfAllTrades.
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