Officials have been going after homes upstream of Paris and houseboats on the Seine that were emptying their sewage and wastewater directly into the river. An Olympic law adopted in 2018 gave moored boats two years to hook up to Paris’ sewage network. Sewage treatment plants on the Seine and its tributary, the Marne, are also being improved.

As part of an ambitious plan to open up the Seine to public swimmers by 2025 — after being illegal for 100 years because of dangerously high pollution levels — France has spent 1.4 billion euros ($1.5 billion) on altering and upgrading the city’s antiquated sewage system, which now includes a 13-million-gallon overflow tank called the Austerlitz Basin.