Paris has pledged to make the Seine swimmable by the 2024 Summer Olympics, investing in a $1.6 billion stormwater holding tank to curb sewage pollution

This is a nice article. The core feature being worked on is tucked away in the middle:

a big part of the plan is to stop the majority of polluted stormwater from entering the river via an underground tank. During periods of high rainfall, Paris’s combined sewer system leads to 44 storm flows ready to dump rainwater and raw sewage into the Seine if the sewer system is overwhelmed. To this end a huge storage tank is being built underneath Square Marie-Curie on the left bank. It can hold 12 million gallons of stormwater (think 30 Olympic-sized swimming pools), which will be stored until the rain ends before being sent back into the sewer system for treatment.

These mingled storm / sewer systems are pretty common, especially in older systems, and they can't cope with modern rainfall patterns, nor with some city building techniques where large areas are paved with impermeable surfaces. Increased rain has nowhere to go, and so runs off into the sewer and, as levels rise, the storm drain. But because there's so much water this mingled sewage and rainwater overflows the storm drains, and they discharge onto beaches or into rivers.

It's a real problem, and it's really expensive to try to fix it, so it's great to see Paris trying something.

Here's Steve Mold talking about it (and I've not eaten shellfish since watching this) https://www.youtube.com/watch?v=VGJLKhsLx18