What I distilled. Paris like most older cities has a combined storm and sewage system. The problem always is when there is too much rain the treatment plant ca

What I distilled. Paris like most older cities has a combined storm and sewage system. The problem always is when there is too much rain the treatment plant can't handle the volume so it get dumped into the river untreated. To fix that they're building a huge holding reservoir so they can capture sewage + storm water and feed it slowly into the waste treatment plant instead of dumping it into the Seine. Added. I've read elsewhere that the Seine is often safe to swim in already. Hope is this will turn often safe to to always or usually safe.