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Comment by lou1306 | Hacker News Reader

Since all other replies are a bit too concise, I'll try and dive deeper (ha ha).

1) Naturally, rains wash out street grime and small pieces of rubbish which may easily bypass the sewage system altogether and end up in the river.

2) Cities have systems in place to treat wastewaters before releasing them into the rivers. However, these systems need to store yet-untreated water in reservoirs, and heavy rain may fill these up. Then, the only solution (if you don't want sewage to blast out sewer holes) is to release the water without treating it. In fact, one of the main upgrades made by Paris for the recent Olympics was a massive, 30-meter-deep new basin [1] built exactly to mitigate that risk.

[1]: https://www.reuters.com/world/europe/giant-reservoir-ready-s...

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So naive question.. but how can they ensure all the water upstream is clean?

I guess in Copenhagen its water is coming from the Baltic so maybe it's less of a concern (short of container ships having issues near the city). But the Seine river flows through many communities before it reaches Paris. Many opportunities for actors to illegally dump things in the river and for all sorts of nasty runoff agricultural and otherwise to make it in periodically along the whole stretch.

Continuous water testing at the city limits also sounds impractical

> Continuous water testing at the city limits also sounds impractical

Why? Surely a small team of a few people could have that as their job, and it would be completely worth it, to help keep all the people that want to swim safe.

And like, aside from the manual testing of the water that this team would do once or twice per day probably, they could have a few different sensors to catch some of the most likely kinds of pollution.

On top of that add hefty fines and maybe even a year or two of jail time for anyone caught dumping pollutants into the river, and you have a good disincentive against polluting the water.

And have a strong sense of pride of nature in the country so that people will naturally want to be mindful of the environment we live in. Promote this kind of thinking in schools and in public service announcements (billboards, ads, etc) from the government.

It's not technically impossible to continuously monitor a river, but as the neighboring comments shows, it's kinda of cutting edge stuff and basically nobody is doing it. It's also very much not cheap. Pinpointing where upstream someone dumped stuff in the middle of the night is virtually impossible

Yeah, it is helped by the geography. The channel in Copenhagen isn't a river, it is directly open to the North+Baltic seas, so we don't have this runoff type challenge of other European cities.

It seems to be normal practice. Two weeks ago Russia poisoned a river flowing across the border into Ukraine. The standard monitoring tools and infrastructure seemed sufficient to monitor the situation.

https://www.msn.com/en-us/news/world/toxic-substances-flowin...

Advances in remote sensing are making continuous water quality monitoring much easier. At our company we use a combination of satellite imagery and in situ optical sensors to monitor rivers and reservoirs. As well as a general water quality index, we can estimate concentrations of harmful nutrients, chlorophyll-a, phycocyanin, suspended matter, dissolved organic carbon to name a few.

A further advantage of satellite remote sensing for water quality is that it can be done by a neutral third party, without permission or access to the land, which can help bring a bit of objectivity to the politicized narrative around water quality we are seeing at the moment.

https://www.gybe.eco/

multiple daily satellite images of the whole length of a river... I know you can purchase these kinds of satellite products, but as I understand it - it's incredibly expensive. Doing that all year round seems cost prohibitive

Considerably more affordable than the alternatives:

- multiparameter sondes (submersible sensors): More expensive, maintenance heavy, only measure at one point.

- Manual grab-samples: Again more expensive when you factor in labour, unreliable, single point.

Remote sensing has its disadvantages though; cloud cover can be problematic for example.

> only solution

Other than "don't put sewage in the rain collector".

Copenhagen’s sewage system was built in the 1850s and it was indeed considered to build a two-pronged solution. There was much resistance against this because it would eliminate a whole industry of “night men” (poop collectors), as well as people drying and trading it as fertilizer. So they build a single-pronged solution that was initially only allowed to be used for rain water.

https://ing.dk/artikel/ingenioerer-maatte-kaempe-haardt-faa-...

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