Is this not normal state-of-affairs on most countries where rivers are part of the storm water system?
In New Zealand where I am, rivers are frequently closed for swimming after very heavy rain. We have separated sewage and storm water but in heavy rain, storm water ends up getting into the sewage system which can overflow. Short of making a sewage system capable of handling the maximum flow (very expensive and not possible for an established city), or preventing any water getting into sewage (tricky to do this in a foolproof manner, as sewage needs to be vented otherwise gases build up and needs to be accessible) not much can be done about it beyond ensuring the sewage system is not overflowing for other reasons such as blockages.
EU mandates storm retention tanks, i.e. a buffer, to minimize this problem.
For instance, Madrid has built a massive 400,000 m3 tank and the problem is now solved. Lots of other main cities are doing / have done the same.
Without those, in the event of heavy rain, there is not only raw sewage but also lots of contaminants from e.g. traffic that leak into rivers and kill everything there periodically.
Wow, I looked it up. I guesstimated it would have a cylinder diameter of 160m and depth of 20m and I was pretty close at ~22m x ~190m given (its an irregular polyhedron). Its an underground lake.
I guess its a question of scale, I was referring to small rivers that meander through city suburbs and are part of the stormwater network and may have a few overflows on occasion, rather than a city of Madrid which has a population about 60% of my entire country.
In New Zealand stormwater to my knowledge is not treated.
Not really, storm water can enter through drain-waste-vents, gully traps and manhole covers. It can also enter when stormwater is accidentally connected to sewage (it happens). You can seal off parts of a sewage system (eg. vacuum sewage between closed valves), but not all of it for practical and safety reasons.
In New Zealand where I am, rivers are frequently closed for swimming after very heavy rain. We have separated sewage and storm water but in heavy rain, storm water ends up getting into the sewage system which can overflow. Short of making a sewage system capable of handling the maximum flow (very expensive and not possible for an established city), or preventing any water getting into sewage (tricky to do this in a foolproof manner, as sewage needs to be vented otherwise gases build up and needs to be accessible) not much can be done about it beyond ensuring the sewage system is not overflowing for other reasons such as blockages.