Even if the water companies were state owned, dealing with the pollution would still cost money and there are a bazillion other demands on the state's finances which are more pressing than keeping rivers clean, such as providing healthcare. This isn't a hypothetical. Scotland has a state-owned water company, and it'd be impossible to write an article like this about theirs not because they're doing better - all indications seem to be that that they're not - but because it relies on information from monitoring equipment that just doesn't exist there. It was pretty recently added in England too, over the last few years, so it's clear that the Environment Agency does actually care about this... not that you'd even know that had happened from the UK news coverage, especially the Guardian's.

Other places in Europe seem to be even worse. For example, I've seen people from Ireland shocked that thsi would happen and preaching the benefits of their state-owned water and their fight against privatisation and water billing. There are places there which literally have no sewage treatment plants still. As in, their sewage goes straight from their houses into the rivers and the sea untreated, not because of a fault or undersized treatment plants or heavy rain ( all of which they have problems with too) but because the sewage system is literally designed to do that 24/7. The government was meant to have fixed this decades ago according to EU rules but has kept dragging its feet on building sewage plants. And they're probably not unique - they were literally the first country I checked, and the second, Spain, had the same issue as well.