[2020] How can i deal with the Sewege Water?

Sounds like your river doesn't have enough flow, so the sewage output simply changes the flow of the river. This is sadly the case with some maps. :/

You could switch to using water towers to supply your city and ignore the pollution of your river. If you have Green Cities, then you can work towards the cleaner sewage treatment options. If you have Sunset Harbor, then you can use the inland sewage options.

If you don't have any of those DLCs, then there's always workshop assets, like this lovely collection. ;)

https://steamcommunity.com/sharedfiles/filedetails/?id=565043158