You don't need an extreme event for the sewage plants to malfunction ! They do that all the time ! If you look at the concentration per liter, it does seem ok, but if you multiply that concentration (any...NO3, oxygene demand, pesticide, medicine...) by the flow and for one day or one week, it is just monstruous !!!
A big part is collected in the sludge, what they do with it ? They dump it, rare are the farmers unconscious enough to spread the stuff on their land !
If one guy was catch dumping the same amount in the river, he will be sent in jail for a long time ! Our civilisation do that every day, completely unnoticed !