Most of the time, that's true.

Some of Montreal's infrastructure is so old, though, that it uses combined sewers, where stormwater and 'sanitary' (raw sewage) inputs use the same sewer line. When there's a storm, that can max out the capacity of the intended local treatment plant, and some will 'surcharge' directly into the river.

The sewage is diluted by stormwater, but it's still raw sewage.