I think the original comment is referencing cities that built early sewer systems where they do not have separate sanitary and stormwater lines. That is by far

I think the original comment is referencing cities that built early sewer systems where they do not have separate sanitary and stormwater lines. That is by far the more pressing problem: street drains (and often house gutters) dump directly into the same lines. These systems are usually in big cities and were built before sewage treatment existed. Building enough capacity to handle stormwater surges is often not practical: what was relatively clean rainwater is now contaminated with sewage.