I learned about this in passing during an urban planning class during undergrad (it was a great elective, and I really think more people should learn that stuff).

Combined sewer overflow [1] is essentially business as usual in many municipalities. I happened to learn about the Detroit sewer system, but even King Co in the PNW has this issue [2]. I like to think areas like that would think ahead about this, given the high annual rainfall, but separate systems are quite the expense.

[1]: https://en.wikipedia.org/wiki/Combined_sewer#Combined_sewer_... [2]: http://www.kingcounty.gov/healthservices/health/ehs/toxic/cs...