If a city outgrows its wastewater treatment infrastructure, there are overflow issues that need to be looked at. Many communities have overflow systems that deposit raw sewage straight into local waterways.
So why is overflow such an issue? Because many of structures were built 40-70 years ago. Cities can grow a lot in that amount of time. Take Seattle as an example. In 1960, about 550,000 people lived there. In 2016, about 3.7 million people live there. The system was built for the smaller population.