You don't need much of a failure. Many cities all you need is a really rainy day and it overflows into general ocean run off[0]. I'm pretty sure parts of Vancouver still just send sewage directly into the ocean. All of Victoria did for over a hundred and fifty years until during Covid they finished a treatment plant finally[1].

[0] - example - https://www.cbc.ca/news/canada/british-columbia/sewage-overf...

[1] -https://www.cbc.ca/news/canada/british-columbia/victoria-crd...