^ Toronto has something similar under construction right now. The first phase of a $3 billion project is under construction, costing $400 million. It builds a new, 10.5km tunnel in the city's east end.
https://www.cbc.ca/news/canada/toron...gram-1.5396886
When complete in about 10 years, the system will feature 22km of tunnels, 12 wet weather flow storage shafts along the tunnels, 12 connection points to the tunnels for storm water and combined sewer overflows, seven offline storage tanks and the technology necessary to regulate the flows inside the city's sewer system.
The project aims to completely eliminate sewer dumps into Lake Ontario in wet weather storm events.