A combined sewer system uses the same infrastructure to transport sewage and rainwater. So when a big storm fills up that system, the water comes back up and floods. That’s called a combined sewer overflow (CSO), and they can cause significant environmental damage.
Ottawa’s goal was ambitious. They wanted to drastically reduce CSOs into the Ottawa River. We started with an environmental assessment, and found through a triple bottom line analysis that the preferred solution was deep tunnel storage. Basically, there would be reservoir tunnels inside the combined sewer system to capture overflow water and hold it until the system could handle the excess.
We are currently working with the City on the final design. The tunnels are 4.4 km and 1.6 km long respectively and 3 m in diameter each, adding 43,600 m3 of storage. The tunnels range from 20 m to 35 m deep with six major drop shafts 2 m to 3 m in diameter.
A previous project of ours upgraded the City’s flow-regulating structures and implemented an automated real-time control system. That project reduced annual CSO volume by 60-70%, and with the addition of the combined sewage storage tunnels, Ottawa will virtually eliminate combined sewage overflow during a typical year.