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Ottawa Combined Sewage Storage Tunnel

Digging in against sewer overflows

  • 43.6K

    M3 of Storage

  • 6.6KM

    Of Tunnels

  • 3M


  • Ottawa, Ontario

    Ottawa, Ontario

Eliminating Ottawa sewage overflows and keeping the river clean

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, discharges as a means to relieve the system and reduce the risk of flooding. That’s called a combined sewer overflow (CSO), and systems are designed so that they happen infrequently to protect the environment while still protecting the community against flooding.

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.

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. This project is currently in construction and is expected to be operational in 2020.

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.

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