The Combined Sewage Storage Tunnel means a cleaner Ottawa River

November 20th, 2020 saw the official opening event for a brand new piece of Ottawa City infrastructure: the Combined Sewage Storage Tunnel. What does that have to do with the river? The new tunnel is designed to help protect the river from the harmful effects of Combined Sewer Overflows (CSOs).

In 2006, a prolonged sewage release from a City of Ottawa sewer to the Ottawa River occurred due to a malfunctioning sanitary sewage control structure. Two years later, that release and the broader issue of Combined Sewer Overflows into the Ottawa River came to public attention, and spurred the revision of existing plans and upgrades to the City’s sewer system.

Even before 2006, Ottawa Riverkeeper had been actively communicating about the importance of controlling and reducing the frequency of CSOs. As plans for CSO management were revised by the City, we committed to understand how they were proceeding, and what the implications would be for each option.

Throughout the years, we have built a relationship based on collaboration with the City of Ottawa to inform you, our community, on when CSOs occur so that you can protect yourself and your loved ones in the water. When plans started coming together, we championed the inclusion of the Combined Sewage Storage Tunnel in the Ottawa River Action Plan.  

Now, we are so happy to finally see this project become a reality! This ambitious infrastructure project was only possible due to cooperation between Municipal, Provincial and Federal governments. We have been calling for stakeholders at multiple levels to come together for the sake of our river for many years and we are proud to see it come about at such a critical time.


Collaboration is key to bringing about positive change. That’s why at our recent Annual Public Meeting, we were honoured to welcome Hasnaa Zaknoun, Wastewater Collection Manager at the City of Ottawa, to give a presentation to our community on how the CSST will help keep the river clean. Additional City staff involved in the CSST project were also kind enough to engage in a Q & A with our attendees as part of the meeting. 

Following the meeting, Ottawa’s Mayor Jim Watson shared some words about why the CSST strengthens the Canadian capital’s position as a world leader in environmental protection. Check out his video below:

We would like to thank the Mayor for his statement and we look forward to continuing to collaborate with municipal governments in both Ontario and Quebec to protect the entire Ottawa River watershed.

What does the CSST actually do? How will it keep sewage out of the river?

When it rains heavily, combined sewers are filled with stormwater, and these aging sewer systems are forced to dump a mixture of water and untreated sewage into the river. Many old sewer systems in Ottawa have been separated over the years, changing them into modern systems that don’t have this issue, and all newly built sewers are already separated. However, some systems are too difficult to replace. Instead, the new CSST project will act as a buffer, storing the excess water and sewage generated during a heavy rain event so that it can be treated after the rain subsides.

Will CSOs vanish entirely? The storage tunnel has a capacity of 43,000m3, which is a lot, but not infinite. It is still possible that a massive rainfall could overwhelm the system, as could large sequential rainfalls that do not give the system enough time to be drained and treated. However, the chances of CSOs occurring from the City of Ottawa have been dramatically reduced. This is a huge step forward for the protection of the river!

There are still other municipalities along the river that may generate CSOs due to combined sewer infrastructure. We will continue to work to advise municipalities and the public on the risks of CSOs as our mission to protect the river carries on. If you want to get involved in your own municipality, check out our Toolkit for Community Action. This guide is built upon our success in working with the City of Ottawa to help make the CSST project happen!

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