Combined sewage storage tunnels can help protect the environment and keep basements dry
January 06, 2020
A higher frequency of intense storms leads to wet basements and sewage flowing into waterways. Here’s how a CSST could help.
Have you ever worked on something for so long that it became a part of your life? A project or concept can live with you throughout your career, permeating your thoughts as you encounter fresh challenges and consider new solutions.
That’s how I feel when I look at the Ottawa River. I’ve been working on initiatives and projects to help protect the river since 1992. It’s been with me for years, via various infrastructure initiatives. The river is an integral part of the city and Canada’s history. As an Ottawan since 1990, I've fished on the river, boated on it, swam in it, sat on its beach, and even enjoyed a sunset or two on its shores.
I’m especially proud of the Combined Sewage Storage Tunnel project in Ottawa, which I’ve been working on since 2005. The team and I recently celebrated a major milestone: the completion of excavation of the tunnels needed for the system. Of course, tunneling is only a fraction of the work on such a complex system. Construction continues across the city with the goal to make the system operational in 2020. I’m glad to be part of this legacy project, which aims to reduce combined sewer overflows (CSOs) to the river. It’s one of the most important projects of the Ottawa River Action Plan, the City’s long-term strategy to protect the health of the river.
If you’re curious about these acronyms—what exactly is a CSO, and how can a combined sewage storage tunnel (CSST) help?—I’m here to explain.
Author Gerald Bauer has enjoyed the Ottawa River as a resident for many years.
Combined sewer overflows (CSOs)
In simple terms, a CSO is an overflow of combined sewage to a water body. Many cities have combined sewer systems, which collect surface runoff from rainwater and snowmelt, as well as sewage and wastewater from homes and businesses.
These systems usually send that flow to sewage treatment plants, but during intense storms, water levels can surpass the capability of the sewers or treatment plants. In these situations, the system becomes overwhelmed and the flow needs somewhere to go. So, it discharges directly to local streams and rivers to relieve the system (an “overflow”).
One of the results of climate change is a higher frequency of intense storms. These days, “100-year” storm events are occurring much more frequently than once in a century and are contributing to flooding. This coupled with seasonal melt can lead to large floods, like those experienced by Ottawa in 2017 and 2019.
To help prevent sewage from flowing into waterways—and to reduce the number of flooded basements experienced by home and business owners—a CSST could be the answer to your CSO issues.
The north-south tunnel of the CSST project in Ottawa. A CSST can act as a relief for the existing sewer system.
Combined sewage storage tunnels (CSSTs)
Here’s how CSSTs work. I’ll use our Ottawa project as an example. The CSST consists of two interconnected tunnels, which are a total length of just over 6 kilometers. When it’s in operation, the CSST will allow the City of Ottawa to hold more than 43,000m³ of combined sewage during major rainfalls or wet-weather events—the equivalent capacity of approximately 18 Olympic-sized pools—which will reduce the number and volume of CSOs. Once the wet weather event has waned, this water will then be treated and returned safely to the Ottawa River.
These days, “100-year” storm events are occurring much more frequently than once in a century.
A CSST can help protect rivers and waterways, but it can also have some other major benefits. Let’s take the example of basement flooding. As mentioned above, when wet-weather events overwhelm sewer systems, the flow needs somewhere to go. If the system has no outfall to a waterway, it starts to back up into its existing connections, which include domestic sewers. Low-lying areas along a sewer system, where rain and snowmelt tend to pool, often see the backup. In many cases, municipalities implement wastewater storage tanks to help combat basement flooding problems. If designed accordingly, however, a CSST can provide multiple benefits in addition to reducing CSOs—including reducing the risk of basement flooding.
A CSST also can contribute to improving operational flexibility and redundancy of existing sewers by providing relief to older systems. This is largely due to inspection limitations on older infrastructure. Without a backup, these sewers must be bypassed during inspection, which becomes expensive and disruptive. A CSST can provide an additional route or backup for some existing major collector sewers. With a CSST in operation, the ability to inspect and repair older infrastructure connected to the system also becomes more feasible.
The Ottawa River from one of Ottawa’s CSST project sites, behind the Supreme Court of Canada. A CSST can help protect rivers and lakes from CSOs.
CSST in action
It’s obvious there are problems that arise from CSOs and there are benefits of combined sewage storage tunnels. A CSST can act as a relief for the sewer system, protect rivers and lakes from CSOs, reduce basement flooding, and assist regions who may be at risk of facing noncompliance issues.
Combined sewage storage tunnels have been implemented in several North American cities, including Portland, Boston, Milwaukee, Cleveland, and Washington, DC. I find it fascinating to learn about these and other projects like those that I have been working on.
After spending so many years looking at reducing overflows to the Ottawa River, I can’t wait to see Ottawa’s CSST in action. I hold this project close to my heart, and I’m proud of what we’ve accomplished so far.