Skip ribbon commands
Skip to main content

What is a combined sewer system

A combined sewer is a system that simultaneously collects surface runoff and sewage water in a shared collection of pipes and tunnels. Construction of combined sewers in Edmonton was an acceptable practice prior to 1960 as it was more economical than building two separate sewer systems. Construction of the combined sewer collection system was discontinued in 1960 due to environmental concerns. Edmonton's combined sewer system consists of 826 km of pipes, 19 combined sewer overflows, and the North Highland Interceptor Sewer.


How a combined sewer system functions

During severe wet weather, when stormwater and sanitary flows in the combined sewer exceeds the capacity of the system, Combined Sewer Overflow (CSO) events occur. In dry weather, all flows collected by the combined system are delivered to the Gold Bar Wastewater Treatment Plant for treatment. Although the volume of wastewater that bypasses treatment through the CSOs is about 2% of the total volume of all Edmonton discharges, EPCOR considers the environmental impact caused by this to be unacceptable.

The combined sewer system can be a cause of odour problems due to large pipes and slow moving water in some neighbourhoods.  The sewer odour escapes from catch basins (or storm drains) and manholes connected to the combined system. EPCOR has a corrosion and odour reduction strategy in place to reduce the impacts of sewer corrosion and odours, and to improve the quality of life for Edmontonians.


Continued improvement of Edmonton's combined sewer system

For the past several years, EPCOR has focused on maximizing environmental protection. To date, 43 storage tanks with a capacity of 25 million litres have been built to reduce the amount of overflow entering the river. In addition, the corrosion and odour reduction strategy has been developed to further control and reduce CSOs.


Questions or concerns with the combined sewer overflow system?

If you have a concern about sewer odours in your neighbourhood you can contact our 24 hour emergency line at (780) 412-4500 to report the problem.

Does combined sewer overflow (CSO) affect Edmonton's drinking water?
​No, the intakes for Edmonton's E.L. Smith and Rossdale Water Treatment Plants are both upstream from all CSO locations.
Will the river's colour change after the implementation of the long term CSO control plan?
​No, the river is sometimes brown because of naturally-occurring silt.
Is there much of a health risk in doing river sports now?
​No. The risk of infection from coming in contact with the river water is very low. You would have to ingest a fairly large quantity before being subject to illness.
Why can't storm and sanitary sewers be separated?
​The cost for complete sewer separation is about $2.5 billion. We will slowly separate some of the combined sewers when we're doing other neighbourhood renewal work.
Why is it taking so long to develop and implement a solution?
There is not one easy solution. Many options have been reviewed, each with different benefits and costs.