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What is a combined sewer system

The sewer system in Edmonton includes three types of sewers.

  1. The sanitary system moves wastewater from our homes and businesses through underground pipes to the Gold Bar Wastewater treatment plant.
  2. The stormwater sewer system moves the water on our streets from snowmelt and rain storms into the stormwater facilities and underground pipes that takes water to the North Saskatchewan River and other creeks in the city.
  3. A combined sewer system is a single pipe that moves both stormwater and snowmelt from city streets and wastewater from our homes to the wastewater treatment plant. The combined sewer system was developed prior to 1960 and at the time was considered to be more economical that building two separate sewer systems. The construction of combined sewers was discontinued due to environmental concerns and as a result of the population growth in Edmonton.

How a combined sewer system works

During small and medium rain storms or when snow melts in the winter and spring all water in the combined sewer system is treated at the wastewater treatment plant. When storm events are more intense and the system is over capacity the water in the pipes will flow directly into the North Saskatchewan River. These are known as Combined Sewer Overflow (CSO) events. When the combined sewer system overflows to the North Saskatchewan River the frequency and volume of the event is reported to both the provincial and federal government.

EPCOR is committed to reducing the impact of Combined Sewer Overflow events in Edmonton. Our Stormwater Integrated Resource Plan is focused on identifying opportunities to slow stormwater from entering the sewer system during intense rain events by storing it in green infrastructure.


Combined sewer and odours

The combined sewer system can contribute to odour problems in Edmonton neighborhoods. These pipes were designed to move large volumes of untreated wastewater during rain storms. When there are no storms, only wastewater is flowing in the pipes and sewer odour may escape from catch basins 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.


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 water intakes for Edmonton's E.L. Smith and Rossdale Water Treatment Plants are both upstream from all CSO locations.
Will upgrades to the combined sewer system change the color of the river?
​No, the river is sometimes brown because of naturally-occurring silt.
Is there a health risk in doing river sports?
​No. The risk of infection from coming in contact with the river water is very low.
Why can't storm and sanitary sewers be separated?
​The cost to separate the combined sewer system into a sanitary and stormwater system is very high. If there is an opportunity to separate the system during other sewer renewal work and where there are few other options to reduce flooding sewer separation may be undertaken. The combined sewer system is more than 25 meters below ground making it very difficult to fully separate. EPCOR is moving towards achieving the environmental equivalent to full sewer separation through investing on improvements that will provide the most benefit to the system.

For more information you can visit our Stormwater Integrated Resource Plan webpage for details on how EPCOR is planning to use green infrastructure to slow stormwater from entering the system and reduce CSO events.

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.