Construction of combined sewers in Edmonton was an acceptable practice before 1960s because it was more economical than building two separate sewer systems. This system consists of 900 km of pipes, 19 combined sewer overflows, and the North Highland Interceptor Sewer. The construction of the system discontinued by about 1960 due to the environmental concerns.
In dry weather, all flows collected by the system are delivered to the for treatment. To prevent basement and street flooding during severe wet weather, excess water in the combined system is allowed to overflow into the river via 19 Combined Sewer Overflows (CSOs). The CSOs are located downstream from the City's water treatment plants. Although the volume of wastewater that bypasses treatment through the CSOs is about 2% of total volume of all City discharges, Drainage Services considers the environmental impact is unacceptable.
For the past several years, Drainage Services 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, connections between sanitary and storm sewer lines in older parts of the City are being eliminated and a strategy has recently been developed to further control and reduce CSOs.
A Combined Sewer Overflow Strategy has been implemented to reduce the amount of sewer water flowing into the river from CSOs, resulting in cleaner water and a healthier ecosystem.