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​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.

Designed functions

In dry weather, all flows collected by the system are delivered to the Gold Bar Wastewater Treatment Plant 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 EPCOR'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.

Improvement

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.

Why were combined sewers built?
​It was less expensive to build one set of sewer pipes instead of two. Prior to the 1950's, there was less concern about the impact of the overflows on the environment.
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.
Are combined sewers the ones that create odours in neighbourhoods?
​They can be the cause of odour problems due to large pipes and slow moving water.
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 just one easy solution. There are many options that have been reviewed, each with different benefits and costs. It has been a slow process to reach a final approach largely because of the complexity of the engineering alternatives, the comprehensive review of all technical data and the time needed to consult and inform the public and stakeholders.