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The City of Edmonton has 18 Combined Sewer Overflow (CSO) sites, which release excess flows to the North Saskatchewan River when runoff from rain or snow overloads the system. The CSO Strategy is a 16-year plan for improvements to the combined sewer system and the wastewater treatment plant.

The CSO strategy is a cost-effective way to reduce the amount of CSO to the North Saskatchewan River (NSR), compared to a total system-wide sewer separation. The strategy will also improve the health of the river by reducing the number of contaminants introduced to the river from runoff from rain or snow.

Program Scope

The CSO Strategy covers the following components:

  1. Early Action Control Plan
    This plan involves using the storage capacity available in the existing sewer system through the ‘real time control’ of moveable gates/dams to store the flows in the system during small events. The flows will be released slowly through the system to the wastewater treatment plant.
  2. Rat Creek River Crossing Tunnel
    This work involves the construction of a tunnel under the North Saskatchewan River to divert a significant amount of CSO volume to the Gold Bar Wastewater Treatment Plant. Construction of the tunnel is scheduled to be completed in 2013.
  3. Opportunistic Sewer Separation
    This work involves converting combined sewer systems into separate sanitary sewers and storm sewers on an opportunistic and cost-effective manner. To date, 3 out of the 9 identified sewer separations have been completed.
  4. CSO Structure Modifications
    This work involves in contruction of adjusting weir heights to retain more flow in the combined sewer system without causing flooding. Of the 18 CSO sites within the city, 14 are expected to have significant CSO control benefit when modified. Weir modification to 5 CSO sites has been completed, and the remaining 9 CSO sites are scheduled after 2012.


  • cost effective
  • reduces the risk of sewer back-ups
  • improves the health of the river

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