West Edmonton Sanitary Sewer will reduce CSO’s by almost 50%

Aerial view ot Dawson Park working shaft

Many older North American cities have combined sewers which handle both wastewater and stormwater. As the city grows, sewers must handle larger wastewater flows, which, if combined with a large rainfall event, could cause the sewer to become overloaded. To accommodate large combined flows, many cities construct combined sewer overflows (CSO’s), to allow large flows to discharge into a watercourse rather than backup into the sewer system or overload the downstream wastewater treatment facility.

The City of Edmonton, like many North American cities, has a number of combined sewers and combined sewer overflows. To protect receiving waters, the City is reducing the magnitude and frequency of combined sewer overflows to the North Saskatchewan River and its tributaries.

The West Edmonton Sanitary Sewer project is part of the City’s short-term and long-term CSO Control Strategies which involve improvements to the City’s sewage conveyance and treatment systems to reduce the impacts of CSO’s. The new West Edmonton Sanitary Sewer system will collect flows from the west end of Edmonton, on the north side of the river, and convey wastewater to the Gold Bar Wastewater Treatment Plant on the south side of the river. The sewer system will be built in stages, and will include twelve tunnels and a total of approximately 9,000 metres of trunk sewer.

Ultimately, the tunnels will provide conveyance capacity. Initially, the tunnels will provide both conveyance and storage. During dry weather conditions, flow is conveyed directly to the wastewater treatment plant. During wet weather conditions, flows are stored. When dry weather conditions return, stored flows are directed to the treatment facility.

Associated Engineering is currently working on the 12th Stage, W12, of the West Edmonton Sanitary Sewer Strategy.

The overall scope of the W12 project is to provide conveyance of combined wastewater flows from the Rat Creek area under the North Saskatchewan River to the existing South Highlands Interceptor at McNally under wet weather flow conditions. The W12 project includes 1.5 kilometres of 2,500 millimetre tunnel as well as a syphon diversion structure, syphon inlet shaft, a real time flow control structure, and outlet shaft.

Project Engineer, Jason Lueke, advises, “The W12 inverted syphon will improve the average annual capture and treatment of wet weather flows from about 56% to 86% by volume, and decrease the number of annual CSO occurrences from about 89 to 46 events per year on average.”

City crews excavate undercut to facilitate tunnel boring machine installation

Construction of the north tunnel segment began in April 2006 and was completed in January 2007. City of Edmonton crews used a modified machine in the City’s inventory, a 3,454 millimetre open-faced tunnel boring machine (TBM), to excavate the north tunnel section. The tunnel primary liner is ribs and lagging.The secondary liner will achieve the required 2.5 metre finished tunnel size.

Construction of the south tunnel under the river between the construction access shaft and McNally Shaft began in June 2007. This tunnel segment will be constructed using the City’s newest tunnel boring machine, a 3,480 millimetre Lovat earth pressure balance machine. Tunneling of the south segment should be completed by 2008, with the commissioning of the W12 syphon by December 2009.

Associated Engineering completed hydraulic analysis and modelling, and design of civil, structural, ventilation, and odour control systems. We also assisted in obtaining permits and approvals and with public consultation.