Ashbridges Bay Treatment Plant Outfall

City of Toronto | Canada | 2015-2024

Anticipated to be the largest wastewater outfall in Canada.

Will improve the City of Toronto's shoreline and the water quality of Lake Ontario.


  • The Ashbridges Bay Treatment Plant Outfall project involves the design and construction of a new tunneled outfall that will convey treated effluent from the Ashbridges Bay Treatment Plant into Lake Ontario. It is one of the oldest wastewater treatment plants in Canada and is the largest of Toronto’s four sewage treatment plants.
  • The Ashbridges Bay Treatment Plant currently discharges treated effluent into the lake through an existing outfall constructed in 1947. The new outfall will be built to replace the existing outfall which has insufficient capacity, does not meet current regulatory standards, and is nearing the end of its service life.
  • Considered as a critical element of a program that will expand the City of Toronto’s wastewater treatment capacity, the new outfall project is anticipated to improve the city’s shoreline, beaches, and Lake Ontario's water quality.


  • The new outfall is designed to allow treated effluent to flow by gravity from the plant, through the effluent conduits, into the shaft along the tunnel, and dispersed into the lake via fifty vertical in-line risers and ports located at the last 1,000 meters of the tunnel to convey the peak design flows.
  • The available hydraulic head between the liquid level in the outfall shaft and the lake level during peak flow conditions is approximately 1 m and cannot be exceeded. Comprehensive analysis using both Computation Fluid Dynamics (CFD) and spreadsheet hydraulic models was used to achieve these stringent parameters and develop the most economically feasible design that minimizes the head loss through the shaft and tunnel.
  • The new outfall was designed to operate within the physical parameters of the study area while meeting the latest provincial guidance for water quality. A Receiving Water Impact Assessment (RWIA) was performed using the latest near-field and far-field modeling tools (CORMIX and MIKE3) and water quality data. The CORMIX model was also applied to optimize the diffuser by comparing predicted dilutions for varying number of ports/risers to the required dilution for total phosphorus concentrations.


  • Hatch, with other specialist firms, coordinated onshore and offshore geotechnical, geophysical, and hydrogeological investigations, performed archaeological investigations, assimilative capacity studies, fish and fish habitat studies, developed and selected outfall plan and profile alignments, and developed tunnel shaft and staging areas. Our team was directly responsible for tunnel, shaft and riser design, and the preparation of contract documents, including a geotechnical baseline report.
  • Other services provided by Hatch include project management, detailed design, construction management, QA/QC, scheduling control, constructability assessment, and cost estimating. The project was also peer-reviewed by leading subject matter experts across North America.
  • Hatch and other specialist firms acquired all permits and approvals including stakeholder consultation over a three-year period during the design phase. Major permits, approvals, and stakeholders include: Aquatic Habitat Toronto, Toronto Region & Conservation Authority, Ministry of the Environment, Conservation and Parks, Transport Canada, Canadian Coast Guard, and the Department of Fisheries & Oceans Canada.

"The outfall project was completed to the city's satisfaction, on time and under budget, which is commendable."

- | Project Manager , City of Toronto

"On time and under budget, what more can I say?"

- | Manager of ABTP Capital Projects, City of Toronto

Project numbers

The outfall is designed to convey up to 3,923 mega-liters per day (MLD) at its peak design flow
A 16 m diameter shaft approximately 85 m deep will direct treated effluent from the treatment plant into the tunnel
A 3,500 m long by 7 m diameter tunnel will be bored through bedrock approximately 50 m beneath the lakebed
Final tunnel lining will consist of approximately 13,600 precast concrete tunnel lining segments installed as 2,300 rings
Fifty 1,000 mm diameter stainless steel riser pipes will extend 50 m high from the tunnel crown up through the lakebed with FRP ports to disperse treated effluent into Lake Ontario

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