Ashbridges Bay Treatment Plant Outfall

Building Canada’s largest wastewater outfall

City of Toronto | Canada | 2015-2025

Will be the largest wastewater outfall in Canada and one of the largest in North America.

Received several industry awards from the Tunnelling Association of Canada and Bentley Systems.

Will significantly improve the water quality along Toronto’s shoreline and within Lake Ontario. 


  • Ashbridges Bay Treatment Plant is one of the oldest wastewater treatment plants in Canada and the largest of Toronto’s four wastewater treatment plants. 
  • Currently, the plant discharges treated effluent into the lake through an existing outfall constructed in 1947, which is nearing the end of its service life.
  • A new outfall is planned for construction to meet peak design flows under highest recorded lake water level conditions while achieving regulatory standards.
  • The City needed a trusted engineering consultant to lead detailed design and construction management for the largest subaqueous tunnel to be built in Toronto.


  • Facilitated onshore and offshore geotechnical, geophysical and hydrogeological investigations, as well as assimilative capacity and dispersion modelling studies, including permits and approvals.
  • Designed the outfall to achieve a 100-year service life, with outfall components selected to minimize energy losses and allow the outfall to operate by gravity from the plant.
  • Served as Construction Manager to manage changes, quality, claims, schedule, and risks while leading tunnel, shaft and riser design.
  • One of the largest outfalls constructed in North America, which will significantly improve water quality along Toronto shorelines and contribute to overall environmental positive change.


  • On completion, the outfall is anticipated to expand Toronto’s wastewater treatment capacity and significantly improve the city’s shoreline, beaches, and Lake Ontario’s overall water quality.
  • Winner of multiple industry awards including the Tunnelling Association of Canada (TAC) 2023 Canadian Project of the Year (>$100M), the 2021 and 2023 TAC Canadian Innovation Initiative Award, and Bentley’s Year in Infrastructure 2020 Awards for digital delivery advancements.
  • Met stringent flow parameters and minimized head loss through the shaft and tunnel by completing a comprehensive analysis, using both Computational Fluid Dynamics and spreadsheet hydraulic models to develop the most economically feasible design.

"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

85 m deep x 14 m diameter on-shore shaft, which will direct effluent from the treatment plant effluent conduits to the tunnel.
3,5000 m long x 7 m diameter subaqueous tunnel, bored through bedrock approximately 50 m beneath the lake bed.
50 stainless steel riser pipes (1,000 mm diameter), each extending approximately 55 m high from the tunnel crown up through the lake bed, complete with FRP ports.
3923 mega liters per day (MLD) peak flow capacity will be available while operating by gravity flow for the next 100 years.

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