Toronto’s wastewater treatment plants operate according to strict provincial and federal regulatory requirements and all wastewater is disinfected even during a bypass.

During heavy rain and snow melt, the increased volume of rainwater and sewage can be fully treated for a period of time. However, if heavy rain continues, the volume of rainwater and sewage reaching a plant may be more than can be treated and processed in a short period of time. In this case, some of the wastewater may be diverted around the biological process (secondary treatment process) to protect the plant. This diversion around one process is called a “bypass”. The bypassed wastewater still goes through screening, grit removal, primary treatment, phosphorous removal and full disinfection to ensure the treated water always meets strict federal and provincial regulations.

Bypasses are a necessity in a combined sewers system because they help to:

  • Prevent rainwater and sewage in the combined sewers from backing up and potentially causing basement and/or surface flooding;
  • prevent flooding of the plant and protect the plant’s ability to continue treating wastewater;
  • protect the core biological plant process (secondary treatment) from damage – too much flow can “wash out” the microscopic organisms needed for secondary treatment, affecting the plant’s ability to function for several days or weeks; and,
  • prevent the wastewater treatment plant from flooding, which can cause significant damage to mechanical and electrical equipment.

A bypass only occurs when the rain causes enough of an increase in rainwater and sewage to overwhelm the capacity of the plant’s secondary treatment process. This is entirely dependent on the intensity and location of the rainfall:


  • If a rainstorm occurs in an area of the city that has mostly separate storm sewers, there may not be any impact on a wastewater treatment plant.
  • If the rain occurs in an area that has combined sewers (one pipe that carries both sewage and rain to a wastewater treatment plant), there is a greater chance of a bypass during heavy rain.

Frequency and duration

  • Ashbridges Bay Treatment Plant has experienced about 10 events per year over the last three years.
  • Humber Treatment Plant has experienced about 16 events in the same period.
  • The average duration of bypasses for Ashbridges Bay Treatment Plant is about seven hours.
  • The average duration of bypasses for Humber Treatment Plant is about five hours.
  • The amount of rain and sewage that is being bypassed starts to drop soon after the rainstorm ends.

View the most recent Bypass Reports.


  • Wastewater treatment is regulated primarily by the Ministry of the Environment, Conservation and Parks (MECP) which is responsible for compliance, reporting requirements and performing inspections.
  • The federal government also has testing and reporting requirements.
  • There are many other agencies that regulate wastewater treatment plant activities from a non-environmental perspective, including the Ministry of Labour, the Technical Standards and Safety Authority (TSSA), and the Electrical Safety Authority (ESA).
  • Toronto’s wastewater treatment plants continue to operate according to provincial and federal regulatory requirements during a wastewater treatment plant bypass.

Bypass reporting

  • The City notifies the MECP’s Spills Action Centre when a bypass starts and ends using the MECP’s Bypass & Overflow Reporting portal or by telephone.
  • Within 10 business days, the City adds more detailed information to the portal such as duration and volume.
  • The City also submits a monthly (Humber) or quarterly (Ashbridges Bay) summary of bypass events to the MECP, and posts these bypass events monthly on the City’s website.
  • Bypass information is included in the plant’s annual report, submitted to the MECP and posted online by March 31 of the following year.

The City of Toronto advises people to stay out of the water during and for 48 hours after rainfall. This is not because of wastewater treatment plant bypasses. It is because there can be a high level of pollutants in the water after a rainstorm. Learn more about surface water pollution.