A bypass occurs when the volume of rainwater and sewage reaching a plant exceeds the volume that can reasonably be processed in a short period of time. This only occurs due to an extreme rain storm.

During bypass, all preliminary and primary treatment occurs, as well as disinfection, to ensure treated water always meets strict federal and provincial regulations.

Bypasses are a necessity in a combined sewer system. Bypasses:

  • Prevent rainwater and sewage in the combined sewers from backing up and potentially causing basement and/or surface flooding.
  • Help protect the core biological plant process from damage and degradation.
    • 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.
  • Help 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 flow to overwhelm the capacity of the plant’s core biological process, also known as secondary treatment. 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 a combined sewers (one pipe that carries both sewage and rain to a wastewater treatment plant), there is a greater likelihood of a bypass during extreme rain events.

Frequency and duration

  • Ashbridges Bay Wastewater Treatment Plant has experienced approximately 12 events per year over the last 4 years.
  • Humber Wastewater Treatment Plant has experienced approximately 16 events in the same period.
  • The average duration of the bypass for both plants is approximately 5 hours.
  • The amount of flow that is being bypassed starts to drop soon after the rain event 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 per 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.
  • Within 10 business days, the City also submits a written report to the MECP, which contains more detailed information on the event, such as duration and volume.
  • The City also submits a monthly summary of bypass events to the MECP, which is posted 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 for 48 hours after rainfall. This is not because of wastewater treatment plant bypasses, but because there can be a high level of pollutants after a rainstorm. Learn more about surface water pollution.