Wastewater Treatment Plant Bypasses

Why Bypasses Are Needed

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

• prevent stormwater and sewage in the combined sewers from backing up and potentially causing basement or surface flooding
• protect the plant’s ability to continue treating wastewater
• protect the secondary treatment process from damage – too much flow can “wash out” the microorganisms needed for secondary treatment, affecting the plant’s ability to function for several days or weeks
• prevent the plant from flooding, which can cause significant damage to mechanical and electrical equipment

Bypass Location, Frequency & Duration

A bypass only occurs when heavy rain and snow melt cause enough of an increase in stormwater and sewage to overwhelm the capacity of the plant’s secondary treatment process. This is entirely dependent on the intensity and location of the stormwater flow.

Location

If heavy rain or snow melt occurs in:

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

Frequency and duration

• Ashbridges Bay Treatment Plant has experienced about 19 events per year over the last three years.
• Humber Treatment Plant has experienced about 12 events per year in the same period.
• The average bypass duration is about seven hours.
• The amount of stormwater and sewage that is bypassed starts to drop soon after the rainstorm or snow melt ends.

View the most recent bypass reports.

Regulation & Bypass Reporting

Regulation

• 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.
• Many other agencies regulate wastewater treatment plant activities from a non-environmental perspective, including the Ministry of Labour, the Technical Standards and Safety Authority and the Electrical Safety Authority.
• 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 Wastewater Treatment Plant Bypass Reports page.
• Bypass information is included in the plant’s annual report, submitted to the MECP and posted online by March 31 of the following year.

Swimming During or After Rain

The City advises people to stay out of the water during and for 48 hours after rainfall, as pollutants from different sources can wash into rivers, creeks and Lake Ontario, affecting the water quality. This is not because of wastewater treatment plant bypasses.

Learn more about why you should not swim during or 48 hours after heavy rain.