Stormwater Management

About the sewers on your street

There are two types of sewer designs in the city – separated and combined. 
  1. Separated sewers, found in the newer parts of the city, in which:
      • sanitary sewers carry sewage to wastewater treatment plants, and
      • storm sewers carry stormwater (rain and melted snow) directly to nearby waterways
  2. Combined sewers, prevalent in the older parts of the city, are designed take both sewage and stormwater directly to wastewater treatment plants in one pipe.  

Sanitary sewers 
Sanitary sewers transport wastewater released from a drain, toilet, sink or appliance such as a clothes or dishwasher. This wastewater from residences and businesses flows to treatment plants where it is cleaned before being released into Lake Ontario.

Storm sewers
Separated storm sewers capture rainwater or snowmelt. Much of this is captured through catch basins, the square grates on the side of the road. This water flows directly into nearby waterways including streams, rivers and Lake Ontario.

Combined sewers 
Some of the City's older areas (approximately 23%), where the sewer system was built as long as a century ago, have combined sewers in which there is only one pipe that carries both sewage and stormwater, a common sewer design used by many municipalities at the time. Most of the time, combined sewers carry all contents (rain, melted snow and sewage) to wastewater treatment plants for full treatment.

During periods of intense, heavy rainfall however, the volume of stormwater that enters these combined sewers may exceed the system's capacity and some of the combined sewer flow (a mix of stormwater and sewage) must be diverted (or overflow) untreated, directly into creeks, rivers and the Lake. Combined sewer overflows (CSOs) were designed to act as a relief valve, preventing overloading which could lead to flooding of properties, public spaces or even the sewage treatment plants. 

Watch this video for an example of what happens during a combined sewer overflow.

Eliminating the adverse effects of combined sewer overflow (CSOs):
Toronto has been a leader in identifying combined sewers as an issue and developing a comprehensive, multi-year, multi-billion dollar infrastructure plan to reduce the water quality impact of this once-common infrastructure design. Learn more:

History of Combined Sewer Overflows
Some of the City's older areas, where the sewer system was built as long as a century ago, have combined sewers in which there is only one pipe that carries both sewage and stormwater -- a common sewer design used by many municipalities at the time. Most of the time, combined sewers carry all contents (rain, melted snow and sewage) to wastewater treatment plants for full treatment. During periods of intense, heavy rainfall however, the volume of stormwater that enters these combined sewers may exceed the system's capacity and some of the combined sewer flow (a mix of stormwater and sewage) must be diverted (or overflow) untreated, directly into creeks, rivers and the Lake. Combined sewer overflows (CSOs) were designed to act as a relief valve, preventing overloading which could lead to flooding of properties, public spaces or even the sewage treatment plants. Many older North American municipalities with sewer systems built during or before the 1940s operate with some combined sewers.

However, today we know CSO discharges contain harmful bacteria, pathogens, heavy metals, oils, pesticides, as well as nutrients that cause excessive algae growth and degrade the health of the Toronto's waterways. 

Watch this video for an example of what happens during a combined sewer overflow.

 

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