Toronto has a comprehensive, multi-year, multi-billion dollar infrastructure plan to reduce the impact of combined sewers on water quality, combined sewer overflows and plant bypasses.

  • 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.
  • 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, 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 sewer overloads, 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.

Today we know CSO discharges contain harmful bacteria, pathogens, heavy metals, oils, and pesticides, as well as nutrients that can increase algae growth and degrade the health of the Toronto’s waterways. Learn what the City is doing to help eliminate combined sewer overflow.

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