Combined Sewer Overflows

In many cities stormwater and sanitary sewers are combined into a single pipe, the combined sewer. In Toronto, many areas that were built before the 1950’s are serviced by this type of sewer. During dry weather, combined sewers carry both stormwater and sanitary sewage to treatment plants. However, during wet weather, the volume of stormwater may exceed the treatment plant's capacity releasing untreated sewage into our rivers and lake. This is called a combined sewer overflow (CSO). CSOs occur about 50 to 60 times a year in Toronto but the city is working to disconnect combined sewers and create holding tanks to eliminate this problem.

CSOs contribute to high levels of E. coli bacteria in affected bodies of water. In Toronto, high levels of E.coli result in beach closures. Water at Toronto beaches is tested daily from June to Labour Day weekend. If the water is contaminated notices are posted at the beach advising people of the hazards of swimming. Visit TorontoBeach.ca for more information and up to date status of beach closures

Understanding the role of CSOs in river and lake pollution is a key step to understanding the challenges of reducing water pollution. The best way you can help to reduce CSOs is to reduce stormwater at the source. Check out 5 Things You Can Do to find out how.

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