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Combined Sewer Overflows

What are combined sewers?

Combined sewer systems are sewers that are designed to collect both land drainage (rainwater and snowmelt) and wastewater (sewage from homes and businesses) in the same pipe. Most of the time, combined sewer systems transport all of the land drainage and wastewater to a sewage treatment plant, where it is treated and then discharged to the river. There are approximately 1,037 kilometres of combined sewers in the city, which is 31% of the total sewer system. Typically, they were built before the 1960s and serve older areas of the city.

Illustration of operation of combined sewer system

What is a combined sewer overflow?

During periods of heavy rainfall or snowmelt, the additional volume in combined sewers system can exceed the capacity of the sewer system. During these occasions, combined sewer systems are designed to overflow and discharge the excess directly to the river without reaching the sewage treatment plant. These overflows, called combined sewer overflows (CSOs), contain not only land drainage, but also wastewater and debris.

Why do we have CSOs?

Historically combined sewers discharged directly to rivers. In the 1930s diversion weirs were put in place to divert flows to the North End sewage treatment plant. The outfalls were retained to offer a level of protection against large wet weather events that could overload the system and result in basement flooding.

Combined sewers carry all of the wastewater flow to the sewage treatment plants during dry weather conditions. During wet weather events, the wastewater diversion weirs cannot handle all of the land drainage that enters the system and it flows over the weirs directly to the rivers to protect basements from flooding.

Why do we still have CSOs?

During periods of heavy rainfall or snowmelt, there is the potential for basement flooding due to high volumes of land drainage entering the sewer system. Combined sewer overflows protect basements from flooding by relieving excess flow to the river.

How often do combined sewers overflow in Winnipeg?

Combined sewer overflows occur, on average, 22 times during the year at each outfall. To check the current probability of an overflow, visit our sewer overflow information system.

Are combined sewers and CSOs unique to Winnipeg?

No. Many North American cities, including others in Manitoba, have similar sewer systems. Hundreds of communities built combined sewers because they were a cost-effective way to provide sewer service and improve drainage. Combined sewers in our city date from 1880. They were the first sewer infrastructure.

How many CSO outfalls are in the city?

There are 77 combined sewer outfalls or outlets to the river system.

Do CSOs affect the rivers' colour?

No. The large amounts of suspended soils give the Winnipeg rivers their natural murky brown appearance.

Do I need to use special precautions when using the river?

Yes. The Chief Provincial Public Health Officer with the Winnipeg Regional Health Authority advises:

  • Never drink river water, whether overflows are occurring or not.
  • Do not swim in the river system at any time because of fast currents, cloudy water, and slippery, muddy banks.
  • Wash your hands if they come in contact with river water, particularly before you touch food.
Sometimes I see garbage floating in the rivers. Is this from a combined sewer overflow?

Yes, there can be debris floating in the rivers when an overflow occurs. However, most of the time, litter from streets is carried into sewers (separate and combined) during a rainstorm. Residents can help reduce floating debris by keeping their yards clean and not putting garbage down their household drain or toilets.

I see foam on the river. Is this from CSOs? Is it harmful?

Foam on the river:

  • is not harmful to the environment,
  • occurs naturally,
  • is caused by materials such as pollen and algae,
  • is sometimes brown in colour, and
  • is similar to the foam you often see when waves crash on a beach.
Is it safe to eat fish caught in the rivers?

Yes, as long as they are cooked thoroughly.

What are the impacts of CSOs on Lake Winnipeg?

High levels of nutrients like Total Nitrogen and Total Phosphorous can cause excessive algae and weed growth, which lowers water quality, harms fish and other aquatic life as a result of lower oxygen levels, and affects the appearance and recreational enjoyment of our rivers and lakes.

Nitrogen and Phosphorus loading to Lake Winnipeg chart

As you can see from the graph above, Winnipeg CSOs deposit very little Total Nitrogen (0.1%) and Total Phosphorous (0.3%) to Lake Winnipeg, leaving a minimal impact.

What is the City doing to reduce the number of CSOs?

We have been improving the combined sewer system operation, and have been studying options to further reduce CSOs, including:

  • completed a CSO Management Strategy in 2002,
  • implemented a Monitoring Program in 2009 to identify and quantify CSO events,
  • proactively eliminated wet weather flows from the combined sewer district through infrastructure upgrades, and
  • invested more than $290 million since 1977 improving the city's sewer system.

In addition, the City supports the Yellow Fish Road Program. More information regarding the program can be found at FortWhyte Alive's website.

Are CSOs regulated?
Yes. The City is regulated by the Province of Manitoba through Environment Act Licence No. 3042.

What can I do to prevent the impact of CSOs?
Even when CSO solutions are implemented, land drainage from our city will continue to flow to streams and rivers during wet weather events. This stormwater picks up pollutants as it flows across the land, whether it comes from streets, open areas, or rooftops. You can help to reduce this pollution by:

  • disconnecting downspouts from the sewer system,
  • using rain barrels,
  • disposing of household chemicals and used oil properly, and not pouring them down the drain or in storm sewers on the street,
  • picking up your pet waste,
  • fixing fluid leaks from your vehicles, and
  • applying lawn chemicals in a way that minimizes runoff to storm sewers
Last updated: August 14, 2017

Combined Sewer Overflow Animation

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