Combined sewer overflow (CSO) annual results
- Background
- Results
- Timeline
- CSO Outfall Locations
Like many cities in North America, Winnipeg has a combined sewer system. Combined sewers are designed to collect both land drainage (rainwater and snowmelt) and wastewater (sewage from homes and businesses) in the same pipe, and transport both to the sewage treatment plant before being released into the river. In large wet weather events, however, the sewers get overloaded with excess rainwater, causing the contents of the combined sewers to overflow into the rivers.
In 2009, the City began investing ($12 million to date), in CSO outfall event monitoring and volume calculation tools to improve CSO estimations. This decision was the result of a recommendation from the Clean Environment Commission. Prior to the installation of these instruments, estimates were based solely on analytical data, utilizing several assumptions.
As a result of these instrumentation investments:
- estimated overflows from all CSO outfalls using a single hydraulic model (2016)
- provided estimates from 39 outfalls, which had been validated based on instrumentation installed at these sites (2015)
- marked the first year the City was able to provide estimates of combined sewer overflow volumes from a portion of the outfalls (2013)
The City has committed over $4 million from 2013 to 2021 for outfall and gate chamber rehabilitation projects. Additional instrumentation has been added to three outfalls since 2020, increasing the total monitored CSO outfalls to 45.
Prior to 2013, CSOs were estimated to occur 22 times on average from each outfall. In 2021, the average number of overflows was 12 (910/76 = 12) primarily based on the validated CSO results.
This bar chart shows the amount of rainfall and the number of events per year that are greater than or equal to five millimeters (mm).
Annual and Average 9-Year Rainfall for the Combined Sewer Area
This bar chart shows the number of CSOs per year along with the volume of overflow per year.
Annual and Average Nine-Year CSO Events and Volume 
CSO volumes vary from year to year depending on a large number of factors. The most critical are: the amount of rain that falls in the city, the intensity, frequency and duration of the rainfall, and where it falls in the city. These factors and river level can greatly influence the volume of sewage within an overflow.
The amount of wastewater that is treated at our sewage treatment plants will vary based on the intensity and frequency of wet weather events. Using an example from a historic intense rain event, four percent of the CSO was wastewater and 96 percent was rainfall runoff. The bar chart below illustrates the total treated wastewater and CSO volume breakdown using the four percent value for the wastewater component.
Annual and Average Nine-Year Total Treated Wastewater, CSO Rainfall Runoff and Wastewater Volume Breakdown Estimate
One megalitre or ML = 1,000,000 litres.
The City Winnipeg has been working to reduce the amount and impact of CSOs in Winnipeg for many years. This work is very expensive and lengthy. You can learn more about CSOs by visiting Winnipeg.ca/cso.
Data values on this page are representative of the 2013 Baseline Sewer Network.
Results
2021
2020
2019
2018
2017
2016
2015
2014
2013
Timeline
Federal Government required reporting on estimated sewerage lost from the sewer network under the Canadian Environmental Protection Act, 1999 as part of the National Pollutant Release Inventory (NPRI).
1999National Pollutant Release Inventory (NPRI) report estimation of percent sewerage volume lost from CSOs was based on purely analytical assumptions as no permanent CSO outfall instrumentation was in place.
2000 - 2002A recommendation from the Clean Environment Commission (CEC) was to instrument the CSO outfalls.
2003Planning began on the CSO Outfall Monitoring Program for installing instrumentation at CSO outfall sites. From 2009 to 2015, 39 outfall locations have been instrumented for CSO monitoring. The cost of the CSO monitoring equipment and hydraulic models necessary to measure and estimate CSO volumes is approximately $12 million.
2008In 2013, a hydraulic model of the instrumented CSO outfall districts was developed and calibrated based on the instrumentation. This allowed for model estimation to be validated based on observed data. This proof of concept work allowed for reliable validated estimation of CSO event and volume for the first time.
Federal Wastewater System Effluent Regulations (WSER) mandated the City to keep CSO records from 2013 and provide annual CSO Reporting due February 15 each subsequent year.
Province Environment Act Licence No. 3042 required annual reporting of CSO to commence in 2014.
2013Estimates of wastewater flows from CSOs have been validated based on instruments installed at 39 CSO Outfall locations.
Since 2013The Department submits CSO discharge data in two Federal reports and one Provincial report on annual CSO discharges: National Pollutant Release Inventory (NPRI) report, Wastewater System Effluent Regulations (WSER) report, and Environment Act Licence No. (EAL No. 3042)
Since 2014The CSO MP Preliminary Proposal was submitted to the Province on December 18, 2015 recommending an 85% CSO volume capture long term control plan.
2015On November 24, 2017 the Manitoba Government directed the City to complete the next phase of the CSO Master Plan by August 2019 with a recommended control of 85% capture in a representative year.
2017The CSO Master Plan was submitted to the Province on August 28, 2019, consisting of conceptual plans to meet the approved control of 85 percent capture in a representative year for 43 combined sewer districts by December 31, 2045.
On November 13, 2019 the Manitoba Government approved the City’s CSO Master Plan.
2019Estimates of wastewater flows from CSOs at instrumented CSO Outfall locations increased from 39 to 42.
2020Estimates of wastewater flows from CSOs at instrumented CSO Outfall locations increased from 42 to 45.
2021