Combined sewer overflow (CSO) annual results
- 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)
This bar chart shows the volume of rainfall and number of events greater than 5 millimetres (mm), or events big enough to cause an overflow.
Annual 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 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 an intense rain event in 2015, 4% of the CSO was wastewater and 96% was rainfall runoff. The bar chart below illustrates the total treated wastewater and CSO volume breakdown using the 4% value for the wastewater component.
Annual Total Treated Wastewater, CSO Rainfall Runoff and Wastewater Volume Breakdown Estimate
One mega litre 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.
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).1999
National 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 - 2002
A recommendation from the Clean Environment Commission (CEC) was to instrument the CSO outfalls.2003
Planning 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.2008
In 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 2014.2013
Estimates of wastewater flows from CSOs have been validated based on instruments installed at 39 CSO Outfall locations.Since 2013
The CSO MP Preliminary Proposal was submitted to the Province on December 18, 2015 recommending an 85% CSO volume capture long term control plan.2015