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Combined sewer overflows (CSOs)

What is a combined sewer overflow (CSO)?

The City of Ottawa has three types of sewers: wastewater, stormwater, and combined. 

  • Wastewater sewers: Collect wastewater from homes, businesses and industries, and transport the wastewater through a network of sewers, pumping stations and forcemains to main or trunk sewers, and direct it to the Robert O. Pickard Environmental Centre for treatment. 
  • Stormwater sewers: Carry rainfall and other surface water run-off directly to the nearest creek, stream or river, generally without treatment.
  • Combined sewers: Collect and transport both sanitary wastewater and stormwater runoff in a single pipe. This type of collection system was typically installed from 1880 to 1960 and is still in operation in older areas of most Canadian cities.

During dry weather, wastewater is collected and carried from a combined sewer to the City's wastewater treatment plant. During heavy rainstorms or snowmelts, the combined sewer may not be able to handle the high volume of stormwater runoff entering the system. Most of the wastewater is transported to treatment plants via the wastewater pipe, but to prevent flooding and sewer backups, some of the rain and wastewater mixture is diverted as overflow into the river.

The practice of discharging overflows during the normal operation of combined sewer systems is accepted by the Ministry of the Environment, Conservation and Parks (MECP). In Ottawa, these events are monitored and reported to the MECP and downstream users such as water purification plants and special interest groups.

What is the City doing to prevent overflows?

The City of Ottawa has implemented the following plan to mitigate the impact of combined sewer overflows into the Ottawa River:

  • The Combined Sewage Storage Tunnel (CSST) is the most significant infrastructure project that the City has undertaken to mitigate the impact of CSOs into the Ottawa River.
  • Real time controls to ensure maximum capture of overflows before they occur
    • Real time controls permit the remote activation and control of overflow equipment combined with continuous monitoring of pipe flow data to maximize the capture and treatment of wastewater flow from combined sewer systems
  • Ongoing sewer separation projects
  • Review of the effectiveness of the current sewer separation program, in comparison to other projects
  • Development and implementation of a Wet Weather Infrastructure Management Plan
  • Installation of devices to remove floatable material from wastewater and prevent it from reaching the river in the event an overflow occurs

The City has been working for many years to separate remaining combined sewers. The value of the work completed to date is estimated at $750 million. With the current funding framework, it is expected the planned separation work will be completed in approximately 25 years.

The City has made the following improvement to existing systems:

  • Water level, flow monitoring and alarm devices installed in major combined sewer overflows and monitored daily to alert staff if an overflow occurs
  • Strengthened protocols to provide prompt notification to the MECP Spills Action Centre and Council in the event of an overflow
  • Construction upgrades are in process for the overflow sites that were the major contributors to the total volume of sewage released during combined sewer overflows
  • Improved regulator inspection and maintenance are improved and now scheduled monthly, weekly after each rain and within 24 hours of receiving an alarm

Sewer overflow sites

There are 13 locations in Ottawa where sewer overflows may enter the river:

Map of Ottawa combined sewer overflow outfall locations
  1. Ladouceur CSO Outfall
  2. Merton CSO Outfall
  3. Booth-Wellington Regulator Overflow
  4. CSST-Kent Overflow
  5. Rideau Canal Regulator Overflow
  6. Keefer Regulator Overflow
  7. Dufferin CSO Outfall
  8. Hemlock CSO Outfall
  9. Sandridge Storage Tank Overflow
  10. Alvin Heights CSO Outfall and RCAF CSO Outfall
  11. RCAF West CSO Outfall
  12. Springhurst CSO Outfall
  13. Clegg CSO Outfall

Combined sewer overflows are generally diverted to flow into the Ottawa River.

The three combined sewer overflow sites that contribute the largest volumes of CSOs are:

  • Rideau Regulator Overflow
  • Booth Regulator Overflow
  • Keefer Regulator Overflow

Overflow activity (January 1, 2020 to date)

PLEASE NOTE: The volume indicated in the chart below (CSO) denotes the volume of combined overflow mixture which is comprised of storm water and wastewater. The majority of overflows occur during heavy rain and snowfall/melting periods. The chart below indicates the date, cause, and volume of each overflow event.

  • CSO: Combined Sewer Overflow
  • SSO: Sanitary Sewer Overflow
Date Event Cause Volume (m3)
01-11-2020 CSO Rain 58,520 m3
03-03-2020 CSO Rain 870 m³
03-20-2020 CSO Rain 23,990 m3
03-29-2020 CSO Rain 26,300 m³
05-15-2020 CSO Rain 4,940 m³
05-29-2020 CSO Rain 11,450 m³
05-30-2020 CSO Rain 80 m³
06-28-2020 CSO Rain 20 m³
07-17-2020 CSO Rain 2,110 m³
07-19-2020 CSO Rain 30,830 m³
07-29-2020 CSO Rain 150 m³
07-30-2020 CSO Rain 1,200 m³
08-02-2020 CSO Rain 44,980 m³
08-04-2020 CSO Rain 12,540 m³
08-11-2020 CSO Rain 590 m³
08-16-2020 CSO Rain 39,550 m³
08-23-2020 CSO Rain 11,920 m³
08-29-2020 CSO Rain 34,900 m³
09-02-2020 CSO Rain 3,780 m³
09-13-2020 CSO Rain 200 m³
09-29-2020 CSO Rain 680 m³
10-07-2020 CSO Rain 6,440 m³
10-21-2020 CSO Rain 3,680 m³
12-24-2020 CSO Rain 24,300 m³
03-26-2021 CSO Rain 55,730 m³
08-29-2021 CSO Rain 680 m³
09-06-2021 CSO Rain 10 m³
09-15-2021 CSO Rain 1660 m³
10-16-2021 CSO Rain 61,870 m³
11-16-2021 SSO Faulty Instrumentation 310 m³
05-16-2022 CSO Rain 12,450 m³
05-21-2022 CSO Rain 910 m³
05-27-2022 CSO Rain 20  m³
06-01-2022 SSO Rain 17.68 m³
07-24-2022 CSO Rain 17.44 m³
08-09-2022 CSO Rain 31,370 m³
04-05-2023 CSO Rain 28,230 m³
04-30-2023  CSO Rain 52,280 m³
06-24-2023 CSO Rain 34.74 m³
06-26-2023 CSO Rain 67,310 m³
07-01-2023 CSO Rain 190.3 m³
07-13-2023 CSO Rain 10 m3
07-18-2023 CSO Rain 130 m3
07-28-2023 CSO Rain 160 m3
08-03-2023 CSO Rain 20 m3
08-10-2023 CSO Rain 315,840 m3
09-18-2023 CSO Rain 20 m3

Historical overflow activity (CSOs only)

CSO activity during the Annual Reporting Period (April 15 to November 15).

Year Number of Events Volume Precipitation (mm)
2019 26 810,000 m3 635 mm
2018 58 536,000 m3 524 mm
2017 79 1,611,000 m3 923 mm
2016 85 413,000 m3 495 mm
2015 80 180,000 m3 332 mm
2014 101 552,000 m3 640 mm
2013 179 213,000 m3 581 mm
2012 142 237,000 m3 523 mm
2011 161 230,000 m3 533 mm
2010 322 673,000 m3 646 mm
2009 384 851,000 m3 649 mm
2008 325 877,000 m3 600 mm
2007 253 730,000 m3 555 mm
2006 271 1,090,000 m3 796 mm