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What are sewer overflows?


View this video to learn about sewer overflows, the history of the problem and what we're doing about it.

A sewer overflow is when sewage from the sanitary and combined collection systems overflows to the environment. It is often a result of the sewers being too full - typically because of excessive groundwater and stormwater entering the sewer system during heavy rains or rapid snowmelt. 

Some of this extra water is collected on purpose, by combined sewers. Some of it comes from leaky pipes and maintenance holes and illegally-connected sump pumps, foundation drains and roof downspouts. 

The majority of sewer overflows that occur in the city of Kingston are due to inherited combined sewers – sewers that collect both sewage and stormwater runoff.  These are referred to more specifically as "combined sewer overflows" or CSOs. 

Overflows of sewage ultimately make it to the environment. When the overflow reaches the lake or river, it contributes pollutants to the environment. This is not only damaging to the environment itself, but also to the health and safety of the flora and fauna that use or live in the water. Typically what overflows from the system is combined sewage, or in other words, sewage that has been highly diluted by stormwater. 

Why not just stop them?

Wastewater system operators do not have control over when and where overflows happen. The ability for a sewer to overflow, and the structures that allow it to happen, are inherited through the evolution of sewage collection. This is the case in almost all cities in Canada and abroad.

The ability to overflow is purposely built into the system, and occurs to prevent sewage from flooding the basements of homes and properties. Learn more from our section Frequently Asked Questions.

What are the solutions?

There are a number of technical solutions available to reduce and eliminate overflows. While Utilities Kingston uses these approaches where it makes sense, a recent update to Pollution Prevention and Control Plans has directed us to focus on source control strategies as the preferred path towards sewage overflow reduction. 

Here are the general categories to address sewer overflows over the long term:

  • Source control strategies focus on implementing measures to keep the storm- and groundwater out of the sanitary sewer in the first place by controlling at the source. Options include separating sewers, reducing ‘extraneous flow’ and eliminating illegally-connected sources (both private and public). In terms of effectiveness, it's been estimated during Pollution Prevention and Control Planning that if all combined sewers in the city were replaced with separated sewers, the majority of overflows would be eliminated.

    The Water and Wastewater Master Plan details projects to the year 2036 that have been identified as required to reduce sewer overflow volume and continue to work towards Utilities Kingston’s goal of “virtual elimination” of combined sewer overflows. View the recommended sewer overflow reduction projects in Table 1-5.
  • Conveyance strategies focus on accommodating the extra flows that occur during wet weather, by providing more flow conveyance and treatment capacity. In other words, focus on collecting and treating sewage to return natural resource quality water to the lake. Generally, to collect the extra sewage that results from heavy rainfall or snowmelt, all the pipes, pump stations and treatment facilities from the source to the treatment plant need to be oversized. Often, there is a lot of infrastructure along the path, and upsizing to have enough capacity for extreme conditions that happen very infrequently may not be cost effective. 
  • End-of-pipe strategies focus on capturing overflows just before they discharge to the environment, temporarily storing the sewage and then discharging it back to the system when the event passes and capacity is available again. Typically, this means the construction of large storage tanks on the waterfront.

What is Utilities Kingston doing about it?

We are proud of the work we are doing in this area and demonstrating leadership in many ways. 

  • Construction of overflow storage tanks. The City of Kingston has nine storage tanks.  Six of these are smaller ‘passive’ tanks that service the local combined sewers in Sydenham Ward, and three are bigger ‘active’ tanks that serve the overflows on the trunk sewer system.  Once overflow is intercepted, it either flows back by gravity, or it is pumped back to the system to return to its rightful flow path to the wastewater treatment plant. These tanks were built in the 1990's and early 2000's.  
  • Sewer separation. Utilities Kingston, in cooperation with the City of Kingston, has been working hard to separate combined sewers. It is the preferred method for overflow reduction, in accordance with Pollution Prevention and Control Plans. Since 2008, we have separating sewers at a rate of about three to four per cent, per year, in coordination with full road reconstruction projects. In 2023, City Council has endorsed a 20-year separation plan to eliminate all combined sewers remaining in the City. Check out the following map to see the progress since 2000.
  • Additional capacity. In some cases, Utilities Kingston has been increasing pipe or pumping capacity to help decrease overflows.  For example, the two sewage forcemains from River Street Sewage Pump Station were recently twinned all the way across the Great Cataraqui River to the start of the Ravensview Trunk Sewer. This increases the amount the River Street Sewage Pump Station can pump to the Ravensview Wastewater Treatment Plant. 
  • Extraneous flow reduction. Utilities Kingston has undertaken extraneous flow reduction projects to reduce the impact of leaky joints, cracks and illegal storm connections to the sanitary sewer system. These programs are complete and focused on the North End Trunk Sewer service area and the Portsmouth area. 
  • Better overflow monitoring. Utilities Kingston has been monitoring overflows for quite some time, but in 2016, a project was completed to equip all remaining overflows in the collection system with new reliable and accurate monitoring equipment. Now, Utilities Kingston is able to identify and quantify overflows anywhere they are possible in the sewer network.  
  • Transparency and ownership. Utilities Kingston wants to lead the way in Canada with ownership of the problem in our municipality and full transparency on the issue and its consequences. While Utilities Kingston has maintained an overflow log for over a decade, recent monitoring improvements are allowing for real-time information to be provided to the public. Before you swim, fish or boat at Kingston's waterfront, check our sewer overflow map.

Check our page on sewer overflow reduction to learn about other ways we are actively reducing the problem.

Construction projects in Kingston that contributed to reducing sewer overflows

Many constructions projects have been undertaken in the city to help reduce the occurrence of sewer overflows.

Where can I find out more?

Being open and transparent about this problem is important. Our web resources are intended to help improve understanding and to answer your questions.  Below is a summary of available resources from Utilities Kingston. If you don't find what you're looking for, contact customer service, Monday to Friday from 8 AM to 5 PM by calling 613-546-0000.

Looking for other sources of information? Try the following: