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What are sewer overflows?

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 Town of Midland are due to combined sewers – sewers that collect both sewage and stormwater runoff.  

Overflows of sewage ultimately make it to the environment. When the overflow reaches Georgian Bay, 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. 


  • A sewage or sewer overflow is defined as a discharge of diluted raw sewage from the wastewater collection system.  These can happen at locations in the underground pipe network (the collection system) as well at lift/pumping stations. 

  • A sewer bypass is defined as a discharge of partially treated sewage from a wastewater treatment plant. Typically, this is caused by a bypass or short circuiting of the secondary treatment process and results in a discharge of blended primary treated and diluted raw sewage. ​​

Why not just stop them?

Wastewater system operators do not have control over when and where overflows happen. These events are largely weather driven (fast snow melts and or rain storms). The ability for a sewer to overflow, and the structures that allow it to happen, have been inherited through the evolution of sewage collection. In the early years it was standard practice to allow stormwater to enter the Collection System to flush and clean the system. Today standard practise is to perform controlled cleaning of the system with specialized equipment. 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. As the ability to bypass secondary treatment prevents damage to the wastewater treatment plants bacteriological processes and equipment.

What are the solutions?

The Town of Midland has invested in; an Infiltration and Inflow study (I&I) to better understand where problems exist and, a Master Wastewater Plan (2019). The information created and summarized by these studies will ultimately lead to source control strategies, a financially viable and a preferred path towards sewage overflow reduction. 

There are a number of technical solutions available to reduce and eliminate overflows;

  • Increase the capacity of sewage lift stations to handle larger flows.
  • Storage tanks that hold mixed sewage until the lift stations and treatment plant is ready to accept the excess.
  • Replace combined sewers with separate pipes for stormwater and sanitary waste.
  • Improve leak detection to reduce the infiltration.
  • Reduce the number of roof leader and sump pump connections.
  • Explore green infrastructure to slow down and efficiently reduce sudden charges of runoff.
  • Reduce sanitary sewage with the promotion of water conservation programs and outreach. 

​Our commit​ment...

In addition to the solutions identified on this page, the Town of Midland will also report overflow occurr​ences below, including the times they occur and the volume of waste involved. 

Our goal is to notify the public, both through this page and on social media, within 12 hours of the occurrence.​

 Overflow Reports

​​​Overflow Location Map

Overflow Location Map​(Click to view larger image)