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Basement flooding

Overflows and Bypasses

What are sewer overflows and plant bypasses?

A sewer system overflow occurs when sanitary sewers are overwhelmed by storm water inflow and infiltration during wet weather events. Sewage mixed with storm water discharges directly into local waterways through an outlet to assist in the prevention of basement flooding.

A wastewater treatment plant bypass occurs when flow exceeds capacity of the treatment plant. The raw sewage and storm water discharge directly into local waterways through an outlet to assist in preventing basement flooding. The wastewater treatment plant treats as much sewage as possible prior to any plant bypass by directing a portion of the flow to receive primary treatment but it bypasses secondary treatment.

What is the issue?

Sewer overflows in London discharge untreated sewage directly into the Thames River. Plant bypasses also directly discharge untreated sewage and secondary bypasses discharge partially treated sewage to prevent basement flooding.

Weeping tile connections, roof drainage connections and infiltration discharging into a homeowner’s sanitary sewer line during a major rain or thaw event can cause the main sewer line to overflow.


What can be done?

To reduce overflow and bypass issues reduction of weeping tile drains to the sanitary sewer is required.

Weeping tile connections to the sanitary sewer occur in about 48% of the homes in London.  The City has undertaken the Sherwood Forest Weeping Tile Disconnect Project – a pilot program designed to reduce the risk of basement flooding by disconnecting foundation drains from the sanitary sewer system and installing backwater valves for homes on Blanchard Crescent.

Work is planned to provide primary treatment (partial treatment) to the peak flows during snow melt and rain events.  This does not solve the problem, but reduces the impact of rain getting into our sanitary sewers.

How can you help?

If you are concerned with the problem of basement flooding and raw or partially treated sewage being released into the Thames River, you can be a part of the solution!

If your weeping tile is connected to the sanitary sewer inside your home, you can have your weeping tiles disconnected from the sanitary sewer and reconnected to the City’s storm water system.

The City has the Basement Flooding Grant Program in place to assist homeowners with the cost.


We need your help London!

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  • Why are there overflows and bypasses?

    The City of London has had sewers built since the 1850s.  The sewers were installed to prevent epidemics that occurred in London Ontario and killed numerous people.  During the next 50 years the sewers ran directly into the Thames River and the epidemics ceased.  In the 1890s, a massive project was started that required the Ontario Government to pass a law to allow the City to borrow sufficient money to finance the building of trunk sewers and a "sewage farm" out in the country, which is the present location of the Greenway Pollution Control Centre.

    Sewage treatment efficiencies have improved over the past 100 years but along the way decisions were made to protect homeowner’s basements and allow overflows to occur.  Some of these decisions included:

    • in the 1950s to allow relief sewers to be constructed that would allow sanitary sewers to overflow to a sewer that discharges to the Thames;
    • in the 1960s to allow weeping tile around homes to go to the sanitary sewer instead of the storm sewer.  Overwhelmed storm sewers allowed flow back into the weeping tile around the house with the water seeping through the concrete floors.
    • in the 1980s the practice of allowing new homes discharge weeping tile (ground water) into the sanitary sewer and made pumping of sumps mandatory for weeping tile.  Although this was required, the practice in the building sector has circumvented the requirements in some homes by drilling holes in the homes sanitary sewer or leaving caps off to allow ground water drainage into the sanitary sewer to persist.
  • What is a combined sewer?

    In some cases, both sanitary and storm water drain to a common pipeline which conveys the mixed water to a nearby wastewater treatment plant.  During wet weather events, these combined sewers can reach high flows which sometimes lead to an overflow to a receiving stream. This type of sewer is no longer constructed, but is occasionally found in old sewer systems.

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