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Inflow and Infiltration (I & I)

Inflow and infiltration is a term which refers to stormwater or ground water which enters a sanitary sewer.  Inflow refers to a direct connection, such as a catchbasin, whereas infiltration generally refers to an indirect connection such as weeping tile, cracks, etc.  Both inflow and infiltration increase the amount of flow in the sewers and also increases the risk of basement flooding.

Sources of (I & I)

Weeping Tile

Weeping tiles are a piping system that is located around the perimeter of a basement to collect rain water or ground water.  In newer homes, this water is directed to a sump pump system in the basement and the water is to be discharged to the storm sewer system or to the lawn, away from the house.  If your home was built between the 1920’s to 1980’s, it is very likely that your weeping tile is connected to the sanitary sewer.  Older homes have their weeping tiles connected to the sanitary sewer system.  The City would like to see as many of the sanitary connections eliminated to reduce the amount of storm water into the sanitary sewers and decrease the risk of basement flooding.  The City of London has a grant program to help pay for the costs of installing a sump pump provided the weeping tiles are no longer connected to the sanitary sewer system. 

Cross section diagram of a basement including the location of the weeping tile.

Combined Sewers

Combined sewers carry both stormwater, from road catchbasins, and sanitary flows at the same time.  The City of London has a long term goal of installing separate storm and sanitary sewers in front of every property in the City, eliminating all combined sewers.  Once the stormwater flow can be eliminated from the sanitary sewers there is less risk that the sanitary sewers will surcharge and need to overflow to another sewer system or back-up into basements.  Approximately 1% of the City’s sewers are combined sewers.

A dug up road during a reconstruction project.Cracks in Pipe, Joints, etc.

The City undertakes numerous construction projects each year to replace aged or failing infrastructure, and/or separate flows from one combined system into two independent systems.  Replacing these sewers decreases the I & I therefore reducing the amount of flow that needs to be treated at the sewage treatment plants as well as decreasing the risk of basement flooding.





A dug up road during a reconstruction project.Downspouts

Another source of I & I are downspouts which are connected directly to the sanitary sewer system.  During the 1960’s it was not uncommon for the downspouts from your roof to directly connect to the sanitary system but today it is not permitted.  Over the years the City has managed to eliminate the majority of these connections but occasionally a few are located.  If your downspouts are connected into the sanitary system they must be disconnected at the surface.  If your downspouts are directed into the ground but not connected into the sanitary system it is not as critical that they are disconnected.


Maintenance Hole CoverMaintenance hole covers

Maintenance hole covers have small holes in them that allow gases to continually escape the sewer system; however, this also allows rain water to enter the sewer system.  Although these holes are not a large source of I & I, it still increases the flow levels in the sewers.

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