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Inflow and Infiltration

Inflow and infiltration happens when water from the environment enters the sewage system for treatment.

York Region has two sewer systems:

  • Sewage system (sanitary sewer)
  • Storm sewer

The sewage system is designed to carry sewage to treatment plants for processing before returning it to the environment. The storm sewer prevents streets from flooding by draining and carrying excess stormwater (water from rainfall and snowmelt) to lakes and streams.

Image of broken ground with water seeping through to sewer


What is Inflow and Infiltration?

Inflow happens when stormwater enters the sewage system through sump pumps or downspouts still connected to the sewage system instead of the storm sewer.

Infiltration happens when water or groundwater from beneath the earth’s surface enters the sewage system through damage or defects such as holes and cracks in manholes and sewer pipes.

A diagram of the sources of inflow caused by a house that is not connected to the sewage system properly and infiltration caused by damage to the system, and a diagram of a house that is connected properly and free of damage for comparrisson.

View videos showing common sources of inflow and infiltration in:


Why is Inflow and Infiltration a Problem?

Inflow and infiltration are problems because they:

  • Reduce the capacity of the sewage system leaving less for existing residents and future growth
  • Make sewage treatment less efficient as the sewage is diluted by water
  • Increase the cost of water to residents because sewage treatment plants are required to treat a higher volume of flow
  • May cause sewage overflows or overwhelm treatment plants leading to
    • Health risks
    • Property and environmental damage

Videos are courtesy of Capital Regional District (CRD).


The Inflow and Infiltration Reduction Program

York Region considers reducing inflow and infiltration a high priority and has budgeted $100 million over a 20 year period.

Finding Sources of Inflow and Infiltration

Finding sources of I and I through smoke testing, dyeing and inspection

York Region monitors rainfall and the sewage system to find areas with higher than expected flow.

The Region further investigates to find the sources of inflow and infiltration through:

Smoke Testing

This process involves forcing a vegetable based, non-toxic, odourless smoke into sewer maintenance holes. Places where smoke escapes through sources not connected to the sewage system may indicate the presence of  inflow and/or infiltration.

Dye Testing

This process involves adding a non-toxic dye to an upstream freshwater source believed to be contributing to inflow and infiltration. Finding traces of the dye in the sewage system confirms the existence of a storm sewer connection sending water into the sewage system where it does not belong.

Visual Inspections

Trained personnel inspect the sewage system for possible sources of inflow and infiltration. They also use mobile video cameras to perform a closed circuit television inspection.

Reducing Inflow and Infiltration

Repairing sewer pipes

Once a source of inflow and infiltration has been found, York Region begins to fix the problem. 

Local municipalities in York Region conduct work regularly to inspect and repair inflow and infiltration. Please visit your local municipality’s website to learn about activities in your area.

Some of the techniques used to reduce inflow and infiltration are:

  • Repairing defects such as cracks or holes in pipes
  • Disconnecting known inflow sources, such as downspouts or storm sewers from the sewage system
  • Reviewing and updating design standards
  • Developing new standards and guidelines for inspecting and testing the sewage system

In the past, replacing or rehabilitating defective pipes involved digging a trench which destroyed lawns and sidewalks in the process. Today, trenchless technology is used to repair or replace these pipes with minimal digging.

Examples of trenchless technology include:

  • Pipe bursting - Replacing an older pipe by sending a new pipe through the old one and “bursting” (destroying) it from the inside
  • Relining - Creating an epoxy lining inside of an existing pipe
  • Cured-in-place pipe - Creating a pipe within an existing pipe

In addition, local municipalities continue to thoroughly inspect and repair manholes and sewer lines, reducing inflow and infiltration in the public portion of the sewage system.

These techniques are used instead of building greater capacity into the current sewage system due to:

  • Expense
  • Impractical engineering
  • The cost to unnecessarily treat stormwater in addition to sewage




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