Inflow And Infiltration
Inflow and infiltration (I&I) happen when water from the environment that does not need to be treated enters the sewage system. Infiltration refers to groundwater entering the system. Inflow refers to rainwater (stormwater) entering the system.
The RDN and member municipalities have a separated sewer made up of two sewer systems:
- Sanitary sewer: designed to carry sewage to facilities that treat the water before returning it to the environment.
- Storm sewer: prevents streets from flooding by draining and carrying excess stormwater (from rainfall and snowmelt) to a stream, lake or the ocean.
Understanding I&I (from the Capital Regional District)
Part 1: Introduction
Part 2: The Impact of Rainwater Entering the Sanitary Sewer System
Part 3: How Property Owners Prevent Rainwater from Entering the Sanitary Sewer System
Part 4: How Municipalities Prevent Rainwater from Entering the Sanitary Sewer System
What are sources of I&I?
Examples of sources of I & I include:
- Stormwater cross connections
- Roof rain lead connections
- Leaky pipe joints and structural defects
- Leaky manhole covers.
Why is I&I 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 since the sewage is diluted by water
- Increase the cost of water to residents because sewage treatment plants are required to treat a higher flow volume
- May cause sewage overflows or overwhelm treatment plants leading to health risks and property and environmental damage.
Historically, the RDN trunk sewer system has received very high wet weather flows from the City of Nanaimo's sanitary sewer collection system. The largest wet weather flow on record, from December 1997, had a peak flow nearly six times the average dry weather flow. However, since 1997, peak wet weather flows have been significantly lower. While rare, sanitary sewer overflows have occurred from the RDN system in the past - during extreme rainfall events. The frequency and likelihood of these overflows are greatly diminished as I & I are reduced.
How can we fix I&I?
The City of Nanaimo has undertaken a rehabilitation program for the City's sanitary sewer collection system. Replacement or repair of leaking pipes and manholes provides positive benefits in terms of I&I reductions. However, a quantifiable trend is not yet apparent in the RDN's flow records due to the variability in annual rainfall, intensity and geographic distribution of individual storms, as well as gaps in flow and rainfall data records.
- Know where your sewer lateral is on your property
- Avoid planting trees and shrubs over sewer laterals as roots seek water and can damage sewer pipes
- Check your gutters and drains to ensure they are not connected to the sanitary sewer system.
- Disconnect roof downspouts from the sanitary sewer system. Even better, install a rain barrel under your roof leader, if conditions permit
- Disconnect foundation drains and groundwater sump pumps from the sanitary sewer system
- Have your property's sewer lateral inspected. Replace any known broken, leaky or problem sections of your property's sewer lateral. Maintaining and repairing the lateral is the homeowner's responsibility and can reduce I&I and prevent sewer backups into your home.
If you need help, contact your local plumber, municipality or the RDN for assistance.