Spring has arrived, and with spring comes rain, often lots of it. While seasonal rainfall brings greener landscapes and replenishes water supplies, it can also create challenges for municipal sewer systems. One of the most significant issues communities face during heavy rains is inflow and infiltration (I&I).
Understanding how I&I affects sewer systems helps municipalities reduce the risk of sewer backups and protect residents’ homes and property from costly damage and repairs.
What is Inflow and Infiltration?
Inflow and infiltration refer to stormwater and groundwater that enter sanitary sewer systems through unintended pathways.
Inflow occurs when rainwater enters the sewer system directly through openings such as roof drains, yard drains, sump pumps, uncapped cleanouts, or manhole covers.
Infiltration occurs when groundwater seeps into sewer pipes through cracks, deteriorated joints, or damaged manholes.
These flow sources can allow large volumes of water to enter sewer systems that were designed to carry only wastewater from homes and businesses.
Spring Rain Increases the Risk of Sewer Backups
The risk of sewer backups and overflows increases during significant rainfall events, especially when large amounts of rain fall in a short period of time. Excess rainwater entering the system can quickly raise water levels in sewer pipes, potentially overwhelming the system’s capacity or pushing sediment and debris that create blockages in the line.
When this occurs, sewer backups and overflows can happen, sometimes forcing wastewater back into homes through floor drains, sinks, or toilets. These events are disruptive and costly to repair, making it essential to keep stormwater out of sanitary sewer systems whenever possible.
Most I&I Comes from a Small Portion of the System
I&I problems are rarely spread evenly throughout an entire collection system. Studies of wastewater infrastructure consistently show that:
Approximately 80% of inflow and infiltration originates from less than 20% of the collection system.
This means that identifying and repairing the most problematic areas can significantly reduce excess water entering the system during rain events.
How Municipalities Identify I&I Problems
Utilities use several proven methods to locate sources of inflow and infiltration:
Flow monitoring to track sewer flow changes during rain events
Smoke testing to reveal improper stormwater connections
CCTV inspections to identify cracks, roots, and pipe defects
Dye testing to confirm suspected inflow sources
Manhole inspections to locate leaks or structural damage
Public awareness also plays an important role. Residents should ensure sewer cleanouts have secure caps, as missing or damaged caps can allow stormwater to enter the system.
Protecting Homes and Infrastructure
With spring rains increasing the potential for excess water in sewer systems, identifying and correcting I&I is critical. Targeting the areas responsible for most inflow and infiltration helps municipalities:
• Reduce the risk of sewer backups into homes
• Improve wastewater system reliability
• Lower treatment costs
• Extend the lifespan of sewer infrastructure
By working together, municipalities and residents can help keep stormwater out of sanitary sewer systems and ensure they operate safely and efficiently throughout the rainy spring season.
