Inflow and Infiltration

If you have ever worked in the water resources industry you have no doubt heard the term Inflow and infiltration. Inflow and infiltration commonly known as I&I are components of flow in sewers. I&I originates from two sources, stormwater, and groundwater. Inflow refers to surface runoff from stormwater into sewer systems. Infiltration refers to groundwater or stormwater entering the sewer through cracks in the pipes, manhole defects, or other defects in the pipe. Because of this inflow impacts the system more rapidly than infiltrations which is a slow seep. I&I can cause overflows, system strains and interruptions.

Do you have an Inflow & Infiltration Problem?

It is common for peak flows in a sewer system to reach 5-20 times the average flow during a heavy rainfall due to I&I. You can identify existing I&I issues within your network by looking for these clues. Initially pumps that are running for hours to deal with increased inflow during storm events. The hydraulic loading of wastewater treatment plants drastically increasing after a storm event. A warning sign is an increase in inflow during dry weather compared to previous months is. Municipalities need to create a dry weather baseline. Metering data from dry days help develop the dry weather baseline flow.

Advantages to fixing Inflow & Infiltration

Municipalities can benefit from identifying I&I early on and reducing their levels. Having controlled overflow points such as pump stations and treatment plants can reduce overflows. Keeping the sewer network capacity unrestricted will improve operational efficiency. I&I is clear water that is being pumped and treated alongside wastewater. Consequently the added volume of unnecessary treated water not only reduces the sewer network compacity. It also increases costs for the municipality and the residents within it.

How to reduce I&I

the first step is to identify the general area of the issue through flow monitoring. The next steps are to identify the exact problem, is it inflow or infiltration. Then locate the area(s) where this is occurring through source detection. Then taking steps to improve and rehabilitate the problem areas. Once the problem areas have been identified it is time to find the exact issue with inspection equipment and specialized testing.

Types of source detection

The inspection of private properties through visual assessment for illegal connections. Illegal connections can include a stormwater down pipe that is directly connected to a wastewater gully trap. Manhole inspection can identify leaks caused by root intrusion or design issues as well as leaky covers. Smoke testing locates I&I sources by identifying broken pipes, cross connections and unsealed manhole covers. CCTV allows municipalities to have the best visualization of the inside of the pipes. Crews use small cameras that travel down the pipe to collect a visual of the conditions within the sewer system. CCTV inspections can revel the need for rehabilitating leaky pipes or broken laterals.  


Crew should now know where and why they have I&I in their system. Now it is time to stop and prevent this I&I from occurring. Rehabilitation is the final step to fixing a system with an I&I issue. Sealing manholes and replacing leaky covers. Applying a cured in place piping (CIPP), a trenchless rehabilitation or chemical compound to seal broken and leaky pipes. Redirecting illegal private connections back into the proper stormwater system. After that the sewer system will have an insignificant amount of I&I. To keep I&I at bay continued inspections and rehabilitation will be needed.

Contact us for info on inspection equipment to add to your effective I&I maintenance program. call U-Rock