Why Identifying Infiltration and Inflow is Critical

underground inspection

What Is Infiltration and Inflow (I&I)?

Sanitary sewer systems are designed to move wastewater from residences and businesses to a treatment facility. However, leaking pipes and manholes, as well as illegal connections, allow groundwater and stormwater into the sanitary sewer system. These sources of relatively clean water, which overtax both the sewer system and wastewater treatment facility, are referred to as infiltration and inflow (I&I). 

Infiltration is excess groundwater entering the collection system through unintended entry points such as open joints, cracks, root intrusion, breaks in the pipes, or other deficiencies. Cracked or collapsed sewer lines are generally caused by deterioration over time, poor design, installation, or maintenance. The aging pipe, for example, might crack and let underground aquifers bubble into the system — a natural consequence of creating a low-resistance flow path.

Inflow is caused by storm sewer system connections to sanitary sewers, leaking manholes and covers, and leaky clean-out caps. Inflow usually comes after excess rain and stormwater from control runoffs like gutters, downspouts, residential driveways, streams, building foundations or other artificial and natural structures that are piped directly into the collection system.

Problematic inflow connections are prohibited by local zoning ordinances and building codes. That doesn’t stop them from doing damage. However, builders and property owners can easily cause problems by connecting unapproved drainage outlets to municipal sewers.

I&I Can Lead to Serious Issues

I&I can lead to a plethora of problems for communities, and if there’s too much I&I in a system during wet weather, potential damage can happen. 

Excess water flowing to the treatment plant can peak beyond the treatment plant’s capacity, causing lift station malfunctions, sewage backups in the collection system and into homes, and sanitary sewer overflows (SSO) or partially treated wastewater being discharged from the treatment facility.

SSOs bring public and environmental health risks, liability, and oftentimes fines from state and federal environmental agencies.

Even if conditions stop short of being severe enough to cause flooding and large-scale catastrophes, I&I puts an unnecessary burden on treatment facilities and contributes to shorter wastewater plant lifespans. Additionally, the “clean” groundwater and stormwater is treated at the wastewater treatment facility at the community’s expense.

Negative Impact of Sewer Infiltration and Inflow

  • Environmental issues: I&I can lead to environmental issues. When sewer pipes succumb to water infiltration or inflow, raw sewage back-ups and overflows can occur, releasing potential pathogens into homes, streets, and waterways.
  • Public health problems: I&I can also lead to public health problems. When sewer pipes succumb to water infiltration or inflow, raw sewage back-ups and overflows can occur, releasing potential pathogens into homes, streets, and waterways.
  • Expensive repairs: I&I can lead to expensive damage caused by backups especially when a sewer pipe breaks, or a lift station malfunctions. Repairs or replacement can be costly. Additionally, the groundwater or stormwater leaking into the sewer system takes up extra space that could be carrying wastewater and causes havoc with the wastewater collection system or treatment plant. If left untreated, I&I could also lead to the need to fund a plant upgrade, because influent flows are exceeding permit and design capacity.

Where Does Infiltration Occur?

The conditions in which infiltration & inflow occurs are many. It can seep in with pipe defects, cracks, or joints that aren’t watertight. It can also come from groundwater being pumped into the system through an illicit sump pump connection, or from a connection to the storm sewer system.

Causes of Infiltration

  • Cracked pipes
  • Poor construction
  • Poor design

Causes of Inflow

  • Illegal system connections (sump pumps, roof drains)
  • Manhole leaks
  • Leaky clean-out caps
  • Connection to storm sewer

Signs of Infiltration & Inflow

  • Backups
  • Flooding buildings
  • Flooding manholes

Is I&I Inevitable?

Some amount of I&I will occur in all sewer systems, even newly constructed ones. And that’s okay. You can’t get all the I&I out! Sewer manholes are not watertight. Pipes don’t come with lifetime guarantees.

Low levels of I&I won’t hurt, and the quest for zero I&I is nearly impossible. But you can control I&I and make sure that you are not getting huge peak flows into the plant or collection system during big rain events by maintaining your sewer systems and repairing manholes or repairing or replacing pipes.

Mitigating Sewer Infiltration

F&V engineers are experts in identifying I&I issues and mitigating the damage it can cause.

We rely on our vast experience! We’ve successfully completed hundreds of projects that have identified and removed infiltration and inflow. The right tools are needed to locate I&I. Our toolbox includes: 

  • Performing manhole assessments
  • Smoke testing
  • Closed circuit televising (CCTV)
  • Flow monitoring

We’ll evaluate flow and weather data, along with staff input and work with you to develop rehabilitation and implementation strategies to get those sources out.

Infiltration and Inflow Studies

The first step in an I&I reduction program is a flow monitoring evaluation to quantify the magnitude of the problem and identify the priority areas. F&V’s specialized technique—mass flow monitoring (MFM)—is the most cost-effective and the most advanced technology. We are also the sole provider for MFM in Michigan and Indiana. 

MFM uses flow monitors that are installed in multiple manhole locations to quickly differentiate between and estimate I&I. It is much less expensive than the cumbersome metering equipment used in the past and provides data at more locations compared to installation of a few expensive flow meters. The monitors are deployed for three to six months, in dry and wet seasons, to monitor where peak flows are coming.

MFM can be easily deployed through small or large portions of your sewer system to locate I&I and is extremely cost effective and versatile. Flow monitors can be easily moved to further refine hot spot areas in the sewer system and no “confined space” entry is required, making the process of installing the monitors much safer.

Lift station flows and site rain data are also incorporated as part of the data for analysis. Results can be used to prioritize additional field investigations such as smoke testing and televising. 

Infiltration and Inspection

F&V uses mass flow monitoring (MFM) to best determine areas of excess infiltration and inflow. After determining the wet-weather flows, the next step includes physical inspections of the system. This includes manhole inspections, televised inspection of pipes, dye testing, smoke testing, and house to house inspections.  

Manhole field inspections identify structural defects and leaks within the manholes. A comprehensive assessment can usually be completed at the top of the manhole without entry. 

Smoke testing is a relatively fast, economical, and effective method of identifying rainwater coming from cross connections. An odorless, nontoxic smoke is pumped into the sewer system to locate damaged or improper connections, like a catch basin connected to the sanitary sewer or a broken sewer lateral clean-out cap. 

Televising an entire sewer system is a more expensive option but televising a small amount at a time and prioritizing suspect areas provides insight into what causes any backups or blockages. The data can be stored in the sewer system GIS and updated as you complete future inspections. 

Reducing & Resolving I&I Issues

F&V has experience in state-of-the-art methods to combat I&I issues from rehabilitation, including trenchless cured-in-place liners and pipe bursting, to repair of joints and manholes.

Combat Infiltration and Inflow Issues with F&V

Wet weather and high groundwater can raise havoc for municipal leaders trying to keep excess clear rainwater or groundwater from coming into the sewer system. Don’t let infiltration and inflow from another wet spring weather season cause you grief!

Our experts at F&V can help you accurately identify the source of I&I in sanitary sewers and save your community frustration, time, and money. Call us today!