Introduction to I&I

The term “infiltration and inflow” (also called I & I) refers to the various ways groundwater and stormwater make its way into a sanitary sewer system. You see, in an ideal world, a sanitary sewer system contains wastewater only. However, we don’t live in an ideal world and excess water from sources other than toilets, sinks, and showers finds its way into sewers, unfortunately. This excess water is called ‘clearwater’ even though it’s mainly dirty groundwater and stormwater.

Clearwater is a problem because…

When clearwater gets into a sewer system it causes an increase in the amount of wastewater in the sewer system. This is because clearwater – once it’s in the sewer system – is seen as wastewater and treated as wastewater. 

This is bad because wastewater costs money to transport and treat. When there’s an increased amount of wastewater the cost associated with transporting and treating it is passed along to taxpayers. This results in higher sewer bills.

There are also health risks. An increased amount of wastewater can cause sewer backups and overflows. When this happens we end up with sewage on our streets, lawns, and other public places.

Keep in mind that in older sewer systems it’s not easy to determine how much of the clearwater is the result of infiltration and how much is entering via inflow. (We’ll explain what these terms mean in a bit.) This is why professionals use the combined term “infiltration and inflow” (I & I). Clearwater – whether from infiltration or inflow – doesn’t belong in sanitary sewers.  

Now let’s take a closer look at the difference between infiltration and inflow. 

What is infiltration?

Infiltration refers to groundwater that finds its way into sewers via broken or cracked pipes, broken side sewers, root intrusion, faulty lateral connections, and deteriorated manholes.  As the term implies, infiltration is about groundwater slowly seeping into the sewer system.

The amount of infiltration will usually vary according to season. For example, infiltration will be higher during rainy or snowy seasons because of the increased amount of groundwater.

What is inflow?

Inflow refers to surface stormwater that rapidly flows into a sewer system from above-ground sources like faulty manhole covers, foundation drains, uncapped cleanouts, storm drain cross-connections, and downspouts.

Inflow will naturally be greater after heavy rainfall or snowmelt

All sewer systems experience infiltration and inflow

Just about every sewer system in use today experiences a certain amount of infiltration and inflow and that’s OK. In fact, a small amount of I & I is even expected. However, problems arise when infiltration and inflow start causing backups, overflows, and when the cost of transporting and treating it is greater than the cost of getting rid of it. Of course, if infiltration and inflow is causing health concerns, it needs to be dealt with even if the cost of transporting and treating it isn’t excessive.

Let’s take a closer look now on exactly why infiltration and inflow is a problem.

Why Infiltration and Inflow is a problem

Infiltration and inflow is a problem because…

  • Wastewater collection and treatment systems aren’t designed to handle a large amount of infiltration and inflow. This could affect a community’s ability to grow.
  • Communities may need to build new sewer facilities to keep up with the increased amount of wastewater.
  • The increased amount of wastewater moving through a system decreases its ability to properly treat it. This can impact a community’s health and make it difficult to comply with water quality standards.
  • Overflowing sewers are a public health risk.
  • The increased amount of wastewater means higher operating costs for collecting and treating it.

Costs associated with infiltration and inflow

Most collection systems and wastewater treatment facilities weren’t built to handle the additional volume generated by heavy infiltration and inflow. The financial impact of I&I includes the cost of general operations, treatment, and repairing pipes that have ruptured or collapsed because they couldn’t handle the additional wastewater. A municipality or clean water agency can easily spend into the hundreds of millions of dollars dealing with infiltration and inflow.

The environmental effects of infiltration and inflow

As long as the increased volume of wastewater generated by infiltration and inflow doesn’t exceed a system’s capacity to collect and treat it, there is no direct environmental impact. However, sometimes the wastewater volume generated by I&I is so great that a system needs to release untreated wastewater back into the environment. This is a serious problem.

What are we doing about infiltration and inflow?

All municipalities and clean water agencies regularly maintain and repair their wastewater collection systems, including pipes and pump stations. They inspect sewer pipes using CCTV video to locate areas where infiltration and inflow might be able to find its way into the collection system. When they find a problem, they fix it. However, repairs can be expensive and most areas are more focused on their immediate problems rather than infiltration and inflow. Yes, it’s possible to detect I&I when it’s in its early stages. However, it takes money, the right equipment, and trained staff. 

How can infiltration and inflow be minimized?

It’s impossible to entirely eliminate I & I. However, we can minimize it by…

  • Eliminating stormwater connections to sewer pipes. These connections allow water from sources other than toilets, sinks, showers, etc. to enter the sewer system instead of soaking into the ground or entering a storm sewer.
  • Providing incentives for property owners to inspect and repair sewer laterals. This is important because laterals contribute a sizable percentage of a system’s infiltration and inflow.
  • Ensuring that both a system’s pipes and treatment facilities can handle the additional wastewater generated by infiltration and inflow.

About t4 Spatial

Here at t4 Spatial we design and develop cloud-based software for managing underground infrastructure. When you use our software you get easy access to your sewer inspection data and make decisions based on fact rather than intuition. 

t4 Underground and t4 Vault enable you to visualize your data on a map, store it in the cloud, and even index it for easy retrieval. We exist so you can find what you need, when you need it, from anywhere, and from any device. 

Case Studies

Learn how the Mission Springs Water District uses our technology to monitor its wastewater systems.

Find out how the City of Santa Cruz uses t4 Underground to conduct the preventative maintenance necessary to protect the city from wastewater spills.

The Los Angeles County Sanitation Districts use t4 Spatial to streamline its pipeline inspection and decision making.

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