Inflow & Infiltration Reduction

 What is Inflow and Infiltration?

Inflow & Infiltration or “I&I”, are terms used to describe the ways groundwater and stormwater
enter the sewer system.


Inflow: Inflow is stormwater that enters the sewer system directly through connections such as roof downspouts, driveway drains, sump pumps, and cross connections with storm drains. Peak inflow can occur during heavy storm events or snow melts causing sewer surcharges resulting in overflows and basement backups.

sewer vs storm drain diagram

Infiltration: Infiltration is groundwater or rainwater that enters sewer pipes and manholes through holes, breaks, joint failures, connection failures and other openings. Infiltration quantities often exhibit seasonal variation in response to groundwater levels. Storm events can trigger a rise in groundwater levels and increase infiltration flows. The highest infiltration flows are observed following significant storm events or following prolonged periods of precipitation.

VIDEO Part 1 – What is Inflow and Infiltration?


What is being done to find a solution?

The City of Columbia has contracted with TREKK Design Group LLC to conduct a Sanitary Sewer Evaluation Study (SSES).  The purpose of an SSES is to identify issues within the  sewer system that could cause problems leading to overflows and basement backups caused by a surcharged  sanitary sewer system (overload due to stormwater runoff in system). During surcharge conditions, the sewer can back up into homes that are lower in elevation or it can overflow from the  sewer manholes.  The ultimate goal of the SSES is to find the sources of inflow and infiltration (I & I); this will lead to the improvement in sanitary sewer capacity and result in the reduction of some basement backups and overflows.

VIDEO Part 2 – What is being done to find a solution?

* Locations mentioned in video have expanded and are for educational purposes.*


Smoke Testing

Smoke testing consists of introducing a harmless, non-toxic theatrical-type smoke into the sewer.  Smoke will escape from defects in the sewer system and those locations will be identified and marked by TREKK personnel.  Types of defects may include roof downspouts, uncapped cleanouts, driveway drains, stairwell drains or yard and area drains. smoke testing

Building Sewer Assessments

This phase is focused on the private side of the sewer system.  In the past, the private side has not been given as much attention as the public, but in recent years it has been determined that the private side is a major contributor of  I & I. The building sewer assessment takes 10-15 minutes. TREKK will come to your house with a two person crew; they will have ID badges, orange TREKK shirts, and be in a vehicle clearly marked with the “TREKK” logo.

If the home has a basement, the crew will look for a connection between a sump pump the sewer.  They will take pictures of the sewer pipe leaving the home, the sump pump, and any other direct connection to the sanitary sewer. They will ask brief questions about any history of water backing up, sewer related problems or leaking basement walls. They will also take a measurement of the basement floor in relation to the finish floor elevation to compare the main and basement elevations to see how prone the basement is to backing up. The investigation is similar for a crawl space. If the home does not have a crawl space or basement the interior investigation is very quick.  Often slab homes do not have sump pumps, the major contributor of I & I from the private side.

The crew will also investigate the outside of the house looking for downspouts or outside drains connected to the sewer.  If defects are found affecting the sanitary sewer, in most cases, the City’s Inflow and Infiltration Reduction Program will pay to remove the defect. The building Sewer Assessments are a key component to the overall investigation. They allow the City to build a better picture of troubled areas and help find defects that are inexpensive to remove and are large contributors of I & I.

Manhole and Mainline Sewer Investigations

 In the City of Columbia, there are approximately 16,000 manholes and over 650 miles of gravity sanitary sewer pipe.  Due to the complexity and age of the system there are numerous opportunities for I & I to find its way into the sewer. The City has been rehabilitating the public side of the sewer for years and will continue to do so. It is important to note that in recent studies conducted in Columbia, a larger quantity of inflow and infiltration comes from defects on the private side.

Sewer Modeling

Sewer modeling is where all the information obtained from the SSES is compiled and analyzed so that City staff can plan the rehabilitation projects for upcoming years on a priority basis. The success of this project is entirely dependent upon individual property owner participation. The more individual participation we have, the more accurate our picture of the sewer system becomes.

What is the Inflow & Infiltration Reimbursement Program?

As part of the Inflow & Infiltration Program, the Sewer Utility has designed 2 Cost Reimbursement Programs to assist homeowners with expenses associated with repairing or removing the defects or direct connections to the sanitary sewer. The removal of these defects will reduce instance of I & I, “freeing up” capacity in the sewer mains to avoid additional and/or future basement backups and sewer overflows. The sanitary sewer system is designed to carry only wastewater.

Any and all stormwater runoff that makes its way into the sanitary sewer system contributes to backups and overflows.  Removing stormwater runoff from the sanitary sewer system creates a more efficient sanitary sewer system and in the long term will save the ratepayers money.

Who is eligible?

Anyone with property in the study area that is an owner-occupied residence having three (3) units or less, that was constructed before 1996, and has a private inflow and infiltration source listed in the table in section 22-217.3(b) of the City Ordinance.


For more information about the program contact:

City of Columbia – Sewer Utility

573.874.CITY (2489)

Sewer@CoMo.govcreate new email