Inflow and Infiltration Glossary

Average Daily Flow

The volume of water that travels through a wastewater system per day, typically measured in millions of gallons per day (MGD).

Breaks, Cracks and Open Joints

Locations where a sanitary sewer pipe has cracked, or separated at the connection joint between two pipes, allowing infiltration to occur.

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Clear Water

Stormwater or groundwater that would normally not require any treatment but enters the wastewater system through inflow and infiltration sources.

Cured-in-Place Pipe (CIPP)

A low-impact method to repair sewer pipes without having to excavate and replace the pipe. A resin-filled flexible cloth is inserted and pulled into the pipe through a maintenance hole or access point. The cloth liner is inflated to fit snugly inside the existing pipe and hardened by either hot water or steam.

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Downspout, Roof Drain, or Rain Leader

Drain pipes that move stormwater from the roof of a home or business to the ground below. This is a significant source of inflow when illegally connected to the wastewater system.

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Dye Water Testing

To test for sources of I/I, water containing a nontoxic coloring is flushed through drains or onto the ground over a sewer pipe. If the colored water is found to be present in the wastewater system, there are sources of I/I that need to be repaired. 

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Excess Inflow and Infiltration (I/I)

Wastewater flow rate that exceeds the peak-flow limit for a metershed, caused by excessive clear water entering the wastewater system.

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Flow Monitoring

Using flow meters and other devices inside sewer systems to determine flow rates and volume, usage needs, exceedances, and to design new systems. 

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Foundation Drain

Typically located around a building foundation, these pipes are designed to drain subsurface water away from a building to the ground surface or storm sewer system. This is a significant source of inflow when illegally connected to the wastewater system. 

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Clear water that gradually enters the wastewater system below ground through cracks and openings in sewer service lines and joints, and public sewer mains and deteriorated maintenance hole covers.

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Clear water that quickly enters the wastewater system after rainfall events from sources such as sewer cleanouts, sump pumps, gutters, building foundation drains, and broken maintenance hole covers.

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Inflow and Infiltration (I/I)

Clear water — such as groundwater and surface water — that enters the wastewater sewer system from sources such as defective sewer service laterals, sump pumps, downspouts, building drains, and aged or broken sewer main and maintenance holes. Clear water would normally not require any type of treatment; however, once it blends with wastewater it cannot be separated and must be treated as wastewater.

Inflow and Infiltration (I/I) Goal

The maximum allowable peak hourly flow discharged from a metershed to the regional wastewater system. These are calculated by MCES annually for each metershed in the region using metershed-specific data including historical wastewater flows, community growth, and a standard peak hourly flow factor.

Inflow and Infiltration (I/I) Toolbox

An online guide of tools and resources to assist communities planning and implementing inflow and infiltration reduction programs available at

Maintenance Hole, Manhole (MH)

A utility structure that extends from the ground surface to the system and pipes below ground, allowing human access for maintenance of the system. Wastewater maintenance holes often include the following components: 
  • Casting – The removable lid and metal frame at ground surface, typically in a roadway.
  • Chimney – The cylindrical portion that can be adjusted so that the casting is level with the surrounding ground surface.
  • Invert – The bottom of an incoming or outgoing sewer pipe at the bottom of the structure. 

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An area of a community that discharges wastewater to the regional wastewater system. Some communities have multiple metersheds.

Metropolitan Disposal System

The wastewater system owned and operated by MCES that provides service to communities throughout the region. The system includes interceptors, treatment plants, maintenance holes, lift stations and pressurized forcemains.

Municipal Wastewater Charge

The wholesale charge communities pay to MCES for wastewater treatment, based on the portion of wastewater flow discharged from the community to the regional system. Year-to-year changes are affected by growth, water conservation, and inflow and infiltration.

Peak Hour Flow

The highest one-hour flow rate of wastewater that travels through a sewer pipe. Many pipes are designed to handle the peak flow. Excessive I/I increases the peak flow, increasing the size and cost of sewer pipes. 

Pipe Joint

The connection between two adjacent pieces of sewer pipe, typically existing every 20 feet or less along a sewer main or service lateral. These joints do not last forever and need maintenance over time to reduce I/I. 

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Root Intrusion

The presence of roots inside a sewer pipe from adjacent trees and other vegetation that have grown into cracks, breaks or joints. 

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Sewer Service Lateral

Sewer pipes that connect homes and businesses to the municipal sewer system. They are a major source of I/I throughout the region, due to cracks, roots intrusion, and separated joints. The ownership and maintenance responsibilities of these pipes varies depending on local rules.

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Sanitary Sewer Interceptors

A large sanitary sewer pipe that collects wastewater from municipal systems and conveys it to a wastewater treatment plant. 

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Sanitary Sewer Main

A municipal pipe that connects homes within a local sanitary sewer system to a regional sewer interceptor.

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Service Connection

The connection between a sewer service lateral and a local sewer main. Depending on the community, this may be owned by the property owner or the community. 

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Sewer Cleanout

A vertical pipe that extends from the ground surface down to a sewer pipe, intended for access to maintain and clean the sewer. Most buildings have a sewer cleanout inside, and long service laterals will also have them outside. A missing cap on the sewer cleanout is a source of I/I that is easy to find and repair.

Smoke Testing

A test that forces nontoxic smoke through pipes underground to find leaks in the wastewater system. Plumes of smoke emerge where there are defects in the pipes. A very efficient and cost-effective way to locate and identify where inflow and infiltration may be entering the public sanitary sewer system.  

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Stormwater System

A drainage system designed to convey surface water and precipitation away from homes and buildings to a discharge point like a treatment pond or waterway. 

Sump Pump

A pump that pulls water out of the ground, typically under a basement floor, and discharges it away from a building or into a storm sewer system. This is a significant source of inflow when illegally connected to the wastewater system. 

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Video Inspection

Video recordings or closed-circuit television video (CCTV) used to inspect the inside of sanitary sewers to identify locations of breaks, root intrusions, joint separation, and overall system condition.

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Water that has been used in the home, business, or as part of an industrial process and needs to be treated before returning to the environment.

Wastewater Treatment Plant

A facility used to treat wastewater discharged from communities. MCES owns and operates nine facilities in the metro area.

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