Glossary

Clean Water Act
Approved by Congress in 1972, the Clean Water Act established the basic structure for regulating discharges of pollutants into the waters of the United States.

Combined Sewer Overflow (CSO) and CSO-Related Bypass
A combined sewer system conveys both sanitary sewage and stormwater in one piping system. During normal dry weather conditions, sanitary wastewater collected in the combined sewer system is diverted to the wastewater treatment plant before it enters natural waterways. During periods of significant rainfall, the capacity of a combined sewer may be exceeded. When this occurs, excess flow, a mixture of stormwater and sanitary wastewater, is discharged at CSO points, typically to rivers and streams. Release of this excess flow is necessary to prevent flooding in homes, basements, businesses, and streets.

Combined Sewer System (CSS)
Wastewater collection systems that combine sanitary sewage from our homes, businesses, and industries with stormwater from rainfall into one pipe that flows to a treatment facility or to a permitted outfall. Nashville’s combined sewer system primarily serves the downtown / urban core of Nashville. Nashville’s combined sewer system dates to the late 1800’s. Prior to the construction of the city’s first wastewater treatment plant in 1958, combined sewer flows discharged directly into streams and rivers. View Nashville’s combined sewer system

Consent Decree
The legally binding document that represents the result of negotiations between the U.S. Department of Justice, the State of Tennessee, and MWS. It specifies the activities and timetables required to address combined and separate sewer overflows in Metro Nashville.

Corrective Action Plan/Engineering Report (CAP/ER)
A plan developed by MWS to address the conditions causing SSOs in the separate sanitary sewer system.

Erosion
The degradation or displacement of the upper layer of soil by the action of natural (e.g. water, ice, snow, air, plants, animals) or man-made disturbances. Erosion – natural or man-made – can impact water quality.

Fully Treated Effluent
Water taken from any waste stream and treated to a high standard so it can be used for a new activity or discharged into receiving waters.

Grease Blockage
A blockage in a sewer pipe caused by the accumulation of fats, oils, and/or grease.

Groundwater Infiltration (GWI)
Groundwater entering the collection system through defective pipes, pipe joints, and manhole walls. The magnitude of GWI depends on the depth of the groundwater table above the pipelines, the percentage of the system that is submerged, and the physical condition of the sewer system. The variation in groundwater levels is seasonal, which typically results in seasonal variation in GWI.

Infrastructure Renewal
Upgrading, enhancing, and/or improving the fundamental facilities and systems serving a country, city, or area such as water/wastewater, transportation, and communication systems; power plants; and schools.

Infiltration
Infiltration refers to rainwater and groundwater that enters the sanitary sewer system. A certain amount of infiltration is unavoidable and is accounted for in routine sewer design. However, when infiltration exceeds design allowances, sewer capacity is consumed and may result in overflows, risks to health, damage to the environment and increased conveyance costs.

Inflow
Water other than wastewater that enters the separate sanitary or combined sewer system from sources such as, but not limited to, roof leaders, cellar drains, yard drains, area drains, drains from springs and swampy areas, manhole covers, cross connections between storm sewers and sanitary sewers, catch basins, cooling towers, storm water, surface runoff, street wash waters, or drainage. A certain amount of inflow is unavoidable and is accounted for in routine sewer design. However, when inflow exceeds design allowances, sewer capacity is consumed and may result in overflows, risks to health, damage to the environment and increased conveyance costs.

Long Term Control Plan (LTCP)
A plan developed by MWS to reduce the occurrence and impact of CSOs during wet weather events.

National Pollutant Discharge Elimination System (NPDES) Permit
The NPDES permit program addresses water pollution by regulating point sources that discharge pollutants to waters of the United States.

Non-Point Source Pollution
Natural and man-made pollution carried by rainfall or snow melt moving over and through the ground that is eventually deposited in lakes, rivers, wetlands, coastal waters, and ground waters.

Rainfall-Derived Infiltration and Inflow (RDII)
Stormwater that enters the sanitary sewer system in direct response to the intensity and duration of rainfall events. RDII includes stormwater entering the sanitary sewer system through direct connections, such as roof downspouts illegally connected to the sanitary sewers, yard and area drains, holes in manhole covers, or cross-connections with storm drains. RDII also includes the more delayed response of stormwater that enters the collection system through defective pipes, pipe joints, and manhole walls after percolating through the soil.

Sanitary Sewer Overflows
Sanitary Sewer Overflows are a condition where untreated sewage is discharged into the environment prior to reaching sewage treatment facilities. The main causes include: infiltration of excessive stormwater into sewer lines during heavy rainfall; rupture or blockage of sewerage lines; malfunction of pumping station lifts or electrical power failure.

Sanitary Sewer System (SSS):
A wastewater collection system that is designed to collect and convey only sanitary wastewater from residential, commercial, industrial, and institutional users. This may also be referred to as a separate sanitary sewer system to distinguish it from a combined sewer system. The separate sanitary sewer system, as displayed by the light green pipes in the Urban Wet Weather Flows diagram, collects the wastewater from individual homes and businesses and transports it through a series of pipes to the wastewater treatment plant. At the plant, wastewater is treated and released into a stream or river called the receiving water. The majority of Nashville is served by a sanitary sewer system.

Separate Storm Sewer System
The City also has a system of culverts, drains, and pipes to carry stormwater to streams and rivers. Stormwater is rainwater that runs off rooftops, streets, and parking lots. The separated storm sewer system (as shown by the light blue set of pipes in the diagram) collects the rainwater and snow melt from street catch basins and roof drains and carries it directly to the receiving waters.

Snow Melt
Snow melt is stormwater runoff in the form of melted snow.

Stormwater Point Source
Any discernible confined and discrete conveyance including but not limited to a pipe, ditch, channel, or conduit from which pollutants are or may be discharged. Point source discharges are required to have either a state NPDES or a Federal NPDES permit.

Street Catch Basin
An opening to the storm drain system that typically includes a grate or curb inlet at street level where storm water enters the catch basin and a sump captures sediment, debris and associated pollutants. Catch basins are able to prevent trash and other floatable materials from entering the drainage system by capturing such debris by way of a hooded outlet.

Sump Pump
A sump pump is a pump used to remove water that has accumulated in a water collecting sump basin, commonly found in the basement of homes. The water may enter via the perimeter drains of a basement waterproofing system, funneling into the basin or because of rain or natural ground water, if the basement is below the water table level.

Wastewater Treatment Plant
A facility designed to remove contaminants from municipal wastewater and convert it into an effluent that can be returned to the water cycle with minimum impact on the environment. Learn more about the Wastewater Treatment process →