Cleanwater Nashville

Metro Water Services

Resources

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 Overflows (CSOs) 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

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.

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.

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

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 (SSOs) An occasional unintentional discharge of raw sewage from a municipal sanitary sewer.

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 majority of Nashville is served by a sanitary sewer system.

Sanitary and Combined Sewer Systems Nashville’s combined sewer system dates to the late 1800s. Prior to the construction of the city’s first wastewater treatment plant in 1958, combined sewer flows discharged directly into streams and rivers.

The graphic below illustrates Metro Nashville’s wastewater collection system – storm sewers, a separate sanitary system, and a combined sewer system – and how the system functions during extended rain events. Hover over items on the diagram to learn more about how the systems work.