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.
Snow MeltSnow Melt is stormwater runoff in a different form.
Separate Storm Sewer SystemThe 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 snowmelt from street catchbasins and roof drains and carries it directly to the receiving waters.
Street Catch BasinAn 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.
Sanitary WastewaterUnderground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal.
Sewer SystemThe 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.
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 refers to rainwater and groundwater that enters the separate storm sewer system. A certain amount of inflow is unavoidable and is accounted for in routine storm 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.
Combined Sewer OverflowThe discharge of wastewater and stormwater from a combined sewer system directly into a river, stream, lake or ocean.
Non-Point Source PollutionThe 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.
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 are:
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
CSO Related BypassDischarge from a combined sewer system from a point prior to the Wastewater Treatment Plant.
Fully Treated EffluentWater taken from any waste stream and treated to a high standard so it can be used for a new activity or discharged receiving waters.
Wastewater Treatment PlantTo learn more about the Wastewater Treatment process, please visit http://www.cleanwaternashville.org/treatment
Sump PumpA 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.
ErosionErosion – natural or manmade – can impact water quality.
Stormwater Point SourceAny 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.
Combined Sewer SystemWastewater collection systems that combine sanitary sewage from our homes, businesses, and industries with stormwater from rainfall and surface water into one pipe that flows to a treatment facility or to a permitted outfall.