Knox County Tennessee

The National Pollutant Discharge Elimination System (NPDES) Permit

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The National Pollutant Discharge Elimination System (NPDES) permit program: Water pollution degrades surface waters making them unsafe for drinking, fishing, swimming, and other activities. As authorized by the Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States. Point sources are discrete conveyances such as pipes or man-made ditches. Individual homes that are connected to a municipal system, use a septic system, or do not have a surface discharge do not need an NPDES permit; however, industrial, municipal, and other facilities must obtain permits if their discharges go directly to surface waters. In most cases, the NPDES permit program is administered by authorized states. Since its introduction in 1972, the NPDES permit program is responsible for significant improvements to our Nation's water quality. (

What is the Knox County’s National Pollution Discharge Elimination System Requirements?

Municipal Separate Storm Sewer System:  As a Phase II MS4 community, Knox County is charged with reducing the discharge of stormwater pollutants to the maximum extent practicable, to protect water quality, and to satisfy the appropriate water quality requirements of the Clean Water Act.

6-minimum measures:  In order to meet these aims, EPA has defined six “minimum control measures” (MCMs) that are to be addressed. Each of these requirements satisfies a portion of the Knox County Phase II NPDES Permit requirements.

The Tennessee Department of Environment and Conservation (TDEC):  TDEC implements the Phase II MS4 permit coverage for stormwater discharges from certain communities in Tennessee. About 100 cities and counties are required to obtain coverage under the permit and implement programs to manage the quality of stormwater runoff from their storm sewer systems. This includes Knox County. Knox County was issued the most recent NPDES General Permit for Discharges from Small Municipal Separate Storm Sewer Systems on September 1, 2022, and it became effective on February 1, of 2023. This permit expires on August 31, 2027, and may be administratively continued until the next permit is issued.

Knox County is a Phase II Municipal Separate Storm Sewer System (MS4)

Knox County Map

The Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES), Phase II Stormwater Program for Municipal Separate Storm Sewer Systems (MS4s) is the second part of a program designed to improve the quality of the nation's streams, rivers, lakes, and estuaries by managing stormwater runoff from urban and suburban areas, construction projects, and industrial sites. 

OVERVIEW:  Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it is often discharged untreated into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a NPDES permit and develop a stormwater management program.

PHASE I AND PHASE II:  Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges. There are approximately 750 Phase I MS4s.

Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. There are approximately 6,700 Phase II MS4s.

Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.

WHAT IS AN MS4?  An MS4 is a conveyance or system of conveyances that is:

  • Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.;
  • Designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.);
  • Not a combined sewer; and
  • Not part of a Publicly Owned Treatment Works (sewage treatment plant).

(This information was taken in part from the website)