National Pollutant Discharge Elimination System (NPDES)

Municipal Wastewater

On this page:


The collection and treatment of domestic sewage and wastewater is vital to public health and clean water. It is among the most important factors responsible for the general level of good health enjoyed in the United States. Sewers collect sewage and wastewater from homes, businesses, and industries and deliver it to wastewater treatment facilities before it is discharged to water bodies or land, or reused.

Wastewater Treatment Facilities

NPDES permits establish discharge limits and conditions for discharges from municipal wastewater treatment facilities to waters of the United States. Resources for discharge requirements include:

Collection Systems

Historically, municipalities have used two major types of sewer systems.

Combined Sewers

Combined sewers are designed to collect both sanitary sewage and stormwater runoff in a single-pipe system. These systems were designed to convey sewage and wastewater to a treatment plant during dry weather. Under wet weather conditions, these combined sewer systems would overflow during wet weather conditions when large amounts of stormwater would enter the system. State and local authorities generally have not allowed the construction of new combined sewers since the first half of the 20th century.

Separate Sanitary Sewers

The other major type of domestic sewer design is sanitary sewers (also known as separate sanitary sewers). Sanitary sewers are installed to collect wastewater only and do not provide widespread drainage for the large amounts of runoff from precipitation events. Sanitary sewers are typically built with some allowance for higher flows that occur when excess water enters the collection system during storm events.

Sanitary sewers that are not watertight due to cracks, faulty seals, and/or improper connections can receive large amounts of infiltration and inflow (I/I) during wet weather. Large volumes of I/I can cause sanitary sewer overflows (SSOs) and/or operational problems at the wastewater treatment facility serving the collection system. In addition, sewage overflows can be caused by other problems such as blockages, equipment failures, broken pipes, or vandalism. 

Resources for overflows and peak flows at treatment plants include:

Peak Flows at Treatment Facilities

Significant increases in flow at wastewater treatment facilities caused by wet weather conditions can create operational challenges and potentially adversely affect treatment efficiency, reliability, and control of unit process operations at the treatment facility. The CSO policy encourages municipalities with combined sewers to maximize wet weather flows to the treatment plant in order to decrease uncontrolled overflows in the collection system. Resources for peak flows at wastewater treatment facilities include:

Additional Information

  • Integrated Planning - integrated planning offers a voluntary opportunity for a municipality to propose to meet multiple CWA requirements by identifying efficiencies from separate wastewater and stormwater programs and sequencing investments.
  • National Pretreatment Program - The national pretreatment program identifies specific discharge standards and requirements that apply to sources of nondomestic wastewater discharged to a POTW. 
  • Biosolids - Biosolids are the nutrient-rich organic materials produced by wastewater treatment facilities.