Official websites use .gov
A .gov website belongs to an official government organization in the United States.
Secure .gov websites use HTTPS
A lock (Lock A locked padlock) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.
Municipal Wastewater
On this page: * Overview * Wastewater Treatment Facilities * Collection Systems * Peak Flows at Treatment Facilities * Additional Information
Overview
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: * (4.19 MB) - Overview of municipal processes used to treat domestic wastewater before discharge to the nation's waters. * NPDES Permitting Framework – Framework for establishing water quality and technology-based NPDES permit limits. * Secondary Treatment Standards - Minimum, technology-based standards for discharges from municipal wastewater treatment facilities.
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: * Combined Sewer Overflows (CSOs) * Sanitary Sewer Overflows (SSOs)
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: * Peak Flows at Treatment Plants – Wet weather conditions can result in peak flows to treatment plants.
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.
National Pollutant Discharge Elimination System (NPDES)
Open Sidenav Menu Close Sidenav Menu Contact Us About NPDES Contact Us About NPDES to ask a question, provide feedback, or report a problem. Last updated on March 10, 2026 Welcome! Please share your feedback on how we can make www.epa.gov work better for you. A red asterisk (*) indicates a required field.
Success
Thank you. Your feedback has been received.
Error
alert message Required Questions - page 1 of 2 question_55203_answer_01 How would you rate your experience on www.epa.gov today? *
0 Stars 1 Star 2 Stars 3 Stars 4 Stars 5 Stars Please explain the reason for your rating. * 400 characters allowed Would you like to take two more minutes to answer more questions to help us improve our services? Yes, keep going → No, only submit these responses Optional questions - page 2 of 2 How would you describe yourself? Business Government employee Scientist Student Other Enter other text Please tell us why you visited www.epa.gov today. 400 characters allowed Did you visit www.epa.gov to accomplish a specific task? If yes, please explain why you were able or unable to accomplish your task. 400 characters allowed How can we make www.epa.gov a more helpful website? 400 characters allowed Submit []: required