What Are MS4s in Stormwater Management? Ultimate Guide

There are multiple discharge points for stormwater runoff across any geographic area, each of which has its own meticulous rules and regulations.

While stormwater runoff into public water systems via rivers and streams is well understood among many in the construction industry, many are surprised that many small and municipal discharge points also require proper NPDES permitting.

If you are undergoing construction in an urban area, then you need to be aware of the rules and regulations surrounding stormwater discharge in an MS4.

What Are MS4s in Stormwater Management?

MS4s, short for “Municipal Separate Storm Sewer System,” regulate any municipal-owned separate sewer system that is not connected to a sewage treatment facility–hence the word “separate.”

These drainage systems are typically designed to release built-up stormwater and discharge it directly into public water systems.

The main goal of MS4s is to regulate the amount of stormwater runoff without causing flooding or overloading treatment facilities.

The most common examples of MS4s include:

  • Retention basins
  • Drainage pipes that run underground
  • Ditches
  • Swales

Since a municipal facility does not treat MS4s, it is very important that only stormwater discharge and no “illicit discharge” flows through MS4s.

Acquiring an NPDES for MS4 Discharge

Under Phase II of the Clean Water Act, all construction sites that plan to discharge into municipal separate sewage systems are required to obtain an NPDES permit.

This permit sets allowable standards and best practices to filter out any sediment, debris, or toxins that may be discharged from a construction site directly into public waters.

A Notice of Intent (NOI) will be required to obtain a Construction General Permit if you plan to disturb one or more acres of land within an area that discharges into an MS4.

Post-Construction Requirements for Managing MS4s

Under NPDES permitting regulations, each municipality is required to develop a post-construction stormwater management plan (PCSM) protecting MS4s from illicit discharges.

In turn, this could alter regulations between different municipalities of what you are allowed to discharge within MS4s and what you are not.

Researching local codes and contacting municipal authorities to see what additional requirements you will need to follow will be important in acquiring proper local permitting.

Best Management Practices for Protecting MS4s

For the most part, construction managers and local municipalities can work together to create a list of best management practices to protect MS4s. Some common examples of positive BMPs include:

  • Infiltration practices: Permeable pavements, rain gardens, and grass swales.
  • Filtration practices: Green roofs, vegetated filter strips, wet swales, and sand filters.
  • Retention measures: Dry retention basins and wet ponds.
  • Onsite treatment and harvesting: Internal filtration systems and recycling stormwater runoff.

Working with municipal authorities, as well as third-party environmental consultants, to develop a proper SWPPP and post-construction stormwater plan will ensure your regulatory compliance.


What is the purpose of an MS4 program?

The primary purpose of an MS4 program is to manage stormwater within urban areas to prevent pollution and protect water quality. MS4 programs aim to control the discharge of stormwater into local water bodies, ensuring that it meets environmental standards and regulations.

By implementing MS4 programs, municipalities can reduce the harmful effects of stormwater runoff, such as flooding and the contamination of rivers, lakes, and oceans.

Who needs to comply with MS4 regulations?

Municipalities, often referred to as “permittees,” are the entities required to comply with MS4 regulations. These municipalities are typically located in urbanized areas and must obtain permits from the Environmental Protection Agency (EPA) or their state environmental agencies.

The permits outline the specific requirements and responsibilities for managing stormwater within their jurisdictions. However, construction sites and all dischargers will need to follow municipal rules to ensure proper compliance.

How are MS4 permits obtained?

MS4 permits are obtained through a regulatory process governed by the EPA and their state environmental agencies. To obtain a permit, municipalities must submit an application that outlines their stormwater management plan.

This plan details how the municipality intends to address the six minimum control measures required for MS4 compliance. After review and approval, the permit is issued, and the municipality is responsible for implementing and adhering to the permit’s requirements.

What are the penalties for MS4 non-compliance?

Non-compliance with MS4 regulations can lead to significant penalties. Penalties may include fines, legal action, and, in severe cases, the revocation of the MS4 permit. Municipalities failing to meet the minimum control measures and other compliance requirements can face costly consequences. Additionally, non-compliance can harm the local environment, potentially leading to environmental damage and public health concerns.

How can communities benefit from MS4 compliance?

Communities can benefit from MS4 compliance in several ways. Firstly, compliance ensures improved water quality, reducing the risk of contamination in local water bodies, which directly benefits residents.

Secondly, effective stormwater management also leads to decreased flooding and erosion, making communities safer and more resilient. Compliance can also enhance a community’s reputation, attracting businesses and investors.

Furthermore, it fosters a sense of environmental responsibility and can lead to community involvement in sustainability efforts. Overall, MS4 compliance helps create healthier, more prosperous, and sustainable communities.