What is an MS4?

This article is guest authored by Jason Dorney, regional sales director for Millwood’s CORE division, which specializes in sediment and erosion control products and services.
In our last CORE-related blog post, we discussed Combined Sewer Overflows (CSOs). We detailed how CSOs present a significant challenge to stormwater management in modern communities and allow wastewater to pollute local bodies of water. How do we tackle the issue of CSOs and make our water cleaner and safer?
Today, we are going to talk about Municipal Separate Storm Sewer Systems, or MS4s. While Combined Sewer Systems combine sewer water and stormwater in the same pipe, MS4 systems do just what the name implies; they separate the storm sewers from the sanitary sewers. As a result, stormwater can flow directly to the receiving waters and the sanitary sewer leads directly to the treatment plants.
MS4s are conveyance systems owned by public entities that discharge to waters of the United States. They are designed to collect stormwater only. In order to prevent pollutants from being discharged into our receiving waters, the MS4 operators are required to obtain a National Pollution Discharge Elimination Systems (NPDES) permit.
The operators are also required to develop Stormwater Management Programs (SWMPs). These programs describe the practices that will be implemented to eliminate or reduce pollutants from entering the receiving waters.
MS4s are separated into two categories, Phase I and Phase II. The Phase I regulations were put in place in 1990, and the Phase II regulations were put in place in 1999.
Phase I MS4s are in medium or large cities or certain counties whose populations are 100,000 or more. According to the EPA, there are approximately 855 Phase I MS4s, which are covered by 25 individual permits.
Phase II MS4s are defined as, “U.S. Census Bureau defined urbanized areas, as well as MS4s designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. Phase II also includes non-traditional MS4s such as public universities, departments of transportation, hospitals and prisons.”
These regulations are in place because large quantities of pollutants enter U.S. receiving waters through stormwater runoff, especially in urbanized places.
Consider the road salt that washes away in winter, bacteria from dog waste, soaps from washing cars, fertilizer runoff from lawns and farms and many other sources of pollution that end up washing into curb inlets, drainage ditches and ultimately end up in our creeks, streams, rivers, lakes, bays and oceans.
Each SWMP is unique, but there are six Minimum Control Measures (MCMs) that each SWMP contains. These six MCs are:
  • Public Education and Outreach
  • Construction Site Erosion Control
  • Public Participation and Involvement
  • Post Construction Stormwater Management
  • Illicit Discharge Detection and Elimination
  • Pollution Prevention and Good Housekeeping
This is one of the ways the Federal Government, in conjunction with state and local governments, regulates stormwater runoff. It decreases pollution in our waters and meets the central tenets of the Clean Water Act, which works to provide swimmable, fishable and drinkable water for all.
Author: Jessica Chizmar