Municipal Separate Storm Sewer System (MS4) is a publicly owned conveyance system or network of systems that is designed to collect and transport stormwater runoff and discharge it to surface waters, while remaining physically separate from sanitary sewer systems that carry wastewater. In the United States, MS4s are regulated under the National Pollutant Discharge Elimination System.
An MS4 includes a wide range of infrastructure components used to manage stormwater, such as streets, highways, catch basins, curb and gutter systems, ditches, swales, storm drains, pipes, culverts, and outfalls. These systems are owned and operated by public entities, including municipalities, counties, state departments of transportation, and other governmental agencies.
The term “separate” distinguishes MS4s from combined sewer systems, where stormwater and sanitary sewage are conveyed together. In an MS4, stormwater is typically discharged directly to receiving waters, such as rivers, lakes, or streams, without treatment. As a result, pollutants carried by runoff, including sediment, nutrients, metals, oils, bacteria, and debris, can enter surface waters unless properly managed.
MS4s are categorized based on size and regulatory designation, commonly including small and large systems, with corresponding permit requirements. Operators of MS4s are required to obtain coverage under an MS4 General Permit and implement a comprehensive stormwater management program to reduce pollutant discharges to the maximum extent practicable.
From a stormwater management perspective, MS4s represent the primary infrastructure through which urban runoff is conveyed. Their design, operation, and maintenance are critical to controlling flooding, reducing erosion, and protecting water quality. Effective MS4 management involves not only maintaining physical infrastructure but also implementing regulatory, operational, and educational measures to address both point and nonpoint sources of stormwater pollution.