MS4 (Municipal Separate Storm Sewer System)

An MS4 (Municipal Separate Storm Sewer System) is a stormwater drainage system — pipes, ditches, inlets — owned and operated by a city, county, or other public entity, kept physically separate from the sanitary sewer. MS4 operators hold their own NPDES permits and, in turn, require construction sites discharging into their system to control pollutants like concrete washout before it reaches an inlet.