What is an MS4?

MS4 is short for, "Municipal Separate Storm Sewer System:, where the word "Municipal" refers to a unit of local government like a borough or a township. And the number 4 refers to the four words that start with the letter "S", Separate, Storm, Sewer, System."

A separate storm sewer system is a collection of structures, including retention basins, ditches, roadside inlets and underground pipes, designed to gather stormwater from built-up areas and discharge it. without treatment, into local streams and rivers. It's called a separate system because its' not connected to the sanitary sewer system which drains waste water from inside a home to a sewage treatment facility or a private septic system. Some communities in Pennsylvania do have combined storm and sanitary sewers. The area served by these "combined systems" are not covered by the MS4 program.