Municipal Separate Storm Sewer System (MS4)
The Municipal Separate Storm Sewer System (MS4) Program is a permit program to regulate nonpoint source stormwater runoff at the municipal level. It is administered by the US Environmental Protection Agency (EPA) and Pennsylvania Department of Environmental Protection (DEP) under the National Pollutant Discharge Elimination System (NPDES). Operators of a small MS4 must obtain an NPDES permit and develop and implement a stormwater management plan (SWMP) according to the details of their specific permit. Mandatory elements of the SWMP include six (6) Minimum Control Measures (MCMs); each MCM has a number of associated BMPs.
An MS4 is a conveyance or system of conveyances that is:
- Not part of a publicly owned treatment works (sewage treatment plant)
- Owned by a state, city, town, village, or other public entity that discharges to waters of the US
- Designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.)
- Not a combined sewer
Stormwater runoff in urban areas is transported either through Municipal Separate Storm Sewer Systems (MS4s) or, in a Combined Sewer System (CSS). In an MS4, stormwater and wastewater are carried in separate systems – stormwater empties straight into rivers and streams, while wastewater is treated at a treatment plant before discharging into the river. In a CSS, which is typical of older cities, both wastewater and stormwater are carried in the same pipes to the treatment plant during normal weather conditions, but overflow into rivers during periods of heavy rain or snow melt.
- Phase I MS4 permits, issued in 1990, require areas with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges. There are approximately 750 Phase I MS4s in the US.
- Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. There are approximately 6,700 Phase II MS4s in the US.
Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.