Municipal Separate Storm Sewer System (MS4)

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Municipal Separate Storm Sewer System (or MS4) is a system of pipes, ditches, or gullies that is owned and/or operated by a municipal government entity for collecting and conveying stormwater and is separate from the collection system for municipal wastewater.

Pursuant to the Federal Water Pollution Control Act (Clean Water Act) section 402(p), storm water permits are required for discharges from an MS4 serving a population of 100,000 or more. The State Water Board’s Municipal Storm Water Program regulates storm water discharges from municipal separate storm sewer systems (MS4s) throughout California.

Learn more about MS4 permits from the State Water Board by clicking here.

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