About 220 municipalities in Wisconsin are currently required to have a Municipal Separate Storm Sewer System (MS4) permit under NR 216, Wis. Adm. Code A MS4 permit is required for a municipality that meets one of the following criteria:

1. It is located within a federally-designated Urbanized Area
2. Its population equals 10,000 or more based on the latest decennial census; or
3. When the Department of Natural Resources designates the municipality for permit coverage in accordance with s. NR 216.025. The MS4 permits are effective for a period of up to five years, at which point the permits are updated and re-issued.

Thiensville MS4

2014 Annual Report