I'm interested in understanding what types of permits my local wastewater treatment plant needed ...
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"There are two kinds of permits issued under NPDES: individual and general. Individual permits are issued for major point sources, such as a city’s wastewater treatment plant or a large industrial facility. General permits are issued for smaller, more common sources, such as city stormwater or industrial sources that are expected to discharge sufficiently low amounts so as not to disrupt a water body’s physical, chemical, or biological state."
The general permits also come in different variants, and the Municipal Separate Storm Sewer Systems (MS4) permit is a big one. For example, my municipality has a Phase II NPDES MS4 permit on the basis of having a separate sewer systems for stormwater and sanitary waste. This requires my municipality to do certain things like public outreach, Illicit Discharge Detection and Elimination (IDDE), and green infrastructure projects relating to impervious surfaces. Other places, like portions of Washington, D.C., have an older Combined Sewer system and an individual permit, which is stricter since those sewers can overflow during a storm and cause health risks.
I'm trying to remember the name of the industrial NPDES permit type that various industries in the city had, but I do remember anecdotally that the state authority did not inspect these very strictly, unfortunately.