Illicit Discharge

What is an illicit discharge?

An illicit discharge is defined as any discharge to the municipal separate storm sewer system (MS4) that is not composed entirely of stormwater, except for discharges allowed under a TPDES permit. Non-stormwater discharges can originate from direct connections to the storm drain system from business or commercial establishments (illicit connections), or indirectly as improper surface discharges to the storm drain system.

What Types of Discharges to the Storm Drain System are Prohibited?

The following table lists some of the prohibited and allowed discharges to the storm drain system.

Prohibited Discharges Allowed Discharges
Industrial process water Water utility line flushing
Commercial car wash wastewater Runoff from firefighting
Sanitary sewer flows Landscape irrigation
Wash-down of loading areas Uncontaminated ground water infiltration
Wastewater treatment plant effluent Discharges from potable water sources
Dumping of liquid waste Dechlorinated pool water
Chlorinated pool water Street wash water from cleaning/maintenance (excluding street sweeper water waste)
Water softener brine backwash Residential car washing
Used oil Air conditioning condensation


What are Indications of Illegal/Illicit Discharges to the Storm Drain System?

The following table lists some of the physical indicators that may signal an illicit discharge; physical indicators are those that can be observed or sensed during dry weather field screening and routine inspections.

Unusual color or cloudiness
Strong pungent or musty odor
Floating debris
Surface scum or foam
Oil sheen
Dead vegetation or inhibited growth
Dead animals
Stains on channel bottom or sides
Pipe corrosion


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