Industries with a potential to violate pretreatment standards and regulations, or that are defined as categorical by the EPA, are called Significant Industrial Users (SIU) and are issued discharge permits. Permitted industries must adhere to discharge and/or pollutant limits and are subject to compliance monitoring and reporting requirements.

All industrial users must have an industrial discharge permit if they:

• discharge an average of 25,000 gallons per day or more of process wastewater to the wastewater treatment plant (excluding sanitary, noncontact cooling and boiler blowdown wastewater)
• contribute a process wastestream which makes up 5% or more of the average dry weather hydraulic or organic capacity of the wastewater treatment plant
• have a reasonable potential for adversely affecting the wastewater treatment plant's operation or for violating any pretreatment standard or requirement
• are defined as a categorical industry by federal regulation

SIUs must obtain an industrial discharge permit.