Significant Industrial Users (SIUs) are classified as either Federal Categorical or Non-Categorical. Federal Categorical Industrial Users are also known as Categorical Industrial Users (CIUs). A Significant Industrial User is defined as:
If the preliminary information indicates that the industrial user is a potential SIU, a joint audit by Inspection and Engineering staff will be conducted. During the joint audit of the facility, inspectors gather the basic information, while the permitting engineers examine in detail the industrial unit processes, sources and volume of wastewater, pretreatment system, etc., required to create the Fact Sheet.
Certain industries fall into the categories regulated by the National Categorical Standards as defined by the Federal Codes of Regulation. The EPA has developed regulations for over twenty industrial categories. Examples of facilities classified as CIUs include: