Significant Industrial Users (SIUs) are sources of industrial wastewater that, by federal definition, require a specific level of regulatory attention. This attention consists of periodic sampling and inspections of the SIU by the pretreatment program, as well as the issuance of specialized discharge permits to the SIU. These permits stipulate self-monitoring and reporting requirements as they apply to effluent limitations, sample collection, reporting schedules, record keeping, hazardous materials management and more. An SIU is defined as an industrial user that meets any one of the following criteria:

• Is subject to Categorical Pretreatment Standards under 40 CFR 403.6 (Exit to gpoaccess.gov) and 40 CFR Chapter I, Subchapter N. (List of categorically regulated processes)
• Discharges an average of 25,000 gallons per day or more of process wastewater to the city's sanitary sewer (excluding sanitary, non-contact cooling and boiler blow-down wastewater).
• Contributes a process waste stream that makes up 5 percent or more of the average dry weather hydraulic or organic capacity of the city's wastewater treatment plant.
• Is designated on the basis that the industrial user has a reasonable potential for adversely affecting the operation of the city's treatment plants or collections system, or for violating any pretreatment standards or requirement.