An industrial user (IU) will be classified as a significant industrial user (SIU) if it meets any of the following:
Is subject to categorical pretreatment standards under 40 CFR 403.6 and 40 CFR chapter I, subchapter N;
Discharges an average of 25,000 gpd or more of process wastewater to the POTW (excluding sanitary, noncontact cooling, and boiler blowdown wastewater);
Contributes a process wastestream that makes up 5 percent or more of the average dry-weather hydraulic or organic capacity of the POTW treatment plant;
Is designated as such by the POTW on the basis that the IU has a reasonable potential for adversely affecting the POTW's operation or for violating any pretreatment standard or requirement [in accordance with 40 CFR 403.8(f)(6)].
A categorical industrial user (CIU) is an SIU [see first bullet point], but an SIU is not always a CIU. Categorical users have specific limits and requirements that are determined by the Federal government. States and local governments can develop requirements that are more restrictive, but not less restrictive.