What is a significant industrial user (SIU)?

Two groups. First, every categorical industrial user: a facility in an industry for which the EPA has written national pretreatment standards. There are more than 30 such industries, among them metal finishing and electroplating, organic chemicals, pharmaceuticals, pulp and paper, iron and steel, and centralized waste treatment. Second, any other user that discharges an average of 25,000 gallons per day or more of process wastewater (sanitary, cooling and boiler water do not count), that makes up 5 percent or more of the plant’s dry-weather hydraulic or organic capacity, or that the plant designates because it could disrupt treatment or violate a standard.

A categorical user that never discharges more than 100 gallons per day of categorical wastewater can be classed as a non-significant categorical industrial user, with lighter duties.

The treatment plant, as control authority, issues each SIU a permit or equivalent control mechanism, valid for at most five years, with discharge limits and self-monitoring, reporting and notification duties. It inspects and samples each SIU at least once a year. The user reports its own monitoring at least twice a year, must report a violation within 24 hours of becoming aware of it and resample within 30 days, and must warn the plant of slug discharges. Each year the plant publishes the users that were in significant noncompliance and reports on its programme to the state.

Pretreatment is where new pollutants are controlled first, because a treatment plant cannot remove what it was not designed for. PFAS is the current example. Michigan required every plant with a pretreatment programme to sample its probable industrial sources, and the results show which industries send PFOS to the sewer, with landfills and metal finishers the most frequent. Klarifi publishes the results by plant and by industry.