Pretreatment Standards for Existing Sources (PSES) apply to which category of dischargers?

• A. New Indirect dischargers
• B. Existing indirect dischargers
• C. Priority pollutants
• D. Appendix A to 40 CFR 423

Answer: (B)

PSES apply to existing indirect dischargers—facilities that were already discharging to a POTW through the municipal sewer system. The pretreatment program distinguishes between existing sources and new sources to tailor limits that protect POTWs from pass-through and interference. Existing indirect dischargers must meet prescribed limits under PSES, while new indirect dischargers are governed by Pretreatment Standards for New Sources (PNS). The other options don’t fit because priority pollutants are a pollutant category, and Appendix A to 40 CFR 423 is a regulatory list, not a discharger category.