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National Pollutant Discharge Elimination System (NPDES)

Secondary Treatment Standards

EPA establishes secondary treatment standards for publicly owned treatment works (POTWs), which are minimum, technology-based requirements for municipal wastewater treatment plants. These standards are reflected in terms of five-day biochemical oxygen demand (BOD5), total suspended solids (TSS) removal, and pH.

Secondary treatment standards also provide for special considerations regarding:

  • treatment plants that receive flows from combined sewers,
  • treatment plants that received industrial wastes,
  • waste stabilization ponds, and
  • treatment plants that receive less concentrated influent wastewater for combined and separate sewers.

In addition, secondary treatment standards provide alternative standards established on a case-by-case basis for treatment facilities considered equivalent to secondary treatment (trickling filters and waste stabilization ponds).

More detailed information related to secondary treatment standards can be found in the following publications: