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Water: Permitting (NPDES)

Secondary Treatment Standards


An important aspect of municipal wastewater is that it is amenable to biological treatment. The biological treatment component of a municipal treatment plant is termed secondary treatment and is usually preceded by simple settling (primary treatment). Secondary treatment standards are established by EPA for publicly owned treatment works (POTWs) and reflect the performance of secondary wastewater treatment plants. These technology-based regulations apply to all municipal wastewater treatment plants and represent the minimum level of effluent quality attainable by secondary treatment, as reflected in terms of 5-day biochemical oxygen demand (BOD5) and total suspended solids (TSS) removal.

The secondary treatment standards also provide for special considerations regarding combined sewers, industrial wastes, waste stabilization ponds, and less concentrated influent wastewater for combined and separate sewers. In addition, the secondary treatment standards also 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:

U.S. EPA NPDES Permit Writer's Manual (37 pp, 166K, About PDF) - Chapter 5, Section 5.2, provides information related to application of secondary treatment standards for municipal discharges.

Petition Requesting Secondary Treatment Nutrient Limits and Performance Data (14 pp, 667K, About PDF) - On December 14, 2012, EPA responded to a petition from the Natural Resources Defense Council (NRDC) requesting EPA to undertake rulemaking to establish numeric limits for nutrients in discharges from publicly owned treatment works as part of the Agency's secondary treatment regulations, and to publish information on the performance of secondary treatment at publicly owned treatment works.

Report on the Performance of Secondary Treatment Technology (4 pp, 81K, About PDF)  describes the performance achieved by secondary treatment facilities in treating for Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD5), and carbonaceous BOD5 (cBOD5). This report summarizes data reported in the EPA data bases: Permit Compliance System (PCS); and Integrated Compliance Information System -NPDES (ICIS -NPDES).

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