Secondary treatment is a technology-based standard and is the minimum treatment requirement for most municipal sewage treatment plants. Secondary treatment means treatment beyond the settling of solids, removal of 85% of the conventional pollutants (materials which deplete oxygen from the water: biochemical oxygen demand and suspended solids), and proper acidity control (pH). Municipal sewage treatment plants are sometimes referred to as publicly-owned treatment works or "POTWs," but will hereinafter be referred to as municipal sewage treatment plants. A municipal sewage treatment plant is a system, owned by a state or municipality, used in the treatment of liquid municipal sewage or liquid industrial wastes.

Do The Secondary Treatment Regulations Apply To Your Community?

Yes, all treatment facilities that discharge to waters of the United States must comply with the secondary treatment standards.

Actions Your Community Should Be Taking

If your municipal sewage treatment plant discharges directly to waters of the United States (such as streams, lakes, wetlands, etc.), it is required to have a National Pollutant Discharge Elimination System (NPDES) permit. For information on where to obtain a permit, consult the section on NPDES permitting on page 39 of this Guide. Your local municipal sewage treatment plant must ensure that any effluent discharged from the treatment plant meets the secondary treatment standards outlined below. Failure to meet the secondary treatment standards will constitute a violation of your municipal sewage treatment works' NPDES permit.

Specific Requirements of Secondary Treatment

To meet the secondary treatment standards, the quality of the effluent must be maintained at the following specific levels:

Biochemical Oxygen Demand (BOD)

• Cannot exceed 30 milligrams per liter (mg/l) over a 30-day average.
• Cannot exceed 45 mg/l over a 7-day average.

Suspended Solids

• Cannot exceed 30 mg/l over a 30-day average.
• Cannot exceed 45 mg/l over a 7-day average.

Acidity (pH)

Must be maintained between 6 and 9.

(Note: averages must be in consecutive days)

The secondary treatment regulations require that, at a minimum, the chosen treatment process be a stabilization pond. Beneficial uses of the receiving waters may necessitate that higher quality effluent be discharged. In this instance, some communities elect technologies with no discharge (such as seepage lagoons) due to the higher cost of advanced treatment. In certain cases, treatment facilities that discharge to marine waters may be eligible for less stringent secondary treatment requirements.

Local water treatment authorities should be aware that special considerations apply to combined sewers (sewers designed to transport both storm water and sanitary sewage), waste stabilization ponds, less concentrated influent wastewater, and industrial wastes.

For specific information regarding secondary treatment, contact your state NPDES permitting office or the Water Division of your U. S. EPA Regional Office (see the Resource Section at the back of this Guide).

Additional Information

The Secondary Treatment Regulations are published in Title 40 Code of Federal Regulations, Part 133.

(See Resource Section for wastewater contacts