Discharge limits for domestic wastewater facilities

If you have a wastewater permit, you must meet certain discharge standards and effluent limits.

Domestic wastewater facility discharge standards

Discharge standards can vary, but most of Washington's wastewater treatment plants (WWTPs) must meet technology-based effluent limits.

See full standards and alternative discharge limit details in Chapter 173-221 WAC.

Discharge standards

Pollution indicator Average monthly limits Limit per sample
Five-day biochemical oxygen demand (BOD) 30 mg/L 45mg/L (7 day average)
Total suspended solids (TSS) 30 mg/L 45mg/L (7 day average)
Fecal coliform 200/100 mL 400/100mL


Between 6 and 9* Between 6 and 9

*Average monthly limits for pH must be between six and nine, unless the permittee demonstrates all of the following:

  1. Inorganic chemicals are not added to the waste stream as part of the treatment process.
  2. Contributions from industrial sources do not cause the pH of the effluent to be less than 6.0 or greater than 9.0.
  3. The discharge does not cause water quality violations outside of an approved dilution (for example, mixing zone).

Alternative discharge standards exist for approved facilities

Some alternative discharge standards include:

  • Trickling filters constructed or expanded prior to Nov. 1984.
  • Principal waste stabilization ponds with less than 2mg/day design or that have received exemption.
  • Wastewater treatment plant with combined sewers on a case-by-case basis.
  • Wastewater treatment plant receiving minimally concentrated influent wastewater.