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 |
pH
|
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:
- Inorganic chemicals are not added to the waste stream as part of the treatment process.
- Contributions from industrial sources do not cause the pH of the effluent to be less than 6.0 or greater than 9.0.
- 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.