NOTICE: Ecology's web applications will be unavailable Wednesday, Nov. 29 from 5:30 to 7:30pm, for scheduled maintenance.

Palouse agriculture slice


Facilities Eligible for Coverage

The primary function of a covered water treatment plant must be the treatment and distribution of potable or industrial water. Water treatment facilities that are part of a larger operation, such as a pulp and paper mill, are covered under the individual NPDES permit issued to the facility as a whole and are not eligible for coverage under this general permit.

The actual production rate of the water treatment plant must be at least 35,000 gallons per day of finished drinking water and/or industrial water as determined on an average monthly basis. “Actual production rate” is the amount of finished water that a treatment facility actually produces on any given day. To calculate the value of the actual production rate on an average monthly basis, add the value of each daily production rate during a calendar month, and divide the sum by the total number of days in the month.

Water treatment filtration processes must produce the wastewater. The wastewaters may include filter backwash, sedimentation or pre-sedimentation basin washdown, sedimentation and clarification wastewater, and filter-to-waste. Filtration processes may include, but are not limited to:

  • Sedimentation
  • Coagulation
  • Flocculation
  • Oxidation
  • Diatomaceous earth filtration
  • Green sand filtration
  • Membrane filtration
  • Rapid sand filtration
  • Pressure sand filtration

Microfiltration and ultrafiltration may also be eligible for coverage if the characteristics of their wastewater discharge are similar to conventional filter backwash.

The wastewater must be discharged to surface water. Surface waters include all lakes, rivers, ponds, streams, inland waters, salt waters, estuaries, wetlands, stormwater drainage systems, and all other surface waters and water courses within the jurisdiction of the state of Washington. Discharge to a settling pond or basin is also a discharge to surface water if an overflow from the pond or basin can flow to surface water.

Facilities Not Eligible for Coverage

The following operations are not covered by this general permit. These activities typically produce no discharge subject to a permit, or produce a discharge beyond the scope of this general permit.

  • Production of wastewater from ion exchange regeneration (water softening)
  • Production of wastewater from reverse osmosis
  • Discharges to sewage treatment facilities (POTWs)
  • Discharges to land only if no potential exists for runoff or overflow
  • Slow sand filtration