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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

More Contact Info

Glossary of Terms

  • Headworks The door to the treatment process where contaminants larger than 3/4" in diameter are removed.
  • Screenhouse Grit removal and screening devices are installed here.
  • Clarifier A large, circular or rectangular tank that separates solids from the waste stream by settling or flotation.
  • Flights Slats that move along the bottom or surface of a rectangular clarifier to transport settled or floating material to removal points.
  • Skimmers A device that removes grease or scum from the surface of wastewater in a clarifier or tank.
  • Aeration Tank A tank normally associated with secondary treatment where air is injected to provide oxygen for microorganisms and to bring them into contact with wastewater through the mixing action.
  • Blowers Low pressure air compressors that inject air into the aeration tank.
  • Anoxic Selector The section of an aeration tank where oxygen is available in a combined form rather than as dissolved O2 to select out undesirable types of bacteria.
  • Anaerobic Digester A closed vessel where solids, or sludges, produced by other treatment processes are stabilized through a biological process which takes place in the absence of oxygen.
  • Thickener Equipment or process that increases the concentration of solids in sludge after gravity sedimentation.
  • Biosolids Treated wastewater sludges that meet stringent quality control standards.
  • Suspended Solids Insoluble solids that either float on the surface of or are in suspension in wastewater or other liquids.
  • Return Activated Sludge (RAS) Sludge that is recycled between secondary clarifiers and aeration tanks to maintain an active biomass for secondary treatment.
  • Thickened Waste Activated Sludge (TWAS) Solids removed from the activated sludge process and passed through a thickening process or thickening device.