Secondary Treatment

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Air Diffusers

Fine-bubble -Porous aluminum oxide or silica is fused into plates or domes.

Fine Bubble Air Diffuser
Fine Bubble Air Diffuser

Medium-bubble – Tubes are covered with perforated saran or fabric socks.

Coarse-bubble – Air is forced through orifices or around rigid or flexible disk.

Mechanical Aerators

Surface – Multi-bladed turbines or brushes churn air into the liquid.

Submerged – A sparger feeds air underneath a rotating impeller.

Radial Submersible Aerator
Surface Aerator

Oxygen Dissolution

Surface – Multi-bladed turbines in covered tanks churn oxygen into liquid.

Submerged – Compressors recirculate oxygen to spargers in covered tanks.

Porous Turbine – Oxygen goes into rotating porous disks submerged in open tanks.

Oxygen Generation

Cryogenic – compressed air is liquefied by cooling and then distilled into oxygen and nitrogen.

Cryogenic Oxygen Plant
Cryogenic Oxygen Plant

Pressure-Swing Adsorption (PSA) – Vessels of granular media adsorb nitrogen from compressed air, letting oxygen pass through.

PSA Plant
Pressure Swing Adsorption Plant

Sludge Collectors

Orifice Tube – A rotating tube with orifices picks up the activated sludge.

Multiple Tubes – Several separate tubes on a rotating collector pick up sludge and discharge to the center well.

Traveling Bridge – Some rectangular clarifiers have rolling bridges and pickup tubes with siphons or airlifts.

Chain and Flight – Some smaller rectangular tanks have sprocket-driven scrapers.

Biofilter Media

Rock or Slag – Chunks with low surface-to-volume ratio are used in shallow standard-rate trickling filters.

Plastic – Corrugated plastic sheets have a high surface area for high-rate treatment or nitrification.

plastic biofilter media
plastic biofilter media

Redwood – Wood slats are used in high-rate filters or activated biofilters.

Biofilter Distributors

Rotary – Influent flowing through orifices in the arms pushes them around.

Nozzles – A uniform flat spray pattern is achieved with pumps or siphons.

BIofilter Distributor
Biofilter Distributor

Rotating Biological Contactors


Motor-drive – Motors rotate each shaft of disks partially submerged in vats.

Air-drive – The shafts rotate as cups on the disk catch diffused air bubbles.

Rotating Biological Contactor
Drawing of an RBC