Which of the following is NOT an attached growth wastewater treatment system?

• A. Rotating Biological Contactor (RBC)
• B. Trickling Filter
• C. Biotower
• D. Stationary Biological Contactor (SBC)

Answer: (D)

Attached-growth processes rely on a biofilm that forms on a solid surface and remains attached as wastewater flows over or through that surface. The rotating biological contactor uses discs whose surfaces host a biofilm that rotates in contact with the wastewater; the trickling filter has a fixed bed of media with biofilm growing on the surfaces as liquid is distributed over it; and the biotower is a vertical tower packed with media where biofilm develops on those surfaces and treats the water as it passes.

The stationary biological contactor differs in that its design does not depend on a surface-attached biofilm in the same way. Its operation isn’t based on a stable biofilm growing on fixed media to treat the flow, so it isn’t categorized as an attached-growth system. Hence it is the option that is not an attached-growth wastewater treatment system.