Which of the following is not a suspended growth biological treatment system?

A trickling filter is a fixed-film biological treatment system, which differs fundamentally from suspended growth systems. In a trickling filter, wastewater is distributed over a bed of rocks or plastic media where microorganisms grow on the surface. As the water trickles over the media, these organisms break down organic matter. This contrasts with suspended growth systems, where microorganisms are suspended in the wastewater and maintain a microbial population in the liquid itself.

In suspended growth systems like activated sludge, membrane bioreactors, and sequencing batch reactors, the microorganisms are kept in suspension and are part of the liquid phase. This allows for a different operational dynamic and treatment process compared to fixed-film systems like the trickling filter. Therefore, identifying the trickling filter as a non-suspended growth system is based on its principle of employing a solid medium for microbial growth rather than relying on suspended microorganisms in the treatment process.