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

A trickling filter does not represent a suspended growth biological treatment system because it operates on a different principle. In a trickling filter system, wastewater is passed over a fixed structure that is covered with a biological layer of microorganisms. These microorganisms adhere to the media and treat the wastewater as it flows over and through it. This is a method of attached growth treatment rather than suspended growth, where microorganisms are kept in suspension within the liquid.

In contrast, systems like the SBR, activated sludge, and membrane bioreactor rely on microorganisms that are suspended in the wastewater throughout the treatment process. In these systems, the biological agents are mixed with the wastewater, and their growth takes place in the liquid phase, highlighting the suspended growth characteristics.