Which item is NOT a suspended growth process?

• A. Trickling filters are fixed-film and not suspended growth.
• B. Activated sludge is suspended growth.
• C. Aerated lagoons are suspended growth.
• D. Sequential batch reactors are suspended growth.

Answer: (A)

This question tests the difference between fixed-film and suspended-growth secondary treatment. In fixed-film systems, microorganisms grow on surfaces of media and stay attached as a biofilm while wastewater passes over them. In suspended-growth systems, the biomass is dispersed in the wastewater as suspended solids or flocs, mixed with the water and later separated in a clarifier.

Trickling filters operate with a biofilm on media surfaces, so they are fixed-film rather than suspended-growth. Activated sludge keeps the biomass in suspension as flocs in an aerated mixture, which is the classic suspended-growth approach. Aerated lagoons also rely on having microorganisms in suspension within the lagoon water. Sequential batch reactors suspend the biomass in the mixed liquor during the reactor cycles, another suspended-growth process.

Therefore, trickling filters are not a suspended-growth process, since the biomass remains attached to the media rather than floating in the water.