What describes fixed film or attached growth reactors?

• A. Bacteria suspended freely in water
• B. Chemical coagulation
• C. Rapid mixing with air
• D. Wastewater brought to bacteria attached to substrate

Answer: (D)

Fixed film or attached growth reactors rely on microorganisms that cling to a solid surface to form a biofilm. Wastewater is brought into contact with a substrate so the bacteria attached to that surface can degrade organic matter as the water passes by. This is what makes the description of wastewater being brought to bacteria attached to a substrate the best fit. It contrasts with suspended growth, where microbes float freely in the water, not attached to any surface. The other ideas—chemical coagulation and rapid mixing with air—describe non-attached biological systems or physico-chemical processes, not fixed-film reactors. Fixed-film examples include trickling filters and packed-bed reactors, where the biomass stays on the surface and treats the water as it flows.