In fixed-film reactors, biomass growth occurs on the media or substrate.

• A. In the air
• B. In the effluent
• C. Sludge blanket
• D. On the media or substrate

Answer: (D)

Biomass in fixed-film reactors grows as a biofilm attached to the packing media inside the reactor. Microorganisms attach to the surfaces and form a slimy biofilm that degrades pollutants as wastewater flows over the media. This attached growth is what keeps the biomass in place, providing a large surface area for contact with the wastewater. It contrasts with suspended-growth systems, where biomass is floating in the water. So the biomass is on the media or substrate, not in the air, not in the effluent, and not as a sludge blanket.