In fixed film reactors, where do bacteria primarily grow?

• A. In the bulk liquid
• B. On the media or substrate
• C. In the effluent
• D. In sludge blanket

Answer: (B)

In fixed-film reactors, the microorganisms attach to surfaces and grow as a biofilm on the media. This attached community is the main site of biological activity, creating a stable layer that treats the wastewater as it flows over or through the media. The bulk liquid contains comparatively fewer free-floating bacteria because most cells are attached to the surface. The effluent is simply the water leaving the reactor, not where growth occurs. The term sludge blanket refers to suspended-growth systems, not the fixed-film setup, where growth is anchored to the media.