A conical tank in which the softening reactions take place as the water passes upward through the unit is described as what type of reactor?

• A. Clarifier, rather slowly
• B. Thickener cone, quite rapidly
• C. Softener reactor, rather slowly
• D. Pellet reactor, quite rapidly

Answer: (D)

The concept being tested is recognizing a reactor where chemical reactions occur rapidly due to contact with a solid media in an upflow, packed-like bed. In this conical tank, water moves upward through a bed of pellets, so the softening reactions take place on the surfaces of these pellets. This configuration creates a high surface-area-to-volume contact and good mixing, which accelerates the reactions, so the softening occurs quite rapidly. The idea is that the pellet-filled, upflow arrangement is designed to maximize contact and mass transfer, unlike a clarifier or thickener, which rely on gravity settling rather than chemical reaction, and unlike a generic softener reactor term that doesn’t emphasize the media-supported, rapid-contact nature.