Question

Which of the following principal types of reactor is related to Waste stabilization pond?

• A. Plug flow
• B. Complete-mix
• C. Packed bed
• D. Fluidized bed

Hint:

Natural treatment systems with wind mixing are generally modeled as complete-mix reactors.

Answer 1

The Correct Option is B

The classification of a reactor mainly comes down to how well its contents are mixed. A stabilization pond is a large, shallow, open body of wastewater exposed to wind and temperature-driven currents, both of which stir the pond's contents so effectively that concentration differences from one point to another largely disappear. This uniform-concentration behaviour is precisely what defines a complete-mix (or completely stirred) reactor, in contrast to a plug flow reactor where fluid moves through with minimal mixing along its length, or packed and fluidized bed reactors which depend on a solid medium that a pond simply does not have.

Therefore, the correct answer is Complete-mix.

Answer 2

One way to settle this is to picture what would happen to a drop of dye added at one corner of each reactor type.

1. Plug flow: A dye drop would move through the reactor largely intact, staying concentrated as it travels, since there is minimal mixing along the flow path; this is not what happens in an open pond.
2. Complete-mix: In a pond, wind and convection would spread that same drop of dye through the entire volume fairly quickly, until its concentration became roughly the same everywhere, which is the signature behaviour of a complete-mix reactor.
3. Packed bed: The dye would move through channels between fixed packing material, a physical setup that does not exist in a pond.
4. Fluidized bed: The dye would interact with particles suspended by an upward flow, again a mechanism absent from a stabilization pond.

Since the dye-drop behaviour of a stabilization pond matches rapid, wind-driven mixing to uniformity rather than orderly flow-through or particle-based mixing, it behaves as a complete-mix reactor.

Therefore, the correct answer is Complete-mix.