In a dual media filter, it is best to have ____ backwash rates because each type of media has a different ____.

• A. three, particle size and thus surface area
• B. two, specific gravity
• C. three, specific gravity
• D. two, particle size and thus surface area

Answer: (D)

Two backwash rates are needed in a dual-media filter because each layer has a different particle size and thus a different surface area, which changes how the bed expands and how easily fines are washed out. Backwashing should fluidize and clean each medium effectively, and a single rate can't accommodate the hydraulic behavior of both layers. Using a rate suited for the finer medium alone may not sufficiently expand or scour the coarser layer, while the opposite rate could damage or eject the finer media. The key factor driving this is particle size (and the resulting surface area and pore structure), not density, so tailoring the backwash to each medium ensures thorough cleaning of both layers.