The generally accepted maximum filtration rate for a single-media rapid sand filter is approximately what? (Variant wording)

• A. 1-2 gpm/ft^2
• B. 4-5 gpm/ft^2
• C. 2-3 gpm/ft^2
• D. 3-4 gpm/ft^2

Answer: (C)

Water moves through the filter bed at a rate measured in gallons per minute per square foot, and this rate determines how much time solids have to be captured as water passes. In a single-media rapid sand filter, the sand bed is uniform, so there’s a limit to how fast water can flow before filtration becomes ineffective. If the rate gets too high, solids aren’t removed efficiently, effluent turbidity rises, and backwashing becomes more frequent and less effective. For a single-media RSF, the practical upper limit is about 2 to 3 gpm per square foot. This range provides enough contact time for filtration mechanisms (interception, straining, and some biological action) while keeping headloss and backwash requirements reasonable. Higher rates are more typical of multi-media filters, where deeper media can accommodate faster flow, but that advantage doesn’t apply to a single-media bed.