In a rapid gravity filtration test, the filter is 14x14 feet. The influent valve is closed and the water level drops 6 inches in 56 seconds. What is the filtration rate in GPM?

• A. 6.0
• B. 4.0
• C. 2.0
• D. 8.0

Answer: (B)

In a rapid gravity filtration test, the rate is found from how fast water level on the filter head drops over the bed surface, which translates the drop into gallons per minute per square foot.

The water level drops 6 inches, which is 0.5 feet, in 56 seconds (0.933 minutes). The volume corresponding to a 0.5-foot drop per square foot of bed is 0.5 ft × 7.48 gal/ft^3 = 3.74 gal/ft^2. Dividing by the time gives the rate per square foot: 3.74 gal/ft^2 ÷ 0.933 min ≈ 4.0 gpm/ft^2. So the filtration rate is about 4.0 gpm per square foot. For reference, the total gpm for the 14×14 ft filter would be 4.0 × 196 ≈ 784 gpm.