A filtration rate is the number of gallons that passes through one square foot of filter surface in one minute. Filtration rates are always expressed in "gpm per square-foot." Because of regulations, which limit maximum filtration rates, the answers to filtration rate problems will always be limited to single-digit numbers. The typical range of answers for filtration rate problems is 1.5 gpm/ft2 to 8 gpm/ft2, with the 2 gpm/ft2 to 4 gpm/ft2 range being the most probable range.
To calculate filtration rates, divide the gpm that passes through the filter by the square footage of the filter's surface. Always remember this, if a problem provides the the gpm of the entire plant, divide the plant gpm by the number of filters to obtain the gpm for the individual filters. You must use only the gpm for the individual filter to calculate the filtration rate for that filter. Below are two representative filtration rate practice problems.
1. A rapid-sand filter is 15 feet wide by 20 feet long. The flow through the filter is 36,000 gph. What is the filtration rate of the filter?
Answer: First, multiply 15 x 20 to obtain a filter surface area of 300 square-feet. Next, divide 36,000 by 60 to obtain a filter flow of 600 gpm. Lastly, divide 600 by 300 to obtain a filtration rate of 2 gpm/ft2.
2. A water treatment plant has eight 12-feet by 16-feet mixed media filters and has a plant flow of 6.6 MGD. What is the plant's filtration rate?
Answer: First, multiply 12 x 16 to obtain 192 square-feet as the square footage of one filter. Next, divide 6,600,000 by 1440 to obtain 4583 gpm as the flow of the entire plant. Then divide 4583 gpm by eight to obtain 573 gpm as the individual filter flow. Finally, divide 573 gpm by 192 to obtain a filtration rate of 2.98 gpm/ft2 ( 3 gpm/ft2, if rounded off).
NOTE: Filtration rates are sometimes called Filter Loading Rates.