An impurity in a water stream at a very small concentration is to be removed in a charcoal trickle bed filter. The filter is in a cylindrical column that is $2 \mathrm{ft}$ in diameter, and the bed is $4 \mathrm{ft}$ deep. The water is stored at a level that is $2 \mathrm{ft}$ above the top of the bed, and it trickles through by gravity flow. If the charcoal particles have a geometric surface area to volume ratio of $48 \mathrm{in} .^{-1}$ and they pack with a porosity of 0.45 , what is the flow rate of water through the column, in gpm?