A rotary drum filter is to be used to filter a lime slurry. The drum rotates at a rate of $0.2 \mathrm{rpm}$, and $30 %$ of the drum surface is submerged beneath the slurry. The filter operates at a constant $\Delta P$ of 10 psi. The slurry properties were determined from a lab test at a constant flow rate of $0.5 \mathrm{gpm}$ using $1 / 2 \mathrm{ft}^{2}$ of the filter medium. The test results indicated that the pressure drop rose to 2 psi after $10 \mathrm{~s}$ and to $10 \mathrm{psi}$ after $60 \mathrm{~s}$. Calculate the net filtration rate per unit area of the drum under these conditions, in $\mathrm{gpm} / \mathrm{ft}^{2}$.