A rotary drum filter is used to separate a suspension of solid particles in a liquid. The filter drum has a diameter of 1 meter and a length of 2 meters. The drum rotates at a speed of 0.5 revolutions per minute. The suspension to be filtered has a concentration of 20% solids by weight and a viscosity of 0.01 Pa·s. The drum is initially filled with the suspension to a depth of 0.5 meters. a) Calculate the time required for the filter cake to build up to a thickness of 10 mm on the drum surface, assuming that the cake is a porous medium with a permeability of 5 × 10^-12 m^2.