The fixed-bed adsorber series arrangement of Exercise 15.24 is to be used to treat 250gpm of water containing 0.185 mg/L of benzene (B) and 0.583 mg/L of m-xylene (X). However, because the two solutes may have considerably different breakthrough times, more than two operating beds in series may be needed. The adsorption isotherms are given in Exercise 15.22, where q is in mg/g and c is in mg/L. From laboratory measurements, the widths of the mass-transfer zones are estimated to be MTZB = 2.5 ft and MTZx = 4.8 ft. Once the cycle is established, how often must the carbon in the bed be replaced?