Abstract: Alum, used as a coagulant in water treatment, is recovered from clarifier sludge by adjusting the pH of the sludge downward to produce an aqueous clarifier sludge solution, and contacting the aqueous clarifier sludge solution with one side of a semi-permeable cation exchange membrane while contacting the other side of the membrane with an acidic sweep solution. By virtue of the Donnan co-ion exclusion phenomenon, aluminum ions, which are trivalent, pass readily through the membrane, in preference to divalent and monovalent cations, and consequently heavy metal carryover is relatively low. Organic matter carryover is substantially excluded, and consequently, the recovered alum can be reused without the potential for trihalomethane formation. The reactor is preferably in the form of a stack of spaced membranes, with the aqueous clarifier sludge solution and the acid sweep solution flowing through alternate spaces. The same process can be used for recovery of ferric iron coagulants.