AWWA ACE91097

Acidic extraction of aluminum hydroxide from alum sludges has two problems: it is nonselective (it recovers all other substances that are soluble under highly acidic conditions or exist as colloids) and concentrations of heavy metals could warrant problems if recycled through recovered alum. This paper describes an alternative, two-step process: selective sorption of Al(III) followed by desorption or regeneration of composite membranes. Results of laboratory experiments show that the two-step process can selectively recover Al(III) from the background of natural organic matter, manganese, and other heavy metals. Organics and heavy metals carry over into the recovered alum but were, on a percentage basis, always much lower than Al(III) recovery. An increase in Cd(II) concentration in the spiked sludge did not increase Cd(II) concentration in the recovered alum, indicating excellent rejection of heavy metals. The composite membrane can be regenerated very efficiently with dilute sulfuric acid. After 30 cycles of batch operation, there was practically no reduction in Al(III) uptake by the composite membrane, indicating durability of the membrane.