| Over 1000 water treatment plants in the United States use alum, Al 2(SO4)3.14H2O, as a coagulant for efficient removal of particulate solids and colloids from surface water supplies. In the process, they generate over 2 million tons of aluminum-laden disposable Water Treatment Residuals (WTR) every day. The need for alum recovery from the WTR is well documented.; A non-pressure driven Donnan Membrane Process (DMP), also referred to as Donnan dialysis, was studied in this research for alum recovery. This process is based on the Donnan Exclusion Principle, according to which negatively charged cation-exchange membranes are impermeable to anions and likewise, positively charged anion-exchange membranes are impermeable to cations. Unlike pressure driven processes such as reverse osmosis, nanofiltration and ultrafiltration, DMP is free from surface fouling by Natural Organic Matter (NOM). The process is able to recover alum virtually free of dissolved organic carbon, which has a trihalomethane formation potential during the chlorination step in water treatment.; Residuals from the Allentown, PA water treatment plant were partially acidified to dissolve aluminum ions from the solid phase to the aqueous solution and the resultant slurry was used as the feed. It was separated from a sweep solution of acid using a cation-exchange membrane (homogeneous Nafion 117 membrane and heterogeneous Ionac MC 3470 membrane). The transfer of aluminum ions from feed solution to the sweep solution took place with simultaneous transfer of hydrogen ions in the opposite direction. At the end of a 24 hour operation, over 75% of aluminum ions was recovered in the sweep solution at a concentration of over 5000 mg/L, which was over 2.5 times the concentration in the feed solution, based on Donnan equilibrium conditions. In accordance with the principle of Donnan exclusion, the dissolved organic carbon concentration in the sweep solution was found to be negligible. The process selectively recovered aluminum ions over "heavy metal" ions, leading to a high purity alum product, which was as effective as commercial alum for coagulant recovery, as found in jar tests. The process was applied to ferric chloride based WTR generated and the recovery was approximately 75% in one day. |
