AWWA WQTC57014

AWWA WQTC57014 Recovery of Alum From Water Treatment Plant Residuals Using Nafion 117 Membrane in Donnan Dialysis Process
Conference Proceeding by American Water Works Association, 11/01/2002

Prakash, Prakhar; SenGupta, Arup K.

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Water treatment plants use alum, Al2(SO4) 3.14H2O, as a coagulant, alum being converted into insoluble aluminum hydroxide, Al(OH)3 ,during the coagulation process. It is recognized that the disposal of Al3+-laden solids from water treatment plants will receive a close scrutiny in coming years and therefore, the prospect of alum recovery from Water Treatment Residuals (WTRs) has merits. Several processes for alum recovery have been considered in the past, each having its own limitation. Donnan Dialysis promises to be an appropriate one, as it can concentrate Al3+ into a small volume of recovered sweep solution and simultaneously reject Natural Organic Matter (NOMs) or Dissolved Organic Carbon (DOC) in accordance with the principle of Donnan exclusion. This allows recovered Al3+ to be free from trihalomethane formation potential and can therefore be used in a plant. In this study, feed solution of WTRs exchanged Al3+ ions with H+ ions from a sweep solution of H2SO4, across Nafion 117, a cation exchange membrane. It was possible, using Nafion 117, to concentrate Al3+ from 74 moles/m3 to 250 moles/m3 in 24 hours with a total recovery of 75%. The Donnan exclusion of DOC was complete, resulting in a total DOC concentration of 2-3 mg/l in recovered Al3+ solution, as opposed to 200-225 mg/l in commonly recognized Acid Digestion Process. The fact that Al3+ forms the bulk of the cations in WTRs solution and also because cation exchange membranes bind more strongly to higher valence cations led to higher selectivity of trivalent Al3+ ions, as compared to bivalent "heavy metal" ions such as Zn2+ and Cu2+. Jar tests were performed using recovered solution on Lehigh River water and the turbidity dropped from 246 NTU to 10 NTU. Settling rate for post-treatment WTRs feed was high, suggesting that it could be easily dewatered. The post-treatment WTRs solution at a pH of 1.2-1.5 needed to be neutralized, before disposal. Includes 30 references, figures.