Journal of Membrane Science 237 (2004) 131–144 Application of homogeneous and heterogeneous cation-exchange membranes in coagulant recovery from water treatment plant residuals using Donnan membrane process Prakhar Prakash, David Hoskins, Arup K. SenGupta ∗ Department of Civil and Environmental Engineering, 13 E. Packer Avenue, Lehigh University, Bethlehem, PA 18015, USA Received 18 July 2003; received in revised form 30 January 2004; accepted 12 March 2004 Abstract The Donnan membrane process (DMP) was applied for recovery of alum from water treatment plant residuals or sludge. Alum is widely used as a coagulant in drinking water treatment plants for efficient removals of suspended solids and colloidal particles. The application was studied for two different types of membrane viz. the homogeneous Nafion 117 membrane and the heterogeneous Ionac 3470 membrane. Homogeneous membranes are coherent ion exchanger gels while heterogeneous membranes consist of colloidal ion exchanger particles embedded in an inert binder. In the process, the recovered Al 3+ could be concentrated to a high value of over 4500mg/L (80% recovery) with Nafion 117, but the recovery was relatively low (25% recovery) with Ionac 3470. Since the Donnan membrane process is driven by the electrochemical potential gradient, the presence of high turbidity and natural organic matters in the sludge did not influence alum recovery and no noticeable membrane fouling was observed even after multiple runs for long hours of operation. The Al 3+ recovery profile led to identification of three zones of mass transport viz. kinetically driven linear zone, equilibrium driven saturation zone and osmosis driven dilution zone. All the three zones were observed in Nafion 117 during a 24 h experimental run. For Ionac 3470, only the linear zone was observed during the experimental period. Osmosis effect was not noted for Ionac 3470. The interdiffusion coefficient value of D Al–H was found to be one order of magnitude greater for Nafion 117. Within an ion exchange membrane, a diffusing ion hops from one charged site to the next and that constitutes the primary intramembrane ion transport mechanism. Scanning electron microphotograph and x-ray fluorescence showed clusters of non-conducting inert phases within the Ionac 3470 membrane containing no ionogenic groups. The lower interdiffusion coefficient for Ionac 3470 was attributed to its larger fraction of the non-conducting phase compared to Nafion 117. © 2004 Elsevier B.V. All rights reserved. Keywords: Donnan membrane process; Ion exchange membrane; Homogeneous and heterogeneous membrane; Alum recovery; Coupled transport of ions; Coagulant; Water treatment residuals 1. Introduction Ion-exchange membranes have been used industrially for separation processes such as in recycling and in water and wastewater treatment processes employing electrodial- ysis [1–3]. They have also been applied for recovery of spent acids by diffusion dialysis [4,5]. Several researchers have also utilized Donnan membrane process (DMP) using ion-exchange membranes as a pretreatment technique in wastewater and drinking water treatment, and in hydromet- allurgical operations [6–9]. In most of the DMP applications ∗ Corresponding author. Fax: +1-610-758-6405. E-mail address: arup.sengupta@lehigh.edu (A.K. SenGupta). discussed in literature, ionic contaminants (heavy metal ions and anions) have been concentrated from a low concentra- tion feed solution to a high concentration sweep solution, for further handling. Although information on applications of DMP in pure solutions is available in the open literature, no work is reported on the use of DMP to treat a sludge or slurry with high concentration of suspended solids or large organic molecules. Recent findings strongly suggest that a single-step Donnan membrane process can selectively re- cover alum, a widely used coagulant, from water treatment plant sludge, henceforth referred to as water treatment plant residuals (WTR) [10]. There are over a thousand (1000) drinking water treat- ment plants in USA which use alum, Al 2 (SO 4 ) 3 ·14H 2 O, as 0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.03.016