Application of homogeneous and heterogeneous cation-exchange membranes in coagulant recovery from water treatment plant residuals using Donnan membrane process

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 Al3+ could be concentrated to a high value of over 4500 mg/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 Al3+ 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 DAl–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.