Divalent Cation Competition in Donnan Dialysis for the Recovery of Ammonium

Donnan dialysis as a continuous ion exchange process is potentially an economic approach to remove and concentrate ammonium from aqueous streams. However, multivalent ions have more favorable Donnan equilibria within ion-exchange membranes and are ubiquitous in wastewater streams, where ammonium would be ideally recovered from. We investigate the effect of calcium on the ammonium exchange for sodium in both flat sheet and hollow fiber module geometries. A strong decrease in the ammonium flux was found compared to the monovalent-only ammonium-sodium exchange case. Our theoretical modelling indicated that this was caused mainly by two effects: calcium occupying approximately up to 50% of the membrane capacity due to favorable Donnan equilibrium and the slower diffusion of calcium. Increasing the flow velocity in either module increased liquid boundary layer mixing and consequently the competitive effect of divalent ions. This has large implications for the required membrane area and the ease of ammonium recovery, also in terms of a higher risk of carbonate scaling. Modelling work showed that the dominant transport resistance frequently was in the fluid compartments, although the membrane itself was often of a similar order of magnitude. Our results highlight the negative impact of divalent cations on ammonium recovery processes using Donnan dialysis and suggest that simultaneous recovery of calcium or the use of monovalent-selective ion-exchange membranes may be advantageous for the process.