Electrodialysis softening: Feasibility study with mathematical model

Feasibility of an ED (electrodialysis) application to a drinking water softening process has been assessed using a mathematical model. A one-dimensional analytical model was built to describe the steady state ionic transport in an ED system by solving the Nernst-Planck equations with the electroneutrality assumption. The model simulates three ions (one anion and two cations with different charges) through a diluate boundary layer, cation exchange membrane, and concentration boundary layer. The model study found that the relatively low ionic concentration required for the effluent of the softening process causes the ED system to operate near the limiting current density. Near the limiting current density, the ionic transport is rate-limited at the diluate boundary layer, and it has been shown that the relative calcium flux can be accelerated by shrinking the diluate boundary layer. The preference of the cation exchange membrane to divalent ions is proven not to affect the relative calcium flux when the ED softening is operated near the limiting current density. © 2009 American Water Works Association Membrane Technology Conference Proceedings. All Rights Reserved.