Abstract: With an electric current passing through an ion exchange membrane, electrolyte concentration on a desalting surface of the membrane is decreased due to concentration polarization (cf. Chapter 5) and reaches zero at limiting current density. At the limiting current density, voltage drop across the boundary layer increases drastically resulting in higher energy consumption. At overlimiting current density, water dissociation occurs (cf. Chapter 6) and it decreases current efficiency and gives rise to scale troubles. In Chapter 5, we introduced the following familiar limiting current density equation of an ion exchange membrane: δttFDCilim (8.1) in which C is electrolyte concentration in bulk, D is the diffusion constant of an electrolyte, F is the Faraday constant, t- and t are respectively transport number of counter-ions in the membrane and in a solution. δ is the thickness of a boundary layer.