Many ion exchange processes exist for a variety of industrial water and wastewater applications.
The ion exchange process consumes large quantities of regeneration chemicals, such as brine, acid, and caustic materials that can present significant handling and disposal problems.
In recent years, membrane processes have been used increasingly for the production of “pure” waters from fresh water and seawater.
Membrane processes are also being applied in process for industrial uses and in waste water systems.
Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electrodeionization (EDI). These processes (with the exception of UF) reduce most ions; RO and UF systems also provide efficient reduction of non-ionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to reduce contaminants, and suspended solids.
Electrodeionization (EDI) processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields.
A negative elec-trode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes.
Alternating compartments carry concentrated brine and filtered permeate. Typically, 40-60% of dissolved ions are removed or rejected. Further improvement in water quality is obtained by staging (operation of stacks in series).
EDI processes do not remove particulate contaminants or weakly ionized contaminants, such as silica.
Usually upstream an EDI a Reverse Osmosis (RO) or and Ion Exchange Systems is installed therefore pretreament of those systems are applied to EDI process.
Below figure shows a simplified schematic of an ED process:
Figure: In electrodialysis and electrodialysis reversal ions pass through alternating cation and anion transfer membranes.