A voltage is
used to move salts through a membrane, leaving fresh water behind.
History of use
Commercially introduced in the early 1960s about ten years before reverse
Although originally was conceived as a seawater desalination process, it
has been generally used as a cost-effective way to desalt brackish water.
Electro dialysis depends on some general principles:
Most salts dissolved in water are ionic, meaning they contain a positive (cation)
or negative (anion) charge.
These ions are attracted to electrodes with an opposite electric charge
Semi-permeable membranes allow selective passage of either an anion or a
cation but not both.
Electrodes connected to battery are placed in container of saline water.
As electrical current is applied through the solution, the ions (Na+, Cl¯,
Ca²+, Carbonate ²¯ ) migrate to the electrode of opposite charge.
Anions (Cl¯, Carbonate ²¯) travel opposite of current and pass through an
anion semi-permeable membrane.
Once through this their path is blocked by a cation semi-permeable
membrane and is trapped.
The same event happens to the cations, (Na+, Ca²+) which pass through a
cation semi-permeable membrane and are trapped by an anion semi-permeable
The result is two flows of fresh water and three flows of concentrated
brine are formed.
Automatic operation of the plant, requiring very little maintenance and
supervision, except for membrane replacement.
Very effective process to remove and purify salt concentrate as a valuable
Applications in many commercial industries
o Cheese whey demineralization
o Tartaric wine stabilization
o De-acidification of fruit juices
Pretreatment is required to prevent materials which could harm or clog
Works most efficient in low-salinity applications
o Brackish water desalination
o Surface water desalination (streams)