|
Electrodialysis
• A voltage is
used to move salts through a membrane, leaving fresh water behind.
History of use
• Commercially introduced in the early 1960’s about ten years before reverse
osmosis.
• 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.
Mechanism
• 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
membrane.
• The result is two flows of fresh water and three flows of concentrated
brine are formed.
Benefits
• 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
product.
• Applications in many commercial industries
o Cheese whey demineralization
o Tartaric wine stabilization
o De-acidification of fruit juices
Cons
• Pretreatment is required to prevent materials which could harm or clog
membranes.
• Works most efficient in low-salinity applications
o Brackish water desalination
o Surface water desalination (streams)
Sources
http://www.eurodia.com/html/ele.htm
http://www.ionics.com/technologies/ed/index.htm
http://www.serve.com/damien/home/solarweb/desal/electrodialysis.html
|