Electrodialysis (ED) and Electrodialysis Reversal (EDR)
▶ Electrodialysis (ED) is a voltage-driven membrane process. An electrical potential is used to move salts through a membrane,
leaving fresh water behind as product water. ED was commercially introduced in the 1960s, about 10 years before reverse
osmosis (RO), Although ED was originally conceived as a seawater desalination process, it has generally been used for brackish
(ED Process and EDR Process)
① Left : Electrolysis Process, ② Right : Electrodialysis Process
※ CEM = Cation-Exchange Membrane, AEM = Anion-Exchange Membrane
ED depends on the following general principles:
o Most salts dissolved in water are ions, either positively charged (cations), or negatively charged (anions).
o Since like poles repel each other and unlike poles attract, the ions migrate toward the electrodes with an opposite electric charge
o Suitable membranes can be constructed to permit selective passage of either anions or cations
(ED System Process)
▶ In a saline solution, dissolved ions such as sodium (+) and chloride (-) migrate to the opposite electrodes passing through
selected membranes that either allow cations or anions to pass through (not both). Membranes are usually arranged in an
alternate pattern, with anion-selective membrane followed by a cation-selective membrane.
During this process, the salt content of the water channel is diluted, while concentrated solutions are formed at the electrodes.
Concentrated and diluted solutions are created in the spaces between the alternating membranes, and these spaces bound by
two membranes are called cells.? ED units consist of several hundred cells bound together with electrodes, and is referred to
as a stack. Feed water passes through all the cells simultaneously to provide a continuous flow of desalinated water and a
steady stream of concentrate (brine) from the stack.
▶ In the early 1970s, the Electrodialysis Reversal (EDR) process was introduced (Buros, 2000). An EDR unit operates on the same
general principle as an ED unit, except that both the product and concentrate channels are identical in construction.
At intervals of several times an hour, the polarity of the electrodes is reversed, causing ions to be attracted in the opposite direction
across the membranes.? Immediately following reversal, the product water is removed until the lines are flushed out and desired
water quality restored. The flush takes just a few minutes before resuming water production.
The reversal process is useful in breaking up and flushing out scales, slimes, and other deposits in the cells before they build up.
Flushing helps in reducing the problem of membrane fouling.
▶ Because of the inherent characteristics of the electrical process used in ED units, they are normally used to desalinate brackish
water, rather than high salinity water such as seawater. The few ED units that are located in Texas are those that are used in
low-salinity applications such as surface water desalination (e.g. Lake Granbury and Sherman).
(Introduction to Desalination Technologies; Texas Water Development)
Development of ED