Encyclopedia of Environmental Science and Engineering, Volume I and II

212 DESALINATION

amount of salt present. Usually, at least 50 to 100% excess of

each is used. This will represent a very high and impossible

cost for large-scale desalination of seawater.

In special cases, particularly for desalinating brackish

water, ion-exchange beds have been used, since only a

relatively smaller amount of chemicals is required to inter-

change ionically with the salt in the feed solution. Thus

water containing 1750 ppm of salt would be regarded as

non-potable, but it would require only about 5% of the

chemicals that seawater would require. On the other hand,

the energy cost for its desalination by evaporation or

RO would not be so greatly different from that required for

seawater because in these processes it is the water which is

being separated.

For emergency kits, packages of ion-exchange resins

have been made to have only a single use, e.g. by avia-

tors downed at sea. Seawater is passed through these small

“beds” to make a small amount of drinking water. Provision

for regeneration of the resins would be complicated and the

resins are discarded when charged with sodium and chlorine,

respectively.

The simplicity of ion exchange has attracted much effort

to finding less expensive methods of generation of the ion-

exchange resins. Carbon dioxide is a weak but cheap acid,

as lime is a cheap alkali, and special resins and processes

have been developed to use them and also to use the major

differences of ion exchange equilibria at 80C as compared

to those at room temperature.

Ion exchange processes depend upon good resins.

Excellent resins are available and they are not the limiting

factor. The major cost is that of the chemicals required for

the much less than stoichiometric replacement of salt by two

chemicals. In desalination the ion-exchange process is used

mainly for pre-treatment of brackish water for electrodialy-

sis or reverse osmosis.

Electrodialysis, ED

Electrodialysis is the transport of ions through ion-selective

membranes as a result of an electrical driving force. The

process takes advantage of the ability of these membranes

to discriminate between differently charged ions, allowing

for free passage to either cations or anions and being imper-

meable to ions of the opposite charge. Electrodialysis is a

desalination process of brackish water and, under certain

circumstances for seawater as well.

The electrical mechanism of ion removal is much more

complicated, and much cheaper, than ion exchange since it

uses electrical energy rather than chemical equivalence to

replace the two ionic changes of a molecule salt, since an

electric current assists greatly the dialysis or movement of

the ions through membranes permeable to the positive ions

and to the negative ions, respectively. A membrane which

is permeable to sodium ions forms the wall on the side of

a channel of flowing saline water and a membrane perme-

able to chlorine ions forms the wall on the other side. The

deionized water flows between the two membranes, and the

electric current may be regarded as flowing at right angles.

The other aqueous streams on the other side of the respec-

tive membranes may flow out counter-currently in the other

direction (Figure 12).

The electrodialysis process is performed in cells consist-

ing of many compartments formed alternatively by an anion

and a cation exchange membrane placed between an anode

and a cathode (Figure 12). Multicompartment electrodialy-

sis cells are usually termed as electrodialysis stacks. A mem-

brane pair is called a “cell pair” and consists of a:

• Cation transfer membrane


• Anion transfer membrane


• Demineralized water flow spacer


• Concentrate water flow spacer

A typical membrane stack contains 300 to 500 cell pairs,

depending on feed salinity. If the feed water has a low salin-

ity, it is possible to obtain an acceptable potable water in

O 2 Na+

Cl 2

H 2

H+

Na+

Na+

Cl Cl– Na+ Na+ Na+

• Cl–

Cl– Cl–

Cl–

OH–

feed water

+ –

brine

brine

brine

current

desalinated water

feed

water

electric DC

A C A CAC

SSSSS

1 2 3 4 5 6 7

FIGURE 12 Flow diagram of a multicomponent electrodialysis

cell, with 7 compartments showing the principle of the process

operation. The saline water feed is pumped through the compart-

ments, s, of the membrane stack and, when a direct current poten-

tial is applied, cations pass easily through the cation- permeable

membrane and are stopped when they reach an anion-permeable

membrane. Similarly anions have free passage through the anion-

permeable membrane and are stopped at the cation-permeable

membrane. The ion concentration increases in the alternate compart-

ments.3,5 Simultaneously the compartments between them become

depleted of ions.2,4 Hence two streams of water are extracted from the

electrodialysis stack: one stream with low ion concentration, which

is the product water, and one stream with high salt concentration,

which is the reject brine. Separate feed and brine blowdown is pro-

vided for the first and last compartments containing the anode and

cathode respectively, since corrosive oxygen, chloride and hydrogen

gases are released.

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