Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 213

a single pass through the electrodialysis stack. If the feed

water is high in salinity this is not normally practical. Feed

velocities have to be maintained above a certain minimum

value and this requirement is handicapped by the necessity

to provide also for a minimum residence time of the fluid

in the compartment. An increase of the current density may

lead to concentration polarization, a phenomenon that can

stop process operation.

Figure 13 is a view of an electrodialysis stack before

assembly, that shows the main components. Typically the

design is based on the configuration used in the plate and

frame filter press. The end frames have provisions for hold-

ing the anode or cathode and are usually made relatively

thick and rigid that pressure can be applied to hold the stack

components together. The inside surfaces of the end frames

are recessed to form an electrode-rinse compartment and pro-

visions are made for introducing and withdrawing the solu-

tions. Spacer frames with gaskets at the edges and ends are

placed between membranes to form the solution compart-

ments when ion exchange membranes and spacer frames are

clamped together.

These are various electrodialysis systems:

• The conventional batch-type which was the first
  commercially developed system.


• The continuous-type unidirectional electrodialysis
  system.

Both types of the electrodialysis systems have some

operation disadvantages and limitations. Ionic movement is

unidirectional. In such systems cations are moving toward a

fixed cathode and anion toward a fixed anode. Sealing form-

ing salts, colloidal particles or slimes, slightly electronega-

tive are accumulated on the surface of the anion-exchange

membrane causing membrane fouling.

• Electrodialysis reversal (EDR) is a system designed
  for continuous operation. The polarity of the elec-
  trodes is reversed 3 to 4 times per hour.

This operation system reverses the direction of ion movement

within the membrane stack, controlling thus scale formation

and fouling.

To day almost all electrodialysis cells are constructed

to operate with the reversal arrangement. Figure 14, gives a

flow diagram of a reversal electrodialysis system, in a vertical

arrangement.

The electrodialysis process is related to Faraday’s law

which states that the passage of 96,500 Ampere-s transfers

theoritically one gram-equivalent of salt. The needed current

for transferring a specific quantity of salt is given by the

equation:

I

Fm c

eN

 d.

(5)

If current efficiency e is 100%, 96,500 A-s will transfer one

gram-equivalent of sodium ions, or 23 g Na^ ^ , to the cathode

and one gram-equivalent of chloride ions, or 35.5 g Cl^ ^ to

the anode.

Electrodialysis is applied usually to brackish water

desalination up to 7 g/kg salinity. Sea water can be used also

to produce water around 0.50 g/kg or less but energy con-

sumption is very high. In reversal electrodialysis scaling is

very low, almost zero, but in conventional ED pretreatment

of feed water is necessary.

Energy input for electrodialysis depends on feed water

salinity and varies from 19 to 20 kWh/m^3 (72 to 76 kWh

per 1000 gallons) for seawater feed. For brackish waters the

required energy is 2.4 kWh/m^3 (9 kWh per 1000 gallons) for

Spacer

frames

Anode feed

solution

Electrode

rinse solu-

tion

Concentrating

Solution

Product water

Feed solu-

tion

Spacer

frames

Cation exchange

membrane

More membranea, spacer

Anion exchange frames, and cathode

membrane

A

N

O

D

E

FIGURE 13 View of part of an electrodialysis stack before assembly.

Component 1 is the one end frame, each of which holds an electrode

and has opening for feeding and withdrawal of the depleting, the con-

centrating and the electrode rinse solutions. Compartments 3 and 5

are spacer frames which have gaskets at the edges and ends and form

solution compartments with the membranes when they are clamped

together. Many membranes and spacers clamped together form an

electrodialysis unit.

raw water

feed

raw water

feed

• 
  

+ –

+

raw water

feed

raw water

feed

desalinated

water

desalinated

water

brine

brine

Na+

A

A

A A

A

A

C

C

C

C

C

C

C

C

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

FIGURE 14 Flow diagram of an electrodialysis reversal cell.

B represents the one-way operation and D the operation after

the reversing of the polarity. Cathode or anode are reversing and

become anode and cathode respectively. Ion movement reverses as

well and concentration compartment acts as dilution compartment.

Blowdown brine circulates in fresh water tubes and vice-versa.

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