Encyclopedia of Environmental Science and Engineering, Volume I and II

214 DESALINATION

feed water having 2,500 ppm and about 3.9 kWh/m^3 (14.8

kWh per 1000 gallons) for feed water containing 4,500 ppm

total dissolved solids. The pumps operate at low pressure of

350 to 700 kPa (51 to 101 psi).

Electrodialysis has a variety of applications, in addition

to that of fresh-water production, i.e., for the treatment of

wastes, for the recovering of trace elements from effluents and

for concentration of various solutions, such as fruit juices.

To the end of 1993 world capacity of electrodialysis

plants producing more than 100 m^3 /d/unit was 1,070,005 m^3 /d

covering only 5.7% of the total desalination capacity, and

14.8% of the membrane processes.

Ion Exchange Membranes The ion-exchange membranes

are membranes selective either to cations or to the anions.

When a cation-selective membrane is immersed in an elec-

trolyte solution the cations in solution will enter in the resin

matrix and replace the cations present. The anions are pre-

vented from entering the matrix by the repulsion of the

anions affixed to the resin. The opposite phenomenon takes

place when an anion-selective membrane is immersed in an

electrolyte solution. Ion exchange membranes are essentially

ion-exchange resins cast in sheet form.

Membranes of synthetic resins have been developed

which are highly selective to the passage of positive ions,

and others have been developed which are highly selective

to the passage of negative ions. Hundreds of membranes

form passages in parallel between somewhat like a plate and

frame filter press., which is the electrodialysis stack. A stack

is presented in Figure 15.

Counter-ions within an ion exchange resin or membrane

are ions with a charge opposite to the charges affixed to the

membrane matrix. Co-ions are ions with the same charge as

the fixed charge of the matrix. Hence, ion selective mem-

branes are selectively permeable to counter-ions and selec-

tively impermeable to co-ions. The selectivity, which might be

expressed in terms of the transference number of counter-ions

in the membranes, is not generally restricted to all ions of the

charge. Membranes may well be more selective to some ionic

species than to others. There are commercially available mem-

branes that will selectively transport univalent ions in electro-

dialysis. With these membranes not only the concentrations

but also the composition of electrolyte solutions can be altered.

A membrane possessing specific selectivity between divalent

and univalent ions would be useful for removing sulfate from

a solution of chloride and sulfate, or to fractionate a mixture

of the ions into two solutions, each one containing only one

of the ions.

Transport numbers of ions in the ion exchange mem-

brane are different from those in the solutions on both sides

of the membrane. Because of the lower transport number of

ions in the solution, the number of ions transported to the

membrane surface by the electrical current is in deficiency to

the ions removed from that surface and transferred through

the membrane. The opposite phenomenon occurs on the

other side of the membrane. A greater number of ions are

transferred from the entering to the outgoing membrane sur-

face than can be carried away by the electrical current. Two

boundary layers with the opposite concentration gradients

are formed at both sides of the membrane. This tendency

for concentration and depletion is opposed by diffusion and

physical mixing. Hence, the thickness of the boundary layers

depend on hydrodynamic conditions and on the degree of

turbulence. However, there remain layers adjacent to the

membrane in which the solutions are completely static.

Increasing the current density has the effect of increas-

ing the concentration gradients at both surfaces of the

membrane and the point may be reached at which the con-

centration of ions at the entering side of the membrane

approaches zero. This is the limiting current density. When

the limiting current density is exceeded, hydrogen and

hydroxyl ions are transported through the solution and the

membrane causing changes of pH inside the membrane

and at the boundary layers of the solutions, as well as an

increase in the overall electrical resistance. The desired

ions participate with only a small amount in the transport.

The phenomenon is termed concentration polarization and

is the major limitation of the production rates achievable

by electrodialysis.

The increase in the pH of the solution associated with

polarization promotes the formation of alkaline precipitates

such as calcium carbonate and magnesium hydroxide on

the membrane surface. Membrane scaling causes additional

electrical and flow resistance, a decrease in electrodialysis

FIGURE 15 Photograph of an electrodialysis reversal stack

with 500 cell pairs in vertical arrangement. (Courtesy Ionics Inc.,

Watertown, Mass. USA.)

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