Membranes - Principles of Chemical Separations with Environmental Applications - page 260

Solution:
(a) The current, I , is calculated for 100 cells:

FQNE r
nE c

I =

96,500 A · s 100,000 L

day

0.066 g · eq · wt 1 day

86

0 . 50

=

g · eq · wt L , 400 s 100 0 . 90

= 41 A .

Since the normality equals the number of gram equivalent weights per liter,

normality 0.066.

The current density is therefore equal to (400)(normality)

=

= (400)(0.066) =

cm 2 .
Thus, (b) the membrane area is:

33 mA /

(41 A)(cm 2 1240 cm 2 Area = / 33 mA)(1000 mA / A) = .

(c) The power required is:

RI 2 5 ohms)(41 A) 2

Power = = (4 . = 7 . 6 kilowatts.

Applications and characteristics

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Electrodialysis is well-suited for demineralization of waters which contain 5 g L or less

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of dissolved solids. Water can be purified to about 500 mg L dissolved solids. The process

is often too expensive to demineralize seawater, but there are full-scale plants in operation
for the purpose of purifying brackish water.
Electrodialysis is also useful in secondary wastewater treatment. Since the membranes
are susceptible to scaling and organic fouling, the wastewater must first be treated to reduce
or eliminate scaling and fouling. Primary treatment can include coagulation, settling,
acid addition, filtration or activated-carbon adsorption. The energy consumption of the
process increases as scale deposits on the membrane (precipitated CaCO 3 , for example).
Membranes that have been fouled with organic compounds can be cleaned by treating them
with an enzyme detergent solution. Pretreatment and cleaning is very important because
about 40% of the total cost is for power consumption and membrane replacement. New
developments are being made to try to reduce the complications involved with cleaning
and replacing membranes [13]:

A recent innovation by a U.S. manufacturer of ED systems has significantly minimized the
need for pretreatment. This innovation, known as electrodialysis reversal (EDR), operates
on the same basic principle as the standard electrodialysis unit, except that both the product
and the brine cells are identical in construction. At a frequency of 3-4 times

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hr, the po-
larity of the electrodes is reversed and the flows are simultaneously switched by automatic