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Electrodialysis

Chapter 1

1.1. OVERVIEW OF TECHNOLOGY

Industrial application of ion exchange membranes started at first in thefield of electrodialysis (ED) (cf. Preface) and it induced the development of thefundamental theory. This fact is easily understandable from those phenomenaexplained in Fundamentals, which are described by taking the ED into account.The development of the fundamental theory led to further development of theED technology. After that ion exchange membrane technology developed in thesucceeding technology such as electrodialysis reversal (EDR), bipolar membraneelectrodialysis (BP), electrodeionization (EDI), electrolysis (EL), fuel cell (FC)etc. describing in the succeeding chapters. Looking over these historical details,we notice that the ED becomes the fundamental technology and it is applied tothe succeeding technologies based on the ion exchange membranes. In thischapter, we discuss the main subjects such as the structure of electrodialyzer, EDprocess, practical application of ED etc.

1.2. ELECTRODIALYZER

1.2.1 Structure of an Electrodialyzer

The basic structure of the vertical sheet-flow type module consists ofstacks in which cation exchange membranes, anion exchange membranes, gas-kets (desalting cells and concentrating cells) are arranged alternately (Fig. 1.1).Fastening frames are put on both outsides of the stack which is fastened uptogether through cross bars setting in the frames. The deformation of the mem-branes is prevented by regulating hydrostatic pressure in the fastening frames.Inlet manifold slots and outlet manifold slots are prepared at the bottoms andheads of the gaskets, respectively. Spacers are incorporated with the gaskets toprevent the contact of cation exchange membranes with anion exchange mem-branes. Many stacks are arranged through the fastening frames. Electrode cellsare put on both ends of the electrodialyzer, which are fastened by a press puttingon the outsides of electrode cells (Fig. 1.2).

An electrolyte solution to be desalinated is supplied from solution feedingframes to entrance manifolds, flows through entrance slots, current passingportions and exit slots, and discharged from exit manifolds to the outside of thestack (Figs. 1.1 and 1.2). A concentrated solution is usually supplied to con-centrating cells in a circulating flow system, and discharged to the outside of thestack through an overflow extracting system.

DOI: 10.1016/S0927-5193(07)12015-5

dx.doi.org/10.1016/S0927-5193(07)12015-5.3d

−

kehge

kl

+

l

j fd

ai

bc

jf

l

Figure 1.1 Structure of a stack (filter-press type). a, Desalting cell; b, concentrating cell;c, manifold; d, slot; e, fastening frame; f, feeding frame; g, cation exchange membrane; h,anion exchange membrane; I, spacer; j, feeding solution; k, desalted solution; l, concen-trated solution (Azechi, 1980).

Anodechamber

Press (fix)

Feedingframe

Stack Stack

Fasteningframe

Feedingframe

Cathode chamber

Press (move)

Figure 1.2 Filter-press type electrodialyzer (Azechi, 1980).

Ion Exchange Membranes: Fundamentals and Applications322

Electrodialysis 323

Effective membrane area is in the range from less than 0.5m2 to aboutmaximum 2m2. In order to reduce energy consumption, it is desirable to de-crease the electric resistance of the membrane and gasket thickness. Gasketmaterial is selected from synthesized rubber, polyethylene, polypropylene,polyvinyl chloride and ethylene–vinyl acetate copolymer etc. The spacer is usu-ally incorporated with the gasket and a solution flows dispersing along thespacer net.

1.2.2 Parts of an Electrodialyzer

The electrodialyzer is composed of the parts as follows (Urabe and Doi,1978).

1.2.2.1 Fastening Frame

Maximum 2000 pairs of membranes are arranged between electrodes in anelectrodialyzer, so as to let disassembling and assembling works be easy. Themembrane array is divided further into several stacks consisting of 50–400 pairs.Fastening frames are fixed by bolts on both ends of the stack. The fasteningframe is usually served as a solution feeding frame, so that a desalting and aconcentrating solution are supplied to each gasket cell from the feeding frameincorporated in every stack. Material of the fastening frame is selected frompolyvinyl chloride, polypropylene and rubber-lining iron etc.

1.2.2.2 Solution Feeding Frame

A solution feeding frame is integrated for feeding solutions to each de-salting and concentrating cell. Manifold holes are prepared at correspondingpositions of the holes fitted in the gasket. Solutions are usually supplied throughthe manifolds to each stack, but as the case may be supplied to each plural stack.

1.2.2.3 Gasket

The shape of the gasket is presented in Fig. 1.3. A solution is suppliedfrom the inlet manifold put at the bottom, flows through the slot and is fed intothe current passing portion. Then the solution is discharged through the outletslot to the manifold fitted to the head. The gasket has the following functions:(1) prevents solution leakage from the inside to the outside of the electrodialyzer,(2) adjusts the distance between a cation exchange membrane and an anionexchange membrane, (3) prevents solution leakage between a desalting cell and aconcentrating cell occurring at slot sections. In order to prevent the solutionleakage, it is desirable to adopt a soft material for the gasket. On the other hand,it is desirable to adopt a hard and stable material to avoid dimension changesduring long-term operation. The material of the gasket is selected from rubber,ethylene–vinyl acetate copolymer, polyvinyl chloride, polyethylene etc. Thethickness of the gasket is in the range of 0.5–2.0mm.

Deformation of a membrane

Membrane

Gasket

Slot

Figure 1.4 Deformation of an ion exchange membrane (Urabe and Doi, 1978).

Manifold

Gasket

Spacer

Slot

Manifold

Figure 1.3 Gasket (Urabe and Doi, 1978).

(a) (b) (c)

Figure 1.5 Structure of slots (Urabe and Doi, 1978).


1.2.2.4 Slot

It is important to reduce the inside solution leakage (cf. Section 12.2 inFundamentals), which arises through pinholes and cracks in the membranes orthrough gaps due to the membrane deformation at the slot as shown in Fig. 1.4.In order to prevent these troubles, a lot of devices are proposed as exemplified inFig. 1.5 in which (a) decrease the width of the slot, (b) bend the slot, (c) insert thesupport in the slot.

(a) Expanded PVC

(c) Diagonal net (d) Mikoshiro texture (e) Honeycomb net

(b) Wave porous plate

Figure 1.6 Structure of spacers (Urabe and Doi, 1978).

Electrodialysis 325

1.2.2.5 Spacer

The function of a spacer is to keep the distance between the membranes.In addition, the spacer increases the limiting current density due to solutiondisturbance (cf. Sections 10.2 and 10.3 in Fundamentals). The spacer is selectedtaking account of the requirement such as; (1) low friction head loss, (2) lowelectric current screening effect, (3) easy air discharge, (4) less blocking of flow-pass caused by the precipitation of fine particles suspended in a feeding solution.The structures of a spacer are classified in Fig. 1.6 as (a) expanded polyvinylchloride, (b) wave porous plate, (c) diagonal net, (d) mikosiro texture and (e)honeycomb net.

1.2.2.6 Electrode and Electrode Chamber

Platinum plated titanium, graphite or magnetite is used for anode materialand stainless or iron is used for cathode material. The shape of electrodes isclassified into net, bar and flat. A partition is inserted between an electrodechamber and a stack for preventing the mixing of solutions. In an anode cham-ber, oxidizing substances such as chlorine gas evolve. An ion exchange mem-brane is easily deteriorated by contact with the oxidizing substances, so it isnecessary to use two sheets of partitions and put a buffer chamber between thetwo partitions. Material of the partition is an ion exchange membrane, an as-bestos sheet or a battery partition.


An acid solution is added into a cathode solution and the electrodialyzer isoperated under controlling pH of the cathode solution for preventing the pre-cipitation of magnesium hydroxides in the cathode chamber. A feeding solutionor a concentrated solution is supplied into the electrode chamber. The concen-tration of oxidizing substances in the anode solution is reduced by adding so-dium sulfite or sodium thiosulfate into the solution being discharged.Sometimes, a sodium sulfate solution is supplied to an anode and a cathodechamber, achieving the neutralization by mixing the effluent of both chambers.

1.2.2.7 Press

An oil pressure press is usually used adjusting the pressure to be 5–10 kgcm�2.

1.2.3 Requirements for Improving the Performance of an Electrodialyzer

In order to improve the performance of an electrodialyzer, membranecharacteristics should be naturally improved. At the same time, the circum-stances in an electrodialyzer in which the membranes work should be better.Here, we describe the definite problems lowering the circumstances in an elect-rodialyzer and requirements for improving the circumstances and performanceof an electrodialyzer (Urabe et al., 1978).

1.2.3.1 Solution Velocity Distribution between Desalting Cells

In an electrodialyzer, ion exchange membranes and desalting and con-centrating cells are arranged alternately and a solution is supplied into desaltingcells. In this flow system, the solution velocity distribution in desalting cells doesnot become uniform. This phenomenon causes the concentration distributionand current density distribution in the electrodialyzer, and gives rise to thedecrease of the limiting current density of the electrodialyzer (cf. Sections 9.1,11.6 and 11.7 in Fundamentals). In order to operate the elctrodialyzer stably, itbecomes necessary to make the solution velocities between the desalting cellsuniform.

1.2.3.2 Solution Leakage in an Electrodialyzer

The dimensions of all parts of an electrodialyzer are not always consistentwith the values in the specifications. Small pinholes can open in an elect-rodialyzer because the strength of ion exchange membranes is relatively low.Gaps may occur between the materials composing the electrodialyzer in theassembly works of an electrodialyzer. If a pressure difference between the de-salting cells and concentrating cells exists in these circumstances, solutions leakthrough the membranes and lower the performance of the electrodialyzer(cf. Section 12.2 in Fundamentals). In order to avoid these troubles, we have toremove the pinholes and gaps in the electrodialyzer and control the pressuredifference between desalting cells and concentrating cells.

Electrodialysis 327

1.2.3.3 Distance between the Membranes

Decrease of the distance between the membranes brings about the de-crease of electrical resistance and energy consumption. On the other hand, itbrings about the increase of friction loss of solution flow, blocking of the ma-terials suspended in a feeding solution and the increase of pumping motivepower. Accordingly, it becomes necessary to realize the optimum distance be-tween the membranes. The optimum distance is decided further taking accountof electric resistance of ion exchange membranes and that of electrolyte solutionin desalting and concentrating cells.

1.2.3.4 Spacer

Main functions of a spacer in to create space between a cation exchangemembrane and an anion exchange membrane. When solution velocity and theReynolds number are decreased, hydrodynamic pattern exhibits laminar flow,which means that disturbing effect of the spacer is low. In order to increase thelimiting current density, turbulent flow should be induced by increasing theReynolds number (cf. Sections 10.3.4 and 10.5 in Fundamentals).

1.2.3.5 Electric Current Leakage

A part of an electric current flows through slots and manifolds causingineffective current leakage. Current leakage is increased by the increases of thenumbers of cell pairs integrated in a stack and the increase of sectional area ofslots and manifolds (cf. Section 12.1 in Fundamentals). These events, however,related with the solution velocity distribution between the cells described inSection 1.2.3.1.

1.2.3.6 Simplicity of Structure of an Electrodialyzer

Disassembling and assembling work is peculiar characteristics in operat-ing an electrodialyzer (cf. Section 1.5.3 in Applications). Excellent durability ofion exchange membranes is owing to careful treatment in this work. So, thesimplicity of the structure is a requirement for performing this work.

1.3. ELECTRODIALYSIS PROCESS

The ED process had been explained in detail in several articles (Mintz,1963; Shaffer and Mintz, 1966; Itoi et al., 1978; Yawataya, 1986; Tanaka, 1993).The following is overall description of the process.

1.3.1 One-Pass Flow Process

An electrolyte solution is fed to an electrodialyzer and desalted solution isdischarged to the outside of the process (Fig. 1.7). When the concentration ofthe feeding solution is invariable, the performance of the system becomes stable.Joining the process in Fig. 1.7 to the succeeding process, a one-pass flow

Feeding solutionConcentrated discharge

Feeding solution

Desalted solution

Concentrating cell

Desalting cell

Ion-exchangemembrane

Pump

Figure 1.7 One-pass flow process (Tanaka, 1993).

ab

l

x

x+dx

Cout

Cin

C-dC

C

0

Figure 1.8 Electrolyte concentration change in a desalting cell (Tanaka, 1993).


multiple continuous system suitable for a large-scale plant is formed. The de-salination in Fig. 1.7 is proceeded as below.

In the desalting cell shown in Fig. 1.8, a, b and l are the flow-pass depth(distance between membranes), the flow-pass width and the flow-pass length,respectively. Cin and Cout are the electrolyte concentration at the inlet and theoutlet of the desalting cell. Passing an electric current across the membranes,ions in the desalting cell are transferred toward the concentrating cell. Assumingthe transport of water to be negligible across the membranes under an electriccurrent passing, the material balance between at x and x+dx in Fig. 1.8 is

Electrodialysis 329

indicated by the following equation, including current density i, linear velocity ina desalting cell u, current efficiency Z and electrolyte concentration at x distantfrom the inlet of a desalting cell C.

�au1

CdC ¼

ZF

i

Cdx (1.1)

Voltage applied to a membrane pair (cell voltage) consists of a membrane po-tential and Ohmic loss of a cation exchange membrane, an anion exchangemembrane, a desalting cell and a concentrating cell. In the desalting process,Ohmic loss of a desalting cell iRde (Rde: electric resistance of desalting cell) isdominant in the cell voltage and voltage difference between electrodes is inde-pendent of x (cf. Section 9.1 in Fundamentals). Accordingly, iRde in the desalt-ing cell is estimated to be invariable in the range of x ¼ 0�l in Fig. 1.8. Further,Rde is inversely proportional to C, so that i/C is assumed to be nearly constantand we can integrate Eq. (1.1) as follows:

�ua

Z Cout

Cin

1

Cdx ¼

ZF

i

C

Z l

0

dx (1.2)

Solving Eq. (1.2)

Cout

Cin¼ exp �

lZaFu

i

C

� �(1.3)

Desalting ratio a is defined as follows:

a ¼ 1�Cout

Cin¼ 1� exp �

lZaFu

i

C

� �(1.4)

Here, we can adopt the average value i=C instead of i/C (Yawataya, 1986) in Eq.(1.4) as follows:

i

C¼

i

C¼

2i

Cin þ Cout(1.5)

From Eqs. (1.4) and (1.5)

i

C¼

2i

Cinð2� aÞ(1.6)

Substituting Eq. (1.6) into Eq. (1.4)

a ¼ 1� exp �2Zi

aFCinðu=lÞð2� aÞ

� �(1.7)

a is calculated using Eq. (1.7), assuming a ¼ 0.1 cm, Cin ¼ 0.05 eq dm�3,Z ¼ 0.90, and plotted against u/l taking i as parameter (Fig. 1.9). l vs. u iscomputed setting a ¼ 0.90 (Fig. 1.10) indicating u is proportional to l. Ina practical-scale sheet-flow electrodialyzer, flow-pass length l is from less than

0.00 0.01 0.02 0.03 0.04 0.05 0.060.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

�

u/l (s-1)

2.5

2.0

1.5

1.0

i=0.5A/dm2

Figure 1.9 Influence of linear velocity, flow-pass length and current density to desaltingratio.

0 1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30

35

40

u (c

m s

-1)

l (m)

2.5

2.0

1.5

1.0

i=0.5A/dm2

Figure 1.10 Flow-pass length and linear velocity in a desalting cell.


Electrodialysis 331

1–2m, and linear velocities in desalting cells u are 3–10 cm s�1. In a tortoise-flowtype electrodialyzer, however, l and u are larger than those in the sheet-flow type(cf. Section 2.2 in Application).

1.3.2 Batch Process

In Fig. 1.11, the feeding solution is prepared in the circulation tank at first.Next, open valve V1, close valve V2 and circulate the solution between the tankand the electrodialyzer. The solution is electrodialyzed applying constant volt-age until electrolyte concentration of a desalted solution attains a definite value,and then the desalted solution is discharged. The process mentioned above isrepeated periodically. This system is usually adopted in a small-scale plant andthe desalination is proceeded as follows.

A definite volume V of electrolyte solution is assumed to be circulatedbetween the circulating tank and the electrodialyzer and it is electrodialyzedapplying a constant voltage between the electrodes. An electric current I andelectrolyte concentration C of the solution decrease with elapsed time t, butI/C does not change with t, because Ohmic loss IRdil is dominant in a cell volt-age and roughly inversely proportional to C. Setting numbers of cell pairs in-tegrated in the electrodialyzer M, the electrolyte concentration in a feedingsolution CF and in a desalted product solution CP, and an operating time inan unit batch cycle t, and assuming the transport of water to be negligible across


Feeding solution Desalted solution

Circulation tank

V1

V2

Figure 1.11 Batch process (Tanaka, 1993).


the membranes under an electric current passing, the material balance in adesalting cell in this batch system (Fig. 1.11) is expressed by the followingequation.

�V

Z CP

CF

dC

C¼

MZF

1

C

Z t

0

dt (1.8)

Integrating Eq. (1.8)

CP

CF¼ exp �

MZFV

I

Ct

� �(1.9)

Expressing an electric current I as I ¼ bli and the solution volume V as V ¼ QPt,

(Qp is product solution volume, water transport across the membranes is as-sumed to be neglected) in Eq. (1.9), the desalting ratio a in the batch system isintroduced as follows:

a ¼ 1�CF

CP¼ 1� exp �

MblZFQP

i

C

� �(1.10)

Accordingly, numbers of cell pairs M integrated in the electrodialyzer is

M ¼FQP lnðCF=CPÞ

blZði=CÞ(1.11)

Here, we estimate M in the following case:QP ¼ 1m3 h�1

¼ 106/3600 cm3 s�1, CF/CP ¼ 10, a ¼ 1�CP/CF ¼ 0.90,b ¼ 50 cm, l ¼ 50 cm, Z ¼ 0.90, i ¼ 0:01 A cm�2; CF ¼ 5� 10�5 eq cm�3,F ¼ 96,500C eq�1.

From Eq. (1.6), i/C is calculated as:

i

C¼

2i

Cinð2� aÞ¼ 364

Substituting these values into Eq. (1.11), we obtain M ¼ 75 pairs.

1.3.3 Partially Circulation (Feed and Bleed) Process

An electrodialyzer is operated at constant current density supplying adefinite amount of solution and circulating a part of feeding solution (Fig. 1.12).Joining Fig. 1.12 to the succeeding process, a multiple partially circulation sys-tem (Fig. 1.13) suitable for a middle-scale plant is formed. The desalination inFig. 1.13 is achieved as below.

Assuming the linear velocity in desalting cells u, current efficiency Z andi/C to be constant in each electrodialyzer, Cout/Cin in each electrodialyzer isexpressed using Eq. (1.3) as follows:

ðCoutÞ1

ðCinÞ1¼

ðCoutÞ2

ðCinÞ2¼ � � � ¼

ðCoutÞn

ðCinÞn¼

Cout

Cin¼ b (1.12)


Feeding solution

Desalted solution

Figure 1.12 Partially circulation process (Tanaka, 1993).

QR-Q QR-Q QR-Q

(Cout)1 (Cout)2 (Cout)n

QF=Q QR Q QR QR

QR QR

Q QP=Q

(Cin)1 (Cin)2 (Cin)n CP=(Cout)n

QR

(Cout)1 (Cout)2 (Cout)n

1 2 n

Figure 1.13 Multiple partially circulation process (Tanaka, 1993).

Electrodialysis 333

From the material balance in Fig. 1.13

CFQF þ ðCoutÞ1ðQR �QÞ ¼ ðCinÞ1QR

ðCoutÞ1Qþ ðCoutÞ2ðQR �QÞ ¼ ðCinÞ2QR

..

.

ðCoutÞn�1Qþ ðCoutÞnðQR �QÞ ¼ ðCinÞnQR

(1.13)

Substituting Eq. (1.12) to Eq. (1.13)

CFQF ¼ ðCinÞ1fQR � bðQR �QÞg

ðCinÞ1 ¼ðCinÞ2

bQfQR � bðQR �QÞg

..

.

ðCinÞn�1 ¼ðCinÞn

bQfQR � bðQR �QÞg

(1.14)


Putting Eq. (1.14) together

CFQF ¼ðCinÞn

ðbQÞn�1

fQR � bðQR �QÞgn (1.15)

Substituting QF ¼ QP ¼ Q, CP ¼ (Cout)n (water transport across the membranesis neglected) and Eq. (1.12) into Eq. (1.15), and rearranging the equation

CF

CP¼

QR

QP

Cin

Cout� 1

� �þ 1

� �n

(1.16)

Desalting ratio a of the process is introduced as follows from Eq. (1.16).

a ¼ 1�CP

CF¼ 1�

QR

QP

Cin

Cout� 1

� �þ 1

� ��n

(1.17)

In one-path flow system, QR ¼ QP holds, so Eq. (1.17) becomes

CF

CP¼

Cin

Cout

� �n

(1.18)

A large-scale plant is realized by assembling a multi-stage multiple partiallycirculation process arranging in each stage N units of electrodialyzer incorpo-rated with M cell pairs as indicated in Fig. 1.14. The amount of solution QR

circulating in each stage is

QR ¼ abuMN (1.19)

QR QR(Cin)1 (Cout)1 (Cin)2

QR(Cout)2 (Cin)N (Cout)N

QF=Q Q Q QCF CP=(Cout)N

QR-Q QR-Q QR-Q

N

2

1

N

2

1

N

2

1

Figure 1.14 Multi-stage multiple partially circulation process (Tanaka, 1993).

Table 1.1 Arrangement of electrodialyzers in a multi-stage multiple partially circulationprocess

n QR (cm3 s�1) N n�N

1 514,530 25.7 1� 262 123,617 6.2 2� 63 65,999 3.3 3� 34 44,494 2.2 4� 25 33,438 1.7 5� 26 26,744 1.3 6� 1

Electrodialysis 335

We estimate the numbers and arrangement of electrodialyzer in a multi-stagemultiple partially circulation process (Fig. 1.14) putting the following param-eters: a ¼ 0.1 cm, b ¼ 100 cm, l ¼ 100 cm, u ¼ 5 cm s�1, i/C ¼ 364A cm eq �1,CF/CP ¼ 10, QP ¼ 200m3 h�1

¼ 200 � 106/3600 cm3 s�1, Z ¼ 0.9, M ¼ 400pairs, F ¼ 96,500Ceq�1. At first, Cin/Cout is computed as follows:

Cin

Cout¼ exp

lZaFu

i

C

� �¼ exp

100� 0:90� 364

0:10� 96500� 5

� �¼ 1:9718

Using Eq. (1.16), CF/CP is

CF

CP¼

QR

QP

Cin

Cout� 1

� �þ 1

� �n

¼QR

200� 106=3600ð1:9718� 1Þ þ 1

� �n

¼ 10

Accordingly, QR is expressed as follows:

QR ¼ ð101=n � 1Þ200� 106=3600

0:9718cm3 s�1 (1)

From Eq. (1.19), numbers of cell pairs per unit stage are

MN ¼ 400N ¼QR

abu¼

QR

0:1� 100� 5

So, we have N as follows:

N ¼QR

20000(2)

Changing the values of n, QR, N and nM are computed as indicated in Table 1.1using Eqs. (1) and (2).

Concentrated

Feeding solution

Desalted solutionC ′out

C ′′

q ′′

C ′in

q0

q0

C0

q ′

q ′

Figure 1.15 Concentration or separation process.


1.3.4 Concentration Process

Fig. 1.15 gives a single-stage concentration unit process. The output of amulti-stage multiple process X is expressed by the following equation.

X ¼i

F

� �bl�ZMnN (1.20)

Here, we calculate numbers of electrodialyzers in the process for concentratingseawater by ED and crystallizing NaCl by evaporation. Putting as NaCl outputin the evaporation process: 200,000 t y�1, operating time of electrodialyzers:8000 h y�1, NaCl yield rate in the evaporation process: 0.97 and NaCl molecularweight: 58.5, we obtain:

X ¼200000

8000� 3600� 0:97� 58:5¼ 122:38 eq=s

Substituting X ¼ 122.38 eq s�1, i ¼ 0.03A cm�2, b ¼ 100 cm, l ¼ 100 cm,e ¼ 0.92, Z ¼ 0.90, M ¼ 2000 pairs and F ¼ 96,500C eq�1 into Eq. (1.20),nN ¼ 23.8 is obtained. Accordingly, the numbers of the electrodialyzers in thismulti-stage multiple processes are known to be 24.

Electrolyte concentration in a concentrated solution C00 is expressed by thefollowing equation introduced from the overall mass transport equation(cf. Section 6.1 in Fundamentals).

C00 ¼1

2r

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA2 þ 4rB

q� A

� �

A ¼ fi þ m� rC0

B ¼ li þ mC0

(1.21)

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

C"

(eq

dm-3

)

i (A dm-2)

9 7 5 3 1

C' (eq dm-3)

Figure 1.16 Dependence of C00 on i and C0.

Electrodialysis 337

The overall transport number l, the overall solute permeability m and the over-all electro-osmotic permeability f are expressed by the following empiricalequations of the overall hydraulic conductivity r (cf. Section 6.1 in Fundamen-tals).

l ¼ l1 þ l2r l1 ¼ 9:208� 10�6 l2 ¼ 1:914� 10�3 (1)

m ¼ mr m ¼ 2:005� 10�4 (2)

f ¼ n1r0:2 þ n2r n1 ¼ 3:768� 10�3 n2 ¼ �1:019� 10�2 (3)

Here, we calculate l, m and f by substituting r ¼ 1� 10�2 cm4 eq�1 s�1 intoEqs. (1)–(3) as; l ¼ 9.399� 10�6 eqC�1, m ¼ 2.005� 10�6 cm s�1,f ¼ 1.398� 10�3 cm3C�1. Dependence of C00 on i for this membrane pairs iscomputed as shown in Fig. 1.16 by substituting current density i, electrolyteconcentration in a feeding solution C0, l, m, f and r into Eq. (1.21).

1.3.5 Separation Process

An ion exchange membrane shows the permselectivity between ionshaving the same charged sign, which is defined by the permselectivity coefficientEq. (1.22) for ion A against ion B, TB

A (cf. Section 3.2 in Fundamentals).

TBA ¼

ðC00B=C

00AÞ

ðC0B=C

0AÞ

(1.22)


C00i ; is concentration (eq dm�3) of ion i in a concentrating cell and it is assumed

to be invariable in the cell. Concentration of ion i in a desalting cell C0i is the

average of the values at the inlet and the outlet as follow:

C0i ¼

1

2ðC0

i;in þ C0i;outÞ (1.23)

C0i;in and C0

i;out are concentrations of ion i at the inlet and outlet of a desaltingcell. Ion A is separated from ion B by applying the permselectability of themembrane in the separation process indicated in Fig. 1.15.

Here, we define the separation factor of ion B against ion A, in a desaltedsolution S

0BA and in a concentrated solution S

00BA by the following equations.

S0BA ¼

ðC0B;out=C

0A;outÞ

ðC0B=C

0AÞ

(1.24)

S00BA ¼

ðC00B=C

00AÞ

ðC0B=C

0AÞ

(1.25)

C0A; C

0B: Concentration of ion A and ion B in a feeding solution.Desalting ratio of ion i (ai) in Fig. 1.15 is defined as:

Ci;out ¼ ð1� aiÞC0i (1.26)

The material balance of ion i in Fig. 1.15 is shown by the following equationsassuming q0 � q00 hold.

C0i q

0 ¼ C0i;outq

0 þ C00i q

00 (1.27)

C0i;inq

0 ¼ C0i;outq

0 þ C00i q

00 (1.28)

q0 is the amount of feeding solution to the process, q0 the amount of a desaltingsolution being supplied to the electrodialyzer and q00 the amount of a concen-trating solution flowing out from the electrodialyzer.

Following equations are introduced from Eqs. (1.22)–(1.28):

S00BA ¼

1� aBð1� ðq0=2q0ÞÞ

1� aAð1� ðq0=2q0ÞÞ(1.29)

S00BA ¼ S

0BAT

BA ¼

1� aBð1� ðq0=2q0ÞÞ

1� aAð1� ðq0=2q0ÞÞTB

A (1.30)

When q0 � q0 hold, Eqs. (1.29) and (1.30) become

S0BA ¼

1

1� aAð1� TBAÞ

(1.31)

Electrodialysis 339

S00BA ¼ S

0BAT

BA ¼

TBA

1� aAð1� TBAÞ

(1.32)

In Eqs. (1.31) and (1.32), the following phenomena are found:

when TBA41 S

0BAo1 S

00BA oTB

A

when TBA ¼ 1 S

0BA ¼ 1 S

00BA ¼ TB

A

when TBAo1 S

0BA41 S

00BA 4TB

A

(1.33)

Equation (1.33) means that the separatability is inferior to the permselectability.Further, we find the following events in Eqs. (1.31) and (1.32):

limaA!0

S0BA ¼ 1 lim

aA!1S

0BA ¼

1

TBA

(1.34)

limaA!0

S00BA ¼ TB

A limaA!1

S00BA ¼ 1 (1.35)

aA vs. S0BA and S

00BA is computed using Eqs. (1.31) and (1.32) and shown in

Fig. 1.17, in which the relationships in Eqs. (1.33)–(1.35) are confirmed.

0.0 0.2 0.4 0.6 0.8 1.010-2

10-1

1

101

102

0

S′ A

B

S′′ A

B

�A

104

0.4

0.01

10

4

1

0.4

TAB =0.01

Figure 1.17 Relationship between permselectivity coefficient, desalting ratio and sep-aration factor. Open, S

0BA ; filled, S

00BA :


1.4. ENERGY CONSUMPTION AND OPTIMUM CURRENT DENSITY

In multi-stage multiple partially circulation system (Fig. 1.14), rectifiersare assumed to be put in each stage, in which an electric current I is supplied toelectrodialyzers through a parallel circuit. I is expressed as follows:

I

FZMN ¼ QRfðCinÞn � ðCoutÞng (1.36)

Voltage V applied to an electrodialyzer is indicated by the following equationputting the voltage in the electrode cell as VP.

V ¼ MV cell þ VP (1.37)

Vcell is cell voltage as follows (cf. Section 13.2 in Fundamentals):

V cell ¼ IðRK þ RA þ Rdil þ RconcÞ þ EM (1.38)

RK and RA are electric resistance of a cation and an anion exchange membrane.Rdil and Rconc are electric resistance of a desalting and a concentrating cell. RM ismembrane potential.

A reasonable estimate of the optimum current density is introduced byassuming that the major costs are divided into three categories: those directlyproportional to current density, those inversely proportional to current density,and those independent of current density (Eq. (1.39)).

Z ¼ ai þb

iþ c (1.39)

where Z is total cost, and a, b, c are the relative proportionality constants. Theoptimum current density iopt is defined by the minimum of Eq. (1.39), so it isintroduced by differentiating Eq. (8.37); dZ/di ¼ 0 and expressed as (Leitz,1986):

iopt ¼P

QR

� �1=2

(1.40)

P is depreciation cost ($/m2s), Q the energy cost ($/Ws) and R the cell pairelectric resistance (O m2).

1.5. SURROUNDING TECHNOLOGY

1.5.1 Filtration of a Feeding Solution

Fine materials such as sand, clay, iron components, humus soil and mis-cellaneous inorganic and organic colloid are usually suspended in a raw feedingsolution. In order to avoid invasion of these materials into an electrodialyzer, afeeding solution is filtrated using sand, fibers or cohesive agent and turbidity ofthe solution is decreased less than 0.1–0.2 ppm. First of all, the following valve-less sand filter is broadly applicable (Tsunoda, 1994). In Fig. 1.18, a raw feeding

H3

10

9

6 H2

H1

5

11

7

4

2

3

1

8

Figure 1.18 Valve-less filter. 1, Feeding solution inlet; 2, filtrating chamber; 3, sandfilter; 4, collecting chamber; 5, filtrate flow out pipe; 6, filtrate outlet; 7, connecting duct;8, washing chamber; 9, siphon pipe; 10, siphon breaker; 11, control valve (Tsunoda,1994).

Electrodialysis 341

solution is supplied into the filtrating chamber, filtrated through the sand filterand collected in the collecting chamber. The filtrate flows out through the flow-out pipe and supplied to an electrodialyzer at solution level H2. At the sametime, a part of filtrate flows through the connecting pipe into the washingchamber put on the filtrating chamber and is accumulated there at H2. Pro-ceeding with the operation, solution level in the siphon pipe goes up with theincrease of flow resistance in the sand filter. When the level surpasses H3, thesolutions in the filtrating chamber, the collecting chamber and the washing tankare discharged to the outside of the process at one stroke through the siphonpipe due to the siphon function. In this instance, sand is washed and the level inthe washing tank goes down to H1.

1.5.2 Scale Trouble Prevention

1.5.2.1 Acid Dosage

Total carbonic acid dissolved in brine is equilibrated to CO2 gas(3� 10�4 atm.) in air. Total carbonic acid concentration in seawater is2–5� 10�2mol dm�3 at pH ¼ 7.0 –7.4. 15–30% of this total value is carbonicacid molecules (CO2 and H2CO3), and the remainders are ionic carbonic acidconsisting of HCO3

� (more than 95%) and CO32� (less than 5%). In a concen-

trating cell in an electrodialyzer HCO3� ions decompose as follows and combine


with Ca2+ ions to form CaCO3 precipitation.

2HCO�3 3CO2�

3 þH2Oþ CO2 " (1)

CO2�3 þ Ca2þ ! CaCO3 (2)

In order to avoid CaCO3 precipitation, HCO3� ions are decomposed into CO2

gas by dosing with an HCl or a H2SO4 solution into concentrating cells.

HCO�3 þHþ ! CO2 " þH2O (3)

1.5.2.2 Precipitation Controlling Agent Dosage

Small amount of precipitation controlling agents such as condensedsodium phosphate Na2[Na4(PO3)6] are dosed into a concentrating cell, resultingwith the absorption of crystalline nuclei to the agents and dissolution of CaSO4

or CaCO3 due to the following chelate reaction.

Na2½Na4ðPO3Þ6� þ CaX ! Na2½Na2CaðPO3Þ6� þNa2X

CaX : CaSO4 or CaCO3(4)

Carboxyl methyl cellulose (CMC) or poly-acrylic acid is also available instead ofcondensed sodium phosphate.

1.5.3 Disassembling and Assembling Works

In spite of the filtration described in Section 1.5.1, a very small quantity offine particles passes through a filter and invades into an electrodialyzer. Some-times, fine organisms pass through the filter and breed in an electrodialyzer. Thefine particles are adhered on the surface of membranes and spacers in desaltingcells (cf. Section 14.2.2 in Fundamentals), causing the increase of flow resistanceof a solution in the desalting cell with acceleration of concentration polarizationon the membrane surface (cf. Section 14.2.1 in Fundamentals). In order to avoidthese troubles, an electrodialyzer is usually disassembled and washed periodically.The disassembling and washing process is generally as follows (Tanaka, 1987):

(1)
Electric current interruption (2) Solution feeding interruption (3) Solution discharge (4) Stack extraction (5) Stack disassembling (6) Membrane surface washing (7) Desalting cell, concentrating cell and spacer washing (8) Stack assembling (9) Leak test
(10)
Integrating stack into an electrodialyzer (11) Solution feeding (12) Electric current passing.

Electrodialysis 343

1.6. PRACTICE

1.6.1 Potable Water Production from Brackish Water

Residents in an isolated island suffer from serious water shortage becauseof their high dependence on rainwater. In order to solve this problem, AsahiChemical Co. constructed the ED plant (2500m3/day) illustrated in Fig. 1.19 in1990 in Ohshima island, Tokyo (Fukuhara et al., 1993). Ohshima Town suppliespotable water to the residents even now. The specifications of this plant areshown in Table 1.2.

Raw brackish water pumped up from wells is fed to the suspended solidfilter (SSF), and then it is supplied to the first stage electrodialyzer unit (EDU-A1 and EDU-B1) through the first stage desalination tank (DST-1). A part ofthe filtrated solution is supplied to the concentrated solution tank (CST). Thedesalted solution in DST-1 is further desalted at the second stage electrodialyzerunit (EDU-A2 and EDU-B2) to be about 400 ppm, and then supplied to thewater cleaning tank (WCT).

P P P PP

P

P P P P P P

Wells ST SSF CST AT EST DST-1 DST-2 WCT DRPWT

EDU-A 1 EDU-A 2 EDU-B 1 EDU-B 2

DS-A 1ACS-A FS-A CS-AACS-B FS-B CS-B

DS-B 1 CCS-ADS-A 2 DS-B 2 CCS-B

+ − + − + − + −

Figure 1.19 Saline water desalting process (Ohshima island). ACS, Anode compartmentsolution; AT, acid tank; CCS, cathode compartment solution; CS, concentrated solution;CST, concentrated solution tank; DR, distributing reservoir; DS, desalinated solu-tion; DST, desalination tank; EDU, electrodialyzer unit; EST, electrode solution tank;FS, frame solution; PWT, product water tank; SSF, suspended solid filter; ST, stock tank;WCT, water cleaning tank (Fukuhara et al., 1993).

Table 1.2 Specifications of the electrodialysis plant

Site Ohshima, Tokyo

Capacity, product water 2500m3/day, from 1200 ppm TDS raw water1000m3/day, from 3000 ppm TDS raw water

Raw water source 4 brackish wellsProduct water TDS: 450 ppm or less

Chloride ions: 150 ppm or lessElectrodialyzers Asahi Chemical SS-0Conductive area 1780 m3 (318 pairs/stack, 4 stack)Thickness Dilution compartment: 0.5mm

Concentration compartment: 0.5mmMode of operation Dual-train, two steps per line automatic, continuous

Source: Fukuhara et al. (1993).

Table 1.3 Operational results

Production rate 2610m3/day, from 1200 ppm TDS raw waterProduct water TDS: 440 ppm

Chloride ions: 145 ppmpH: 6.8

Electric consumption 0.8 kWh m�3



Organic fouling due to organic acid including in raw brackish water isavoided by integrating anti-fouling anion exchange membranes Aciplex A-201into the electrodialyzer instead of standard type Aciplex A-101 membranes(cf. Section 14.3.2 in Fundamentals).

Operational results are shown in Table 1.3. Analysis of raw and productwater are shown in Table 1.4. The values listed in both tables meet the potablewater standard established by the Ministry of Welfare, Japan. Actual operatingcost is determined based on the plant operation in a year as indicated in Table 1.5.

1.6.2 Electrodialysis Desalination System Powered by Photo-Voltaic Power

Generation

Babcock-Hitachi K.K. and Hitachi Ltd. established ED system combinedwith sunlight photo-voltaic power generation in Oshima island and at Saki-yama, Nagasaki (Inoue and Kuroda, 1993). In this system, the electrodialyzerwas operated using direct current power generator using solar batteries andproduced potable water.

Fig. 1.20 gives the system in Oshima island, showing a two-stage seawaterdesalting process for producing 103/day of potable water. In the first stage, if it isfine, seawater (35,000mg l�1-TDS) is desalted during the daytime to obtain an

Table 1.4 Analysis of raw and product water

Raw Watera Product Water

Visual Inspection Colorless, Transparent Colorless, Transparent

Turbidity (1) o1.0 o1.0Color (1) o1.0 o1.0pH 7.2 7.3Electrical conductivity (mS cm�1) 2500 650Total hardness (CaCO3, ppm) 685 83Calcium hardness (CaCO3, ppm) 406 33Evaporation residue (ppm) 1631 418Silica (SiO2, ppm) 43 42Chloride ion (Cl, ppm) 632 131Sulfuric ion (SO4, ppm) 262 40Hydrocarbonate ion (HCO3, ppm) 210 82Calcium ion (Ca, ppm) 51 14Potassium ion (K, ppm) 17 3Magnesium ion (Mg, ppm) 48 12Sodium ion (Na, ppm) 368 94

Source: Fukuhara et al. (1993).aFrom single well.

Table 1.5 Operating cost

Units Projected Actual

Volume of product water m3/day 1950 1853Raw water TDS mg l�1 1600 1250Water recovery % 80.0 86.5Electric consumption yen m�3 27.82 10.04Stering agents yen m�3 0.38 3.73Membrane replacement yen m�3 11.80Total yen m�3 40.00 13.77


Electrodialysis 345

intermediately desalted solution (5000mg l�1-TDS) and at the same time, elec-tric power surpluses are stored in batteries (STB). In the second stage, theintermediately desalted solution is further desalted to obtain potable water(400mg l�1-TDS) in the rainy or cloudy daytime or in the nighttime using thestored electric power.

Fig. 1.21 shows the system in Sakiyama for producing 200m3/day ofpotable water from brackish water. In this system, electric consumption forpumping up raw brackish water is larger. So, in the fine daytime, raw brackishwater (1500mg l�1-TDS) is pumped up and is desalted to obtain potable water(400mg l�1-TDS). At the same time, brackish water and electricity are stored in

SOB

CBO STB

DAT

FIL

DST

IST

DSP ED CSPCSTBrine

Sea

PWTProductwater

− +

FSTSWP

Sea

t

Figure 1.20 Seawater desalination process powered by solar generation (Oshima island).CBO, Control board; CSP, concentrated solution pump; CST, concentrated solutiontank; DAT, direct/altering current transducer; DSP, desalted solution pump; DST, de-salted solution tank; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; IST,intermediately concentrated solution tank; PWT, product water tank; SOB, solar battery;STB, storage battery; SWP, seawater pump (Inoue and Kuroda, 1993).


the fine daytime for operating the electrodialyzer in the rainy or cloudy daytimeand in the nighttime. The specifications of both systems are shown in Table 1.6.

An electric power consumption pattern of the Oshima plant is illustratedin Fig. 1.22. In a batch desalting system, electric power consumption is large atbeginning and it decreases with elapsed time. So, when a large amount of electricpower is generated in the fine daytime, seawater is electrodialyzed to obtainintermediately desalted solution, which is temporarily stored in a tank (high-concentration operation). At the same time, electric power surpluses are storedin STB. In the cloudy or rainy daytime and in the nighttime, the stored solutionis further desalted to obtain potable water (low-concentration operation) pass-ing the stored electric power.

Fig. 1.23 shows an electric power consumption pattern of the Sakiyamaplant, which is automatically controlled with a sunlight photo-voltaic powergeneration system, a storage battery system and an ED system. An electricpower generated in the solar system is directly consumed in the electrodialyzer,and the electric power surpluses charge the battery.

Operating performance of both plants are indicated in Table 1.7.

SOB

t

CBO STB

DCLDATACLAlternating current

FIL

WEL

SWP

DST

DSP

ECM fromFST

PWT

ProductwaterCST

SeaCSP

ED

− +

to CSTFST

Saline water

Direct current

Figure 1.21 Saline water desalination process powered by solar generation (Sakiyama).ACL, Altering current load; CBO, control board; CSP, concentrated solution pump;CST, concentrated solution tank; DAT, direct/altering current transducer; DCL, directcurrent load; DSP, desalted solution pump; DST, desalted solution tank; ECM, electro-conductivity meter; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; PWT,product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump;WEL, well (Inoue and Kuroda, 1993).

Electrodialysis 347

1.6.3 Electrodialytic Recovery of Wastewater from a Metal Surface Treatment

Process

Electrodialysis is applicable to recovering wastewater and nickel in a metalsurface treatment process. The process was developed by Asahi Glass Co. asfollows (Itoi et al., 1986).

A nickel plating process includes several rinsing processes. In this process,Ni concentration in the effluent from the first rinsing stage is high. So that theeffluent is usually returned to the electro-plating bath and the effluent from thefinal rinsing stage is discharged. The ED process is designed to collect Ni ions inthe first rinsing stage and return them to the electro-plating bath for the purposeof increasing recovery ratio of Ni and decreasing Ni content in the waste fromthe final rinsing stage.

Fig. 1.24 is a continuous process operating in the car components man-ufacturing factory. The process includes two stages of rinsing baths and one unit

Table 1.6 Specifications of the desalting process

Oshima Sakiyama

Solar generatorType Silicon single crystal Silicon single crystalCapacity 25 kW 65 kWModule number 64W� 390 module 47W� 1380 module

Storage batteryType Lead storage battery Lead storage batteryCapacity 115 kWh 230 kWhVoltage 96V 192V

ElectrodialyzerCapacity 10m3/day 200m3/dayRaw water Seawater (35,000mg l�1-TDS) Saline water (1500mg l�1-

TDS)Type Filter-press Filter-pressGasket dimension 185� 2000mm 370� 2000mmEffectivemembrane area

0.238m2 0.476m2

Distance betweenmembranes

0.8mm 0.8mm

Number of cell pair 250 600System control 3 modes operation 4 modes operation

1. High-concentrationoperation

1. Pump up+desaltingoperation

2. Low-concentration operation 2. Pump up operation3. Stand by operation 3. Desalting operation

4. Stand by operation

Source: Inoue and Kuroda (1993).

Potable water production

Intermediately concentratedsolution production

Solar battery outputElectrodialysis

Ele

ctric

cur

rent

(A

)

6 8 10 12 14 16 18 20 22 24Time

Auxiliary machine

Figure 1.22 Electric power consumption pattern of the seawater desalination plant(Oshima island) (Inoue and Kuroda, 1993).


Auxiliary machinecurrent

Electrodialysis current

Pump up current

Solar battery output

Stand by

Time

Desalination

Desalination + Pump up

20181614121086

Ele

ctric

cur

rent

(A

)

Figure 1.23 Electric power consumption pattern of saline water desalination plant(Sakiyama) (Inoue and Kuroda, 1993).

Table 1.7 Operating performance of the electrodialyzer

Oshima Sakiyama

1986 1987 1988 1900 1901

Sunlight quantity (kWh m�2) 3.60 3.73 3.86 4.08 3.60Generation quantity (kWh/day) 49.3 50.6 57.0 162 145Water production (m3/day) 3.45 3.44 4.04 229 194Electric power consumption (kWh m�3) 14.3 14.7 14.1 0.71 0.75

Source: Inoue and Kuroda (1993).

Electrodialysis 349

of electrodialyzer, which is designed to maintain the Ni concentration in the firststage rinsing bath to about 5 g l�1 when the concentration in the Ni plating bathis about 84 g l�1. The specifications of the electrodialyzer are presented inTable 1.8. The limiting current density for NiSO4 or NiCl2 is extremely lowcomparing to that for Na2SO4 or NaCl. So, it is important to control the currentdensity to prevent the precipitation of Ni(OH)2 caused by water dissociation.Further, the organic substances in the solution added into the electro-platingbath possibly give rise to generate the organic fouling of anion exchange

2nd RIT1st RITEPB

Chemicaldosing

*

PRT+ −

*

ED

CXC AXC

FIL

DIL CON ELR 2nd RIW DEW

1st RIW

Figure 1.24 Electrodialysis Ni2+ electro-plating wastewater recovery process. AXC,Anion exchange column; CXC, cation exchange column; CON, concentrate; DEW, de-mineralyzed water; DIL, diluate; ED, electrodialyzer; ELR, electrode rinse; EPB, electroplating bath; FIL, filter; PRT, pretreatment; RIT, rinse tank; RIW, rinse waste (Itoi et al.,1986).

Table 1.8 Specifications of the electrodialysis plant

Electrodialyzer Model DU-111Ion exchange membrane Selemion CMV/AMVSize of membrane 0.49� 0.98mEffective membrane area 0.336m2

Number of membrane pairs 40 pairsDistance between membranes 2.0mmFlow velocity 3.0 cm s�1

Current density 1.0A dm�2

Maximum voltage 50V

Source: Itoi et al. (1986).


membranes. So, it is necessary to remove the organic substances by means of aspecial pretreatment.

Material balance of Ni2+ ions and water in the process is indicated inFig. 1.25 showing that the recovery ratio is larger than 90% and a dilutedsolution is perfectly recycled to the first stage rinsing bath. Current efficiencyand electric power of ED were, respectively, greater than 90% and 2 kWh kg�1

Ni ion.Cost estimation is presented in Table 1.9.

Take out from plating bath 13 l/h (Ni2+ 84.2 g/l)

1 st stage Rinsing tank

Take out to 2nd stage 13 l/h (Ni2+ 5g/l)

Feed to ED 3 m3/h (Ni2+ 5g/l)

Concentrated stream To plating bath 12.4 l/h (Ni2+ 83 g/l)

Electrodialyzer Diluate stream 3 m3/h Ni2+ 4.65 g/l

Figure 1.25 Ni2+ and water balance in the electrodialysis process (Itoi et al., 1986).

Table 1.9 Cost estimation of electrodialytic recovery of rinsing waste in the nickelelectroplating process

Nickel recovery rate (as NiSO4 � 6H2O) 3460 kgmonth�1

Electricity consumption (as NiSO4 � 6H2O) 0.7 kWhkg�1

Equipment installation cost 15million yen�1

Electricity unit cost 10 yen kWh�1

Purchasing price of nickel salt (as NiSO4 � 6H2O) 360 yen kg�1

Profit of Nickel salt recovery360 yen kg�1

� 3460 kgmonth�1¼ 1,245,600 yenmonth�1

Runnig costElectricity 10 yen kWh�1

� 0.7 kWhkg�1

� 3460 kgmonth�1¼ 24,220 yenmonth�1

Maintenance and consumable item (annually 3%)15,000,000� 0.03/12 month ¼ 37,500 yenmonth�1

Amortization (7 years)15,000,000 yen (7 year� 12 month)�1

¼ 178,570 yenmonth�1

Interest (annually 9%)15,000,000 yen� 0.09/12 month ¼ 112,500 yenmonth�1

Running cost total 352,790 yenmonth�1

Profit 892,810 yenmonth�1

Source: Itoi et al. (1986).

Electrodialysis 351

1.6.4 Reuse of Wastewater by Electrodialytic Treatment

Ion exchange membrane ED technology is applicable to wastewater treat-ment for realizing resource saving and protection of environment. TokuyamaInc. developed the technology to establish a closed system by means of

HCl, H2SO4

NaOH

Flocculant

Desalted solution

Designed process is surrounded by the dotted line.

Process

Acid waste

Neutralization

Clarifier

Traider receiving

Filter

Electrodialysis

Concentrated solution

Figure 1.26 Electrodialytic treatment process of industrial waste (Matsunaga, 1986).

Solution Na Ca Cl SO4

Circulating solution 0.255N 13 ppm 0.168 N 0.087 NConcentrated solution 3.870 220 3.329 0.541

to process

from clarifier

MITSAF

DSTCHF

ED

1

2CST Concentrated

solution

HCl

65.4 l /h

Figure 1.27 Electrodialytic treatment process of industrial waste. CHF, Check filter;CST, concentrated solution tank; DST, desalted solution tank; ED, electrodialyzer; MIT,middle tank; SAF, sand filter (Matsunaga, 1986).


Electrodialysis 353

electrodialytic reuse of wastewater in a plating process as shown in Figs. 1.26and 1.27 (Matsunaga, 1986).

�
Typical components of a raw feeding solution in this system.pH: 9.5, NaCl: 17.6 g l�1, Na2SO4: 7.1 g l�1, Ca: 48mg l�1, SS: 20mg l�1.
Main component: Fe(OH)3. The others: Fe, Cu, Zn, P etc.
� Conditions of electrodialytic treatment.
Quantity of a desalted solution: 10 ton/month. Degree of desalination:maximum. Degree of concentration: maximum. Temperature: normal. Op-erating time: 24 h/day, 30 days/month, 12months/year. Operating system:partially circulation. Ion exchange membrane: Neocepta C5S-8T (monova-lent cation selectively permeable cation exchange membrane) and NeoceptaACH-45T (anion exchange membrane).

�
Operating results.Current density: 3.0A dm�2. Cell voltage: 0.5V/pair (18–20 1C). Electro-
lyte concentration in a desalted solution: Cde,Na ¼ 0.25–0.32 eq dm�3,Cde, Ca ¼ 13–15mg l�1.

Electrolyte concentration in a concentrated solution Ccon,Na ¼ 3.80–4.02 eq dm�3, Ccon,Ca ¼ 220–240mg l�1. Quantity of a concentrated solu-tion: 60–66 l h�1 (1.44–1.58m3/day). Quantity of a desalted solution 10.9–12.1 ton/month.

�
Running cost.Electric power: 262 kWh/day� 15 yen/kWh ¼ 3930 yen/dayLabor: 0.2 people/day� 15,000 yen/people ¼ 3,000 yen/dayThe others (Ion exchange membrane, HCl, electrode, filter): 1330 yen/dayTotal: 8260 yen/day.
1.6.5 Simultaneous Treatment of Wastewater by Electrodialysis and Reverse

Osmosis

Yunichica Ltd. developed a treatment process of wastewater includingtoxic metallic ions discharged from a semi-conducting material manufacturingprocess (Ishibashi, 1986).

The process is illustrated in Fig. 1.28 consisting from the following system.

1.6.5.1 High-Concentration System

HS gas dissolving in the wastewater is deaerated and neutralized addingNaOH. Fine particles suspending in the wastewater are removed using a micro-osmosis filter. Then, a clear salt solution is supplied to an ion exchange mem-brane ED unit and concentrated to 3.5–4.0M. The concentrated solution isevaporated using a vacuum evaporator (EV) to obtain Na2SO4 and NaCl crys-tals, which are reused after re-purification. The desalted solution obtained fromthe ED unit is further desalted using a reverse osmosis (RO) unit. Concentrated

Concentratedsolution

EV Separated salt 570 kg/day

TDSTDS 240g/l TDS

High concentration system

36g/l 1.5g/lED RO

TDS0.1g/l

Lowconcentration system

IX

Pure water Heigh pure water

Process

ED: Electrodialysis RO:Reverse osmosis IX: Ion exchange EV:Evaporation

Polishing

Figure 1.28 Simultaneous treatment process of wastewater by electrodialysis and re-verse osmosis (Ishibashi, 1986).


solution from the RO unit is returned to the ED unit. The specifications of bothunits are as follows:

�
Electrodialysis unitTokuyama TS-25-160 type integrated with Neocepta C66-5T/AFSBatch wise operating systemDesalting performance 18m3/dayConcentration of a desalted solution 1.5 g l�1
�
Reverse osmosis unitMiddle pressure hollow fiber B-9Desalting performance 15m3/dayConcentration of a desalted solution 100 ms
1.6.5.2 Low-Concentration System

After deaeration, the solution is filtered passing through a carbon filterand neutralized using weak basic ion exchange resins. Then the solution is sup-plied to a cation exchange and an anion exchange column to obtain pure water.The pure water is fed to the semi-conducting material manufacturing process

Anion exchange layer Cation exchange membrane

Mn+

H+ H+

Anode Cathode

+ −

++++++

−−−−−

−−−

−−

−

Figure 1.29 H+ ion permeable cation exchange membrane. Mn+, n valent metalic ion(Katayama, 2004).

Electrodialysis 355

directly or via a polishing column filled up with cation and anion exchangeresins and nonionic resins.

1.6.6 Electrodialytic Recovery of Acid

Acid is recovered from a waste acid applying a H+ ion permselectivecation exchange membrane placed an anion exchange layer on a cation exchangemembrane as illustrated in Fig. 1.29. Here, multivalent cations Mn+ do not passthrough the membrane due to repulsive effects between the cations and theanion exchange layer. The concept mentioned above is applicable to recover anacid from an aqueous solution dissolving for instance Fe(NO3)3 with HNO3 asshown in Fig. 1.30. In this system, cation exchange membranes correspond tothe membrane illustrated in Fig. 1.29, and they permeate H+ ions selectively anddo not permeate Fe3+ ions. On the other hand, anion exchange membranepermeate NO� ions selectively rather than H+ ions. Consequently, HNO3 isrecovered in the concentrating chamber and Fe(NO3)3 is remained in the de-salting chamber.

Table 1.10 shows the material balance in an electrodialyzer (effectivemembrane area: 90m2) developed by Tokuyama Inc., which treated a wastedacid solution (0.54M acid solution including H2SO4 and HCl with 0.30M ofAl3+ ions) generated in an etching process of aluminum products (Katayama,2004). Acid concentration in a recovered acid is 1.90M including 0.01M of Al3+

ions. Acid recovering ratio and leak ratio are, respectively, 88% and 0.6%.Energy consumption and current efficiency are, respectively, 12 kW and 55%.

1.6.7 Seawater Concentration for Salt Production

Electrodialysis is applied to concentrating seawater for producing salt inJapan. The seawater concentrating process is illustrated in Fig. 1.31 (Tomita,1995). Seawater pumped up from sea is filtered and supplied to the elect-rodialyzer via the filtered solution tank. Concentrated seawater is circulated

Table 1.10 Material balance in an electrodialyzer for acid recovering

Solution SolutionQuantity (l h�1)

Composition

H+ (M) Al3+ (M) SO42� (M) Cl� (M)

Waste acid (beforeelectrodialysis)

742 0.54 0.30 0.65 0.16

Deacid solution(afterelectrodialysis)

671 0.07 0.33 0.50 0.08

Water 115 0 0 0 0Recovered acid 186 1.90 0.01 0.79 0.35

Source: Katayama (2004).

−+

Anode

Water

Waste acid HNO3 Fe(NO3)3

H+ H+ H+

Fe3+

C A C A C

De-acidified solution Fe(NO3)3

Recovered acid HNO3

NO3− NO3

−

H+

NO3−

H+

Fe3+

Cathode

NO3−

Figure 1.30 Acid recovery by means of ion exchange membrane electrodialysis. C, H+

ion permselective cation exchange membrane; A, H+ ion low-permselective anion ex-change membrane (Katayama, 2004).


between the concentrated seawater tank and the electrodialyzer, and its gain issupplied to an evaporating process to obtain salt crystals. CaCO3 scale precip-itation in concentrating cells is prevented by adding hydrochloric acid to theconcentrated seawater to decompose HCO3

� and CO32� ions (cf. Section 1.5.2 in

Applications). A part of filtrated seawater is supplied to anode chambers.Titanium is adopted as the anode material. In order to avoid membrane de-struction due to Cl2 and HClO generated by an anode reaction, a perfluorinatedion exchange membrane is integrated between the cathode chamber and the

2

1

6

910

3 4 5

8

7

C

+ −

Figure 1.31 Electrodialytic seawater concentration process. 1, Diluted seawater tank; 2,cathode solution tank; 3, concentrated seawater tank; 4, washing solution tank; 5, HCltank; 6, filtrated seawater; 7, concentrated seawater output; 8, HCl; 9, electrodialyzer; 10,anode solution (Tomita, 1995).

Electrodialysis 357

adjacent stack. The wasted solution from the anode chamber is mixed with thefiltrated seawater to suppress the growth of microorganisms in seawater. Cath-ode material is plated with Pt. An HCl solution is supplied to the cathodechamber to neutralize OH� ions generated by the cathode reaction. A washingsystem is provided for washing the inside of desalting cells by acid or chemicalreagents and dissolving adhered substances (cf. Section 14.2.3 in Fundamentals).

When the turbidity of raw seawater is 2 ppm, it is decreased to about0.05 ppm by filtering through two-stage sand filters (cf. Section 1.5.1 in Appli-cations). In spite of such filtration, fine particles pass through the filter andinvade into the electrodialyzer and precipitate on the membrane surfaces.Fe(OH)3 components precipitated on the membrane possibly give rise to waterdissociation (cf. Sections 8.8.4 and 8.9 in Fundamentals). Sometimes, fine or-ganisms pass through the filter and breed in the electrodialyzer. These troublesare avoided by disassembling and washing the electrodialyzer at the interval of4–6 months (cf. Section 1.5.3 in Applications). In order to increase currentefficiency and avoid CaSO4 scale precipitation, membrane surfaces are treated togive monovalent ion permselectivity (cf. Section 3.7 in Fundamentals).

Figure 1.32 Composition of an electrodialyzer (Tokuyama). 1, Fastening bolt; 2, fas-tening and feeding frame; 3, concentrated seawater inlet; 4, diluted seawater inlet; 5,diluted seawater outlet; 6, concentrating cell; 7, desalting cell; 8, desalting cell manifold; 9,concentrated seawater outlet; 10, cation exchange membrane; 11, anion exchange mem-brane; 12, concentrating cell manifold (Tomita, 1995).


Compositions of electrodialyzers developed by Tokuyama Inc., AsahiGlass Co. and Asahi Chemical Co. are illustrated in Figs. 1.32–1.34 (Tomita,1995). Typical performance of an electrodialyzer is exemplified in Table 1.11(Tanaka, 1991).

1.6.8 Salt Production Using Brine Discharged from a Reverse Osmosis Seawater

Desalination Plant

Concentrated brine is discharged from a RO seawater desalination proc-ess. It seems advantageous to use this brine as raw material for salt production.

Figure 1.33 Composition of an electrodialyzer (Asahi Glass Co.). 1, Cathode chamber;2, anode chamber; 3, cathode plate; 4, anode plate; 5, intermediate frame; 6, desalting cell;7, concentrating cell; 8, cation exchange membrane; 9, anion exchange membrane; 10,packing cell frame; 11, intermediate packing; 12, blind cell frame (Tomita, 1995).

Figure 1.34 Composition of an electrodialyzer (Asahi Chemical Co.). 1, Diluted sea-water; 2, special gasket; 3, concentrating cell; 4, turn-Buckle; 5, desalting cell; 6, con-centrated seawater; 7, fastening frame; 8, cation exchange membrane; 9, anion exchangemembrane (Tomita, 1995).

Electrodialysis 359

Table 1.11 Performance of an electrodialyzer for concentrating seawater (plant A, 1987)

Current density (A dm�2) 2.66Temperature (1C) 25.3Cl current efficiency (%) 88.6Na current efficiency (%) 80.8Energy consumption (kWh t�1 NaCl) 178.6Concentrated solution puritya (%) 90.90Constitution of concentrated solutionNaCl (g dm�3) 190.6Cl (eq dm�3) 3.568SO4 (eq dm�3) 0.015Ca (eq dm�3) 0.064Mg (eq dm�3) 0.169K (eq dm�3) 0.095Na (eq dm�3) 3.256

Source: Tanaka (1991).aNaCl(g)/Total electrolyte (g).


The operating performance and energy consumption in a salt manufacturingplant were investigated for an ion exchange membrane ED system to whichdischarged brine from a RO plant is supplied as follows (Tanaka et al., 2003).

The salt manufacturing process (NaCl production capacity: 200,000ton/year) is illustrated in the dotted frame in Fig. 1.35. Discharged brine (elec-trolyte concentration: 1.5 eq dm�3) from a RO plant is assumed to be supplied toan ion exchange membrane electrodialyzer. The concentrated solution obtainedfrom the electrodialyzer is supplied to a multiple-effect EV, in which salt iscrystallized. The salt obtained from the evaporator is supplied to an ionexchange membrane electrolytic bath, in which sodium hydroxide and chlorineare produced. Energy consumption in the salt manufacturing process is assumedto be supplied by a simultaneous heat-generating electric power unit consistingof a boiler and a back-pressure turbine.

Fig. 1.36 shows the flows of electricity and steam in a salt manufacturingplant. Boiler steam is introduced to a turbine and generates electricity, which isdistributed to electrodialyzers, etc. The back-pressure of a back-pressure turbineis supplied to a heater in a No. 1 evaporator in multiple-effect evaporators.Evaporated steam in a No. 1 evaporator is supplied in turn to the followingevaporators. Pressure and temperature of boiler steam are set as 6Mpa and4801C in this study. The temperature difference between heating steam andevaporated steam is fixed to 201C at each evaporator. The number of evaporatoris kept to a minimum, but the quantity of electricity does not exceed the electricpower consumption in this salt manufacturing plant. An electric power shortfallis assumed to be made up by purchased electric power, which is generated by acondensing turbine.

Seawater 0.6 eq/dm3

Desalted solution Discharged brine

C′in=1.5 eq/dm3

ROC′′

C′out

ED

Cl2 T B

EV

NaOHNaCl

crystals

IMSalt manufacturing process

H2O

Figure 1.35 RO-ED combined salt manufacturing process. RO, Reverse osmosis; ED,electrodialyzer; EV, evaporation; IM, ion exchange membrane electrolysis; B, boiler; T,turbine. Diagonals in RO, ED and IM unit box represent the membranes (Tanaka et al.,2003).

Electrodialysis 361

The energy required for producing salt F is plotted against current densityI/S in both cases of RO discharged brine ED and seawater ED. The plots areshown in Fig. 1.37, which indicates that the energy consumption in a salt man-ufacturing process using RO discharged brine is 80% of the energy consumptionin the process using seawater. The optimum current density at which the energyconsumption required in the salt manufacturing process being minimized is 3Adm�2 for both RO discharged brine ED and seawater ED.

1.6.9 Desalination of Amino Acid and Amino Acidic Seasonings

Amino acid is an amphoteric electrolyte, and its behavior is different fromthat of usual electrolytes. Itoi and Utsunomiya (1965) electrodialyzed aqueoussolutions of methionine and glysine containing sodium formate as follows; (a)methionine 25 g l�1, HCOONa 20 g l�1. (b) glysine 70 g l�1, HCOONa 20 g l�1.

The solution was supplied to the electrodialyzer (membrane pairs: 9,effective membrane area: 209 cm2) incorporated with Selemion CSG and AST.Changing pH and maintaining cell voltage at 15V (average current density:nearly 1A dm�2), amino acid permeation ratio (the ratio of amino acid

Purchsed electric power

∆G

Ggen+∆G=Etotal × PNaCl

ROH*S*

Discharged brine1.5 eq/dm3solutionGenerated

electric powerGgen

B BPT ED

W = w × PNacl

50°C 30°C70°C

St

Qcond EV NO1 EV NO2 EV NO3

Tcond=90°CHcond

Scond

Figure 1.36 Energy flow in a salt manufacturing process. B, Boiler; BPT, back-pressureturbine; EV, evaporator; ED, electrodialyzer; RO, reverse osmosis unit (Tanaka et al.,2003).


transported across a membrane pair against that dissolving in a feeding solu-tion) was measured and shown in Fig. 1.38, indicating that the amino acidpermeation ratio becomes minimum near the isoelectric point of amino acid.Changing current density and keeping pH near the isoelectric point of the aminoacid, the amino acid permeability (quantity ratio of amino acid transportedacross a membrane pair against electricity) was measured and shown inFig. 1.39. Inspecting Fig. 1.39 and the limiting current density measured for anHCOONa solution, it is concluded that the amino acid permeability becomesminimum by applying the limiting current density. Concentration changes ofNaCl and essence in the batch system ED of soy sauce are indicated in Tables1.12 and 1.13.

0 1 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

2 3 4 5 6 70

10

20

30

40

50

60

�(R

O d

isch

arge

d br

ine

elec

trod

ialy

sis)

/ �

(sea

wat

er e

lect

rodi

alys

is)

�(1

03 M c

al/h

)

I/S (A/dm2)

: RO discharged brine electrodialysis : Seawater electrodialysis

: RO discharged brine electrodialysis / Seawater electrodialysis

Figure 1.37 Energy consumption in a salt manufacturing plant (Tanaka et al., 2003).

Electrodialysis 363

1.6.10 Desalination of Natural Essences

In an extraction process of natural essences, several kinds of salt, acid andalkali are added. Accordingly salt is obtained as a by-product in a final stage.Salt content in the natural essences is expected to be controlled for maintaining ataste and human health. Tokuyama Inc. developed the following desalinationprocesses of natural essences (Ideue, 1986; Yamamoto, 1993).

An ED system was set up using ion exchange membranes met the foodhygiene standard established by the Ministry of Welfare, Japan, and incorpo-rated with polyvinyl chloride or polypropylene materials suitable for food pro-duction. It was further necessary to pay attention to prevent solution stagnationand cultural contamination. Further cleaning in place (CIP) was necessary tooperate the apparatus stably. The electrodialyzer was operated in a batch systemusing ED system indicated in Fig. 1.40, in which a conductivity control indicatorwas set at an exit of the electrodialyzer for detecting the concentration of adesalted solution. The solution feed and solution discharge were automaticallycontrolled by switch valves operating with the conductivity control indicator.

Natural essences include meat essences, seafood essences, flesh essencesetc. They were extracted with the aid of a NaCl solution. Salt added in theextraction process was desalinated by means of ED mentioned above. Constit-uent changes in desalination of extracted meat essences, fish essences and fruit

Met

hion

ine

Gly

cine

Cell voltage : 15 V

0 2 4 6 8 10 12 14pH

80

70

60

50

40

30

20

10

0

Am

ino

acid

per

men

tatio

n ra

tio (

%)

Glicine 70 g/l, HCOONa 20 g/lMethionine 25 g/l, HCOONa 20 g/l

Figure 1.38 Relationship between solution pH and amino acid permeation ratiothrough ion exchange membranes (Itoi and Utsunomiya, 1965).


flesh essences are shown in Table 1.14. Specifications of an electrodialyzer forseafood essences are shown in Table 1.15.

1.6.11 Electrodialysis of Milk and Whey

1.6.11.1 Composition of Milk and Whey (Ideue, 1986; Tomita et al., 1986)

Whey is obtained as a by-product of a cheese production process. Theoutput of whey amounts to nine times of that of cheese, so it is an importantsubject in the dairy industry to utilize the whey effectively. The effective utilizationof permeates derived from an ultra-filtration process of milk and whey is also abig problem in the dairy industry. The composition of milk, whey and ultra-filtration permeates is indicated in Table 1.16. Ash content in dry matter of thewhey and permeate is so high that it is expected to reduce their ash content by ED.

Powdered milk for baby rising is prepared using cow milk as main rawmaterials. The constituents of breast milk and cow milk are not same as shownin Table 1.17. Total ash and casein compositions of cow milk are, respectively,3.4 and 4.4 times of those of the breast milk. Accordingly, baby rising powdered

Gly

cine

Met

hion

ine

pH: 6.3-6.9

3.0

2.5

2.0

1.5

1.0

0.5

00 0.5 1.0 1.5 2.0

Average current density (A/dm2)

Am

ino

acid

per

mea

bilit

y (g

/Ah)

Glycine 70 g/l, HCOONa 20 g/l

Methionine 25 g/l, HCOONa 20 g/l

Figure 1.39 Relationship between current density and transport rate of amino acid (Itoiand Utsunomiya, 1965).

Table 1.12 Electrodialytic demineralization of soy sauce

Start End

Solution quantity (l) 11.6NaCl concentration (g l�1) 191 31Essence concentration (g l�1) 163 191pH 4.7 4.7Density 1.181 1.103Current efficiency (%) 90

Note: Effective membrane area: 209 cm2; Numbers of membranes: 10 pairs; Currentdensity: 3.5A dm�2; Applied voltage: 25–58V.Source: Itoi (1983).

Electrodialysis 365

Table 1.13 Concentration changes of components in electrodialysis of soy sauce

Component Ratio (Before ED/After ED)

NaCl 0.800Total amino acid 0.94Glutamic acid 0.93Aspartic acid 0.95Lysine 0.98Leucine 0.98Isoleucine 0.98Alanine 0.96Phenylalanine 0.87Valine 0.95Total nitrogen 0.96Essence part 1.009

Source: Itoi (1983).


milk is prepared by adding whey to cow milk. Before this mixing operation,however, it is necessary to extract excessive ash from the milk or whey by ED.

1.6.11.2 Limiting Current Density in Electrodialysis of Milk and Whey

(Nagasawa et al., 1973; Nagasawa et al., 1974)

In ED of a dairy product solution, protein is condensed and attached onthe desalting surface of an anion exchange membrane at above limiting currentdensity due to occurrence of water dissociation. At the same time, insoluble saltsuch as calcium phosphate etc. precipitates on the concentrating surface of theanion exchange membrane. Accordingly, it is important to increase the limitingcurrent density to operate a practical electrodialyzer stably. Nagasawa et al.evaluated the limiting current density as follows.

In ED of a skim milk solution, limiting current density was evaluated basedon the relationship between current density and voltage. The relationship betweenconductivity of the skim milk solution k and the limiting current density ilim wasexpressed by ilim ¼ 1.08 � 10�3k. (i/k)lim was known to be proportional to the0.6th power of solution velocity of skim milk V. (i/k)lim/V

0.6 vs. temperature T

and dry matter content of skim milk S is given in Fig. 1.41 which indicates that(i/k)lim/V

0.6 decreases with the increase of T and S. Flow length and distancebetween the membranes did not influence to ilim. From the investigation men-tioned above, the following limiting current density equation was introduced as

ilim ¼ QS�1=6ð1:01665S0:1

Þ�TV0:6k (1)

Q is a constant. S, T and V can be voluntarily fixed, so the above equation isexpressed by the following equation.

ilim ¼ Rk (2)

1 2

10

8

3

4

5

3

7

3

9

LC

6P1

LA

F1F1

AI VI

P1

P1P1

F1

+ _

LAF1

CCI

Figure 1.40 Electrodialysis process of natural essences. 1, Raw solution tank; 2, desalted solution tank; 3, drain; 4, concentratedsolution tank; 5, concentrated waste solution; 6, electrode solution tank; 7, water supply; 8, ammeter; 9, electrodialyzer; 10, desaltedsolution; Vi, voltage indicator; Ai, ampere indicator; Fi, flow indicator; Pi, pressure indicator; CCi, conductivity control indicator, LC,level control; LA, level alarm (Yamamoto, 1993).

Electro

dialysis

367

Table 1.14 Desalination of fish essences

Raw Essence Desalted Essence Desalting Ratio

Conductivity 15.4mS cm�1 2.74mS cm�1 82.2%Viscosity 4 cp 6.4 cpAshNa 2975 ppm 560 ppm 81.2%K 241 ppm 19 ppm 92.1%Ca 38 ppm 2.4 ppm 93.7%Mg 45 ppm 1.9 ppm 95.8%Cl 3299 ppm 583 ppm 82.3%

Source: Yamamoto (1993).

Table 1.15 Specifications of an electrodialyzer for desalinating seafood essences

RequirementRaw solutionNaCl 14 g l�1

Protein 100 g l�1

Specific gravity 1.05Viscosity 4.0 cppH 5.5

Treating amount 0.5 t h�1

SpecificationsEffective membrane area 256m2

Number of membrane pair 200 pairsIon exchange membrane Neocepta CM-1, AM-1Model TS-25-200Operating system Automatic batch system

Running cost 1517 yen t�1

Source: Yamamoto (1993).

Table 1.16 Typical composition of milk and UF-permeate

Fat (%) Protein(%)

Lactose(%)

Ash (%) Water(%)

Ash/DryMatter(%)

Cow milk 3.3 2.9 4.5 0.7 88.6 6.1Cheese whey 0.3 0.7 4.5 0.6 93.7 9.5Permeate derivedfrom UF of milk

0 0.2 4.4 0.5 94.8 9.6

Source: Tomita et al. (1986).


Table 1.17 Comparison of composition between breast milk and cow milk

Water(%)

Ash (%) WheyProtein (%)

Casein(%)

Fat (%) Lactose(%)

Breast milk 88.0 0.2 0.68 0.42 3.5 7.2Cow milk 88.6 0.7 0.69 2.21 3.3 4.5

Source: Tomita et al. (1986).

10

5

2

0 10 20 30 40Temperature (°C)

(i/�)

lim/V

0.6

x 10

2

S % : 0.3, : 8.3, : 16, : 25Figure 1.41 Effects of temperature and total solid content on (i/k)lim/V

0.6 (Nagasawaet al., 1974).

Electrodialysis 369

R is a constant. We know ilim from k, and it was confirmed that the elect-rodialyzer is operated stably at under ilim estimated from Eq. (2).

1.6.11.3 Ion Exchange Membrane (Okada et al., 1975) for the Demineralization

of Milk or Whey

In the demineralization of skim milk or whey including rich proteinaceousmaterials or other organic matters, conventional membranes applied to thetreatment of saline water are not feasible because the life span of the membranesis shortened and the performance is deteriorated. In order to make membranesapplicable to the demineralization process of milk or whey, the investigation wasperformed to give the following characteristics to the membranes.

Anti-Organic Fouling

The specific conductance of various membranes k was determined in ademineralization experiment of Gouda cheese whey. The membranes are,Aciplex K-101 (conventional cation exchange membrane, Asahi Chemical Co.),A-101 (conventional anion exchange membrane) and A-201 and A-211 (both are

A-101

100

80

60

40

20

00 2000 4000 6000 8000

A-211A-201

Operating time (hr)

Rem

aini

ng s

peci

ficco

nduc

tanc

e(%

) K-101

Figure 1.42 Membrane conductivity changes with operating time (Okada et al., 1975).


newly developed anion exchange membranes). k of K-101 is not changed largelywith running time as indicated in Fig. 1.42. k of A-101 shows very rapid de-crease. However, k of A-201 and A-211 membranes does not change signifi-cantly for 6000 h. A-201 and A-211 membranes show excellent anti-fouling(cf. Section 14.3.2 in Fundamentals).

Anti-Alkaline Circumstance

Organic fouling due to deposition of proteinaceous materials or fattysubstances from the dairy products is cleaned by washing in an alkaline deter-gent by means of CIP method. Exposure of the membrane to OH� ions gen-erated from water dissociation vitiates its performance. Therefore, life span ofthe membrane is shortened without the durability against a basic solution incontact. Fig. 1.43 shows the durability evaluated by measuring specific con-ductance k of the membranes immersed in a 1% NaOH solution. k of conven-tional cation exchange membrane K-101 does not change, however, that ofanion exchange membrane A-101 decreases remarkably with immersing dura-tion. On the other hand, decrease of k is suppressed for anti-organic foulinganion exchange membrane A-201 and A-211. Particularly, A-211 membraneexhibits excellent anti-alkaline performance.

Permeability to Larger Organic Ions of an Anion Exchange Membrane

An anion exchange membrane which does not permeate larger organicanions causes pH lowering in demineralization of dairy products. This is becauseOH� ions caused by water dissociation generated on a desalting surface of ananion exchange membrane permeate the membrane instead of the larger organicanions toward a concentrating cell. This phenomenon induces pH increase in theconcentrating cell and gives rise to the precipitation of inorganic salts such asCa3(PO4)2, CaCO3, CaSO4 etc. On the other hand, pH in the desalting cell is

100

90

80

70

60

50

40

30

20

10

0

Rem

aini

ng s

peci

fic c

ondu

ctan

ce(%

)

0 100 200 300 400 500

A-101

Immersing duration (hr)

A-201

A-211

K-101

Figure 1.43 Membrane conductivity changes with time in alkaline circumstances. 1 %NaOH at 371C (Okada et al., 1975).

Electrodialysis 371

estimated to be lowered because H+ ions are remained in the desalting cell.Fig. 1.44 shows pH changes upon 90% ash reduction of Gouda cheese wheyusing three types of anion exchange membranes. A-211 membrane scarcelybrings about the decrease in pH. This is because pore radius in A-211 membraneis large enough (cf. Section 14.3.2.1 in Fundamentals), so that organic acids suchas citric acid, lactic acid, amino acid as well as phosphoric acid easily permeatethe membrane and that water dissociation does not easily occur.

From the experiment described here, it is concluded that A-211 is the mostsuitable anion exchange membrane. CIP operations of an electrodialyzer incor-porated with A-211 membranes realize 90% ash reduction and life span exten-sion.

1.6.11.4 Electrodialysis System (Okada et al., 1975) for the Demineralization of

Milk or Whey

Fig. 1.45 illustrates the four stack ED process MED SV1/2 4-4 developedby Morinaga Milk Industry Co. The specifications and operating conditions ofthis system are shown, respectively, in Tables 1.18 and 1.19. In Fig. 1.45, the

pH

A-211

A-201 (and A-101)7

6

5

40 10 20 30 40 50 60 70 80 90

Relative deashing rate [%]

Figure 1.44 pH changes of whey in demineralization (Okada et al., 1975).


pump P1 feeds the whey to the balance tank WB-1. Succeedingly, the whey issupplied to the 1st stack of the ED system, demineralized to some extent andoverflows from WB-1 to WB-2. The whey in WB-2 circulates through pump P3

and second stack, then part of which overflows into WB-3. In the similar way,the whey flows via P4, third stack, WB-4, fourth stack, WB-5, and finally de-mineralized whey flows out via P6 and supplied to the succeeding process such aspasteurization, evaporation and drying. At each balance tank, the conductivityof the whey is monitored to control the desalting rate. The feed rate of the wheyto this system is automatically controlled by means of conductivity measurementin the effluent of demineralizing stream.

1.6.12 Desalination of Sugar Liquor

The sugar manufacturing system is classified into the cane sugar–finesugar manufacturing system and the beet sugar manufacturing system. The rawmaterial for the former is sugar canes and that for the latter is beet sugar. In thesugar manufacturing process, the greater part of organic nonsugar componentsin raw sugar is removed by means of defecation, carbonation, adsorption (bonechar, active carbon, ion exchanger etc.). In these treatments, inorganic compo-nents are not removed and transferred to an evaporation process, in whichresidual organic nonsugar components are separated from sugar crystals andremained in molasses. In the course of repeating evaporation and separation, theinorganic and organic nonsugar components are gradually accumulated in the

CB-5 CB-4 CB-3 CB-2 CB-1

P7P8P9P10P11

Con Con

I4

V4V3V2

2nd1st

V1

DilDil

P2

P1Whey

P3 P4 P5

P12

H2SO4

Va1 Water

Demineralizedwhey

Rinse solutionforfastening frame&cathode frame

DC : Rectifier

V1-4 : Voltage meter

I1-4 : Amperage meter

Con. : Concentrating compartmentsDil. : Diluting compartments

P6

PB-5

1st : Stack - 1

2nd : '' - 2

3rd : '' - 3

4th : '' - 4

WB-4WB-3WB-2WB-1

DC

foranode frame(one pass & waste)

ConCon

4thDC

DilDil

DCDC3rd

I3I2I1

Figure 1.45 Flow diagram of Morinaga continuous electrodialysis process (Okada et al., 1975).

Electro

dialysis

373

Table 1.18 Specifications of MED SV1/2 4-4

1. Center-press 12. Stack 43. Electrode 4 pairs4. Cell pair 150 cell pairs/stack5. Distance between membranes 0.75mm6. Effective membrane area 50 dm2/cell7. Spacer Sheet-flow type8. No. of channel 19. Width of the channel 500mm10. Length of the channel 1000mm

Source: Okada et al. (1975).

Table 1.19 Operating conditions of demineralization of whey (Morinaga ED system)

1. Quality of whey to be treated1.1 Total solids 20%1.2 Ash content 1.60%1.3 Specific conductance 0.013 S cm�1

1.4 Sediment test (200ml, 1000 G) less than 0.1ml

2. Operating conditions2.1 Level of applied d-current density 3000� ka (mA cm�2)2.2 Linear velocity in the compartment 12 cm s�1

2.3 Operating temperature 201C

Source: Okada et al. (1975).ak: Specific conductance.


molasses and finally they are discharged to the outside of the system as wastemolasses. The waste molasses includes considerable amount of sugar compo-nents, so it is utilized as raw materials for fermentation or animal food, however,its economical value is extremely low comparing to that of sugar itself. Becauseof the background described above, the technology development was expectedfor preventing sugar component transfer to waste molasses and increasing sugarrecovering ratio. Further desalting technology by means of ion exchange mem-brane ED came to be attracted because the sugar recovering ratio is influencedby residual inorganic components in syrup.

Application of ED in sugar manufacturing industry was investigated fromthe latter half of 1950s. However, it was difficult to put this program intopractice. This is because water dissociation and organic fouling are apt to occuron anion exchange membranes (cf. Section 14.3 in Fundamentals). In order toavoid these troubles, Taito Co. and Asahi Chemical Co. developed the tech-nology using neutral membranes consist of polyvinyl alcohol instead of anionexchange membranes (Sugiyama et al., 1982; Kokubu et al., 1983). The

Electrodialysis 375

advantages and disadvantages of this method (Transport depletion method) areas follows.

Ta

DeCuCud

MoMo(

MopH

AsCMKCSSPCS

NoCaSo

1.

ble 1

saltinrrentrrentm�2

lasselasseoBx)lasse

h (%aOgO

2OliO2

O3

2O5

O2

ulfa

te: I:Cl2 aurce:

Advantages(a) Neutral membranes do not deteriorate due to organic fouling.(b) Sugar does not decompose, because pH decrease caused by water

dissociation does not occur on the neutral membrane.(c) Current density can be increased, because water dissociation does

not occur.

.20

g reffide)

s vs c

s p

on

te a

DuddK

E

atiociennsity

olumonce

urity

soli

sh

ple-ing.okub

2.
Disadvantages(a) Removing efficiency of anions is low.(b) Current efficiency Z is low, because transport number of anions of
a neutral membrane tAA0:5: If we assume the transport number ofa cation exchange membrane tK ¼ 1:0; Z ¼ tK þ tA � 1 ¼ 0:5:

In the first stage in a sugar manufacturing process, raw molasses (originalsyrup) is treated to remove organic materials and evaporated to obtain A sugar

ffect of electrodialysis on pretreatment for molasses

I II I�II

(%) 66.18 63.60 2.58cy (%) 43.55 34.01 9.54(A 3.04 3.07 �0.03

Start End Difference Start End Differencee (l) 10.00 9.76 D 0.24 10.00 9.76 D 0.24 0.00ntration 51.35 46.85 D 4.50 50.45 46.45 D 4.00 D 0.50

(%) 51.70 59.01 7.31 52.66 58.45 5.79 1.526.35 6.35 0.00 6.40 6.40 0.00 0.00

Start End Desaltingratio

Start End Desaltingratio

d)0.19 0.06 68.42 0.22 0.13 40.91 27.510.74 0.33 55.41 0.72 0.42 41.67 13.745.47 1.59 70.93 5.98 2.01 66.39 4.543.99 0.15 96.24 3.65 0.19 94.79 1.450.41 0.33 19.51 0.44 0.41 6.82 12.690.69 0.41 40.58 1.22 1.00 18.03 22.550.18 0.19 D 5.56 0.21 0.18 14.29 D 19.840.19 0.14 26.3212.42 4.73 61.92 12.78 5.19 59.39 2.53

stage centrifuging after CaCl2 adding; II: Duple-stage centrifuging without

u et al. (1983).

B sugar

B molasses Ca(OH)2 CaCl2

1st sludge 1st centrifuged molasses

Water

2nd sludge 2nd centrifuged molassesSeparated molasses

Final sludge

Seawater

Discharged water Water

Desalted molasses (C molasses)

C sugar

Heating

Water

2nd boiling

3rd boiling

1st centrifuging

Refrigerator

Electrodialyzer

Steam

Steam

2nd centrifuging3rd centrifuging

Mixing

Figure 1.46 Desalination of B molasses (Kokubu et al., 1983).


and A molasses. In the next stage, B sugar and B molasses are obtained fromA molasses through the similar process. Finally, C sugar and C molasses areobtained from B molasses. Table 1.20 shows ED experiment of B molasses,which is centrifuged two times after adding CaCl2 (Case I) and without addingCaCl2 (Case II). The experiment indicates that desalting ratio and current effi-ciency in Case I are increased comparing those in Case II. This is because theminerals such as CaO, SiO2, SO3, P2O5 etc. are removed by the CaCl2 treatmentin Case I.

Based on the experiment described above, Taito Co. designed the EDtreatment process of B molasses as shown in Fig. 1.46. In this process, B

Table 1.21 Bacterial strains and media for cell count

Strains Media (Agar)

Staphylococcus aureus Mannitol saltSalmonella heidelberg DeoxycholateEscherichia coli K-12 DeoxycholatePseudomonas aeruginosa DeoxycholateProteus vulgaris DeoxycholateKlebsiella pneumoniae DeoxycholateBacillus subtilis Nutrient

Source: Sato (1989).

C+ A A −

VIVIIIIII

FS

P

AS BSS CS

P P

Anode Cathode

FS

C

Figure 1.47 Schematic diagram of an electrodialytic disinfection diagram. C, Cationexchange membrane (Selemion CMV); A, anion exchange membrane (Selemion AMV);CS, cathode solution; AS, anode solution; BSS, bacteria cell suspending solution; dis-tance between the membranes, 1 cm; membrane area, 18.4 cm2 (Sato, 1989).

Electrodialysis 377

molasses is treated at first by defecation mixing with Ca(OH)2 and CaCl2. Afterheating and two stages of centrifugation, the second centrifuging molasses issupplied to the electrodialyzer via the refrigerator to obtain desalted molasses(C molasses). C sugar is crystallized in the evaporation of C molasses. Byintegrating the ED step mentioned above in the sugar manufacturing process, itbecame possible to obtain D sugar from C molasses.

Cel

l via

bilit

y (%

)

100

90

80

70

60

50

40

30

20

10

0

Current density (A/dm2)

0.54 0.81 1.08 1.35 1.65

: S. aur.,

: K. pneu., : B. sub (spore).

: Sal. heid, : E. coli, :Ps. aerug., :Pr. vulg.,

Figure 1.48 Relation between bacteria cell viability and current density. Flow rate,3 cm3/min; duration time, 60min (Sato, 1989).


1.6.13 Electrodialytic Disinfection

Sato et al. (1984) investigated water disinfection by means of ion exchangemembrane ED. The merits of this method are as follows.

(1)
The operation is proceeded at normal temperature. (2) Comparing to chlorine disinfection, the electrodialytic disinfection is
more powerful and proceeded during shorter time.
(3) The process is not harmful to human body.
Bacterial strains in Table 1.21 were cultivated at 371C for 18 h. The cul-tivated solution suspending 108 cells cm�3 of bacteria cells was supplied into thedesalting chamber (chamber III) in an ED system in Fig. 1.47 and electrodi-alyzed for 60min. Viability of the cells is plotted against current densities andshown in Fig. 1.48. Here, limiting current density is 0.81A dm�2. Bacteriaviability is seen to be decreased with the increase of current density in a range ofover limiting current densities and becomes zero at 1.63A dm�2. Electron

E. coli cellWater dissociation

OH- H+ H+

Anode Cathode

Anion exchange membrane

Desalting chamber Cation exchange membrane

Figure 1.49 Mechanism of disinfection in electrodialysis (Sato, 1989).

Electrodialysis 379

microscope observation revealed that bacteria cell conformation is shrank underapplying over limiting current density. The mechanism of electrodialytic disin-fection in this study is estimated as follows.

In Fig. 1.49, Escherichia coli cells are suspended in a solution in the de-salting chamber (Chamber III). Passing over limiting current in this system, theelectrolyte concentration in the desalting chamber is decreased and electric re-sistance of the solution is increased. In this situation, water dissociation is gen-erated on the anion exchange membrane (cf. Section 8.8.1 in Fundamentals) andH+ ion concentration in the desalting chamber is increased. An E. coli cell is anelectron conducting substance, so H+ ions pass through the E. coli cells. Thisphenomenon is similar to that in electrodeionization (cf. Chapter 4, Fig. 4.8 inApplication). E. coli cells are estimated to be destroyed by H+ ions passingthrough the cells.

The investigation described here should be analyzed from the biologicaleffects of alkaline electrodialyzed water at the cellular level (Takahashi, 2006;Kikuno, 2006).

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Electrodialysis 381

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