Demineralisation of vinasse by electrodialysis

Jourt~ul of

Food Engineering 7 (1988) 177-196

Demineralisation

of Vinasse by Electrodialysis

Janusz A. Milewski Institute of Fermentation

Industry, ut’.Rakowiecka 32,02-532

Warsaw, Poland

and Piotr P. Lewicki Department

of Food Technology, Warsaw Agricultural University (SGGW-AR). ut Nowoursynowska 166,02-766 Warsaw, Poland

(Received 27 May 1986; revised version received 6 February 1987: accepted 4 August 1987)

ABSTRACT The purpose of this study was to examine the possibility of demineralising vinasse by electrodia~vsis. A prototype laboratory electrodialyzer was used with ‘.I‘elemion’ (Asahi Glass Co) ion-exchange membranes at a constant current density between 5 and 15 mA cm-‘. Electrodialysis with cation- and anion-exchange membranes CUFI remove 70-85% of ash from vinasse at a current density in the range j-I.5 mA cnt 2 at 35°C and at a vinasse concentration of 6-2O”Brix. The current density exerts most influence on vinasse demineralisation kinetics. The process ran with highest intensity at maximal current dens@. that is 15 mA cm-‘. An increase in vinasse concentration from 6 to 2O”Bri.xreduces the ioss of drv organic matter in the demir~eralisatio~l process. An increase in temberature from 15 to 3YC increased ash removlul byi about 5%. The vinasse velocity through the desalting compartments within the range of 2-10 cm s- ’ has no significant influence on the demineralisation process. During electrodialysis the pH of vinasse decreases. When the vinasse is demineralised to the extent of 80% K+ ions are completely removed, the Na’ ion content is decreased by about S.?%, that of Ca” by about 40% and that of A@“’ by about 19%. The sulphate content decreases by about 38% when vinasse is demineralised by 76%. During vinasse demineralisation its betaine content does not change, and the percent of total amino 177 Jouwzul

of Food Engimwing 0260-8774/88/$03.50

Publishers Ltd. England. Printed in Great Britain

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178

J. A. Milewski, P. P. Lewicki acids in the remaining dry matter increases. With increasing demineralisation the energy consumption per gram of ash removed increases. To obtain 75% demineralisation about 10 Wh g- ‘of ash were required.

INTRODUCTION Electrodialysis Although the first publications concerning electrodialysis with ionexchange membranes go back to the beginning of this century, it was only as late as the 1950s that rapid development in the relevant technology and its practical application took place. A classic example of electrodialysis applications in the food industry is the demineralisation of water (Tuwiner, 1962; Wilson, 1960) and milk products (Higgins and Short, 1980; Okada et al., 1975). Since 1975 large industrial plants have been in operation in Japan to purify water for use in boilers (Nakamura and Itoi, 1976) One of these plants has an output of 2000 m3 per day and another 12 000m3 per day. The chloride ion concentration is reduced from 845 to 190 ppm, and energy expenditure for electrodialysis is 1.83 kWh rnp3. A system has been developed for the production of table salt from seawater by way of electrodialysis (Lacey and Leob, 1972). By electrodialysis of whey, valuable raw material can be obtained for use in baby foods (Van Steeden and Hoeting, 1970), alternatively the properties of whey can be modified to provide a non-caking product after drying (Francis, 197 1), or its acidity can be decreased (Williams and Kline, 1980). The stability of liquid whole milk and skim-milk may be increased by demineralisation (Lonergan, 198 1; Morinaga Milk. 1970). The electrodialysis of milk products permits 90% demineralisation at about 20°C and plant cleaning by CIP (clean-in-place) ensures nearcontinuous operation of the plant (Hiraoka et al., 1979). Investigations into the use of electrodialysis in processing sugar industry by-products indicate that elimination of 40% of K+ is possible from a molasses of 5O”Brix concentration (Giorgi et al., 1980). Reduction of the potassium content of wines and fruit juices by electrodialysis (Wucherpfennig, 1973, 1974) prevents the formation of oxalate and tartrate sediments. Electrodialysis can easily be automated and is economically advantageous in many cases. According to Japanese data, the cost of brine obtained by electrodialysis of seawater is comparable to that of brine produced from imported salt. Since 1972 two Japanese firms have

Dimineralisation

of vinasse

179

offered seawater concentration plants, both with yields of 3000 m3 h-l. The energy requirement for this process is about 300 kWh per ton of salt. Vinasse Vinasse is a by-product of molasses fermentation and distillation. It contains all the non-sugar solids of molasses and the other chemicals added to molasses during processing. Sulphuric acid is usually added so the corresponding sulphates form in the vinasse. The chemical composition of vinasse varies with the sugar beet variety, soil fertilisation and the amount of precipitation. A typical composition of vinasse is shown in Table 1. Vinasse contains many valuable organic components such as nitrogen-free organic acids, betaine, amino acids and glycerol. Inorganic compounds can be determined as ash, the amount of which varies from 26 to 34% of the dry matter. The main component of the ash is potassium, varying from 45 to 59% (Zagrodzki, 195 1). The proteinTABLE 1 Typical Composition of Filtered Vinasse Component

Coi2centration ( %)

Water Dry matter

8X5-905 115-95

The dry matter contains: nitrogen-free organic acids glycerol betaine amino acids unknown total organic matter

G20 5 15 7-10.5 > 15 =70

potassium calcium sodium Mg, Fe, Zn, Cu, Pb, Ni non-metallic components

5.8-15.6 1.0-3.2 1.3-2.5 0.1-0.6 21.8-8.1

(S,N,C,O,H,Cl) total ash In the case of incomplete sugar

= 30 fermentation

vinasse contains some

1x0

J. A. Milewski. I? I’. Lewicki

aceous substances, coloured components and the composition of the microflora in vinasse are not fully known. Betaine is one of the interesting components of vinasse. It is a good antioxidant for lipids and can replace choline in poultry feedstuffs. Betaine is also a component of some pharmeceutical preparations. Many publications point to the substantial advantages of using vinasse in fodder mash (e.g. Braun, 1977; Krzyianiak, 1977). A limiting factor, however, in feeding vinasse direct is its high mineral, especially potassium, content. One possible way of desalting vinasse is by precipitation of calcium and potassium as sulphates (Stros, 1967). Other methods use mineral acids and organic solvents (Robertiello and Degen, 1980), extraction with pyridine or ion exchange (Gwardys, 1979). It is possible to utilise vinasse as substrate for the biosynthesis of fodder vitamin B,?, or as a source of glutamic acid, betaine, glycerol and potassium carbonate. Most vinasse in practice is used for the production of microorganic biomass or is concentrated for use as fodder. The aim of the work described here was to ascertain the possibility of demineralising vinasse by electrodialysis. The influence of current density, temperature, concentration and vinasse flow rate through the electrodialyzer were examined.

MATERIAL

AND METHODS

As basic raw material, the vinasse was taken from the stripping column of an industrial distillery. The concentration of the vinasse after clarification in a self-cleaning centrifuge (Westfalia Separator AG, type SA 1-Ol175, BRD) was about 6”Brix and this was increased to either 13 or 20” f 1 Brix in a rotary evaporator (Buchi, type R-20, Switzerland). A multi-compartment laboratory electrodialyzer with 200 cm2 of active surface on each membrane, constructed by the Wroclaw Institue of Technology, Poland was used, equipped with ‘Selemion’ type AMV and CMV membranes (Asahi Glass Co. Ltd, Japan). The compartments containing the anode and cathode were charged with a sulphuric acid solution, and a sodium chloride solution was passed through the concentrating compartment. A diagram of the cell showing the principle of the demineralisation process and the flow of solutions is shown in Fig. 1. The electrodialysis process was run periodically at constant temperature and at constant current density. Before starting the experiments the limiting current density ilimwas determined by the method described by Cowan and Brown (1959).

IXminerulisution

Electrode

Demineralised

of vinasse

solution

vinasse

1

Concentrated solution

II

Raw

vinasse

0 cation 0 anion Fig. 1.

HP SO4 The principle of the electrodialysis

The following determinations

Solution

process.

were carried out:

(4 electric conductivity, by a Mera-Elwro Type N-571 conductivity meter with a type PS-2z Energopomiar,

Gliwice sensing device:

(‘4 pH with a pH meter (Radiometer, type PHM 64a, with type GK (c)

2301C electrode, Copenhagen); vinasse concentration, by Brix hydrometer accuracy of 0.1 O;

with

a reading

J. A. Milewski, P. P. Lewicki

182

dry matter content by drying at 105°C to constant weight; in a Motgos resistance furnace, type WKe idI ash after incineration 105-C at 550-600°C to constant weight; (f) cations (sodium, potassium, calcium, iron, copper, magnesium) directly from the vinasse by atomic absorption spectrometry with an AP-1900 spectrometer (Pye Unicam); (g) sulphates by precipitation as barium sulphate and incineration of the precipitate in a furnace at 700-800°C; betaine by photocolorimetry at A= 525 nm after precipitation of (h) betaine with ammonium reineckate (BDH Chemicals Ltd, Poole,UK); (i) amino acids by gas chromatography. RESULTS AND DISCUSSION Characteristics

of the vinasse

The composition of the vinasse used in this work is given in Table 2. The values obtained are typical and consistent with data in the literature. Zabrodskij (1972) states that the dry matter of vinasse produced in the USSR contains between 24 and 45% ash. In two successive years Polish vinasses had a mean ash content of 25.9 and 2 1.4% of the dry matter and their pH was between 4.5 and 5.0, with an arithmetic mean of 4.8 (Gwardys, 1979). TABLE 2 Characteristics of the Vinasse Used Arithmetic mean

Dry matter (%) Ash (%) PH Dry matter contains (%) betaine amino acids sulphates

5.08 1.42 4.83 20.66 8.21 2.84

Range

4.92-5.56 1.15-1.68 4.63-4,94 14.98-26.9 1 5.09-12.66 2.04-3.3 1

The electrical conductivity may characterise the state of the vinasse since it increases with concentration (Fig. 2) and reaches a maximum at about 3O”Brix. A relationship of the type shown in Fig. 2 is due to the composite effect of the degree of ionisation, the ionic interference and

Diminerahation

cfvinasse

183

. f5’C o 25'C x 35'C

Concentration,

Fig. 2.

Influence of vinasse concentration

O&ix

and viscosity on electrical conductance.

the concentration of ions (Brown, 1964). Moreover, viscosity of the solution affects an ion’s velocity in the electric field. An increase in vinasse concentration increases the total number of solute particles which, in turn, increases electric conductivity. The decrease in the degree of ionisation of weak electrolytes, the increase of the ionic interference in strong electrolytes (the ionic freedom decreases) and the decrease of ion velocity due to viscosity increase, which accompany vinasse concentration, decrease electric conductivity. A balance between these opposite

J. A. Milewski, P. P. Lewicki

184

effects is reached at a concentration of 30”Brix and the maximum electric conductivity at that concentration is observed. Intermediate products in sugar beet processing also show a maximum electric conductivity at a similar concentration of 30-35% dry matter (Houssiau and Pieck, 1976; Zagorodnii et al., 1975). Limiting current density The values of the limiting current density determined for three vinasse concentrations and three flow rates are shown in Fig. 3. The limiting

0-z

L

5

10

15

Concentration, Fig. 3.

Dependence

20

*Brc’x

of limiting current density, i,,,, on concentration vinasse.

and flow rate of

current density increases with the vinasse concentration and with increase in flow rate. The change in limiting current density with vinasse concentration is greater the greater the flow rate. The range of limiting current densities was 12.75-35.00 mA cmp2. The greater the vinasse concentration and the flow rate, the greater the change in limiting current density.

185

Dimineralisation of vinasse

The process of electrodialysis should be carried out at current densities less than the limiting current density. This is because current densities above the limiting value cause concentration polarisation and water electrolysis. The increased concentration of ions at the membrane surface and formation of H’ and OH- changes the pH of the solution, precipitates are formed and the membrane selectivity decreases. On the basis of the results shown in Fig. 3, 15 mA cme2 was adopted as the upper limit of current density for further experiments. All the demineralisation trials were performed without difficulty with the exception of those at a concentration of 6”Brix and a flow rate of 2 cm s- ‘, for which the current density of 15 mA cme2 was above or at the limit. At this concentration white sediments of calcium and magnesium sulphates were observed. Vinasse demineralisation The changes in ash content with demineralisation by electrodialysis are shown in Fig. 4. Within the range tested (up to 75-85% demineralisation) the graphs are linear. Demineralisation occurs rapidly in a medium with a low initial ash content. Further, when different current densities are

6 &ix l

f3

l

Brix

20 o &ix

Fig. 4.

Vinasse demineralisation

kinetics at equal energy input of 10 mA cm-’ and a temperature of 35°C.

186

J. A. Milewski, P. P. Lewicki 0

5

mA/un)

A 0 tl 10 mA/cm’ x = i5 mA/cma

Time, Fig. 5.

Vinasse demineralisation

kinetics at constant concentration temperature of 35°C.

h of 20”Brix and a

used at constant vinasse concentration, the process proceeds most rapidly at the highest energy input (Fig. 5) in accordance with Faraday’s Law. During electrodialysis ionic organic materials also migrate. A decrease in the dry organic matter content by about 20% of the initial values was observed (Fig. 6), depending on the current density, the degree of demineralisation and the vinasse concentration. During electrodialysis, the pH of the vinasse falls (Fig. 7). This is due to the different mobilities of cations and anions in the solution and through the membranes. The changes in pH were also observed during demineralisation of whey (Okada et al., 197.5). Influence

of vinasse concentration

on the demineralisation

process

Figure 8 shows the influence of vinasse concentration on the amount of ash removed during dimineralisation. It is evident that, under the experimental conditions of this work, the vinasse concentration has no influence on the amount of ash removed at constant current density. This result could be expected as long as the appropriate quantity of electricity was transferred across the solution and at the electrodes.

Dimineralisation of vinasse

187

. 6°8rix xo f3*0rix AO 20*Brix

2

4

6

Time, h Fig. 6.

Changes in dry organic matter content during vinasse demineralisation current density of 10 mA cm-’ and a temperature of 35°C.

I

01

q

.

1

2

3

I

at

6O &ix M’ BnL

5

6

Time,h Fig. 7.

pH of vinasse during demineralisation (arithmetic mean of several experiments at 10 mA cmm2, 35°C).

J. A. Milewski, P. P. Lewicki

188

1

x

5 mA/cm’

0 8

10 mA/cml 15 mA/cm’

l

w

0 0

20

n

0

0

t

X

X

10 -

0

L

X

, 6

13

20

Concentration, Fig. 8.

O&ix

Influence of vinasse concentration on the mass of ash removed in the process of demineralisation (35”C, 2 h).

The passage of an electric current through an electrolyte causes a transfer of ions and discharge of them at electrodes. There are, therefore, ions carrying the current to the electrode and those which are actually discharged at the electrode. The carriage of current depends on the concentration and speeds of the various ionic species present in the solution, whereas the discharge potential is determined essentially by the reversible potential in the given solution of the particular ion discharged (Glasstone and Lewis, 1960). Hence, if the total quantity of electricity carried through the solution is appropriate the two processes are quite independent, and the mass of discharged ions is not affected by the electrolyte concentration. The decrease in dry organic matter is dependent on vinasse concentration, being less the greater the vinasse concentration. In unconcentrated vinasse the amount of dry organic matter removed is twice as much as that at 20”Brix. According to Faraday’s first law of electrolysis the amount of any substance deposited or dissolved at an electrode is gived by the equation: m=

kit

(1)

Dimineralisation of vinasse

0,

f

x

6 @&ix

0

U.&ix

189

c

I’, 4

Charge Fig. 9.

Relationship

21, IS

of electricity,

C-IO”

between the quantity of electricity and the mass of ash removed in the process of demineralisation (35”C, 2 h).

where VZ=mass in grams, I= current strength in amperes, t= time in seconds and k= electrochemical equivalent of the substance in grams per coulomb. Since it has been shown that m is not dependent on vinasse concentration Faraday’s Law is rigorously valid and is presented in Fig. 9. The vinasse is a complex mixture of ionic and non-ionic species and for this reason eqn ( 1) should be modified as follows: m= qklt where 7 is the current efficiency (Milazzo, 1963). The calculated value of q k is 15 x 10 -4 g C- ’ for the vinasse electrodialysis at 35°C and a raw material velocity of 2 cm s- ‘. Effect of temperature A rise in temperature leads to an increase in the electrical mobility of ions. However, if the total quantity of electricity carried through the

190

J. A. Milewski, P. P. Lewicki

solution is appropriate and the amount of ash removed is not affected by the vinasse concentration, the increase in ion mobility will have little effect on the process of electrodialysis. In unconcentrated vinasse a rise of 10°C causes an increase in ash removal of about 3%, and in concentrated vinasse the influence of temperature is even smaller. Ash removal increases by about 2% for each rise of 10°C within the range of 15-35°C. The slope of all three lines shown in Fig. 10 is practically the same and independent of the concentration.

0 2O’Briw l

13’ B?i#

x 6OBrix

Fig. 10.

Influence of temperature

on ash content of demineralised vinasse (10 mA cme2, 2 h).

There are some reports on the benefit of elevated temperature on demineralisation both of wine in the range of 6*5-25°C (Postel and Prasch, 1977) and of syrup from hydrolyzed maize starch at 26-60°C (Robertiello and Degen, 1980). The results of this work show that temperature affects electrodialysis little and, therefore, the choice of temperature should, in the first place, be determined by the characteristics of the raw material and the temperature resistance of the membranes.

Diminerahation of vinasse

191

Effect of vinasse velocity The effect of different vinasse velocities on demineralisation degree is shown in Fig. 11. There is very little effect evident. These results are similar to those obtained with the laboratory electrodialyzer produced by Ionica USA (Johnson et al., 1976), on whey demineralisation.

too t P

a

0 5 mA/cmz IOmAlcm2 n 15 mA/cm’

l

0

6

10

c

Velocity, cm/a Fig. 11.

Influence of vinasse velocity on relative ash content (35”C, 13”Brix, 2 h).

A greater velocity increases turbulence, reduces the thickness of the laminar hydrodynamic layers and reduces the adhesion of solid particles to the membranes. The electrical resistance of the boundary layers is reduced and the probability of sediment precipitation is also reduced, due to flushing of the membrane surfaces. Hence, the velocity should affect the process of electrodialysis, and, probably, the effect of linear velocity in the desalting compartments on the electrodialyzer performance would be observed during a long run. In short experiments, with the appropriate quantity of electricity transferred across the solution, this effect is negligible. The typical range of linear velocities in industrial equipment is 5- 15 cm s-r for parallel-flow type stacks, and 30-50 cm SC’ in tortuous-path

192

J. A. Milewski, P. P. Lewicki

stacks (Lacey and Loeb, 1972). The linear velocity in the desalting compartments of the present authors’ apparatus was 2- 10 cm s - I. Changes in the content of some vinasse components during electrodialysis The relative change in the content of particular cations during electrodialysis depends on the degree of demineralisation (Fig. 12). Large changes occur in monovalent cations, especially sodium and potassium.

Co

01

\K, 20

40

60

80

&mineralization Fig. 12.

,$

Changes in content of some cations in vinasse in relation to the degree of demineralisation. Arithmetic means of several experiments.

Complete removal of potassium ( < 1 mg dmP3) occurs when 80% demineralisation is reached. At that point about 45% of the original sodium, 70% of the original calcium and 85% of the original magnesium remain in the solution. No changes were found in the iron and copper content. If an electrolyte contains a number of different positive and negative ions then each ionic discharge will take place as the appropriate potential is reached. The discharge potential or deposition potential at any

193

Dimineralisation of vinasse

electrode depends on many factors, such as current density, the chemical nature of the electrode material, roughness of the electrode surface and the nature of the electrolyte. In general, however, it can be accepted that ions having the largest reduction potential are those which are discharged most readily (Glasstone and Lewis, 1960). The standard reduction potentials of certain cations at 25°C are given in Table 3. According to these data the deposition of metals on the cathode should be in the order given in the first column of Table 3, provided there are no disturbing factors. The cation-exchange membrane can affect the electrical mobility of cations and can change the order of deposition.

Standard

No.

TABLE 3 Reduction Potentials of Certain Cations at 25°C (Glasstone and Lewis, 1960; Milazzo, 1963) Element

K+/K Ca’+/Ca Na+/Na Mg?+/Mg Fe2 +/Fe Fe3 +/Fe cu* +/cu cu+/cu

Potential ( V)

+ +

2.925 2.87 2.714 2.37 0.44 0.036 0.337 0.52

Similar differences in the rate of migration of the particular ions were also noted in whey (Ton-ma et al., 1980) and skim-milk electrodialysis (Okonogi, 1974). The most valuable component of vinasse is betaine. Its amount does not change in the course of demineralisation by electrodialysis, because the betaine molecule is relatively large and in aqueous solution has both a positive electrical charge associated with the ammonium group and a negative charge on the carboxyl group [(CH,),N +CH,COO -1. The amino acid content of vinasse diminishes during electrodialysis but its proportion in the dry matter increases with the degree of demineralisation (Table 4). All amino acids have at least two groups that are involved in proteolytic reactions in aqueous solution. At pH values between about 4 and 9, the amino acid exists in a dipolar form, the carboxyl group being dissociated and the amino group associated. Over this pH range the

194

.I. A. Milewski, P. P. Lewicki

TABLE 4 Some Amino Acids in Demineralized Vinasse (% of Dry Matter) Degree of demineralization ( %) Specification

0

44

0

Vinasse concentration (“Brix) Dry matter (%) Amino acids (g kg- ‘) Amino acids (%of dry matter) Glutamic acid Serine Alanine Lysine Methionine

19.8 17.3 8.8 5.09 3.01 0.45 0.35 0.13 0.01

16.1 15-O 9.3 6.17 3.33 0.91 0.41 0.15 0.01

19-l 17.0 12.3 7.25 3.83 1.22 0.50 0.13 o-01

54

0

13.9 12.5 13.3 10.9 9.9 8.5 8.24 7.80 4.29 4-32 1.41 1.19 0.60 0.53 0.17 0.13 0.01 0.01

57

0

79

9-3 8.2 7.6 9.20 4.78 l-45 O-67 O-18 0.01

6-4 5.3 6.7 12.66 7.53 2.02 0.62 0.30 0.01

3.8 3.4 4.9 19.87 1028 3.23 1.31 0.45 0.01

amino acids bear little net charge (Mahler and Cordes, 1966) and thus, move little in an electric feild. The side chain of aspartic acid, glutamic acid, lysine, histidine, arginine, cysteine and tyrosine has appreciable acidic or basic properties. The aqueous mixture of amino acids subjected to an electric field undergoes separation which is affected by the pH of the electrolyte, the deposition potential and the permeability of membranes to cations and anions. During vinasse processing sulphuric acid is used so that, of the anions, sulphate predominates. In the course of demineralisation the amount of sulphate decreases. Irrespective of electrodialysis conditions, a mean degree of demineralisation of 73% corresponds to the removal of 38% of the sulphate as SO:-. Electric energy expenditure In the initial phase of electrodialysis

the energy consumption depended on the current density used and varied between 3 and 10 Wh g- 1 of ash removed. Toward the end of the process it increased to between 15 and 40 Whg-‘.. In the removal of potassium from grape juice about 5 Wh g-l were consumed (Wucherpfennig, 1973). In the demineralisation of whey 0.79 Wh g- l ash were required (Delbeke, 1975) for a 70% demineralisation. The quoted results and the values obtained in the present experiments differ widely. This may result from the character of the raw material used and the construction of the dialyzer as well as the scale of the process.

Dimineralixztion of vinasse

195

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Higgins, J. J. and Short, J. L. (1980). Demineralisation by electrodialysis of permeates derived from ultrafiltration of wheys and skim milk. N. Z. J. Dairy Sci. TechnoL, 15 (3), 277-88.

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Houssiau, J. and Pieck, R. (1976). On the use of electrodialysis in the sugar industry. Sucretie Be&e, 95 (4), 143-55. Johnson, T. K., Hill, C. G. and Amundson, C. H. (1976). Electrodialysis of raw whey and whey fractionated by reverse osmosis and ultrafiltration. J. Food Sci., 41(4),

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Lacey, R. E. and Loeb, S. (1972). Zndustrial Processing with Membranes, J. Wiley-Interscience, New York. Lonergan, D. A. ( 198 1). Use of electrodialysis and ultrafiltration procedures to improve protein stability of frozen concentrated milk. Diss. Abstract Znt., B40 (3), 1107.

Mahler, H. R. and Cordes, E. H. (1.966). Biological Chemistry, Harper & Row, New York, pp. 15-36. Milazzo, G. (1963). Electrochemistry Theoretical Principles and Practical Applications, Elsevier, London. pp. 24-274. Morinaga Milk Ind. Co. Inc. ( 1970). Frozen condensed milk. Jap. Patent 1 221/ 70. Nakamura, J. and Itoi, S. (1976). Construction and operation experiences of large scale electrodialysis of water desalination plant. Fifth Znt. Symp. on Fresh Waterfrom Sea, Alghero, Italy. Okada, K., Tomita, M. and Tamura, Y. ( 1975). Electrodialysis in the treatment

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