Defluoridation during desalination of brackish water by electrodialysis

Desalznotwn, 71 (1989) 301-312 Elsevler Science Publishers B V , Amsterdam -

301 Pnnted m The Netherlands

Defluoridation During Desalination of Brackish Water by Electrodialysis* S K ADHIKARY, U K TIPNIS, W P HARKARE and K P GOVINDAN Central Salt & Marine Chemrcals Research Instrtute, Bhuunagar 364 002 (I&u), 24714, Telex 0182-230 SALT IN (Received September 29,1988, m rewed

Tel 0278-

form January 12,1989)

SUMMARY

Stules have been conducted to defluorldate brackish water containing 2120, 3020, 4260 and 4800 ppm total dissolved solids (TDS) and 5, 10, 15 and 20 ppm fluoride by means of electrodialysrs. A laboratory electro&alysis stack contannng 15 cell pairs of cation- and anion-exchange membranes of 80 cm2 effective cross-sectional area is used Data have been collected under different flow rates and current densities Optimum operational conditions have been determined for obtaining product water contannng < 600 ppm TDS and < 15 ppm fluoride, which IS acceptable for use as potable water Brackish waters up to 5000 ppm TDS contannng fluoride up to 15 ppm can be reduced to ca 600 ppm TDS and < 15 ppm fluoride. This approach is found suitable for desahnation and defluorrdatlon of water having TDS up to 5000 ppm and fluoride up to 10 ppm with an energy requirement of c 1 kWh/kg of salt removed Keyulords brackish water, defluondatlon,

desahnatlon, electrodlalysls

INTRODUCTION

In many parts of Andhra Pradesh, %Jasthan, PunJab, Haryana, Uttar Pradesh, Madhya Pradesh, Gujarat, Maharashtra and Tamilnadu, water is brackish and contains fluoride mostly m the range of 3-8 ppm [ 1,2] At a fluoride concentration of l-l 5 ppm, the teeth of only a small percentage of children are affected A fluoride concentration of 1.5 ppm is therefore not to be exceeded in public water supplies because it causes the well-known dental defect called “mottled teeth” [ 31. The I&an Muustry of Health has therefore prescribed 1 ppm as the permitted concentration of fluoride m drmkmg water Several methods for fluoride removal are. (1) the precipitation method mvolvmg the addition of chemicals and precipitation or copreclpltatlon of the *Thuspaper was presented at the 5th meetmg of the Indian Membrane Society held at Indum Institute of Technology, Bombay, May 13-14,1988

ooll-9184/89/$03

50

0 1989 Elsevler Science Pubhshers B V

302

fluonde [ 41, (11) the adsorptlon method mvolvmg passage of the fluonde-contaming water through a contact bed [ 41 and (ni ) the Nalgonda techmque developed by scientists at the National Environmental Engmeermg Research Institute (NEERI), Nagpur, In&a [ 5-71, where water 1s treated with hme and alum successively followed by flocculation and sedlmentatlon. Bleachmg powder for &smfectlon can be added simultaneously with hme and alum Desahnatlon of brackish water by electro&alyns (ED) IS a well-known process for removal of salt. Extensive mvestlgatlons and field trials have been carried out m this mstltute [ 8-121. Some demonstration-cum-experimental ED plants have also been installed m villages of RaJasthan, GuJarat and Tamllnadu It was found during the field trials that the water sources m many villages were not only brackish, but that the fluoride content was also high and varied from 3 to 8 ppm During this mvestlgatlon, stu&es have been carried out on defluorldatlon of saline water contammg fluorides and TDS at various levels to get potable water having TDS well below 1000 ppm and a fluoride content of ca 1.5 ppm or less. EXPERIMENTAL

PROCEDURE

The salient features of the ED stack 1s gven along with the flow hagram m Fig 1 The stack is packed with 15 cell pairs of cation- and anion-exchange membranes prepared from mterpolymerlc films based on high-density polyethylene (HDPE)-linear low-density polyethylene (LLDPE)-styrene-&vlnylbenzene The ratio of HDPE LLDPE in the mterpolymer 1s kept at 80 20 )______<

I

l-

FEED WATER

I

ELECTRODE WASH

DILUATE CONCENTRATE

C=Catlon exchange membrane A=An~on exchange membrane

ELECTRODE

WASH

IN

I

Fig 1 SchematIc flow diagram and sahne features of electrodlalysls stack ED stack 15 cell pairs of cation- and amon-exchange membranes made from mterpolymer fihns based on HDPE-LLDPEstyrene-dlvmyl benzene Areal resistance catlon-exchange membrane, 2-3 Ohm/cm’, amon-exchange membrane, 3-4 Ohm/cm’ Cell thickness, 0 2 cm Gaskets bmlt-m flow arrangements and spacers Electrodes expanded titannun metal coated with precious metal oxide Housmg for electrode npd PVC with built-m-flow dlstnbutor and outlets Pressmg assembly threadmg rods with nuts for leakproof assembly Flow arrangements parallel-cum-senes flow m three stages

A parallel-cum-series flow m three stages is employed m the stack, which has a total membrane area of 0 24 m*. Brackish water contauung known quantities of TDS and F- IS fed to the ED stack by gravity flow through a distribution panel After removal of air from the ED stack, flow rates of diluate and concentrate streams are adjusted A small quantity of brackish water at a flow rate of 200 ml/h IS passed through the electrode compartments to remove the products of electrolysis. An electric potential of 25 V IS apphed between the two electrodes After attaining a steady current, the product water, concentrate water and electrode wash are collected at hourly intervals for analysis of TDS, Cl -, total hardness and FTDS is measured gravimetncally, Cl- by titration with standard AgN03 solution, total hardness by EDTA titration and F- by calorimetry [ 131 From the voltage, current, TDS of feed and product water, flow rates of treated stream and number of cell pairs, current efficiency and energy consumption m kWh/kg salt removed are calculated A material balance for TDS, Cl-, total hardness and F- before and after ED is calculated and found to be matching After runnmg the unit contmuously for 6 h, the electrical polarity 1s reversed and the stack is run for another hour to prevent any scale formation due to hardness of brackish water inside the stack Synthetic brackish waters having TDS values of 2000,3000,4000 and 4800 ppm and contaunng 5,10,15 and 20 ppm F- are prepared by addmg an appropriate amount of NaCl and NaF to tap water which contains 700 ppm TDS and 290 ppm total hardness as CaCO, For the brackish water contaming fixed TDS and F- experiments are conducted under different flow rates of &luate stream varying from 1 0 l/h to 4 8 l/h A ratio of flow rates of dduate to concentrate streams has been fixed at 3 1 in all the experiments RESULTS AND DISCUSSIONS

The results of defluoridation by ED of brackish water having different TDS and fluoride contents are shown m Tables I-IV For all brackish waters, reduction m TDS and F- content Increases with decreasing flow rate A product water having TDS well below 900 ppm and fluoride content of 15 ppm can be achieved from brackish water contaunng TDS up to 4260 ppm and fluoride up to 15 ppm Table IV shows that product water of 620 ppm TDS and 15 ppm fluoride can be obtained from brackish water contaunng 4800 ppm TDS and fluoride up to 9 0 ppm. Barring some cases, the current efficiency is above 80% and energy consumption m most cases is below 1 kWh/kg salt removed Current efficiency IS higher with higher flow rates. This is because at lower flow rates the TDS of brackish water is also low - sometimes below 500 ppm (Tables I-IV). At low TDS of water the energy consumption per kg salt removed becomes higher, hence lowermg the current efficiency It has been found that % reduction m TDS, Cl- and total hardness IS almost

(cm/mm)

Whr)

20 0

150

95

192 165 137 120 105

85

59 44

31

50

39 21

20 14

110

35

110

137 120

206 172

16

21 20

123 92 59 44

112

37

56 42

152 129

75 53

34 24 17

210 175

150 120

12 7 96

44

210 168

(d)

Current

58 44

32 20

12 5 88 70

velocity

rate

(ppm)

51 40

Lmear

Flow

60

Dduate stream

F- 1x1

feed water

150 131

206 1 71

2 40

137

150

2 57 2 15 171

1 40

190 1 61

400 220

910 790 580

340

600 440

950 890

350

880 750 530

1020

800 690 420

2 62 2 19

990

2 10 187 150

TDS

25 OS

60 40

70

10

20

60 50 35

10

40 25

50 45

15

40 30 2 25

F-

Product water (ppm)

2 62

densely (mA/cm’)

Current

Defluondatlon dunng electrodlalysls of bracklsb water TDS=2120ppm, Cl-= 1060 ppm, apphed potentlal=25 V

TABLE I

81 2 86 4

62 8 12 I

512

840

71 8 79 3

55 3 58 1

64 7 75 1 83 5

52 0 586

67 5 80 2

62 3

53 4

TDS

Reduction

0 90 090 093 0 89

87 5 97 5

088

090

091

092 0 87 0 93

093

1 12 102

1 10 1 11

108 106

1 60 1 25

(TDS F-j

rat10

‘5 Reduction

650 700 80 0

93 3

76 7 86 7

60 0 66 7

89 5

52 6 519 73 7

47 4

500 625 75 0

33 3

F-

(%)

3 3 1 2

73 3

91 1 86 0

94 6 94 4

90 2 84 2 77 7

95 6 90 2

887 806

93 9 92 0

91 5

92 94 91 85

( %)

efhency

Current

087 099

0 79 0 82

0 79

096

0 83 0 89

0 79 083

080 081 084 093

082

088

081 0 79 082

removed )

(km/kg salt

Energy

20 0

14 0

100

26 19 11

46 37

10

18

34 22

42

16

42 32 22

42 24

10 1 81 57

48 39 22

V

2 46 206 160

128

3 62 3 10

2 25 200 1 55

3 37 292

2 25 187

3 37 2 81

3 00 2 25 187

3 37

(mNcm*)

Current density

197 165

290 248

234 180 160 124

270

180 150

48 35

92 75

270 225

180 150

270 240

(InA)

Current

92 70

48 35

16

22

(I/h)

92 79

(cm/mm)

rate

(ppm)

42 36

Lmear velocity

Flow

feed water

50

Dduate stream

F- ,n

Defluondatlon durmg electrochalysls of brackish water TDS=3020 ppm, Cl- = 1580 ppm, apphed poLenM=25

TABLE II

700 360

1400 1260 950

300

620

1080 750

1250

720 480

1200 1000

700 500

1300 1180

TDS

88 1

53 6 583 685 768

80 65 50 30 10

79 5 901

64 2 75 2

586

84 1

603 669 76 2

83 4

609 768

57 0

TDS

% ReducLmn

75 0 85 0 95 0

600 67 5

77 0 78 8 819 80 1 68 6

0 89 0 86 091 090 093

65 8

091 093 097 85 7 92 8

82 0 840 82 7 80 5

80 7

093 109

095 091

097

090 0 93 1 14

0 91 0 89

0 88 0 87 0 89 0 92

84 4 85 6 83 8

82 1

700 800 85 0

0 93 801

091 090

loo 095 095 099

600

093 091 0 88

79 7

(kWh/kgsalt removed )

Energy

82 2 84 5

(R)

Current effirlency

64 3 714

1 28 1 19

1 90 1 52

(TDS F-j

rat10

600 700

300 400

F-

Reduction (%)

20 10

50 40 25

15

40 30 20

20 15

35 30

F-

Product water (ppm)

20 0

1500

10 0

50

35 20

160 090

39 26

18 12 206 160

280

336

235 175 154

48 31 24

2 00

3 50 2 58

4 20

2 19 1 92

850 600

1160

1410

1400 1300 980 700 s70

770 430

1320 1150 920

720 450

1150 1080

1420

TDS

20

60 40

80

30 20 15

50 40

2 (1 10

35 30 25

20 10

30 25

15

F-

Product water (ppm)

4 20

V

3 78 2 94

160

128

3 88

2 40 1 76

3 50 3 30

4 60

(mA/cm’)

3 37 2 75 2 40

336 302

79 61

28

Current densw

310 270 220 192

264 224 141

368 280

(mA)

Current

79 68

35 17

36

11

36 31 22 14

26 20 16 08

70 57 44

62 57

32

88

284 260

Lmear velocltv (cm/mm)

40

(I/h)

Flow

rate

(ppm)

Dduate stream

waler

feed

F- ,n

Defluorldatlon durmg electrochalysw of brackwh water TDS=4260ppm, Cl- =2320 ppm, applied potentlal=25

TABLE III

859

72 8 80 0

669

86 6

695 77 0 83 6

67 1

800 90 0

60 0 700

900

73 3 80 0 86 7

667

75 0 80 0 900

78 4 819 89 9

650 700

80 0

50 0 600

300 400

F-

(%)

69 0 730

83 1 89 4

73 0 74 6

66 7

TDS

Reduclwn

Current

095

1 04 1 00

111

096 096

0 95 096

100

1 02 100

89 4 82 3

916 93 0

85 5 79 0

919 91 2 92 1

91 0 89 8 91 1 87 2 718

104 104

75 8 729

946 93 9

92 6

effiicwncy (%u)

106

1 38 1 12

1 82 149

2 22

F-j

% Reduction ratlo (TDS

091

0 81 0 84

0 82

0 88 097

0 81 082 081

1 06

082 0 78

0 82 0 83

0 80 099 103

0 71

0 81

Energy CkWh/kg salt removed I

200

150

90

330 285 240

79

59 48 33

24

27 22 15

1I

4 87 4 12 3 56 281 2 25

225 180

3 56 300 2 44

4 12

4 71

190 330 285

195

382

2 25

180

5 06 4 21

1230 960

3 75 309 2 25

400

1610 1350 1090 720

700

860

1600 1380 1170

620

1160 840

1840 1560

640

1820 1540

TDS

40

80 70 50

90

30 25

50 40

60

20 15

35 25

40

20 15

30 25

35

F-

Product water (ppm)

4 87 4 31

(mA/cm’)

3 37 2 81

59

48 35 26

Current density

270 226

408 337

247 180

345 300

390

(m“I)

Currerll

77

33 24

83 61 44

36

12

22 16

35 27

20 15 11

38 28

29 23 17 11

24

79

63 50 37

36

(cm/mm)

Whr)

50

Lmear velocity

Flow

rate

(ppm)

Dduate stream

water

feed

F- In

TDS=4800 ppm, Cl- =2640 ppm, apphed potentlal=25V

Defluondatlon dunng electrodudysls of brackish water

TABLE IV

91 I

719 77 3 85 (I

66 5

756 82 1 85 4

66 7 712

87 1

75 8 82 5

61 I 67 5

867

800

62 1 67 9 74 4

TDS

65 0 75 0 80 0

550 600

83 3

667 73 3 800

600

17 8 83 3

72 2

55 5 61 1

70

1 49

50 60

1 14

1 20 1 19 1 19

121

1 03 1 02 102

111 1 07

1 06 104

1 05

111 1 10

1 33 1 24

207 1 70

(TDS F-1

I Reduction rat.10

30 40

F-

Reduction (“u)

81 6 80 7

84 7 85 9

88 3

78.8 75 7

83 9 84 1

880

76 6

80 8 809 79 2

83 3

79 3 76 3

82 2 82 1

82 5

(%)

Current effinency

092 093

089 0 87

085

089 095 099

089

085

098

093 095

093

090

098

091 095

091 091

Energy (kWh/kg salt removed )

60 95 15 0

2120

0 8”

04’

200 08

24 24 17

24 15

11 07”

11 11

28 26

31 28

13 12

90 15 0

50

200

100 15 0

60

14 13

140 20 0

35 33

39 37

18 17

16 15

44 42

20 19

88 44

(cm/mm)

Whr)

40 20

velocity

Flow rate

Lmear

Dduate stream

50 10 0

“Extrapolated value

4800

4260

3020

50 100

1000

200

F-

TDS

feed water (ppm )

lmtml concentrations m

1 IO”

2 28 184”

2 28

192 1 54”

2 10 2 00

1 82 1 76 170

1 88

1 46 143 1 40

150

188 1 12

460 200

640 620

380

620 570

620

440

500 450 440

390 330

420 410

200

540

(ppm)

(mA/cm’)

Fmal TDS of product water

density

Current

Optunum condltlons for obtammg product water contammg Q 1 5 ppm F-

TABLE V

90 4 95 8

86 7 87 1

91 1

85 4 866

85 4

85 4 85 4

83 4 851

80 7 816 84 4

802

460 800

TDS

92 5

83 3 900

700

900 92 5

85 0

70 0

850 89 3 92 5

700

92 5

84 2 900

750

80

70

F-

Reduction (“4)

100 1 03

124 1 04

096 0 98

100

122

092

100 096

1 19

091 0 91

107 0 96

0 66 100

62 7

099 0 98 106 1 19

75 8 708

0 95 1 13

0 89 091

75 4

818 790 66 2

84 4

76 8 739

095 097 101

0 93

092 092

819 81 5 806 79 2

088 090

(kWh/kgsslt removed )

Energy

85 0 83 3

(R)

(TDS F-j

Currenl

efiiclency

% Reduction rat10

46 55

4 14 4 14

3 49 44

3 23 3 33

2 51 2 61

2 35 2 43

1 64

1 54 1 59

150

1 12

094

(kWh/m’of product water)

309

Flow

rste, 1

h

Fig 2 % Fkductlon ratlo of TDS end F- vs floa rate for feed water of 4800 ppm TDS F- 111feed water (1) 5ppm, (2) 9ppm, (3) 15ppm, (4) 20ppm

the same However, percentage F- reduction IS not always equal to the percentage reduction of TDS (Tables I-IV ). The plot of the ratio of % reduction vs Row rate for feed water of 4800 ppm TDS (Fig 2 ) shows that % reduction m the ratio of TDS F- increases with increasing flow rates of the d&ate stream This means that at a higher flow rate, TDS reduction IS much higher than fiuorlde reduction For a particular flow rate, the reduction ratlo 1s much higher when brackish water contains 5 ppm of F- This ratlo decreases with mcreasmg lnltlal F- contents In brackzsh water and becomes least when mltlal F- content 1s 15 ppm The reduction ratlo for water with 4800 ppm TDS and F- content up to 20 ppm 1salways > 1 It may be due to the presence of a large quantity of Cl- m the water, which prevents the transport of F- through the membranes at the uutlal stages. For lower TDS ( ~4260 ppm) of brackish water contammg F- between 10 and 20 ppm, the reduction ratlo IS < 1 In most cases, as can be seen from Tables I-III. Since under Indian con&tlons a TDS of 1000 ppm and 15 ppm F- In drinkmg water IS accepted, It 1s necessary to find the optimum con&tlons for obtaining product water havmg a maximum F- of 1.5 ppm. The results under optimum con&tlons are presented m Table V The data presented tn this table are experimental except those for the brackish water contammg 4800 ppm

Fig 3 (a) TDS m product water and (b) F- m product water vs flow rate for brackish water having 4800 ppm TDS F- m feed water (1) 5 0 ppm, (2) 9 0 ppm, (3) 15 0 ppm, (4) 20 0 ppm

10

5 F-

I”

feed

15 water,

20

PP~

Fig 4 % Reduction ratio vs F- m feed water contammg tierent TDS under optimum operatlonal condltlons for 15 ppm F- TDS of feed water (1) 2120 ppm, (2) 3020 ppm, (3) 4260 ppm, (4) 4800 ppm

TDS and 15.0 and 20 ppm F- The data for the latter are extrapolated from the graph m Fig. 3. It is seen that to obtain product water contauung 1.5 ppm F-, the flow rate 1sto be reduced wrth mcreasmg uutlal TDS and F- content. The TDS of product water - though wrthm acceptable hmlts - also increases with increasing TDS and decreasing initial F- content. Current efficiency 1s very low ( < 74% ) for the brackish water contanung 3000-4300 ppm TDS and

311

20 ppm F-. A plot of % reduction ratio of TDS and F- content vs F- content of feed water under the optimum comhtrons for 1.5 ppm F- (Fig. 4) shows that the reduction ratio decreases with increasing F- content up to 15 ppm for the same TDS of brackish water However, the reductron ratio increases wrth increasing TDS m brackish water. CONCLUSIONS

( 1) The F- concentratron can be reduced to ,< 15 ppm when the uutlal concentration m brackish water of TDS IS up to 4260 ppm and of F- up to 15 ppm The TDS of product water contaunng ,< 1.5 ppm F- 1s < 600 ppm (2) From brackish water contammg 4800 ppm TDS and F- up to 9 ppm, product water of ca. 600 ppm TDS and 1.5 ppm F- can be obtained directly by ED. (3 ) Even from brackish water contauung TDS as low as 1000 ppm and Fup to 10 ppm, product water contammg ca 600 ppm TDS and 15 ppm F- can be obtained by ED (Table V ) Normally the TDS content of brackish water m areas which have problems with fluoride hazard 1s 3000-5000 ppm and the fluorrde content m these area varies from 3 to 10 ppm So ED can be helpful m reducmg both the bracklshness and the fluoride content to < 600 and < 15 ppm, respectively (4) The energy cost for defluorrdatlon and desalmatlon of the brackish water as mentroned above 1s 2.5-5 kWh/m3 of product water depending on the uutral TDS of brackish water (5 ) The system of ED 1s more compact compared to conventronal chemical processmg techniques

REFERENCES T G Shnmvasan, Cent Pubhc Health R.es Inst Bull, l(2) (1959) 30-54 P R Mehta, B K Shukla, M C Valdya and D J Mehta, Proc of the Symposium on Problems m Water Treatment (CPHERI, Nagpur), 1964, p 147 Paul L Bishop and George Sansoucy, Amencan Water Works Assoc J ,70 (1978) 554 Won-Wock Chow and Kenneth Y Chen, Am Water Works Assoc J (71 (1979) 562 W G Nawlakhe, D N Kulkarm, B N Pathak and K R Bulusu, In&an J Enwon Health, 16(l) (1974) 4 W G Nawlakhe, D N Kulkarm, B N Pathak and K R Bulusu, In&an d Environ Health, 17(l) (1975)26 K R Bulusu.B B Sundaresan,B N Pathak, W G Nawlakhe. D N Kulkarmand W P Thergaonkar, J Inst Eng (India), Part EN, 60 (1979) 1 K P Govmdan and P K Narayanan, Proc VIth Int Conf on Fresh Water from the Sea, 3 (1978) 75 W P Harkare, S K Adhlkary, P K Narayanan, V B Bhayam, NJ Dave and K P Govmdan, Desahnatlon, 42 (1982) 97

312 10 11 12 13

P K Narayanan. W P Harkare, S K Adhlkary, NJ Dave, D K Chauhan and K P Govmdan, Desalmatlon, 54 ( 1985) 145 S K Adhlkary, W P Harkare and K P Govmdan, Indian J Technol ,25 ( 1987) 79 R A Mahabala, SK Adhlkary, P K Narayanan, W P Harkare, SD Gomkale and K P Govmdan, Desalmatlon, 67 (1987) 59 A Handbook of Colorlmetnc Chemical AnalytIcal Methods, Part 3, Inorgamc Chemical Analysis, The Tmtometer Ltd , Sahsbury