ABC of Desalination

SALINE WATER CONVERSION CORPORATION

RESEARCH DEPARTMENT

RIYADH, SAUDI ARABIA

The DesaltingABC ' s

Prepared by 0. K. BurosFor the InternationalDesalination AssociationModified and Reproduced byResearch DepartmentSaline Water Conversion Carporation1990

The Desalting ABCs

This publication has been prepared by the International Desalination Association (IDA)with a grant from the Saline,Water Conversion Corporation (SWCC) in the Kingdom ofSaudi Arabia, to provide a simplified explanation of some of the basics of desalting. Itwas edited and modified by the Research Department at Saline Water ConversionCorporation. Additional information about desalination and its use in Saudi Arabia andother Arab countries provides the reader with the extent of the use of this technology forthe benefit of the people.

Material in this booklet may be used freely. If portions are used, then credit must begiven to this publication or the original source of the material.

Contents

Introduction Page No.

Desalting: A Treatment ProcessThe Development of DesaltingWorldwide AcceptanceAll Countries Producing Desalinated Waterin Order of CapacitiesAll Countries Producing Desalinated WaterWith Processes in Alphabetical OrderArab Countries Using Desalination TechnologyMap of Saudi Arabia Showing Swcc ActivitiesExisting Desalinated Water Pipe LineExisting Desal Plants Along the Red SeaExisting Desal Plants Along the Gulf

Desalting TechnologiesThermal Processes

Multi-Stage Flash DistillationMultiple Effect DistillationVapor Compression Distillation

Membrane ProcessesElectrodialysisReverse Osmosis

Other ProcessesFreezingMembrane DistillationSolar Humidification

Other Solar and Wind-Driven Desalters

Other Aspects of DesaltingCogeneration

1 Concentrate Disposal2 Hybrid Facilities4 Economics

5

92024252628

AcknowledgementsBibliography

29303132333437

42424344

45

Introduction

Desalting: A Treatment Process

Desalting, as discussed in this booklet, refers to a watertreatment process that removes salts from water. It isalso called desalination or desalinization, but it meansthe same thing. Desalting can be done in a number ofways, but the result is always the same: fresh water isproduced from brackish or sea water. Desalting technolo-gies can be used for a number of applications, but thepurpose of this booklet is to discuss the use of desaltingto produce potable water for domestic or municipalpurposes.

The ability to treat salty water so that it can be used fordrinking and agriculture is one people have sought for along time. Over three quarters of the earth’s surface iscovered by salt water and, although this water is impor-tant for transportation and fisheries, it is too salty tosustain human life or farming. Desalting techniques haveincreased the range of water resources available for useby a community.

Previously, only water with a dissolved solids (salt)content generally below about 1000 milligrams per liter(mg/l) was considered for a community water supply.This limitation has sometimes restricted the size andlocation of communities around the world and often ledto hardship to many who could not afford to live near aready supply of fresh water. The application of desaltingtechnologies over the past 40 years has changed this in

MSF Plant

many places. Villages, cities, and industries have nowdeveloped or grown in many of the arid and water-shortareas of the world where sea or brackish waters areavailable and have been treated with desaltingtechniques.

This change has been noticeable in parts of the aridMiddle East, North Africa, and some of the islands of theCaribbean, where the lack of fresh water severely limiteddevelopment. Now, modern cities and major industrieshave developed in some of those areas thanks to theavailability of fresh water produced by desalting seawater.

The Development of Desalting

Desalting is a natural, continual process and an essen-tial part of the water cycle. Rain falls to the ground.Once on the ground, it flows to the sea, and people usethe water for various purposes as it makes this journey.As it moves over and through the earth, the waterdissolves minerals and other materials, becomingincreasingly salty. Once it arrives in the world’s oceansor other natural low spots like the Dead Sea or the GreatSalt Lake, a part of the water is evaporated by the sun’senergy. This evaporated water leaves the salts behind,and the resultant water vapor forms clouds that producerain, continuing the cycle.

The technical process for desalting sea water has beenknown for a long time. The problem was that the pro-cess was costly and inconvenient. One of the continuingneeds for desalted water was on ocean-going ships thatspent prolonged periods of time away from land. Asidefrom carrying stores of water, attempts were made todistill water with heat from the ship’s cook stove orengine. This technique was used during the 19th cen-tury and was only partially successful because of theexpense involved to produce fresh water.

A major step in development came in the 1940s duringWorld War II when various military establishments inarid areas needed water to supply their troops. Thepotential that desalting offered was recognized morewidely, and work was continued after the war in variouscountries. One of the most concentrated efforts wastaken by the American government through the creation

and funding of the Office of Saline Water (OSW) in theearly 1950s and its successor organizations like theOffice of Water Research and Technology (OWRT). The

I I I -4

Worldwide growth of A

2

American government actively funded research anddevelopment for over 30 years, spending about 300million dollars in the process. These monies helped toprovide much of the basic investigation and developmentof the different technologies for desalting sea and brack-ish waters.

By the late 1960's commercial units of up to 8000 cubicmeters per day (cum/d) [2 million gallons per day (mgd)]were beginning to be installed in various parts of theworld. These mostly thermal-driven units were used todesalt sea water, but in the 197Os, commercial mem-brane processes began to be used. Originally, the distil-

Thermal Process, MSF Plant

Commercially Available Desalting Processes

Major Processes

- Multi-Stage Flash Distillation- Multiple Effect Distillation- Vapor Compression Distillation

l Membrane- Electrodialysis- Reverse Osmosis

Minor Processes

l Freezingl Membrane Distillationl Solar Humidification

lation process was used to desalt brackish water. Thistechnique was expensive and inhibited the developmentof this type of water resource. When electroclialysis wasintroduced, it could desalt brackish water much moreeconomically, and many applications were found for it.Similarly, reverse osmosis (KO) was used originally fordesalting brackish waters although the process hasproven to be suitable for sea water desalination also.

By the 198Os, desalination technology was a fully com-mercial enterprise. The technology benefitted front theoperating experience (sometimes good, sometimes bad

3

achieved with the units that had been built and operat-ing in the previous decades. A variety of desalting tech-nologies has been developed over the years and, based ontheir commercial success, they can be classified intomajor and minor desalting processes shown on page 3.

Worldwide Acceptance

An inventory completed in 1990 for IDA by KlausWangnick indicated that the total capacity of installeddesalination plants worldwide was about 13.2 millioncum/d (3480 mgd). Desalting equipment is now used inabout 120 countries. Almost half of this desalting capac-ity is used to desalt sea water in the Middle East andNorth Africa. Saudi Arabia ranks first in total capacity(about 27 percent of the world’s capacity), with most of itbeing made up of sea water desalting units that use thedistillation process. The United States of America (USA)ranks second in overall capacity, with about 12 percent.

Most of the capacity in the USA consists of plants inwhich the RO process is used to treat brackish groundwater.

Wangnick’s inventory indicates that the world’s installedcapacity consists mainly of the multi-stage flash distilla-tion and RO processes. These two processes make upabout 86 percent of the total capacity. The remaining 14percent is made up of the multiple effect, electrodialysis,and vapor compression processes while the minor pro-cesses amounted to less than one percent.

Summary of the I990 /IDA Inventory

Desalting % of Total CapacityProcess World Capacity million cum/d

Multi-Stage Flash 56 7.4Reverse Osmosis 31 4.1Multiple Effect 5 0.7Electrodialysis 5 0.6Vapor Compression 3 0.4

Capacitymgd

19501080

180160110

TOTAL CAPACITY 100 13.2 3480

4

All countries producing desalinated water in order of capacities from

maximum to minimum amount of fresh water in M3/day.

S/NO. COUNTRY CAPACITY M3/DAY

l-

2-

3-

4-

6-

7-

8-

9-

10-

l l -

12-

13-

14-

15-

16-

17-

18-

19-

20-

21-

22-

23-

24-

SAUDI ARABIA 3503,082

KUWAIT 1,334,650

UAE 1,306,846

USA 1,272,625

LIBYA 576,119

IRAN 368,689

BAHRAIN 311,620

QATAR 308,138

ITALY 261,066

USSR 259,951

SPAIN 218,608

IRAQ 211,707

HONG KONG 183,582

ALGERIA 164,912

NETH. ANTILLES 156,170

JAPAN 148,251

OMAN 129,659

HOLLAND 95,888

VIRGIN ISLANDS 90,666

GREAT BRITAIN 84,869

AUSTRALIA 79,487

MEXICO 77,707

GERMANY D 69,338

MALTA 66,245


25- EGYPT 52,510

26- ISRAEL 51,422

27- GERMANY DDR 50,470

28- SOUTH AFRICA 48,005

29- UNKNOWN 47,917

30- INDONESIA 42,428

31- BAHAMAS 31,758

32- GREECE 27,199

33- FRANCE 24,490

34- INDIA 20,943

35- TUNISIA 20,370

36- ANTIGUA 19,641

37- SINGAPORE 16,509

PERU 14,852

39- VENEZUELA 14,175

40- CHILE 13,240

41- KOREA 12,585

42- CHINA 11,768

43- CUBA 11,356

44- BERMUDAS 9,325

45- GIBRALTAR 8,707

46- CYPRUS 7,710

47- COLUMBIA 7,465

48- NIGERIA 7,070

49- SAHARA 7,002


50-

51-

52-

53-

54-

55-

56-

57-

58-

59-

60-

61-

62-

63-

64-

65-

66-

67-

68-

69-

70-

71-

72-

73-

74-

MALAYSIA 6,968

JORDAN 6,861

CANADA 6,764

CZECHOSLAVAKIA 6,192

SYRIA 5,623

ECUADOR 5,614

CAPE VERDE 5,363

THAILAND 5,284

AUSTRIA 5,229

YEMEN 5,214

POLAND 5,032

CAYMAN ISL. 4,876

BELGIUM 4,832

LEBANON 4,691

MAURITANIA 4,654

PORTUGAL 4,420

FRENCH ANT. 4,150

PHILIPPINES 3,787

SWITZERLAND 3,671

MOROCCO 3,324

ASCENSION 2,128

ARGENTINA 2,053

MARSHALL ISLAND 2,050

DOMINICA 1,514

TURKEY 1,374


75-

76-

77-

78-

79-

80-

81-

82-

83-

84-

85-

86-

87-

88-

89-

90-

91-

92-

93-

94-

95-

96-

97-

98-

99-

JAMAICA

YUGOSLAVIA

SUDAN

MADEIRA

NAMIBIA

HONDURAS

HUNGARY

NICARAGUA

CONGO

IRELAND

SWEDEN

PAKISTAN

BRAZIL

ANGOLA

EL-SALVADOR

ANTARCTICA

SOMALIA

DJIBOUTI

DENMARK

MOZAMBIQUE

TRINIDAD

LEICHTENSTEIN

SENEGAL

FINLAND

SOLOMON ISLANDS

TOTAL

1,363

1,102

976

800

775

651

615

600

550

545

542

502

390

380

378

300

288

254

242

189

189

151

132

121

100

11,982,595

All land based desalination plants producing more than 100 m3/day.This data is compiled from the 1988 IDA worldwide desalting plants inventory datedJune 1988.

S. No.Capacitym3Day Process

NO. OF NO. OFUnits Plants Feed Water

l- Algeria 10,570 ED955 ME

60,535 MSF961 Other

78,226 RO13,665 VC

164,912

2- Arab Emirate (U.A.E)5,1022,8008,266

1,195,73277,31417,632

1,306,846

2- Australia

4- Angola 380 M E 1

5- Antigua 9,080 ME10,446 MSF

115 RO

6- Argentina

7- Antarctica 300 MSF

8- Austria 5,229 RO

1,925 ED1,418 ME3,407 MSF

65,387 RO7,350 VC

79,487

19,641

1,046768239

2,053

EDOtherMEMSFROV C

MEOtherRO

124

261

5216

111

121

221007555

265

465

795

99

421

7

2

17

12 Brack2 Sea

13 Sea1 Sea

30 Brack/Sea11 Sea/Brack

69

12 Brack1 Brine

15 Sea/Brine/Brack43 Sea43 Brack/Sea26 Sea

140

424

503

BrackBrackBrack/SeaBrack/Sea/WasteSea/Waste/Brine

63

Sea

SeaSeaSea

6

BrackBrackBrack

Sea

Brack

S. No.Capacitym3/Day Process


16- China

17- Canada

14- Belgium

15- Chile

1l- Bahrain

12- Bermudas

13- Brazil

9- Ascension 3891,739

2,128

lO- Bahamas 114576

24,5056,563

31,758

187,713106,987

13,1562,6291,135

311,620

3,1372,9742,836

378

9,325

125265

390

1,8852,947

4,832

2,443121

6,7713,905

13,240

3,768

11,768

1,4875,277

6,764

VCMSF

EDMEMSFRO

MSFROEDV CM E

EDMSFROV C

ROME

MERO

EDM EMSFRO

EDMSFRO

MERO

1 15 4

6 5

1 13 26 5

15 10

25 18

15 849 2544 4411 6

1 1

120 84

41092

25

11

34

7

3198

21

227

11

413

17

310

71

21

11

2

34

7

3177

18

115

7

310

13

SeaSea

BrackSea/BrackSeaBrack/Sea

SeaSea/BrackBrackSeaBrack

BrackSeaBrack/SeaSea

BrackBrack

Brack/SeaBrack

BrackSeaSeaBrack/Sea

BrackSeaBrack

BrackBrack



18- Cayman Isl.

19- Cyprus

20- Cape Verde Isl.

21- Cuba 11,356 MSF 4 2 Sea

22- Columbia 4,965 RO2,500 VC

6 34 1

10 4

SeaSea

23- Czechoslovakia 6,192 RO 8 5 Brack

24- Congo 550 VC 2 2 Sea

25- Denmark 242 ME 1 1 Brack

26- Dominica 1,135 ED379 RO

11

2

BrackBrack

27- Djibouti

28- Ecuador

29- Egypt

2,000 M E2,876 VC

4,876

110 ME5,283 MSF2,167 RO

150 VC

7,710

4,700663

5,363

7,465

1,514

130 RO124 VC

254

1,334 MSF1,703 RO2.577 VC

5,614

17,984 RO 23 14 Brack22,669 ED 29 27 Brack

2,317 ME 7 6 Sea/Brack5,000 VC 10 6 sea4,340 MSF 12 7 sea

52,510 81 60

MSFRO

1 15

7 6

1 111 710 3

1 1

23 12

43

7

31

4

11

2

3 31 18 7

12 11

SeaSea

SeaSeaBrackSea

SeaSea

SeaSea

seaSeaSea



35- Germany DDR

36- Gibraltar

37- Great Britain

38- Greece

30-El Salvador

31- Finland

32- France

378

121

7,0314,840

3637,5664,690

24,490

RO 2 2 Brack

M E 1 1 Brack

M EMSFOtherROV C

6112

1310

42

5 Sea/Brack7 Sea1 Sea

10 Brack6 Sea

29

33- French Ant. 1,5002,650

4,150

MSFV C

27

SeaSea

9

34- Germany D. 50,8491,300

10,389

59001,300

500

69,338

ROEDMEOtherMSFV C

682

29121

103

54 Brack/Sea/Waste2 Brack

24 Brack1 Sea2 Brack/Sea1 Sea

84

11,26810,99428,108

50,470

MSFROME

316

5

24

2 Brack11 Brack

5 Waste/Brack

18

1,3636,564

780

8,707

MEMSFRO

112

3

16

1102

SeaSeaSea

13

4,558 ED 10 8 Brack43,334 ME 124 42 Brack/Sea

7,355 MSF 4 3 Sea2,725 Other 5 2 Brack

25,617 RO 38 33 Brack1,280 VC 6 3 Sea/Brack

48,869 187- - 91

1,94816,9006,9021,449

27,199

ROEDMSFV C

7 64 47 57 3

25 18

Brack/SeaBrackSeaSea


NO. OF NO. OFunits Plants Feed Water

40- Honduras

41- Hungary

42- Hong Kong 182,0281,554

183,582

43- India

44- Indonesia

45- Iran 5,71018,065

291,199252

32,75420,709

368,689

46- Iraq

39- Holland 6,5804,673

70,23414,401

95,888

EDMEMSFRO

24 3

12 917 14

44 28

BrackSea/BrackBrack/SeaBrack

651

500115

615

MSF 1 1 Sea

MSFRO

BrackBrack

MSFRO

7 24 4

11 6

SeaBrack/Sea

1,69319,250

20,943

MERO

8 423 17

31 21

Sea/BrackBrack

1,00040,484

704240

42,428

MEMSFOtherV C

2 132 18

4 41 1

39 24

SeaSeaSeaSea

EDMEMSFOtherROV C

8 727 1384 35

2 134 2963 19

218 104

BrackishSea/BrackSeawaterSeawaterBrackishSeawater

77,2351,175

10,824122,473

211,707

EDMEMSFRO

42 356 52 1

62 37

112 78

BrackishSea/BrackBrackishBrackish

47- Ireland 545 ME 1 1 Seawater

48- Israel 2,40021,028

7,19118,1992,604

51,422

ED 1 1 BrackishM E 2 2 SeawaterMSF 2 2 Seawater

RO 13 13 BrackishVC 8 8 Sea/Brackish

26 24



49- Italy

50- Jamaica 1,363 RO 2 1 Brackish

51- Jordan 719 ED6,142 RO

1 110 6

11 7

BrackishBrack/Sea

6,861

52- Japan 33,993 ED36,025 MSF66,912 RO10,783 ME

538 Other

77 1819 1788 7312 11

2 2

198 121

Sea/Brack/WasteSeawater/BrackBrack/Sea/WasteSeawaterWastewater

53- Korea

54- Kuwait

55- Leichtenstein 151 RO 1 1 Brackish

56- Lebanon 520 MSF711 RO

3,460 V C

1 14 35 2

10 6

SeawaterSea/BrackSeawater

187,629 MSF15,940 RO32,451 ED6,118 ME

15,300 VC3,628 Other

261,066

148,251

480 ME12,105 RO

12,585

1,295,911 MSF27,786 RO4,133 ED4,904 ME1,766 Other

150 VC

1,334,650

4,691

57- Libya 414,253 MSF 92 46 Seawater83,777 RO 142 59 Brack/Sea/Waste66,894 ED 110 90 Brackish

5,956 ME 7 5 Seawater4,039 VC 14 9 Seawater1,200 Other 1 1 Brine

576,119 366 210

39 2925 2033 31

5 422 13

7 7

131 103

28

10

14

5

70 2834 1412 12

9 22 21 1

128 59

SeawaterBrack/SeaBrackishSeawaterSeawaterSeawater

BrackishBrackish

SeawaterBrack/SeaBrackishSea/BrineBrineSeawater


NO. OFUnits Plants Feed Water

62- Mauritania

63- Morocco

64-Mexico

58- Madeira 300500

vcRO

SeawaterSeawater

59- Malaysia 4,8442,124

6,968

MSFRO

SeawaterBrack/Sea

8

60- Malta 41,66024,585

66,245

ROMSF

216

27

12 Sea/Brack4 Seawater

16

61- Marshall Island 1,100950

2,050

ME 1

MSF 1SeaSea

2

1,654

4,654

MSFV C

14

SeaSea

5

284400676

1,964

3,324

EDOtherROV C

BrackSeaBrackSea/Brack

19 10

4,000

46,20726,321

667

77,707

ED 1 1 BrackME 2 2 Sea/Brack

MSF 35 20 Sea/Brack

RO 46 36 Brack/Waste/Sea

VC 4 4 Sea/Brack

88

65-Mozambique

66- Namibia

189

545

230

775

RO Brack

MSFME

SeaBrack

67- Neth Antilles 30,875 ME 5 9 Sea122,595 MSF 60 24 Sea

1,700 RO 2 2 SeaVC 2 1 Sea

156,170 69 36



70-Oman

72- Peru

73- Philippines

74- Poland 5,032 RO 10 8 Brack

75-Portugal 4,420 RO 6 4 Brack

76-Qatar 4,7151,844

1403,642

297,797

308,138

ROV CEDMEMSF

9 79 71 1

10 427 13

56 32

Brack/SeaSeaBrackSeaSea

77-Sahara 7,002 MSF 2 2 Sea

68- Nicaragua

69-Nigeria

600

434636

7,070

1,504896

4,200108,065

14,994

129,659

71-Pakistan 200302

502

10,9781,6542,220

14,852

928265302

2,292

3,787

MSF 2 1 Sea

M EROV C

2 16 13 1

11 3

BrackBrackSea

V CEDM EMSFRO

9 24 47 5

10 837 26

67 45

SeaBrackSeaSeaBrack/Sea

ROOther

BrackBrack

MSFROV C

4 43 36 2

13 9

SeaBrack/SeaSea

MEMSFOtherRO

4127

14

2117

11

SeaBrackBrackBrack



78-Saudi Arabia MSF 206 70 Sea/BrackRO 830 552 Brack(Sea/WasteED 193 180 BrackM E 53 31 Sea/BrineVC 56 25 Sea/Brack

2,533,639832,913

82,53616,46637,528

3,503,082 1,338 858

79- Senegal 132 Other 1 1 Waste

80-Singapore10,509

16,509

M E 2 2 SeaRO 20 18 Brack/Sea

22 20

81- Soloman Isl 100 RO 1 1 Sea

82- Somalia 288 RO 1 1 Brack

83-South Africa 7,12529,83310,938

109

48,005

RO 8VC 14ED 5M E 1

Brack/Waste/SeaBrine/WasteBrackBrack

28 22

84-Spain 27,06084,84865,260

14347,4512,846

218,608

54 42 Sea/BrackRO 67 54 Sea/BrackMSF 19 12 SeaOther 1 1 SeaED 21 11 BrackM E 6 3 Sea/Brack

168 123

85-Sudan 750226

976

MEMSF

12

SeaBrack

3

86-Sweden 542 M E 2 2 Sea/Brack

87- Switzerland 3213,350

3,671

M E 1 1 BrackRO 13 13 Brack/Waste

14 14

4,4231,200

5,623

RO 8ED 3

BrackBrack

11



89- Thailand

90- Trinidad 189 RO 1 1 Brack

91- Tunisia 9,3974,2206,056

ROV CEDMEOtherMSF

13 612 79 72 11 11 1

38 23

BrackSeaBrackBrackseasea

92- Turkey

93- USA

94- USSR

95- Unknown

96- Venezuela

97- Virgin Islands

2,1761,7981,310

5,284

121336

20,370

2461,128

1,374

52,884982,62577,32954,87261,89343,022

1,272,625

230190,55849,67119,292

259,951

35,92911,738

250

47,917

4,5249,651

14,175

50,33437,729

1,0511,552

90,666

ROMSFOther

MERO

MSFROEDMEV COther

EDMEMSFRO

EDROV C

MERO

MEMSFROV C

1 16 5

7 6

20 15862 531109 5993 7367 4957 52

1,208 779

1 114 12

5 421 9

41 26

150 344 12

2 1

196 16

3 215 6

18 8

11 813 12

7 66 6

37 32

BrackseaBrack

Brack

Sea/BrackWaste/Brack/SeaBrackBrack/SeaBrack/Waste/SeaSea/Brack

BrackSea/WasteSea/WasteBrack/Waste/Sea

BrackBrack/Seasea

Brack/Sea

SeaSeaSea/BrackSea



98- Yemen 125 VC 1 11,336 RO 10 71,800 MSF 5 21,953 ME 4 2

5,214 20 12

99- Yugoslavia 1,102 RO 2 2

SeaSea/BrackSeaSea

Brack

ARAB COUNTRIES USING DESALINATION TECHNOLOGY

COUNTRYNO. OF NO. OF FEED WATER

PROCESS PLANTS UNITS M3/DAY

1- ALGERIA ED

MSFOTHER

RO

V C

2- ARAB EMIRATES ED

ME

MSF

OTHER

RO

V C

3- BAHRAIN ED

ME

MSF

RO

V C

4- EGYPT

5- IRAQ

ED

ME

MSF

RO

VC

ED

ME

MSF

RO

6- JORDAN ED

RO

12

2

13

1

30

11

69

12

15

43

43

26

140

44

1

8

25

6

84

27

6

7

14

6

60

35

5

1

37

78

12 10,570 BRACK

4 955 SEA26 60,535 SEA

1 961 SEA52 78,226

16 13,665 SEA/BRACK

111 164,912

12 5,102 BRACK22 8,266 SEA/BRINE/BRACK

100 1,195,732 SEA1 2,800 BRINE

75 77,314 BRACK/SEA

55 17,632 SEA

265 1,306,846

44 13,156 BRACK1 1,135 BRACK

15 187,713 SEA49 106,987 SEA/BRACK11 2,629

120 311,620

29 22,669 BRACK7 2,317 SEA/BRACK

12 4,540 SEA23 17,984 BRACK

10 5,000 SEA

81 52,510

42 77,235 BRACK

6 1,175 SEA/BRACK

2 10,824 BRACK

62 122,473 BRACK112 211,707

1 719 BRACK10 6,142 BRACK/SEA

11 6,861

COUNTRYNO. OF NO. OF CAPACITY FEED WATER

PROCESS PLANTS UNITS

7- KUWAIT ED

MSF

OTHER

RO

V C

8- LEBANON MSF

RO

V C

9- LIBYA ED

MSF

OTHER

RO

V C

l0- MOROCCO ED

OTHER

RO

V C

ll- OMAN -ED

MSF

RO

V C

12- QATAR ED

MSF

RO

V C

12

2

28

2

14

1

59

90

5

46

1

59

9

210

2

1

2

5

10

4

5

8

26

2

45

1

4

13

7

7

32

12 4,133 BRACK

9 4,904 BRINE/SEA

70 1,295,911 SEA

2 1,766 BRINE

34 27,786

1 150 SEA

128 1,334,650

1 520 SEA

4 711 BRACK/SEA

5 3,460 SEA

10 4,691

110 66,894

7 5,956 SEA

92 414,253 SEA

1 1,200 BRINE

142 83,777 BRACK/SEA/WASTE

14 4,039 SEA

366 576,119

2 284 BRACK

4 400 SEA

4 676 BRACK

9 1,964 SEA/BRACK

19 3,324

4 BRACK

7 4,200 SEA

10 108,065 SEA

37 14,994 BRACK/SEA

9 1,504 SEA

67 129,659

140 BRACK

10 3,642 SEA

27 297,797 SEA

9 4,715 BRACK/SEA

9 1,844 SEA

56 308,138

COUNTRYNO. OF NO. OF CAPACITY FEED WATER

PROCESS PLANTS UNITS

13- SAUDI ARABIA ED 180

ME 31

MSF 70

RO 552

VC 25

14- SYRIA ED 1

RO 6

15- TUNISIA ED

ME

MSF

OTHER

RO

V C

16- YEMEN ME

RO

V C

858

7

7

1

1

1

6

7

23

193 82,536 BRACK

53 16,466 SEA/BRINE

206 2,533,639 SEA/BRACK

830 832,913 BRACK/SEA/WASTE

56 37,528 SEA/BRACK

1338 3,503,082

3 1,200 BRACK

8 4,423 BRACK

11 5,624

9 6,056 BRACK

2 240 BRACK

1 336 SEA1 121 SEA

13 9,397 BRACK

12 4,220 SEA

38 20,370

4 1,953 BRACK

5 1,800 SEA

10 1,336 BRACK

1 125 SEA

20 5,214

TOTAL NO. OF PLANTS IN ARAB COUNTRIES WITH CAPACITY & PROCESS

PROCESS

NO. OF NO. OF CAPACITY % OF

PLANTS UNITS TOTAL

ED

ME

MSF

OTHERS

(Thermal Process)

RO

V C

428 474 291,590 3.67

79 132 51,209 0.64

241 567 6,111,665 76.92

7 10 7,248 0.09

1362 1,389,854 17.50

108 208 93,760 1.18

1700 2753 7,945,326 100.00

Water produced by desalination in Arab countries = 7,945,326 M3/day

% of water produced by desalination in Arab countries in comparison to overall worldwide production ofdesalinated water = 66.3 %

% of water produced by desalination in Saudi Arabia in comparison to overall worldwide production ofdesalinated water = 29.1%

Existing Desalinated Water PipelinesNAME OF DESIGN PIPELINETHE PROJECT FLOW RATE LENGTH

PIPE MATERIAL PUMP STATIONSAND DIAMETER TERMINALS MISCELLANEOUS

Jubail-RiyadhTransmission

830,000 m3/day 465 kms- Steel API Std: 6 pump stations From Jubail DesalOr Double line 60 in. dia. 430 MW capacity

H Dplants to Riyadh, via

219 million Terminal at PT, hahran., S hIJSG/D Riyadh

edgum,Hofuf, Khurais

300,000 rn3 storage and Wasia,in 6 Conc. tanks

Shoaibah-Makkah-Taif 182.000 m:‘/day 247 kms - Steel API Std: Tunnel: 13.2 km inWater Transmission 96 km Double line 56 in. dia.

4 pump stations

Facility 48 million 55 km Single line 42 in. dia.85 MW capacity

Two terminalslength. through Taif30 million IJSG to

IJSG/D 4 X 50,000 m3 storage Makkah2 X 25,000 m,’ storage 18 million I JSG to

Taif

Assir Phase One 113,636 m3/day 215 kms - Steel API Std: A series of tunnelsWater Transmission o r 140 km Pressure line

4 pump stations

Facility 30 million 75 km Gravity lineConcrete cyl. pipes42 in. 8r 36 in. dia.

108 MW capacity Total length 10.5 kmLarge terminal

USGID 48 in. 8r 20 in. dia. A abhStorage Total-125,000 m3

Smalll - Bin NaumanIJkad

Yanbu-Madina 104.000 m3/day 176 km - Madina Steel API Std: 2 pump stations 22 million IJSG/DWater Transmission o r 50 km - Yanbu 32 in. dia. 17 MW capacity to MadinaSystem 28 million (Pressure lines)

IJSG/DAsbestos Cement 6 million IJSG/DPipe 24 in. dia. to Yanbu

Al Khobar 2 190,000 m’/day 84.5 kmWater Transmission or

Concrete c I pipes1100 mm x

1 pump station Stora eat:ia. - 30 km and 1. Al Khobar-96,000 ml1a

System 50 million 1000 mm dia. - 71.5 km 7 blending stations 2. Dhahran-15,000Mr3USC/D 900 mm dia. - 37.0 km Storage capacity

500 mm dia.-25.25 kmAirport

= 459,000 rn’ 3. Dammam-200,000 m3

4. Sayhat-20,000 rn”5. Qatif-88,000, rn”6. Safwa-20,000 rn”7. Rahemah-20,000 rn’j

Water Transmissionfrom Jubail toWashem-Sudayr& Qasim

380,000 m3/day 389 km Steel API Std: 4 pump stations Branch linesor Single line pressure 60 in. dia. - 389 km between 1. Zulfi line100 million 384 km Prestressed concrete Jubail-Riyadh 400 mm dia. CCP

IJSG/D Double line pressure 16,OO mm dia.-205 km Qasim 37.4 km& Gravity 2,000 mm dia.-165 km Storage at 2. Sha

Steel 48 in.-163 km Burayda

dah = 300,000 m3 lqra Steel

Pipe ine 20 in.Riya h = 300,000 m3 96.8 km

25

Existing Desal Plants Along The Red Sea

HAQL

DUBA

AL WAJH

Two units ReheatFour units Reheat

UMM LAJJ

YANBU

MEDINAH

Phase

I

II

I

II

III

I

II

I

II

I

II

Process

MSF

RO

MSF

MSF

RO

MSF

MSF

RHRH

MSF

R-0

MSF

MSF + RO

Installed CapacityNo. of Unit , Year of

Megawatts Operation

1 900 1979*

1989

1 230 1968*

1 550 1978*

- 4400 1989

1 230 1968*

1 550 1978(Transfered from

960 Ummlajj)

1200 (Transfered fromAl-Khafji)

1 550 1974*

3788 1986

(5) 107954 (5)250 1980

4 + R O 227000 150 1991

(*) Plant is not Operating l

Continued :

RABIGH

JEDDAH

MAKKAHTAIF

Phase

I

I

IIRO

III

IV

I

r

Installed CapacityProcess No. of Unit _ Year of

Cubic m/day Megawatts Operation

- -

MSF 2 1400 - I 1980

RO Rehab 56800 1989

MSF 4 43181 w 4 1977

Reverse Osmosis 12120

MSF 10 87878 (5)240 1978

MSF 10 220075 1980

MSF 10 181818 (5)320 1988

AL BIRK

ASSIR

FARASAN

Al-LEETH

QUNFUDA

I

I

I

Rush Units

Planned

IPlanned

RO

MSF

MSF

MSF

2272

94696

500

1250

3788

3788

- - 1982

(2)128 1988

. -2.3 1978

1978

- - -

(*) Plant is not Operating.

27

Existing Desal, Plants Along The GULF

AL KHAFJI

- - -

AL JUBAIL

AL KHOBAR

Phase

I

II

I

II

II

Process

MSF

MSF

MSF

MSF

MSF

Installed CapacityNo. of Uni t Year of

Cubic m/day Megawatts Operation

1 550 1972

2 22727 1985

6 136363 (5)360 1980

4 958333 (l0)1295 1982

10 195075 (5)750 1982

(*) Plant is not Operating.

28

Desalting TechnologiesA desalting device essentially separates saline water intotwo streams: one with a low concentration of dissolvedsalts (the fresh water stream) and the other containingthe remaining dissolved salts (the concentrate or brinestream). The device requires energy to operate and canuse a number of different technologies for the separation.This section briefly describes the various desaltingprocesses commonly used to desalt saline water.

Thermal Processes

Over 60 percent of the world’s desalted water is producedwith heat to distill fresh water from sea water. Thedistillation process mimics the natural water cycle inthat saline water is heated, producing water vapor that isin turn condensed to form fresh water. In a laboratory orindustrial plant, water is heated to the boiling point toproduce the maximum amount of water vapor.

For this to be done economically in a desalination plant,the boiling point is controlled by adjusting the atmo-spheric pressure of the water being boiled. (The tempera-ture required to boil water decreases as one moves fromsea level to a higher elevation because of the reducedatmospheric pressure on the water. Thus, water can beboiled on top of Mt. McKinley in Alaska [elevation 6200meters (20300 feet)] at a temperature about 16OC (28°F)less than boiling it at sea level.) The reduction of theboiling point is important in the desalination process fortwo major reasons: multiple boiling and scale control.

To boil, water needs two important conditions: the propertemperature relative to its ambient pressure and enoughenergy for vaporization. When water is heated to itsboiling point and then the heat is turned off, the waterwill continue to boil only for a short time because thewater needs additional energy (the heat of vaporization)to permit boiling. Once the water stops boiling, boilingcan be renewed by either adding more heat or by reduc-ing the ambient pressure above the water. If the ambientpressure is reduced, then the water would then be at atemperature above its boiling point (because of thereduced pressure) and will boil with the extra heat fromthe higher temperature to supply the heat of vaporizationneeded. As the heat of vaporization is supplied, thetemperature of the water will fall to the new boiling point.

To significantly reduce the amount of energy needed forvaporization, the distillation desalting process usuallyuses multiple boiling in successive vessels, each operat-ing at a lower temperature and pressure. This process ofreducing the ambient pressure to promote boiling cancontinue downward and, if carried to the extreme withthe pressure reduced enough, the point at which waterwould be boiling and freezing at the same time would bereached.

Aside from multiple boiling, the other important factor isscale control. Although most substances dissolve morereadily in warmer water, some dissolve more readily incooler water. Unfortunately, some of these substanceslike carbonates and sulfates are found in sea water. Oneof the most important is gypsum (CaSO,), which beginsto leave solution when water approaches about 95OC

29

(203°F). This material forms a hard scale that coats anytubes or containers present. Scale creates thermal andmechanical problems and, once formed, is difficult toremove. One way to avoid the formation of this scale is tokeep the temperature and boiling point of the waterbelow that of temperature.

These two concepts have made various forms of distilla-tion successful in locations cess which accounts for themulti-stage flash distillation MSF process.

Multi-Stage Flash Distillat

around the world. The pro-most desalting capacity iscommonly referred to as the

ion

In the MSF process, sea water is heated in a vessel calledthe brine heater, This is generally done by condensingsteam on a bank of tubes that passes through the vesselwhich in turn heats the sea water. This heated sea waterthen flows into another vessel, called a stage, where theambient pressure is such that the

HEATING

water will immediately boil. Thesudden introduction of the heatedwater into the chamber causes itto boil rapidly, almost explodingor_flashing into steam. Generally,only a small percentage of thiswater is converted to steam(water vapor), depending on thepressure maintained in this stagesince boiling will continue onlyuntil the water cools (furnishingthe heat of vaporization) to theboiling point.

Large MSF plantThe concept of distilling water with a vessel operating ata reduced pressure is not new and has been used for well

FLASH ANDHEAT RECOVERY SECTION , I

2nd STAGE Nth STAGE

SECTION

1st Stage

Condensate

30

developed. In this unit, the feed water could passfrom one stage to another and be boiled repeatedlywithout adding more heat. Typically, an MSF plant cancontain from 4 to about 40 stages.

The steam generated by flashing is converted to freshwater by being condensed on tubes of heat exchangersthat run through each stage. The tubes are cooled by theincoming feed water going to the brine heater. This, inturn, warms up the feed water so that the amount ofthermal energy needed in the brine heater to raise thetemperature of the sea water is reduced.

Multi-stage flash plants have been built commerciallysince the 1950s. They are generally built in units of about4000 to 30000 cum/d (1 to 8 mgd). The MSF plantsusually operate at the top feed temperatures (after thebrine heater) of 90- 120°C (194-248°F). One of the factorsthat affects the thermal efficiency of the plant is thedifference in temperature from the brine heater to thecondenser on the cold end of the plant. Operating a plantat the higher temperature limits of 120°C (248°F) tends toincrease the efficiency, but it also increases the potentialfor detrimental scale formation and accelerated corrosionof metal surfaces.

Multiple Effect Distillation

The multiple effect distillation (MED) process has beenused for industrial distillation for a long time. One popu-lar use for this process is the evaporation of juice fromsugar cane in the production of sugar or the production

of salt witthe evaporativeprocess.Some of theearly waterdistillationplants usedthe MEDprocess,but thisprocess wasdisplacedby the MSF

MED Plant

units because of cost factors and their apparent higherefficiency. However, in the past decade, interest in theMED process has renewed, and a number of new designshave been built. Most of these new MED units have beenbuilt around the concept of operating on lower tempera-tures.

MED. like the MSF process, takes place in a series ofvessels (effects) and uses the principle of reducing theambient pressure in the various effects. This permits thesea water feed to undergo multiple boiling without sup-plying additional heat after the first effect. In an MEDplant, the sea water enters the first effect and is raised tothe boiling point after being preheated in tubes. The seawater is either sprayed or otherwise distributed onto thesurface of evaporator tubes in a thin film to promoterapid boiling and evaporation. The tubes are heated bysteam from a boiler, or other source, which is condensedon the opposite side of the tubes. The condensate fromthe boiler steam is recycled to the boiler for reuse.

31

Only a portion of the sea waterapplied to the tubes in the firsteffect is evaporated. The remain-ing feed water is fed to the secondeffect, where it is again applied toa tube bundle. These tubes are inturn being heated by the vaporscreated in the first effect. Thisvapor is condensed to fresh water

SeawaterFeed

SteamfromBoiler

1st EFFECT 2nd EFFECT 3rd EFFECT Note: P i > P2 > P3

T, > T2 > Tg

P = PressureVacuum Vacuum Vacuum T = Temperature

Brine

Vapor3 - L

NZGEffect

-

product, while giving up heat toevaporate a portion of the remain-

Usually, the remaining sea water in each effect must bepumped to the next effect so as to apply it to the nexttube bundle. Additional condensation takes place in eacheffect on tubes that bring the feed water from its source

MED plants are typically built in units of 2000 to 10000cum/d (0.5 to 2.5 mgd). Some of the more recent plantshave been built to operate with a top temperature (in thefirst effect) of about 70°C (1580F), which reduces thepotential for scaling of sea water within the plant but inturn increases the need for additional heat transfer areain the form of tubes. Most of the more recent applicationsfor the MED plants have been in some of the Caribbeanareas. Although the number of MED plants is still rela-tively small compared to MSF plants, their numbers havebeen increasing.

Pump

Vapor Compression Distillation

The vapor compression (VC) distillation process is gener-ally used forsmall- and medium-scale sea water desalt-ing units. The heat for evaporating the water comes fromthe compression of vapor rather than the direct exchangeof heat from steam produced in a boiler.

The plants which use this process are generally designedto take advantage of the principle of reducing the boilingpoint temperature by reducing the pressure. Two primarymethods are used to condense vapor so as to produceenough heat to evaporate incoming sea water: a mech-anical compressor or a steam jet. The mechanical com-pressor is usually electrically driven, allowing the soleuse of electrical power to produce water by distillation.

VC units have been built in a variety of configurations topromote the exchange of heat to evaporate the sea water.The diagram illustrates a simplified method in which amechanical compressor is used to generate the heat for

32

evaporation.The com-pressorcreates avacuum inthe vesseland thencompressesthe vaportaken fromthe vesseland con-denses itinside of atube bundlealso in the same vessel. Sea water is sprayed on theoutside of the heated tube bundle where it boils andpartially evaporates, producingmore vapor. A portion of the hot brine is

recirculated to the spray nozzles forfurther vaporization on the tube

With the steam jet-type of VC unit, bundle.

venturi orifice at the steam jetcreates and extracts water vaporfrom the main vessel, creating alower ambient pressure in the mainvessel. The extracted water vapor iscompressed by the steam jet. Thismixture is condensed on the tubewalls to provide the thermal energy(heat of condensation) to evaporatethe sea water being applied on theother side of the tube walls in the

VC units are usually built in the 20- to 2000-cum/d(0.005- to 0.5-mgd) range. They are often used for re-sorts, industries, and drilling sites where fresh water isnot readily available.

Membrane Processes

In nature, membranes play an important role in theseparation of salts. This includes both the processes ofdialysis and osmosis that occur in the body. Membranesare used in two commercially important desalting pro-cesses: electrodialysis and RO. Each process uses theability of membranes to differentiate and selectivelyseparate salts and water. However, membranes are useddifferently in each of these processes.

The vapor gains heat energy bybeing compressed by the vapor

COMPRESSOR

A steam jet ejector could replacethe vapor compressor wheresurplus steam is available.

EXCHANGER- - -.::*.::

::*::::*.-.

vessel.

selectively through a membrane, leaving fresh waterbehind as product water. In the RO process, pressureused for separation by allowing fresh water to movethrough a membrane, leaving the salts behind.

is

Both of these concepts have been explored by scientistssince the turn of the century, but their commercializationfor desalting water for municipal purposes has occurredin only the last 30 years.

Electrodialysis

Electrodialysis was commercially introduced in the early196Os, about 10 years before RO. The development ofelectrodialysis provided a cost-effective way to desaltbrackish water and spurred considerable interest in thisarea.

Electrodialysis depends on the following generalprinciples:

l Most salts dissolved in water are ionic, being positively(cationic) or negatively (anionic) charged.

These ions are attracted to electrodes with an oppositeelectric charge.

passage of either anions or cations.

The dissolved ionic constituents in a saline solution suchas sodium (+), chloride (- ), calcium (+ +), and carbonate

(- - ) are dispersed in water, effectively neutralizingtheir individual charges. When electrodes connected toan outside source of direct current like a battery areplaced in a container of saline water, electrical current iscarried through the solution, with the ions tending tomigrate to the electrode with the opposite charge.

For these phenomena to desalinate water, membranesthat will allow either cations or anions (but not both) topass are placed between a pair of electrodes. Thesemembranes are arranged alternately with an anion-selective membrane followed by a cation-selective mem-brane. A spacer sheet that permits water to flow along

34

the face of the membrane is placed between each pair ofmembranes.

One spacer provides a channel that carries feed (andproduct) water, while the next carries brine. As theelectrodes are charged and saline feed water flows alongthe product water spacer at right angles to the elec-trodes, the anions in the water are attracted and divertedtoward the positive electrode. This dilutes the salt con-tent of the water in the product water channel. Theanions pass through the anion-selective membrane, butcannot pass any farther than the cation-selective mem-brane, which blocks its path and traps the anion in thebrine. Similarly, cations under the influence of thenegative electrode move in the opposite direction throughthe cation-selective membrane to the concentrate chan-nel on the other side. Here, the cations are trappedbecause the next membrane is anion-selective andprevents further movement towards the electrode.

By this arrangement, concentrated and diluted solutionsare created in the spaces between alternating mem-branes. These spaces, bounded by two membranes (oneanionic and the other cationic) are called cells. A cell pairconsists of two cells, one from which the ions migrated(the dilute cell for the product water) and the other inwhich the ions concentrate (the concentrate cell for thebrine stream).

The basic electrodialysis unit consists of several hundredcell pairs bound together with electrodes on the outsideand is referred to as a membrane stack. Feed waterpasses simultaneously in parallel paths through all ofthe cells to provide a continuous flow of desalted product

water and brine to emerge from the stack. Depending onthe design of the system, chemicals may be added to thestreams in the stack to reduce the potential for scaling.

An electrodialysis unit is made up of the following basiccomponents:

r

RawFe&water

Basic components of an ED plant.

l Pretreatment train

l Membrane stack

l Low-pressure circulation pump

l Power supply for direct current (a rectifier)

l Post-treatment

The raw feed water must be pre-treated to prevent mate-

35

channels in the cells from entering the membrane stack.The feed water is circulated through the stack with a low-pressure pump with enough power to overcome theresistance of the water as it passes through the narrowpassages. A rectifier is generally used to transformalternating current to the direct current supplied to theelectrodes on the outside of the membrane stacks.

Post-treatment consists of stabilizing the water andpreparing it for distribution. This post-treatment mightconsist of removing gases such as hydrogen sulfide and

Electrodialysis Reversal Process (EDR)

In the early 197Os, an American company commerciallyintroduced the EDR process for electrodialysis. An EDRunit operates on the same general principle as a stan-dard electrodialysis plant except that both the productand the brine channels are identical in construction. Atintervals of several times an hour, the polarity of theelectrodes is reversed, and the flows are simultaneouslyswitched so that the brine channel becomes the productwater channel, and the product water channel becomesthe brine channel.

The result is that the ions are attracted in the oppositedirection across the membrane stack. Immediatelyfollowing the reversal of polarity and flow, enough of theproduct water is dumped until the stack and lines areflushed out, and the desired water quality is restored.This flush takes about 1 or 2 minutes, and then the unitcan resume producing water. The reversal process isuseful in breaking up and flushing out scales, slimes,

and otherdeposits inthe cellsbefore theycan buildup andcreate aproblem.Flushingallows theunit tooperate with

An EDR plant

fewer pretreatment chemicals and minimizes membranefouling.

Application

Electrodialysis has the following characteristics that lendit to various applications:

l Capability for high recovery (more product and lessbrine]

l Energy usage that is proportional to the salts removed

l Ability to treat water with a higher level of suspendedsolids than RO

l Lack of effect by non-ionic substances such as silica

l Low chemical usage for pretreatment

Electrodialysis units are normally used to desalinatebrackish water. The major energy requirement is the

36

direct current used to separate the ionic substances inthe membrane stack.

Reverse Osmosis

In comparison to distillation and electrodialysis, RO isrelatively new, with successful commercialization occur-ring in the early 1970s.

RO is a membrane separation process in which the waterfrom a pressurized saline solution is separated from thesolutes (the dissolved material) by flowing through amembrane. No heating or phase change is necessary forthis separation. The major energy required for desaltingis for pressurizing the feed water.

PRE-’ TREATMENT

HIGHMEMBRANEASSEMBLY

FreshwaterPOST-

*TREATMENT.

StabilizedFreshwater

BrineDischarge

Basic components of a reverse osmosis p/ant.

In practice, the saline feed water is pumped into a closed

a portion of the water passes through the membrane, theremaining feed water increases in salt content. At thesame time, a portion of this feed water is dischargedwithout passing through the membrane.

Without this controlleddischarge, the pressurizedfeed water would continueto increase in salt concen-tration, creating suchproblems as precipitationof supersaturated saltsand increased osmoticpressure across the mem-branes. The amount of thefeed water discharged towaste in this brine streamvaries from 20 to 70percent of the feed flow,depending on the saltcontent of the feed water.

An RO system is made upof the following basiccomponents:

l Pretreatment

l Highpressure pump

l Membrane assembly

l Post-treatment

RO Plant

Pretreatment is important in RO because the feed watermust pass through very narrow passages during theprocess. Therefore, suspended solids must be removedand the water pre-treated so that salt precipitation ormicroorganism growth does not occur on the

37

Rotated 180o

The Hollow Fine Fiber System

39

Internal View of RO Plant

membranes. Usually,the pretreatmentconsists of fine filtra-tion and the additionof acid or other chemi-cals to inhibit precipi-tation.

The high-pressurepump supplies thepressure needed toenable the water topass through themembrane and havethe salts rejected. Thispressure ranges from17 to 27 bar (250 to400 psi) for brackishwater and from 54 to80 bar (800 to 1180psi) for sea water.

The membrane assembly consists of a pressure vesseland a membrane that permits the feed water to be pres-surized against the membrane. The membrane must beable to withstand the drop of the entire pressure acrossit. The semi-permeable membranes are fragile and varyin their ability to pass fresh water and reject the passageof salts. No membrane is perfect in its ability to rejectsalts, so a small amount of salts passes through themembrane and appears in the product water.

RO membranes are made in a variety of configurations.Two of the most commercially successful are spiral-

wound sheet and hollow fine fiber. Both of these configu-rations are used to desalt both brackish and sea water,although the construction of the membrane and pressurevessel will vary depending on the manufacturer andexpected salt content of the feed water.

Post-treatment consists of stabilizing the water andpreparing it for distribution. This post-treatment mightconsist of the removing gases such as hydrogen sulfideand adjusting the PH.

Two developments have helped to reduce the operatingcosts of RO plants during the past decade: the develop-ment of membranes that can operate efficiently with

RO Plant

41

lower pressures and the use of energy recovery devices.The low-pressure membranes are being widely used todesalt brackish water. The energy recovery devices areconnected to the concentrate stream as it leaves thepressure vessel. The water in the concentrate streamloses only about 1 to 4 bar (15 to 60 psi) relative to theapplied pressure from the high-pressure pump. Theseenergy recovery devices are mechanical and generallyconsist of turbines or pumps of some type that canconvert a pressure drop to rotating energy.

Other Processes

A number of other processes have been used to desaltsaline waters. These processes have not achieved thelevel of commercial success that distillation, electrodialy-sis, and RO have, but they may prove valuable underspecial circumstances or with further development. Themost significant of these processes are freezing, mem-brane distillation, and solar humidification.

Freezing

Extensive work was done in the 1950s and 1960s todevelop freezing desalination. During the process offreezing, dissolved salts are naturally excluded duringthe formation of ice crystals. Sea water can be desali-nated by cooling the water to form crystals under con-trolled conditions. Before the entire mass of water hasbeen frozen, the mixture is usually washed and rinsed toremove the salts in the remaining water or adhering tothe ice crystals. The ice is then melted to produce freshwater.

Theoretically, freezing has some advantages over distilla-tion, the predominant desalting process at the time thefreezing process was developed. These advantages includea lower theoretical energy requirement, minimal potentialfor corrosion, and little scaling or precipitation. Thedisadvantage is that it involves handling ice and watermixtures that are mechanically complex to move andprocess.

A small number of plants have been built over the past 40years, but the process has not been a commercial successin the production of fresh water for municipal purposes,The most recent significant example of a freezing desalt-ing plant was an experimental solar-powered unit con-structed in Saudi Arabia in the late 1980s. The experi-mental work has been concluded, and the plant disas-sembled. At this stage, freezing desalting technologyprobably has a better application in the treatment ofindustrial wastes rather than the production of municipaldrinking water.

Membrane Distillation

Membrane distillation was introduced commercially on asmall scale in the 1980s. As the name implies, the pro-cess combines both the use of distillation and mem-branes. In the process, saline water is warmed to enhancevapor production, and this vapor is exposed to a mem-brane that can pass vapor but not water. After the vaporpasses through the membrane, it is condensed on acooler surface to produce fresh water. In the liquid form,the fresh water cannot pass back through the membrane,so it is trapped and collected as the output of the plant.

42

Thus far, the process has been used only in a few areasCompared to the more commercially successful pro-cesses, membrane distillation requires more space andmay use considerable pumping energy per unit of pro-duction. Being essentially a distillation process, it issubject to some of the same performance limitations asare experienced with that process.

The main advantages of membrane distillation lie in itssimplicity and the need for only small temperaturedifferentials to operate. Membrane distillation probablyhas its best application in desalting saline water whereinexpensive low-grade thermal energy is available, suchas from industries or solar collectors.

Solar Humidification

The use of direct solar energy for desalting salinewater has been investigated and used for some time.During World War II, considerable work went intomaking small solar stills for use on life rafts. Thiswork continued after the war, with a variety of de-vices being made and tested.

These devices generally imitate a part of the naturalhydrologic cycle in that the saline water is heated bythe sun’s rays so that the production of water vapor(humidification) increases. The water vapor is thencondensed on a cool surface, and the condensatecollected as product water. An example of this type ofprocess is the green house solar still, in which thesaline water is heated in a basin on the floor and thewater vapor condenses on the sloping glass roof thatcovers the basin.

Variations of this type of solar still have been made in aneffort to increase efficiency, but they all share the follow-ing difficulties, which restrict the use of this techniquefor large-scale production:

l Large solar collection area requirements

l High capital cost

l Vulnerability to weather-related damage

A general rule of thumb for solar stills is that a solarcollection area of about one square meter is needed toproduce 4 liters of water per day (10 square foot/ 1gallon). Thus, for a 4000-cum/d facility, a land area ofabout 100 hectares would be needed (250 acres/mgd).

BASIC ELEMENTS IN A SOLAR STILL

T R O U G H BASIN

The inside of the basin is usually black to efficiently absorb radiationand insulated on the bottom to retain heat.

Diagram of a solar still.

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Experimental Solar Desalination Unit

This operation would take up a tremendous area andcould create problems if located near a city where landwas scarce and expensive.

The stills themselves are expensive to construct, andalthough the thermal energy may be free, additionalenergy is needed to pump the water to and from thefacility. In addition, careful operation and maintenance isneeded to prevent scale formation caused by the basinsdrying out and to repair glass or vapor leaks in the stills.

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An application for these types of solar humidificationunits has been for desalting saline water on a small scalefor a family or small village where solar energy is abun-dant but electricity is not.

Other Solar and Wind-Driven Desalters

Desalting units that use solar collectors or wind energydevices to provide heat or electrical energy also havebeen built to operate standard desalting processes likeRO, electrodialysis, or distillation. The economics ofoperating these plants tend to be related to the cost ofproducing energy with these alternative energy devices.At this point in time, the cost tends to be high, but areexpected to improve as development of these energydevices continues.

At the present time, using conventional energy to drivedesalting devices is generally more cost-effective thanusing solar and wind-driven devices, although appropri-ate applications for solar and wind-driven desalters doexist. An inventory of known wind- and solar-powereddesalting plants (Wangnick, 1990) listed units in about100 locations scattered over 25 countries. Most of theseinstallations had capacities of less than 20 cum/d (0.005mgd). This inventory does not account for many smallsolar stills used by families in many parts of the world.

Other Aspects of DesaltingCogeneration

In some situations, it is possible to use energy so thatmore than one use can be obtained from it as the energymoves from a high level to an ambient level. This occurswith cogeneration where a single energy source canperform several different functions.

Certain types of desalination processes, especially thedistillation process, can be structured to take advantageof a cogeneration situation. Most of the distillation plantsinstalled in the Middle East and North Africa operateunder this principle. These units are built as part of afacility that produce both electric power and desalted seawater for use in the particular country.

The electricity is produced with high-pressure steam torun turbines which in turn power electric generators. Ina typical case, boilers produce high-pressure steam atabout 540°C (1 ,OOOOF). As this steam expands in theturbine, its temperature and energy level is reduced.Distillation plants need steam whose temperature isabout 120°C (248°F) or below, and this can be obtainedby extracting the lower temperature steam at the lowpressure end of the turbine after much of its energy hasbeen used to generate electricity. This steam is then runthrough the distillation plant’s brine heater, where it iscondensed in the tubes, thereby increasing the tempera-ture of the incoming sea water. The condensate from thesteam is then returned to the boiler to be reheated foruse in the turbine.

Large Dual Purpose Plant

The main advantage of a cogeneration system is that itcan significantly reduce the consumption of fuel whencompared to the fuel needed if two separate plants wererequired. Since energy is a major operating cost in anydesalination process, this can be an important economic-benefit. One of the disadvantages is that the units arepermanently connected together and, for the desalinationplant to operate efficiently, the steam turbine must beoperating. This can create a problem with water produc-tion when the turbine or generator is down for repairs.

This type of power and water production installation iscommonly referred as a dual-purpose plant. Since manyof the oil-producing countries of the Middle East andNorth Africa were engaged in building up their total

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infrastructure, these types of installations fit in well withthe overall development program in these countries.

Other types of cogeneration facilities benefitting desalina-tion can derive lower-cost steam from industrial pro-cesses or from burning solid wastes in an incinerator.

Concentrate Disposal

The common element in all of these desalination pro-cesses is the production of a concentrate stream (alsocalled a brine, reject, or waste stream). This streamcontains the salts removed from the saline feed to pro-duce the fresh water product as well as some of thechemicals that may have been added during the process.This stream varies in volume depending on the process,but will almost always be a significant quantity of water.

The disposal of this wastewater in an environmentallyappropriate manner is an important part of the feasibilityand operation of a desalting facility. If the desalting plantis located near the sea, the potential for a problem will beconsiderably less. The major pollutant in the concentratestream is salt, and disposing of salt in the sea is gener-ally not a problem. At the same time, care must be takenrelative to possible problems from added constituents,dissolved oxygen, and water temperature.

The potential for a more significant problem comes whena desalting facility is constructed inland, away from anatural salt water body. Care must then be taken so asnot to pollute any existing ground or surface water withthe salts contained in the concentrate stream. Disposal

may involve dilution, injection of the concentrate into asaline aquifer, evaporation, or transport by pipeline to asuitable disposal point. All of these methods could add tothe cost of the process.

The means of properly disposing of the concentrate flowshould be one of the items investigated first in any studyof the feasibility of a desalination facility. The cost ofdisposal could be significant and could adversely affectthe economics of desalination.

Reject Stream

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Another method of reducing the overall costs of desaltingcan be the use of hybrid systems. Such hybrid systemsare not applicable to most desalination installations, butcan prove to be an economic benefit in some cases. Ahybrid system is a treatment configuration made up oftwo or more desalination processes. An example is usingboth distillation and RO processes to desalt sea water atone facility to use and to combine the different character-istics of each process productively.

An example of such a hybrid system would be wheresteam is used in a dual-purpose plant (electricity anddesalination). The steam is used in a distillation plant todesalt sea water. The product water from the distillationunit has a low level of total dissolved solids, perhaps 20mg/l. Alongside the distillation plant could be a seawater RO plant that would be run only in off-peak powerperiods so as to stabilize the load on the generator andtherefore use lower cost electricity. The RO plant willproduce water with a higher level of total dissolved solidsin the range of 500 mg/l. The water from the two pro-cesses can be combined to produce a water that has areasonable level of total dissolved solids while helping tolevel the electrical load on the generators.

Economics

Desalination facilities exist in about 120 countriesaround the world, so specifying exact costs for desaltingis not appropriate. What can be said with certainty is

that the capital and operating costs for desalination havetended to decrease over the years, although the increasein energy prices during the 1970s affected productioncosts. At the same time desalting costs have been de-creasing, the costs of obtaining and treating water fromconventional sources has tended to increase because ofthe increased levels of treatment being required in vari-ous countries to meet more stringent water qualitystandards. This rise in cost for conventionally treatedwater also is the result of an increased demand for water,leading to the need to develop more expensive conven-tional supplies since the readily obtainable water sourceshave already been used.

Many factors enter into the capital and operating costsfor desalination: capacity and type of plants, plantlocation, feed water, labor, energy, financing, concentratedisposal, and plant reliability. In general, the cost ofdesalted sea water is about 3 to 5 times the cost ofdesalting brackish water from the same size plant.During the past decade in a number of areas in the USA,the economic cost of desalting brackish water has be-come less than the alternative of transferring largeamounts of conventionally treated water by long-distancepipeline.

In 1990, the total production costs, including capitalrecovery, for brackish water systems with a capacities of4000 to 40000 cum/d (1 to 10 mgd) in the USA typicallyranges from $0.25 to $0.60 per cum ($1.00 to $2.40 per1000 gallons). The probable costs for sea water desaltingfor plants in the capacity of 4000 to 20000 cum/d (1 to 5mgd) in the USA is estimated at $1 to $4 per cum ($4 to

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$16 per 1000 gallons). These estimates give some idea ofthe range of costs involved, but site- and country-specificfactors will affect the actual costs.

In any country or region, the economics of using desali-nation is not just the number of dollars, pesos, or dinarsper cubic meter, but the cost of desalted water versus theother alternatives. In many water-short areas, the cost ofalternative sources of water is already very high andoften above the cost of desalting.

Summary

Desalination technology has been extensively developedover the past 40 years to the point where it is reliablyused to produce fresh water from saline sources. Thishas effectively made the use of saline waters for waterresource development possible. The costs for desalinationcan be significant because of its intensive use of energy.However, in many arid areas of the world, the cost todesalinate saline water is less than other alternativesthat may exist or be considered for the future. Desali-nated water is used as a main source of municipalsupply in many areas of the Caribbean, North Africa, andthe Middle East. The use of desalination technologies,especially for softening mildly brackish water, is rapidlyincreasing in the southeastern USA.

There is no “best” method of desalination. Generally,distillation and RO are used for sea water desalting,while RO and electrodialysis are used to desalt brackishwater. However, the selection of a process should bedependent on a careful study of site conditions and the

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application at hand. Local circumstances may play asignificant role in determining the most appropriateprocess for an area.

The “best” desalination system should be more thaneconomically reasonable in the study stage. It shouldwork when it is installed and continue to work anddeliver suitable amounts of fresh water at the expectedquantity, quality, and cost for the life of a project.

AcknowledgementsThe purpose of this publication is to provide an up-to-date replacement for the booklet entitled, The A-B-C ofDesalting, which was published by the U.S. Departmentof the Interior’s Office of water Research and Technologyin about 1977. The text for this new booklet is based, inpart, on a previous paper by the A. K. Buros entitled, AnIntroduction to Desalination (UN, 1987). The diagrams onpages 30, 32, 33, 34, 35, 37,43 and 44 are adapted from TheUSAID Desalination Manual and are used courtesy of theU.S. Agency for International Development.

Bibliography

Buros et al. The USAID Desalination Manual. Produced byCH2M HILL International for the U.S. Agency for Intema-tional Development. 1980.

United Nations. Non-Conventional Water Resource Use inDeveloping Counties. (UN Pub No. E.87.11.A.20. 1987.Wangnick, Klaus. 1990 IDA Worldwide Desalting PlantsInventory. Produced by Wangnick Consulting for theInternational Desalination Association. 1990.

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ABC of Desalination

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