3rd Edition, July 2003

Desalination and Water Purification

Research and Development Program

Report No. 72

United States Department of the Interior

Bureau of Reclamation

Technical Service Center

Water Treatment Engineering and Research Group

Desalting

Handbook

for Planners
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Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction 

Report (0704-0188), Washington DC 20503. 

1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE

 July 2003 

3. REPORT TYPE AND DATES COVERED

 Final 

4. TITLE AND SUBTITLE

Desalting Handbook for Planners, Third Edition 

5. FUNDING NUMBERS

Agreement No. 

98-PG-81-0366 

6. AUTHOR(S)

Ian C. Watson, PE.; O.J. Morin, Jr., PE.; Lisa Henthorne, PE 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

RosTek Associates, Inc., Tampa, Florida 

DSS Consulting, Inc., Blue Ridge, Georgia 

Aqua Resources International, Inc., Evergreen, Colorado 

8. PERFORMING ORGANIZATION

REPORT NUMBER

98-003-01 

 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Bureau of Reclamation 

Denver Federal Center 

PO Box 25007 

Denver CO 80225-0007 

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER 

Desalination Research and 

Development Program Report 

No. 72 

11. SUPPLEMENTARY NOTES

Desalination and Water Purification Research and Development Program 

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Available from the National Technical Information Service, Operations Division, 5285 Port Royal 

Road, Springfield, Virginia 22161 

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

This Handbook was first published by the Office of Water Research and Technology and the Bureau of Reclamation in 1972. A 

second edition was published by the Bureau of Reclamation in 1977. In the intervening years, desalting technology and cost have 

changed radically, raising the need for this Handbook (the third edition). The Handbook contains descriptions of both thermal and 

membrane technologies in common use today, together with chapters on the history of desalination in the U.S. Chapters on 

pretreatment and post-treatment, environmental issues, and a chapter on case histories precede the final chapter on costs and cost 

estimating. 

This Handbook is designed for use by appointed and elected officials, planners, and consultants with a limited knowledge of the

technologies involved, but who have enough familiarity with the general principles to recognize that desalting may have value as a 

viable alternate source of drinking water for their communities. 

14. SUBJECT TERMS-- 

Desalting, thermal, membrane, multi-stage flash distillation, multi-effect distillation, vapor compression, reverse 

osmosis, electrodialysis, electrodialysis reversal, nanofiltration, microfiltration, ultrafiltration, seawater treatment, 

brackish water treatment, wastewater reclamation, pretreatment, post-treatment, capital cost, operating and 

maintenance cost, case studies, concentrate disposal, brine disposal, water chemistry, ASTM standards, worked cost 

examples 

15. 

NUMBER 

OF PAGES

316 

16. PRICE 

CODE

17. SECURITY CLASSIFICATION 

OF REPORT

 UL 

18. SECURITY CLASSIFICATION

OF THIS PAGE

 UL 

19. SECURITY CLASSIFICATION 

OF ABSTRACT

 UL 

20. 

LIMITATION 

OF 

ABSTRACT

 UL 
DESALTING HANDBOOK FOR PLANNERS

Third Edition 

by 

RosTek Associates, Inc., Tampa, Florida 

in association with 

DSS Consulting, Inc., Blue Ridge, Georgia 

Aqua Resources International, Inc., Evergreen, Colorado 

and in cooperation with 

U.S. Department of the Interior 

Bureau of Reclamation 

Technical Service Center 

Water Treatment Engineering and Research Group 

Cooperative Assistance Agreement Number: 98-PG-81-0366 

Desalination Research and Development Program Report No. 72 

July 2003 
Mission Statements 

U.S. Department of the Interior 

The mission of the Department of the Interior is to protect and provide access to our 

Nation’s natural and cultural heritage and honor our trust responsibilities to Indian tribes 

and our commitments to island communities. 

Bureau of Reclamation 

The mission of the Bureau of Reclamation is to manage, develop, and protect water and 

related resources in an environmentally and economically sound manner in the interest of 

the American public. 

Federal Disclaimer 

Information contained in this document regarding commercial products or firms was 

supplied by those firms. It may not be used for advertising or promotional purposes and 

is not to be construed as an endorsement of any product or firm by the Bureau of 

Reclamation. 

The information contained in this document was developed for the Bureau of 

Reclamation. No warranty as to the accuracy, usefulness, or completeness is expressed 

or implied. 

Reclamation Point of Contact 

Contact Susan Martella, Water Treatment Engineering and Research Group, at 

303-445-2257, or use the website at <http://www.usbr.gov/pmts/water_trtmt> 
Acknowledgments

 

The principal author wishes to thank Kevin Price and his staff at the Bureau of 

Reclamation for their patience, forbearance, and understanding during the long and 

sometimes rocky path to completion of this work. Updating a 1977 document to circa 

2000 turned out to be more difficult than first expected. Thanks also to Deena Larsen 

and Teri Manross for their suggestions and help in getting this Handbook into the official 

format with all the proper documentation. 

Also, the authors acknowledge the work of Mr. Sherman May, who spent a considerable 

amount of time and effort to make sure the Handbook units are consistent, and who 

offered great insight into the thermal portions of the work. Finally, there would be no 

Handbook without the support, encouragement, and significant word processing effort of 

Ms. Evelyn Watson. Our grateful appreciation. 
About this Handbook 

The Desalting Handbook for Planners was originally published in 1972 by the 

Department of the Interior, Office of Saline Water (OSW), which is no longer in 

existence. The U.S. Bureau of Reclamation (Reclamation) was largely responsible for 

preparing this original document, which was updated in 1977. Due to the renewed 

awareness and need for desalination, both domestically and internationally, Reclamation 

has taken the initiative to update this document and make it available for public use. This 

effort is funded by the Water Desalination Act of 1996, administered by Reclamation and 

commonly referred to as the Simon Bill. This program is discussed in Appendix A; a 

copy of the legislation is provided in Appendix B. 

The Handbook was originally created to assist municipal planners in understanding how 

desalination could augment their water supply and to assist them in implementing 

desalination in their community. Today, water supplies are more complex and many 

more groups of individuals are involved in making decisions. This Handbook is intended 

for use by: 

• Community leaders considering or currently using desalination 

• Desalination project developers 

• Engineers responsible for assisting communities 

• Project developers considering or implementing desalination 

• Research entities investigating desalination technologies 

• Government and regulatory entities requiring an understanding of desalination 

• Industrial and commercial users of desalination 

• Environmental groups interested in understanding desalination 

• Students interested in learning more about desalination technology and its 

uses 

The purpose of the Handbook is to: 

• Provide an in-depth understanding of the desalination technologies, issues, 

and costs 

• Act as an educational tool for engineers and researchers being introduced to 

desalination 
vii 

Table of Contents 

Page 

Chapter 1: Introduction ...............................................................................................................1 

 1.1 Background ................................................................................................................1 

 1.2 Introduction to Desalination Technologies.......................................................................3 

 1.3 Desalination Market..........................................................................................................4 

Chapter 2: Desalting Applications ..............................................................................................7 

 2.1 Sources of Feed Water......................................................................................................8 

 2.1.1 Industrial Wastewater ...........................................................................................8 

 2.1.2 Municipal Wastewater ..........................................................................................8 

 2.1.3 Brackish Ground Water ........................................................................................9 

 2.1.4 Brackish Surface Water ........................................................................................9 

 2.1.5 Seawater ..............................................................................................................10 

 2.2 Desalting for Water Supply ............................................................................................10 

 2.2.1 Supplemental Public Supplies in the U.S. ..........................................................10 

 2.2.2 Alternative to Reservoir Development ...............................................................13 

 2.2.3 Alternative to Long-Distance Water Transfer ....................................................13 

 2.2.4 Industrial Applications........................................................................................13 

 2.3 Case Histories ..............................................................................................................15 

 2.3.1 Brackish Water Desalting using Reverse Osmosis.............................................15 

 2.3.2 Seawater Desalting using Reverse Osmosis .......................................................17 

 2.3.3 Wastewater Reclamation using Reverse Osmosis ..............................................19 

 2.3.4 Seawater Desalting using Multi-Stage Flash ......................................................22 

Chapter 3: Water Chemistry .....................................................................................................27 

 3.1 Basic Water Chemistry ...................................................................................................27

 3.1.1 Water Cycles and Constituents ...........................................................................27 

 3.1.2 Basic Chemical Terms ........................................................................................28 

 3.1.3 Chemical Formulas and Compounds..................................................................28 

 3.1.3.1 Valences...............................................................................................28 

 3.1.3.2 Ions.......................................................................................................29 

 3.1.4 Salts as Compounds ............................................................................................30 

 3.1.5 Constituents in Water..........................................................................................31 

 3.1.6 Measurements for Water Samples ......................................................................33 

 3.1.6.1 Measuring pH.......................................................................................33 

 3.1.6.2 Measuring Conductivity.......................................................................34 

 3.2 Types of Water and Treatments......................................................................................36 

 3.2.1 Fresh Water.........................................................................................................36 

 3.2.2 Brackish Water....................................................................................................36 

 3.2.3 Seawater ..............................................................................................................38 

 3.3 Water Analyses ..............................................................................................................38 
 viii

Table of Contents 

Page

Chapter 4: Desalting Processes..................................................................................................43 

 4.1 Desalting Plant Processes ...............................................................................................43

 4.2 Distillation Process Fundamentals..................................................................................45 

 4.3 Characteristics of Distillation Processes.........................................................................46 

 4.3.1 Temperature in Distillation Processes ................................................................46 

 4.3.2 Scaling in Distillation Processes.........................................................................46 

 4.3.2.1 Effects of Scaling.................................................................................46 

 4.3.2.2 Calcium Sulfate Scale..........................................................................46 

 4.3.2.3 Calcium Carbonate and Magnesium Hydroxide Scale ........................47 

 4.3.3 Corrosion and Erosion in Distillation Processes.................................................48 

 4.3.4 Heat Transfer in Distillation Processes...............................................................49 

 4.3.5 Post-Treatment in Distillation Processes ............................................................49 

 4.3.6 Energy Requirements for Distillation Processes.................................................50 

 4.4 Multiple Effect Distillation Process................................................................................50 

 4.4.1 MED Operating Principle ...................................................................................50 

 4.4.2 MED Design Configurations ..............................................................................51 

 4.4.2.1 Horizontal Tube Arrangement .............................................................51 

 4.4.2.2 Vertical Tube Arrangement .................................................................55 

 4.4.2.3 Vertically Stacked Tube Bundles.........................................................55 

 4.4.3 MED Process Characteristics..............................................................................58 

 4.4.4 MED Materials of Fabrication............................................................................58 

 4.4.5 MED Process Status............................................................................................59 

 4.5 Multi-Stage Flash (MSF) Distillation .............................................................................61 

 4.5.1 MSF Operating Principle ....................................................................................61 

 4.5.2 MSF Process Arrangements................................................................................63 

 4.5.3 MSF Process Description....................................................................................63 

 4.5.3.1 Once-Through Design..........................................................................63 

 4.5.3.2 Recycle Design ....................................................................................65 

 4.5.3.3 Scaling..................................................................................................65 

 4.5.3.4 Corrosion..............................................................................................66 

 4.5.4 MSF Process Characteristics...............................................................................66 

 4.5.5 MSF Materials of Fabrication.............................................................................67 

 4.5.6 MSF Process Status.............................................................................................68 

 4.6 Vapor Compression ........................................................................................................69 

 4.6.1 VC Operating Principle.......................................................................................69 

 4.6.2 VC Process Arrangement....................................................................................69 

 4.6.3 VC Process Description ......................................................................................69 

 4.6.4 VC Process Characteristics.................................................................................72 

 4.6.5 VC Materials of Fabrication ...............................................................................72 

 4.6.6 VC Process Status...............................................................................................72 

 4.7 Comparing Distillation with Other Desalting Processes ................................................73 
ix 

Table of Contents 

Page

 4.8 Electrodialysis ..............................................................................................................73 

 4.8.1 ED/EDR Process Fundamentals .........................................................................74 

 4.8.2 ED and EDR Stack Design .................................................................................76 

 4.8.3 ED/EDR Power Consumption ............................................................................78 

 4.8.4 ED/EDR Process Variables.................................................................................78 

 4.8.5 ED/EDR Equipment............................................................................................80 

 4.8.6 ED/EDR Plant Layout.........................................................................................80 

 4.8.7 ED/EDR Membrane Scaling and Fouling...........................................................81 

 4.8.8 ED/EDR Membrane Life ....................................................................................83 

 4.8.9 ED/EDR Electrode Life ......................................................................................84 

 4.9 Comparing ED/EDR with Other Desalting Processes....................................................84 

 4.10 Reverse Osmosis and Nanofiltration ..............................................................................85 

 4.10.1 RO and NF Process Fundamentals .....................................................................85 

 4.10.2 Comparing RO and NF Membranes ...................................................................87 

 4.10.3 RO Membrane Configurations............................................................................88 

 4.10.3.1 Spiral Wound .......................................................................................88 

 4.10.3.2 Hollow Fine Fiber................................................................................89 

 4.10.3.3 Tubular Configuration .........................................................................91 

 4.10.3.4 Plate and Frame Configuration ............................................................92 

 4.10.3.5 Considerations in Membrane Configuration Design ...........................92 

 4.10.4 Power Consumption............................................................................................93 

 4.10.5 RO Process Variables .........................................................................................95 

 4.10.6 RO and NF Peripheral Equipment ......................................................................98 

 4.10.7 RO Plant Layout ...............................................................................................100 

 4.10.8 Membrane Life..................................................................................................102 

 4.11 Comparing RO and NF with Other Desalting Processes..............................................102 

Chapter 5: Pretreatment..........................................................................................................105 

 5.1 Introduction ............................................................................................................105 

 5.2 Distillation Processes....................................................................................................105 

 5.2.1 Scaling in Distillation Processes.......................................................................105 

 5.2.1.1 Calcium Sulfate Scaling.....................................................................105 

 5.2.1.2 Calcium Carbonate and Magnesium Hydroxide Scaling...................106 

 5.2.2 Corrosion in Distillation Processes...................................................................106 

 5.2.3 Erosion by Suspended Solids in Distillation Processes ....................................107 

 5.2.4 Impact of Other Constituents............................................................................107 

 5.3 Membrane Processes.....................................................................................................108

 5.3.1 Scaling for Membrane Processes......................................................................110 

 5.3.2 Metal Oxide Fouling in Membrane Processes..................................................112 

 5.3.3 Biological Fouling in Membrane Processes .....................................................114 

 5.3.4 Suspended Solids in Membrane Processes .......................................................116 
 x

Table of Contents 

Page

 5.3.5 Other Considerations for Membrane Processes................................................119 

 5.3.5.1 Silica ..................................................................................................119 

 5.3.5.2 Hydrogen Sulfide...............................................................................119 

Chapter 6: Post-Treatment......................................................................................................123 

 6.1 Introduction ..................................................................................................................123 

 6.2 Stabilization ..................................................................................................................124 

 6.2.1 Chemical Addition............................................................................................125 

 6.2.2 Corrosion Considerations..................................................................................125 

 6.3 Blending Stabilization...................................................................................................127 

 6.4 Dissolved Gas Stripping ...............................................................................................128 

 6.5 Disinfection ..................................................................................................................129 

 6.5.1 Chlorine Treatment ...........................................................................................131 

 6.5.1.1 Chlorine Gas ......................................................................................131 

 6.5.1.2 Liquid Chlorine, Sodium, or Calcium Hypochlorite .........................133 

 6.5.1.3 Chloramine Treatment .......................................................................133 

Chapter 7: Process Selection and Water Cost........................................................................135 

 7.1 Introduction ............................................................................................................135 

 7.2 Product Water Quality ..................................................................................................135

 7.2.1 Salt Concentration.............................................................................................135 

 7.2.2 Composition......................................................................................................137 

 7.2.3 Blending ............................................................................................................137 

 7.3 Feed Water Source Characteristics ...............................................................................138 

 7.3.1 Dependability....................................................................................................138 

 7.3.2 Salinity ............................................................................................................138 

 7.3.3 Temperature ......................................................................................................139 

 7.3.4 Composition......................................................................................................140 

 7.3.5 Physical Quality................................................................................................141 

 7.4 Heat and Electrical Energy ...........................................................................................141 

 7.4.1 Single-Purpose Plant for Distillation Plants .....................................................142 

 7.4.2 Dual-Purpose Plant ...........................................................................................142 

 7.5 Environmental Constraints for Site Selection...............................................................146 

 7.5.1 Planning and Cost Features...............................................................................146 

 7.5.2 Concentrate Disposal ........................................................................................147 

 7.5.2.1 Disposal to Surface Waters................................................................147 

 7.5.2.2 Deep-Well Injection...........................................................................148 

 7.5.2.3 Evaporation Ponds .............................................................................148 

 7.5.2.4 Evaporation to Dryness and Crystallization ......................................148 

 7.6 Factors Influencing Site Location.................................................................................148 

 7.6.1 Source Water.....................................................................................................149 
xi 

Table of Contents 

Page

 7.6.1.1 Source Water Supply .........................................................................149 

 7.6.1.2 Pretreatment Considerations ..............................................................150 

 7.6.1.3 Source Water Quality.........................................................................150 

 7.6.2 Site Location .....................................................................................................152 

 7.6.3 Land Area Requirements ..................................................................................152 

 7.6.4 Concentrate Disposal Issues .............................................................................152 

 7.6.5 Data Collection .................................................................................................153 

 7.6.6 Problematic Issues ............................................................................................153 

 7.7 Relative Desalting Process Costs..................................................................................155 

 7.7.1 Plant Investment and Water Cost Summary .....................................................155 

 7.7.1.1 Assumptions.......................................................................................155 

 7.7.1.2 Comparing Plant Costs ......................................................................157 

 7.8 Optimizing Capital and Operating Costs for Distillation Processes.............................162 

 7.9 Optimizing Costs for Electrodialysis Reversal.............................................................163 

 7.10 Optimizing Capital and Operating Costs for Reverse Osmosis....................................164 

Chapter 8: Environmental Considerations ............................................................................169 

 8.1 Introduction ..................................................................................................................169 

 8.2 Concentrate Disposal Option ........................................................................................170 

 8.2.1 Possible Environmental Issues..........................................................................171 

 8.2.2 Surface Water Discharge ..................................................................................172 

 8.2.3 Discharge to Sewer ...........................................................................................173 

 8.2.4 Deep Well Injection ..........................................................................................173 

 8.2.5 Land Application ..............................................................................................174 

 8.2.6 Evaporation Ponds/Salt Processing Facilities...................................................174 

 8.2.7 Concentrate Concentrators for Zero Discharge Facilities.................................174 

 8.3 Federal Legislation in the U.S. .....................................................................................174 

 8.3.1 National Environmental Policy Act..................................................................176 

 8.3.2 Clean Water Act................................................................................................179 

 8.3.3 Safe Drinking Water Act ..................................................................................180 

 8.3.4 Other Regulations .............................................................................................180 

 8.4 State Requirements .......................................................................................................182 

 8.4.1 Underground Injection Control.........................................................................182 

 8.4.2 Coastal Zone Management ...............................................................................183 

 8.4.3 State Environmental Impact Assessment..........................................................183 

 8.5 Environmental Obligation Assessment.........................................................................184 

Chapter 9: Cost Estimating Procedures.................................................................................187 

 9.1 Background Information for Cost Estimate..................................................................188 

 9.2 Capital Cost ..................................................................................................................189 

 9.2.1 Capital Cost Basis.............................................................................................189 

 9.2.2 Annual Costs.....................................................................................................190 
 xii

Table of Contents 

Page

 9.3 Detailed Cost Estimates ................................................................................................190

 9.3.1 Capital Costs .....................................................................................................190 

 9.3.1.1 Desalting Plant Costs .........................................................................191 

 9.3.1.2 Concentrate Disposal .........................................................................192 

 9.3.1.3 Pretreatment .......................................................................................193 

 9.3.1.4 Feed Water Intake..............................................................................193 

 9.3.1.5 Feed Water Pipe.................................................................................194 

 9.3.1.6 Steam Supply .....................................................................................194 

 9.3.1.7 General Site Development .................................................................194 

 9.3.1.8 Post-Treatment...................................................................................194 

 9.3.1.9 Auxiliary Equipment..........................................................................194 

 9.3.1.10 Building and Structures......................................................................195 

 9.3.2 Indirect Capital Cost .........................................................................................195 

 9.3.2.1 Freight and Insurance.........................................................................195 

 9.3.2.2 Interest During Construction..............................................................195 

 9.3.2.3 Construction Overhead and Profit .....................................................195 

 9.3.2.4 Owner’s Direct Expense ....................................................................196 

 9.3.2.5 Contingency .......................................................................................196 

 9.3.3 Nondepreciating Capital Costs .........................................................................197 

 9.3.3.1 Land ...................................................................................................197 

 9.3.3.2 Working Capital.................................................................................197 

 9.3.4 Annual Cost ......................................................................................................197 

 9.3.4.1 Labor..................................................................................................197 

 9.3.4.2 Chemicals...........................................................................................198 

 9.3.4.3 Energy................................................................................................198 

 9.3.4.4 Replacement Parts and Maintenance Materials .................................200 

 9.3.4.5 Membrane Replacement Cost............................................................200 

 9.3.4.6 Insurance............................................................................................200 

 9.3.4.7 Annual Cost of Capital.......................................................................200 

 9.3.4.8 Plant Factor ........................................................................................201 

Appendices 

A Reclamation’s Desalting Program 

B Water Desalination Act of 1996 

C ASTM Standards Applicable to Membrane Systems 

D Worked Examples 

E Glossary 

F Common Conversions 
xiii 

Table of Contents 

Figures 

Page

1-1 Growth of global industrial and municipal water consumption...........................................2 

1-2 Regional percentages of contracted desalination capacity through the end of 1999 ...........5 

2-1 Global desalination capacity for membrane and thermal plants, 1950-2001.....................12 

2-2 Global desalination capacity usage, 1950-2001.................................................................12 

2-3 Process flow diagram for the Dare County, North Carolina, North RO plant...................17 

2-4 Process flow diagram for the Harlingen Reclamation Plant..............................................21 

2-5 Ghubrah Complex seawater intake lines............................................................................25 

2-6 MSF unit at the Ghubrah Complex....................................................................................25 

3-1 Mineral acidity as a function of pH ...................................................................................35 

3-2 Effect of bicarbonate alkalinity and CO2 on pH ................................................................35 

4-1 General desalting plant schematic – surface supply ..........................................................43 

4-2 General desalting plant schematic – ground water supply.................................................44 

4-3 Conceptual process design drawing...................................................................................45 

4-4 Calcium sulfate solubility ..................................................................................................47 

4-5 Multiple effect schematic...................................................................................................51 

4-6 MED horizontal tube arrangement.....................................................................................52 

4-7 MED process schematic – horizontal tube arrangement....................................................53 

4-8 Calcium sulfate solubility and MED operation .................................................................54 

4-9 Calcium sulfate solubility and vertically stacked MED operation ....................................54 

4-10 MED vertical tube bundle arrangement.............................................................................55 

4-11 MED process schematic – vertical tube arrangement........................................................56 

4-12 MED vertically stacked tube bundles ................................................................................56 

4-13 MED process schematic – vertically stacked tube bundles ...............................................57 

4-14 Process schematic, MWD’s short tube test unit.................................................................60 

4-15 MSF arrangement...............................................................................................................62 

4-16 MSF stage ..........................................................................................................................62 

4-17 MSF long tube design ........................................................................................................63 

4-18 MSF cross tube design.......................................................................................................64 

4-19 MSF schematic, once-through ...........................................................................................64 

4-20 MSF recirculation schematic .............................................................................................65

4-21 Calcium sulfate solubility and MSF operations.................................................................66 

4-22 VC schematic.....................................................................................................................70 

4-23 Mechanical or thermo-compression VC overall process schematic ..................................71 

4-24 Calcium sulfate solubility and VC operations ...................................................................71 

4-25 ED schematic .....................................................................................................................75 

4-26 ED stack assembly .............................................................................................................75 

4-27 Examples of hydraulic and electrical staging ....................................................................76 

4-28 Three-stage, two-line ED/EDR schematic.........................................................................81 

4-29 Osmotic pressure................................................................................................................85 
 xiv

Table of Contents 

Figures 

Page

4-30 Osmosis and reverse osmosis.............................................................................................86

4-31 Spiral wound element construction....................................................................................89 

4-32 Spiral wound membranes and vessel assembly .................................................................90 

4-33 Hollow fine fiber permeator schematic..............................................................................90 

4-34 Tubular membrane schematic............................................................................................91 

4-35 Plate and frame schematic .................................................................................................92 

4-36 Three types of energy recovery devices in use today ........................................................96 

4-37 Solute concentration factor as a function of recovery .......................................................97 

4-38 Horizontal cartridge filters in Jupiter, Florida ...................................................................98 

4-39 Typical chemical feed system............................................................................................99

4-40 Cleaning system, North Hatteras plant, Dare County, North Carolina..............................99 

4-41 Simplified RO process flow diagram...............................................................................100 

4-42 Typical arrangements for staging.....................................................................................101 

5-1 Pretreatment schematic, low temperature design (T = 90 oC) .........................................108 

5-2 Pretreatment schematic, high temperature design (T = 110 oC) ......................................108 

5-3 Langelier Saturation Index nomograph ...........................................................................111 

5-4 How a bacterium sticks to the membrane surface ...........................................................116 

6-1 Blending mass balance flow diagram ..............................................................................127 

6-2 Cross section of typical packed tower .............................................................................129 

6-3 HOCl-OCl-

 equilibrium diagram .....................................................................................132 

6-4 Chlorine addition schematic ............................................................................................132 

7-1 Process schematic – single-purpose MSF arrangement...................................................142 

7-2 Three dual-purpose arrangements – distillation processes ..............................................144 

7-3 Dual-purpose arrangement – RO process ........................................................................145 

7-4a Relative seawater desalting capital costs (metric) ...........................................................158 

7-4b Relative seawater desalting capital costs (U.S.) ..............................................................158 

7-5a Relative brackish water desalting capital costs (metric)..................................................159 

7-5b Relative brackish water desalting capital costs (U.S.).....................................................159 

7-6a Cost of water seawater desalting (metric)........................................................................160 

7-6b Cost of water seawater desalting (U.S.)...........................................................................160 

7-7a Cost of water – brackish water desalting (metric) ...........................................................161 

7-7b Cost of water – brackish water desalting (U.S.) ..............................................................161 

7-8 Energy consumption comparison – RO and EDR ...........................................................164 

7-9 BWRO feed pressure versus feed water TDS..................................................................166 

8-1 Comparison of concentrate disposal methods for all brackish water 

 processing facilities in the U.S...................................................................................171 

8-2 Reclamation’s NEPA flowchart.......................................................................................177 

9-1 Total construction cost—MSF process............................................................................207 

9-2 Process construction cost—MSF .....................................................................................207 

9-3 Total construction cost—MED process...........................................................................208 
xv 

Table of Contents 

Figures 

 Page

9-4 Process construction cost—MED ....................................................................................208 

9-5 Total construction cost—MVC process...........................................................................209 

9-6 Process construction cost—MVC....................................................................................209 

9-7 Total construction cost—SWRO, BWRO, and NF plants with surface 

 water feed...................................................................................................................210 

9-8 Total construction cost—SWRO, BWRO, and NF plants with well 

 water feed...................................................................................................................210 

9-9 Total construction cost—EDR plant................................................................................211 

9-10 Construction cost—concentrate disposal pipeline for distillation processes...................211 

9-11 Construction cost—concentrate disposal pipeline for membrane processes ...................212 

9-12 Construction cost—evaporation ponds for membrane processes ....................................212 

9-13 Construction cost—injection wells for membrane processes ..........................................213 

9-14 Construction cost—pretreatment for distillation processes .............................................213 

9-15 Construction cost—surface water pretreatment for membrane processes.......................214 

9-16 Construction cost—open intake systems for distillation processes .................................214 

9-17 Construction cost—open intake systems for smaller membrane processes.....................215 

9-18 Total construction cost—wellfields .................................................................................215 

9-19 Construction cost—seawater feed water pipeline for distillation processes....................216 

9-20 Construction cost—seawater feed water pipeline for membrane processes....................216 

9-21 Construction cost—package boilers for single-purpose plant .........................................217 

9-22 Construction cost—site development for distillation processes ......................................217 

9-23 Construction cost—site development for membrane processes ......................................218 

9-24 Construction cost—post-treatment for distillation processes ..........................................218 

9-25 Construction cost—post-treatment for membrane processes ..........................................219 

9-26 Construction cost—product storage for distillation systems using steel 

 tank construction........................................................................................................219

9-27 Construction cost—product storage for membrane systems using prestressed 

 concrete tank construction .........................................................................................220 

9-28 Construction cost—product transmission pipeline ..........................................................220 

9-29 Construction cost—emergency generators ......................................................................221 

9-30 Construction cost—step-down transformers....................................................................221 

9-31 Construction cost—distillation process buildings ...........................................................222 

9-32 Construction cost—membrane process buildings............................................................222 

9-33 Owner’s direct expense and COH factors........................................................................223 

9-34 Land requirements—for distillation process plants .........................................................223 

9-35 Land requirements—for membrane process plants .........................................................224 

9-36 Annual cost—labor for distillation process .....................................................................224 

9-37 Annual cost—labor for membrane processes ..................................................................225 

9-38 Annual cost—MSF chemicals .........................................................................................225 

9-39 Annual cost—MED chemicals ........................................................................................226 
 xvi

Table of Contents 

Figures 

 Page

9-40 Annual cost—MVC chemicals ........................................................................................226 

9-41 Annual cost—chemicals for surface water membrane processes....................................227 

9-42 Annual cost—chemicals for ground water membrane processes ....................................227 

9-43 Annual cost—MSF processes electricity.........................................................................228 

9-44 Annual cost—MED processes electricity ........................................................................228 

9-45 Annual cost—MVC processes electricity........................................................................229 

9-46 Annual cost—SWRO processes electricity .....................................................................229 

9-47 Annual cost—BWRO, EDR, and NF plants electricity...................................................230 

9-48 Annual cost—steam for single-purpose plants ................................................................230 

9-49 Annual cost—steam for dual-purpose plants, power credit method................................231 

9-50 Annual cost—steam for dual-purpose plants, available energy method..........................231 

9-51 Annual cost—MSF repairs and spares.............................................................................232 

9-52 Annual cost—MED repairs and spares............................................................................232 

9-53 Annual cost—MVC repairs and spares............................................................................233 

9-54 Annual cost—membrane replacement.............................................................................233 

Tables 

 Page

2-1 Desalting plant capacity by location, contracted through the end of 2001........................11 

2-2 Desalting plant capacity by process, contracted through the end of 2001.........................11 

2-3 Typical quality limits for selected industries.....................................................................14 

2-4 Ghubrah 1-6 main design features .....................................................................................24 

2-5 Ghubrah desalting costs .....................................................................................................24 

3-1 Symbols, atomic weights, and common ionic charges commonly 

 occurring in natural waters...........................................................................................32 

3-2 Major components of natural water ...................................................................................32 

3-3 Analysis of various brackish waters ..................................................................................37 

3-4 Typical brackish water analysis.........................................................................................39 

3-5 Three methods of reporting water analysis........................................................................40 

3-6 Conversion of ionic concentration to CaCO3 equivalents (hardness)................................41 

4-1 Process characteristics of MED systems............................................................................58 

4-2 Materials of fabrication, MED systems .............................................................................59 

4-3 Process characteristics, MSF systems................................................................................67 

4-4 Materials of fabrication, MSF systems ..............................................................................68 

4-5 Process characteristics, VC systems ..................................................................................72 

4-6 Materials of fabrication, VC systems.................................................................................73 

4-7 Selected properties of commercial ED/EDR membranes..................................................77 

4-8 Pretreated feed water quality goals for EDR .....................................................................82 
xvii 

Table of Contents 

Tables 

 Page

4-9 Typical RO feed pump power consumption ......................................................................94 

4-10 Typical recoveries for various vessel lengths ..................................................................101 

5-1 Pretreated water quality requirements for membrane processes......................................109 

5-2 Typical commercial scale inhibitors ................................................................................113 

5-3 Principal events in membrane biofouling processes........................................................115 

5-4 Disinfection techniques for biofouling control................................................................117 

5-5 Techniques for removing suspended solids .....................................................................117 

5-6 Distribution of hydrogen sulfide species as a function of pH..........................................120 

6-1 Effect of mineral addition on water quality .....................................................................125 

7-1 Summary of guidelines for desalting selection................................................................136 

7-2 Suggested water quality analysis .....................................................................................151 

7-3 Unit site areas, for plants of 19,000 m3

/d and larger .......................................................152 

7-4 Disposal of concentrates ..................................................................................................153 

7-5 Data collection checklist..................................................................................................154 

7-6 Plant investment and product water cost for a 37,850 m3

/d (10 mgd) MSF 

 base case plant............................................................................................................162 

7-7 MSF optimization ............................................................................................................163 

7-8 Principal water cost components for ED/EDR ................................................................163 

7-9 Principal water cost components for RO .........................................................................165 

8-1 Desalination processes and characteristics of their concentrate streams .........................170 

9-1 Estimated design and construction periods......................................................................188 

9-2 Operations and maintenance staff – distillation processes ..............................................197 

9-3 Operations and maintenance staff – membrane processes...............................................198 

9-4 Unit costs of chemicals by process and use.....................................................................198 

9-5 Capital recovery factors ...................................................................................................201 

9-6a Design criteria..................................................................................................................202 

9-6b Economic criteria.............................................................................................................203 

9-7 Supporting sheet, computation sheet, and cost summary ................................................204 
 xviii

Acronyms and Abbreviations 

AMTA American Membrane Technology Association 

AWWARF American Water Works Association Research Foundation 

ASTM American Society for Testing Materials 

BCI Building Cost Index 

BPE boiling point elevation 

Btu British thermal unit 

BWRO brine water reverse osmosis 

CA cellulose acetate membrane 

CCI Construction Cost Index 

CEQ Counsel on Environmental Quality 

CF concentration factor 

cm centimeter 

COH construction overhead 

CT concentration and time 

CWA Clean Water Act 

DBP disinfection byproducts 

DCC direct capital cost 

D/DBP disinfection/disinfection byproducts 

EA Environmental Assessment 

ED electrodialysis 

EDR electrodialysis reversal 

EIS Environmental Impact Statement 

EPA Environmental Protection Agency 

ft foot 

IEC ion exchange capacity 

g gram 

gpd gallons per day 

gpm gallons per minute 

HAA haloacetic acids 

HFF hollow fine fiber 

HMS horizontal multi-stage, with energy recovery turbine 

hr hour 

HTC© hydraulic turbocharger 

kg kilogram 

kgal thousand gallons 

kW kilowatt 

kWh kilowatthour 

l liter 

lb pound 

LSI Langelier saturation index 

LTTU long tube test unit 

m meter 

MED multiple effect distillation 

meq/g milliequivalent per gram 

MEW Ministry of Electricity and Water 

MF membrane filtration 
xix 

mgd million gallons per day 

MJ mega joule 

mm millimeter 

MSF multi-stage flash distillation 

MVC mechanical vapor compression 

MWD Metropolitan Water District of Southern California 

NA not applicable or not available 

NDP net driving pressure 

NEPA National Environmental Policy Act 

NF nanofiltration 

NOM natural organic matter 

NPDES National Pollutant Discharge Elimination System 

NSF National Sanitation Foundation 

NTU nephelometric turbidity unit 

OMB Office of Management and Budget 

OSW Office of Saline Water 

PA polyamide membrane 

PLC programmable control systems 

ppm parts per million 

PSD Preventing Significant Air Quality Deterioration 

psi pounds per square inch 

psig pounds per square inch gauge 

PVDF polyvinylidene-fluoride 

RCRA Resource Conservation and Recovery Act of 1976 

RO reverse osmosis 

RSI Ryznar Stability Index 

SDI silt density index 

SDWA Safe Drinking Water Act 

SRB sulfate reducing bacteria 

STTU short tube test unit 

SW spiral wound 

SWDA Solid Waste Disposal Act 

SWFWMD Southwest Florida Water Management District 

SWRO seawater reverse osmosis 

TDS total dissolved solids 

THM trihalomethane 

THMFP trihalomethane formation potential 

TSS total suspended solids 

TVC thermo vapor compression distillation 

UF ultrafiltration 

UIC underground injection control 

USGS U.S. Geological Survey 

UF ultrafiltration 

U.K. United Kingdom 

UV ultraviolet 

VC vapor compression 

w/ft2

-K watts per square foot – degrees Kelvin 
Chapter 1: Introduction 

1.1 Background 

For the purposes of this Desalting Handbook for Planners (Handbook), “desalination” is defined 

as a water treatment process that removes salts from water. Desalination processes can be used 

in various applications including: 

• Municipal desalting of brackish or seawater for drinking water production 

• Industrial and commercial applications for production of high-purity boiler feed 

water, process water, bottled water, and for zero discharge applications; producing 

water for industries including the pharmaceutical, electronics, bio/medical, mining, 

power, petroleum, beverage, tourism, and pulp/paper industries 

• Rigorous treatment of wastewater for reuse applications 

Desalination has now become an accepted water treatment process around the world and is 

becoming a price-competitive option for more communities as the cost of desalination is 

decreasing to the level of new supplies using conventional means. 

Fresh water resources on Earth are limited. Over 97 percent of the world’s water is seawater, 

with an additional 2 percent of the world’s water resources locked up in ice caps and glaciers. 

Saline ground water and inland saline seas reduce available fresh water even further. As a result, 

less than 0.5 percent of the Earth’s water resources is available as fresh water for direct human 

consumption or for agricultural and industrial uses. 

Stress on our fresh water resources is increasing as: 

• Global population increases 

• Irrigation and agricultural demands increase 

• Standards of living improve 

• Industrialization increases 

• Environmental needs require more water 

• Water quality of existing resources declines 

Population.⎯The demand for fresh water and population growth are directly related. The 

world’s population⎯approximately 6 billion people⎯is projected to double in the next 50 to 

90 years, while our renewable fresh water resources remain constant. Currently, over 

400 million people live in regions with severe water shortages. This is estimated to climb to 

2.8 billion people by 2025. This is roughly 35 percent of the projected total population. At least 

1 billion of these people will be living in countries facing absolute water scarcity, defined as less 

than 500 cubic meters (132,000 gallons) per person per year. 
Desalting Handbook for Planners 

2

Agriculture.⎯Agriculture is the largest single user of our fresh water resources, accounting for 

approximately 63 percent of the overall water withdrawal worldwide, but over 86 percent of the 

actual worldwide water consumption. Other water users, such as industry, recycle much of their 

withdrawal and, therefore, do not consume as much of the water they withdraw. As the 

population increases, the agricultural demand also increases. Available land for agricultural 

production is now declining. Raising grain production can only come from higher land 

productivity, which correlates, in many cases, to increased irrigation. 

Standards of living.⎯While overall improvements in standards of living are positive for society, 

the demand for water resources increases. The domestic consumption rate—as well as the 

overall per capita consumption—increases with the standard of living of a society. Additionally, 

an increased standard of living results in the demand for goods and products, which increases 

industrialization. Water recycling has been effectively applied in many segments of industry, yet 

consumption for industrial applications continues to climb. Consumption for industrial and 

municipal use is shown in figure 1-1 for the period 1900 to 2000 to further illustrate our 

increasing global water demand. 

Figure 1-1 Growth of global industrial and municipal water consumption (Gleick, 1993) 

In the U.S., particularly, there is a growing desire to ensure environmental water needs are met. 

Future environmental regulation is expected to ensure that the ecological needs of many rivers, 

lakes, wetlands, and streams are met by maintaining minimum instream flow requirements, 

thereby placing additional stress on the multiple and competing users of these bodies of water. 

0

5000

10000

15000

20000

25000

30000

35000

1900 1940 1950 1960 1970 1980 1990 2000

Year

Billion gallons

 per year

Industry Municipal
Chapter 1: Introduction

3

At the same time that water demand is increasing, water quality is diminishing in many parts of 

the world. The U.S., through the Environmental Protection Agency regulations and programs, 

has improved its overall water quality over the last two decades. Unfortunately, water quality in 

much of the rest of the world is still in decline, particularly in Asia and South and Central 

America. For instance, in China, 82 percent of the rivers are polluted and only 20 percent meet 

the lowest Chinese government standard for agricultural use. In urban areas of China, 80 percent 

of the surface water is contaminated. In India, it is estimated that 70 percent of the surface water 

is severely polluted. Much of this contamination is a result of high industrial activity in densely 

populated areas with poor environmental regulations, insufficient sanitation in rural and 

underdeveloped areas, and agricultural runoff contamination. 

As water is a precious and irreplaceable resource, water has also been a major source of conflict 

throughout the world. The Jordan River was at the heart of the 1967 Arab-Israeli conflict, and 

water issues continue to plague the Middle East peace process. In North Africa, the Nile River is 

the source of tension between Egypt and Sudan. In south Asia, river waters have been a point of 

contention between India, Bangladesh, and Nepal. Even in the U.S., water rights between the 

Western States with access to the Colorado River is contentious, with legal battles and political 

implications. 

Mankind has very limited options by which to reduce these stresses on our water supply. These 

options include: 

• Improved water conservation efforts 

• Additional large infrastructure projects (dams, reservoirs, and water carriers and 

water transfer projects) 

• Increased water recycling and reuse of process and wastewater 

• Desalination of brackish water and seawater 

1.2 Introduction to Desalination Technologies

There are basically two families of desalination technologies used throughout the world today. 

These include thermal technologies and membrane technologies. Thermal technologies are those 

that heat water and collect condensed vapor (distillation) to produce pure water. Rarely are 

distillation processes used to desalinate brackish water (water with less than 

10,000 milligrams per liter of total dissolved solids), as it is not cost effective for this 

application. The thermal technologies include the following specific types of processes: 

• Multiple-stage flash distillation (MSF) 

• Multiple effect distillation (MED) 

• Vapor compression (VC)—mechanical (MVC) and thermal (TVC). 
Desalting Handbook for Planners 

4

The concept of distilling water at reduced pressures, as is practiced in the thermal desalination 

processes, has been used for well over a century, with the early generation of present-day, 

multi-stage thermal desalters being developed in the 1950s. Thermal desalination is most 

commonly practiced in areas with abundant fossil fuel that can capitalize on cogeneration of 

power and water, such as in the Middle East. 

Membrane technologies use thin, semipermeable membranes to separate the feed stream into two 

streams of differing concentration, a product and concentrate stream. In desalination 

applications, the feed is either brackish or seawater. The membrane technologies include the 

following specific types of processes: 

• Reverse osmosis (RO) 

• Electrodialysis (ED) and electrodialysis reversal (EDR) 

The RO process uses pressure as the driving force to separate the saline feed into a product 

stream and a concentrate stream. The ED/EDR processes use opposing electrodes to separate out 

the positive and negative ions of the dissolved salts from a saline stream. 

Nanofiltration (NF) is also a membrane process that is used in some desalting applications, but it 

principally removes the divalent salt ions (such as calcium, magnesium, and sulfate), not the 

more common salts (sodium and chloride). As a result, nanofiltration is most commonly used 

for water softening and other nondesalting applications, such as organics removal. 

In comparison to thermal processes, membrane technologies are much younger in their 

development: ED was developed in the 1950s and RO in the 1970s. The membrane market is 

very fluid—new, improved products are continually introduced to the marketplace. 

Membrane technologies are generally constructed as single-use facilities, but recent 

developments indicate synergistic benefits result from co-locating membrane desalination plants 

with power or other industrial facilities. 

1.3 Desalination Market 

Thermal and membrane installed capacity contracted through the end of 1999 was approximately 

25.74 million cubic meters per day (6.8 billion gallons per day), with 50 percent in distillation 

capacity and 50 percent in membrane capacity. This capacity has been installed over the history 

of modern desalination, beginning in the 1950s, and not all of this capacity is presently in 

operation. Figure 1-2 shows the regional distribution of installed capacity worldwide. 
Chapter 1: Introduction

5

Figure 1-2 Regional percentages of contracted desalination capacity through the end of 1999 

(Wangnick, May 2000) 

On a global basis, the growth rate of desalination capacity from 1972 through 1999 for all 

desalting technologies averaged just under 12 percent per year, with an average of slightly more 

than 1.4 million cubic meters per day (370 million gallons per day) additional capacity installed 

each year. There have been over 8,600 desalination plants installed through 1999, with 

approximately 20 percent of these in the U.S., 16.6 percent in Saudi Arabia, and 11.2 percent in 

Japan. 

References 

Gleick, P., “Water in Crisis – A Guide to the World’s Fresh Water Resources,” Oxford 

University Press, 1993. 

Wangnick, K., “2000 IDA Worldwide Desalting Plants Inventory, Report No. 16,” International 

Desalination Association, May 2000. 

Regional Percentages of Global Desalination Capacity

Middle East

47.8%

C.America

3.2%

Africa

5.5%

Asia

12.2%

Europe

13.0%

S. America

0.7%

N. America

17.2%

Australia

0.4%

Chapter 2: Desalting Applications

7

Chapter 2: Desalting Applications 

Since the early 1970s, desalting has gained a foothold in the U.S. as a practical source of water 

supply. Desalination is now providing high-quality water for municipal and industrial use, 

particularly from brackish ground water sources. While most municipal facilities are located in 

coastal areas, the technology is being used inland in Texas, Colorado, Missouri, Iowa, and 

elsewhere. The major challenge for inland plants is disposal of the concentrate in a manner that 

is compatible with the environment. 

Along with commercial applications, desalting technology has rapidly advanced, and component 

life and system reliability have improved significantly. The cost of desalted water, particularly 

from membrane processes, has declined significantly. Actual experience in full-scale 

commercial operations and in pilot plants of different types has expanded and will continue to 

expand the knowledge base. Further improvement in desalting process technology will continue; 

further reductions in cost will lead to even more growth in desalting as a solution to water supply 

and quality problems. 

Water supply professionals are accustomed to thinking in terms of the hydrologic cycle and how 

civil works may be applied to the problems of water availability. They have been very 

successful in using capital-intensive, long-lasting structures, often quite distant from the point of 

use, in developing water supplies at modest cost to users. 

Desalting, by contrast, may be quite independent of the hydrologic cycle and, therefore, free 

from possible legal or political constraints. Desalting creates new water, as it treats seawater and 

brackish water. Desalting creates more valuable water, as it increases the utility of brackish 

water. Additionally, desalting processes can be used in industrial applications to produce 

ultrapure water or process water of very high quality, thereby greatly enhancing the productivity 

of numerous industries, including electronics, pharmaceuticals, power, food and beverage, 

mining, refining, and paper industries. 

Desalting plants can generally be located close to the point of use. Desalting adds diversity and, 

therefore, insurance to a water system because it is fundamentally different from the 

conventional water sources that it might complement. Desalting requires less investment per unit 

capacity than conventional water treatment facilities, particularly to acquire the source water. 

However, desalting may have higher operating costs. Desalting capacity is available in a variety 

of plant sizes and processes and can be matched closely to the water demand curve, possibly 

making an investment in desalting more suitable for financing than a capital and environmentally 

intensive conventional water project. Desalting is a hardware technology, and continued 

improvements in its efficiency are expected. Prototype plants will provide experience in large 

plant operation. Continued research and development will bring process efficiencies closer to 

theoretical limits. 
Desalting Handbook for Planners 

8

2.1 Sources of Feed Water 

Because desalting converts saline waters into a resource, the technology has broadscale 

application where water has high value. Source waters include ocean waters and numerous 

inland sources of saline water, such as geothermal, brackish ground and surface water, and 

wastewater. 

2.1.1 Industrial Wastewater 

Industrial use of reclaimed wastewater for cooling water and processes has increased in recent 

years. Applications currently include power plant and refinery cooling water and process water 

in such industries as textiles, electronics, and pulp and paper. A 15,142 cubic meters per day 

(m3

/d) (4 million gallons per day [mgd]) plant in Harlingen, Texas, provides high-quality water 

“across the fence” to a Fruit of the Loom facility. Conventional water sources were unable to 

supply the quantity required, but using wastewater reclaimed with reverse osmosis (RO) 

provided the necessary quantity, at a much higher level of quality. The major outcome of this 

project was the creation of 2,000 manufacturing jobs in largely agricultural southeast Texas. 

2.1.2 Municipal Wastewater 

Municipal reuse is a potential application of water renovation programs. Multiple barriers are 

required for both direct and indirect reuse of reclaimed municipal wastewater. Desalting is a 

necessary step in treating wastewater for reuse when salinity exceeds certain levels. Treated 

water may be suitable for ground water recharge or for industrial and agricultural use. 

The use of reclaimed municipal wastewater for nonpotable use has increased dramatically over 

the past 25 years. The first and, possibly, best known example of the use of desalting technology 

to meet ground water recharge rules is Water Factory 21 in Fountain Valley, California. A 

system involving a 18,927 m3

/d (5 mgd) reverse osmosis plant has been in operation since 1977, 

treating secondary effluent to a high level of quality prior to injecting it into the local ground 

water system, both as a salinity barrier and for recharge. In this case, the membrane treatment is 

used to reduce total dissolved solids (TDS) and to provide a barrier against dissolved organic 

matter and pathogens. Currently, a 283,906 m3

/d (75 mgd) system is under design, which will 

combine both membrane filtration and RO to treat secondary effluent for discharge into the Santa 

Anna recharge areas. 
Chapter 2: Desalting Applications

9

2.1.3 Brackish Ground Water 

In water-short areas, even brackish water may be used as a public water supply source, either by 

itself or mingled with other, better water to satisfy user needs. The use of brackish water also 

often presents problems involving water rights. 

Because each source of brackish water is unique, thorough investigation is necessary to 

determine the optimum process design for that site. Most brackish water can be reclaimed by 

membrane-type plants because the salinity is low in comparison to seawater. 

Unlike seawater, the quantity of brackish ground water is often limited. Thus, detailed 

investigation of the aquifer by test wells and modeling should always be conducted—especially 

when the source to be used has not previously been exploited. Key factors in the 

hydrogeological work are the safe yield, long-term storage, and proclivity for significant changes 

in the water chemistry. The latter should not be limited to increases in TDS, but also possible 

changes in ionic distribution with time. 

Maps of the 20 hydrologic regions of the U.S. are available from the U.S. Geological Survey 

(USGS); the USGS has performed ground water studies in almost all of the U.S. In addition, 

some States have made independent ground water surveys, and many States now require ground 

water pumping permits. Such permit applications often require the applicant to perform the 

necessary hydrogeological studies as part of the permit application. 

2.1.4 Brackish Surface Water 

In recent years, there has been an increasing interest in using brackish surface waters as a source 

of potable water, particularly in areas where traditional sources of fresh water are limited in 

capacity or are being stressed by competing uses, such as irrigation and industrial needs. The 

source of these waters may be either naturally brackish (such as the Brazos River and Lake 

Granbury in Texas) or from an estuary (where river waters meet tidal waters). The RO plant in 

Robinson, Texas, uses water from the Brazos River as its feed water, while Lake Granbury 

provides water to an electrodialysis reversal (EDR) plant operated by the Brazos River 

Authority. 

The advantage of surface water supplies over brackish ground water is that, generally, surface 

water supplies are not limited, except in severe cases of drought, when fresh water flows into the 

system may be depleted. In such instances, the salinity of the feed water may increase beyond 

the ability of the desalting plant to produce potable quality water. 

Pretreatment also becomes more complicated with surface water as feed water because 

suspended solids, biological activity, and pathogens must be addressed prior to the desalting 

operation. In addition, the Surface Water Treatment Rule of the Safe Drinking Water Act must 
Desalting Handbook for Planners 

10

be considered in planning these types of facilities. The use of membrane filtration as 

pretreatment addresses most, if not all, of these issues. 

2.1.5 Seawater 

Coastal locations present a virtually unlimited source of water of reasonably uniform quality and 

composition, as well as a convenient and economical sink for disposing concentrate. Also, dualpurpose plants for power and water production are best located near an unlimited supply of 

cooling water. Using seawater as a source is usually indicated when the point-of-use is located 

in or near coastal areas. Desalted seawater can also be used to supply areas a considerable 

distance from the coast, as in Saudi Arabia. The distance that desalted water can be conveyed is, 

of course, limited by the cost of constructing and operating the conveyance system. In some 

areas, it may be possible to supply additional water to areas far removed from the coast by 

exchange of desalted water for river water. For example, studies by the Bureau of Reclamation 

for augmentation of the Colorado River in the Yuma, Arizona, area showed that supplying 

desalted seawater or brackish water either to the metropolitan Los Angeles area, or to the Lower 

Colorado River area near Yuma, could, by exchange, make water available in the Upper 

Colorado Basin, more than 1,000 miles from the desalting areas. 

2.2 Desalting for Water Supply 

Approximately 8,700 land-based desalting plants had been installed or contracted for installation 

throughout the world by the end of 1999. These plants have a capacity above 100 m3

/d or 

26,400 gallons per day (gpd). Worldwide installed capacity in 1999 was 25.74 million m3

/d 

(6.8 billion gpd). Distillation accounts for about 50 percent of worldwide desalting capacity, 

with multi-stage flash (MSF) distillation providing about 84 percent of that quantity. Membrane 

technology accounts for the remaining 50 percent of the worldwide desalination capacity, with 

RO contributing 83 percent of that quantity. 

The following graphs and tables are statistical summaries of data contained in Desalting Plants 

Inventory Report No. 16, published by Wangnick Consulting Gmbh (Wangnick, May 2000). 

Table 2-1 lists the number of land-based desalting plants by location; table 2-2 lists the number 

of plants by process. Figure 2-1 shows that desalination capacity has significantly increased 

since the early 1970s. In addition, it shows that the use of thermal and membrane technology is 

about equal. Figure 2-2 illustrates the global water usage of desalination technology by user type 

over the last 50 years. Figure 2-2 indicates that since the early 1970s, industry and 

municipalities use more desalinated water than the military and power producers. 

2.2.1 Supplemental Public Supplies in the U.S. 

Approximately 150 U.S. municipal water systems use desalting for a part or all of their 

supply. Approximately 94 percent of this capacity is generated using membrane technology. 
Chapter 2: Desalting Applications

11

Together, these processing facilities produce over 2.98 million m3

/d (787 mgd) of water for 

municipal use. 

Table 2-1.—Desalting plant capacity by location, 

contracted through the end of 2001 

Region 

Number 

of plants 

Installed capacity, m3

/d 

(mgd) 

United States 1,863 5.129 x 106 (1,355) 

Central America 167 0.399 x 106 (106) 

Canada/Bermuda 69 0.082 x 106 (22) 

Caribbean 240 0.741 x 106 (196) 

South America 142 0.261 x 106 (69) 

Europe 1,572 4.182 x 106 (1,104) 

Africa 786 1.662 x 106 (439) 

Middle East 2,470 15.855 x 106 (4,189) 

Asia 1,994 3.625 x 106 (958) 

Australia 134 0.239 x 106 (63) 

Russia 32 0.125 x 106 (33) 

Unknown 41 0.071 x 106 (19) 

Total - all locations 9,510 32.373 x 106

* (8,553) 

 (Individual region totals do not add to 25.89 x 106 because of 

rounding.) 

Table 2-2.—Desalting plant capacity by process, 

contracted through the end of 2001 

Process 

Number 

of plants 

Plant capacity, m3

/d 

(mgd) 

Multi-stage flash distillation 667 12.727 x 106 (3,363) 

Vapor compression 652 1.549 x 106 (409) 

Multi-effect distillation 428 1.028 x 106 (272) 

Reverse osmosis 6,463 14.100 x 106 (3,725) 

Electrodialysis/electrodialysis reversal 997 1.533 x 106 (405) 

Nanofiltration 44 1.230 x 106 (325) 

Other 259 0.206 x 106 (54) 

Total - all types of processes 9,510 32.373 x 106 (8,553) 
Desalting Handbook for Planners 

12

Figure 2-1 Global desalination capacity for membrane and thermal plants, 1950-2001 

 

Figure 2-2 Global desalination capacity usage, 1950-2001 

Desalination Capacity Usage Distribution

Municipal

Industrial

Power

Military

Tourism

Other

Global Desalination Capacity

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

19501953

1956

1959

1962

1965

196819711974

1977

1980

1983

1986

19891992

1995

1998

2001

Year

Cumulative Capacity (Mgd)

Membrane

Thermal
Chapter 2: Desalting Applications

13

Municipal benefits, such as dependability and a consistent, high-quality supply, are making 

desalination attractive to a variety of municipalities. The choice of the best process for each case 

will depend principally on local water conditions, water quality, energy costs, and concentrate 

disposal options. 

Desalination processes have grown significantly in the past few years. Further improvements, 

particularly in continued reduction of energy requirements and in-process performance, are being 

pursued by manufacturers and through government-sponsored research, development, and 

demonstration. 

2.2.2 Alternative to Reservoir Development 

A desalting plant may be an attractive alternative to a "last added" reservoir in a system when 

increased firm yield is desired. The last added reservoir may be a marginal investment in terms 

of dollars per unit yield of water. In contrast to the reservoir, desalting has a lower capital 

investment, but it trades capital cost for higher operating cost. Additionally, the environmental 

impact from a desalting facility is much less than adding additional reservoir capacity. 

Alternatively, conjunctive use of reservoirs and desalting facilities can provide the water 

resource planner a useful economic model. A desalting plant can be matched to the water 

demand curve, in modular fashion, much more closely than a dam and reservoir because a dam is 

most economically constructed by relation to the site and stream hydrology, rather than to the 

demand curve. A desalting plant may be more financially desirable. Because the desalting plant 

is less capital intensive, it may be easier for the water supply agency to obtain financing. In 

times of higher interest rates, the desalting plant may be a more feasible investment. 

2.2.3 Alternative to Long-Distance Water Transfer 

Desalination plants are usually close to the point of water use. Therefore, desalted water supply 

probably will have a much shorter lead time than long-distance water transfer. Additionally, a 

desalting facility will often have less environmental impacts than a long-distance pipeline. 

2.2.4 Industrial Applications 

Specifications for industrial water purity vary widely and depend entirely on the intended use. 

Highly saline water is often acceptable for cooling and condensing industrial exchangers. On the 

other hand, requirements for pure process water are so stringent in many industries that plants 

often employ expensive and complex treatment for even high-quality natural water. 

No formal quality standards exist for an entire industry, or even for specific products, although 

estimates of quality tolerances by industries are available in various reports. One such set of 

limits is given in table 2-3. 
Desalting Handbook for Planners 

14

Table 2-3.—Typical quality limits for selected industries 

(mg/l) 

Industry Turbidity Color Hardness Alkalinity pH TDS Fe and Mn 

Baking 10 10 0.5 

Boiler feed 

 0-150 psi 20 80 75 8.0+ 1,000-3,000 

 150-250 psi 10 40 40 8.5+ 500-2,500 

 250-400 psi 3 5 8 9.0+ 100-1,500 

 >400 psi 1 2 2 9.5+ 50 

Brewing 

 Light 10 75 6.5-7 300 0.1 

 Dark 10 150 7.0+ 1,000 0.1 

Canning 

 General 10 0.2 

 Legumes 10 25-75 0.2 

Carbonated 

beverages 

2 10 250 50 850 0.3 

Confectionary 50 100 0.2 

Cooling 50 50 0.5 

Icemaking 1-5 5 30-50 300 0.2 

Laundering 50 0.2 

Clear plastics 2 2 200 0.02 

Pulp and paper 

 Ground wood 50 20 180 1.0 

 Kraft 25 15 100 300 0.2 

 Soda and sulfite 15 10 100 200 0.1 

 Light paper 5 5 50 200 0.1 

Rayon (viscose) 

Production 5 5 8 50 100 0.05 

 Manufacturing 0.3 55 7.8-8.3 0.0 

Tanning 20 10-100 50-135 133 8.0 0.0 

Textiles 

 General 5 20 20 0.25 

 Dyeing 5 5-20 20 200 0.25 

 Wool scouring 70 20 1.0 

 Cotton bandage 5 5 20 0.2 

 Note: psi = pounds per square inch; mg/l = milligrams per liter 
Chapter 2: Desalting Applications

15

Much of the water used in the textile, leather, paper, chemical, and food industries has to have 

very low hardness and low salinity. Water used in the manufacture of textiles and high-grade 

paper also requires extremely low iron content. With textiles and carbonated beverages, water 

must be sparkling, free of organic and inorganic suspended matter, clear, and relatively free of 

dissolved minerals, especially calcium and magnesium. Process water used in the electronics 

and pharmaceutical industries must be free of ions and contaminants. 

The most common industrial water quality problems are caused by suspended solids and 

hardness. Both problems respond to relatively inexpensive treatment by conventional methods. 

However, when other constituents are included, distillation or selective ion exchange processing 

of part or all of the water supply may be a desirable and economical solution. An example of a 

very high water quality need is in supercritical power plants. One such plant, with temperatures 

of 659 o

C (1,200 °F) and pressures of 345 bar (5,000 psi) has the following feed water 

maximums: 

2.3 Case Histories 

To demonstrate both the necessity of alternative water sources and the usefulness of desalting 

technology in meeting that need, four site-specific studies have been provided. Each study 

includes a description of water needs and constraints in each geographic area. In addition, each 

study illustrates the implementation of desalting technology and includes a process flow diagram 

and description for the process facility. Finally, each study contains an economic summary of 

the cost of these technologies for each case. 

2.3.1 Brackish Water Desalting using Reverse Osmosis 

Location.—Dare County, North Carolina 

Background.—The Dare County Water Department, the community, and its governing boards 

made the decision to use desalination in the mid-1980s, prompted by the need for consistently 

available, potable-quality water to strengthen the economic growth and financial well-being of 

the county. 

Total dissolved solids 0.50 mg/l 

Silica 0.02 mg/l 

Iron 0.01 mg/l 

Copper 0.01 mg/l 

Dissolved oxygen .005 mg/l 

pH 9.5 to 9.6 
Desalting Handbook for Planners 

16

The Outer Banks of Dare County, North Carolina, have almost no fresh water resources. 

Limited treated water is available from a 18,925 m3

/d (5 mgd) ion exchange treatment plant on 

Roanoke Island, supplemented in the summer by a small fresh water pond providing an 

additional 3,785 m3

/d (1 mgd). This total capacity of 22,710 m3/d (6 mgd) was insufficient to 

spur economic growth in the area, particularly that afforded by summer tourism and recreation. 

In addition, further withdrawals from the fresh water aquifer on Roanoke Island were not 

possible. 

When desalination of brackish water sources on the islands was proposed in the 1980s, it was 

considered quite an expensive alternative. The value of good quality water in terms of retail and 

commercial success, tax revenues to support services that benefit the residents, and the overall 

quality of life all determined that the benefits provided by desalination outweighed the cost, even 

in 1980 dollars. The first plant was in operation by 1989. As the cost of desalination has 

declined in the 1990s, additional desalination plants were brought online in 1996 and 2000. 

The most significant benefit of this project has been the economic growth afforded by the 

availability of potable-quality water. Growth in total retail sales is one of the most useful 

indicators of development and economic activity. In Dare County, total retail sales increased 

from $216 to $800 million from 1984 to 1998. Over the last 10 years, retail sales have increased 

an average of more than 8 percent per year. Over this same period, the growth in retail sales in 

Dare County has exceeded the growth rate of the State of North Carolina by more than 7 percent 

and exceeded the second ranked county in total retail sales by 30 percent. 

The assessed value of property within Dare County has grown 148 percent over the last 10 years, 

while the per capita income of permanent residents has grown almost 35 percent over this same 

period. While it is difficult to totally credit this economic boon to the advent of desalination in 

Dare County, it is safe to say that this level of growth would not have been possible without the 

consistent availability of high-quality water. The technology did not inhibit financial success of 

the area but, rather, acted as a catalyst for growth and prosperity. 

Project Overview.—The Dare County desalination plants are located on the Outer Banks of 

North Carolina, within the community of Kill Devil Hills and on Hatteras Island. Three 

desalination plants are in operation, with a small fourth plant under construction in the mainland 

community of Stumpy Point. Currently, the Dare County water system has a 20,439 m3

/d 

(5.4 mgd) desalination capacity, serving 25,000 permanent residents and a summer population of 

over 100,000 visitors. 

The Dare County desalination plants all use RO technology. The first plant, the North RO plant, 

in operation since 1989, is a 11,355 m3

/d (3.0 mgd) plant, located in Kill Devil Hills. An 

additional 3,785 m3

/d (1.0 mgd) plant began operation in 1996 on northern Hatteras Island, and 

another 5,299 m3

/d (1.4 mgd) began operation in 2000, serving south Hatteras Island. Figure 2-3 

describes the technology used at the Kill Devil Hills facility. The Kill Devil Hills facility treats 

brackish water from local wells using RO technology that has a recovery of 75 percent. 
Chapter 2: Desalting Applications

17

Figure 2-3 Process flow diagram for the Dare County, North Carolina, North RO plant 

Costs and Conclusions.—Dare County Water Department uses a combination of desalinated 

water from its three desalination plants, in addition to the older, conventional ion exchange 

treatment plant on Roanoke Island, to meet its water demand. Overall production is distributed 

between the plants to minimize costs, with 54 percent of the water supply originating from the 

desalination plants over the past 9 years. The plants on Hatteras Island serve separate 

communities and are not connected to each other or to the Roanoke Island and North systems. 

Dare County Water Department has conducted a thorough cost comparison of its desalinated 

supply and its conventional supply, determining that the cost to produce (without depreciation) 

desalinated water is less than that of the conventional supply. Since 1995, the desalinated water 

has cost less than $0.40 per cubic meter (m3

) ($1.50 per thousand gallons [kgal]), with the cost 

decreasing each year. Electrical costs average $0.06 per kilowatthour (kWh) (204.76 British 

thermal units [Btu]) in this area. Dare County Water Department has negotiated a load 

management plan with the power company, so that power cost at the Kill Devil Hills plant 

consistently averages below $0.045/kWh (153.57 Btu).

As desalination water costs have declined and the community has prospered as a result of the 

availability of high-quality water, it is clear, in retrospect, that desalination has been the right 

approach. 

2.3.2 Seawater Desalting using Reverse Osmosis 

Location.—Tampa, Florida 

Background.—Tampa Bay Water is Florida’s largest water wholesaler, providing water to its 

member governments of Hillsborough, Pasco, and Pinellas Counties, and the cities of New Port 

Brackish 

feed from 

wells

Acidification & 

Scale Inhibition

Cartridge 

Filtration

RO Variable 

Speed Pump

1st Stage RO Storage

Effluent to 

Atlantic 

Ocean

Posttreatment

2nd Stage RO 
Desalting Handbook for Planners 

18

Richey, St. Petersburg, and Tampa. Those member utilities, in turn, serve 2 million people in the 

Tampa Bay area. Tampa Bay Water, in partnership with Poseidon Resources, Tampa Electric, 

and the Southwest Florida Water Management District (SWFWMD), is presently developing the 

nation’s largest seawater desalination plant, initially producing 94,625 m3

/d (25 mgd) of potable 

water. 

The tri-county area, like many coastal cities, has experienced significant growth in recent years. 

Overpumping at the well fields has created environmental stresses. SWFWMD mandated that 

Tampa Bay Water reduce ground water pumping from a current level of 598,030 m3

/d (158 mgd) 

to 457,985 m3

/d (121 mgd) by 2003. Additionally, by the year 2008, the pumping must be 

further reduced to 340,650 m3

/d (90 mgd). To address this significant reduction in ground water 

pumping, a Master Water Plan was developed for the community, which will use seawater 

desalination to provide 10 percent of the total water supply by the year 2007. 

Though desalination, especially seawater desalination, has historically been too expensive for 

most U.S. communities, the economics are changing. The desalination technologies used today 

are cost effective and dependable, with improved energy efficiencies, manufacturing, and 

material improvements contributing to the improved economics. At the Tampa Bay plant, 

additional cost savings were realized though a public/private partnership and collocation with an 

electric utility. Tampa Bay Water has chosen desalination as a drought-proof, cost-effective 

means to diversify its water supply. 

The most significant benefits of this project will be: 

• The protection of the environment by reduction in well field pumping from the ground 

water aquifers 

• Continued economic growth and prosperity of the community from the ongoing 

availability of high-quality water for human consumption, tourism, and industry 

Additionally, the project will generate about $162 million in direct economic benefits to 

Hillsborough County in wages and personal income, with total project revenue of about 

$600 million to the community. 

Project Overview.—The Tampa Bay Water plant will initially produce 94,625 m3/d (25 mgd) of 

drinking water, expandable to 132,475 m3

/d (35 mgd). Seawater will be taken from the adjacent 

Big Bend Power Plant, which uses seawater as a process coolant. The recovery for this 

desalination process is expected to be approximately 57 percent, and the concentrate stream will 

be blended with cooling water before it is discharged into the bay. The concentrate stream will 

have more than twice the salinity of the feed stream. Approximately 71,915 m3

/d (19 mgd) of 

concentrate will be mixed with 5.3 million m3

/d (1,400 mgd) of power plant cooling water in 

order to reduce the salinity of the concentrate stream and ensure that the marine ecosystem will 

not be affected by the discharge. 
Chapter 2: Desalting Applications

19

Environmental Considerations.—Tampa Bay Water and its member governments, along with 

SWFWMD, were very sensitive to the needs of the aquatic life and marine environment in 

Tampa Bay and conducted several studies on the bay. One of the reasons for selecting the Big 

Bend Power Plant site is the volume of data available to predict any changes that might occur 

once the plant is operational. In addition, this site provides the ability to mix the discharge from 

the power plant and desalination plant concentrate flow before returning it to the bay. Though 

all predictions indicate there will be no effect on the Tampa Bay marine environment, a lifetime 

monitoring program will ensure that if there are any impacts, they will be identified and 

operations will be changed accordingly. 

Costs and Conclusions.—The desalination plant will cost approximately $100 million to 

construct. The plant is a public/private partnership with the private partner, Tampa Bay Desal, 

LLC. Tampa Bay Desal, 100 percent owned by Poseidon Resources, will incur much of the 

financial risk for the project. Additionally, the SWFWMD is co-funding the project to offset the 

cost to Tampa Bay Water. As a result, Tampa Bay Water will pay only a 30-year net average of 

$0.55/m3

 ($2.08/kgal) of desalinated water, less the $0.11 - 0.16/m3 ($0.40 - 0.60/kgal) using the 

SWFWMD funds towards the capital cost of the plant. Final cost to Tampa Bay Water 

customers is about $0.42/m3

 ($1.60/kgal). 

The cost for seawater desalination for the Tampa Bay Water project is low, when compared to 

historical seawater desalination costs in other parts of the world. The low cost can be attributed 

to the lower salinity of the Gulf seawater compared to seawater in many other areas, shared 

intake/outfall with the Big Bend Power Plant, reasonable power cost of $0.04/kWh (136.51 Btu), 

and improved economics through a long-term public/private partnership and financial support of 

SWFWMD. 

2.3.3 Wastewater Reclamation using Reverse Osmosis 

Location.—Harlingen, Texas 

Background.—Harlingen, Texas, is a predominantly agricultural community of approximately 

55,000 inhabitants, located near the southern tip of Texas, about 10 miles from the international 

border with Mexico. High unemployment rates in the 1980s and early 1990s prompted the 

community to create the Harlingen Development Corporation to organize and support industrial 

development in the area. 

The first obstacle for the Harlingen Development Corporation in attracting new industry to the 

Harlingen area was the lack of available new water. In 1988, the Harlingen Water Works 

System commissioned a study to determine the most cost-effective method to provide water and 

wastewater service to a potential industrial customer. The results of that study indicated only 

three options were available: 

• Buy existing potable water supplies, followed by further treatment to meet water quality 

criteria. Finding willing sellers was difficult, and water was expensive. 
Desalting Handbook for Planners 

20

• Use irrigation canal returns and treat extensively, although quality and quantity were 

unreliable. 

• Use domestic wastewater and treat extensively (water reuse). 

Cost estimates at the time the study were conducted, which indicated production costs for each of 

the three options: 

 $/m3

 $/kgal 

Potable source (Option 1) 0.44 $1.65 

Irrigation canal (Option 2) 0.30 $1.15 

Water reuse (Option 3) 0. 023 $0.88 

As a result of the preferred economics of reuse, the reliability of the flow, and the conservation 

aspects of this source of water, water reuse was chosen as the preferred option. Reuse of the 

industrial wastewater return from the Fruit of the Loom plant was not included in the initial 

phase of the project but is being considered for future use. 

The Harlingen Water Works System, a city-owned and board-operated corporation that provides 

water and waste treatment services to the city, determined that treating domestic wastewater was 

the most cost-effective method to provide additional water supplies to potential industrial 

customers, based on the options available. The industrial partner, Fruit of the Loom Corporation, 

agreed to locate a major facility in Harlingen if at least 6,056 m3

/d (1.6 mgd) of water were 

available for its use. The Harlingen Reclamation Plant began its successful operation in 1990, 

providing water to the adjacent Fruit of the Loom plant. Supplemental plant expansions since 

that time have brought the total capacity of this facility to 15,140 m3

/d (4 mgd), of which 

11,355 m3

/d (3 mgd) is currently used.

The most significant benefits of this project have been: 

• Creating approximately 3,000 jobs in a community of high unemployment 

• Demonstrating on a national level the value, benefit, and successful economics of water 

reuse, even for small communities 

Project Overview.—The Harlingen Reclamation Plant has been constructed in phases to meet the 

growing needs of the community. The initial phase, which began operation in 1990, produced 

8,327 m3

/d (2.2 mgd), using only municipal wastewater as feed water. A follow-on expansion, 

completed in 1999, added capacity up to about 15,140 m3

/d (4 mgd), of which 11,355 m3/d 

(3 mgd) is currently used. The production from this facility is limited by the available 

wastewater feed, which is currently all municipal wastewater. Future plans to use industrial 

wastewater from the adjacent Fruit of the Loom plant could bring the plant into its full 

15,140 m3

/d (4 mgd) capacity. 
Chapter 2: Desalting Applications

21

The plant uses extensive conventional pretreatment prior to its RO system. Figure 2-4 shows the 

flow diagram of the technologies used at the Harlingen facility. 

Figure 2-4 Process flow diagram for the Harlingen Reclamation Plant. 

The facility discharges the waste stream through the Arroyo Colorado, which is a tidally 

influenced creek that eventually discharges into the estuary of the Rio Grande River. The RO 

discharge is mixed with the treated wastewater discharge from Fruit of the Loom, which is not 

being reused, to dilute the salt content of the concentrate prior to discharge. 

The Harlingen plant has operated successfully for more than 10 years, with an on-line factor of 

greater than 90 percent. Continued economic growth and water deficiencies in the area may 

dictate further expansion and continued technological improvements in this proven leader. 

Costs and Conclusions.— Federal and State grants were obtained to assist in constructing the 

initial project. State loans provided additional financial resources for the project. Water costs 

for this reused source average $0.23/m3

 ($0.87/kgal), with the potential for future reductions as 

the plant implements new, more cost-effective technologies. 

The Harlingen experience has been an excellent example of how a community can improve its 

economic standing through creative solutions to water resources development. Bringing a strong 

industrial customer to the community, with potential for future growth, has played a significant 

role in the economic well-being of a community susceptible to drought impacts from its strong 

agricultural base. Water reuse has played an integral role in meeting this community’s needs, 

and their circumstances are not unique. More communities are considering and utilizing reuse, 

as water demands grow and supplies remain constant or are declining. 

Two - stage filtration Reverse Osmosis

Storage 

Bar 

Screens 

Primary 

Clarifiers 

Trickling 

Filters 

Bio-Towers

Solids Contact 

Chamber

Flocculating 

Clarifiers 

Chlorine 

Contact 

Industrial 

Waste 
Desalting Handbook for Planners 

22

2.3.4 Seawater Desalting using Multi-Stage Flash 

Location.—Ghubrah Complex, Muscat, Oman 

Background.—Desalting in the Sultanate of Oman came later than in most of the Arabian Gulf 

States, with the first municipal seawater desalting plant becoming operational in 1976. Almost 

all the thermal desalination facilities producing water for municipal use in the Middle East region 

are developed in phases. That is, a desalting complex is developed in phases over time, as 

capacity is needed and as the financing is available. The Ghubrah Complex near Muscat, Oman, 

consists of six MSF “plants” or phases, which have been put into service starting in 1976 through 

2000. Four of these plants were supplied by the same Japanese manufacturer, while the other 

two were supplied by German and United Kingdom (U.K.) suppliers. Like most thermal 

desalination plants, the Ghubrah Complex is a cogeneration facility, producing both electrical 

power and desalinated water. The facility currently can produce approximately 189,250 m3

/d 

(50 mgd) of desalinated water and 630 megawatts of electricity. 

The country of Oman has significantly more renewable water resources than its neighbors, with 

about 761 m3

 per capita per year (201,000 gallons per capita per year), based on year 2000 

populations, while the United Arab Emirates has less than 204 m3

 per capita per year 

(54,000 gallons) per capita per year, and Kuwait has less than 83 m3

 (22,000 gallons) per capita 

per year. These renewable water resources are accessed through reservoirs located throughout 

the country, and they greatly offset the dependency on seawater desalination in Oman. 

Additionally, Oman is generally conservative with its water resources and relies heavily on water 

recycling for nonpotable needs, particularly in the more urban greater area of Muscat. During 

the mid-1970s, however, as the greater Muscat area was modernizing, the demand for water and 

electricity increased dramatically and outpaced their renewable water resources. The decision to 

construct the Ghubrah desalination/power facility resulted. 

Desalinated water in Oman is the responsibility of the Ministry of Electricity and Water (MEW), 

and the MEW is responsible for the Ghubrah Complex. The complex is operated by SOGEX, a 

private company. The original MSF unit, Ghubrah 1, produced approximately 22,710 m3

/d 

(6 mgd) of desalinated water. Subsequent additions resulted in the following overall production 

capacity for this complex: 

Ghubrah 1 22,710 m3

/d (6.0 mgd) 

Ghubrah 2 27,252 m3/d (7.2 mgd) 

Ghubrah 3 54,505 m3

/d (14.4 mgd) 

Ghubrah 4 27,252 m3

/d (7.2 mgd) 

Ghubrah 5 27,252 m3

/d (7.2 mgd) 

Ghubrah 6 31,794 m3

/d (8.4 mgd) 

 Total 190,764 m3

/d (50.4 mgd) 
Chapter 2: Desalting Applications

23

Project Overview.—The intake for the Ghubrah desalination plants extends 1.0 kilometer 

(0.6 mile) into the Gulf of Oman. Seawater is drawn by vacuum through 203-centimeter 

(80-inch) pipes into a seawater pump pit, located adjacent to the beach on the plant site. 

Figure 2-5 is a photograph of the intake line. Centrifugal pumps bring the water into the plant 

from this location for distillation. 

Each of the seven distillation units (Ghubrah 3 consists of two units) at the Ghubrah Complex are 

MSF units. The initial unit installed at Ghubrah 1 is significantly different in design from the 

subsequent units. Unit 1 uses a long tube, parallel flow-type evaporator, while the subsequent 

units use cross-tube evaporators. Additionally, Unit 1 employs three blowdown pumps. On all 

other units, the blowdown is taken from the discharge line of the concentrate pump. All units 

utilize concentrate recirculation. Table 2-4 summarizes the design data for the seven Ghubrah 

units, and figure 2-6 is a photograph of one of the Ghubrah MSF units. 

The Ghubrah Complex is operated in four different modes, with steam from: 

• Turbine extracts 

• Back pressure turbine 

• Waste heat recovery boilers directly 

• Fired boilers directly 

Power demand greatly fluctuates between summer and winter, whereas the water demand is 

relatively constant year-round. As a result, in summer, the power and desalination plants are run 

at full load; while in winter, the power plants are run at low loads and the desalination plants are 

run at full load. The Ghubrah plants are designed to accommodate these variations in electric 

load without significant variation in water demand. At low electrical loads, exhaust heat can 

supply only part of the energy needed for desalination, and the balance is provided directly from 

the boilers. As a result, the energy cost allocated to desalting increases in the winter. 

The desalinated water is stabilized before distribution or storage by passing it through a 

limestone bed. The water can be stored in onsite reservoirs prior to distribution to the city of 

Muscat. The final product water fully meets the Omani drinking water standards. 

Costs and Conclusions.—The Ghubrah Complex has efficiently provided desalinated water and 

electricity to the Muscat area for over 25 years. It has made possible the transformation of the 

area from a walled city without paved highways to a modern industrialized city with all the 

conveniences of any contemporary municipality. 

Each of the phases of the Ghubrah Complex vary in their cost of operation, due to their 

differences in design and mode of operation relative to the power plant. Because the boilers 

must be direct fired in the winter to provide sufficient steam for the MSF process, the plants are 

significantly more costly to operate in the winter. During the summer, more waste heat is 

available, due to the heavier loads on the power plant, thereby decreasing the cost allocation of 

fuel to the distillation process. Table 2-5 presents the costs of fuel and other costs (including 

auxiliary power, manpower, consumables, spare parts, depreciation, and chemicals) for the 

summer and winter period, for an average of all phases of the Ghubrah Complex. 
Desalting Handbook for Planners 

24

Table 2-4.—Ghubrah 1-6 main design features 

Item Ghubrah 1 Ghubrah 2 Ghubrah 3 Ghubrah 4 Ghubrah 5 Ghubrah 6 

Type and year 

of installation 

Long tube 

1976 

Cross tube 

1982 

Cross tube 

1986 

Cross tube 

1992 

Cross tube 

1996 

Cross tube 

2000 

Capacity, mgd 6 

(22,720 m3

/d) 

7.2 

(27,252 

m3

/d) 

2 x 7.2 

(54,504 m3

/d) 

7.2 

(27,252 m3/d) 

7.2 

(27,252 m3/d) 

8.4 

(31,794 m3/d) 

Gained output 

ratio, pounds 

dist./pounds 

condensate 

6.8 

(3.08 kg) 

6.42 

(2.91 kg) 

6.42 

(2.91) 

6.52 

(2.96) 

Unknown Unknown 

Distillate quality, 

parts per million 

100 50 50 50 50 50 

Design inlet 

temperature, oC 

Max.35 30-35 30-35 30-35 30-35 30-35 

Seawater -

tempering 

system 

Not provided Duplex 

seawater 

recirculation 

pumps 

Seawater 

supply pump 

with tempering 

falling in the 

suction pit 

One seawater 

recirculation 

pump 

Unknown Unknown 

Make-up 

strainers 

Not provided Duplex strainers system 

Blowdown 

system 

Triplex pumps From discharge of concentrate pumps 

Vacuum system Steam ejectors + 

direct mixing 

barometric 

condensers 

Shell and tube vent condenser + steam ejectors + shell and tube 

inter- and after-condensers + condensate extraction pumps 

Control station Remote control 

room + plant- side 

condensers 

Remote control room + local 

control room + plant side 

control 

Remote control 

room + plant 

side control 

Unknown Unknown 

Number of main 

closed control 

loops 

7 12 12 13 Unknown Unknown 

Number of main 

open control 

loops 

2 13 12 12 Unknown Unknown 

Chemical dosing 

system 

Antiscale, 

antifoam 

sodium hydroxide 

(NaOH) 

Antiscale, antifoam, 

sodium hydroxide (NaOH) 

sodium sulfite 

Table 2-5.—Ghubrah desalting costs 

 Summer Winter 

Cost of fuel, $/m3 ($/kgal) 0.073 (0.275) 0.284 (1.075) 

Other costs, $/m3 ($/kgal) 0.563 (2.130) 0.599 (2.266) 

Total costs, $/m3

 ($/kgal) 0.636 (2.405) 0.883 (3.341) 
Chapter 2: Desalting Applications

25

Figure 2-5 Ghubrah Complex seawater intake lines 

Figure 2-6 MSF unit at the Ghubrah Complex 

 
Desalting Handbook for Planners 

26

References 

Wangnick, K., “2000 IDA Worldwide Desalting Plants Inventory, Report No. 16,” International 

Desalination Association, May 2000. 
Chapter 3: Water Chemistry

27

Chapter 3: Water Chemistry 

3.1 Basic Water Chemistry 

Knowing water chemistry is necessary to: 

• Interpret chemical analyses of saline waters 

• Understand the chemical reactions associated with the desalting processes 

• Select appropriate pretreatment and post-treatment techniques 

Some fundamental concepts of general chemistry must be discussed along with the specific 

aspects of water chemistry. The general chemistry, water chemistry, and electrochemistry 

background in this section provides the basis for the further discussions on water types and 

treatments in section 3.2 and for the various desalting processes and pretreatment methods 

discussed later in this Handbook. 

3.1.1 Water Cycles and Constituents 

The Earth's water supply is constantly being depleted and replenished in a hydrologic cycle. The 

depletion is caused by evaporation from surface waters and by transpiration from plant life. 

Replenishment occurs when the water vapor is condensed in the atmosphere and is returned to 

earth in the form of rain, snow, sleet, or dew. Approximately 4 trillion gallons per day of rain 

fall in the U.S. Of this amount, about 65 percent is returned directly to the atmosphere by 

evaporation and transpiration. The rest flows to lakes and oceans, via runoff, or permeates into 

underground aquifers. 

When water vapor condenses and falls to earth, it absorbs and dissolves certain atmospheric 

gases, principally oxygen and carbon dioxide, in contact with the water. The degree of gaseous 

concentrations depends on the temperature and chemistry of atmosphere, which differ in 

different parts of the Earth. 

All naturally occurring waters contain constituents in the form of dissolved or suspended 

inorganic or organic compounds. Note that references to water in this Handbook refer to this 

mixture, rather than chemically pure water made up of two atoms of hydrogen and one of oxygen 

(H2O). Typically, inorganic compounds are made up of various combinations of metallic 

(e.g., calcium, magnesium, sodium, iron) and nonmetallic (e.g., carbon, nitrogen, oxygen, sulfur, 

chlorine) elements, while the organic compounds are generally more complex structures always 

containing carbon. 
Desalting Handbook for Planners 

28

3.1.2 Basic Chemical Terms 

An atom is the smallest particle of an element possessing all the chemical characteristics of that 

element. 

An element is a simple substance that cannot be decomposed by chemical processes into simpler 

substances (e.g., chlorine, hydrogen, oxygen, sodium). 

A compound is a substance that can be decomposed by chemical processes into two or more 

elements or which can be built up from two or more elements (e.g., sodium chloride, NaCl). 

In a chemical reaction, atoms unite, separate, or exchange places in the ratio of their particular 

atomic weights or simple multiples thereof. 

A valence is a whole number that represents or denotes the combining power of one element 

with another. 

A molecule is the smallest particle of a compound possessing all the chemical characteristics of 

that compound. 

Chemical formulas describe molecules (e.g., the formula for water is H2O). 

Chemical equations describe how atoms combine to form molecules. For example, 

 Mg2+ + 2 Cl-

 = MgCl2 means that one atom of magnesium and two atoms of chlorine bond 

together to form one molecule of magnesium chloride. 

Items on the left side of a chemical equation are the reactants and the right side are the

products. 

When chemical reactions occur, not all of the reactants react to form products. The solution 

attains a chemical equilibrium. At equilibrium, there may be residual reactants or products. 

An ion is one atom, or a group of atoms, with either plus or minus charges. 

3.1.3 Chemical Formulas and Compounds 

Understanding how atoms bond together is crucial to understanding chemical processes. 

3.1.3.1 Valences 

When atoms share electrons, atoms are held together by covalent bonds to form molecules. 

To determine these bonds and combinations of atoms, chemists use valences. Valence is a 

number—either positive (+) or negative (-)—that indicates the capacity of an atom to combine 

with another element. The valence of an ion is always shown by a number in a superscript on the 
Chapter 3: Water Chemistry

29

right side of the element’s name or formula. A hydrogen atom, H+

, has a valence of positive 1. 

All elements that have the same combining capacity as hydrogen have a valence of 1, either 

positive (+) or negative (-). These include chlorine, C1-

; sodium, Na+; and potassium, K+. Other 

elements can have higher valences (up to 7), either positive (+) or negative (-). Calcium has a 

valence of +2, for example, while oxygen has a valence of –2. Items with valences greater than 

1 may be written with the positive (or negative) sign and the number for the valence (e.g., Mg2+

or Mg++). The number is usually omitted with monovalent ions—(H+

) rather than (H1+). 

The total valence of the ions in a compound is always zero. Therefore, when hydrogen and 

chloride combine, the compound is hydrogen chloride, HCl, which has a total valence of zero. 

This would be written as an equation: H+

 + Cl- = HCl. This equation states that one atom of 

hydrogen combining with one atom of chlorine yields one molecule of hydrogen chloride, HCl. 

This is not necessarily a one-to-one relationship—one atom can combine with two atoms of 

another element if the total valence is zero. For example, magnesium has a chemical valence of 

+2, so it would combine with 2 molecules of chlorine (each of which has a valence of –1) to 

form 1 molecule of magnesium chloride, MgCl2,.i.e., 

Mg2+ + 2Cl-

 = MgC12

The nucleus of an atom consists of protons and neutrons, which have virtually identical masses. 

Atoms have varying numbers of protons and neutrons. To have equal numbers of different 

atoms react to form products, the mass of the nuclei (the atomic mass) has to be in the proper 

ratio. For example, the nucleus of the sodium, Na+

, atom is 23, and the nucleus of the chlorine, 

Cl-

, atom is 35. For sodium and chlorine to react to produce sodium chloride, the ratio 23:35 

must be maintained. 

3.1.3.2 Ions 

Many atomic groups can form or produce electrically charged particles called “ions.” Ions 

influence the corrosion, scale, and type of desalting processes that will be effective. 

When atoms exchange electrons, ionic bonds form. For example, Na and Cl react to produce 

sodium ions and chlorine ions. Positively charged ions are called “cations,” and negatively 

charged ions are called “anions.” Positive and negative charges can be greater than one. The 

charges of the cations and anions in a chemical formula must sum to zero. 

When salts are dissolved in water, their molecules dissociate into ions. Salts, acids, and bases 

are called “electrolytes.” The concentrations of electrolytes and their degree of ionization will 

govern the conductivity of a specific water sample. 

In a solution, a typical equilibrium exists between molecules and ions. Therefore, ionic 

equations are written as equilibrium equations: 
Desalting Handbook for Planners 

30

NaC1 o Na+

 + C1-

where the (+) indicates a single positive charge on the sodium ion, and the (-) indicates a single 

negative charge on the chloride ion. 

For example, add calcium carbonate,CaCO3, to water. Some of it will dissolve, and some will 

remain as a solid. Calcium carbonate dissolves into two ionic forms, Ca2+ and CO3

2-: 

CaCO3 

o Ca2+ + CO3

2-

3.1.4 Salts as Compounds 

In general, a salt is a compound made up of a metallic element with a nonmetallic element. 

Metallic elements have a great ability to conduct heat and electricity (high conductivity), are 

ductile, and are shiny. Examples of metals are gold and iron—as well as magnesium, calcium, 

and sodium. Nonmetallic elements, on the other hand, are brittle, dull, and do not conduct heat 

or electricity well. Examples of nonmetals are chlorine, bromine, sulfur, carbon, oxygen, and 

nitrogen. 

When metals react with water, they produce cations, anions, and hydrogen gas. Examples 

include: 

Na + H2O = Na+ + OH- + 1⁄2 H2 ↑

Ca + 2H2O = Ca2+ + 2OH- + H2 ↑

Note the corresponding cation of the metal. 

When nonmetals react with water, they produce acids. For example: 

Cl2 + 2H2O → HCl + HClO 

Note that the hydrochloric acid forms and the anion (chloride) comes from the nonmetal Cl2. 

These anions and cations can combine to form salts. These salts can then precipitate out of 

water. When these salts form on surfaces, they are called “scale.” The list below shows some of 

the common salts: 

• Sodium chloride, NaCl 

• Sodium sulfate, Na2SO4

• Sodium bicarbonate, NaHCO3
Chapter 3: Water Chemistry

31

• Magnesium chloride, MgCl2

• Magnesium sulfate, MgSO 

• Calcium chloride, CaCl2

• Calcium sulfate, CaSO4

3.1.5 Constituents in Water 

Water has been called the universal solvent. To varying degrees, water dissolves gases 

(e.g., oxygen, O2, and carbon dioxide, CO2), inorganic compounds, and some organic 

compounds (e.g., sugar and tannic acid). 

Total dissolved solids (TDS) are the sum of all the constituents that are dissolved in the water, 

either inorganic or organic. TDS is usually expressed as milligrams per liter (mg/l). Water can 

hold finely divided insoluble materials in suspension. Any of these dissolved or suspended 

substances may be classified as constituents. Total suspended solids (TSS), expressed as mg/l, 

are the sum of all material in a water sample that is not dissolved. TSS is one of the 

characteristic properties of the water. These properties are often quite different from the 

characteristics of pure water (H2O). 

Some compounds (e.g., sodium chloride, NaCl) are very soluble in water. Others are soluble 

only to a very limited extent (e.g., calcium carbonate, CaCO3). When the solubility limit of a 

compound is exceeded, it precipitates from solution. Equations expressing the precipitation 

phenomenon are written as follows: 

Ca2+ + CO3

2- → CaCO3↓ 

FeC13 + 3NaOH → 3NaC1 + Fe(OH)3↓

(where the symbol, ↓, represents a precipitate, or solid) 

Table 3-1 is a list of elements that frequently occur in natural waters. The symbol for each 

element, its approximate atomic weight, and usual ionic charges are also shown. 

The major components of an average water analysis generally include only a limited number of 

these elements or of their combinations. The major components that usually constitute almost all 

of the TDS content of the water are listed in table 3-2. 
Desalting Handbook for Planners 

32

Table 3-1.—Symbols, atomic weights, and common 

ionic charges commonly occurring in natural waters 

Name Symbol 

Approximate 

atomic weight 

Common 

ionic 

charges 

Aluminum Al 27 3+ 

Arsenic As 75 3+, 5+ 

Barium Ba 137 2+ 

Boron B 11 3+ 

Bromine Br 80 1- 

Calcium Ca 40 2+ 

Chlorine Cl 35.5 1- 

Chromium Cr 52 3+, 6+ 

Copper Cu 64 2+ 

Fluorine F 19 1- 

Hydrogen H 1 1+ 

Iron Fe 56 2+, 3+ 

Lead Pb 207 2+, 4+ 

Magnesium Mg 24 2+ 

Oxygen O 16 2- 

Phosphorus P 31 5+ 

Potassium K 39 1+ 

Silicon Si 28 4+ 

Sodium Na 23 1+ 

Strontium Sr 88 2+ 

Sulfur S 32 6+, 4+, 2- 

Table 3-2.—Major components of natural water 

Metallic elements Nonmetallic elements or radicals 

Name Symbol Name Symbol 

Calcium Ca2+ Carbonate CO3

-

Magnesium Mg2+ Bicarbonate HCO3

-

Sodium Na+

 Sulfate SO4

-

Potassium K+

 Chloride Cl-
Chapter 3: Water Chemistry

33

3.1.6 Measurements for Water Samples 

Two of the most common measurements for water samples are pH and conductivity. 

3.1.6.1 Measuring pH 

The pH value of a water is a very important factor in pretreatment and desalting plant operations, 

as well as an important factor in pipeline corrosion. Chemists use the pH scale to express how 

acidic (like an acid) or basic (like a base) a substance is. This is a fundamental characteristic that 

drives water chemistry. Chemical reactions, such as corrosion or scale, may depend on whether 

the water is acidic or basic. 

An acid is a compound of a nonmetallic element with a hydrogen ion, H, that dissociates. 

Examples are sulfuric acid, H2SO4, hydrochloric acid, HCl; and carbonic acid, H2CO3. A base is 

a compound of the cation for the metallic element and the hydroxyl ion, OH-

, that dissociates. 

Examples are: sodium hydroxide, NaOH; calcium hydroxide, Ca(OH)2; and magnesium 

hydroxide, Mg(OH)2. Bases can also exist without a metallic component. For example, 

ammonium hydroxide, NH3 + water (H2O) →NH4OH.

Acids and bases react to form water and salts. For example: 

hydrochloric acid, HC1, + sodium hydroxide, NaOH, → sodium chloride, NaC1, + water (H2O) 

The pH concept is particularly well suited to determine and numerically define very small 

hydrogen ion concentrations. An acid solution has more hydrogen ions, H+

, than hydroxyl ions, 

OH-

. A neutral solution has an equal number of each. A basic solution has more hydroxyl ions 

than hydrogen ions. The concentration of hydrogen ions in water is usually expressed in pH, 

which is defined as: 

pH = -log (H+

) 

where: H+ = the hydrogen ion concentration in grams per liter of solution. 

The pH characteristics are: 

Less than 3 3-6 6-8 8-10 Greater than 10 

Highly acidic Mildly acidic Neutral Mildly basic Highly basic 
Desalting Handbook for Planners 

34

Since this is a reciprocal log function, a lower pH number indicates a higher H+

 concentration, or 

a more acidic solution. Even a basic solution with a pH as high as 12 has a very small 

concentration of hydrogen ions, that may be calculated from the following equation: 

-log (H+

) = 12 

log (H+

) = -12 

(H+

) = 1 x 10 -12 moles per liter 

A neutral solution (pH = 7) has a hydrogen ion concentration of 1 x 10-7 moles per liter, while an 

acidic solution (pH of 2) has a H+

 concentration of 1 x 10-2, or 0.01 moles per liter. 

A solution with a pH of 5 has a hydrogen ion concentration 10 times that of a solution with a pH 

of 6. A pH of 4 represents 10 times the hydrogen ion concentration of a pH of 5 and 100 times 

that of a pH of 6. A pH 2 solution has a hydrogen ion concentration 10,000 times that of a pH 6 

solution. Since the lower pH values are attainable only by increased acid concentrations, the 

economic aspect of producing low pH solutions may be visualized. Assuming no other 

variables, it would thus cost 10 times more to decrease from a pH of 6 to a pH of 4 than it would 

to decrease from a pH of 6 to a pH of 5. 

A water with a pH of 4.2 or less contains no alkalinity but is considered an acid water; it contains 

carbonic acid (H2CO3 = water (H2O) + carbon dioxide, CO2) and may contain free mineral 

acids. Figure 3-1 relates the mineral acidity concentration to pH. Within the pH range of 4.2 to 

8.2, the bicarbonate ion co-exists with dissolved CO2. Figure 3-2 illustrates the effect of 

alkalinity and CO2 on pH. The bicarbonate and normal carbonate ions contribute to the 

alkalinity within the 8.2 to 9.6 pH range, while above pH 9.6, the carbonate and hydroxyl ions 

exist. 

3.1.6.2 Measuring Conductivity 

Conductivity is an important factor in desalting. Conductivity provides an approximate 

measurement of how much salt is in the water. The higher the concentration of dissolved salts, 

the greater the solution’s ability to conduct electricity. Pure water is a very poor conductor, 

while seawater is a good conductor. 

The solution as a whole is electrically neutral because the total of positive charges must equal the 

total of negative charges. When two electrodes are immersed in a solution of an electrolyte and a 

direct-current voltage is impressed on the electrodes, the negatively charged anions will migrate 

to the anode, and the positively charged cations will migrate toward the cathode. 

Conductivity is not an absolute measurement, but a relative comparison. The conductivity of a 

water sample is measured against a “standard” sample—usually a potassium chloride, KCl, 

solution. These standards are either made in the laboratory or purchased commercially. Labs 

use standards with the same grade, type, and quality of chemicals. 
Chapter 3: Water Chemistry

35

1

2

6 8 10

1 1

2

2

3 3

3

4 4

4

5 5

5 20 30 40 5060 80 100 200 400 600 1000

Free Mineral Acid as mg/l CaCO3

pH Values

Figure 3-1 Mineral acidity as a function of pH 

Ratio: Methyl orange alkalinity as mg/l CaCo3

Free Carbon dioxide as mg/l CO2

.1 .2 .3.4.5 1 2 3 4 5 10 20 50 100

5.0

6.0

7.0

pH Values

8.0

8.4

 Figure 3-2 Effect of bicarbonate alkalinity and CO2 on pH 
Desalting Handbook for Planners 

36

3.2 Types of Water and Treatments 

Saline waters are characterized as fresh or brackish, or seawater, depending on the amount of 

TDS: 

This section explains the three main types of water and how these types are treated. 

Based on the Environmental Protection Agency Drinking Water Standards, potable water should 

not exceed 500 mg/l of TDS. However, up to 1,000 mg/l of TDS may be considered acceptable. 

3.2.1 Fresh Water 

In most cases, if a source of fresh water is available, its treatment to produce a water supply for 

public potable use is relatively inexpensive. In some regions, simple pH adjustment and 

disinfection may be the only treatment required. In some areas, however, the fresh water 

resource, either surface or ground water, may be of poor quality, requiring extensive treatment. 

In these cases, desalted brackish water may be a more reliable and inexpensive supply. 

3.2.2 Brackish Water 

Compared to desalinating seawater, treating brackish water is very site specific. Inland surface 

and ground waters present varied pictures of TDS and water composition. Individual ionic 

species can vary significantly, even within a single well field. This variation is particularly 

important in ground water, as surface waters are routinely refreshed. Careful and precise 

analyses are required, preferably while wells are being pumped, or while surface water is at its 

poorest quality condition. Note that the characteristics of ground waters can occasionally change 

in unexpected ways. 

The TDS of these waters cover a very wide range, as indicated in the example analyses in 

table 3-3. 

Type TDS 

Fresh Less than 1,000 mg/l 

Brackish 

 Mildly brackish 1,000 to 5,000 mg/l 

 Moderately brackish 5,000 to 15,000 mg/l 

 Heavily brackish 15,000 to 35,000 mg/l 

Seawater Approximately 35,000 mg/l and above 
Chapter 3: Water Chemistry

37

Table 3-3.—Analysis of various brackish waters 

 mg/l as A B C D E F G 

Calcium CaCO3 510 323 1,050 748 99 110 545 

Magnesium CaCO3 376 365 668 148 408 415 1,398 

Sodium CaCO3 1,944 1,123 249 519 582 965 4,861 

Potassium CaCO3 29 29 99 

Total cations CaCO3 2,830 1,811 1,967 1,415 1,118 1,519 6,903 

Bicarbonate CaCO3 355 132 221 274 128 128 223 

Sulfate CaCO3 938 1,310 1,425 998 173 328 173 

Chloride CaCO3 1,537 369 240 123 804 1,058 6,696 

Nitrate CaCO3 81 20 - TR 

Phosphate CaCO3 TR - TR 

Total anions CaCO3 2,830 1,811 1,967 1,415 1,105 1,514 7,092 

Total hardness1

 CaCO3 886 688 1,718 896 507 525 1,943 

Total alkalinity CaCO3 355 133 221 274 226 226 223 

Carbon dioxide CO2 6 6 18 

Total iron Fe 0.35 1.2 -- 0.02 <0.1 0.1 <0.1 

Dissolved iron Fe 

Silica SiO2 32 49 22 -- 22 18 22 

Turbidity NTU <1.0 <1.0 0.3 

Suspended matter <1.0 

Color CoPt 

units 

 <5.0 <5.0 <10.0 

pH 7.95 7.7 7.2 7.4 7.6 7.6 7.4 

Solvent extractables mg/l 

Total organic carbon mg/l <2.0 <2.0 <2.0 

Total dissolved 

solids 

mg/l 3,628 2,478 2,410 1,880 1,328 8,076 

Temperature o

C 28 28 20 

Note: A = Wellton-Mohawk, Arizona; B = Coalinga, California; C = Tularosa, New Mexico; D = Fort Morgan, 

Colorado; E = Cape Coral, Florida, Wellfield #1; F = Cape Coral, Florida, Wellfield #2; G = Cape Hatteras, North 

Carolina 

 1 Hardness is the sum of the Ca and Mg ions, expressed as CaCO3

In other examples, the Moffat diversion source of the Denver, Colorado, surface water supply 

has a TDS of less than 100 mg/l; the city of Roswell, New Mexico, has many well sources for 

municipal water considerably in excess of 1,000 mg/l TDS; at Dalpra farm near Longmont, 

Colorado, a former Office of Saline Water (OSW) site, the aquifer has a salt concentration of 

3,500 mg/l TDS. As an extreme, the salt concentration in the Great Salt Lake is several times that 

of seawater. In Dare County, North Carolina, three reverse osmosis (RO) plants cope with 

completely different feed water quality: about 4,000 mg/l TDS in Kill Devil Hills; about 

1,100 mg/l TDS at the Rodanthe-Waves-Salvo plant; and over 8,000 mg/l TDS at the new plant 
Desalting Handbook for Planners 

38

at the south end of Hatteras Island. In Southwest Florida, Cape Coral uses two well fields 

yielding reliable low TDS water, while 50 miles to the north, after 27 years of operation, the 

Rotonda West RO plant wells yield 7,000-8,000 mg/l TDS water. 

3.2.3 Seawater 

The concentration of dissolved salts in water from the open seas throughout the world is fairly 

uniform at approximately 3.5 percent or 35,000 parts per million (mg/l). In areas of high 

precipitation or runoff from the land (e.g., bays and inlets such as Tampa Bay), the TDS 

concentration of seawater is less, while in areas of high evaporation (e.g., Red Sea, Arabian 

Gulf), it is higher. In all cases, the relative proportion of the major ions compared to the TDS 

content of the seawater remains remarkably constant. 

3.3 Water Analyses 

A typical chemical analysis of the water from Dalpra Well 1, which has been used extensively 

for evaluation of the membrane desalting process, looks as follows: 

Constituent 

Concentration 

(mg/l) 

Calcium (Ca) 107 

Magnesium (Mg) 65 

Sodium (Na) 936 

Potassium (K) 11 

Carbonate (CO3) 0 

Bicarbonate (HCO3) 470 

Sulfate (SO4) 1,958 

Chloride (Cl) 135 

Total 3,682 

Specific conductivity at 25° C = 4,420 micromho per centimeter. 

PH 7.7 

TDS at 105 oF – mg/l 3,512 

Strontium – mg/l 3.35 

Iron – mg/l 0.23 

Manganese – mg/l 0.08 
Chapter 3: Water Chemistry

39

The TDS content can be obtained by summation of the concentration of the individual ions. 

Note that in calculating TDS, the concentration must be expressed as ions —not as calcium 

carbonate. In this example, the TDS content is 3,682 mg/l. TDS can also be obtained by 

evaporating a water sample until dry, at 105° C. In this example, the TDS content obtained by 

this method is 3,489 mg/l. The evaporative TDS is lower because of partial thermal 

decomposition of the bicarbonate. This difference is sometimes estimated by assuming that half 

the bicarbonate is lost during this process. 

Table 3-4 shows analyses of three brackish waters, illustrating another method of reporting water 

analyses. 

Table 3-4.—Typical brackish water analysis 

No. 1 No. 2 No. 3 

mg/l meq/l1

 mg/l meq/l mg/l meq/l 

Calcium 282 14.1 220 11.0 334 17.2 

Magnesium 88 7.2 134 11.0 106 8.7 

Sodium 904 39.3 82 3.6 151 6.6 

Potassium 6 0.2 13 0.3 23 0.6 

Strontium - - 3 0.1 - - 

Bicarbonate 76 1.2 151 205 207 3.4 

Sulfate 771 16.1 1,056 22.0 1,319 27.5 

Chloride 1,460 41.1 42 1.2 66 1.9 

Sum of ions, mg/l 3,587 1,701 2,216 

Cation, meq/l 60.8 26.0 33.1 

Anion, meq/l 58.4 25.7 32.8 

1 milliequivalents per liter 

For practical purposes, when considering fresh water or moderately mineralized water, the terms 

mg/l (milligrams per liter) and ppm (parts per million) may be used interchangeably. Remember 

that mg/l is a measurement of mass per unit volume, while ppm is a measurement of mass per 

unit mass. However, when a water source contains sufficient minerals to increase its specific 

gravity, as is the case with seawater, the conversion from mg/l to ppm must include a factor for 

the increased water density. 

The analytical results may be found expressed as the weight of the chemical species per unit 

weight of solution (e.g., 50 mg Ca2+ per liter). Concentration may also be expressed as milliequivalents per liter (meq/l). This is accomplished by dividing the concentration (mg/l as the 

species) by the equivalent weight (molecular weight/valence). For example, the equivalent 

weight of calcium is 20, that is, 40/2. Therefore, the 50 mg/l Ca2+ can also be reported as 

50/20 mg/l = 2.5 meq/l Ca2+. This is a useful tool for checking analytical balance. Since the 

solution must be charge neutral, the cation meq/l must equal the anion meq/l. If the difference is 

greater than 5-10 percent, the analysis should be done again. 
Desalting Handbook for Planners 

40

The most popular method, although far from universal, is to express the cations and anions in 

mg/l as calcium carbonate, CaCO3. Calcium carbonate has an equivalent weight of 50, which 

provides a convenient factor for making this conversion. Thus, one merely multiplies the 

concentration of the species in milliequivalents per liter (meq/l) by 50 to convert the result to 

mg/l as CaCO3. Table 3-5 compares the three methods of reporting analyses. 

Table 3-5.—Three methods of reporting water analysis 

 mg/l as CaCO3 mg/l as ion meq/l 

Calcium 58 23.2 1.16 

Magnesium 20 4.8 0.40 

Sodium 18 8.3 0.36 

Total cations 96 36.3 1.92 

Bicarbonate (HCO3

-

) 55 67.1 1.10 

Sulfate (SO4

-

) 15 14.4 0.30 

Chloride (Cl-

) 21 14.9 0.42 

Nitrate (NO3

-

) 4 5.0 0.08 

Phosphate (PO4

-

) 0 

Fluoride (F-

) 1 0.4 0.02 

Total anions 96 101.8 1.92 

Alkalinity is the acid-neutralizing capacity of a water. The alkalinity of most natural waters can 

be attributed to the presence of the bicarbonate ion, HCO3

-

, which is formed by the reaction 

between dissolved CO2 gas and the carbonate-bearing mineral formations with which the rain, 

snow, sleet, or hail comes in contact. The alkalinity is frequently reported in mg/l expressed as 

calcium carbonate, CaCO3,. However, alkalinity can include other bases as well as calcium 

carbonate. 

“Hardness” in water is the sum of the calcium and magnesium concentrations, both expressed as 

calcium carbonate, CaCO3, in milligrams per liter. When the alkalinity exceeds the hardness 

concentration, all of the hardness is regarded as "temporary" because it will be precipitated by 

heating the water to its boiling point. This is illustrated by the following equations: 

 Heat 

Ca(HCO3)2 → H2O + CO2 ↑ + CaCO3 ↓

 Heat 

Mg(HCO3)2 → H2O + CO2 ↑ + MgCO3↓
Chapter 3: Water Chemistry

41

 Heat 

MgCO3 + 2H2O → H2O + CO2 ↑ + Mg(OH)2↓

(The symbol, ↑, represents the release of a compound from solution in a gaseous form.) 

When hardness exceeds alkalinity, that portion of the hardness in excess of the alkalinity 

concentration is termed "permanent" hardness. Thus, in table 3-3, water A has a total hardness 

of 886 mg/l and an alkalinity of 355 mg/l, both expressed as CaCO3, and the permanent hardness 

is 886-355 = 531 mg/l as CaCO3. Table 3-6 lists conversion factors for expressing ions, 

including Ca and Mg, as CaCO3. 

Table 3-6.—Conversion of ionic 

concentration to CaCO3 equivalents 

(hardness) 

Ion Multiply by 

Calcium 2.49 

Magnesium 4.10 

Sodium 2.18 

Potassium 1.28 

Strontium 1.14 

Barium 0.73 

Aluminum 5.56 

Bicarbonate 0.82 

Sulfate 1.04 

Chloride 1.41 

Nitrate 5.0 

Phosphate 1.58 

Fluoride 2.63 

Hydroxide 2.94 

The electrical conductivity of a brackish water is a characteristic useful in water analysis and 

desalting process control. Conductivity measurements are very accurate and can be made in a 

fraction of a minute. Measurements are usually reported as microsiemens per centimeter at 

25.0 °C (equivalent to micromho per centimeter). 

For each water, the relation between conductivity and concentration must be derived by accurate 

conductivity measurement and chemical analysis. The relationship between TDS and 

conductivity can then be expressed in the form of an equation. For example, the following 

equations for a well water feed and the corresponding product from a reverse osmosis pilot plant 

were used in the long-term performance evaluation of the unit. 
Desalting Handbook for Planners 

42

Feed: mg/l = (0.8637* K) - 386 

Product: mg/l = (0.6898* K) - 16 

where: K represents conductivity 

With these equations, measurements of conductivity then indicate the amount of TDS. Once 

established, such equations become valuable tools in desalting operations. It is possible to derive 

mathematical expressions from which the TDS of a specific water can be estimated. 

It is also possible to derive approximations of conductivity from ionic strength, and vice versa. 

Ionic strength of a solution is the sum of the concentration of each ionic constituent. 

ionic strength, : = i

Σ

2

1

 (CiZi

2

) 

where: Ci = concentration of ionic species, expressed as moles/liter

Zi = charge of species 

The specific conductance and ionic strength are related thus: 

ionic strength = specific conductance x 1.6 x 10-5

From the ionic strength expression, it is clear that a unit concentration of sodium (charge = 1) 

contributes one-fourth of the ionic strength of unit concentration of calcium (charge = 2). It 

follows then that as the contribution of divalent ions in a solution increases, the relationship 

between TDS and conductivity also changes. This is important to remember when using 

conductivity measurements to calculate salt rejection in a RO system, for example. It is also 

important to remember that conductivity is affected by both concentration and temperature. 

Any desalting process is impacted, to some degree, by the quality of the feed water source and 

the chemistry involved in the process of removing dissolved salts from water. A thorough 

understanding of the basic principles of water chemistry will help in the successful application of 

this technology. 

References 

Faust, S.D., and O.A. Aly, “Chemistry of Water Treatment, 2nd edition,” Lewis Publishers, 1998. 

Snoeyink, V.L., and D. Jenkins, “Water Chemistry,” John Wiley and Sons, 1980. 
Chapter 4: Desalting Processes 

 43

Chapter 4: Desalting Processes

This chapter discusses the two major desalting technologies: thermal distillation and 

membranes. According to the Wangnick 2002 Desalination Plant Survey, the U.S. currently has 

a total installed desalination capacity of over 3.8 million m3

 (one billion gallons) per day. The 

various membrane technologies account for 91 percent of that desalting capacity. 

4.1 Desalting Plant Processes 

General schematics of the processes used to desalt surface and well-field supplies are presented 

in figures 4-1 and 4-2, respectively. In figure 4-1, raw water is first screened to remove debris. 

The raw water is then pumped to the pretreatment system, where the water is prepared for the 

desalting process. Pretreatment for distillation processes involves removing gases, such as 

carbon dioxide, CO2 (if acid is added to the supply), and any sand. Pretreatment for the reverse 

osmosis (RO) process is more rigorous, requiring the removal of suspended particulates, such as 

colloidal material. This type of removal will normally require the use of a coagulant/filtration 

process, acid addition, and/or filtration such as microfiltration (MF) or ultrafiltration (UF). 

Backwashing filters results in the requirement for solids disposal, as shown in figure 4-1. 

Figure 4-1 General desalting plant schematic – surface supply 

Source: DSS Consultants, Inc. 

Screening & 

Water 

Intake 

Equipment

Post-treatment

Equipment

Solids Removal

Reverse Osmosis

Electrodialysis

Concentrate disposal, all processes

Gas Removal

Distillation Gas Removal

Electrodialysis

Product water 

Pretreatment Desalting

Process

Reverse Osmosis
Desalting Handbook for Planners 

44

Figure 4-2 shows the desalting process when the raw water supply is taken from ground water. 

Pretreatment is minimized because the ground water pretreats the supply. Thus, a pretreatment 

filtration step is usually not needed for ground water, but is usually needed with the open-type 

surface intake. 

Figure 4-2 General desalting plant schematic – ground water supply 

(Source: DSS Consultants, Inc.) 

Feed water—no matter what the source—is pumped to the desalting process, where it is 

processed. The feed water is converted to two (and sometimes three) streams: (1) product 

water; (2) concentrate; and (3) at times, a gas stream, depending on the process. The product 

water stream is the primary output. All desalting processes produce a concentrate stream as pure 

water (product) is removed, leaving a more concentrated stream to be disposed of. In some 

processes (notably distillation), a vent stream removes a small portion from the production 

stream to remove gases from the process. Gases are also generated in the electrodialysis (ED) 

process. The product stream is then further treated in the post-treatment step. This is done to 

“stabilize” the water (i.e., render it noncorrosive). This treated supply is then ready for pumping 

to the distribution system. 

In every desalination process, energy is needed. Performance ratio is defined as the mass of 

desalinated water produced per unit of energy input. In English units, this is the number of 

pounds for each 1,000 British thermal units (lb/Btu) of heat input. In metric, this is the number 

of kilograms per mega joule (kg/MJ). Note that the performance ratio in English units is 

2.2 times the performance ratio in metric. 

Solids Removal

Reverse Osmosis

Electrodialysis

Concentrate disposal, all processes

Gas Removal

Distillation

Gas Removal

Reverse Osmosis

Electrodialysis

Product Water Supply

Wells

Pre-treatment

Equipment

Desalting

Process

Post

Equipment

Treatment
Chapter 4: Desalting Processes 

 45

4.2 Distillation Process Fundamentals 

Dissolved solids, salts, which are relatively nonvolatile, remain in solution as the water is 

vaporized when a saline solution is boiled. Water that forms when water vapor condenses on a 

cooler surface is pure—it does not contain any dissolved solids. The basic distillation process is 

shown in figure 4-3. 

Figure 4-3 Conceptual process design drawing 

Distillation is a heat transfer process. The fundamental engineering problem involves finding 

ways to transfer large quantities of water, vapor, and heat economically. 

Approximately 1,000 Btu of energy is used to vaporize one pound of water. If fuel costs $2 per 

million Btu, it would cost $16.90 per hour to supply the heat needed to vaporize 1,000 gallons of 

water per hour. Therefore, for economic reasons, a process that recovers more that one pound of 

water for every 1,000 Btu of energy input is needed. Three different distillation processes have 

been developed to achieve this: 

• Multiple Effect Distillation (MED), discussed in section 4.4 

• Multi-Stage Flash Distillation (MSF), discussed in section 4.5 

• Vapor Compression Distillation (VC), discussed in section 4.6 
Desalting Handbook for Planners 

46

4.3 Characteristics of Distillation Processes 

4.3.1 Temperature in Distillation Processes 

Distillation at high temperatures is generally more economical. The main advantage of raising 

the process temperature is to increase the temperature difference between the highest operating 

temperature and the feed water temperature. This temperature difference is the driving force for 

evaporation. That is, the higher the difference, the greater the amount of water that can be 

produced for a given size of evaporator. This difference could also allow for the use of a higher 

number of flashing stages or effects, used in MSF (section 4.5). Using these temperature 

differences results in an increase in performance ratio and, thus, produces more water for each 

unit of energy input. While more stages increase the capital cost of the process, they reduce the 

heat requirements. 

4.3.2 Scaling in Distillation Processes 

4.3.2.1 Effects of Scaling 

Scale forms when solid materials are deposited on solid surfaces. There are three main culprits 

in distillation plants: calcium sulfate, CaSO4; magnesium hydroxide, Mg(OH)2; and calcium 

carbonate, CaCO3. 

Scale is particularly undesirable when it forms on a surface through which heat must be 

transferred, like a metal tube in a distillation unit. As scale has a much lower thermal 

conductance than the metal of the heat transfer tubes, scale can greatly reduce the overall heat 

transfer. 

 4.3.2.2 Calcium Sulfate Scale 

Formation of calcium sulfate, CaSO4, scale cannot be limited by pretreatment. Therefore, 

calcium sulfate must be controlled by limiting the operating temperature or by limiting the 

concentrate of the calcium and/or sulfate ions in the concentrate. 

Generally, as the temperature of the solution is increased, the solubility of dissolved salts 

increases. However, certain salts, such as calcium sulfate, have an inverse solubility. This 

means that the solubility of these salts decreases with increasing temperature. Calcium sulfate 

must be controlled by limiting temperature or by limiting the concentrate of the calcium and/or 

sulfate ions in the concentrate. 

Calcium sulfate occurs in three crystalline forms, depending on the degree to which the crystal 

is hydrated. These are anhydrous, CaSO4; hemihydrate, CaSO41/2 H2O; and dihydrate, 

CaSO42H2O, also called gypsum. These forms have different solubilities, as shown in 

figure 4-4. 
Chapter 4: Desalting Processes 

 47

Figure 4-4 Calcium sulfate solubility 

The crystalline form with the lowest solubility for a particular temperature is the one that will 

precipitate at that temperature. Distillation processes must be operated such that scaling 

(precipitation) does not occur. This sets a maximum on the concentration of calcium sulfate that 

can be tolerated in the concentrate. As mentioned above, an increase in operating temperature 

will result in improved performance. However, the increase must be accomplished in context of 

the maximum calcium sulfate concentration allowable in the concentrate. To a degree, this 

maximum concentration can be increased by use of additives discussed in chapter 5. The present 

limitation for seawater desalting generally is 110  ̊C (230  ̊F) with additives and 120  ̊C (248  ̊F) 

for plants using acid treatment, with a maximum concentrate concentration of about 1.9 times 

that of normal seawater. 

Because each distillation process can operate at different maximum temperatures and 

concentrations, the pretreatment to prevent scaling will differ. Scaling for each distillation 

process is discussed in the section for that process. 

4.3.2.3 Calcium Carbonate and Magnesium Hydroxide Scale 

Calcium carbonate and magnesium hydroxide are alkaline, soft scale, and can be easily removed 

by adding acid. However, pretreatment of the feed water through pH control, followed by 

decarbonation, can prevent the formation of magnesium hydroxide, Mg(OH)2, and calcium 

carbonate, CaCO3, scale. This pretreatment can minimize the frequency of scale removal. Three 

chemicals are added to control alkaline scale formation: 

• Polyphosphate.—Polyphosphate is nonhazardous, readily stored, and easy to add to the 

make-up water to the plant. Polyphosphates decompose and become ineffective at 

temperatures above 90.6 °C (195 °F). Operations at this temperature can treat only 

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature (C)

CaSO4 Concentration (as Sodium Chloride), %

Hemihydrate (CaSO4 - 1/2 H20) Gypsum (CaSO4 - H20) Anhydrite (CaSO4)

Gypsum 

Scaling 

Anhydrite 

Scaling 

Hemihydrate 

Scaling 

Non Scaling 

Region

Normal Seawater Concentration
Desalting Handbook for Planners 

48

concentrate below about 1.8 times normal seawater concentration. Polyphosphates will 

control scaling from both magnesium hydroxide and calcium carbonate. 

Polyphosphate treatment produces a carbonate sludge which is discharged with the 

concentrate. To avoid gradual buildup of sludge and, thus, minimize the need for 

periodic acid, a ball cleaning system can be used on the tube surfaces. Ball cleaning 

systems put foam balls about the size of the inner tube through the tube to remove 

buildups. The balls are recovered and recirculated. 

• Acid.— Any acid can be used, but sulfuric acid is normally used because it is more 

readily available and costs less than other acids. Acid treatment is carried out at a 

maximum operating temperature of 120 °C (248 °F). This higher temperature increases 

the performance ratio of the process. The pH of the feed stream to the evaporator is 

lowered to about 4.2, so that all carbonate is removed. The carbon dioxide thus formed is 

then removed in a decarbonator. If it is not removed, it will re-dissolve in the vessels, 

creating carbonic acid, which will accelerate corrosion. 

• Polymers.—Polymers have been developed that can operate at higher temperatures than 

polyphosphates (up to 110 °C [230 °F]). While polymers do not operate at temperatures 

quite as high as acid, they have fewer corrosion problems than acid. 

4.3.3 Corrosion and Erosion in Distillation Processes 

Distillation plants are subject to corrosion. Seawater and concentrate stream factors that 

influence corrosion include: 

• pH 

• Temperature 

• High chloride concentration 

• Dissolved oxygen 

Product waters are very aggressive to metal and concrete. Factors that influence corrosion 

include: 

• pH 

• Temperature 

• Lack of minerals 

Corrosion can be minimized by the use of corrosion-resistant materials (e.g., high-performance 

steel) throughout the feed and concentrate streams, and with proper pretreatment through the 

flash chambers, along with the proper choice of materials. 
Chapter 4: Desalting Processes 

 49

4.3.4 Heat Transfer in Distillation Processes 

Heat transfer surfaces—surfaces that either put heat into or take heat out of the system—

represent a major expense, typically: 

Process Expense for heat transfer (PR = 5.17 kg/MJ) 

MSF Up to 40 percent of the evaporator costs 

MED Up to 40 percent of the evaporator costs 

VC Up to 35 percent of the evaporator costs 

The plant design will, therefore, have to balance the cost of the heat exchanger surface against 

the cost of energy (mostly the heat energy to the heat input section). In the MSF design, the bulk 

of the heat transfer surface is in the heat recovery section and effect bundles. In the VC design, 

the bulk is in the vessel. 

The design of the venting system for removing noncondensable gases is essential to maintain the 

heat transfer rates at their design point. If these noncondensable gases are not removed, they will 

blanket the tube surfaces, which will result in a loss of water production. 

4.3.5 Post-Treatment in Distillation Processes 

Product water from distillate plants can be as low as 0.5 milligrams per liter (mg/l) of total 

dissolved solids (TDS) and, generally, does not exceed 5 mg/l. This lack of minerals makes the 

supply very unstable and corrosive. Therefore, before this supply is delivered to the distribution 

system, the water must be stabilized by increasing the mineral content. The following general 

guidelines are used for stabilization: 

Measurement Goal 

pH 8 to 9 

Alkalinity 40 mg/l as calcium carbonate, CaCO3, or greater 

Total hardness 40 mg/l as CaCO3, or greater 

Langelier Saturation Index (LSI) Positive 

These goals can be reached by adding chemicals or blending with a brackish water source. In 

some cases, blending and adding chemicals may be needed. Stabilization is discussed further in 

Chapter 6, Post-Treatment. 
Desalting Handbook for Planners 

50

4.3.6 Energy Requirements for Distillation Processes 

The quantity of steam required will depend upon the performance ratio of the process. MSF and 

MED are primarily steam-driven processes, whereas VC requires more electricity in the 

compression process. Distillation processes use thermal energy at relatively low temperatures 

and pressures. For example, the MSF process requires steam at pressures between atmospheric 

and about 1.76 kilograms per centimeter squared (kg/cm2

) (25 pounds per square inch gauge 

[psig]). MED can use steam at less than atmospheric to 1.76 kg/cm2

 (25 psig), and thermo-vapor 

compression (TVC) requires steam at about 5.27 kg/cm2

 (75 psig) (minimum). 

It is economical to co-locate a desalting plant with a power plant. In these cases, steam can be 

taken from the power plant at low pressure, after the steam has generated electricity. The 

arrangement, known as a “dual purpose” plant, results in lowering the primary fuel cost of the 

desalting plant by 60 to 70 percent, thus reducing the cost of the produced water. 

4.4 Multiple Effect Distillation Process 

This section describes MED processes in more detail, including the vertical and horizontal tube 

arrangements and the newest development in the vertical tube arrangement, stacking the effects 

one on top of the other. 

MED plants currently produce anywhere from 1.7 to 6.4 kg/MJ (4 to 15 lb/1,000 Btu). Vertical 

tube arrangements are designed for a higher performance ratio—9.9 kg/MJ (23 lb/1,000 Btu). 

4.4.1 MED Operating Principle 

In MED, a series of evaporator effects produces water at progressively slightly lower pressures. 

Because water boils at lower temperatures as pressure decreases, the water vapor of the first 

evaporator effect serves as the heating medium for the second evaporator effect, and so on. The 

more effects, the higher the performance ratio. 

Thus, theoretically (assuming no losses), if a single effect evaporator produces 2.2 kg per 

1.055 MJ (1 lb/1,000 Btu), then three evaporator effects will produce about 1.8 kg (4 lbs) of 

distillate with the same amount of heat. Figure 4-5 shows three effects of a multiple effect 

evaporator. Effect 1 is at higher pressure than effect 2, and similarly, the pressure in this effect is 
Chapter 4: Desalting Processes 

 51

 Figure 4-5 Multiple effect schematic 

higher than in effect 3. The heat source in effect 1 is enough to boil a portion of the feed water 

entering at the top of the unit. The vapor formed in this effect heats the next lower pressure 

effect. The process of producing vapor in each effect, and using it to heat the next lower effect, 

continues throughout all the effects until the vapor for the last effect is condensed in the main 

condenser. Concentrate from each effect can be directed to the next lower effect or it can be 

taken out at specific points in the process. Distillate, product water, is obtained from the 

condensate of the vapor in each effect and from the main condenser. 

4.4.2 MED Design Configurations 

Three arrangements have evolved for MED processes. They are based primarily on the 

arrangement of the heat exchanger tubing: 

• Horizontal tube arrangement 

• Vertical tube arrangement 

• Vertically stacked tube bundles 

Each of these designs is described in the following subsections. 

4.4.2.1 Horizontal Tube Arrangement 

In this arrangement, the tube bundles are arranged horizontally in the vessel as shown in 

figure 4-6. The feed water is sprayed over the outside surfaces of the tubing, and the inside 

tubing surfaces contain the heat to vaporize the feed water. The vapor generated in each effect 

is directed to the next lower pressure effect. 

EFFECT EFFECT EFFECT 

SEAWATER SUPPLY HEADER

STEAM SUPPLY

CONDENSATE

RETURN

VAPOR TO EFFECT 2

CONCENTRATE TO 

NEXT EFFECT

CONCENTRATE TO 

NEXT EFFECT

CONCENTRATE TO 

NEXT EFFECT

VAPOR TO EFFECT 3 VAPOR TO NEXT EFFECT 
Desalting Handbook for Planners 

52

Figure 4-6 MED horizontal tube arrangement 

The full process schematic is shown in figure 4-7. Feed water enters the main condenser. The 

condenser can be of the conventional shell and tube type, as depicted, or it can be designed 

similar to the effect design. Most of the feed water flow is for cooling and is returned to the sea. 

A small portion of the feed water is used as make-up for the process. The make-up enters the 

degassifier/deaerator. Normally, there are two vessels at this step: one for removing air and one 

for removing carbon dioxide if acid is used for pretreatment. A make-up pump is required to 

pump the make-up from the vacuum condition to the top of the last effect. Here, the feed is 

pumped through a heat exchanger, where some heat is recovered. The feed stream continues 

through each recovery heat exchanger of each tube bundle, where a small portion of the feed 

water is vaporized. 

The steam used in the first effect is condensed as the steam gives up its heat to the vaporization 

process and is pumped back to the boiler. The diagram in figure 4-7 shows that the feed from the 

first effect is collected in the vessel and piped to the second effect, where the vaporization 

process begins once again. Vapor from the first effect is piped to the second effect, to be used as 

the heat source. This process continues, through each successive effect, until the vapor from the 

final effect is condensed in the main condenser. 

The distillate produced in each effect is joined with the condensate from the main condenser and 

becomes the product water, which is then pumped to the post-treatment system prior to storage 

and pumping to the water distribution system. 

Noncondensable gases enter the unit primarily through leaks in the piping and vessels. These 

gases must be removed in order to prevent “blanketing” of the tube surfaces. Any blanketing of 

the tube surfaces will result in a loss of heat transfer, with consequent loss in production. Using 

steam jet air ejectors to remove the noncondensables is the preferred method. To ensure that 

DISTILLATE FROM

TUBE BUNDLE

VAPOR TO NEXT EFFECT

NEXT EFFECT PREVIOUS EFFECT

VAPOR FROM

PREVIOUS EFFECT

CONCENTRATE 

CHAMBER CONCENTRATE TO NEXT EFFECT 

FEED WATER 
Chapter 4: Desalting Processes 

 53

Figure 4-7 MED process schematic—horizontal tube arrangement 

these gases are removed from the unit, a portion of the vapor generated in the effect is used. The 

condensing unit used can be either the shell and tube type or the barometric condenser. 

Currently, barometric-type condenser designs are preferred because the shells of these units can 

be fabricated from nonmetallic material and do not need heat exchanger tubing. Therefore, 

corrosion is not a problem. 

The process shown in the diagram is termed the “feed-forward” system. That is, concentrate is 

pumped to the first effect, where the concentrate returns through each successfully lower 

pressured effect. This design can also be furnished as a “backward feed” system, in which the 

feed would be pumped to each higher pressure effect. The drawback for using the backward 

feed system to treat seawater is that as the feed enters each successive effect, its concentration 

and temperature have increased. Such increases cause scaling on the tube surfaces. 

This system can be arranged to limit the number of pumps required by feeding the seawater to 

more than one effect at a time. For this case, the concentrate and distillate streams would also be 

modified. 

This process has been designed to operate at two distinct maximum temperatures: low 

temperature at about 71.1 o

C (160 °F) and high temperature at 110 oC (230 °F). Operating at low 

temperatures limits the severity of corrosion. Low temperature units can be made from less 

expensive materials. 

Scaling in the MED process (or any distillation process) is a function of temperature and feed 

water concentration. Operating curves for high temperature and low temperature MED, and their 

STEAM JET EJECTOR

MAIN

CONDENSER

PRODUCT

TO

STORAGE

BLOWDOWN

EFFECT PUMPS

TUBE

BUNDLE

SECTION

(2)

MAKE-UP

PUMP

STEAM SUPPLY

CONDENSATE

RETURN

(1)

(1) DEGASSIFIER/DEAERATOR

(2) (2) (2) (2)

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

HP STEAM SUPPLY

EFFECT PUMPS

FEED WATER SUPPLY

(2) CONCENTRATE CHAMBER

COOLING 

WATER

RETURN
Desalting Handbook for Planners 

54

relationship to calcium sulfate scaling curves, are shown in figure 4-8. Operating curves for 

vertically stacked MED systems are shown in figure 4-9. The design can operate at temperatures 

up to 110  ̊C (230  ̊F) without fear of calcium sulfate scaling. At temperatures of 104  ̊C (220  ̊F) 

or lower, the process operates well below the anhydrite scaling region. 

 Figure 4-8 Calcium sulfate solubility and MED operation 

Figure 4-9 Calcium sulfate solubility and vertically stacked MED operation 

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature (C)

CaSO4 Concentration (as Sodium Chloride), %

Hemihydrate (CaSO4 - 1/2 H20) Gypsum (CaSO4 - H20) Anhydrite (CaSO4)

Gypsum Scaling 

Region

Anhydrite 

Scaling Region

Hemihydrate 

Scaling Region

Non Scaling 

Region

Vertically Stacked

Operation

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature (C)

CaSO 4 Concentration (as Sodium Chloride), %

Hemihydrate (CaSO4 - 1/2 H20) Gypsum (CaSO4 - H20) Anhydrite (CaSO4)

Gypsum 

Scaling Region

Anhydrite 

Scaling Region

Hemihydrate 

Scaling Region

Non Scaling 

Region

Low Temperature

High Temperature
Chapter 4: Desalting Processes 

 55

4.4.2.2 Vertical Tube Arrangement 

The vertical tube bundle arrangement is depicted in figure 4-10. The feed water enters at the top 

of the effect and flows on the inside surface of the tube. The heat for vaporization is on the 

outside surface of the tubing. The advantage of this design over the horizontal tube arrangement 

is higher heat transfer rates. Higher heat transfer rates result from having a thin film on both the 

inside and outside surfaces of the heat exchanger tubing. One drawback to this design, however, 

is the difficulty of ensuring that good flow distribution is achieved for each tube. 

Figure 4-10 MED vertical tube bundle arrangement 

Figure 4-11 is a process schematic of the vertical tube arrangement. Note that the design and 

operation are identical to the horizontal tube arrangement, except that the tubes are arranged 

vertically. 

4.4.2.3 Vertically Stacked Tube Bundles 

The tubing arrangement in the vertically stacked unit is depicted in figure 4-12. For this design, 

the concentrate flows down between effects, thus eliminating the need for pumping. As with the 

vertical unit described above, the feed water is fed to the inside surface of the tubing, and the 

heating for vaporization is on the outside surface of the tube bundle. This drawing depicts two 

sets of bundles, but the unit can consist of many sets of bundles. 

The process schematic is shown in figure 4-13. This design is repreentative of the 303,000 m3

/d 

(80 mgd) unit as conceptually designed by the Metropolitan Water District of Southern 

California (MWD). Feed water supply enters the unit at the bottom and flows by gravity through 

the degassifier/deaerator to remove carbon dioxide (acid pretreatment is employed) and oxygen 

VAPOR TO 

SEAWATER SUPPLY

STEAM SUPPLY

CONDENSATE 

VERTICAL

VAPOR SPACE

TUBE

BUNDLE

NEXT EFFECT

RETURN

CONCENTRATE TO NEXT EFFECT 
Desalting Handbook for Planners 

56

Figure 4-11 MED process schematic – vertical tube arrangement 

Figure 4-12 MED vertically stacked tube bundles 

STEAM JET EJECTOR

MAIN

CONDENSER

COOLING

WATER

RETURN

PRODUCT

TO

STORAGE

BLOWDOWN

EFFECT PUMPS

TUBE

BUNDLE

SECTION

(2)

MAKE-UP

PUMP

STEAM SUPPLY

CONDENSATE

RETURN

(1)

(1) DEGASSIFIER/DEAERATOR

(2) (2) (2) (2)

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

TUBE

BUNDLE

SECTION

HP STEAM SUPPLY

EFFECT PUMPS

SEAWATER SUPPLY

(2) CONCENTRATE CHAMBER 

(2)

CENTERLINE

DEMISTER

VERTICAL

TUBE

BUNDLE

PREHEATER BUNDLE

VERTICAL

BUNDLE

TUBE

VERTICAL

TUBE

BUNDLE

VERTICAL

TUBE

BUNDLE

M

VAPOR SPACE VAPOR SPACE

CENTERLINE

FEED WATER HEATER

CONCENTRATE 

FROM 

UPPER EFFECT CONCENTRATE 

FROM 

UPPER EFFECT

CONCENTRATE 

POOL 

CONCENTRATE TO NEXT EFFECT CONCENTRATE TO NEXT EFFECT 
Chapter 4: Desalting Processes 

 57

Figure 4-13 MED process schematic – vertically stacked tube bundles 

to acceptable levels. The unit uses aluminum tube materials to take advantage of lower cost and 

considerably higher material conductivity. 

If heavy metals enter the unit, severe corrosion of the tubing would occur. Aluminum packing 

inside the degassifier/deaerator serves as an “ion trap” to remove all heavy metals, such as 

copper, nickel, etc., before the seawater enters the tubing proper. After leaving the 

degassifier/deaerator, the seawater is pumped through the preheater to the top of the unit. The 

feed stream is preheated as it passes through each effect. Once the feed reaches the first effect, it 

is distributed throughout the tube bundle by flow devices that are placed at the end of each tube. 

The remainder of the feed flows by gravity through the rest of the effects in the feed-forward 

design. 

Steam at low pressure is the heat source in the first effect. Vapor generated in the first effect is 

directed to the next lower pressure effect (see figure 4-13). The process of vaporization 

continues throughout the unit. Thus, the process operates the same as the processes described 

above. 

The tubing of each effect is furnished in a double-fluted design. The double-fluted tubes 

arranged in the vertical plane offer the advantage of boosting heat transfer to three to four times 

that of smooth tubes carrying water. This characteristic reduces the amount (and cost) of heat 

transfer surface by the same amount. 

164.5m

24m

134m

24m

Grade Grade

STEAM SUPPLY

CONDENSATE RETURN

DEGASSIFIER

DEAERATOR

SEAWATER

SUPPLY

E-1

E-2

E-3

E-n

#1 #2

#3

#4

PUMP #1 FEED 

PUMP #2 CONCENTRATE 

PUMP #3 DISTILLATE 

PUMP #4 CONDENSATE 

DISTILLATE TO 

STORAGE 

CONCENTRATE TO DISPOSAL 
Desalting Handbook for Planners 

58

4.4.3 MED Process Characteristics 

Table 4-1 gives the process characteristics of each of the above-described processes. Although 

the main temperature of operation given for the horizontal and vertical arrangements is 76.7 o

C 

(170 °F), this type of unit can operate at temperatures up to 110 o

C (230 °F). 

Table 4-1.—Process characteristics of MED systems 

Item 

Low 

temperature 

horizontal tube 

design 

Low 

temperature 

vertical tube 

design 

Stacked vertical 

tube design 

High 

temperature 

horizontal 

tube design 

High temperature 

vertical tube design 

Maximum operating 

temperature (o

C) 71.7 71.7 110 110 110 

Process recovery 

(percent) 20 to 35 20 to 35 67 20 to 35 20 to 35 

Performance ratio 

(kg/MJ) 3.44 to 5.17 3.44 to 4.30 10.33 3.44 to 6.46 3.44 to 6.46 

Heat transfer 

coefficient (w/m2

-K) 1,703 to 3,407 1,703 to 3,407 4,542 to 11,356 1,703 to 

4,259 1,703 to 4,259 

Concentrate (mg/l) 54,000 54,000 106,000 54,000 54,000 

Electrical 

consumption 

(MJ/m3

) 

0.00132 – 

0.0026 

0.00132 – 

0.0026 

0.000528 – 

0.00106 

0.00132 – 

0.0026 0.00132 – 0.0026 

Distillate quality (mg/l) 0.5 to 25.0 0.5 to 25.0 0.5 to 25.0 0.5 to 25.0 0.5 to 25.0 

Pretreatment 

chemical 

Polyphosphate Polyphosphate Acid or polymer Polymer Acid or polymer 

Pretreatment dose 

rate (mg/l) 

0.5 to 4.0 0.5 to 4.0 Acid at 140 

Polymer at 1 to 2 

1.0 to 2.0 Acid at 140.0 

Polymer at 5 to 10 

 Note: MJ/m3 = Mega joules per cubic meter, w/m2

-K = watts per square meter-Kelvin. 

4.4.4 MED Materials of Fabrication 

The materials to construct each design discussed above are listed in table 4-2. The materials 

listed in the table assume the following: 

• The horizontal and stacked vertical tube units are designed for operation at 76.7 oC 

(170 °F) (maximum). 

• The standard vertical tube unit is designed for an operating temperature of 110 oC 

(230 °F) (maximum). 

• All designs use 50.8-millimeter (mm) (2-inch) diameter tubing. 
Chapter 4: Desalting Processes 

 59

Table 4-2.—Materials of fabrication, MED systems 

Item 

Horizontal tube 

design 

Vertical tube 

design 

Stacked vertical 

tube design 

Effect vessels Carbon steel, epoxy 

coated 

Carbon steel, epoxy 

coated 

Concrete 

Effect tubing Aluminum Aluminum brass, 

copper nickel 

Aluminum 

Effect tube sheets Aluminum Aluminum brass, 

copper nickel 

Aluminum 

Preheater tubing Aluminum Aluminum brass Titanium 

Pumps Stainless steel, grade 

316 

Stainless steel, grade 

316 

Aluminum brass 

Deaerator Carbon steel, epoxy 

coated 

Carbon steel, epoxy 

coated 

Concrete, 

aluminum 

Decarbonator Carbon steel, epoxy 

coated 

Carbon steel, epoxy 

coated 

Concrete, 

aluminum 

External structural shapes Carbon steel Carbon steel Not required 

Internal supports Carbon steel, epoxy 

coated 

Carbon steel, epoxy 

coated 

Aluminum 

Demisters Stainless steel, grade 

316 

Stainless steel, grade 

316 

Stainless steel, 

grade 316 

4.4.5 MED Process Status 

The status of each of these design configurations is discussed below: 

• Horizontal Tube Arrangement.—This arrangement has been offered for sale 

worldwide for the past 20 years. Approximately 300 units have been sold to date; 

thus, this process has been fully developed. 

• Vertical Tube Arrangement.—One of the first plants constructed with this design was 

installed in the U.S. Virgin Islands in 1968. Since then, two more units have been 

constructed in the U.S. Virgin Islands. These plants were fabricated for operation at 

110 o

C (230 °F). Because these units were developed with insufficient heat 

exchanger surface area, meeting the guaranteed water production was difficult. 

However, the process did produce water in accordance with the required performance 

ratio. Other firms around the world have successfully used this arrangement 

(Wangnick, 2002). 

• Vertically Stacked Tube Bundles.—This design is very new, compared with the 

horizontal and vertical tube arrangements, although it was conceived in the late 1960s 

or early 1970s. The Metropolitan Water District of Southern California (MWD) has 

completed a detailed concept report and has designed and operated two pilot plants at 

Huntington Beach, California, using this design. Each pilot plant was constructed in 
Desalting Handbook for Planners 

60

two effects. One unit, the long tube test unit, was used to evaluate corrosion aspects, 

water quality, and scaling. The second, the short tube test unit (STTU), was used to 

confirm heat transfer data. The process schematic for MWD’s STTU is shown in 

figure 4-14. The operating period of these units was 1-1/2 years. This testing 

confirmed that: 

3⁄4 Heat transfer coefficients were greater than the design projected 

requirements. 

3⁄4 Operation of these units was stable over the temperature range of 37.8 to 

110 o

C (100 to 230 °F). 

3⁄4 Production from the units met design requirements. 

3⁄4 Water quality goals, including the concentration of aluminum in the 

distillate, were met. 

3⁄4 No significant corrosion of the tubing was evident. 

3⁄4 Scaling of the tube surfaces could be easily removed. 

Figure 4-14 Process schematic, MWD’s short tube test unit 

CONCENTRATE PUMP 

CONCENTRATE TO DISPOSAL 
Chapter 4: Desalting Processes 

 61

4.5 Multi-Stage Flash (MSF) Distillation 

Multi-stage flash distillation is another method commonly used for desalting drinking water 

supplies. The practical maximum performance ratio for an MSF plant is 5.17 kg/MJ 

(12 lbs/1,000 Btu). 

4.5.1 MSF Operating Principle 

Each stage of an MSF plant operates at progressively lower pressures, as water boils at lower 

temperatures (figure 4-15). For this technique, the feed water is heated under sufficiently high 

pressure to prevent boiling, until it reaches the first “flash chamber.” In the first flash chamber 

(stage), the pressure is released and sudden evaporation or “flashing” takes place. This flashing 

of a small portion of the feed continues in each successive stage, because the pressure in each is 

lower. Unlike the multiple effect process, this process generates and condenses its vapor in the 

same effect (stage). Thus, this design offers the benefit of heat recovery. That is, the feed water 

passing through the heat exchanger in the upper section of the flash chamber gains heat as it 

condenses the vapor to distillate. 

Two distinct sections of each stage are the flashing chamber (where the vapors are produced) and 

the condensing section (where the vapors are condensed). The amount of water that flashes will 

be in proportion to the temperature difference between stages. Thus, the greater the temperature 

difference, the greater the amount of flashed vapor. Once the vapor has flashed, the concentrate 

temperature will decrease until it is in thermodynamic equilibrium with the pressure in that stage. 

As the vapor is generated, the concentrate stream becomes more concentrated. The condensing 

section contains the heat exchanger tubing where the vapors are condensed by cooler seawater 

feed water. 

Treating water begins with the feed water entering the recovery section tubing. Here, the feed 

water gains heat as it passes through each stage, by condensing the vapors generated. Feed water 

then exits the recovery section and enters the concentration heater, which serves as the heat 

source for the thermodynamic process. This unit raises the feed water temperature to its design 

point. For this discussion, a final temperature of 90.6 o

C (195 °F) is used. Once the feed exits 

the concentration heater, the feed enters the first stage of the MSF evaporator. Figure 4-16 

depicts the flashing that will occur in a stage. 

The feed water entering the first stage is hotter (90.6 o

C [195 °F]) than the temperature in the 

second stage (78.1 o

C [172.5 °F]). As the feed water cannot exist at 90.6 oC (195 °F) in the 

second stage, it immediately flashes to the stage temperature of 78.1 o

C (172.5 °F). The amount 

of flashing will depend upon the temperature difference (in this example, 12.5 o

C [22.5 °F]). As 

the feed water enters the next stage, it is again hotter than the temperature existing in the stage, 

so once again it flashes. However, the amount of flashing will be less because the new stage has 

a lower temperature difference. The concentrate continues to flow from stage to stage only by 

the pressure difference. 
Desalting Handbook for Planners 

62

Figure 4-15 MSF arrangement 

Figure 4-16 MSF stage 

SEAWATER SEAWATER 

DISTILLATE 

MESH DISTILLATE

VAPOR 

FEED WATER OUT FEED WATER IN
Chapter 4: Desalting Processes 

 63

The distillate produced in each stage is sent to the next lower pressure stage. It will exit the 

evaporator at the last stage and be pumped to the post-treatment system. Noncondensable gases 

are removed, as discussed above, for this MED process. 

4.5.2 MSF Process Arrangements 

There are two process arrangements for the MSF process: once-through and recycle. Each of 

these process can be arranged as a “long tube” or “cross tube” design. In the long tube design, 

tubing is parallel to the concentrate flow in the vessel, as shown in figure 4-17. Tubing is 

perpendicular to the concentrate flow in the cross tube design, as shown in figure 4-18. Since 

operations for the two designs are the same, the following paragraphs describe the process for 

both designs. 

4.5.3 MSF Process Description 

4.5.3.1 Once-Through Design 

The once-through design for an MSF process is depicted in figure 4-19. As the term indicates, 

the feed water is pumped through the recovery section and concentrate heater, then passes 

through the flash chambers without recycling. 

The concentrate is then disposed of. 

The drawback to the once-through design is 

that the entire feed must be pretreated before 

entering the unit, to minimize the effects of 

corrosion and scaling. Also, the supply is 

pumped twice: at the intake and after the 

deaerator/decarbonator. This means that the 

entire feed water flow must be pretreated and 

the deaerator/decarbonator equipment is much 

larger than that required for the recycle design. 

A significant advantage of the once-through 

design is that it is not prone to calcium sulfate 

scaling because the seawater passing through 

the recovery section tubing is at standard 

seawater concentration (see figure 4-4). Other 

advantages include higher operating 

temperature and lower boiling point elevation. 

Figure 4-17 MSF long tube design 

TUBE FLOW DIRECTION

BRINE FLOW DIRECTION

MSF SECTION VIEW

Condensing 

Section

Flash Chamber

Section

Condensing 

Flash Chamber

STAGE 1

MSF PLAN VIEW

TUBE FLOW DIRECTION

STAGE 2 STAGE 3 STAGE 4

STAGE 1 STAGE 2 STAGE 3 STAGE 4

 CONCENTRATE FLOW DIRECTION 
Desalting Handbook for Planners 

64

Figure 4-18 MSF cross tube design 

Figure 4-19 MSF schematic, once-through 

DIRECTION

BRINE FLOW

DIRECTION

TUBE FLOW

STAGE 1

MSF PLAN VIEW

STAGE 2 STAGE 3 STAGE 4

CONDENSING

FLASH CHAMBER BRINE FLOW

SECTION

FLASH CHAMBER

CONDENSING

SECTION

MSF SECTION VIEW

STAGE 1 STAGE 2 STAGE 3 STAGE 4

CONCENTRATE FLOW 

CONCENTRATE FLOW 
Chapter 4: Desalting Processes 

 65

4.5.3.2 Recycle Design 

The MSF recycle design was developed to reduce the cost of pumping, chemical treatment, 

deaeration, and decarbonation. Figure 4-20 shows the process schematic for the recycle design. 

The evaporator is broken into two distinct sections: the rejection section and the recovery 

section. The rejection section is provided as the “heat sink” for the process, whereas the 

recovery section serves to raise the temperature of the recycle stream. 

Figure 4-20 MSF recirculation schematic 

In this process, feed water is passed through the heat rejection section tubing. It exits the 

rejection section and is returned to the original source of water. However, a small portion is 

taken as make-up to the process. This stream is pretreated and then passed through the 

decarbonator and deaerator before entering the last stage of the rejection section. In the last 

stage, a portion of the recycle stream is removed as the make-up stream is added. These two 

streams are used to control the concentrations in the recycle stream. Most of what would be the 

blowdown is recycled to form the feed water supply to the tube bundle section. A small part is 

discharged through the blowdown pump and replaced by the make-up stream. The tube bundle 

section then operates as discussed above for the once-through design. 

4.5.3.3 Scaling 

Operating curves for straight-through and recycle MSF, and their relationship to calcium sulfate 

scaling curves, are shown in figure 4-21. This shows that the once-through design, which 

normally operates at a temperature of 90.6  ̊C (195  ̊F) will not cross into the anhydrite scaling 

region. On the other hand, the recycle process curve goes well into the scaling region as shown. 

The point of crossing occurs at about 90.6  ̊C (195  ̊F). Thus, scaling is more likely to occur in 

STEAM JET EJECTOR

CONDENSATE

PUMP

TUBE BUNDLE SECTION

SEAWATER SUPPLY

BLOWDOWN

PUMP

HEAT RECOVERY SECTION

DISTILLATE

HP STEAM SUPPLY

LP STEAM SUPPLY

To Atmosphere

CONCENTRATE RECYCLE

FLASH CHAMBER SECTION

DECARBONATOR

DEAERATOR

COOLING WATER RETURN

HEAT 

REJECTION

SECTION

STAGE 1

MAKE-UP STAGE n STAGE 2

CONCENTRATE 

HEATER
Desalting Handbook for Planners 

66

Figure 4-21 Calcium sulfate solubility and MSF operations 

the recycle design process. Even though the recycle mode operates in the anhydrite scaling 

region, scale inhibitors can be used to prevent scale. Scale also is limited in this process as 

anhydrite forms slowly, and the highest concentration occurs in the flash chamber section and 

not on the tube surface or because of use of additives. 

4.5.3.4 Corrosion 

In addition to the corrosive effects of the environment, MSF plants are also subject to erosion 

and impingement attack. Erosion results from the turbulence of the feed water in the flash 

chamber, particularly in the two-phase flow regime when the concentrate passes from stage to 

stage through the orifice plate (a device that controls the flow into each feeding stage). 

For example, for the long tube design where mass flow rates typically exceed 16,856 kg/hr-m2

(400,000 lb/hr-ft2

), the entire shell should be lined or clad with stainless steel 316L. For the 

cross tube design, where the mass flow rate is kept below 16,856 kg/hr-m2

 (400,000 lb/hr-ft2), 

unclad steel can be used—with the exception of the first 3 to 6 stages where lining or cladding is 

recommended. 

4.5.4 MSF Process Characteristics 

Table 4-3 gives the process characteristics of the once-through and recycle designs. Although 

the once-through design can operate at temperatures up to 110 o

C (230 °F), the maximum 

temperature is usually limited to 90.6 o

C (195 °F). Therefore, table 4-3 gives the process 

characteristics for operating the once-through design at 90.6 o

C (195 °F). 

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature (C)

CaSO4 Concentration (as Sodium Chloride), %

Hemihydrate (CaSO4 - 1/2 H20) Gypsum (CaSO4 - H20) Anhydrite (CaSO4)

Gypsum 

Scaling 

Region

Anhydrite 

Scaling 

Region

Hemihydrate 

Scaling Region

Non Scaling 

Region

Recycle MSF Operation

Once Through MSF Operation
Chapter 4: Desalting Processes 

 67

Table 4-3 Process characteristics, MSF systems 

Item Once-through Recycle 

Maximum operating 

temperature (o

C) 90.6 110 

Process recovery (percent) 10 to 15 10 to 20 

Performance ratio (kg/MJ) 3.44 to 4.30 3.44 to 5.17 

Heat transfer coefficients (watts 

per square meter-Kelvin) 

(W/m2

-K) 

2,271 to 3,407 2,207 to 3,407 

Concentrate concentration 

(mg/l) 58,000 62,500 

Energy consumption (mega 

joules/liter) 

- High-pressure steam 

- Low-pressure steam 

- Electricity 

NA 

0.24 to 0.29 

0.026 

0.20 to 0.29 

NA 

0.026 

Distillate quality (mg/l) 0.5 to 25.0 0.5 to 25.0 

Pretreatment 

- Chemical Polyphosphate Acid or polymer 

- Dose rate (mg/l) 4.0 to 6.0 Acid = 140 

Polymer = 5 to 10 

 NA = not available 

4.5.5 MSF Materials of Fabrication 

The materials required to construct the long tube and cross tube designs are quite different. The 

velocity of the concentrate in the long tube design is more than twice that of the cross tube 

design. Thus, tubes in the long tube design are subject to erosion and impingement attacks. 

Carbon steel materials will not withstand the higher velocities or the resulting impingement 

attacks from the concentrate as it exits the flashing orifice in each stage. Therefore, the long tube 

design must be fully clad with stainless steel grade 316L, or equal material. 

The cross tube design allows a much wider flash stage and, thus, considerably lower concentrate 

velocities. Plant operations have determined that for a mass flow of 16,856 kg/hr-m2

(400,000 lb/hr-ft2

), or less, carbon steel can be used without the need for lining or cladding the 

vessel walls or other internal components. Using the 400,000 lb/hr-ft2 design figure may allow 

the use of cheaper material (e.g., carbon steel). On the other hand, a narrow design figure leads 

to a narrow vessel at the expense of cladding. Current practice in many designs is the use of a 

higher design figure with cladding on the carbon steel to provide a longer design life and lower 

maintenance cost. Note that the first 3 to 6 stages in high temperature operations (e.g., at 

maximum operating temperatures of 110 o

C [230 °F]) must still be lined or clad. Higher 

velocities can be used with cladding. 

The materials of construction of each design are given in table 4-4. 
Desalting Handbook for Planners 

68

Table 4-4 Materials of fabrication, MSF systems 

Item Long tube design Cross tube design 

Flash chambers Carbon steel, 316L stainless steel 

clad 

Carbon steel (first three stages SS 

316 clad) 

Flash chamber internal supports Stainless steel, grade 316 Carbon steel 

Condensing section walls Carbon steel, clad with stainless 

steel grade 316L 

Carbon steel, clad with stainless 

steel grade 316L 

Condenser tubing: 

 - Rejection section 

 - Recovery section 

 - Concentrate heater 

70-30 copper/nickel 

90-10 copper/nickel, to 80 o

C 

70-30 copper/nickel above 80 o

C 

70-30 copper/nickel 

70-30 copper/nickel 

90-10 copper/nickel, to 80 o

C 

70-30 copper/nickel above 80 o

C 

70-30 copper/nickel 

Interconnecting piping and water 

boxes 

Carbon steel, 90-10 copper/nickel 

clad 

Carbon steel, 90-10 copper/nickel 

clad 

Tube plates: 

 - Rejection section 

 - Recovery section 

 - Concentrate heater 

70-30 copper/nickel 

90-10 copper/nickel, to 80 o

C 

70-30 copper/nickel above 80 o

C 

70-30 copper/nickel 

70-30 copper/nickel 

90-10 copper/nickel, to 80 o

C 

70-30 copper/nickel above 80 o

C 

70-30 copper/nickel 

Pumps Bronze Bronze 

External structural shapes Carbon steel Carbon steel 

Demisters Stainless steel, grade 316 Stainless steel, grade 316 

Deaerator/decarbonator Carbon steel, rubber lined Carbon steel, rubber lined 

4.5.6 MSF Process Status 

The development status of the two MSF processes are: 

• Once-Through Design.—The MSF process is the oldest of the distillation processes. The 

once-through MSF design is fully developed. Many of these units have been sold over 

the past 30 years. 

• Recycle Design.—Over the past 20 years, the majority of the MSF plants built have been 

of the recycle design, with the cross tube as the preferred arrangement. The long tube 

type was popular in the early 1970s; however, due to many problems (principally 

corrosion/erosion), it has lost its appeal for those designing new MSF plants. 
Chapter 4: Desalting Processes 

 69

4.6 Vapor Compression 

Vapor compression offers higher recovery ratios. Values of up to 50 percent are possible when 

treating seawater. Process performance ratio is moderately high, at up to 7.7 kg/MJ (18 pounds 

distillate per 1,000 Btu) of heat input. 

4.6.1 VC Operating Principle 

The vapor compression process compresses the vapor generated within the unit itself. Two 

methods of compression are employed: 

• Mechanical (mechanical vapor compression [MVC]) 

• Steam (thermo-vapor compression [TVC]) 

In MVC, the compressor is operated by an electric motor or diesel engine. In the steam type, 

high-pressure steam is used to compress the vapor generated in the vessel. The compressed 

steam is then used as the heat source for further vaporization of the feed water. 

In TVC, vapor is generated in the evaporator by the transfer of heat from the compressed vapor, 

as figure 4-22 shows. Hot vapor lies on the inside of the tubes, while the feed water is sprayed 

on the outside surfaces. The vapor thus generated is then compressed to be used for heat in the 

evaporator. The vapor can be compressed by either a mechanical compressor or by the use of a 

steam jet thermo-compressor. In most cases, a mechanical compressor is used. 

4.6.2 VC Process Arrangement 

Two types of VC systems can be purchased in today’s marketplace: horizontal or vertical heat 

exchanger tubing arrangements. VC systems can also be designed for very low temperature 

operation, at approximately 46.1 o

C (115 °F) or for higher temperatures ranging up to 

approximately 101.7 o

C (215 °F). 

4.6.3 VC Process Description 

Figure 4-23 provides the schematic diagram for the VC process. Feed water enters the process 

through a heat exchanger (generally a plate-type for small systems) and is mixed with a portion 

of the concentrate recirculating in the system. The rates of feed to concentrate will be 

determined by the design concentration required. The feed water is then either sprayed over the 

tube surfaces 
Desalting Handbook for Planners 

70

 Figure 4-22 VC schematic 

(in the case of horizontal tube arrangement) or distributed to the tube ends in the vertical tube 

arrangement. A portion of concentrate flowing down the inside tube surfaces is then vaporized 

through the action of heat on the outside surfaces of the tubes. 

The vapor generated is then compressed by mechanical or thermal means. Compressing the 

vapor raises its temperature by a sufficient amount to serve as the heat source. The concentrate 

is removed from the evaporator vessel by the concentrate recirculating pump. This flow is then 

split, and a portion is mixed with the incoming feed, and the remainder is pumped to waste. 

The distillate stream is formed as the vapor gives up its heat, condenses, and is then pumped to 

the post-treatment system. 

The feed water heater gains its heat from the hot distillate and concentrate leaving the unit. This 

heater preheats the feed water. 

A steam supply is required to initiate startup, but once the system is operational, additional heat 

is not required unless the feed temperature or other operating conditions change. 

Operating curves for VC, and their relationship to calcium sulfate scaling curves, are shown in 

figure 4-24. The VC operating curve crosses the anhydrite solubility curve at about 80  ̊C 

(176  ̊F). This temperature can be exceeded in certain situations as anhydrite forms slowly, or 

additives can be used to lower the scaling potential. 

FEED WATER 

VAPOR

DISTILLATE

CONCENTRATE TO RECIRCULATION

VAPOR SPACE

EVAPORATOR VESSEL

COMPRESSOR

TUBE BUNDLE
Chapter 4: Desalting Processes 

 71

Figure 4-23 Mechanical or thermo-compression VC overall process schematic 

Figure 4-24 Calcium sulfate solubility and VC operations 

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature (C)

CaSO4 Concentration (as Sodium Chloride), %

Hemihydrate (CaSO4 - 1/2 H20) Gypsum (CaSO4 - H20) Anhydrite (CaSO4)

Gypsum 

Scaling Region

Anhydrite 

Scaling Region

Hemihydrate 

Scaling Region

Non Scaling 

Region

VC Operation

MECHANICAL 

DISTILLATE PUMP 

VAPOR SPACE 

EVAPORATOR VESSEL 

COMPRESSOR 

TUBE BUNDLE 

STEAM SUPPLY 

DISTILLATE 

FEED WATER SUPPLY 

CONCENTRATE 

BLOWDOWN 

FEED WATER PUMP 

CONCENTRATE 

PUMP 

CONDENSATE 

PUMP 

 STEAM SUPPLY-FOR STARTUP 

THERMOCOMPRESSOR 

PLATE TYPE 

HEAT EXCHANGER 
Desalting Handbook for Planners 

72

4.6.4 VC Process Characteristics 

Table 4-5 gives the process characteristics for low and high-temperature mechanical VC 

systems, and a low temperature unit using a steam thermo-compressor. 

Table 4-5 Process characteristics, VC systems 

Item Low temperature 

(MVC) 

High temperature 

(MVC) 

Low temperature 

(TVC) 

Maximum operating temperature 

(

o

C) 

46.1 101.7 46.1 

Process recovery (percent) 40 40 40 

Performance ratio (kg/MJ) 3.44 to 5.17 NA NA 

Heat transfer coefficient (W/m2

-K) 1,703 to 2,271 NA NA 

Concentrate (mg/l) 58,000 58,000 58,000 

Energy consumption (MJ/m3

) 

 High-pressure steam 

 Electricity use 

None 

0.0172 to 0.0252 

None 

0.0172 to 0.0252 

0.0159 to 0.0238 

0.00132 

Distillate quality (mg/l) <25 <10 <25 

Pretreatment 

 Chemical 

 Dose rate (mg/l) 

Polyphosphate 

0.5 

Acid or 

polyphosphate 

4 to 10 

Polyphosphate 

0.5 

NA = not available 

4.6.5 VC Materials of Fabrication 

The materials required to construct vapor compression units depend on the operating temperature 

and the concentrate. Since VC systems can operate over a wide range of conditions, materials 

will vary greatly. Table 4-6 gives typical materials for the low and high temperatures. 

4.6.6 VC Process Status 

Both the low temperature and the high temperature processes are offered in today’s marketplace: 

• Low Temperature Unit.—The low temperature unit is typically sold in unit sizes up to 

1,900 m3

/d (500,000 gpd). Many plants have been sold around the world, since they have 

special appeal for small remote sites. 

• High Temperature Unit.—The high temperature unit is used in areas where water is 

scarce. Such sites are at power plants, where cooling water has to be conserved. Many 

systems using mechanical vapor compression have been sold. A diesel engine is the 

driver for this type of unit in sizes up to 3,800 m3

/d (1.0 mgd). 
Chapter 4: Desalting Processes 

 73

Table 4-6.—Materials of fabrication, VC systems 

Item Low temperature operation High temperature operation 

Evaporator shell Carbon steel, epoxy coated Carbon steel, clad with 316L stainless 

steel or all 316L stainless steel 

Heat exchanger tubing Aluminum Titanium 

Tube plates Aluminum Carbon steel, clad with Titanium 

Interconnecting piping Nonmetallic Stainless steel, grade 316L 

Feed heater Titanium Titanium 

Pumps Bronze Stainless steel, grade 316 

External structural shapes Carbon steel Carbon steel 

Demisters Stainless steel, grade 316 Stainless steel, grade 316 

4.7 Comparing Distillation with Other Desalting Processes

This section compares the three distillation processes (MSF, MED and VC) with the membrane 

processes. Electrodialysis/electrodialysis reversal (ED/EDR) is compared to distillation and RO 

in section 4.9; and RO is compared to distillation and ED/EDR in section 4.10. 

Distillation is the oldest of the desalting processes and, collectively, the largest selling process, 

with MSF being the technology most often used for seawater desalination. Currently, about half 

of all of the desalting capacity worldwide uses MSF. MSF, however, has its drawbacks. It has a 

higher capital cost and higher operating and maintenance costs than other desalting processes. 

For this reason, MSF is generally the process of choice for dual-purpose (electric power and 

potable water production) facilities and for applications that cannot be performed by RO or EDR 

treatment, such as highly saline feed waters (greater than 50,000 mg/l TDS) and for situations 

where the feed water conditions would adversely affect the performance and life of the 

membrane. 

Recent improvements in the MED and VC technologies have combined to lower capital costs 

and reduce the amount of auxiliary power consumed, making these processes economically 

competitive with MSF distillation for all but the largest dual-purpose installations. The higher 

performance ratios offered by the new MED designs have simplified the plant design and 

contributed to lower capital cost, compared with MSF. 

4.8 Electrodialysis 

Electrodialysis is one of the two common membrane processes in desalination. ED is based on 

selective movement of ions in solutions. ED uses a direct electric current to transfer ions 

through a membrane that possesses fixed ionic groups chemically bound to the membrane 

structure. ED is primarily used in desalting brackish waters. Electric energy is consumed in 
Desalting Handbook for Planners 

74

proportion to the quantity of salts to be removed. Economics usually limit its application to feed 

waters of less than 10,000 mg/l TDS. However, in certain applications, ED’s special 

characteristics may promote the technology over RO. 

The electrodialysis reversal process is based on the same principles of electrochemistry as ED. 

The fundamental difference in operation is the periodic automated reversal of polarity and cell 

function. This change is typically done three to four times per hour to reverse the flow of ions 

across the membrane. This action improves the tolerance of the technology to operations 

treating scaling-prone or turbid feed waters. EDR has largely replaced ED in the U.S. and in 

some overseas markets. 

4.8.1 ED/EDR Process Fundamentals 

Electrodialysis is a membrane process governed by the ASTM standards listed in Appendix C. 

The ED/EDR process is based on the ability of semipermeable membranes to pass select ions in 

a solution of ionized salts, while blocking others. Salts are in solution as ionized particles with 

positive or negative charges (for example, sodium chloride as Na+

 and C1-). See Chapter 3 for a 

basic discussion. When a direct current is imposed on the solution, the positive ions migrate to 

the negative electrode, or cathode. The negative ions migrate to the positive electrode, or anode. 

A cation permeable membrane allows positive ions to pass, but blocks negative ions. An anion 

permeable membrane does the opposite—allows negative ions to pass, but blocks positive ions. 

ED does not remove colloidal matter, matter that is not ionized, or bacteria. 

Figure 4-25 shows an electrodialysis schematic. The two types of membranes create alternately 

salt-depleted and salt-enriched solution streams. Multiple cell pairs between an anode and a 

cathode comprise a “stack.” An anion membrane, a diluting spacer, a cation membrane, and a 

concentrating spacer comprise a repeating unit called a “cell pair.” 

The introduction of polarity reversal (EDR) increased the utility of the ED process. During the 

reversal, the concentrated layers that have formed against the membranes in the concentrate 

compartments are dissipated. The reversals also reduce the tendency for scaling. Product water 

is not collected during a short interval immediately after reversal. Periodic polarity reversals and 

simultaneous interchanging of the product and concentrate streams provide better control of 

scaling and colloidal buildup. This permits operation at higher levels of supersaturation for 

scale-forming compounds without using anti-scalent chemicals. 

Figure 4-26 gives an example of an ED stack assembly. The manner in which the membrane 

stacks are arranged is called “staging” (based on Meller, 1984). The purpose of staging is to 

provide sufficient membrane area and retention time to remove a specified fraction of salt from 

the demineralized stream. Two types of staging are used: hydraulic and electrical. In a stack 

with one hydraulic stage and one electrical stage, each increment of water makes one pass across 

the membrane surface between one pair of electrodes and exits. In membrane stacks (such as 

those manufactured by Ionics), water flows in multiple parallel paths across the membrane 

surfaces, and a single pass consists of flowing through one water flow spacer, between 
Chapter 4: Desalting Processes 

 75

Figure 4-25 ED schematic 

two membranes, and 

exiting through the outlet 

manifold. In a sheet-flow 

stack (such as those 

manufactured by Asahi 

Glass), water enters at 

one end of the stack and 

flows as a sheet across 

the membrane to exit at 

the other end in a single 

pass. 

Figure 4-27 gives 

examples of hydraulic 

staging. Additional 

hydraulic stages must 

be incorporated to 

increase the amount of 

salt removed in an 

ED/EDR system, as 

shown in the left 

schematic of figure 4-27. 

Figure 4-26 ED stack assembly 

Feed In

Concentrate In

Electrode Feed

(-) Cathode

Top End Plate

Electrode Waste

Cation Transfer Membrane

Demineralized Flow Spacer

Anion Transfer Membrane

Concentrate Flow Spacer

(+) Anode

Electrode Feed

Bottom End Plate

Electrode Waste

Product

Concentrate Out
Desalting Handbook for Planners 

76

Figure 4-27 Examples of hydraulic and electrical staging 

Electrical staging is accomplished by inserting additional electrode pairs into a membrane stack. 

This gives flexibility in system design, which provides maximum salt removal rates while 

avoiding polarization and hydraulic pressure limitations. An example of electrical staging is 

shown in the middle schematic of figure 4-27. 

4.8.2 ED and EDR Stack Design 

The basic ED stack consists of an inlet feed water channel, semipermeable membranes, spacers, 

two electrodes, and end plates to form a rigid device. The edges of the cell pairs (two 

membranes and one spacer) are sealed by the pressure applied to the end plates by tie rods. Each 

electrode is connected to a source of direct current. The spacers separate the membranes and 

contain and direct the flow of water uniformly across the exposed face of the membrane. 

Spacers are generally about 1 mm thick and designed to cause turbulent mixing. Stacks are 

arranged either vertically or horizontally. 

Cation membranes usually consist of an appropriate polystyrene that has been sulfonated to 

produce fixed ionic groups within the membrane. The anion membrane has quaternary 

ammonium groups fixed in a similar polymer. In water, these fixed ionic groups ionize to form 

mobile counter ions. Under an applied direct current, the mobile ions readily exchange with ions 

of the same charge from the water solution. Table 4-7 shows selected properties of some 

commercial ED/EDR membranes. 

Hydraulic

stage 1

Hydraulic

stage 2

Hydraulic

stage 1

Electrodes

Electrode

Hydraulic

stage 2

Hydraulic

stage 1

Electrode

Electrode

Hydraulic

stage 3

Electrode

Electrical

stage 1

Electrical

stage 2

Electrode

Electrode

SOURCE: ROSTEK ASSOCIATES, INC. 2000.
Table 4-7.—Selected properties of commercial ED/EDR membranes 

 Note: IEC = Ion Exchange Capacity, meq/g = milliequivalent per gram 

 Source: Water Treatment Membrane Processes, McGraw-Hill, 1996. Ionics data updated 2002

 77 

Membrane Type 

Structure 

properties 

IEC 

(meq/g) Backing 

Thickness 

(mm) 

Gel water, 

(percent) 

Area resistance 

0.5 N NaCl 

(25 oC, Ω cm2) 

Asahi Chemical Industry Company Ltd. Chiyoda-ku, Tokyo, Japan K 101 Cation Styrene/DVB 1.4 Yes 0.24 24 2.1 A 111 Anion Styrene/DVB 1.2 Yes 0.21 31 2-3 Asahi Glass Company Ltd., Chiyoda-ku, Tokyo, Japan CMV Cation Styrene 2.4 PVC 0.15 25 2.9 AMV Anion Butadiene 1.9 PVC 0.14 19 2-4.5 ASV Anion Univalent 2.1 0.15 24 2.1 Ionics Inc., Watertown, MA 02172 67 HMR Cation Acrylic 2.1 Acrylic 0.57 46 2.8 64 LMP Cation Acrylic-DVB 2.4 Polypropylene 0.56 42 6.5 61 CMR Cation Styrene-DVB 2.1 Acrylic 1.2 40 15 69 HMP Cation Acrylic-DVB 2.1 Polypropylene 0.63 49 6 204 SZRA Anion Acrylic 2.4 Acrylic 0.56 46 3.5 204 UZRA Anion Acrylic 2.8 Acrylic 0.57 40 3.7 103 QDP Anion Styrene-DVB 2.18 Polypropylene 0.54 36 4.1 Tokuyama Soda Company Ltd., Nishi-Shimbashi, Minato-Ku, Tokyo 105, Japan CL-25T Cation 2.0 PVC 0.18 31 2.9 ACH-45T Anion 1.4 PVC 0.15 24 2.4 CMS Cation Univalent >2.0 PVC 0.15 38 1.5-2.5 ACS Anion Univalent >1.4 PVC 0.18 25 2-2.5 
Desalting Handbook for Planners 

78

4.8.3 ED/EDR Power Consumption 

The ED/EDR process uses electrical power to transfer ions through the membranes and to pump 

water through the system. Two, or sometimes three, pumping stages are involved in the process. 

Although high heads are not required, pumping power can sometimes be significant. To desalt 

brackish water at economical current densities, about 2 kilowatthours (kWh) are used for ion 

transfer per 3.785 m3

 (1,000 gallons) of product for each 1,000 mg/l reduction in salinity. For 

pumping, the power required depends on the concentrate recirculation rate, the need for both 

product and waste pumping to discharge, and the pumping equipment’s efficiency. 

Rectifiers are used to convert alternating current to direct current for ion transport through the 

membranes. The ability to adjust individual stack voltage is important, and variable transformers 

are generally incorporated into the power supply for this purpose. The direct-current power 

required is proportional to the quantity of salt being removed. 

Power requirements are affected by temperature. Hot feed water lowers resistance and, 

therefore, reduces power consumption. As a rule of thumb, one can assume a 1 percent decrease 

in power consumption with each 0.5 °C (0.9 °F) rise above 21 °C (70 °F), and a 1 percent 

increase with each degree below 21 °C (70 °F). Membranes available today are limited to 38 °C 

(100.4 °F) feed water temperature; the practical lower feed water temperature is 10 °C (50 °F). 

4.8.4 ED/EDR Process Variables 

Stack current can be predicted by the theoretical relationships of Faraday's and Ohm's laws: 

One Faraday is the amount of electric energy required to transfer 1 gram equivalent of salt. 

F = 96,500 ampere-seconds = 26.8 ampere-hours 

For ED, Faraday’s Law can be written thus (Ionics, 1984): 

 I = F Qp ) N

 e Ncp

where: 

I = direct current, amperes 

)N = change in normality of dilute stream between inlet and outlet 

F = Faraday’s constant 

Qp = flow of dilute stream 

e = current efficiency 

Ncp = number of cell pairs 
Chapter 4: Desalting Processes

79

For Ohm’s Law: 

 E = IR 

where: 

E = potential across stack 

I = direct current, amperes 

R = system resistance 

For an ED/EDR stack, R is the combined resistance of the membranes and the water filling the 

spacer flow paths. 

In practice, the actual current required is proportional to the reduction in salt content for a given 

flow rate. The actual current includes losses for current leakage through the stack manifold and 

water transfer through the membrane. Other process variables include polarization, current 

density, and stack voltage. 

Polarization.—This phenomenon occurs when the ion transport through the membrane exceeds 

the arrival of replacement ions at the membrane surface. Several results are possible: 

• Changes in pH throughout the solution 

• Loss of current efficiency 

• Increase in resistance 

• Ionization of the water with H+ and OH-, causing severe stack malfunction 

Current Density.—Current density is the current per unit area of available membrane through 

which the current passes. In theory, the higher the current density, the smaller the membrane 

required and the lower the capital cost. Beyond a limiting current density for each application, 

polarization can occur. 

Stack Voltage.—The required voltage depends on the resistance of the stack and on current 

density. Voltage is varied manually, based on the required current density. 

The extent to which the feed water is desalted depends on the residence time within the stack and 

the current density. Current densities are kept as high as possible to increase production. The 

high electrical resistance that results when the salt content of the liquid is sharply reduced limits 

the actual current flow. On the concentrate side of the membrane, high salt content may cause 

scale formation. 
Desalting Handbook for Planners 

80

4.8.5 ED/EDR Equipment 

In addition to the ED/EDR stacks themselves, a complete process train requires the equipment 

described below. In some cases, a system for preconditioning the feed water may be required. 

These requirements are in addition to the normal equipment supplied as part of the ED/EDR 

process. Pretreatment requirements are addressed in chapter 5. 

Cartridge Filters.—These filters are usually supplied as a last line of defense against large 

suspended solids entering the process. The nominal separation is typically 10-20 microns. The 

design flow rate for a 25-cm (10-inch) long filter is generally 15-19 liters per minute (4-5 gpm) 

to minimize pressure loss. 

Electrode System.—A chemical system typically supplied with the ED/EDR equipment to 

control the process conditions at the electrodes, where chemical reactions occur. Hydrogen ions, 

chlorine, and oxygen can be formed at the anode; hydrogen gas and hydroxyl ions can be formed 

at the cathode. Therefore, the pH is depressed at the anode and elevated at the cathode. In ED, 

hydrochloric acid is typically added to the cathode electrode stream to prevent scaling. In EDR, 

frequently alternating polarity largely eliminates the need for adding acid to the cathode stream. 

However, in both ED and EDR, the electrode streams are degassed to remove the hydrogen, 

oxygen, and/or chlorine that are generated. Electrode streams are then vented to atmosphere 

with the appropriate environmental controls. 

Cleaning System.—In most cases, with a properly designed plant, membrane cleaning should be 

infrequent. However, for both ED and EDR, clean-in-place systems are normally provided to 

circulate either hydrochloric acid solution for mineral scale resolution or sodium chloride 

solution with pH adjusted for organics removal. In most plants, facilities and space are also 

provided for stack disassembly and hand-cleaning operations. 

Control System.—Most ED/EDR systems today are controlled by microprocessor-based 

programmable control systems (PLC). These systems measure and/or control stack voltage, flow 

rates and pressures of the flow streams, conductivity of the dilute and concentrate streams, 

system recovery, and pH. For EDR plants, the PLC also will time polarity and flow reversal, 

operate valves, and provide sequencing for multi-unit operation. 

4.8.6 ED/EDR Plant Layout 

A single electrodialysis stack can remove from 25 to 60 percent of the TDS, depending on the 

feed water characteristics. Further desalting requires that two or more stacks be used in series; 

these series are referred to as stages. A separate power supply is used for each stage. 

Figure 4-28 shows a schematic of an ED plant operating with three stages. Feed water is 

pumped through a pretreatment section and then through successive membrane stacks. A portion 
Chapter 4: Desalting Processes

81

Figure 4-28 Three-stage, two-line ED/EDR schematic 

of the resulting concentrate is recycled to improve system performance. The concentrate, which 

is removed from the system, is replaced by make-up feed water. Using an equal number of 

stacks in parallel, as shown, can double production. 

The number of stages required to treat a given water is usually determined by an economic 

analysis, based on the feed water chemical analysis and the desired product water quality. 

Primary factors are hardness, alkalinity, TDS, temperature, and the presence of particularly 

troublesome ions, such as iron and manganese. 

4.8.7 ED/EDR Membrane Scaling and Fouling 

ED/EDR only removes ions. Therefore, any bacteria, colloidal material, or silica present in the 

feed water stream will remain in the product stream. Table 4-8 lists pretreatment requirements 

for EDR systems. With continuous ED plant operation, fouling and scale deposits may form on 

the membrane surfaces. The amount of deposits depends on feed water quality. Fouling and 

scale result in an increase in stack resistance and power requirements. Techniques for cleaning 

in place have been developed to increase the interval between stack disassembly for manual 

cleaning. 

Polarity reversal (EDR) reduces or eliminates the need for adding acid to the feed water; thus, 

EDR has less stringent pretreatment requirements. The periodic reversing of polarity and cell 

function helps to clean surfaces of scaling and fouling materials. Periodically or continuously 

rinsing the electrodes removes gases formed at the electrodes. 

Blowdown

Feed water Product

Concentrate Recycle

2nd

Stage

1st

Stage

3rd

Stage

1st

Stage

2nd

Stage

3rd

Stage

Source: RosTek Associates, Inc. 2001
Desalting Handbook for Planners 

82

 Most waters containing up to 150 mg/l of silica do not impair ED/EDR operation. 

Table 4-8.—Pretreated feed water quality goals for EDR 

Issue Measurement Pretreatment goal 

Suspended matter Turbidity <0.5 nephelometric turbidity units 

Silt Density Index <15 (15 minutes) 

Iron 0.3 mg/l 

Manganese 0.1 mg/l 

Ionic content 

Sulfide 0.1 mg/l 

Organics Consult equipment supplier 

Residual chlorine 0.5 mg/l free or 2.0 mg/l total 

Scale inhibitor (mg/l) May be used if required. Consult 

equipment supplier. 

Chemical additives 

Acidification (pH) May be used to reduce concentrate 

LSI. Consult equipment supplier. 

Minimum feed temperature 1 o Temperature C 

Maximum feed temperature 43 oC 

Saturation LSI +2.1 maximum in concentrate 

Calcium sulfate, CaSO4 Up to 6.25 x Ksp in concentrate 

Barium sulfate, BaSO4 Up to 150 x Ksp in concentrate 

Strontium sulfate, SrSO4 Up to 8 x Ksp in concentrate 

Solubility product 

Calcium fluoride CaF2 Up to 500 x Ksp in concentrate 

Solubility Silica Up to saturation in feed 

The cost of pretreating the feed water for electrodialysis will vary with feed water quality. A 

well producing soft water with no oxidized iron or manganese will require minimum 

pretreatment. Pretreatment costs are related to the salt concentration that can be permitted in the 

concentrate stream. Scaling occurs when the concentrate stream becomes saturated with the less 

soluble alkaline scale or nonalkaline scale, such as calcium sulfate. Pretreatment is required to 

remove the scale potential. 

Alkaline scale—calcium carbonate and magnesium hydroxide—tends to form on the concentrate 

side of the membranes. Adding acid to the feed water so that the LSI does not exceed 2.5 in the 

concentrate stream is considered adequate control for EDR systems. Calcium sulfate scale is 

prevented by limiting the concentrate’s concentration to levels no greater than 150 to 200 percent 

of the saturation level, unless operating with scale inhibitor in the recirculating concentrate. 

Pretreatment for removing iron is recommended where feed waters contain more than 0.3 parts 

per million of iron. Iron and manganese are removed with potassium permanganate, or lime 
Chapter 4: Desalting Processes

83

softening. Filtration removes the insoluble ferric and manganese hydroxides, Fe(OH)3 and 

Mn(OH)4, that form. 

Since natural waters contain a certain amount of suspended solids, most electrodialysis plants 

have 5 to 25 micron polishing filters immediately before the feed inlet to the unit. 

The following lists normal pretreatment of the feed waters of electrodialysis units in the order 

normally used: 

1. Iron and manganese removal 

2. Acid addition 

3. Final polishing filters 

It may be desirable to chlorinate the feed water to control slime or to oxidize certain constituents. 

Normal practice involves removing residual chlorine, prior to the membrane units, by the use of 

activated carbon filters or sulfite addition. Some modern membranes, however, exhibit some 

chlorine tolerance, making dechlorination unnecessary in some instances. 

4.8.8 ED/EDR Membrane Life 

Membrane life significantly influences the process economics of ED/EDR. Membrane 

replacement can be tedious and time consuming, particularly if not all membranes in a stack are 

to be replaced. 

Since ED/EDR membranes are flat sheet forms of ion exchange resins, they are subject to the 

same characteristics as ion exchange resins, except mechanical bead damage. There are two 

different types of membranes, anion and cation. Cation membranes typically last longer than 

anion membranes. Anion exchange membranes are particularly susceptible to oxidation by 

chlorine and other strong oxidants. Under some circumstances, styrene-based anion membranes 

can become irreversibly fouled with organics found in surface and reclaimed wastewater 

(Elyanow, et al., 1991). Acrylic-based membranes introduced with EDR in 1981 solved these 

problems, and they generally outperform their predecessors. 

Because of the conditions existing within the spacer flow paths, some waters will induce 

bacterial growth in the stacks. Attempts at disinfecting the stacks may cause membrane damage. 

‘Hot spots” or shorts inside the stack will also damage the membrane, requiring stack 

disassembly and replacement of membranes and spacers damaged by the influence of the “hot 

spot.” 

In general, a 10-year overall membrane life is realistic for determining membrane replacement 

needs. In clean well water applications, membrane life over 10 years is possible, given a 

properly and conservatively designed plant. Effective and timely cleanings-in-place will also 

extend membrane life and improve product quality and power consumption. 
Desalting Handbook for Planners 

84

4.8.9 ED/EDR Electrode Life 

Electrode materials have changed over the years as experience and process understanding have 

increased. Material science has developed new techniques for plating and deposition. Electrode 

life in the last decade varied with the application and type of feed water, capability of the 

operators, and other factors. It was not unusual to get a consistent 5-year life from a platinumplated electrode in an unidirectional plant. Anode life was typically less than cathode life. 

However, with the advent of EDR, the electrode life was reduced at both ends of the stack. 

Ionics, Inc., undertook an extensive research and development program designed to produce an 

electrode that has reasonable life, is reasonably inexpensive, and is relatively electrically 

efficient. Typically, electrode life is now 2 to 3 years. Electrodes can be reconditioned. 

4.9 Comparing ED/EDR with Other Desalting Processes 

Although ED was originally conceived as a seawater desalting process, the major success of the 

technology has been in desalting brackish water. 

ED has several advantages over low-pressure RO, its major competitor in the brackish water 

market. ED does not separate nonpolar substances, so ED is not as constrained by silica 

concentration in the feed water. Because of the open-channel design, a significant level of 

suspended materials can be tolerated in the feed water if the reversal feature is incorporated. 

With improved spacer design and membrane formulas, electrical efficiency has been much 

improved. The reversal process permits inherently higher recovery with a given feed water, and 

product quality can be tailored to a certain extent. The process operates at relatively low 

pressure, and stacks are fabricated predominantly from corrosion-resistant material. 

On the negative side, the ED process is not a barrier to organics or micro-organisms. Therefore, 

ED receives no credit for reduction in cyst or virus populations when it comes to compliance 

with the Safe Drinking Water Act, and ED does not remove taste and odor compounds. Energy 

consumption increases rapidly with increases in TDS. There is limited tolerance for oxidizable 

components in the feed water, such as hydrogen sulfide, H2S, and iron, Fe. 

The ED/EDR process should always be considered as an alternative to RO if the application can 

be addressed competently by both processes. In many cases, the advantages of ED/EDR may 

outweigh its disadvantages because of a particular factor, such as enhanced recovery. This was 

the case in Suffolk, Virginia, in 1990 (Werner and Gottburg, 1998). The amount of feed water 

was limited and under regulatory control, and high silica limited RO recovery to 85 percent. As 

a result of side-by-side pilot testing, a decision was made to construct a 14,400 m3

/d (3.8 mgd) 

EDR plant. This plant has a recovery rate of 94 percent. 
Chapter 4: Desalting Processes

85

4.10 Reverse Osmosis and Nanofiltration 

The RO and nanofiltration (NF) processes use hydraulic pressure to force pure water from saline 

feed water through a semipermeable membrane. Current RO technology governed by the ASTM 

standards listed in Appendix C can be used for desalinating both seawater and brackish water. 

The membranes used in the RO process are generally either made from polyamides or from 

cellulose sources. 

Cellulose acetate membranes in both flat sheet and hollow fine-fiber configuration are still 

manufactured. The composite polyamide flat sheet product of several manufacturers dominates 

modern membrane technology. New formulas are constantly being developed. Unfortunately, 

many fail to meet the basic criteria for commercial success: stable performance for a long period 

of time, inexpensive to make with high yields, and repeatable characteristics. For further details 

on membranes, see Chapman-Wilbert (1998). 

4.10.1 RO and NF Process Fundamentals 

Osmosis is a natural process in which water passes through a semipermeable membrane from a 

solution with low salt concentration to a more concentrated salt solution. Plants use this 

phenomena to draw in water from the soil. The driving force for this passage of water is known 

as the osmotic pressure. Osmotic pressure depends on the difference in salt concentrations of the 

two solutions, as shown in figure 4-29. The pressure head is equal to the osmotic pressure at the 

point at which there is no net pressure of water through the membrane. If pressure greater than 

the osmotic pressure of the solution is applied to the concentrated solution, pure water passes 

through the selectively permeable membrane from the concentrated solution but not the 

dissolved salts. Figure 4-30 shows how this reverse osmosis works. 

Figure 4-29 Osmotic pressure 

Osmotic Pressure Head }

Semipermeable 

Membrane

Freshwater Saline

Water
Desalting Handbook for Planners 

86

Figure 4-30 Osmosis and reverse osmosis 

Semipermeable membranes are not perfect. Some passage of salt will accompany the passage of 

water. Flux and salt passage are the two key descriptive parameters of RO and NF membranes 

that affect membrane performance. Flux is often characterized by the water (solvent) coefficient, 

and salt passage is referred to as salt (solute) transport coefficient. The water transport 

coefficient is proportional to the net applied pressure, while the salt transport coefficient is a 

function of the membrane material itself. Flux and rejection depend on operating conditions, 

while transport coefficients are supposed to be intrinsic qualities of the membrane. Thus, these 

terms are not really synonymous with their respective transport coefficient terms. 

In simple terms, flux is the rate at which the solvent passes through a unit area of the membrane, 

and it is usually expressed as liters/square meter/hour (gallons/square foot/day). Salt passage 

relates the product water quality for a specific ion or TDS to the feed water quality and is merely 

a percentage. Care must be taken in stating the rules under which this calculation takes place. A 

salt passage of 1 percent of feed means that a 1,000 mg/l of feed water will yield a product water 

TDS of 10 mg/l. However, a system of salt passage incorporates the concentration factor and 

will be a lower value for the same membrane and feed water. 

Some expressions describe the passage of solvent and solute through a membrane. The 

following equation expresses how a solvent (such as water) passes through a RO membrane: 

Low

Concentration

High 

Concentration

High 

Concentration

Osmosis

Reverse 

Osmosis

Semi-permeable membrane Semi-permeable membrane

If a solution of high concentration is separated from an equal mass of 

low concentration solution by a semi-permeable membrane...

When both vessels are at the same pressure, pure water migrates through the 

semi-permeable membrane from the low concentration vessel to the high 

concentration vessel. The solution in the high concentration vessel is diluted. 

Flow will stop when the hydraulic head on the high concentration side is equal to 

the solution's osmotic pressure.

If pressure is applied to the high concentration side in excess of the osmotic 

pressure, pure water will migrate through the semi-permeable membrane from 

the high concentration vessel to the low concentration vessel. Flow will stop 

when the hydrostatic head in the low concentraiton vessel equals the osmotic 

pressure of the high concentration solution.

Source: Chapman-Wilbert et al, 1998

Low

Concentration
Chapter 4: Desalting Processes

87

Qw = Kw ()P-)Π)A/T 

where: 

Qw = water flow rate through membrane 

Kw = membrane water permeability coefficient 

)P = hydraulic pressure gradient across the membrane 

)Π = osmotic pressure differential across the membrane 

A = active membrane surface area 

T = membrane thickness 

Solute passage through a membrane is also described. The following equation expresses how a 

solute (such as salt) passes through an RO membrane:

Qs = Ks ()C)A/T 

where: 

Qs = salt flow rate through membrane 

Ks = membrane salt permeability coefficient 

)C = salt concentration difference across membrane 

Clearly, pressure plays an important role in the flow of solvent, but there is an absence of any 

pressure component in the solute passage equation. As pressure increases, the solvent 

permeation rate increases, but the solute passage remains constant, thus giving an improved 

product water quality. 

Several mechanisms impede salts from passing through a membrane—referred to as salt 

rejection. Feed water properties that have the most influence on membrane salt rejection are: 

• Ionic valence.—Rejection increases with valence number; di- and tri-valent ions are more 

thoroughly rejected than mono-valent ions. 

• Molecular size.—Rejection increases with increased molecular size. 

• Hydrogen bonding tendency.—Rejection decreases for compounds with strong hydrogen 

bonding (e.g., water and ammonia). 

• Dissolved gases.—Gases are permeable in their free state (e.g., carbon dioxide and 

hydrogen sulfide. 

4.10.2 Comparing RO and NF Membranes 

The RO and NF membranes available today are very similar. It seems that, rather than 

marketing RO and NF membranes, manufacturers are producing different membranes to cover a 

wide range of rejection characteristics. For this discussion, any membrane with greater than 
Desalting Handbook for Planners 

88

95-percent rejection of sodium chloride is considered to be an RO membrane. How does one 

choose which membrane to use for a particular situation? The following questions can be 

answered with a good water analysis and will help with the discussion. 

• What is the TDS? If the TDS is over about 1,500 mg/l, RO will probably be best. 

• What is the target product water quality? If a reduction in TDS greater than 

95 percent is needed, RO will be necessary. 

• What is the percentage of multi-valent ions? If the TDS is made up of mostly multivalent ions, NF may be best. 

• Which constituents exceed primary or secondary drinking water standards? RO 

permeate can often be blended with water from another source. However, if there are 

contaminants that will exceed drinking water standards, even when blended with RO 

permeate, the cost of treating the blend water must be considered. If the cost is high 

and/or there is a high percentage of multi-valent ions, then NF may be a better choice. 

• Are there size and cost restrictions? Systems can be designed to minimize any 

parameter, but there are tradeoffs. 

• What are the concentrate disposal options? In some locations with only marginal 

water quality, surface disposal may be possible if the concentrate TDS is not too high. 

What is considered “too high” depends on the local soils, amounts of precipitation, and 

vegetation grown in the area. In other locations, concentrate volume will be the most 

important factor. 

Note: This discussion comes from Chapman-Wilbert et al. (1998), which contains a more 

indepth comparison of RO and NF membranes. 

4.10.3 RO Membrane Configurations 

Two membrane configurations dominate today’s marketplace for both brackish and seawater RO 

and NF desalting: spiral wound (SW) and hollow fine fiber (HFF). Two other configurations, 

tubular and plate and frame, are rarely found in desalting applications but are used extensively in 

food processing and industrial applications. 

4.10.3.1 Spiral Wound 

Spiral wound elements are constructed from flat sheet membranes. Modern flat sheet 

membranes consist of a backing material to provide mechanical strength, a thin dense active 

layer, and a porous spongy layer to support the active layer. Membrane material may be 

cellulosic (cellulose acetate membrane) or noncellulosic (composite membrane). For cellulose 

acetate membranes, the two layers are different forms of the same polymer, referred to as 
Chapter 4: Desalting Processes

89

“asymmetric.” For composite membranes, the two layers are completely different polymers, 

with the porous substrate often being polysulphone. 

In the spiral wound design, the membrane is formed in an envelope that is sealed on three sides. 

A supporting grid, called the product water carrier, is on the inside. The envelope is wrapped 

around a central collecting tube, with the open side sealed to the tube. Several envelopes, or 

leaves, are attached with an open work spacer material between the leaves. This is the 

feed/concentrate, or feed-side spacer. The leaves are wound around the product water tube, 

forming spirals if viewed in cross section. Each end of the unit is finished with a plastic 

molding, called an “anti-telescoping device,” and the entire assembly is encased in a thin 

fiberglass shell. Feed water flows through the spiral over the membrane surfaces, roughly 

parallel to the product water tube. Product water flows in a spiral path within the envelope to the 

central product water tube. A chevron ring around the outside of the fiberglass shell forces the 

feed water to flow through the element. A diagrammatic sketch of the device is shown in 

figure 4-31, with assembly in a pressure vessel shown in figure 4-32. 

4.10.3.2 Hollow Fine Fiber 

The HFF design places a large number of hollow fiber membranes in a pressure vessel. The 

hollow fiber material may be a polyaramid or a blend of cellulose acetates. The membranes have 

an outside diameter of about 100 to 300 microns and an inside diameter between 50 and 

150 microns. Normally, the fibers are looped in a U-shape, so both ends are imbedded in a 

plastic tubesheet. The pressurized saline water is introduced into the vessel along the outside of 

the hollow fibers. Under pressure, desalted water passes through the fiber walls and flows down 

the inside of the fibers for collection. The HFF design is illustrated in figure 4-33. 

Figure 4-31 Spiral wound element construction 
Desalting Handbook for Planners 

90

Figure 4-32 Spiral wound membranes and vessel assembly 

 Figure 4-33 Hollow fine fiber permeator schematic 

SNAP RING

O-RING SEAL

FEED

CONCENTRATE 

END PLATE

FIBER

FLOW SCREEN

OPEN END OF FIBERS

PERMEATE

POROUS BLOCK

POROUS FEED DISTRIBUTOR

EPOXY TUBE SHEET

OUTLET
Chapter 4: Desalting Processes

91

The DuPont Company, an RO pioneer, and the only major U.S. manufacturer of HFF permeators 

since Dow Chemical withdrew in the 1980s, recently announced that it would no longer 

participate in the RO membrane market. This leaves only Toyobo as a major HFF supplier, but 

its products have rarely been used in the U.S. 

4.10.3.3 Tubular Configuration 

The tubular configuration (figure 4-34) uses a porous tube. The tube’s diameter varies, 

depending on the application and manufacturer. The membrane is cast, usually on the inside of 

the tube, from a wide variety of polymers. Some membranes are dynamic (i.e., the membrane 

material itself can be placed on the support medium in situ). These devices can be used with 

many different membrane types, and the membrane material can be “repaired,” an advantage in 

many applications. Tubes are usually bundled and inserted into a housing with single feed, 

permeate, and concentrate ports. 

Figure 4-34 Tubular membrane schematic 

Tubular membrane devices are generally more costly and less space efficient than HFF or SW 

devices. Consequently, this design is used primarily in industry and food processing, 

applications for which the configuration is well suited. 
Desalting Handbook for Planners 

92

4.10.3.4 Plate and Frame Configuration 

The fourth configuration is the plate and frame. This configuration (figure 4-35) is not widely 

used for the production of potable water, although it does appear with some frequency in several 

industrial niche markets, such as food processing and treating landfill leachate. 

Figure 4-35 Plate and frame schematic 

The plate and frame configuration looks like a plate and frame filter press. A flat sheet 

membrane (typically the same polymers as used in SW elements) is placed in the device with an 

open feed spacer and a once-through flow path. This design feature permits introducing feed 

water with relatively high suspended solids. These devices typically have small capacity and the 

highest cost of any RO device per unit capacity. In Kuwait, a significant effort was expended to 

develop cost-effective plate and frame systems for seawater desalting, but without success. 

4.10.3.5 Considerations in Membrane Configuration Design 

One of the major considerations in membrane configuration design is concentration polarization. 

As pure water is forced through a membrane, a layer of high salt concentration builds up against 

the feed side of the membrane. This local concentration of salt can reach levels several times 

greater than that in the bulk stream. This buildup, called “concentration polarization,” can cause 

several problems: 

• The osmotic pressure at the membrane surface is much higher than the pressure in the 

bulk stream, thereby reducing the net driving pressure 

FEEDWATER

PERMEATE

CONCENTRATE

IN

OUT

OUT

MEMBRANE ENVELOPE

WITH POROUS SUPPORT

O-RING SEALS

TIE RODS
Chapter 4: Desalting Processes

93

• Salt transport through the membrane increases (because of the increased salt 

concentration gradient), resulting in product quality deterioration 

• Compounds like calcium sulfate and calcium carbonate are more concentrated and more 

likely to precipitate 

One effective way to combat concentration polarization is to maintain adequate feed/concentrate 

velocities through the unit. This can be done by carefully adhering to the membrane 

manufacturer’s recommendations for minimum flow rates and maximum pressure drops during 

the process design phase. 

4.10.4 Power Consumption 

The electrical energy consumed by the RO process is primarily for pumping. In the last 

10 years, the net driving pressure (NDP) required for permeation has been significantly reduced, 

and the in-process electrical energy needs have declined. 

The NDP required for any given membrane application in RO and NF is a function of both the 

osmotic pressure change and hydraulic resistance, and can be calculated from the following 

expression: 

)Pnet = PF – ()P/2 + Πave + Pp - Πp) 

where: 

)Pnet = net driving pressure 

 PF = membrane feed pressure 

)P = pressure drop across the feed/concentrate side 

Πave = average osmotic pressure, feed/concentrate side 

 Pp = product water back pressure 

Πp = product water osmotic pressure 

An approximation of this equation is: 

)P = PF – PC 

PC = concentrate pressure 

Product water osmotic pressure, Πp, is generally ignored, since it is small compared to the other 

terms. Product water back pressure, PP, is becoming more significant as the NDP for 

commercial brackish water membranes declines. In a typical brackish water application, PP can 

range from 3 to 10 m (approximately 10 to 30 ft) of water column, as the water is directed to 

either a ground storage tank or a degassifier. 
Desalting Handbook for Planners 

94

The NDP required is influenced by the concentration factor in the process and, thus, the water 

recovery. Recovery (R) is the term used to describe the efficiency of the process in terms of 

yield, and is usually measured as the fraction of feed water recovered as permeate. That is: 

R = (QP/QF ) 100 percent 

Temperature also impacts energy consumption in RO feed pumps, since temperature affects flux, 

and flux impacts NDP. Flux increases as NDP increases. 

High efficiency of both the motor and the pump should be the primary guideline in selecting feed 

pumps. Variable frequency drives now are commonplace in brackish water RO plants. These 

frequency drives should be selected on the same basis. Typical feed pump energy requirements 

for brackish water RO plants range from 0.5 to 2 kWh/m3 (1.9 to 7.6 kWh/kgal). Requirements 

for seawater RO plants are now typically less than 3 kWh/m3

 (11.4 kWh/kgal) with the use of 

energy recovery devices. 

Some examples of power requirements are shown in table 4-9. 

Table 4-9.—Typical RO feed pump power consumption 

Location 

Feed TDS 

(mg/l) 

Recovery 

(percent) 

Temperature 

(

o

C) 

Feed 

pressure 

(bars) 

Feed pump 

power 

(kWh/m3

) 

Feed pump 

type 

Jupiter, FL1 (Phase I) 5,000 75 21 24 1.125 VT 

Jupiter, FL2 (Phase II) 5,000 75 21 614.4/17.2 0.650 VT 

Cape Coral, FL Plant 2 1,300 85 28 12.5 0.454 VT 

Kill Devil Hills, NC3 3

2,300 75 20 18.2 0.828 VT 

Santa Barbara, CA SSW 40 10-15 60-65 3.5-4.0 HMS 

Key West, FL4

 SWW 30 20-28 55-60 4.0-4.5 VT 

Arlington, CA4

 1,200 77 21 14.5 0.515 VT 

Marco Island, FL5 ~10,000 75 21 6

23.1/27.2 1.111 VT 

 1

 Hydranautics CPA-2 

 2

 Hydranautics ESPA with interstage boost 

 3

 Feed water now ~ 4,000 mg/l TDS 

 4

 With energy recovery, reverse running turbine between pump and motor 

 5

 Uses hydraulic TurbochargerTM as interstage boost 

 6

 First and boosted second stage pressures 

 Note: VT = vertical turbine, can type 

 HMS = horizontal, multi-stage with energy recovery turbine 
Chapter 4: Desalting Processes

95

Energy recovery has become fairly commonplace in the specification of seawater RO systems. 

There are four basic types: 

• Pelton Wheel 

• Work exchanger 

• Pressure exchanger 

• Hydraulic Turbocharger 

Figure 4-36 shows photos of three of these energy recovery devices in operation. In general, the 

energy recovered is between 25 and 35 percent of the input energy for seawater RO (Oklejas, et 

al., 1996). 

The large seawater plants being built today in Spain, Trinidad, and at Tampa Bay, Florida all use 

Pelton Wheel energy recovery devices. In these sizes, 454 m3

/hr (2,000 gpm) and larger, 

recovery efficiency is high, above 80 percent in most cases. The pressure exchanger is currently 

used for smaller systems and has even higher efficiency (above 90 percent). Turbocharger 

efficiency is currently between 60 and 70 percent and is also size limited, with the largest unit 

currently in production sized for 409 m3

/hr (1,800 gpm). 

Until the introduction of “ultra” low-pressure brackish water membranes, there was little 

incentive to include energy recovery devices in the process design, except where the TDS was 

high and recovery was low. However, given the characteristics of current low-pressure 

membranes, reduced feed pressure with interstage boost using energy recovery devices has been 

successfully employed at some locations (Duranceau et al., 1999; Nemeth and Tomkins, 2001). 

4.10.5 RO Process Variables 

Recovery is the most important process variable, and the one the other variables largely depend 

on. Recovery controls the concentration factor, CF: 

 CF = 1/(1-R) 

where: 

 R = recovery expressed as a fraction 

Figure 4-37 shows the concentration factor as a function of recovery, assuming a perfect 

membrane. Using these values will result in a slightly conservative design. 
Desalting Handbook for Planners 

96

Figure 4-36 Three types of energy recovery devices in use today

Small Pelton Wheel, serving two small SWRO units

Hydraulic Turbocharger Heat Transfer Chamber (HTC®

)

Work exchanger
Chapter 4: Desalting Processes

97

Figure 4-37 Solute concentration factor as a function of recovery 

Recovery is limited by water chemistry for brackish systems and pressure requirements for 

seawater systems. Other process variables that must be considered are: 

• Flux.—The value of flux will determine the membrane area to be used and the feed 

pressure requirement. Flux also impacts the rate of fouling. 

• Feed rate per vessel.—This rate will also impact the feed pressure requirement by 

impacting the system pressure drop. It establishes cross-flow velocity and affects 

concentration polarization. 

• Minimum concentrate flows.—Flow across the membrane surface is an important 

consideration in controlling concentration polarization. 

• Product water quality.—Quality can be impacted by the selection of membrane type and 

flux. 

• Temperature.—Temperature is very important in the process design stage, since 

temperature has a significant impact on flux and, therefore, the capital and operating 

costs. While ground water temperatures are relatively constant, seawater temperature 

from open intakes can vary widely. 

0.00

10.00

20.00

30.00

40.00

50.00

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Recovery, Permeate as % of Feed

Concentration Factor
Desalting Handbook for Planners 

98

4.10.6 RO and NF Peripheral Equipment 

In addition to the membrane assemblies themselves, other process equipment includes: 

• Fine filtration.—This usually involves the use of cartridge filters of 5-10 microns 

nominal retention. Figure 4-38 shows horizontal cartridge filters. These filters are not 

designed to perform routine filtration, but to prevent unusual changes in the quality of the 

feed water source (upsets) from damaging either the feed pump or the membranes. 

Sometimes, these cartridge filters are located downstream of the feed pumps. In such 

cases, a fine mesh strainer would be included to protect the pump. 

• Scale control systems.—Most RO systems require adding a scale inhibitor and, 

sometimes, acid for scale control in the membrane array, since at least one constituent of 

the feed water will probably be supersaturated in the concentrate. Systems for storing 

and introducing these chemicals include bulk and day storage tanks, metering pumps, 

calibration tubes, and monitoring and control systems. Proper safety equipment must 

also be provided. Figure 4-39 shows a typical chemical feed system. 

• Cleaning system.—It is good practice to include a cleaning system in the requirements for 

a commercially sized RO system. A typical cleaning system consists of one or two tanks, 

a recirculation pump, a fine filtration device, and monitoring instrumentation. Figure 4-

40 shows a cleaning system. 

Figure 4-38 Horizontal cartridge filters in Jupiter, Florida 
Chapter 4: Desalting Processes

99

 Figure 4-39 Typical chemical feed system 

Figure 4-40 Cleaning system, North Hatteras plant, Dare County, North Carolina 

Acid feed system at Jupiter, 

Florida, showing metering pumps, 

day tank, and calibration columns. 

This acid system serves two 

separate RO plants, each one 

processing 22,700 m3

/d 

(6.0 mgd). 
Desalting Handbook for Planners 

100

4.10.7 RO Plant Layout 

Figure 4-41 shows a simple flow diagram of an RO system. Incoming feed water is given the 

required pretreatment, the RO feed pump raises pressure in the feed water, and the pressurized 

water then enters the membrane assembly. The arrangement of vessels in parallel and series 

groups determines the recovery and capacity. 

Figure 4-41 Simplified RO process flow diagram 

Spiral wound membranes are loaded into pressure vessels. These vessels are made from either 

fiberglass-reinforced plastic or stainless steel. The pressure vessels can hold from one to eight 

standard 1,016-cm (40-inch) elements. Typically, for large brackish water and NF systems for 

potable water production and wastewater reclamation, vessels are selected to hold six or seven 

standard 203-cm (8-inch) diameter by 1,016-cm (40-inch) long membrane elements. For 

seawater systems, it is now more common to find systems using seven-element-long vessels in 

one or two stages, as shown in figure 4-42. A seven-element vessel system requires a space 

about 7.62 m long (25 ft). In addition, at least 1.2 m (4 ft) is required at each end for vessel 

handling, for a total length requirement of 10-10.7 m (33-35 ft). The width depends on the 

capacity of the unit and the type of feed water. Review of actual installations indicates that an 

average plant area of 0.022-0.029 m2 per m3

 (900-1,200 ft2 per million gallons) of output. Table 

4-10 gives guidelines for selecting vessel length based on recovery. 
Chapter 4: Desalting Processes

101

Figure 4-42 Typical arrangements for staging 

Table 4-10.—Typical recoveries for various vessel lengths 

No. of elements 

per vessel 

Maximum recovery per 

stage standard pressure 

(percent) 

Maximum recovery per 

stage low pressure 

(percent) 

4 40 35 

5 50 45 

6 55 50 

7 65 60 

8 75 N/A 

• The membrane product tubes of elements in a vessel are connected together with O-ringsealed interconnectors. Vessels are then grouped in parallel to form stages, and the stages 

are arranged in series to maintain adequate cross flow and minimum concentrate flow. 

• The concentrate control valve maintains the back pressure on the membrane system so 

that the NDP requirements of the stages can be met. In some ultra-low pressure 

applications, product water back pressure on the first stage or interstage boost pumping 

may be required to control flux in the lead membranes and to prevent premature fouling. 

product

concentrate

concentrate

product

product

concentrate

(Taper = 2:1 typical)

(Taper = 3:2:1 typical)

a) Single stage ~ maximum recovery 55 percent

b) Two stage ~ maximum recovery 80 percent

c) Three stage ~ maximum recovery 90 percent
Desalting Handbook for Planners 

102

4.10.8 Membrane Life 

The useful life of RO and NF elements is usually a function of the membrane material and the 

application. For cellulose membranes, the lifespan most commonly used in planning is 3 to 5 

years. However, there have been examples of much longer life, particularly when RO elements 

were used to treat anaerobic ground water. 

Noncellulosic membranes can operate over a much wider range of pH. Most, however, have a 

limited resistance to chlorine and other strong oxidants. Typically, for planning purposes, the 

lifespan for noncellulosic elements is estimated at 5 to 7 years. However, some systems have 

spiral wound elements that are 10 years old and still operational, while it has been reported that 

in some HFF systems, permeator performance has been maintained for over 15 years. 

4.11 Comparing RO and NF with Other Desalting 

Processes 

The RO process is now in daily operation worldwide in applications ranging from producing 

ultrapure water to converting seawater to drinking water. Sizes range from a few m3

/d to the 

massive Yuma Desalting Plant, at 272,500 m3

/d (72 mgd). The largest operating membrane 

plant in the U.S. is an NF plant in Boca Raton, Florida. 

RO for brackish water can remove organics and microorganisms from the feed water. NDP 

requirements have been greatly reduced in recent years, resulting in lower operating costs. 

Conversely, today’s RO membranes have higher salt rejection characteristics, permitting higher 

blend ratios. Suitably treated raw water is sometimes blended with product water to reduce RO 

equipment size and to remineralize low TDS permeate. Specialized membranes, such as NF, and 

ultra-low pressure have widened the application spectrum. 

Generally, RO has lower capital and operating costs than distillation processes. RO feed pumps 

may be driven by steam turbines, allowing coupling to steam producers such as cogeneration 

facilities. Materials selection is important, but not as critical as it would be for the high 

temperature thermal processes. Pretreatment is especially critical for RO, particularly because 

pretreatment adds to the capital and operating costs of the process. 

References 

Chapman-Wilbert, M. et al., “The Desalting and Water Treatment Membrane Manual: A Guide 

to Membranes for Municipal Water Treatment,” (2nd edition). Water Treatment Technology 

Program Report No. 29, Bureau of Reclamation, Denver, Colorado, 1998. 
Chapter 4: Desalting Processes

103

Duranceau, S.J., et al., “Interstage Turbines as Self-Regulating Pressure Control Valves in a 

Desalting Facility,” Proceedings of the American Water Works Association Membrane 

Technology Conference, Long Beach, California, February 1999. 

Eisenberg, T.N., and E.T. Middlebrooks, “Reverse Osmosis Treatment of Drinking Water,” 

Butterworths, 1986. 

Elyanow, D, et al., “Desalination of Brackish Ground Water for a Prairie Community Using 

Electrodialysis Reversal,” Proceedings of the Twelfth International Symposium on Desalination 

and Water Re-Use, Malta, April 15-18, 1991.

Fluid Systems, “Seawater Reverse Osmosis Guidelines,” July 1997. 

Mallevialle, J., et al., “Water Treatment Membrane Processes,” McGraw-Hill, 1996. 

Meller, F., ed., “Electrodialysis and Electrodialysis Reversal Technology,” Ionics, Inc, 1984.

Nemeth, J.E., and B. Tomkins, “Energy Recovery Devices: Comparison and Case Studies,” 

Proceedings of the American Water Works Association Membrane Technology Conference, San 

Antonio, Texas, March 2001. 

Oklejas, E., et al., “Improvements in the Economics of Reverse Osmosis through Advanced 

Pumping and Energy Recovery Technology,” Proceedings of the American Desalting 

Association, Monterey, California, August 4-8, 1996. 

Speigler, K.S., and A.D.K. Laird, “Principles of Desalination, 2nd edition,” Academic Press, 

1980. 

Speigler, K.S., and Y.M. El-Sayed, “A Desalination Primer,” Balaban Desalination Publications, 

1994. 

Wade, N.M., “Technical and Economic Evaluation of Distillation and Reverse Osmosis 

Desalination Processes,” Desalination 93, 1993. 

Wangnick, K., “2000 IDA Worldwide Desalting Plants Inventory, Report No. 16,” International 

Desalination Association, May 2000. 

Wangnick, K., “2002 IDA Worldwide Desalting Plants Inventory, Report No. 17,” International 

Desalination Association, May 2002. 

Watson, I.C., “Membrane System Applications and Economics,” Proceedings of the American 

Water Works Association Conference, Orlando, Florida, 1988. 

Werner, T.E., and A.T.M.Gottburg, “5 Billion Gallons Later – Operating Experience at the City 

of Suffolk EDR Plant,” Proceedings of the American Desalting Association, Williamsburg, 

Virginia, August 2-6, 1998. 

Chapter 5: Pretreatment

105

Chapter 5: Pretreatment 

5.1 Introduction 

Some pretreatment of the feed water for a desalting plant is generally always required. 

Pretreatment ensures that constituents that are present in the raw water supply do not result in a 

loss of performance or a reduction in the output of the facility during normal operation of the 

plant. 

Each desalting technology has different requirements for the quality and condition of water 

entering the process. For distillation processes, the concerns are: 

• Scaling of the heat exchanger tube surfaces 

• Corrosion of the plant components 

• Erosion by suspended solids 

• Effect of other constituents 

For membrane processes, the major concerns are: 

• Membrane fouling and scaling 

• Suspended solids plugging 

• Biological fouling or attack 

• Membrane degradation by oxidation or other means 

5.2 Distillation Processes 

5.2.1 Scaling in Distillation Processes 

In distillation processes, calcium sulfate, CaSO4, magnesium hydroxide, Mg(OH)2, and calcium 

carbonate, CaCO3, in the water can cause scaling on the tube surfaces. Scales form as these 

compounds precipitate out of the feed water. 

5.2.1.1 Calcium Sulfate Scaling 

Removing either the calcium or the sulfate from the water supply is one way to prevent calcium 

sulfate scaling in the pretreatment process. This approach, however, is generally not cost 

effective. The traditional means of controlling calcium sulfate scaling is to operate the plant at 

lower temperatures or use antiscalants. Nanofiltration (NF) pretreatment is another technique 

that is sometimes used to reduce the potential for calcium sulfate scaling for both distillation and 

reverse osmosis (RO) systems (Hassan, 1999). 
Desalting Handbook for Planners 

106

Calcium sulfate has a reverse solubility curve. This means that the solubility decreases with an 

increase in temperature. In other words, the higher the temperature, the greater the rate of 

precipitation and the greater the potential for scale formation. 

5.2.1.2 Calcium Carbonate and Magnesium Hydroxide Scaling

These chemical compounds predominate (or form) at lower temperatures. Scale formation can 

be prevented by removing the bicarbonate ions. The bicarbonate ion, HCO3

-

, is responsible for 

calcium carbonate and magnesium hydroxide scale formation. Therefore, eliminating this ion 

prevents scale from these two compounds. The bicarbonate ion is readily removed by treating 

the feed water with acid. Sulfuric acid, H2SO4, reacts with the bicarbonate ion to produce an 

unstable carbonic acid, H2CO3, which, in turn, decomposes to form water and carbon dioxide 

gas. In order to bring this reaction to completion, the carbon dioxide gas is stripped from the 

seawater feed water in a decarbonator. Alternatively, additives can be used as discussed in 

chapter 4. 

5.2.2 Corrosion in Distillation Processes 

Corrosion within the evaporator depends on: 

• Amount of gases entering unit 

• Temperature of operation 

• Concentration of chloride ion 

• pH 

Pretreating the feed water helps influence these variables to prevent corrosion. 

Amount of gases.—The amount of corrosive gases entering the evaporator can be minimized by 

using a decarbonator and deaerator. The decarbonator will remove carbon dioxide, while the 

deaerator removes oxygen. Sodium bisulfite, or a similar oxygen scavenger, is then added to the 

make-up stream as it exits the deaerator to ensure all oxygen has been removed. 

Temperature of operation.—The maximum temperature of operation will be determined by the 

process type and chemicals used for pretreatment. Polyphosphate is effective for temperatures 

up to 90.6 o

C (195 oF). For operation at higher temperatures, acid or carbolic-type polymers are 

required. These pretreatment systems do not prevent corrosion within the unit. 

Concentration of the chloride ion.—Corrosion due to the concentration of chloride cannot be 

eliminated by pretreatment. To prevent corrosion from chloride ions, construction materials that 

resist this corrosion are required. For example, SS316L cannot be used in applications where the 

chlorine content exceeds 1,000 parts per million (ppm). 

pH.—By adding acid, the pH of the feed water can be lowered enough to lower the concentrate 

of the bicarbonate ion. This is accomplished by lowering the pH to a value of 4.2 to 4.5. 

However, if the carbon dioxide produced by adding acid is allowed to enter the evaporator, it 
Chapter 5: Pretreatment

107

could result in severe corrosion. Carbon dioxide gas is removed in a decarbonator before the 

feed water enters the evaporator. This step results in further elevating the pH to between 5.5 and 

6.0. 

5.2.3 Erosion by Suspended Solids in Distillation Processes 

Sand is the only suspended solid of concern for distillation processes. If sand is allowed to enter 

the evaporator, the tubing surfaces will erode. Such processes will lead to early replacement of 

the tube bundles, with consequent increase in the cost of water. In addition to causing tube 

erosion, sand can plug spray nozzles, which can result in frequent unit shutdowns for cleaning.

5.2.4 Impact of Other Constituents 

Hydrogen sulfide.—Some feed waters (particularly ground water) contain hydrogen sulfide, H2S, 

which must be removed from the feed water before it enters the unit. Hydrogen sulfide reacts 

with materials such as copper and nickel, which are typically used for heat transfer surfaces in 

distillation processes. The sulfides react with the copper/nickel to reduce the heat transfer and 

ultimately lead to tube failure. 

Oil.— This contaminant must also be removed in the pretreatment system. If oil is not removed, 

it will foul the evaporator tube surfaces, resulting in a loss of heat transfer. 

Marine growth.—Marine growth can occur in the feed water intake and supply line. This is 

generally taken care of by adding chlorine early in the feed water supply. Chlorine is added at 

low dose rates of 0.5 milligram per liter (mg/l) or less. Periodic shock treatment at dose ratios as 

high as 5.0 mg/l may also be required. 

Heavy metals.—If the evaporator is constructed with aluminum parts, metals such as copper, 

nickel, and mercury must be removed from the feed water before it enters the evaporator. This is 

accomplished using an “ion trap.” The ion trap is simply a vessel containing small “chips” of the 

same aluminum material used for the fabrication of the unit. The metals react with this 

sacrificial aluminum and are removed in the trap. 

Figures 5-1 and 5-2 show typical pretreatment schematics for operation at low temperature with 

polyphosphate addition or high temperature with acid addition. 
Desalting Handbook for Planners 

108

Figure 5-1 Pretreatment schematic, low temperature design ( T = 90 oC) 

Figure 5-2 Pretreatment schematic, high temperature design ( T = 110 oC) 

5.3 Membrane Processes 

Pretreatment is essential for proper operation of RO and electrodialysis reversal (EDR) 

equipment and can add substantially to the project’s capital and operating costs. Three main 

types of membrane scaling and fouling are considered in this section: 

Pre-chlorination

Oil/Sand Removal

(if required)

Acid Treatment

Ion Trap

To Evaporator Oxygen Scavenger

Intake Chamber

Feed Pump De-Aerator

To Air Ejector

(if required)

Travelling Screen

& Trash Rack

Decarbonator

Pre-chlorination

Oil/Sand Removal

(if required)

Polyphosphate treatment

Ion Trap

To Evaporator Oxygen Scavenger

Intake Chamber

Feed Pump De-Aerator

To Air Ejector

(if required)

Travelling Screen

& Trash Rack
Chapter 5: Pretreatment

109

• Scaling: mainly alkaline and nonalkaline scale, such as calcium sulfate 

• Fouling by metal oxides: mainly iron or manganese, but oxidized hydrogen sulfide is 

also a concern 

• Fouling by suspended solids, colloids, and/or biological growth 

Table 5-1 lists the general pretreatment requirements for RO and EDR membrane systems. 

Pretreatment goals are to control: 

• Biological activity and fouling 

• Metal oxide fouling 

• Mineral scale 

• Silica precipitation 

• Colloidal fouling 

• Particulate fouling 

Table 5-1 Pretreated water quality requirements for membrane processes 

 Spiral CA Spiral PA EDR 

Suspended matter 

a) Turbidity, NTU <1.0 <1.0 <5 

b) SDI <4.0 <4 <15 

Ionic content 

a) Iron, mg/l (ferrous) <2.0 <2.0 <0.1 

b) Manganese, mg/l <0.5 <0.5 <0.1 

c) Strontium, in conc percent saturation 2,000 percent 3,000 percent 4,000 percent 

d) Barium, in conc percent saturation 5,000 percent 5,000 percent 7,000 percent 

e) Silica, mg/l (w/o inhibitor) in concentrate <160 <160 <saturation in feed 

Chemical Additives 

a) Residual Chlorine, ppm <1.0 ND ND 

b) Scale Inhibitor, mg/l in concentrate 12-18 12-18 As required 

c) Acidification, pH 5.5-6.0 4-10 As required 

Temperature, saturation, solubility 

Maximum feed temperature oC 40 45 43 

Maximum LSI with scale inhibitor Note +2.4-+2.8 2.1 

Solubility product (CaSO4) with scale inhibitor, 

percent saturation 

150 percent 150 percent 650 percent 

NA = Not applicable 

ND = Nondetectable 

SDI = Silt Density Index 

NTU = Nephelometric turbidity units 

Type of membrane: 

PA = Polyamide 

CA = Cellulose acetate 

EDR = Electrodialysis reversal 

 Note: CA membranes require acidification to pH 5.5 to 6.0 to reduce the rate of hydrolysis. Therefore, the 

Langelier Saturation Index of the exiting concentrate tends to be low enough that scale inhibitor for calcium 

carbonate is not necessary. 
Desalting Handbook for Planners 

110

5.3.1 Scaling for Membrane Processes 

Calcium carbonate, CaCO3, is the most common form of mineral scale found in membrane 

process operations. CaCO3 precipitation takes place toward the end of an RO system or in the 

later stages of an electrodialysis/electrodialysis reversal (ED/EDR) system. The tendency for 

CaCO3 scaling has been traditionally predicted by the Langelier Saturation Index (LSI) method 

(Langelier, 1936). 

LSI = pH (actual) – pHs 

where: 

pHs = pH of solution if it were in equilibrium with CaCO3, i.e.: 

pHs = pCa + pAlk + C(T,TDS) 

where: 

pca = log of Ca++ concentration 

pAlk = log of HCO3 alkalinity 

C(T,TDS) = constant to include temperature and TDS 

At higher ionic strengths (e.g., seawater), the Stiff and Davis Index is a more accurate predictor 

of scaling tendency. 

SD = pH (actual) – pHSD

where: 

 SD = Stiff and Davis Index 

 pHSD = pCa + pAlk + K(T,IS)

where: 

 K = constant to include temperature and ionic strength. 

If pH > pHs (or pHSD), then the water is saturated with calcium carbonate, CaCO3; if pH < pHs, 

then the water is unsaturated. A positive value indicates a tendency toward precipitation. 

In RO systems using modern scale inhibitors, an LSI of < +2.4 in the exiting concentrate is easily 

controlled. Some manufacturers claim an even higher threshold of +2.8 to +3.0. Carbonate scale 

is readily redissolved by circulating a muriatic acid solution, or lowering the pH of the feed 

water during operation. An LSI nomograph is provided in figure 5.3. 
Chapter 5: Pretreatment

111

Figure 5-3 Langelier Saturation Index nomograph 

For EDR plants, CaCO3 scale is rarely a problem. If it occurs, cleaning with hydrochloric acid 

and adding a small amount of scale inhibitor to the concentrate recirculation system can be used 

to reduce and eliminate the problem. 

Sulfate scales of calcium, strontium, and barium are also important factors in RO and ED/EDR 

operations. Before synthetic scale inhibitors were available, the recommended procedure was to 

keep the calcium sulfate saturation to less than 100 percent. Nowadays, a value of twice 

saturation is controllable, provided a reliable, monitored system for scale inhibitor addition in the 

right amount is included in the design. Barium and strontium sulfates, BaSO4 and SrSO4, are 

extremely insoluble but, fortunately, they tend to form very slowly and, thus, are controllable at 

high levels of supersaturation. Commonly used values for BaSO4 are 30 to 70 times saturation. 

In EDR systems, a much higher level of supersaturation is possible in some cases. 

In predicting the solubility limits of sulfates, it is important to remember two points: 

1. Modern RO membranes reject divalent ions very well. Therefore it is reasonable to 

assume a zero percent salt passage when calculating the concentration factor, CF. 

2. Compounds are more soluble in the concentrate than in the feed water. The solubility 

product constant, KSP1, of each compound increases with ionic strength. 

3 Ca ++ as mg/l CaCO

mg/l CaCO3

HCO as 3

3 pHCO

o

o

o

o

o

o o

o o

o

o o

o o

o o

o o

43.3 C (110 F)

37.8 C (100 F)

32.2 C (90 F)

26.7 C (80 F)

21.1 C (70 F)

15.6 C (60 F)

10 C (50 F)

4.4 C (40 F)

0 C (32 F)

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

1.8

1.7

1 2 5 10 20 50 100 200 500 1000 5000

3 as CaCO

= 774.8 mg/l

LSI = +1.91

b pH

 = 6.28 pHS

C = 2.37

 = 1.81 3 pHCO

pCa = 2.10

 (use 5000 mg.l)

TDS = 7853.6 mg/l

_

HCO3

3 CaCO

 = 800 mg/l as

 

++ Ca

pH = 7.6

o Temp = 20 C

Example

1.0

2.0

3.0

4.0

5.0

pHCO and pCa Scale 3

= 8.19

mg/l

pCa

TDS Constant "C"

"C" Scale
Desalting Handbook for Planners 

112

As a rule of thumb, scale inhibitor dosages for RO systems are calculated as concentrations in 

the concentrate of 12-18 mg/l. This value is then converted to a feed water dosage using the CF 

for the design recovery, and assuming 0 percent salt passage. In ED/EDR systems, the scale 

inhibitor is added to the recirculating concentrate stream, while an equivalent amount is “wasted” 

in the concentrate blowdown. 

SI(f) = SI(c) / CF 

where: 

SI(f) = scale inhibitor concentration in the feed water, mg/l 

SI(c) = scale inhibitor concentration in the exiting concentrate, mg/l 

CF = the concentration factor at the operating recovery 

Table 5-2 shows the characteristics of some commercially available scale inhibitors. It is 

important to remember that any product added to drinking water systems must have National 

Sanitation Foundation (NSF) certification under NSF Standard 60, Drinking Water Additives. 

Although temperature is seldom an influence in membrane scaling control strategies, it is 

important to remember that CaCO3 has a somewhat inverted solubility curve after about 35 o

C. 

In some U.S. locations (e.g., Laredo, Texas; and Mount Pleasant and Hilton Head, South 

Carolina), the feed water temperature for RO is >35 o

C (95 oF). 

In addition to calcium carbonate and sulfate scales, the potential exists, at least theoretically, for 

other soluble compounds (e.g., calcium fluoride and calcium phosphate) to cause problems in 

membrane desalting applications. These can usually be controlled by using the same techniques 

that work to control the more common scaling salts. 

5.3.2 Metal Oxide Fouling in Membrane Processes 

The oxides of several metals are extremely insoluble. Oxides of ferric iron, Fe3+, and 

manganese, Mn3+, are of principal concern in membrane plants. RO is generally more tolerant of 

feed waters containing iron and manganese than EDR, since the electrochemical nature of the 

EDR process results in releasing oxidants in the EDR stack. However, the reversal process has 

increased the ED process’ tolerance to these metals in the feed water. 

In RO applications, it is recommended that the amount of iron and manganese in the feed water 

be limited to no more than 2.0 mg/l of iron and no more than 0.5 mg/l of manganese. Higher 

concentrations may cause problems of co-precipitation with other constituents, such as silica. 

This is particularly important for RO drinking water systems, since the concentrations of iron 

and manganese are limited by the secondary drinking water standards. 
Chapter 5: Pretreatment

113

Table 5-2. —Typical commercial scale inhibitors 

Active ingredient 

Trade name Designation Type % wt 

Specific 

gravity pH Appearance Application 

PC-191 Blend NA 1.36 10.5 Pale yellow liquid General 

PC-391 Blend NA 1.10 11.0 Clear, colorless 

liquid 

Small flow systems/ 

recycle systems PermaTreatTM1

PC-510 Blend NA 1.22 9.7 Amber liquid Silica control 

100 NA 35 1.17 3.5 Pale yellow liquid Inorganic scale FloconTM 

260 NA 35 1.16 <2.0 Pale yellow liquid Dispersant 

600 Blend 35 1.2 3.0-4.0 Water white to 

amber 

Inorganic scale 

control 

820 Blend 37 1.2 4.3-5.3 Water white to 

amber 

Dispersant and 

metal ion stabilizer 

1025 Blend 36.5 1.15 2.5-5.3 Water white to 

amber 

Inorganic scale 

control and 

dispersant 

Aquafeed 

Antiscalent TM2

1405 Blend 29 1.1 4.1-5.1 Water white to 

amber liquid 

Silica and silicate 

control 

MDC 120 Poly-acrylic 

acid NA 1.05-1.15 3.56 Pale to amber liquid General 

MDC 150 

Polyelectrolyte 

blend 

NA 1.1-1.2 2.56 Pale to dark amber 

liquid Inorganic scales 

HypersperseTM3

MSI310 

Polyelectrolyte 

polymer 

blend 

NA 1.05-1.15 4.8-5.5 Pale to amber liquid Silica 

0100 Blend NA 1.03-1.09 1-2 Colorless liquid Inorganic scales and 

gels 

1100 Blend NA 1.04-1.13 2.6 Colorless liquid Inorganic scales 

NSF Blend NA 1.35-1.55 5.3 Light brown liquid Inorganic scales 

Silica Blend NA 1.03-1.09 1-2 Colorless liquid Reactive silica 

2000 Blend NA 1.03-1.13 10.0 Light brown liquid Inorganic scales 

Pretreat PlusTM4

Y2K Blend NA 1.04-1.14 1-2 Colorless liquid Inorganic scales and 

gels 

0200 Blend NA 1.14 1.5 Pale yellow liquid Inorganic scales and 

gels 

0300 Blend NA 1.07 2.0 Amber liquid Inorganic scales and 

gels 

0400 

Concentrate Blend NA 1.3 7.0 Pale yellow liquid Inorganic scales and 

gels 

ProTec RO Blend NA 1.03 5.2 Straw colored liquid Colloidal silica and 

silt 

ProTec RO-B Blend NA 1.04 7.5 Colorless liquid Microbial colloids 

ProTec RO-C Blend NA 1.07 8.0 Yellow liquid Colloidal organics 

and sulfur 

ProTec ROTM5

ProTec RO-D Blend NA 1.02 6.0 Colorless to light 

yellow liquid Mixed colloidals 

 1 Information from Nalco, Inc., 2002. PermaTreat is a registered trademark of Ondeo Nalco, Ltd. 

 2

 Information from Noveon, Inc., 2002. 

 3

 Information from GE Betz, Inc., 2002. 

 4

 PreTreat Plus are antiscalants. Information from King Lee Technologies, 2002. 

 5

 ProTec RO are antifoulants. Information from King Lee Technologies, 2002. 
Desalting Handbook for Planners 

114

For EDR, the iron concentration should be limited to 0.3 mg/l, and manganese to 0.10 mg/l. 

Relatively high levels of iron may also influence the effectiveness of scale inhibitors, possibly 

leading to premature precipitation of scale-forming compounds. 

When designing the membrane plant, the feed water system must be kept anaerobic, so that iron 

and manganese remain in the soluble divalent or “-ous” state, rather than be oxidized to the 

insoluble trivalent, or “-ic” state. This condition assumes no other design influences. 

5.3.3 Biological Fouling in Membrane Processes 

Biofouling is a significant concern resulting in a loss of flux and an appreciable increase in the 

hydraulic pressure drop through the feed concentrate spacer. Although biofilm formation is 

generally a precursor to biofouling, biofilm can exist on the membrane surface in the absence of 

detectable biofouling (Costerton et al., 1985). 

Even in clear, brackish well waters, the potential for biofouling can exist. In the early days of 

RO development, cellulose acetate membranes were susceptible to attack from a variety of 

bacteria. Holes appeared in the membrane surface, resulting in a rapid increase in both flux and 

salt passage. More typical, however, was the loss of flux due to biofouling over days or weeks, 

resulting in a change in salt passage and a significant increase in power requirements. Chlorine 

disinfection can be used to control biofouling in situations where it can be shown that chlorine in 

small dosages does not harm the membranes. 

A list of the mechanisms that may lead to biofouling is incorporated into table 5-3. A diagram of 

the adhesion to the membrane surface is shown in figure 5-4. 

With the advent of polyamide and other noncellulosic membrane materials, the use of chlorine, 

or other strong oxidants within the membrane system becomes unacceptable, due to the potential 

for membrane degradation. However, these membrane materials typically are resistant to 

biological attack. The challenge then is to prevent or control the formation of biofilm on the 

membrane surface (Costerton et al., 1985). 

A significant body of work exists, describing efforts to develop techniques for preventing the 

adhesion of biofilm in a dynamic, operating environment. Evaluating disinfection strategies and 

increasing focus on mechanical and process design issues are alternative approaches. Biofilm 

adhesion interference has been demonstrated, but very few techniques are available that do not 

cause significant and permanent loss of membrane performance. Feed water disinfection, 

usually with chlorine followed by dechlorination, likewise has had mixed results. A significant 

amount of work with CA membranes has been undertaken by the Bureau of Reclamation in 

conjunction with maintenance of CA membranes at Yuma, Arizona (Henthorne and Lichtwardt, 

1996). 
Chapter 5: Pretreatment

115

Table 5-3 Principal events in membrane biofouling processes 

Event Time To onset1

 Description/explanation 

Primary organic 

film 

Seconds/minutes Typically referred to as the conditioning film; defined as the 

rapid adsorption of dissolved organic macromolecules and 

inorganic substances at the membrane/liquid interface. 

Primary cell 

adhesion

 

Seconds/minutes Refers to pioneer bacterial attachment; depends on nature 

of cell surface, membrane type, feed water chemistry, and 

system hydrodynamics; provides major contribution to early 

biofilm accumulation. 

Cellular 

detachment

 

Seconds/minutes Influences biofilm accumulation rate; detachment is 

sometimes enhanced by micro-biocidal agents, 

dispersants, etc. 

Cell growth/ 

multiplication 

Minutes/hours Occurs at expense of soluble and sorbed feed water 

nutrients; may provide greatest contribution to biofilm 

formation where biocides are not present. 

Biopolymer 

(EPS) synthesis 

Minutes/hours Provides for greater biofilm structural integrity; acts as a 

reactive transport barrier to chemical biocides; promotes 

nutrient concentration/storage. 

Particle/colloid 

entrainment 

Seconds/minutes Secondary effect where suspended particles and colloidal 

material are passively entrained in the biopolymer matrix or 

within biofilm void spaces. 

Secondary cell 

adhesion 

Days/weeks Commences after primary biofilm formation by pioneer 

cells; probably strongly influenced by surface properties 

and physiology of primary biofilm and leads to greater 

species diversity. 

Biofilm sloughing Days/weeks Refers to cell and biomass detachment; occurs in response 

to changes in hydrodynamic shear or turbulence forces, or 

introduction of biocides, dispersants, etc. 

Biofilm 

senescence 

Weeks/months Refers to accelerated cell die-off in old biofilms; cell death 

is in equilibrium with biofilm growth in continuous flow 

systems; may result in release of soluble nutrients via cell 

lysis. 

1

 Time after a membrane is placed into operation. 

Prevention and/or control of biofouling are typically site specific. Some process and design 

steps to assist in biofouling control: 

• Thorough source water investigation 

• Good process design 

• Good mechanical design 

• Thorough understanding of water quality 

• Good operator training 

• Selection of effective disinfectants and biocides 

• Routine system disinfection 

• Maintain anaerobic conditions throughout process 
Desalting Handbook for Planners 

116

Figure 5-4 How a bacterium sticks to the membrane surface 

5.3.4 Suspended Solids in Membrane Processes 

While generally not a consideration with properly constructed and maintained ground water 

systems, suspended solids can be a major concern when treating surface water or reclaiming 

wastewater. 

Suspended solids in feed water for membrane processes can consist of inorganic materials, such 

as clays, insoluble metal oxides, and organic substances such as colloidal color. 

Conventional water treatment practices (shown in table 5-5) can be used to reduce or remove the 

suspended solids from the feed water, with varying degrees of success. Early examples of this 

are the pretreatment system for Water Factory 21, operated by the Orange County Water District 

in Southern California, and the two-stage filtration systems commonly employed as pretreatment 

for seawater RO in the Middle East and elsewhere. 

Cartridge filtration is generally not used for suspended solids removal because of the cost. 

However, for small seawater applications, single-stage direct filtration followed by two-stage 

cartridge filtration has been employed with some success. 

Precoat filters, using diatomaceous earth, were used for seawater RO pretreatment for a short 

time. This technique was very effective, but difficulties with equipment resulted in the release of 

material through the septum into the membranes, causing rapid membrane fouling. A high level 

of operator attention was needed to avoid this, and the process is no longer used. 

Cell approaches membrane

Initial weak

attachment

Time Time

RO membrane

Cell 

multiplication

EPS1 biosynthesis

stabilizes attachment

1 Source: RosTek Associates, Inc. Extracellular Polymeric Substance
Chapter 5: Pretreatment

117

Table 5-4.—Disinfection techniques for biofouling control 

Biocide 

category Examples 

Concentration 

range 

Membrane 

compatibility Comments 

Chlorine 0.1-1.0 mg/l CA, PS 

Monochloramine 0.5-5.0 mg/l All 

Peracetic acid 0.1-1.0 mg/l CA, PS 

Hydrogen 

peroxide 0.1-1.0 mg/l All 

Oxidizing 

Iodine 

The oxidizing biocides listed are 

used primarily as feed water 

additives. Because 

monochloramine exhibits reduced 

oxidizing activity compared to free 

chlorine, it does not harm 

performance of PA membranes. It is 

an excellent biocide, especially with 

respect to biofilms. 

Formaldehyde 0.5-5.0 percent All 

Glutaraldehyde 0.5-5.0 percent All 

Bisulfite 1.0-100 mg/l All 

2-methyl-4-

isothiazolin-3-one 

0.01-1.0 

percent All 

Quaternary 

amines 

0.01-1.0 

percent CA, PS 

Benzoate 0.1-1.0 percent All 

Nonoxidizing 

EDTA 0.01-1.0 

percent All 

Sodium bisulfite is the only reducing 

agent currently used as a feed water 

additive. Sodium bisulfite acts to 

prevent growth. 

All others listed are used primarily to 

preserve membranes from 

biodegradation/biodeterioration 

during plant inactivity. 

Ultraviolet 1-2 MR1 All 

(conditional) 2 Irradiation 

Gamma All 

(conditional) 2 

Disinfection by irradiation is very 

effective but leaves no biocide 

residual following exposure; thus, 

surviving bacteria may regrow on 

membrane surfaces. Gamma 

radiation is particularly suited for 

disinfection of new membrane 

modules (sealed in plastic wrap prior 

to long-term storage). 

 1 MR = megarads 2 Compatibility depends on radiation dosage, temperature, pH redox potential, and other factors. 

Table 5-5.—Techniques for removing suspended solids 

Technique Unit operation 

Cartridge filters 

Precoat filters 

Screening Physical straining 

Membrane filtration (MF)/Ultrafiltration (UF) 

Multimedia filters – up flow Gravity and pressure granular filtration Multimedia filters – down flow 

Sedimentation 

Gravity separation Clarification 

Chemically assisted clarification 

Dissolved air flotation Other 

Cyclone separators 
Desalting Handbook for Planners 

118

Cyclone separators are routinely used to remove sand from ground water. These devices are 

effective, relatively inexpensive, and particularly well suited to anaerobic waters in which there 

is the potential for metal oxide or sulphur fouling.

If the source water is subject to periodic surges of high turbidity and suspended solids loading, it 

may be necessary to include some kind of settling basin prior to filtration. 

Organics are typically removed from the feed water by using either a powdered activated carbon 

in a sedimentation basin; coagulation with alum, ferric chloride, or other coagulant prior to 

filtration; or direct filtration with an in-line coagulant feed. Ultrafiltration has also proven to be 

effective in reducing colloidal organic material, given appropriate pore size selection. 

Membrane filtration devices are commercially available in several common configurations:

• Spiral wound 

• Tubular 

• Hollow fiber 

• Capillary 

• Plate and frame 

Of these, the most commonly used in water treatment are the capillary, hollow fiber, and spiral 

wound. Pressure requirements are usually below 2 bar (30 pounds per square inch gauge (psig)) 

with typical transmembrane pressures of 1-1.5 bar (15-22 psig). Pressure requirements for spiral 

wound membrane filters tend to be at the higher end of the range. 

Membrane filtration systems are normally supplied complete with automatic controls for 

backwashing and, in the case of some hollow fiber devices, with on-line membrane integrity 

testing. 

Common characteristics are: 

Microfiltration Pore Size 0.05-0.5 micron 

Materials polypropylene, polysulphone, polyvinylidenefluoride (PVDF), ceramic 

Ultrafiltration Pore Size 0.001-0.1 micron 

Materials polysulphone, PVDF, cellulose acetate, ceramic 

 

Recently, membrane filtration has gained acceptance as a pretreatment method for preparing 

secondary and tertiary effluent as feed water for RO. A significant amount of work has been 

done at the Orange County Water District; at Aqua 2000 in San Diego, California; at Livermore, 

California; and at Scottsdale, Arizona. The only large-scale application of membrane filtration 

for seawater is the recent renovation of the Ad Dur SWRO plant in Bahrain, using UF 

membranes. 
Chapter 5: Pretreatment

119

Over the past several years, research and pilot testing have been conducted in Europe, principally 

the Netherlands, combining MF with RO or NF to treat river water. As a result, a large plant 

consisting of UF followed by low pressure RO is now operational at Heemskerk in the 

Netherlands (Kamp, et al., 1999). 

5.3.5 Other Considerations for Membrane Processes 

Each source of feed water to a membrane plant presents its own unique treatment challenges. 

Silica and hydrogen sulfide are particularly troublesome when it comes to causing operational 

problems and membrane damage. 

5.3.5.1 Silica 

Silica occurs in varying concentrations in all natural waters. RO membranes reject silica, but 

silica is largely unaffected by passage through an EDR system. Consequently, silica 

precipitation is not of concern in EDR plants. 

In RO feed water sources, silica can occur in three different forms: 

• Monomer silica, or silicic acid, Si(OH)4: commonly referred to as soluble or reactive 

silica 

• Polymerized silicic acid: commonly referred to as colloidal or unreactive silica 

• Particulate silica 

Generally, RO operations require the silica concentration in the concentrate to be less than 

120 mg/l. However, there are RO facilities that have operated successfully at higher 

concentrations. This can be done by operating at high pH. Moftah (2002) states that “as pH 

values increase, silica solubility increases and thus silica scaling is alleviated.” 

Heavy metals in the feed water in small concentrations can cause premature polymerization of 

silica as a complex with the metal. 

Because reducing silica in the feed water can be very costly, the primary strategy for controlling 

silica fouling has been by water recovery. Currently, inhibitors are available (table 5-2) that 

allow RO systems to operate at concentrate silica concentrations up to 220 mg/l. It is cautioned 

that, like all operating parameters, site-specific conditions must be considered, and pilot testing 

at design conditions is advised. 

5.3.5.2 Hydrogen Sulfide 

Hydrogen sulfide, H2S, is a frequent constituent of ground water along the coast or at other 

locations where wetlands have existed. Hydrogen sulfide is a by-product of the life cycle of 

sulfate reducing bacteria (SRB) that are common in many ground water systems. 
Desalting Handbook for Planners 

120

Table 5-6 shows the distribution of hydrogen sulfide as a function of pH. H2S occurs in an 

anaerobic environment as both a dissolved gas and (weakly) ionized electrolyte: 

H2S ↔ H+ + HS2HS-

 ↔ 2H+ + S2 - 

Table 5-6.—Distribution of hydrogen sulfide 

species as a function of pH 

Concentration, percent 

pH H2S HS- S2-

4 100 - - 

7.1 (pKa,1) 50 50 - 

9 0 100 - 

14 (pKa,2) 0 50 50 

Hydrogen sulfide in RO plants.—Like all gases, H2S is passed by RO membranes and appears in 

both the permeate and the concentrate. Experience has shown that special attention must be paid 

to the wellhead and raw water piping design, plant piping, and pressure vessel port orientation to 

make sure that air is not admitted prior to the membrane process. It is common practice with RO 

not to deal with H2S in the feed water pretreatment system, since removal from the permeate is 

more effective and less costly. This also reduces the risk of virtually irreversible membrane 

fouling with colloidal sulfur, which is produced as the result of oxidation of H2S, thus: 

2H2S + O2 → 2S↓ + 2H2O 

Hydrogen sulfide in ED/EDR plants.—H2S must be removed from the feed water to ED/EDR 

plants to prevent oxidation in the stack and the resultant membrane fouling. Oxidation occurs by 

reaction with both the chlorine and oxygen produced by electrolysis in the process. 

The most common treatment is air stripping, followed by (usually) chlorination to complete the 

oxidation of the remaining sulfide, and granular filtration. This process has been used with 

success at the 12-mgd EDR plant in Sarasota County, Florida. For efficient air stripping, the pH 

should be between 5.5 and 5.8 with adequate carbon dioxide to maintain a low pH partway down 

the packed tower. Open packing and efficient distribution on both the air and water side is also 

critical. 

In many locations, odor control is required to treat offgas from the air strippers. Typical 

techniques include counter-current scrubbing with sodium hypochlorite or caustic soda and 

proprietary technologies such as the U.S. Filter “Lo-Cat” process. Investigation into using strong 

oxidants such as ozone and hydrogen peroxide as in-pipe pretreatment steps has produced 

encouraging results, but full-scale facilities using this technique have yet to be constructed. 
Chapter 5: Pretreatment

121

References 

Costerton, J.W., et al., “Phenomenon of Bacterial Adhesion,” Bacterial Adhesion: Mechanisms 

and Physiological Significance, Plenum Press, New York, 1985. 

Faust, S.D., and O.A. Aly, “Chemistry of Water Treatment, 2nd Edition,” Lewis Publishers, 1998. 

Hassan, A.M. et al., “Optimization of NF Pretreatment of Feed to Seawater Desalination Plants,” 

IDA World Congress, San Diego, California, 1999. 

Henthorne, L.R., and M.A. Lichtwardt, “Long-Term Storage Materials Compatibility Testing of 

Three Biocides for the Yuma Desalting Plant,” Proceedings of the American Desalting 

Association, Monterey, California, August 4-8, 1996. 

Kamp, Peer C., et al., “Integrated Membrane Systems for Surface Water Treatment: Productivity 

and Fouling Control,” Proceedings of the American Water Works Association Membrane 

Conference, Long Beach, California, 1999. 

Langelier, W.F., “The Analytical Control of Anti-Corrosion Water Treatment,” Journal of the 

American Water Works Association, 28:1500, October 1936. 

Moftah, K., “High pH RO for Wastewater Treatment,” Pollution Engineering, October 2002. 

Chapter 6: Post-Treatment 

123

Chapter 6: Post-Treatment

6.1 Introduction 

Product water from a desalting process requires post-treatment to prepare it for potable uses and 

some industrial uses. Post-treatment practices are generally mandated by law or are included as 

a process step toward regulatory compliance. Regardless of what other post-treatment steps are 

employed, disinfection and maintenance of a chlorine residual in the water distribution system 

are required for all municipal drinking water systems. 

Each State is required to comply with the minimum requirements of the Safe Drinking Water Act 

of 1974, as reauthorized and amended in 1996. Some States have adopted specific, more 

stringent standards of their own. For example, a sodium limit of 160 milligrams per liter (mg/l) 

is a primary standard in Florida. Florida also enforces, as primary, almost all of the Federal 

secondary standards. 

Applicable water quality rules, all of which have been published in the Federal Register, include: 

• Surface Water Treatment Rule 

• Lead and Copper Rule 

• Disinfection/Disinfection By-Product Rule 

The mineral content (dissolved solids in the product water) from desalting processes is quite low. 

Product water from distillation processes typically ranges between 0.5 and 50 mg/l total 

dissolved solids (TDS). Product water from membrane processes can range from 25 to 500 mg/l 

TDS, depending on the application. 

Low concentrations of minerals or lack of the “stability” components calcium and bicarbonate 

alkalinity in any water supply results in water that is “aggressive” or “unstable.” If such a supply 

is not treated, it will attempt to stabilize itself by dissolving (corroding) materials it comes in 

contact with. Therefore, product water must be treated either to reintroduce minerals or to add 

corrosion inhibitors to render it benign. 

Adding calcium and bicarbonate, along with a change in pH, will result in a water supply that 

will not corrode piping, storage tanks, and other components in the distribution system, 

including the end user’s plumbing system. 

Other factors that may promote or enhance corrosion in a desalted water supply are: 

• High temperature 

• Low silica content (although this constituent itself does not promote corrosion, silicates 

are known to provide some corrosion protection) 
Desalting Handbook for Planners 

124

• High oxygen content 

• Free carbon dioxide 

• Low ratio of alkalinity to chloride and sulfate content 

To yield a stable product water, each factor must be evaluated to determine its possible effect on 

the corrosiveness of the water supply. Water quality goals that generally meet these 

requirements include: 

• Total hardness of 40 mg/l (as CaCO3) or higher 

• Alkalinity concentration of 40 mg/l (as CaCO3) or higher 

• Final pH of 8.0 to 9.0 

In addition, corrosion indices, such as the Langelier Stability Index (LSI) should be analyzed to 

evaluate the corrosion potential of the product water. Note that these indices are not definitive. 

The post-treated product should, within reasonable limits, match the existing water supply to 

prevent problems such as aggressive water and excess gas. 

Aggressive water.—Product water can lack minerals and be very aggressive. The mineral 

content of the product water can be increased by either adding chemicals or blending the product 

water with raw water or a combination of these two treatments. Each of these post-treatment 

methods is discussed in this chapter. 

Excess gas.—In some areas of the U.S., gas stripping is also required to remove excess carbon 

dioxide (CO2) and hydrogen sulfide (H2S). 

6.2 Stabilization 

The goal is to make sure that the finished water has been stabilized before it is pumped to the 

distribution system. Stabilized water is water that calcium carbonate does not precipitate from or 

dissolve in. Reintroducing minerals to the water helps reduce the corrosivity of the product 

water. The following chemicals (among others) are commonly used for stabilization: 

• Caustic soda, NaOH 

• Sodium bicarbonate, NaHCO3

• Soda ash, Na2CO3

• Chemical lime (Quicklime, CaO) 

• Hydrated lime, Ca(OH)2

These chemicals are normally dissolved in water before they are injected into the product water. 
Chapter 6: Post-Treatment 

125

Calcium carbonate (marble) “chips” held in pressure vessels have also been found to be an 

effective means to increase the alkalinity and hardness of a water supply. The product water is 

simply pumped through the pressure vessels. As the product water dissolves the calcium 

carbonate, the hardness and alkalinity of the water increase until saturation is reached and the 

reaction stops. 

Limestone beds have sometimes been used for thermal post-treatment, particularly in the Middle 

East. The benefit of this approach is that both calcium hardness and alkalinity are added to the 

water at the same time. 

6.2.1 Chemical Addition 

The amount of chemicals to be added can be determined by the chemical reaction formula for 

alkalinity increase, carbon dioxide reduction, and total hardness increase. This information is 

summarized in table 6-1. 

Table 6-1.—Effect of mineral addition on water quality 

Chemical 

(1 mg/l addition) 

Alkalinity 

increase 

(as CaCO3) 

(mg/l) 

Free CO2

decrease 

(mg/l) 

Total hardness 

increase 

(as CaCO3) 

(mg/l) 

Caustic (98.06 percent) 1.23 1.08 -- (no effect) 

Sodium bicarbonate (100 percent) 0.60 -- (no effect) -- (no effect) 

Soda ash (99.16 percent) 0.94 0.41 -- (no effect) 

Quicklime (90 percent) 1.61 0.41 1.61 

Hydrated lime (93 percent) 1.26 1.11 1.26 

Review of this information indicates that all chemicals do not increase both alkalinity and 

hardness. For example, caustic increases alkalinity but not hardness. Thus, the chemical to be 

used will depend on the water quality to be treated. In other words, if the hardness of the water 

is suitable, caustic, sodium bicarbonate, or soda ash can be used. However, if the hardness must 

be increased, a chemical containing calcium, such as lime, must be used. In some cases, lime 

should be used in combination with caustic or another chemical. 

6.2.2 Corrosion Considerations 

Corrosion indices are measures used to evaluate the corrosivity of the water supply. There are 

many contributing factors to corrosion, including temperature, oxygen content, pH, alkalinity, 

calcium, TDS, organic matter, and specific ions such as chloride, sulfate, and silica. Further, 

these factors affect different materials through a variety of mechanisms. 

As corrosion phenomena are complex, a single index to accurately predict corrosion cannot be 

developed. The following indices provide some indications of the corrosivity of the water: 
Desalting Handbook for Planners 

126

Langelier Stability Index (LSI):

LSI = pH – pHs 

 where: 

pHs = pH of solution if it were in equilibrium with 

 CaCO3 

(see chapter 5) 

Evaluation Factor: 

LSI values greater than zero indicate that water is scale 

forming, and values less than zero indicate that water is 

aggressive. 

Ryznar Stability Index (RSI): 

RSI = 2pHs – pH Evaluation Factor: 

RSI values less than 6.5 indicate that water is scale forming, 

and values greater than 6.5 indicate that water is aggressive. 

Aggressivity Index (AI): 

AI = pH + Log (A*Ch) 

where: 

A = bicarbonate, or Methyl Orange (MO), 

 alkalinity mg/l, as CaCO3 

Ch = calcium hardness mg/l as CaCO3

Evaluation Factor: 

AI values greater than 12 indicate that water is scale forming, 

values less than 10 indicate that water is aggressive, and 

values between 10 and 12 indicate that water is moderately 

aggressive. 

A coupon study is the best method for determining the corrosivity of water. Thin coupons are 

made of the same materials as those of the piping and distribution system. They are inserted at 

various points in the system and are allowed to remain there for a long period of time. 

Periodically, they are removed, examined, and weighed to determine weight loss. From this, the 

corrosion rate can be determined. 

However, since the water supply must be operational before the corrosion rate can be accurately 

determined, corrosion may be mitigated by adding chemicals that form a protective layer (film) 

on the surfaces of pipes and tanks. These chemicals, called inhibitors, reduce the corrosion but 

do not totally prevent it. The three types of inhibitors approved for use in drinking water systems 

are: 

• Chemicals that cause calcium carbonate scale formation 

• Inorganic, or glassy phosphates 

• Sodium silicate 
Chapter 6: Post-Treatment 

127

The choice of a particular type of inhibitor to use in a corrosion control program depends upon 

the specific water quality and the materials to be protected.

6.3 Blending Stabilization 

Stabilization is achieved by increasing the calcium and bicarbonate in the product water. Mixing 

the product water with brackish water containing significant concentrations of calcium or 

bicarbonate from another source can be an effective method for stabilizing the product water. 

The blending ratio must be selected on the basis of a controlling constituent that will limit the 

amount of that constituent (e.g., sodium, organics) in the finished product. Optimum 

stabilization may not be achieved by blending in all cases, and supplemental means, such as 

adjusting the pH, may be necessary. 

A mass balance calculation is used to determine the amount of blending that can be carried out. 

A complete water analysis of each potential blending source must be performed first in order to 

carry out the mass balance. Figure 6-1 shows the mass balance diagram. 

Figure 6-1 Blending mass balance flow diagram 

If the total hardness is less than the water treatment goal of 40 mg/l, calcium will have to be 

added to the finished product water. This can be done by adding Quicklime, hydrated lime, 

calcium chloride, dolomite, or similar soluble calciferous mineral. 

 (Cb,Wb) Stream 3 

Concentration (Cb) and flow rate 

(Wb) for blended water. 

(C1W1) Stream 1 

Concentration (C1) 

and flow rate (W1) 

for the product 

water.

(C2W2) Stream 2 

Concentration (C2 ) 

and flow rate (W2) 

for the blending 

water. These equations may be used for any 

specific ion, or component:: 

W1 + W2 + . . . + Wn = Wb

C1W1 + C2W2 + . . . + CnWn = CbWb
Desalting Handbook for Planners 

128

6.4 Dissolved Gas Stripping 

The product water from electrodialysis reversal (EDR) plants may contain carbon dioxide. 

Product water from reverse osmosis (RO) plants may contain carbon dioxide, CO2, and/or 

hydrogen sulfide, H2S. In EDR plants, hydrogen sulfide must be removed in pretreatment and, 

therefore, will not exist in the product water. 

Blending for TDS stabilization may be done with untreated well water. This blending normally 

occurs prior to air stripping, so the equipment must be sized for the blended flow, rather than for 

just the product water. 

Hydrogen sulfide stripping is not a post-treatment concern for distillation processes or EDR. In 

both instances, hydrogen sulfide must be removed during the pretreatment process. 

Carbon dioxide stripping is also typically a post-treatment process. Natural ground waters 

sometimes contain high concentrations of CO2, and in some desalting applications the feed water 

is acidified, releasing CO2: 

HCO3

-

 + H+ ↔ H2O + CO2↑

Carbon dioxide passes through RO membranes and appears in the permeate in approximately the 

same concentration as in the feed water. 

The most common technique for dissolved gas stripping is to use a packed tower with either 

forced or induced draft. Both CO2 and H2S are typically removed by air stripping in a packed 

tower. The tower must be designed for the worst-case scenario, which is normally removing 

hydrogen sulfide. Removal should be specified to a residual concentration of not more than 

0.1 mg/l. If higher residuals are permitted, the combination of aeration and chlorination may 

generate sufficient colloidal sulfur that turbidity regulations will be violated. Induced draft 

towers tend to be a little quieter, but since the blower and motor are in the gas stream, material 

selection is extremely critical. These towers must be made of some metal or plastic that will 

resist the corrosive effects of the components of the gas stream. 

Figure 6-2 shows a typical forced draft tower. The key factors for specification are: 

• Concentration of CO2 and H2S entering the tower 

• Required residual concentrations leaving the tower 

• Liquid loading rate, usually 30 gallons per minute per square foot maximum 

• Air flow rate 

• Packing type 

• Adequate space above the packing for efficient liquid distribution 
Chapter 6: Post-Treatment 

129

Figure 6-2 Cross section of typical packed tower 

• Adequate space below the packing for efficient air distribution 

• Materials selection 

6.5 Disinfection 

Product water from a desalting process is essentially free of all viruses, pathogens, etc. 

However, such systems must be designed to provide some form of disinfection of the product 

stream to: 

• Ensure a disinfection residual in the distribution system 

• Prevent contaminants from entering the distribution system due to membrane tears or 

leaks 

Blending untreated brackish water for stabilization will also impact the disinfection strategy. 

Since this water has not been exposed to the membrane barrier, it will contain any biological or 

organic species that exist in the raw water. 

Investigators have found that for each contaminant present, a specific time period and 

disinfectant concentration is required. The contaminant(s), rate of inoculation required, and 

concentration of the disinfectant supply all must be known. These combine to give the required 

contact time and dose rate of the disinfectant to apply (concentration and time, or CT). 

AIR OUTLET SCREEN

INLET

AIR DISENGAGEMENT 

ZONE

DEMISTER

DISTRIBUTOR

PACKING

PERFORATED FRP

PLATE

VORTEX BREAKER

OUTLET

AIR DISTRIBUTION 

ZONE

FRP BLOWER

FRP AIR INLET 

DUCT W/DAMPER
Desalting Handbook for Planners 

130

Proper operation of the disinfection system at optimum cost requires the operator to: 

• Monitor the disinfection dose rate 

• Monitor microbial activity 

• Monitor residual disinfection concentration 

Disinfecting the product water can be accomplished in a number of ways: 

• Chlorine application using either gas chlorination, sodium hypochlorite, or calcium 

hypochlorite 

• Chlorine dioxide 

• Ultraviolet (UV) light 

• Monochloramine 

• Ozone 

In the U.S., a chlorine disinfectant residual must be present at all points in the distribution 

system, regardless of the method. One of the difficulties in using chlorine and ozone is the 

potential for the production of disinfection by-products (DBPs) in the distribution system. DBPs 

are formed by the reaction of chlorine or ozone with natural organic material (NOM) that is in 

the water. Chlorinated NOM forms two classes of DBPs of concern: trihalomethanes (THMs) 

and haloacetic acids (HAAs). Both of these classes of compounds are regulated by the Safe 

Drinking Water Act. Desalting processes remove NOM. Therefore, chloramine disinfection is 

probably only suitable for blending applications where NOM may be a problem. 

The potential for THM formation in product water is very low, with the exception of the product 

from EDR. NOM is rejected by RO and nanofiltration (NF) membranes, and organics are 

removed in the distillation processes through the noncondensable venting system. Since EDR 

does not present a barrier to the water, NOM reduction is limited to that fraction of the water 

which is ionized, typically less than 50 percent. 

Use of ozone as a disinfectant has been practiced in Europe for many years and is becoming 

more widespread in the U.S. Ozonation does not form THMs or HAAs, but does create DBPs, 

which must be removed from the water prior to distribution. Removing these DBPs is typically 

performed by absorption on activated carbon, prior to final disinfection with chlorine. 

(AWWARF, 1991) 

Of the treatment schemes listed above, the use of UV light is not considered practical. UV does 

not provide a residual in the distribution system. Chlorine residual in the distributed water is 

required in the U.S. Ozone, although effective, also does not remain as a residual. If ozone is 

used, the disinfection scheme would have to include chlorine or other disinfectant to provide a 

chlorine residual. Since the THM formation potential (THMFP) of product water, except for 

EDR product, is very low, ozonation is not generally required for treating product water. 
Chapter 6: Post-Treatment 

131

Chlorine dioxide can be used for disinfection in those situations where the blending water carries 

a significant THMFP. It is effective for the destruction of organics without forming THMs or 

HAAs in significant amounts. However, regulations limit the chlorite residual in the finished 

water, which limits the dose of chlorine dioxide that can be used. Where THMFP could be a 

problem, monochloramine can be used. Monochloramine is formed in situ by first adding 

chlorine to the water, then quenching the chlorine with ammonia. The contact time of chlorine 

must be carefully controlled to prevent the formation of excessive THMs and HAAs. 

Monochloramine is an effective disinfectant, but its contact time for virus inactivation is much 

longer than the contact time for chlorine. 

In summary, from a practical standpoint, gas chlorination, sodium hypochlorite, and chloramine 

treatment are considered best for disinfection post-treatment. As in most situations, site-specific 

conditions will dictate final disinfection strategies. 

6.5.1 Chlorine Treatment 

6.5.1.1 Chlorine Gas 

When chlorine gas is dissolved in water, it quickly reacts to form hypochlorous acid, HOCl, as 

shown in the following equation: 

Cl2 + H2O = H+

 + HOCl + ClThe weak hypochlorous acid will then dissociate according to the following equation: 

HOCl = OCl-

 + H+

The formations of hypochlorous acid and hypochlorite ion, OCl-

, are pH dependent. The 

distribution of free chlorine between HOCl and OCl-

 is shown in figure 6-3. Both forms act as 

disinfectants, but the acid, HOCl, is 80 to 100 times more effective as a disinfectant (based on

E.Coli kill) than OCl-

 (Snoeyink and Jenkins, 1980). 

For more effective disinfection, the chlorine should be injected into the product stream prior to 

final pH adjustment. It can be seen from figure 6-3 that hypochlorous acid is more prevalent at 

low pH. The lower the pH of the reaction, the more effective disinfection would be. If 

pretreatment results in a stabilized permeate water not requiring decarbonation, chlorine should 

be injected before adding corrosion inhibitors. 

Product water from the process will not contain any reducing agents or other dissolved 

constituents that may react with the chlorine, other than a trace of hydrogen sulfide, H2S, if 

present in the feed. Also, microbial activity will not be present in the product water from 

distillation or RO. For these processes, 1 mg/l of chlorine injected results in approximately 

1 mg/l of free chlorine residual. 
Desalting Handbook for Planners 

132

Figure 6-3 HOCl-OCl- equilibrium diagram 

For EDR, a chlorine demand study may be necessary to establish the demand of any NOM in the 

water. To ensure proper contact time after the primary chlorine addition (as defined by the 

U.S. Environmental Protection Agency for surface water sources), a minimum storage volume 

must be provided. Normally, the finished water storage tank will have sufficient volume unless 

chloramines are used for disinfection. If so, a contact chamber may be required ahead of the 

finished water storage. Figure 6-4 shows the options for adding disinfectant in a typical 

desalting plant. 

Figure 6-4 Chlorine addition schematic 

Blending Stream

Product from 

Desalter

Indicates Operation

without Decarbonator

Decarbonator

Finished Water Storage

& Chlorine Contact 

Secondary

Chlorine Feed

Finished Water 

To Distribution

Primary 

Chlorine Feed

α2=[OCl-]/CT,OCl α0=[Cl2]/CT,OCl α1=[HOCl]/CT

0.00

0.20

0.40

0.60

0.80

1.00

0 1 2 3 4 5 6 7 8 9 10

pH

Fraction of C

T
Chapter 6: Post-Treatment 

133

6.5.1.2 Liquid Chlorine, Sodium, or Calcium Hypochlorite 

The use of so-called “liquid” chlorine has become more and more widespread, as the safety 

requirements associated with the use of gaseous chlorine have become more and more stringent. 

Liquid chlorine is usually found in water utilities as a sodium hypochlorite solution. In some 

cases, using calcium hypochlorite is beneficial, as calcium hypochlorite provides a source of 

calcium hardness. 

Sodium hypochlorite can be generated onsite, using a sodium chloride solution as feedstock, in 

an electrolytic cell. The resulting hypochlorite solution is approximately 0.8 percent, by weight. 

Commercial bleach can be obtained in bulk at about 11 to 15 percent strength, but attention must 

be paid to proper storage and the effective shelf life of the solution. Calcium hypochlorite is 

generally purchased in solid form and dissolved at the point of application. 

About 70 percent of the sodium hypochlorite in a commercial solution is available as chlorine. 

This roughly can be assumed to be 120 grams of chlorine per liter (1 pound of chlorine per 

gallon) of solution. Standard liquid chemical feed systems are used, very similar to those used 

for acid and scale inhibitor in desalting plants. 

6.5.1.3 Chloramine Treatment 

Chloramines are formed by reactions between chlorine and ammonia in the correct proportions. 

Chloramines do not react with any NOM present in water. This treatment scheme would, 

therefore, not produce THM’s in the drinking water supply. In many applications then, 

chloramine treatment may offer the most cost-effective solution to limit the formation of THMs. 

References 

Bryant, E.A. et al., “Disinfection Alternatives for Safe Drinking Water,” Von Nostrand 

Reinhold, 1992. 

Connell, G.F., “The Chlorination/Chloramination Handbook,” American Water Works 

Association, 1996. 

Culler, P.L., “Taming the Aggressiveness of Membrane Desalted Water,” Proceedings of the 

American Desalting Association, Monterey, California, August 4-8, 1996. 

Faust, S.D., and O.A. Aly, “Chemistry of Water Treatment,” 2nd Edition, Lewis Publishers, 1998. 

Snoeyink, V.L., and D. Jenkins, “Water Chemistry”, John Wiley and Sons, 1980. 

 

135

Chapter 7: Process Selection and Water Cost 

7.1 Introduction 

This chapter compares the available desalting processes and their applications to varying 

project situations. These guidelines are designed to aid the planner in making a preliminary 

choice of a suitable process. The cost data furnished here can be refined by the methods 

described in chapter 9. 

The primary factors to be considered are: 

• Desired water quality 

• Dependability and quality of the feed water source 

• Energy availability 

The following considerations will also have a significant influence: 

• Concentrate disposal 

• Environmental factors 

• Site restrictions 

Table 7-1 summarizes these factors for easy comparison and reference. 

7.2 Product Water Quality 

7.2.1 Salt Concentration 

Distillation processes produce high purity product water, normally ranging from 0.5 to 

25 milligrams per liter (mg/l) total dissolved solids (TDS). Distillation product water can be 

blended with other supplies to improve overall water quality. 

Electrodialysis (ED) and electrodialysis reversal (EDR) product water is generally 200 to 

500 mg/l TDS. A lower TDS product is technically possible. However, electrical resistance 

of the desalted stream increases as the salt concentration decreases; therefore, an ED plant is 

normally designed to produce product water at the maximum desired salt concentration 

without blending. The increase in power consumption and the necessity for more stages for 

the additional desalting impose an economic limit. 

Reverse osmosis (RO) product water from brackish water generally ranges from 25 to 

500 mg/l TDS, depending on the TDS of the feed water and choice of membrane. Today’s 

high performance membranes can reject more than 99 percent of the dissolved minerals in 
Table 7-1.—Summary of guidelines for desalting selection 

Maximum unit design capacity, m3/d (mgd), 

versus time period 

Feed water 

characteristics favorable 

to economic applications Normal product quality 

Principal energy sourc

Process 1972 1985 2002 TDS (mg/l) 

Hardness Temp. (oC) mg/l 

Lowpressure 

steam 

Mediumpressure 

steam 

Highpressure 

steam 

Low-temperature MSF (once through) 3785.0 (1) 18,930 (5) 18,930 (5) 30,000-60,000 Any 1.7-35.0 0.5-25 A A A High-temperature MSF (once through) 3785.0 (1) 18,930 (5) 37,850 (10) 30,000-60,000 Any 1.7-35.0 0.5-25 NA A A Low-temperature MSF (recirculation) 3785.0 (1) 22,710 (6) 56,800-75,700 (15-20) 30,000-60,000 Any 1.7-35.0 0.5-25 A A A High-temperature MSF (recirculation) 3785.0 (1) 30,285 (8) 56,800-75,700 (15-20) 30,000-60,000 Any 1.7-35.0 0.5-25 NA A A Low-temperature horizontal tube multieffect 1892.5 (5) 9,465 (2.5) 56,800-75,700 (15-20) 30,000-60,000 Any 1.7-35.0 0.5-25 A A A High-temperature vertical tube multieffect 1892.5 (5) 7,570 (2) 56,800-75,700 (15-20) 30,000-60,000 Any 1.7-35.0 0.5-25 NA A A Stacked vertical tube multi-effect NA NA 37,850-378,500 (10-100) 30,000-60,000 Any 1.7-35.0 0.5-25 NA A A MVC 378.5 (0.1) 1,892.5 (0.5) 2,840-5,680 (0.75-1.25) 30,000-60,000 Any 1.7-35.0 0.5-25 NA NA NA Thermal vapor compression 378.5 (0.1) 946 (0.25) 2,840-5,680 (0.75-1.25) 30,000-60,000 Any 1.7-35.0 0.5-25 NA NA A SWRO NA 5,680 (1.5) 26,500 (7) 15,000-45,000 Any 1.7-45.0 <500 NA NA A (2) Brackish water RO 946.0 (0.25) 11,360 (3) 18,930 (5) 500-15,000 Any 1.7-45.0 <5003 NA NA NA ED/EDR 946.0 (0.25) 1,892.5 (0.5) 5,680 (1.5) 500-3,000 Any 1.7-43.3 100-500 NA NA NA NF 

NA NA 18,930 (5) 500-50,000 Any 1.7 

Depends on application 

NA NA NA 

1 Startup time depends on the size and control of the boiler and the size of the hogging ejector. 2 This process can be arranged so that the high-pressure booster pump can be driven by a steam turbine, diesel, or gas turbine. 3 The permeate from brackish water RO plants is usually blended with a portion of the feed water to produce a finished water close to 500 mg/l TDS. 

 Note: m3/d = cubic meters per day; mgd = million gallons per day; A = applicable; NA = not applicable; MSF = multi-stage flash; MVC = mechanical vapor co

reverse osmosis; NF = nanofiltration 

136
Chapter 7: Process Selection and Water Cost

137

the water. High salinity feed waters produce a higher salinity permeate. Low salinity feed 

waters desalted with these membranes will produce permeate much lower in salinity than was 

previously produced. Considering “standard” seawater of 35,000 mg/l, permeate quality at 50-

percent recovery or less will be less than 500 mg/l. As the seawater TDS increases, recovery 

must be reduced to maintain permeate quality less than 500 mg/l. 

7.2.2 Composition 

A complete discussion on removing the numerous specific ions or compounds found in waters 

suitable for desalting is beyond the scope of this Handbook. Some objectionable materials may 

be removed in pretreatment processes, some are removed in the desalting process, and others 

depend on post-treatment for reduction to the desired levels. An integrated design of the 

pretreatment, desalting, and post-treatment processes is usually necessary to obtain the desired 

quality of product from a given feed water. 

In general, the various processes affect the product composition in the following ways: 

• The purity of ED product water is affected by the characteristics of the membranes used. 

Both anion and cation selective membranes are variously selective in passing monovalent 

or divalent ions and, in general, do not pass nonelectrolytes. 

• Reverse osmosis membranes pass a greater percentage of monovalent ions than divalent 

ions. The RO process removes most larger nonelectrolytes very effectively. 

In conjunction with appropriate "pre" and "post" treatments, RO and ED/EDR have been used 

effectively on a variety of wastewaters, including sewage effluents, acid mine waters, and 

industrial effluents, to reduce TDS, organics, toxic compounds, and acidity, producing treated 

water quality ranging from potable to high purity industrial. 

In the distillation processes, essentially all suspended substances, and nearly all dissolved 

substances, remain in the concentrate. Some volatile organic compounds and gases such as 

carbon dioxide, CO2, transfer to the product water. 

7.2.3 Blending 

The ED process is designed to produce water of the desired quality. Seawater product typically 

meets the final product quality criteria. However, brackish water membrane product and 

distillation product have very low TDS and require stabilizing. Blending raw water with the 

product water helps in the post-treatment process and helps stabilize the finished product. 

Blending can reduce both capital and operating costs. The amount of water that can be blended 

in distillation or membrane systems will depend on the blend water quality and the product water 

quality goals. 
Desalting Handbook for Planners 

138

With most nanofiltration (NF) applications, however, blending is not used, and all of the water 

produced is treated in the membrane system. Because of NF membranes’ unique characteristics, 

the product water is generally stable, in terms of its tendency to scale or corrode, and is 

essentially free from natural organic material. 

7.3 Feed Water Source Characteristics 

Before selecting a desalting process and beginning plant design, analyze the feed water source. 

The cost of this work must be included in the estimate of the water cost. 

Annual and seasonal variations in quantity and quality must be accommodated by the desalting 

system design. All desalting processes have a degree of flexibility with certain modifications. 

Pretreatment and post-treatment can also be varied to suit the condition of the feed water. 

7.3.1 Dependability 

The existence of a dependable supply of feed water must be established before selecting a 

desalting process and beginning plant design. Records of annual and seasonal changes in 

availability should be obtained, studied, and recorded. The effect of withdrawal of feed waters 

on the feed waters’ continued availability must also be researched. For ground water systems, 

potential quality degradation, due to intrusion from more saline sources, should be investigated. 

The ability of a ground water system to provide water in the quantity required and of predictable 

quality is one of the most important design considerations. 

7.3.2 Salinity 

The ranges of feed water salinity ordinarily handled by the various desalting processes are: 

Feed concentration 

(TDS) Process 

30,000 - 60,000 mg/l Distillation processes 

500 – 2,500 mg/l ED 

1,500 - 45,000 mg/l SWRO/brackish water RO 

Any NF 

For comparing costs, feed water salinity outside these ranges should be considered if particular 

circumstances warrant it. 

Distilling water from a 50,000-mg/l salt solution does not require a significantly larger amount of 

energy than is required to distill water from a 1,000-mg/l solution. The cost in distillation, 

therefore, varies in direct proportion to the amount of water produced. 
Chapter 7: Process Selection and Water Cost

139

In contrast, the separation energy required by electrodialysis is roughly proportional to the 

salinity of the feed. Technically, electrodialysis can be used to desalt high salinity waters. 

However, due to the resulting electrical energy consumption and capital investment, 

electrodialysis today is used only for brackish waters with TDS levels up to about 2,500 mg/l.

For reverse osmosis, the maximum concentration of TDS in feed water that is acceptable to 

produce water of a given salinity depends mainly on the membrane properties, particularly its 

salt rejection. Reverse osmosis is now commonly used for desalinating seawater. At the other 

end of the salinity spectrum, RO is widely used for demineralizing potable quality water in the 

production of ultrapure industrial water. Reverse osmosis can be considered for all ranges of 

feed salinity. However, combinations of ion exchange or ED with RO may offer advantages in 

some applications. 

Nanofiltration can be used with a wide range of salinity levels, due to NF membranes’ unique 

rejection characteristics. NF can be used with low salinity feed water to remove heavy metals or 

reduce hardness. NF can be used even with very high salinity waters to remove hardness without 

significantly reducing the TDS. This process was used successfully in Paradox, Colorado, to 

enable deep well injection of a concentrate waste stream. 

7.3.3 Temperature 

Feed water temperature for desalting plants throughout the world ranges from about 1.7 oC 

(35 o

F) to 35 oC (95 oF). This temperature will influence plant capacity. Distillation processes 

benefit from lower temperatures, while membrane plants benefit from higher temperatures. 

For distillation processes, the driving force for evaporation is the total temperature difference 

(i.e., the maximum process operating temperature minus the feed water temperature). The 

greater this temperature difference, the greater the amount of water that can be produced from 

the supply. Therefore, for two plants operating at the same top temperature, the one with the 

lower feed water temperature can produce more water. In addition, the cost of pumping the 

cooling water supply and its return to the feed water source will be less as the feed water 

temperature decreases. If low steam costs are available, the multi-stage flash (MSF) process 

becomes more competitive with other desalting processes. 

In ED and EDR, warmer feed increases electrical conductivity and salt diffusion within the 

stacks. These translate into economic benefits because of increased limiting current densities 

and decreased power consumption at a given current density. Generally, however, studies have 

shown that the additional cost of feed water heaters more than offsets the savings in power 

consumption for desalting. 

Membrane flux in reverse osmosis increases with feed water temperature. For cellulose acetate 

membranes, the temperature should not exceed 35 o

C (95 oF), except for brief periods, to prevent 

accelerated hydrolysis and compaction of the cellulose acetate membrane. Thin film composite 
Desalting Handbook for Planners 

140

membranes can operate up to 45 o

C (113 oF). However, higher temperature operations can 

increase biofouling potential. In general, the temperature advantages are: 

Process 

Feed 

temperature 

MSF Cool 

Multiple effect distillation 

(MED) 

Cool 

Vapor compression Warm 

ED/EDR Warm 

RO Warm 

Ion exchange Warm 

7.3.4 Composition 

Pretreatment to control various substances in the feed water has been discussed in previous 

chapters of this Handbook. Feed water hardness, however, is so significant that it merits further 

discussion. The hardness components in water are calcium, Ca, and magnesium, Mg. Normal 

pretreatment prevents the detrimental effects of Mg and calcium carbonate, CaCO3, precipitation. 

The limited solubility of calcium sulfate, CaSO4, imposes a limit on the allowable discharge 

concentrate concentration and, therefore, recovery. 

For membrane processes, dissolved solids are concentrated roughly in proportion to the recovery 

ratio, according to the equation: 

CF = 1 / (1-R) 

where: 

CF = concentration factor 

R = recovery 

Hence, at 75-percent recovery, the dissolved solids are concentrated by a factor of 4. At 

90-percent recovery, the factor is 10. 

For RO units, feed water pH must be controlled to prevent CaCO3 scale formation. If the CaSO4

concentration does not exceed 200 percent of the saturation level, adding enough scale inhibitor 

to the feed water will control CaSO4 scaling. 

Electrodialysis processes require scale inhibitor and/or acid addition to the concentrate. 

Concentration factors must be controlled in ways similar to the RO process. For ED/EDR units, 

the calcium carbonate concentration may approach or exceed 250 percent of the saturation level, 
Chapter 7: Process Selection and Water Cost

141

without adding any acid to reduce the pH of the feed water. Calcium sulfate concentration may 

approach 200 percent of the saturated limit without adding scale inhibitor. 

Some water sources, particularly surface sources, are subject to sudden changes in turbidity. In 

such cases, the pretreatment plant must be designed for the worst condition. The process best 

able to cope with high turbidity may be favored over alternative processes. 

7.3.5 Physical Quality 

Feed water to distillation facilities should be free from debris, marine flora and fauna, and any 

component that could interfere with pumps or process operation. In some instances, intake 

design will include trashracks and traveling screens. 

Similar precautions must be taken with open sea feed water to RO plants. Special attention must 

be paid to the low turbidity requirements in the feed water and the potential for membrane 

system biofouling. Pretreatment selection will be predicated upon the range of conditions 

expected at the intake site. 

Most brackish water systems will operate with well water feeds. In general, biofouling will not 

present a problem, although absence of bacteria capable of producing biofilms should be 

verified. Some forms of bacteria and fungi are capable of destroying cellulose acetate 

membranes. 

Well design should be predicated upon a philosophy of quality, not quantity. Screens and gravel 

pack must be carefully selected, and wells must be properly developed prior to commissioning. 

7.4 Heat and Electrical Energy 

All desalting processes require electrical energy to operate. Most distillation processes also 

require heat. The basic requirements for heat or energy for each desalting process are discussed 

in chapter 4. 

Section 7.6 of this chapter describes the energy requirements for each major process in terms of 

cost related to cost of product water. Analysis of the source data shows that heat and electrical 

energy costs for each process are a significant percentage of the desalting water cost. The cost of 

heat energy is a major factor in determining the cost of water for the MSF and MED processes. 

Also, the cost of electricity plays an important role for the cost of MVC and the membrane 

processes. The choice of the heat source should be based primarily on economics. 
Desalting Handbook for Planners 

142

7.4.1. Single-Purpose Plant for Distillation Plants 

The heat (steam) for distillation can be taken from a boiler or from a power plant cycle. The 

boiler-only option is called a “single-purpose” plant arrangement because it produces only water. 

The option that joins the desalting plant with a power plant is called a “dual-purpose” or 

“cogeneration” plant arrangement because it produces power as well as water. The water cost 

from a dual-purpose plant arrangement will always be less than the cost from a single-purpose 

arrangement. 

The single-purpose arrangement is shown in figure 7-1. For this scheme, fuel is used to produce 

steam, which is then used in the desalting process. The electrical energy required is purchased 

from the local electrical grid system. 

Figure 7-1 Process schematic – single-purpose MSF arrangement 

7.4.2 Dual-Purpose Plant 

The dual-purpose arrangement for water and power can be accomplished in three distinct ways: 

• Steam is taken from the power cycle steam turbine at an extraction point after it has been 

used to generate electricity 

• Steam is taken from a back-pressure turbine exhaust 

• Steam is generated in a heat recovery boiler and used in the desalting process 

MSF

PROCESS

BOILER

STEAM SUPPLY

CONDENSATE RETURN DISTILLATE PUMP

FUEL SUPPLY

CONCENTRATE

HEATER
Chapter 7: Process Selection and Water Cost

143

Each of these options are shown in figure 7-2. The most economical combination of power and 

desalting plant will depend upon site-specific factors such as fuel cost, plant capacities (power 

and water), equipment cost, etc. Process studies must be carried out for each arrangement to 

determine which arrangements will be most cost effective. 

The cost of steam to the desalting plant in the dual-purpose arrangement is less than that for the 

single-purpose arrangement because the steam used in the desalting plant provided useful work 

before being sent to the desalting facility. The cost of steam for this arrangement can be 

determined in a number of ways. These are explained in chapter 9, “Desalting Cost Estimating 

Procedures.” 

An often overlooked, further benefit offered by the dual-purpose arrangement is the significant 

reduction in primary fuel used. That is, when calculating the fuel use for single-purpose power 

and single-purpose desalting plants, compared with the primary fuel use in a dual-purpose 

facility, it is found that a 60 to 70 percent reduction in primary fuel use can be realized. 

Vapor compression distillation has a low energy requirement compared to other distillation 

processes. Most of the energy in this process is used for vapor compression. A variety of drivers 

and energy sources can be used, including electric motors, diesel engines, gas turbines with heat 

recovery from turbine exhaust gas, and high-pressure steam turbines. 

The RO and NF processes are not generally thought to be amenable to the dual-purpose 

arrangement. Generally, these plants are thought of as only single-purpose plants, purchasing 

electricity from the grid. However, there are a number of methods that can be employed to result 

in a truly dual-purpose arrangement. These include: 

• Steam is extracted from a steam turbine (as with the method explained above) and 

used in a steam turbine driving the high-pressure booster pump 

• Diesel generators are arranged to produce electricity, while driving the high-pressure 

booster pump 

• Gas turbines are arranged to produce electricity, while driving the high-pressure 

booster pump 

These arrangements are shown in figure 7-3. Although these arrangements would not be 

economical for brackish water RO and NF, there is a potential application for RO and NF 

systems treating seawater or highly saline brackish waters. In these arrangements, the cost of the 

water supply can be reduced. The steam pressure required to use a steam turbine to drive the 

high-pressure pump is considerably higher than the steam pressure required for the distillation 

processes. That is, the steam is taken from the power cycle before it has done as much useful 

work in generating electricity. Thus, the cost savings is not as great, when compared with the 

distillation processes. 
Desalting Handbook for Planners 

144

Figure 7-2 Three dual-purpose arrangements – distillation processes 

GENERATOR

PROCESS HEAT 

CONDENSER

MAIN

CONDENSATE RETURN

TURBINE

STEAM 

INPUT

DISTILLATE

PUMP

PROCESS

THERMAL

STEAM SUPPLY

HEAT 

FUEL SUPPLY

COMBUSTION

TURBINE

GENERATOR

RECOVERY

c) Combined steam power cycle schematic

BOILER

THERMAL

PROCESS

BOILER

STEAM SUPPLY

CONDENSATE RETURN

DISTILLATE

PUMP

FUEL SUPPLY

PROCESS HEAT 

INPUT

GENERATOR

STEAM 

TURBINE

MAIN

CONDENSER

a) Condensing steam power cycle schematic

b) Back-pressure steam power cycle schematic

FUEL SUPPLY

BOILER

GENERATOR

PROCESS HEAT 

CONDENSATE RETURN

TURBINE

STEAM 

INPUT

DISTILLATE

PUMP

PROCESS

THERMAL

STEAM SUPPLY
Chapter 7: Process Selection and Water Cost

145

Figure 7-3 Dual-purpose arrangement — RO process 

BOILER

STEAM SUPPLY

RO FEED

PUMP

FUEL SUPPLY

GENERATOR

STEAM 

TURBINE

MAIN

CONDENSER

a) Condensing steam power cycle schematic

TURBINE

STEAM 

CONCENTRATE

TO DISPOSAL

PERMEATE

TO 

RO FEED WATER

STEAM SUPPLY

DRIVER

RO TREATMENT SYSTEM

POST- 

 TREATMENT

GENERATOR

DIESEL

ENGINE

c) Diesel engine power cycle schematic

CONCENTRATE

TO DISPOSAL

PERMEATE

TO 

RO FEED WATER

RO TREATMENT SYSTEM

POST- 

 TREATMENT

EXHAUST

FUEL SUPPLY

RO FEED PUMP

RO FEED GENERATOR PUMP

COMBUSTION

TURBINE

b) Combustion turbine power cycle schematic

CONCENTRATE

TO DISPOSAL

PERMEATE

TO 

RO FEED WATER

RO TREATMENT SYSTEM

POST- 

 TREATMENT

FUEL SUPPLY EXHAUST
Desalting Handbook for Planners 

146

In addition, although not a dual-purpose arrangement, co-locating a seawater or brackish water 

RO plant with a power plant or other similar industrial facility can provide several advantages, 

including warm feed water, joint use of discharge facilities for concentrate disposal and, 

possibly, the availability of suitably zoned land that has minimal environmental constraints. This 

approach is currently being studied for seawater RO plants in California, as well as seawater and 

brackish water plants in Florida. The Tampa Bay Desal 1 plant is co-located with a power plant 

on Tampa Bay. 

7.5 Environmental Constraints for Site Selection 

Environmental constraints imposed by both local and national regulations greatly influence the 

site selection, construction, and operation of desalting plants. Although typically used for the 

production of municipal water supplies, desalting plants may often seem to resemble industrial 

facilities in their potential impact on the environment. This is particularly true of large dualpurpose power and distillation desalting facilities. 

7.5.1 Planning and Cost Features 

Depending on the type of plant, liquid and solid waste disposal, air quality, noise, and thermal 

issues must all be addressed in the project planning process. In many communities, aesthetics 

will also be an important consideration. 

Some or all of these factors may have a significant impact on the cost of product water. An 

environmentally compatible facility may be located at a site where land cost, concentrate 

disposal options, and location have a significant impact on the water cost, whereas the cost of 

environmental compliance could rule out a more conveniently located site. 

Desalting plants must be planned, designed, and operated to minimize pollution. Thermal 

pollution exists only with the distillation processes. Air pollution is not normally a factor in 

process selection, except where a dual-purpose plant generating electricity is considered. 

Chemical wastes generated by a process generally require suitable preparation and disposal. 

Effluents have to conform to specified Federal, State, and local standards to prevent 

environmental and ecological degradation. 

Solid waste disposal is becoming a more significant issue with the rapidly expanding use of 

membranes for desalting and for pretreatment (membrane filtration). With distillation plants, the 

most significant solid waste produced is failed heat transfer tubing. This waste is a suitable 

candidate for recycling. However, there is not yet a suitable recycling method for used 

membranes, and most are put into sanitary landfills. This can be a significant cost factor, 

particularly in some locations. Spent cartridge filters are also put in landfills, and such filters can 

also be a significant disposal challenge as plants become larger. 
Chapter 7: Process Selection and Water Cost

147

7.5.2 Concentrate Disposal 

Concentrate disposal can present significant engineering and economic problems. There are 

often environmental and legal constraints against discharging liquid wastes from a desalting 

plant into surface waters or underground. 

For example, concentrate from seawater RO plants is generally at the 70,000 to 80,000 mg/l TDS 

level. It may contain corrosion or erosion products from the plant, have a pH from 7 to 9, be low 

in oxygen, or have varying degrees of turbidity. 

Concentrate from brackish water plants may be high in heavy metal content (such as iron), 

contain sulfides, and be completely anaerobic, requiring that it be aerated prior to discharge. 

Four major methods of disposal now in general use are discussed below. For a more detailed 

discussion of concentrate disposal options, see Mickley and Associates (2001). 

7.5.2.1 Disposal to Surface Waters 

In general, direct discharge without treatment into a river, lake, or other watercourse cannot be 

made without degrading surface water quality. Water quality control laws of most political 

bodies prohibit such discharge. The effluent from a desalting plant located near a coast would 

probably be discharged into the ocean or large estuaries. Some considerations for discharge are 

listed below: 

• For all surface water discharges, diffusers will probably be required. Occasionally, a 

pipeline discharging into a high-ocean energy zone may be required for adequate 

dispersion of the concentrate. 

• Fresh and marine water biota can be harmed by corrosion byproducts, higher 

temperatures, or low oxygen concentrations. Effluent from a distillation plant may 

contain copper from corrosion. 

• Toxicity of the effluent stream can possibly be reduced to acceptable levels by dilution 

with the receiving water. Some dilution is gained from power plant cooling water in a 

dual-purpose or co-located plant. 

• Outfalls to the ocean should be located on the open coast. Locations on estuaries and 

areas with restricted interchange of water should be avoided. 

• Concentrate from brackish water facilities may require additional treatment prior to 

discharge, such as aeration or pH adjustment. Mixing zones in the receiving water may 

be required. 
Desalting Handbook for Planners 

148

7.5.2.2 Deep-Well Injection 

Injection into subsurface strata is frequently used to dispose of wastewaters at inland sites. 

However, subsurface injection is not permitted in some jurisdictions. Such disposal is feasible 

only at locations where underground formations for receiving the effluent are suitable. Each 

potential site must be evaluated individually. The cost of concentrate disposal by injection can 

be a substantial part of the total cost of desalting and must be considered in the initial design of a 

desalting facility. The cost of deep-well injection primarily depends on disposal volume, well 

depth, system design, and injection pressure. A properly designed system, based on sound 

engineering and geologic principles, should place wastes where ground water will not be 

contaminated. 

7.5.2.3 Evaporation Ponds 

Evaporation from surface concentrate disposal ponds is a method of concentrate disposal that is 

only suited to inland plants in hot, arid locations with inexpensive land available. The 

construction of such ponds is subject to increasingly strict regulation to prevent pollution of both 

underground and surface environments and to prevent other adverse effects. Acquisition of land, 

earthwork for berms, and the impervious liner required in most jurisdictions make this method of 

disposal costly when combined with a plant of significant size. Basic rules are available under 

which the design of concentrate disposal ponds may be developed. 

The use of evaporation ponds is only feasible in hot, dry climates with high net evaporation rates, 

relatively level terrain, and low land costs. It is not an option for seawater desalting systems. 

7.5.2.4 Evaporation to Dryness and Crystallization 

Evaporation to dryness and crystallization of the effluent into disposable solid salts is probably 

the most expensive approach to the concentrate disposal problem. It is used only if: 

• Legal or site restrictions eliminate other disposal techniques 

• A valuable byproduct could be recovered 

• The process is technically feasible 

This method has been used for many years in the process industry. Salts produced are either sold 

or transported to a disposal area. 

7.6 Factors Influencing Site Location 

The selection criteria for choosing a proper site vary, depending on the process used for 

desalting. For example, the RO process requires a water supply that is much “cleaner” than a 

supply for a distillation process. Also, the TDS concentration of the supply is not as important 

for a distillation process as it is for the membrane processes. 
Chapter 7: Process Selection and Water Cost

149

The site selection process often results in an evaluation of two or more possible sites. When this 

is the case, the final choice can be made by performing a cost study that addresses: 

• Cost of land 

• Location in proximity to the power supply 

• Location in proximity to the water distribution connection 

• Location in proximity to a highway or railroad (i.e., for chemical deliveries) 

• Cost of source water supply and pretreatment 

• Process cost of water 

• Location in proximity to the concentrate disposal point 

The key criteria that must be addressed in the selection of a desalination site generally can be 

grouped into the following categories, discussed in this section: 

• Source water 

• Site location 

• Land area requirements 

• Concentrate disposal issues 

• Data collection 

• Problematic issues 

7.6.1 Source Water 

7.6.1.1 Source Water Supply 

There must be enough raw feed water available at the site to sustain the desired production 

throughout the life of the desalination plant. The quantity of water available from a surface 

water supply is usually easily determined. However, if the surface source is impacted by runoff 

from the watershed, there could be significant impacts from droughts. These impacts could 

either reduce the available quantity of water, change its character, or both. 

Water supplies for desalting plants may be taken from surface water sources (e.g., rivers, lakes or 

reservoirs, the sea, or ground water systems). In many locations, seawater is taken from beach 

wells and from Ranney-type collectors for smaller plants. Open intakes are used for the largest 

plants. 
Desalting Handbook for Planners 

150

For a ground water aquifer, pumping and modeling of the supply must be carried out to: 

• Determine what the “safe yield” of the well field will be. The safe yield of the well field 

is defined as the amount of water that can safely be pumped without affecting adjacent 

aquifers or nearby surface water supplies. Pumping tests and modeling are carried out to 

determine the safe yield. 

• Determine if a change in water quality over time is expected. Pumping and modeling of 

the aquifer are also important for determining if the water quality will change over time 

(Missimer and Watson, 1994). 

7.6.1.2 Pretreatment Considerations 

Ground water sources are usually naturally filtered, whereas open intake supplies will be 

unfiltered. 

For the RO process, water taken from an open intake will have to be pretreated by some form of 

filtration before the supply enters the process itself. Filtration is required to remove suspended 

solids, as well as colloidal material.1

 If not removed, colloidal material will foul the membrane 

surfaces, which will reduce production and require frequent membrane cleaning and premature 

replacement. The amount of colloidal material and, therefore, fouling potential in a water supply 

can be evaluated by the Silt Density Index (SDI) test (ASTM, 2002). 

For the ED/EDR process, the pretreatment requirements are not as stringent as they are for RO. 

However, the guidelines specified by the manufacturer must be followed. It is noteworthy that 

one of the advantages of the ED/EDR process is that the plant recovery is not affected by any 

amount of silica in the source water. Thus, should the supply contain a significant amount of 

silica (for example, at 30 mg/l or higher), the ED/EDR process might be a better choice over the 

RO process, as recovery would be higher. A drawback to using ED/EDR, however, is that it 

does not remove organics from the source water. As organics are still in the water, all feed water 

is treated with chlorine, and trihalomethanes form. Thus, if the source water is high in 

trihalomethane formation potential, ED/EDR cannot be used as a stand-alone process. 

For distillation processes, the pretreatment requirements are generally mandated by the amount 

of bicarbonate in the feed water supply. The pretreatment for these processes has traditionally 

been acid, followed by decarbonation to prevent calcium carbonate, CaCO3, and magnesium 

hydroxide, Mg(OH)2, scale from occurring on the tubing surfaces. More recent designs use 

synthetic scale inhibitors in lieu of acid for scale control. 

7.6.1.3 Source Water Quality 

The water quality to be treated is profoundly important for the design of RO plants. It is of less 

importance for ED plants and has minimal importance for distillation processes. Nevertheless, a 

 

1

 Colloidal material is finely dispersed particulates of extremely small size. 
Chapter 7: Process Selection and Water Cost

151

comprehensive water analysis must be performed to get a complete understanding of the 

chemistry of the water to be treated. If a change in TDS or character of the water is expected 

over time, then the RO or ED processes will have to be designed to treat the future water quality. 

Table 7-2 gives the typical analysis to be performed. The designer can then project the product 

water quality expected from the process and the amount of pretreatment to be carried out before 

the process. The process design and pretreatment requirements, coupled with the auxiliary 

equipment to be furnished, will establish requirements for the site area. 

Table 7-2.—Suggested water quality analysis 

Constituent 

Cations(+) Anions (-) Other constituents 

Calcium 

Magnesium 

Sodium 

Potassium 

Ammonium 

Barium 

Strontium 

Iron 

Manganese 

Aluminum 

Copper 

Lead 

Zinc 

Chromium 

Carbonate 

Bicarbonate 

Sulfate 

Chloride 

Nitrate 

Fluoride 

Total phosphate 

Orthophosphate 

Bromide 

TDS 

pH 

Silica 

SDI 

Alkalinity 

Color 

Total organic carbon 

Dissolved organic carbon 

Conductivity 

Turbidity 

Temperature 

Boron 

Standard plate count 

Yeast and mold count 

Total coliform 

Sulfide generating bacteria 

Total suspended solids 

 

The TDS of the water to be treated has a direct bearing on the cost of water from the ED and the 

RO processes. If two sites with a different TDS concentration are under consideration, the site 

selection should include a process cost evaluation. As the TDS increases, the operating pressure 

increases for RO. Therefore, more energy is expended for making the same amount of water. 

The TDS also plays an important part on the cost of water from the ED process. As TDS 

increases, more energy is required to separate the charged particles (ions) from the supply. Less 

energy is needed to move Na+

 and Cl- than to move Ca2+ and SO4

2-. However, membrane 

characteristics determine transport rate. In waters high in sodium chloride, ED will normally use 

less energy than RO for TDS up to 2,000 mg/l. In water sources where sodium chloride is not 

the major constituent, ED will use less energy than RO over a concentration of up to 1,000 mg/l. 

TDS plays a negligible part in the cost of water from a distillation process. The distillation 

processes produce pure water (i.e., 0.5-25.0 mg/l TDS) from any supply, no matter what the 

source water TDS is. 
Desalting Handbook for Planners 

152

7.6.2 Site Location 

Every plant should be reasonably close to the water distribution point, electrical supply, 

concentrate disposal point, and highway or rail access. The location of the site in proximity to 

housing developments will depend on the process type. For example, RO and EDR systems can 

be located in buildings whose exterior can be integrated with commercial buildings in the area. 

Distillation process equipment has to be located out in the open and, therefore, away from 

residential areas. 

7.6.3 Land Area Requirements 

The area required for the facility will vary, depending upon the process used and the source 

water to be treated. For example, RO plants treating seawater need larger process and 

pretreatment areas than RO plants treating brackish water. Table 7-3 gives approximate unit 

areas for each process and the area required for the auxiliary equipment (i.e., storage tanks, 

pretreatment, post-treatment, etc.). These figures are typical for a 19,000 m3

/d (5 mgd) process 

production rate. 

Table 7-3.—Unit site areas, for plants of 19,000 m3

/d and larger 

Process 

Access and 

process building, 

m2

 (ft2) 

Auxiliary 

equipment1

,

m2 (ft2) 

Total area, 

m2

 (ft2) 

Reverse osmosis 

Seawater 

Brackish water 

1,800 (19,375) 

1,200 (12,917) 

7,800 (83,959) 

6,800 (73,195) 

9,600 (103,334) 

8,000 (86,111) 

Electrodialysis 

ED 1,500 (16,146) 6,800 (73,195) 8,300 (89,340) 

Distillation 

MSF 

MED 

4,800 (51,667) 

6,000 (64,583) 

7,800 (83,959) 

7,800 (83,959) 

12,600 (135,625) 

13,800 (148,542) 

 1 Assumes area for product water storage at 1-day production capacity, 

distribution system pumping, and electrical substation. 

7.6.4 Concentrate Disposal Issues 

The concentrate disposal point must be close to the plant to be economical. Table 7-4 gives 

typical disposal points and major requirements. Other requirements may be dictated by 

regulations to be met. These issues are more fully discussed in chapter 8. 
Chapter 7: Process Selection and Water Cost

153

Table 7-4.—Disposal of concentrates 

Disposal point Guideline 

Brackish surface water TDS of concentrate not greater than disposal point. 

Oxygen and pH match disposal point. 

Diffusion system is employed. 

Sewage system Combination of quantity and quality of concentrate does not disturb 

wastewater treatment plant operations. 

Concentrate quality does not impact wastewater treatment plant 

effluent standards. 

Deep-well injection The disposal zone does not have a direct or indirect connection with 

any aquifer of lower TDS or any aquifer designated as a drinking 

water source by local or State regulation. 

Ocean Concentrate discharge located to prevent recirculation to intake. 

Diffuser system is employed. 

Oxygen and pH match disposal point. 

Evaporation ponds Double lining with leachate collection system required. 

Depth of pond suitable to hold all precipitated solids over the life of 

the plant. 

Sewage treatment plant outfall 

 

Pipe size suitable to handle concentrate disposal, as well as sewage. 

The outfall would require a permit revision. 

7.6.5 Data Collection 

The data to be taken for evaluation of the site location is contained in table 7-5. This information 

will be used for performing the necessary studies required for site selection. 

7.6.6 Problematic Issues 

There are certain findings that may be considered “fatal flaws” to the siting of a desalting plant. 

Major items are listed below: 

• Land use constraints (e.g., site is too close to an environmental preserve) 

• Distance to any of the following points may make the plant too expensive: 

♦ Source water 

♦ Concentrate disposal 

♦ Electricity supply 

♦ Water distribution point 

♦ Highway access 
Desalting Handbook for Planners 

154

Table 7-5.—Data collection checklist 

Date: 

Information taken by: 

Location: 

Purpose of treatment plant:1 

 

Item Result 

1. General 

 Yearly ambient temperature profiles 

 Yearly relative humidity profile 

 Electrical voltage and amperage available 

 Steam supply pressure, temperature and flow rate 

 Water distribution characteristics2

2. Utility distances in reference to site location 

 Water distribution point 

 Electrical supply 

 Highway access point 

 Source water supply point 

 Wastewater treatment plant location 

 Concentrate disposal point location 

 Nearest housing development 

3. For surface water supplies 

 Depth at 100 feet offshore 

 Depth at 500 feet offshore 

 Depth of 1,000 feet offshore 

 Prevailing current direction 

 Prevailing wind direction 

4. Water quality sampling 

 Source water type3

 Source water quality4

 Disposal point quality5

 Condition of water at source and disposal points6

 Temperature and pH at source and disposal points 

 Source water SDI 

 Existing water supply at distribution point5 

5. Site characteristics7

 Area available 

 Flat or gradual slope 

 Description 

6. Permitting issues 

 Meet with regulators to determine permitting8

 requirements 

7. Local costs 

 Construction labor 

 Operating labor 

 Taxes or duties 

 Chemical costs 

8. Other significant findings 

 1 Refers to what type of treatment will be carried out: seawater, brackish water, etc. 2 Water distribution characteristics refer to quantity and quality of new water that can be accommodated at the distribution 

point. 

 3 Fresh, brackish, or seawater. 4 A number of source water quality analyses are required to document quality over time. 5 Complete water analysis is required. 6 Condition refers to the turbidity, silt, color, etc., at those points. 7 Provide a description: site is flat, sloping, hilly; contains trees, bushes, etc. 8 Provide a summary of discussions. 
Chapter 7: Process Selection and Water Cost

155

• Electricity is not available in the voltage or amperage required by the process 

• Topography presents problems (e.g., a ravine, cliffs, potential flooding) 

• Source water temperature is too high (this only applies to membrane treatments) 

• Surface water supply depths are too shallow. A suitable depth for a seawater intake is 

6 m (20 feet) or more 

• Geological conditions (soil bearing characteristics) present problems (e.g., a marshy area) 

7.7 Relative Desalting Process Costs 

The purpose of this section is to give the reader some knowledge of the relative magnitude of 

desalting process costs, in order to compare different processes for various water sources and 

situations. 

The data presented in this chapter are based on cost data taken from many tenders over the past 

30 years and computer programs developed for costing desalting systems. It is further based on 

recent vendor quotations from actual plant designs. The selected data illustrates, in general, the 

relative cost of water for each of the processes and the relative importance of the major sitespecific items, such as power and fuel, in each case. The numbers and assumptions in this 

section are based on Morin (1990, 1996, and 1999), which are some of the most recently 

published analyses between distillation and membrane processes. 

Cost numbers and assumptions in this chapter differ from chapter 9. As site-specific factors vary 

widely, assumptions also vary widely. This Handbook thus provides different scenarios. Design 

and costing programs for desalting processes, such as Determining Costs of Desalting Processes

(available from DSS Consulting Inc.) provide more analyses for site-specific conditions. 

7.7.1 Plant Investment and Water Cost Summary 

7.7.1.1 Assumptions 

These costs were based on the following assumptions: 

• The MSF process is assumed to be the high-temperature recycle type. Maximum 

operating temperature is 110 °C (235 °F). This is the process most used for MSF. 

• The concentrate rejection temperature was assumed at 43.3 °C (110 °F). 
Desalting Handbook for Planners 

156

• All distillation processes assumed the same performance ratio of 5.17 kg/MJ) (12 lb 

distillate/1,000 Btu) heat input. 

• All processes were costed as single unit sizes, except for the MVC process. The largest 

capacity MVC unit was taken at 1.0 mgd. In the larger plant sizes (i.e., 5, 8, and 

10 mgd), multiple units of 1.0 mgd were used. This process was not costed at the 50-mgd 

capacity, due to the large number of units that would have to be furnished. 

• The costs include the following items: 

♦ Intake systems. For the distillation processes, an open intake was assumed. For the 

RO process, a well field was assumed. 

♦ Pretreatment systems. All processes, with the exception of ED/EDR, assumed using 

acid and scale inhibitors. 

♦ Desalting process. 

♦ Buildings. 

♦ Post-treatment. This cost included only caustic and chlorine addition. In most cases, 

further post-treatment would be required. 

♦ Product storage. Product storage tanks were costed at full plant capacity for 1day. 

♦ High service pumps (distribution system pumps). 

• Land costs are not included in general site costs, as land costs vary widely. 

• All processes were costed using a plant factor of 85 percent. Plant factor is the amount of 

time the process is onstream during the year at the percent of design capacity. Thus, a 

plant could be onstream for 85 percent of the year at full capacity, or at 85 percent of 

design capacity for the full year. 

• All costs can be assumed to be based on year 1999. 

• No blending of brackish water was used for any process. 

• Seawater Reverse Osmosis (SWRO) recovery is 45 percent and assumes the use of coarse 

and polishing dual-media filters for pretreatment. 

• The distillation processes all assumed a dual-purpose plant installation. Thus, no cost is 

included for these processes for a boiler. Steam cost was allocated from a nominal cost 

of steam of $1,422/MJ ($1.50/million Btu). 

• The electrical cost was assumed at $0.033 per kilowatthour. 
Chapter 7: Process Selection and Water Cost

157

• A service life of 25 years was used for all processes. 

• Interest was assumed at 5 percent. 

• Indirect costs were taken as a percentage of total construction costs as follows: 

♦ Freight and insurance at 5 percent 

♦ Construction overhead and profit at 15 percent 

♦ Owners cost at 10 percent 

♦ Contingency cost at 10 percent 

7.7.1.2 Comparing Plant Costs

The unit capital costs for seawater desalting processes are given in figure 7-4. The unit brackish 

water capital costs are given in figure 7-5. The cost of water for each desalting process is shown 

in figures 7-6 and 7-7 for seawater and brackish water systems, respectively. 

These costs are based on the assumptions given above and will change as these assumptions 

change. The following sections give examples of how these costs are affected by the various 

“site-specific” items. 

Table 7-6 compares plant investment costs and product water costs for desalting and indicates 

how scale-up affects capital and operating costs. The base case for these costs is the hightemperature MSF design, recirculation mode with a 10-mgd capacity. In table 7-6, the two base 

cases are shaded. This table is particularly useful in determining which processes are most cost 

effective for treating the various water supplies. For example, in the same process for a 3,785 

m3

/d (1-mgd) unit size, the cost per gallon per day (gpd) is 213 percent more (about 2 times as 

much) than the base case cost of 37,850 m3

/d (10 mgd). If the base case cost is $7.56/gpd, then 

the cost for a 1-mgd unit would be $16.11/gpd. Conversely, a 189,250 m3

/d (50-mgd) unit size 

would cost 57 percent of that price (about half as much). If the base case cost is $7.56/gpd, the 

cost for the 50-mgd unit would be $4.31/gpd. 

Review of this information shows that RO is the most cost-effective treatment for both seawater 

and brackish water sources. Of the distillation processes, the MVC process is most economical 

for smaller plants, and the MED process is the most economical for larger plants. For treating 

brackish water supplies in the feed water salinity range of 2,500 to 5,000 mg/l, the RO process is 

most economical. Note that new technologies may change these ratings. 

This table is based on 1999, second quarter costs. This table also assumes that all distillation 

processes use a dual-purpose plant installation and a plant factor of 85 percent. The cost of fuel 

was taken as $1,422/MJ ($1.50/million Btu). Blending has not been included for any process. 
Desalting Handbook for Planners 

158

Figure 7-4a 

Relative Seawater Desalting Capital Costs (Metric) 

Figure 7-4b

Relative Seawater Desalting Capital Costs (U.S.)

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0 10 20 30 40 50 60

Plant Capacity (mgd)

Unit Capital Costs ($/gpd)

Hi-Temp MSF-R Lo-Temp MSF-R Hi/Lo Temp MED MVC SWRO

0

2,000

4,000

6,000

8,000

10,000

,

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000

Plant Capacity (cubic meters/d)

Unit Capital Cost ($/cubic meter)

HI-Temp MSF-R Lo-Temp MSF -R Hi-Lo Temp MED MVC SWRO
Chapter 7: Process Selection and Water Cost

159

Figure 7-5a

Relative Brackish Water Desalting Capital Costs (Metric)

0

500

1,000

1,500

2,000

2,500

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000

Plant Capacity (cubic meters/d)

Capital Cost ($/cubic meter)

Reverse Osmosis - 2,500 mg/L Reverse Osmosis - 5,000 mg/L

ED/EDR - 2,500 mg/L ED/EDR - 5,000 mg/L

Figure 7-5b

Relative Brackish Water Desalting Capital Costs (U.S.)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0 10 20 30 40 50 60

Plant Capacity (mgd)

Unit Capital Costs ($/gpd)

Reverse Osmosis 2,500mg/L Reverse Osmosis 5,000 mg/L

ED/EDR 2,500 mg/L ED/EDR 5,000 mg/L
Desalting Handbook for Planners 

160

Figure 7-6a

Cost of Water Seawater Desalting (Metric)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000

Plant Capacity (cubic meters/d)

Cost of Water ($/cubic meter)

Hi-Temp MSF-R Lo-Temp MSF-R Hi/Lo MED MVC SWRO

Figure 7-6b

Cost of Water Seawater Desalting (U.S.)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0 10 20 30 40 50 60

Plant Capacity (mgd)

Cost of Water ($/kgal)

Hi-Temp MSF-R Lo-Temp MSF-R Hi/Lo Temp MED MVC SWRO
Chapter 7: Process Selection and Water Cost

161

Figure 7-7a

Cost of Water - Brackish Water Desalting (Metric)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000

Plant Capacity (cubic meters/d)

Cost of Water ($/cubic Meter)

Reverse Osmosis 5,000mg/L Reverse Osmosis 2,500mg/L

ED/EDR 5,000mg/L ED/EDR 2,500mg/L

Figure 7-7b

Cost of Water Brackish Water Desalting (U.S.)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 10 20 30 40 50 60

Plant Capacity (mgd)

Cost of Water ($/kgal)

Reverse Osmosis 5,000mg/L Reverse Osmosis 2,500mg/L

ED/EDR 5,000mg/L ED/EDR 2,500mg/L
Desalting Handbook for Planners 

162

Table 7-6.—Plant investment and product water cost for a 37,850 m3/day (10 mgd) MSF base case plant 

Process Feed water Relative plant unit capital costs, percent 

relative to base, m3

/d (mgd)1

Relative unit water costs2

, percent 

relative to base, m3

/d (mgd) 

Plant size, m3/d 3,785 18,925 37,850 189,250 3785 18,925 37,850 189,250 

Plant size, mgd (1) (5) (10) (50) (1) (5) (10) (50) 

Low-temperature MSF 

(once through) Seawater 302 172 135 77 211 141 119 79 

High-temperature MSF 

(once through) Seawater 213 126 100 57 191 128 100 72 

Low-temperature MSF 

(recirculation) Seawater 302 172 135 77 211 141 119 79 

High-temperature MSF 

(recirculation)3 Seawater 213 126 100 (basis) 57 191 128 100 (basis) 72 

Low-temperature 

horizontal tube multieffect 

Seawater 135 79 63 36 136 87 79 53 

High-temperature vertical 

tube multi-effect Seawater 135 79 63 36 136 87 79 53 

Stacked vertical tube 

multi-effect4 Seawater NA NA NA 21 NA NA NA 32 

MVC5 Seawater 96 86 82 NA 119 84 81 NA 

SWRO Seawater 78 44 35 20 75 50 44 25 

BWRO 5,000 mg/l 55 26 22 11 30 20 16 8 

Brackish water RO 2,500 mg/l 38 17 14 7 28 17 12 6 

ED/EDR 5,000 mg/l 66 52 49 36 122 100 85 68 

ED/EDR 2,500 mg/l 41 29 23 18 60 46 38 27 

 1 All costs include the intake, pretreatment, process, post-treatment, product storage, buildings and distribution system 

pumps. 

 2 Water costs are annual costs divided by annual production. 3 Assumes recycle type, at 112.8 °C (235 °F) top operating temperature. 4All distillation processes use the same performance ratio, except the stacked vapor thermal compression MED 5 The MVC process assumes that the largest unit size at 3,785 m3

/d (1.0 mgd). All other costs are multiples of 

3,785 m3

/d (1.0 mgd). 

7.8 Optimizing Capital and Operating Costs for Distillation 

Processes 

To optimize a distillation system, determine the minimum cost of water from the process. The total 

water cost is a sum of capital and operating costs (mostly for energy). More capital investment is 

needed to lower energy costs, so optimizing costs involves a tradeoff between capital and operating costs. 

As performance ratio2

 increases, less heat is required. The amount of heat required, or the heat consumed 

is based on: 

 

2 The performance ratio is the amount of water produced per amount of heat consumed.
Chapter 7: Process Selection and Water Cost

163

• MSF—the number of stages used 

• MED—the number of effects used 

• MVC —the amount of tubing or surface area 

That is, is the number of stages or effects or surface area increases, the performance ratio 

increases and the heat required decreases. As performance ratio increases, capital costs increase 

and steam use (or operating costs) decreases. Thus, a higher capital investment will result in 

lower energy use. Where low-cost energy is available, the capital investment required for higher 

performance ratios may be unnecessary. Conversely, where energy costs are high, costs for 

higher performance ratios would be warranted. If these two costs were plotted against the 

performance ratio, then the optimum or lowest cost of water would be where these two lines 

intersect. 

The MED process optimization is similar to the MSF process. Table 7-7 shows this for steam 

costs of $947, $1,894, and $2,841/MJ ($1.00, $2.00, and $3.00/million Btu). 

Table 7-7.—MSF optimization 

Steam cost, $/MJ 

($/million Btu) 

Optimum performance ratio 

kilograms distillate/MJ 

Total water cost 

$/m3

 ($/kgal) 

 947 (1.00) 3.5 <1.24 (4.69) 

 1,894 (2.00) 3.9 1.50 (5.68) 

 2,841 (3.00) 4.6 1.74 (6.58) 

7.9 Optimizing Costs for Electrodialysis Reversal 

Modern commercial application of ED is limited to desalting brackish water, and the dominant 

process is EDR. The principal water cost components are given in table 7-8. 

Table 7-8.—Principal water cost components for ED/EDR 

 Desalting 

equipment1

 

(percent) 

Electric power 

(percent) 

2,500 mg/l TDS feed water 

 3,785 m3

/d (1.0 mgd) 

 37,850 m3

/d (10 mgd) 

39 

52 

14 

22 

5,000 mg/l TDS feed water 

 3,785 m3

/d (1.0 mgd) 

 37,850 m3

/d (10 mgd) 

59 

48 

23 

32 

 1 Excludes replaceable membranes, but includes pretreatment equipment. 
Desalting Handbook for Planners 

164

For practical hydraulic reasons, EDR capacity has a finite limit. Depending on the desalting load 

required, the biggest single unit available is about 5,680 m3

/d (1.5 mgd). The most units 

combined are in Sarasota County, Florida, where there are ten 4,540 m3

/d (1.2 mgd) units. 

ED/EDR operating cost is very sensitive to feed water properties, such as TDS, with energy 

consumption proportional to the solids removed. However, because of improved membranes, it 

is an economical method of desalting low TDS brackish waters. Energy consumption versus 

TDS is shown in figure 7-8 for EDR and RO. For high salinity brackish water feed, the 

equipment cost and the electrical energy cost increase considerably. 

Figure 7-8 Energy consumption comparison – RO and EDR 

Overall capital costs for ED plants versus plant capacity are shown in figure 7-5, with total water 

cost in figure 7-7. 

7.10 Optimizing Capital and Operating Costs for 

Reverse Osmosis 

Using reverse osmosis to desalt seawater is now a well-accepted practice worldwide. The 

increasing use of SWRO has resulted in technology improvements in equipment and materials 

that have combined to substantially reduce the cost of water in the past 20 years. Membrane 

development has played a role, by allowing operation at higher pressure and higher recovery. In 

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

Feed water TDS, mg/l

Energy Consumption, kWh/m3 produced

RO: 75% Recovery, 25 °C, 27 l/m2/hr (16 gfd)

EDR: 75% Recovery, 25 °C

Standard Pressure RO

Low energy RO

EDR

# of EDR stages 

changes from 3 to 4 @ 

feed TDS of 2,500-

3,500 mg/l
Chapter 7: Process Selection and Water Cost

165

most applications including elevated TDS, such as the Red Sea, single pass desalting to produce 

potable water is possible. In the last few years, the use of micro- or ultrafiltration as a 

pretreatment has demonstrated its worth, providing for minimal RO membrane flux decline, less 

frequent cleanings, and extended membrane life. The cost of this technology is also declining 

rapidly. The principal components of the cost of RO processes are shown in table 7-9. 

As with all desalting processes, a tradeoff exists between the capital and operating costs. For the 

RO system, this tradeoff results from the recovery rate (i.e., the amount of product water 

produced per amount of feed water to the process) used for design. As recovery increases, the 

amount of membranes required decreases, resulting in a decrease in capital costs. However, with 

increasing recovery, the required pressure increases, resulting in an increase in operating costs. 

Table 7-9.—Principal water cost components for RO 

 Desalting 

equipment1

 

(percent) 

Electric 

power 

(percent) 

Membrane 

replacement 

(percent) 

Seawater: 

 SWRO 3,785 m3/d (1.0 mgd) 36 27 14 

 SWRO 37,850 m3

/d (10 mgd) 31 7 14 

2,500 mg/l TDS: 

 3,785 m3

/d (1.0 mgd) 21 4 8 

 37,850 m3

/d (10 mgd) 24 12 8 

5,000 mg/l TDS: 

 3,785 m3

/d (1.0 mgd) 43 6 8 

 37,850 m3

/d (10 mgd) 59 9 3 

 1 Excludes replaceable membranes, but includes pretreatment equipment. 

Membrane replacement, including labor, has become a less significant factor in recent years 

because average membrane life has increased from 3 years or less to more than 5 years. Also, 

membrane prices have declined radically and now account for only 5 to 10 percent of desalting 

equipment direct capital costs. The larger the plant size, the higher the percentage of equipment 

costs for membranes. Chemical cost can be high if extensive pretreatment of the seawater feed is 

required. 

Overall, plant capital costs vary with plant capacity. A range of these costs is given in figure 7-

4. Figure 7-6 shows the variation in water costs with plant size for the lower and higher range of 

capacities. 

Scale-up in plant size does impact the distribution of desalting equipment, electric power 

consumption, and chemical use. This is largely due to the modular construction of RO plants. 

To double the capacity of a plant, the number of membranes must be doubled, and twice as much 

water must be pumped. Special pretreatment equipment can also add significantly to the capital 

cost. However, modern, high-performance membranes have lowered costs because they reduce 
Desalting Handbook for Planners 

166

pressure, the number of membranes required, and the replacement cost. Membrane cost as a 

percent of the equipment cost and membrane operating life is similar to that previously noted for 

seawater RO. 

Overall, capital plant costs are given in figure 7-5, with total water cost in figure 7-7. Feed water 

pressure versus TDS is given in figure 7-9. Higher values of TDS in the feed water result in 

greater osmotic pressure difference, resulting in a greater feed pressure. 

Figure 7-9 BWRO feed pressure versus feed water TDS 

References 

ASTM, 2002. “D4189-95—Standard Test Method for Silt Density Index (SDI) of Water,”

ASTM Book of Standards (Print and CD-ROM), Volume 11.02, May 2002. 

Mickley and Associates, 2001. “Membrane Concentrate Disposal Practices and Regulations,” 

Bureau of Reclamation, Desalination Water Purification Report Series No. 69, Denver, 

Colorado. 

0

10

20

30

40

0 2000 4000 6000 8000 10000 12000

Feed water TDS, mg/l

Applied Pressure, bar

75% Recovery, 25 °C, Flux = 27 l/m2/hr (16 gfd)

Standard Pressure RO

Low energy RO

Low energy systems are typically 

limited with respect to feed TDS.
Chapter 7: Process Selection and Water Cost

167

Missimer, Thomas M., 1994. “Water Supply Development for Membrane Water Treatment 

Facilities,” with description of the membrane process design by Ian C. Watson, Lewis 

Publishers, 1994. 

Morin 1991. “Desalting Costs Update,” Proceedings of the First American Water Works 

Association Membrane Processes Conference, March 1991. 

Morin, 1996. “Optimizing the Design of RO Facilities,” Proceedings of the American Water 

Works Association Membrane Processes Conference, May 1996. 

Morin, 1999. “Desalting Plant Cost Update: 2000,” International Desalination Association, 

San Diego Conference, August 1999. 

Chapter 8: Environmental Considerations

169

Chapter 8: Environmental Considerations 

8.1 Introduction 

All desalination processes generate a low-salinity product stream and a high-salinity concentrate 

stream. Disposal of the concentrate, or waste stream, is the most significant environmental 

consideration for desalination facilities. The composition of the concentrate stream varies 

drastically, according to the desalination process used. Therefore, the environmental impact of 

the concentrate also varies significantly. 

The desalting industry has a long history of environmentally safe operations. However, in some 

parts of the world, particularly in the U.S., regulating concentrate disposal can be a major factor 

for potential desalination users. In recent years, many articles have been published on the overall 

environmental impacts of desalting plants. Additionally, the European Desalination Society 

sponsors a biannual conference, “European Conference on Desalination and the Environment,” 

which focuses primarily on the effect of concentrate disposal on the environment. 

The water quality of the concentrate stream depends on: 

• Feed water quality 

• Pretreatment chemicals—polymer additives, acids, chlorination, corrosion inhibitors 

• Dechlorination chemicals 

• Water recovery 

• Temperature 

• Post-treatment or cleaning chemicals 

• Amount of concentrate blending 

Unlike most industrial processes, the concentrate stream produced from the desalting process is 

not characterized by process-enhanced chemicals. Instead, the concentrate stream reflects the 

characteristics of the feed water and is primarily only the feed water at a more concentrated 

level. Feed water is typically pretreated with chemicals to control scaling, fouling, and corrosion 

of internal equipment, as described in earlier chapters. These chemicals are present in relatively 

low levels—typically less than 10 milligrams per liter (mg/l), and, therefore, the constituents in 

the feed water primarily define the concentrate stream. 

The easiest method to consider the impact of the concentrate is to consider the extent of 

concentration (concentration factor) in the desalination process. The definition of concentration 

factor is: 
Desalting Handbook for Planners 

170

 CF = 1/(1-R) 

A plant, for example, with a 30-percent recovery would result in a concentration factor of 1.43, 

which means that the concentrate is 43 percent higher in salinity than the feed water. 

Table 8-1 lists the different desalination processes and their approximate concentrate 

characteristics. According to the table, the reverse osmosis (RO) process results in significantly 

higher concentrations of salts in their concentrate, compared to thermal processes, due to the 

higher recoveries practiced in the membrane processes. Though brackish water RO results in the 

highest concentration factor (2.5 to 6.7), keep in mind that this is relative to the feed water 

quality and is still less than seawater in salt content, even at the higher recoveries. Seawater 

reverse osmosis (SWRO) results in the highest concentrate salinity content of any desalination 

process. This high-salinity concentration can be reduced by blending water with other lower 

saline wastewaters in the proximity of the desalination plant. 

Thermal processes usually blend their concentrate streams with cooling water prior to discharge, 

which causes the concentration factors to be lower, compared with the other processes. 

However, this may also increase the discharge temperature. The concentrate streams are 

generally less than 15 percent above feed water concentration. 

Table 8-1.—Desalination processes and characteristics of their concentrate streams (Mickley, 1995) 

Process Brackish RO Seawater RO 

Multi-stage flash 

(MSF) 

Multiple-effect distillation 

(MED) 

Feed water Brackish Seawater Seawater Seawater 

Recovery, percent 60 to 85 percent 30 to 60 percent 30 percent 20 percent 

Concentrate 

temperature 

Ambient Ambient 5.5 to 15.5 o

C 

above ambient 

5.5 to 15.5 oC above 

ambient 

Concentrate 

blending 

Possible, not 

typical 

Not typical, but 

becoming more 

common 

With cooling water 

discharge 

With cooling water 

discharge 

Final concentration 

factor 

2.5 to 6.7 1.4 to 2.5 < 1.15 < 1.15 

8.2 Concentrate Disposal Options 

Several methods are used to dispose of the concentrate streams. Surface water discharge is the 

most frequent disposal method used for brackish water plants and is the disposal method for 

nearly all seawater plants. Surface water discharge would include discharge to bodies of surface 

waters such as the ocean or a sea, a river or stream, or to the effluent end of a sewage treatment 

plant. Effluent disposal options for concentrate disposal that have been used include: 
Chapter 8: Environmental Considerations

171

• Surface water discharge 

• Disposal to the front end of a sewage treatment plant for processing 

• Deep well injection 

• Land application 

• Evaporation ponds/salt processing ponds 

• Concentrate concentrators for zero discharge facilities 

Figure 8-1 illustrates the relative predominance of disposal methods for brackish RO plants 

within the U.S. 

Figure 8-1 Comparison of concentrate disposal methods for all brackish 

water processing facilities in the U.S. (Mickley, 2001) 

8.2.1 Possible Environmental Issues 

In the U.S., a significant amount of time is required to permit a new desalination plant. In 

addition, if surface water discharge is not an option, concentrate disposal cost will have a 

significant impact on the overall cost of the facility. Other environmental issues related to 

desalination facilities include: 

• Disposal of used equipment and membrane vessels 

• Disposal of cleaning chemicals 

Surface 

55% Sewer 

26%

Deep 

well

18%

Other 1%
Desalting Handbook for Planners 

172

• Antifoam agents 

• Biocides 

• Air quality issues, resulting from emissions from the associated power provider 

• Aesthetic concerns 

The permitting process is an important element of the overall planning process. Through the 

permitting process, planners can gather the data and analyses needed to ensure good plant 

designs and good stewardship. Considering the environmental needs and sustainability issues, 

and incorporating these considerations is crucial when designing new desalination plants or 

renovating existing facilities. 

The permitting process differs widely, depending on the State, type of feed water, type of plant, 

and type of discharge. 

8.2.2 Surface Water Discharge 

The most common discharge concentrate involves discharging the effluent directly into a larger 

body of water. Regulations for discharging concentrate into surface waters mandate that no 

alternate beneficial use of the water may be significantly hindered and that the discharge does 

not harm the ecosystem in question. 

The Clean Water Act (CWA) has mandated the development of standards and regulations for all 

wastewater discharges to surface water. To discharge concentrate to surface water, a National 

Pollutant Discharge Elimination System (NPDES) permit must be filed. For desalination, 

NPDES permits are based solely on discharge water quality standards, which are both narrative 

and numeric. To assess the feasibility of obtaining a NPDES permit for desalination concentrate 

discharge to surface water, the concentrate must meet water quality standards that apply to the 

surface water in question. 

An example of a facility that discharges concentrate directly to surface water is a reverse osmosis 

plant in the city of Newport News, Virginia, which began operation in 1998. This facility was 

designed to produce 21,577 cubic meters per day (m3

/d) (5.7 million gallons per day [mgd]) of 

potable water from a brackish ground water supply, which has a feed water total dissolved solids 

(TDS) value of 2,900 mg/l and fluoride contamination. The membrane concentrate is discharged 

directly into a nearby river.

Water Factory 21 in Orange County, California, brought online in 1977, is another RO facility 

that discharges concentrate to surface water. Operating in an area that receives less than 

15 inches of rainfall annually, this facility was intended to decrease the salinity of the ground 

water and regenerate the water table. This is a water reuse facility, with concentrate that is 

disposed through an ocean outfall. Seawater intrusion has been a concern since the 1950s, when 

overpumping caused the water table to sink below sea level, and subsequently, ground water 
Chapter 8: Environmental Considerations

173

salinity began to increase. Water Factory 21 processes municipal wastewater, with a feed TDS 

value of 935 mg/l, to produce a designed capacity of 22,713 m3

/d (6 mgd) of potable quality 

water, which is injected into the water table to prevent further seawater intrusion. 

8.2.3 Discharge to Sewer 

Discharging concentrate to the sewer system has become prevalent in recent years. A NPDES 

permit is not required as long as the membrane processing facility discharges to a publicly 

owned treatment facility. If concentrate is discharged to a publicly owned treatment facility, this 

facility may have trouble meeting treatment standards, due to high concentrations of 

contaminants. The CWA has established standards for all discharge water going into public 

sewer systems, and these standards should be met before any concentrate leaves the membrane 

processing facility. This may require pretreating the concentrate prior to disposal or obtaining 

special permitting from the local sewer system. 

An example of membrane concentrate disposal to sewer is the brackish desalting/softening 

facility in Hollywood, Florida. This is a reverse osmosis/nanofiltration plant that processes 

brackish and hard water with visible coloration. This facility has a design capacity of 

136,274 m3

/d (36 mgd) of which presently 68,137 m3/d (18 mgd) is currently operating (seven 

trains of 7,570 m3

/d [2 mgd] nanofiltration capacity and two trains of 7,570 m3/d [2 mgd] RO 

capacity). 

8.2.4 Deep Well Injection 

Deep disposal wells are defined as wells that inject fluids beneath the lowest geologic formation. 

The wells must be at least 0.4 kilometer (km) (0.25 mile) from an underground drinking water 

source. Deep well injection has declined as a disposal method in recent years. Only the State of 

Florida practices deep well injection, within the context of membrane concentrate disposal. 

Many States do not permit deep well injection at all. Permits for deep well injection are 

regulated by the U.S. Environmental Protection Agency (EPA) and also mandated by the State, 

in most cases. Permits are evaluated on a site-specific basis because feasibility is highly 

dependent on regional geology. A NPDES permit may be sufficient for permitting deep well 

injection, although the Underground Injection Control (UIC) program and State agencies may 

require additional permitting. 

An example of a desalination facility that uses deep well injection to dispose of concentrate is the 

RO/softening plant in North Collier County, Florida, which began operation in 1993. This 

facility processes brackish ground water, with a TDS value of 600 mg/l, and has a design 

capacity of 75,709 m3

/d (20 mgd) of potable water. Monitoring wells ensure compliance of this 

deep well disposal system. 
Desalting Handbook for Planners 

174

8.2.5 Land Application 

In land applications, just as in surface water discharge, concentrate applied to soils may affect 

either surface or ground water resources. Spray irrigation may be implemented if there is a need 

for irrigation close to the desalination plant and if the concentrate TDS is acceptable for crop 

growth. Blending the concentrate with available lower salinity wastewaters may be necessary. 

This method of disposal is practiced infrequently.

8.2.6 Evaporation Ponds/Salt Processing Facilities 

Evaporation ponds are extremely land intensive and are, therefore, only applicable in areas with 

low land value. Ponds are most appropriate for smaller desalination facilities located in arid 

climates such as the Southwestern U.S. Evaporation ponds are simple to construct and have low 

maintenance cost. They generally require an impermeable liner, which usually constitutes most 

of the cost, over and above the land cost. Evaporation ponds do not require permits under the 

NPDES or UIC, as long as the responsible party can provide conclusive evidence that no leakage 

will occur. Commonly, users of evaporation ponds acquire a NPDES permit, rather than prove 

that no leakage will be possible. 

8.2.7 Concentrate Concentrators for Zero Discharge Facilities 

If zero liquid discharge facilities are required, the resulting residue from the concentrate will be 

sludge or dry salts. Solid waste disposal methods are required, which stipulate that the waste is 

stored in an impervious area to prevent contamination of drinking water sources. Concentrate 

concentrators were developed in the 1970s to achieve zero discharge of wastewater for thermal 

power stations. Currently, there are at least 75 concentrate concentrators in operation worldwide. 

About 12 are used to concentrate RO effluent (Mickley, 2001). 

8.3 Federal Legislation in the U.S. 

This introduction to the applicable environmental regulations is provided to briefly explain the 

regulatory framework for desalination facilities in the U.S. The first congressional efforts to 

control environmental pollution on a comprehensive basis began with the enactment of the 

Federal Water Pollution Control Act of 1956. Air pollution control legislation followed in 1964, 

with the Clean Air Act. In 1965, the Solid Waste Disposal Act (SWDA) completed the 

legislative scheme intended to manage the sources of environmental pollution. 

These legislative initiatives were partially successful in improving the quality of the 

environment, but they did not represent a coordinated effort, nor did they satisfy growing public 

clamor for the enforcement of a balance between human activity and the natural ecosystem. In 
Chapter 8: Environmental Considerations

175

recognition of this fact, Congress enacted the National Environmental Policy Act (NEPA) in 

1969. NEPA has since become the cornerstone of the nation's environmental protection effort. 

NEPA sought to establish a national purpose by stating three specific goals to: 

• Foster a harmonious relationship between man and his environment 

• Promote prevention of damage to the national environment and stimulate human health 

and welfare 

• Enrich man's understanding of the ecosystem and the earth's natural resources 

Congress moved swiftly to fully implement these goals. After several years, all of the original 

major environmental statutes were re-evaluated and refined. As time went by, it became obvious 

that some additional refinement of the major components of the underlying authorizations was 

needed. The CWA was amended in 1987, and it greatly increased the scope of the 1977 statute. 

The Resource Recovery Act, enacted in 1970, substantially modified the old Safe Drinking 

Water Act (SDWA). In 1986, the language of the CWA was completely replaced. Finally in 

1996, the reauthorization of the SDWA established additional environmental control 

mechanisms to help prevent contamination of the Nation's drinking water supply. 

All of these major statutory environmental management initiatives were similar in that they: 

• Called for the establishment of national environmental quality standards 

• Placed primary responsibility for the attainment of those standards on the States while 

providing for Federal and citizen overview of State activities and direct Federal 

intervention of citizen action through the Federal courts, when warranted 

• Defined uniform program policies, approaches, and procedures to be followed by 

responsible government agencies 

• Provided for technical, program management, and financial assistance for those 

government agencies 

• Supported basic research in the areas of pollutant effects and control technologies 

• Provided for substantial civil and criminal penalties, including fines and imprisonment, 

for those who willfully disregard the statutory pollution control laws 

State environmental protection programs have generally followed close behind the Federal 

programs. By statute, State legislatures must implement Federal law as a minimum standard. In 

many cases they have incorporated more stringent provisions into State environmental laws. At 

the same time, States have provided for their own unique environmental needs. With very few 

exceptions, every important national and State environmental value is protected to some extent in 

a cooperative Federal and State environmental protection program. 
Desalting Handbook for Planners 

176

It has become virtually impossible for anyone to engage in an activity which has significant 

environmental effects without being subject to Federal, State, and local environmental 

restrictions. All potential pollution must be prevented or controlled so that air, water, and land 

resources are protected for their intended uses both now and in the future. Further, any waste 

products resulting from pollution control must be handled and treated or disposed of safely, so 

that the solution of one pollution problem does not lead to the creation of another. 

Every new industrial, commercial, or public works project is subject to these requirements 

through a regulatory system which includes requirements for environmental impact assessments, 

permits for construction and operation, and compliance monitoring and verification. Desalting 

plants are subject to these environmental requirements and will be scrutinized to ensure their 

overall public benefit. 

8.3.1 National Environmental Policy Act 

The intent of NEPA is to establish the Nation's environmental protection policy. NEPA seeks to 

ensure that all major actions of the Federal government are consistent with that policy. Major 

Federal actions range from new statutory and regulatory initiatives to the licensing of individual 

projects. 

NEPA requires every major Federal action to be evaluated for its potential environmental impact. 

The Federal agency planning a project must prepare (or have some other entity prepare) a 

detailed environmental assessment (EA). If the EA indicates the possibility of significant 

environmental effects, a comprehensive environmental impact statement (EIS) must be prepared. 

Federal action, such as licensing projects, may not be taken until the Federal, State, and local 

governmental agencies with jurisdiction over pertinent areas of environmental concern have 

reviewed the EIS and determined that the action will be acceptable. Environmental acceptability 

means that there will be no significant environmental risks, that the risks that are presented are 

justified by the benefits to be attained, and that no viable alternatives to those risks are available. 

Figure 8-2 provides a flowchart for NEPA activities. 

EAs and EISs, if required, are prepared in accordance with guidelines issued by the Council on 

Environmental Quality (CEQ), which oversees EIS preparation and review. These guidelines 

have been replaced by formal CEQ regulations, which were promulgated on November 29, 1978, 

and took effect on July 30, 1979. 

The review of draft EISs is coordinated through the Office of Management and Budget A-95 

(OMB, A-95) clearinghouse review process to ensure that all appropriate government agencies 

have adequate opportunity for review and comment. When disputes arise that cannot be resolved 

among agencies, CEQ provides mediation services to help complete the EIS and reach final 

determinations on environmental acceptability. 
Chapter 8: Environmental Considerations

177

Fi

gure 8-2 Reclamation’s NEPA flowchart (Source: Reclamation, 2000)
Desalting Handbook for Planners 

178

NEPA requirements are applicable to desalting plants because of their involvement with one or 

more of the following: 

• Federal funding in the form of technology demonstration grants, or economic 

development grants, or Federal construction loans or loan guarantees. 

• Wastewater discharge into navigable inland, coastal, or marine waters requiring a 

NPDES permit, pursuant to the provisions of Section 404 of the CWA. 

• Location in a designated wetlands area requiring a "dredge and fill" permit from the 

U.S. Army Corps of Engineers, pursuant to Section 404 of the CWA. 

• Location in a flood plain or flood hazard area requiring flood insurance from the 

Department of Housing and Urban Development. 

• Location in, or adjacent to, the habitat of a species of animal listed pursuant to Section 4 

of the Endangered Species Act of 1973 by the U.S. Fish and Wildlife Service of the 

Department of the Interior. 

• Licensing by the Federal Power Commission for the location, construction, and operation 

of a power plant or related facility. 

• Requests to the Economic Regulatory Administration of the Department of Energy for 

exemptions from the requirement of the Power Plant and Industrial Fuel Use Act of 1978, 

which requires all major fuel burning installations to use coal as a primary source of fuel. 

Major installations are defined as those with a heat input capacity equal to or greater than 

29,287.47 kilowatts (100 million British thermal units) per hour. 

Environmental assessments for desalting plants must, as a minimum, consider: 

• Purpose and need for the facility 

• Alternatives considered 

• Affected environment 

• Environmental consequences 

The “Affected Environment” and “Environmental Consequences” sections of an EA must give 

specific consideration to air, water, land, and noise pollution. Consideration must also be given 

to the relationship between the facility and existing or potential general land use, comprehensive 

waste treatment management plans, State implementation plans, solid waste management plans, 

and, if appropriate, coastal zone management plans, flood plain management plans, and wetland 

protection plans. 
Chapter 8: Environmental Considerations

179

If no significant environmental impact is found, a "negative declaration" (sometimes called a 

Finding of No Significant Impact [FONSI]) is issued for public scrutiny. If there are no 

legitimate objections to the "negative declaration," then no further analysis is required, and the 

project is determined to be environmentally sound. On the other hand, if the EA identifies 

potentially harmful effects, or if a negative declaration does not stand up to public scrutiny, an 

EIS may be required. As mentioned above, draft EISs are subject to OMB A-95 clearinghouse 

review, and comments from this review are incorporated into a final EIS. The final EIS is 

subjected to public hearing and is finally issued with a finding of environmental acceptability or 

unacceptability. EPA makes recommendations to the lead agency for improving the draft EIS, 

based on a set of criteria for rating draft EISs. For these ratings, visit the EPA website: 

<http://www.epa.gov/compliance/nepa/comments/ratings.html> 

The most serious effect NEPA may have on desalting projects pertains to the project schedule. 

The preparation of an EA normally takes from 3 to 6 months. If an EIS is required, an average 

of 12 months will be required for preparation and review. Another 6 months will normally be 

required for finalization, public hearing, and a final determination. For projects that are subject 

to the entire EIS process, approximately 2 years are required to secure approval. Complex or 

controversial projects may take longer. 

8.3.2Clean Water Act 

The long-term goal of the Clean Water Act is the elimination of the discharge of pollutants into 

the navigable waters and coastal areas of the Nation. The CWA seeks to preserve water quality, 

which provides for domestic water supply, recreation (both in and on the water), and for the 

propagation of fish, shellfish, and wildlife. The CWA has created a NPDES program to regulate 

all significant pollutant discharges. The NPDES program is based on a comprehensive system of 

effluent guidelines and standards, standards of performance for new sources, and—where no 

published standards exist—best engineering judgment. 

Most of the States have taken over complete program authority, including authority to issue and 

reissue NPDES permits. EPA's current responsibility is to monitor both overall program 

effectiveness and individual permit actions in the delegated States, and to continue to administer 

the program in those States which have not yet requested or received delegation of program 

authority. 

Desalting plants of all types produce significant quantities of concentrated water of different 

quality, depending upon sources such as pretreatment, blowdown, and cleaning wastes. 

Discharge of these materials into, or in close proximity to, surface water requires an NPDES 

permit. 

Water quality standards for TDS in inland waters rarely exceed 500 mg/l. This fact alone would 

preclude the use of direct discharge. Concentrate discharge to the ocean may be permissible, 

depending on quantity, salinity, temperature, contamination, and a number of other factors, 

including the nature of the marine or estuarine ecosystem in the discharge area. 
Desalting Handbook for Planners 

180

8.3.3Safe Drinking Water Act 

The scope of the SDWA is to guarantee a plentiful supply of safe drinking water. An essential 

part of that effort is the protection of underground sources of drinking water through the 

development of State UIC programs. These UIC programs are designed to regulate deep well 

disposal of fluids so as to prevent, in any potential underground source of drinking water, the 

presence of any contaminants that may result in the failure of that source of supply to conform to 

the national drinking water standards that have been established by the EPA. 

The EPA has developed regulations for State UIC programs. The EPA is also developing 

regulations to administer a national UIC program responsibly. In the interim, many States have 

already put into effect comprehensive UIC programs that include many of the control concepts. 

The SDWA also provides for the EPA to study the impact of surface disposal methods on current 

and potential underground drinking water supplies. This study may lead to control requirements 

for surface waste disposal.

Deep well injection is one method to dispose of concentrate from inland desalting processes. 

EPA regulations require permits for all waste disposal wells, including concentrate disposal 

wells. Presently, all States have the authority to determine whether to permit deep well injection 

wells, as long as they are in compliance with the EPA Federal regulations regarding waste 

disposal wells. 

UIC permits for deep well disposal of concentrate impose requirements on well construction 

involving casing and cementing, to prevent migration of fluids into or between underground 

sources of drinking water. Permits also impose requirements for operating, monitoring, and 

reporting information to the EPA. 

Where the climate is suitable, concentrate may be disposed of in evaporation ponds. It is 

possible that these ponds will come under government control in the future, if they are found to 

threaten underground water supplies. Under most statutory requirements in existence today, any 

disposal pond must be fully lined with an impermeable liner and have sufficient freeboard to 

ensure complete containment under all conditions. In cases where the surficial ground water is 

suitable, infiltration into brackish or saline wetlands may be permitted. 

Casing or cementing requirements for wells, or liner requirements for evaporation ponds, will 

have an impact on the cost of producing water for all but the smallest desalting plants. The 

increasing requirements for operation, monitoring or reporting, and operations will also burden 

smaller systems. However, the most significant effect of SDWA requirements on desalting 

projects may be on project schedule. 

8.3.4Other Regulations 

The purpose of the Clean Air Act is to protect and enhance the quality of the Nation's air 

resources to promote and protect the health and welfare of its population. This act created a 
Chapter 8: Environmental Considerations

181

balanced strategy for the Nation to attack problems. The act requires integrated strategies for 

preventing and controlling air pollution from both new and existing stationary and mobile 

sources. The States are to provide for the attainment and maintenance of national clean air 

standards established by EPA to protect public health, property, and other air quality related 

values. 

States have been delegated enforcement authority, but the EPA maintains an overview of State 

activities and has the authority to step in and take direct Federal action when States fail to carry 

out their responsibilities. The EPA also administers new regulatory measures intended for 

eventual incorporation by the States, while States are acquiring resources and developing rules 

and procedures in preparation for these programs. Federal New Source Performance Standards, 

National Environmental Standards for Hazardous Air Pollutants, and the Prevention of 

Significant Air Quality Deterioration (PSD) program are examples of these EPA measures. 

A major PSD requirement is for computer modeling of air pollution dispersion, along with 

ambient monitoring of the proposed site, to demonstrate that no ambient air quality standards 

will be violated. PSD permit application preparation can take more than 1 year, if ambient 

monitoring data must be collected. PSD permit application review by the EPA can take as long 

as 1 year, and typically takes 4 to 8 months. State permit review procedures can take just as long 

when the EPA is involved but, fortunately, State review can proceed concurrently with the EPA's 

PSD review, so that the reviews can be coordinated and total review time minimized. 

One of the major objectives of the Resource Conservation and Recovery Act (RCRA) of 1976 is 

to promote the protection of health and the environment. Two ways in which the RCRA seeks to 

achieve this objective are to eliminate open dumping of nonhazardous wastes on land and to 

regulate the treatment, storage, transportation, and disposal of hazardous wastes, so they will 

have no adverse effects on human health or the environment. 

Since RCRA represents a major statutory initiative intended to close the circle of environmental 

control that began with the enactment of the generation of air and water pollution control laws, 

the EPA has been very deliberate in its implementation. The development of necessary 

regulations is behind the statutory timetable. The regulations have upgraded disposal 

requirements for nonhazardous waste to the standards of sanitary landfills and have identified a 

list of hazardous materials, so that all such materials may be stored and transported safely and, 

ultimately, may be properly treated and disposed of in facilities licensed by the government. A 

number of core regulations have been proposed—the most recent in 1997. 

Desalting plants may produce both hazardous and nonhazardous solid waste. An example of 

nonhazardous waste is debris collected at seawater intake screens, which must be hauled away 

for disposal. Replaced membranes and dirty cartridges from filters may also fall into this 

category. Though not common, the primary source of hazardous wastes from a desalting plant 

could be sludge or residue from the pretreatment of hazardous types of feed water. Examples of 

potentially hazardous waste include treating wastewaters from industrial manufacturing facilities 
Desalting Handbook for Planners 

182

or recycling wastewaters with heavy metals or other contaminants. Sludge from pretreating 

certain industrial return flows or agricultural runoff may contain substances that meet EPA's 

hazardous materials criteria. 

Desalting plants that produce hazardous pretreatment sludge may be classified as hazardous 

waste generators or treatment and disposal facilities. As such, they would be subject to RCRA’s 

licensing, monitoring, and reporting requirements. Compliance with RCRA requirements related 

to nonhazardous wastes means that such wastes must be disposed of in permitted sanitary 

landfills, rather than in open dumps. 

Hazardous pretreatment sludge must be incinerated, chemically treated, placed in a landfill, or 

impounded according to EPA standards. If desalting plants that produce such sludge are 

considered treatment facilities, or partial treatment facilities, within the meaning of Section 3004 

of RCRA, they must be licensed by the EPA or the State. 

Licensing requirements involve general facility standards that may apply to selection, security, 

emergency procedures, operator training, manifests, recordkeeping, reporting, inspections, 

closures and post-closure responsibilities, ground water monitoring, and financial requirements. 

Licensing requirements also involve the treatment and disposal standards referred to above. 

Utility wastes, such as fly ash, bottom ash, and scrubber sludge, although hazardous, are 

classified as special wastes because their large volume prohibits normal hazardous waste 

treatment and disposal. Special waste treatment and disposal facilities must meet RCRA's 

general facility standards, but are not yet subject to any specific treatment and disposal 

requirements. 

The effect of the RCRA on desalination plants is the cost of safe disposal of hazardous 

pretreatment sludge and special utility wastes, if any. The treatment and disposal facility 

licensing process may take some time and should be provided for in the project schedule. 

8.4 State Requirements 

Most State environmental requirements of concern to desalination facilities planners are modeled 

after the Federal requirements. This section focuses on State requirements that are substantially 

different from Federal requirements or are otherwise unique. There are three areas in which 

State regulations merit additional concern. 

8.4.1 Underground Injection Control 

In most States, well disposal is not acceptable. Many States in the western part of the country 

have had serious problems with contamination of ground water by well disposal. Well disposal 

is prohibited in Missouri and only permitted under extraordinary circumstances in Washington. 
Chapter 8: Environmental Considerations

183

Arizona does not allow any disposal wells that will penetrate water-bearing strata, while 

Nebraska, Oregon, and a number of other States have very stringent permit requirements. 

8.4.2Coastal Zone Management 

As a result of the environmental degradation of a few coastal areas in the past 10 years, as well 

as the passage of the Coastal Zone Management Act, many States, such as New Jersey, have 

developed special regulations that require EISs and special permits before a new source can be 

located in their coastal zones. Other States, such as Delaware, prohibit locating any industries 

with large external equipment in their coastal zones. 

8.4.3State Environmental Impact Assessment 

Just as environmental assessments are required for major action taken by Federal agencies, many 

States have similar requirements for State agencies. (Many States require environmental 

assessments for various actions, and at least five States have limited or special environmental 

assessment requirements.) The requirements of the State of Michigan are typical. They require 

an environmental impact assessment to be prepared for activities that meet one of the following 

criteria: 

• Major policies, projects, administrative actions, and programs 

• Any activity, regardless of size, that could have a significant environmental impact 

• Controversial activities of significant impact 

In general, a Federal EIS is required when an activity will result in one of the following: 

• A potential significant impact on the human environment that could adversely affect the 

public health and welfare or could degrade the quality of life 

• Alteration or destruction of a significant element of the human, natural, or historic 

resources of the State 

• Significant alteration in existing land-use patterns 

• Significant alteration in population distribution 

• Significant impact on the maintenance and enhancement of the long-term productivity of 

the State's natural resources 
Desalting Handbook for Planners 

184

• Imposition of an alteration to the ecological integrity of a significant element of the 

environment 

• Significant additional use of energy resources or the acquisition thereof 

Any of these special State requirements may affect desalting projects and may be as critical 

during desalting facility planning as any of the Federal requirements. 

8.5 Environmental Obligation Assessment 

Federal and State environmental requirements are not only comprehensive and substantial, but 

can be closely interrelated. Unfortunately, the interrelationships among the various statutory and 

regulatory requirements are not always obvious, which often makes proper assessment of the 

environmental obligation of a particular project extremely difficult. It can be a complicated 

process to identify all requirements that apply, and then to determine if, and how, those 

requirements interrelate. 

This discussion presents a rational approach for assessing environmental obligations, developing 

compliance strategies, and securing all necessary approvals in the shortest possible time. The 

approach has 10 basic steps: 

(1) At the earliest stage in project planning, when little more than the objectives and, perhaps, 

the outline of an approach is understood, the basic project should be tested by determining its 

general consistency with the various State resource management plans. 

(2) If the concept appears to be feasible at the planning level, an assessment should be 

developed, based on the preliminary concept. This assessment should address the following 

factors: 

• Potential site locations and characteristics 

• Process type and size 

• Identification of waste streams 

• Rough estimate of waste quantities 

• Possible control options 

• Environmental impact 

(3) Locations and characteristics, process type and size, and environmental impacts should 

then be varied to optimize the process within the project goals and environmental limitations. 

At this level, variations can be performed relatively quickly and easily. 
Chapter 8: Environmental Considerations

185

(4) Options that satisfy both project objectives and environmental requirements should be 

discussed with appropriate State and Federal environmental officials to obtain preliminary 

confirmation of acceptability and to determine which preconstruction approvals will be 

necessary. 

(5) With this input, the project plan, including completion schedule, can be finalized and 

detailed process flow diagrams can be developed. 

(6) Once the flow diagrams have been completed, rough estimates of waste quantities, 

control options, and environmental impacts of the assessment should be updated to precisely 

define all pertinent environmental factors. 

(7) The updated assessment should be reviewed with government officials to reconfirm 

acceptability and decide if an EIS is necessary. 

(8) EIS preparation, if necessary, and permit application preparation should proceed 

concurrently with detailed engineering, so that the EIS and permit applications can be 

submitted for approval well ahead of the scheduled date for commencement of construction. 

(9) Consolidated EIS and permit reviews should be requested, so that reviews will be 

coordinated, rather than duplicated, and will culminate in a single and conclusive hearing. 

(10) Construction should begin before construction permits expire or requirements change. 

This approach is widely applicable, even in the case of small desalting projects that may face 

only a few State or local environmental requirements. It is an essential approach for major 

projects that may be subject to many requirements and that have a number of substantial 

environmental impacts. 

References 

Mickley, M., “Environmental Considerations for the Disposal of Desalination Concentrates,” 

1995 International Desalination Association World Congress on Desalination, Proceedings, 

Abu Dhabi, United Arab Emirate, 1995. 

Mickley, M., “Membrane Concentrate Disposal: Practices and Regulation,” U.S. Bureau of 

Reclamation Desalination and Purification Research Development Program, Report No. 69, 

Denver, Colorado, USA, September 2001. 

Reclamation, “National Environmental Policy Act Handbook,” Public Review Draft, 2000. 

Wangnick, K., “2000 IDA Worldwide Desalting Plants Inventory, Report No. 16,” International 

Desalination Association, May 2000. 

Chapter 9: Cost Estimating Procedures

187

Chapter 9: Cost Estimating Procedures 

This chapter provides cost estimates for the desalting processes discussed in previous chapters. 

The capital (construction) cost data presented are derived from actual bids, vendor quotations, 

experience, and personal cost data files. The operating costs are derived, in part, from actual 

plants in service, supplemented by performance estimates. The cost estimating information 

should not be used for procurement negotiations or for seeking construction funding. The data 

should be used only to compare alternative schemes for water supply at a planning level, or for 

similar purposes. The implied level of accuracy of the data presented in this chapter is 

approximately ± 30 percent. 

The cost basis year is year 2000 in the United States. For that year the values of two relevant 

indices were: 

• Engineering News Record Building Cost Index 3539 

• Engineering News Record Construction Cost Index 6221 

Data contained in this chapter are based on the assumption that the owner will award a contract 

that includes preparing design and specifications, procurement, construction engineering, 

construction supervision, and construction of the facility, including furnishing all of the required 

materials and equipment. 

In many cases, the base conditions and costs associated with a planned project may be 

significantly different from those presented in this chapter. In such cases, adjustments to the data 

presented here may be made by using a ratio of the unit costs used in this chapter and the unit 

cost for a specific project. For example, if the electric power cost at the planned site were $0.09 

per kilowatthour, the annual power cost would be 1.5 times the value derived in this chapter. 

The Bureau of Reclamation, with the assistance of I. Moch & Associates and Boulder Research 

Enterprises, has developed a computer program, WTCost©, that evaluates the capital and 

operation and maintenance costs for membrane systems with a wide variety of pretreatments and 

post-treatments. The model contains cost algorithms for reverse osmosis/nanofiltration 

(RO/NF), ultrafiltration/microfiltration, electrodialysis, and ion exchange. Intake and outfall 

infrastructures, pretreatment and post-treatment unit operations, pumps, tanks, and control 

instrumentation are also included so that plant costs can be fully estimated. All inputs needed for 

the calculations, such as water analyses, energy and chemical usages and prices, labor staffing 

and rates, construction indices, and amortization have default values that can be overridden when 

more complete information is available. The program includes an editable membrane database 

but is supplier neutral. The program is available from I. Moch at imoch@aol.com. For a further 

discussion of cost comparisons for membranes, see Chapman-Wilbert (1998). 

Costs developed from the graphs in this chapter generally yield a higher water cost compared to 

the output of WTCost©, indicating that the graphs are somewhat conservative. Worked 

examples using these graphs are presented in Appendix D for six different applications: 
Desalting Handbook for Planners 

188

• Multi-Stage Flash Evaporation 

• Multi-Effect Distillation 

• Mechanical Vapor Compression 

• Surface Seawater Reverse Osmosis 

• Ground Water Nanofiltration 

• Brackish Surface Water EDR 

9.1 Background Information for Cost Estimate 

Estimating costs for a desalting system begins with identifying the water source and its 

characteristics, plant capacity, and site-specific design and cost criteria. The design criteria 

establish the plant size, water quality requirements, etc., which are used in performing the 

conceptual technical design of the facility. The cost criteria establish the key parameters used to 

calculate the capital and operating costs. Using standard forms for presenting cost data will aid 

in comparing costs for alternative plans and ensure that the required features for each plant are 

included. This is particularly important because “site-specific” features, such as the intake type, 

location, electricity costs, etc., will vary between scenarios. These standard forms, tables 9-6 and 

9-7, are included before the graphs at the end of this chapter for your convenience. The forms 

can be modified when required to meet the needs of each particular investigation. For a 

background comparison, table 9-1 provides estimated design and construction periods for the 

various desalination technologies. 

Table 9-1.—Estimated design and construction periods 

Process Capacity, m3

/d (mgd) Design time (months) Construction time (months) 

378.5-1,892.5 (0.1-0.5) 3 6 

3,785 (1) 6 9 

18,925 (5) 9 12 

37,850 (10) 12 18 

MSF 

189,250 (50) 18 36 

378.5-1,892.5 (0.1-0.5) 3 6 

3,785 (1) 6 9 

18,925 (5) 9 12 

37,850 (10) 12 15 

MED 

189,250 (50) 18 30 

378.5-1,892.5 (0.1-0.5) 3 6 

3,785 (1) 4 9 

18,925 (5) 6 12 

37,850 (10) 9 18 

MVC 

189,250 (50) 12 24 

378.5-1,892.5 (0.1-0.5) 3 6 

3,785 (1) 4 9 

18,925 (5) 6 12 

37,850 (10) 9 18 

SWRO 

189,250 (50) 12 24 

378.5-1,892.5 (0.1-0.5) 3 6 

3,785 (1) 4 9 

18,925 (5) 6 12 

37,850 (10) 9 15 

BWRO/EDR/NF 

189,250 (50) 12 18 

 Note: MSF = multi-stage flash distillation, MED = multiple effect distillation, MVC = mechanical vapor compression, 

SWRO = seawater reverse osmosis, BWRO = brackish water reverse osmosis, EDR = electrodialysis reversal 
Chapter 9: Cost Estimating Procedures

189

9.2 Capital Cost 

Capital cost (construction cost), as used in this document, refers to all costs associated with 

building the entire facility. In this sense, capital costs are often referred to as investment costs. 

The total capital (investment) cost estimate should represent a workable plant in place. Use 

tables 9-6 and 9-7 to record cost assumptions for your specific site. The same tables can be used 

for membrane process plants or distillation process plants. When cost detail is obtained from 

sources other than this chapter, supporting information should be shown in backup sheets. 

9.2.1 Capital Cost Basis 

The following cost basis was used for preparing the curves contained in this chapter. 

Direct Capital Costs 

The direct capital cost is the sum of: 

• A desalting plant – distillation or membrane 

• Concentrate discharge piping and disposal 

• Pretreatment 

• Water intake structure 

• Feed water supply piping to the desalination units 

• Steam supply for distillation plants 

• Site preparation, paving, and grading 

• Post-treatment equipment 

• Process building for membrane systems 

• Auxiliary equipment 

• Product storage 

• Emergency generators 

• Step-down transformer and control building for thermal systems 

• Product water distribution (high service) pumps 

• Process building for membrane systems 

Indirect Capital Costs 

Indirect costs are based on a percentage of direct capital costs. They include: 

• Freight and insurance 

• Construction overhead 

• Owner’s direct expense 

• Contingency 
Desalting Handbook for Planners 

190

Depreciating Capital Costs 

Depreciating capital costs are costs amortized over the life of the plant. They may include: 

• Land cost 

• Working capital 

Total Capital Cost is the sum of direct, indirect, and depreciating capital cost. 

9.2.2 Annual Costs 

The annual operating costs are for: 

• Operations and maintenance labor 

• Chemicals 

• Energy 

• Supplies and maintenance materials 

• Replacement parts and membranes 

• Insurance 

• Fixed charges 

Costs for these items are estimated for a 1-year period and divided by the quantity of water to be 

produced during the year to determine the overall cost of water. All of these charges, with the 

exception of insurance and fixed charges, depend on the amount of water produced. The plant 

factor, the percentage of time the plant is planned to be on-line, is used to adjust these dependent 

costs when calculating the annual cost of water. 

9.3 Detailed Cost Estimates 

9.3.1 Capital Costs 

The following sections correspond to the items in tables 9-6 and 9-7. Use the cost parameters 

determined in each section to obtain cost from the indicated table. The total direct capital cost is 

the sum of items 9.3.1.1 – 9.3.1.10. 
Chapter 9: Cost Estimating Procedures

191

9.3.1.1 Desalting Plant Costs 

Multi-Stage Flash Distillation (MSF) 

Figure 9-1 gives the total estimated construction cost for a complete MSF distillation system 

(recycle configuration). Total costs are given for a performance ratio1

 of 3.44, 4.30, and 

5.17 kilograms per mega joule (kg/MJ) (8, 10, and 12 lb of distillate/kBtu]). Figure 9-2 gives 

costs for the process only (for the plant proper) for the same performance ratios. 

Multiple Effect Distillation (MED) 

Figure 9-3 gives the total estimated construction cost for a MED plant. This cost assumes the 

low temperature, horizontal tube configuration. Total construction costs are given for 

performance ratios of 3.44, 4.30, and 5.17 kg/MJ (8, 10, and 12 lb/kBtu). Figure 9-4 shows the 

process-only capital costs for performance ratios of 3.44, 4.30, and 5.17 kg/MJ (8, 10, and 

12 lb/kBtu). 

Mechanical Vapor Compression (MVC) 

Figure 9-5 gives the total estimated cost and cost basis for an MVC plant. Total costs are given 

for performance ratios of 3.44, 4.30, and 5.17 kg/MJ (8, 10, and 12 lb/kBtu). Figure 9-6 gives 

the process-only costs for the MVC process. 

Seawater Reverse Osmosis (SWRO) 

Figure 9-7 gives the total estimated construction cost for a SWRO plant with surface water feed. 

Figure 9-8 shows the total estimated construction cost for a SWRO plant using seawater wells, 

with a capacity limit of 10 million gallons per day (mgd) (37,850 m3

/d). Using seawater wells to 

extract more than 20 mgd (75,700 m3

/d) of feed water would normally not be considered feasible 

in the U.S. Total costs are based on single-pass plants, 50-percent recovery, 35,000 mg/l of total 

dissolved solids (TDS), and incorporating energy recovery devices. 

Brackish Water Reverse Osmosis (BWRO) 

Figure 9-7 gives the total estimated construction cost for a BWRO plant with surface water feed, 

and figure 9-8 gives the estimated construction cost for well water feed. Total construction costs 

are based on 75 percent recovery with no blending. The use of appropriately treated raw water 

for blending with the RO product will reduce the required RO capacity on a one-to-one basis. 

Raw water TDS is assumed to be 2,000 - 3,000 mg/l. 

 

1

 The performance ratio is the amount of water produced per unit of energy. 
Desalting Handbook for Planners 

192

Nanofiltration/Membrane Softening (NF) 

Figure 9-7 gives the total estimated construction cost for a NF plant with surface water feed, and 

figure 9-8 gives the estimated construction cost with well water feed. Total construction costs 

assume an 85 percent recovery, feed water TDS of 750 mg/l, and with no blending. 

Electrodialysis Reversal (EDR) 

Figure 9-9 gives the total estimated construction cost for an EDR plant with surface and well 

water feed. Total construction costs assume an 80 percent recovery, with no blending. Cost 

curves are given for 3-stage (1,500-2,000 mg/l TDS feed water) and 4-stage (2,500-3,000 mg/l 

TDS feed water) systems. Each stage will remove approximately 50 percent of the TDS. A 

practical limitation on the capacity of EDR is about 57,000 m3

/d (15 mgd). The Sarasota 

County, Florida, EDR plant, at 45,420 m3

/d (12 mgd) is the largest example of this technology 

built to date. 

9.3.1.2 Concentrate Disposal 

This section describes the cost parameters for discharge costs. Mickley and Associates (2001) 

describe in detail several other methods for concentrate disposal with methods for determining 

costs. 

Surface Water Discharge 

In most cases, surface water discharge consists of a simple pipe. Cost is based on pipe length 

and diameter. Figure 9-10 gives construction costs for concentrate for distillation disposal. 

Pipeline costs will depend upon the distance of the plant from the disposal site. The charts are 

prepared for three distances: 457 m (1,500 ft); 762 m (2,500 ft); and 1,067 m (3,500 ft). 

Figure 9-11 gives construction costs for surface water concentrate discharge for membrane 

systems. Pipeline costs for surface disposal will depend upon the distance to the discharge point. 

Three distances were chosen for estimating costs. 

Disposal to Sewer 

Where possible, this means of disposal is simple and usually cost effective. Factors that 

determine cost and feasibility are distance between the desalting plant and the sewage treatment 

plant, whether the two facilities are owned by the same entity, and future anticipated capacity 

increases. 

Land Application 

Cost depends on the loading rate, days of storage, cost of land, and the cost of clearing the land 

for planting. The values depend on the site, the evapo-transpiration rate, and the type of crops to 

be grown. 
Chapter 9: Cost Estimating Procedures

193

Evaporation Ponds 

Parameters for estimating costs of evaporation ponds are the cost of land, cost for excavation, 

area of the pond, height of the dike, and the liner thickness. These parameters depend on the 

evaporation rate, quality of the water, and capacity. Evaporation ponds and injection wells are 

assumed for brackish water systems only. Figure 9-12 gives costs for evaporation pond 

concentrate management for membrane systems. 

Concentrate Concentrators for Zero Discharge 

Costs depend on concentrate chemistry, energy usage, evaporator surface area, construction 

materials, and the need for chemical additives. 

Deep-Well Injection 

Deep-well injection is only appropriate where it is geologically feasible. The idea is to inject the 

concentrate down a well into a confined aquifer that cannot be used for drinking water or 

irrigation. The aquifer must not contact a fault and must be situated where there is little or no 

seismic activity. Cost depends on the diameter and depth of the well. The injection well costs 

are based on typical Florida injection well construction, with a 7,570 m3

/d (2 mgd) capacity per 

well, and 100 percent redundancy. Figure 9-13 gives costs for deep-well injection of concentrate 

from membrane systems. 

9.3.1.3 Pretreatment 

Figure 9-14 gives the construction costs for pretreatment for distillation processes, which include 

chlorine disinfection, sodium bisulfate for dechlorination, acid, anti-foam, decarbonator, and 

deaerator. Construction cost for surface water pretreatment systems for membrane facilities 

includes chlorine disinfection, sodium bisulfate dechlorination, two-stage filtration, and sludge 

treatment. Figure 9-15 presents these costs. 

9.3.1.4 Feed Water Intake 

Seawater and Brackish Surface Water 

An open intake was assumed for costing surface water systems. Figure 9-16 shows the 

construction cost for the intake for a distillation system. Figure 9-17 shows the construction cost 

for the intake for membrane systems. 

Seawater and Brackish Well Water 

Figure 9-18 gives costs for seawater and brackish well fields. Seawater wells are patterned after 

the Ranney method, with a yield of 15,140 m3

/d (4 mgd) per collector. Brackish well field costs 

are based on 7,570 m3

/d (2 mgd) per well, with a spacing of 305 m (1,000 ft). This includes 

submersible pumps, controls, security fence, and collection piping. 
Desalting Handbook for Planners 

194

9.3.1.5 Feed Water Pipe 

Figure 9-19 gives the construction costs for various lengths of seawater intake pipes for 

distillation processes. Figure 9-20 gives the construction costs for the feed water intake for a 

seawater membrane plant for different lengths of pipe. 

9.3.1.6 Steam Supply 

Steam supply for distillation may be provided by a boiler that produces steam for the thermal 

desalting process. Backpressure turbines are used to produce steam for power and desalting in 

dual-purpose systems. Dual-purpose desalting systems offer attractive economics. However, 

evaluation of cost advantages requires a detailed study that is beyond the scope of this chapter. 

Cost data included in chapter 7 give a general insight into capital costs, steam costs, and electric 

power costs involved. 

Figure 9-21 gives the construction costs for a boiler and controls (package boilers) for use with a 

single-purpose plant. 

9.3.1.7 General Site Development 

General site development includes excavation, site preparation, roads, and paving. Figure 9-22 

shows costs for distillation site development, and figure 9-23 shows costs for membrane 

facilities. 

9.3.1.8 Post-Treatment 

Post-treatment for distillation includes caustic and lime addition for stabilization and chlorine for 

disinfection. Degasification is also included for post-treatment of membrane system product 

water. Figure 9-24 shows product post-treatment construction costs for distillation, and 

figure 9-25 shows product post-treatment costs for membrane systems. 

9.3.1.9 Auxiliary Equipment 

Auxiliary equipment includes product storage and transmission, step-down transformers for 

distillation processes, and emergency generators. 

• Product storage tank construction costs are shown in figure 9-26 for distillation 

systems using steel tank construction and in figure 9-27 for membrane systems using 

prestressed concrete tank construction. 

• Product transmission piping construction costs are shown in figure 9-28, for 

pipeline lengths of 3,657 m, 7,315 m, and 14,630 m (12,000 ft, 24,000 ft, and 

48,000 ft, respectively). 
Chapter 9: Cost Estimating Procedures

195

• Emergency generator construction costs, including the fuel supply and piping, are 

given in figure 9-29. These costs assume the use of a diesel engine to drive the 

generator. 

• Step-down transformer costs are shown in figure 9-30. 

9.3.1.10 Building and Structures 

Figure 9-31 gives construction costs for distillation buildings, and figure 9-32 gives construction 

costs for membrane systems for unit costs of $807, $1,076, and $1,614 per square meter ($75, 

$100, and $150 per square foot). 

9.3.2 Indirect Capital Cost 

Indirect, or depreciating, capital costs are based on a percentage of the direct capital cost 

calculated in section 9.3.1. Indirect capital costs include freight and insurance, interest during 

construction, construction overhead, owner’s expenses, and contingency. These costs are added 

to the direct capital cost to calculate the estimated total capital cost. 

9.3.2.1 Freight and Insurance 

The total construction costs do not include the shipping costs for delivering plant components to 

the site. For plants erected in the U.S., these costs are generally taken at 5 percent of the 

construction cost. 

9.3.2.2 Interest During Construction 

Interest during construction is computed on the basis that the owner borrows money as needed to 

finance construction and pays simple interest on this money after it is spent. The computation of 

this cost is simplified by assuming a constant rate of expenditure of funds over the construction 

time for the process selected and applying the interest rate to the direct capital costs for a period 

of one-half of the construction time. 

9.3.2.3 Construction Overhead and Profit 

The contractor’s overhead includes: 

• Fringe benefits 

• Labor burden 

• Field supervisor 

• Temporary facilities 

• Construction equipment 

• Small tools 

• Miscellaneous 
Desalting Handbook for Planners 

196

Construction overhead is generally 15 percent of the direct capital costs for membrane systems. 

This percentage is modified for distillation processes using the adjustment factors in figure 9-33. 

The contractor’s profit is assumed at 3 percent. 

9.3.2.4 Owner’s Direct Expense 

The owner’s direct expenses include: 

• Project investigation 

• Land acquisition 

• Engineering design 

• Contract administration 

• Legal expenses 

• Administrative expenses 

• Commissioning and/or startup costs 

These costs vary considerably, depending on the owner’s method of accounting. The normal 

value is 10 percent of the direct capital costs, but this value depends on the specifics of a project 

and the size, both of which impact the total capital costs. Figure 9-33 shows adjustment factors. 

The commissioning period is used to: 

• Clean and flush piping 

• Check rotation of motor drivers 

• Perform hydrostatic testing 

• Set up and calibrate controls 

Membrane systems also include: 

• Disinfect the system 

• Load membranes 

This period will require between 2 weeks and 6 months, depending upon the size, complexity, 

and number of units to be operated. The costs associated with commissioning are borne by the 

contractor as part of the total capital cost of the facility. 

Startup costs are costs incurred by either the owner or the contractor. This period involves actual 

operation of the plant by conducting the performance and reliability testing. The duration of this 

period can be from 1 week to 1 month. During this time period, the contractor, equipment 

suppliers, and the owner will have personnel onsite. The costs for the owner’s personnel can be 

prorated from the annual labor cost for this facility. Note that there may be many more people 

and expenses during the startup. 

9.3.2.5 Contingency 

Contingency allocation is generally 10 percent of the direct capital cost. 
Chapter 9: Cost Estimating Procedures

197

9.3.3 Nondepreciating Capital Costs 

Nondepreciating capital costs are costs that do not lose value or expense, such as land and 

working capital cost. 

9.3.3.1 Land 

Figure 9-34 shows the land requirements for distillation process plants. These land requirements 

include roads and product storage areas. Local land costs should be used for pricing the site 

acquisition cost. Figure 9-35 shows the land requirements for membrane process plants. 

9.3.3.2 Working Capital 

Working capital is the ready cash on hand to cover the day-to-day expense of operating the 

facilities. This expense includes salaries (and salary overhead), chemicals, energy, maintenance 

materials, and insurance. For this item, an allowance of 2 months’ operating and maintenance 

cost is suggested. This allowance will result in a working capital cost of one-sixth of the total 

annual costs. 

9.3.4 Annual Cost 

Annual costs vary directly with the quantity of water produced. All costs are to be indexed to the 

price levels at the date of the estimate. 

9.3.4.1 Labor 

Figure 9-36 gives the annual labor costs for distillation processes, and figure 9-37 gives the 

annual labor costs for membrane processes. The labor category includes routine maintenance, as 

well as operations. Table 9-2 gives the total number of staff required for single and dual-purpose 

plants. Table 9-3 gives the total number of staff for surface and well water plants. These costs 

assume that the plant will be staffed 24 hours per day, 7 days per week for plants of 18,925 m3

/d 

(5 mgd) and larger and 16 hours per day, 5 days per week for plants smaller than 18,925 m3

/d 

(5 mgd). The actual staffing requirements will vary from State to State. The figures assume a 

labor rate of $25,000 per year. 

Table 9-2.—Operations and maintenance staff – distillation processes

Capacity, m3/d (mgd) 

Process 3,785 (1) 18,925 (5) 30,280 (8) 37,850 (10) 189,250 (50) 

Single purpose 14 14 18 20 35 

Dual purpose 12 12 13 14 28 
Desalting Handbook for Planners 

198

Table 9-3.—Operations and maintenance staff – membrane processes

Capacity, m3 Process /d (mgd) 

3,785 (1) 18,925 (5) 37,850 (10) 94,625 (25) 189,250 (50) 

SWRO 6 12 15 18 20 

BWRO, EDR, NF 6 10 12 15 17 

9.3.4.2 Chemicals 

Figures 9-38, 9-39, and 9-40 give the annual costs for chemicals at distillation plants with 

performance ratios of 3.44, 4.30, and 5.17 kg/MJ (8, 10, and 12 lb/kBtu) for MSF, MED, and 

MVC, respectively. Figures 9-41 and 9-42 give the chemical costs for pretreatment, posttreatment, and cleaning for membrane systems for surface water and ground water sources, 

respectively. For more accurate estimates based on usage, the current unit cost of chemicals is 

listed in table 9-4. 

Table 9-4.—Unit costs of chemicals by process and use 

Process Chemical Use 

Unit cost, 

$/kg ($/lb) 

All Chlorine (1-ton cylinders) Pretreatment and 

post-treatment 0.49 (1.08) 

All Lime (Quicklime) Post-treatment 0.17 (0.37) 

All Caustic (50-percent solution) Post-treatment 0.75 (1.65) 

Membrane Scale inhibitor Pretreatment 1.47 (3.24) 

Distillation Anti-foam Pretreatment 1.21 (2.67) 

All Sodium bisulfate Pretreatment 2.04 (4.50) 

Thermal Polyphosphate Pretreatment 2.78 (6.13) 

Distillation Ferric chloride Pretreatment 0.55 (1.21) 

Distillation Corrosion inhibitor Post-treatment 0.88 (1.94) 

All Sulfuric acid, 93 percent Pretreatment 0.11 (0.24) 

9.3.4.3 Energy 

Figures 9-43, 9-44, and 9-45 show the cost of electricity at $0.06/kWh for MSF, MED, and 

MVC, respectively. Figures 9-46 and 9-47 show membrane system costs for electricity at 

$0.06/kWh. 

Steam 

Steam costs for distillation vary not only with fuel costs and performance ratio but also with the 

plant type (i.e., single or dual-purpose). The dual-purpose plant steam costs also vary with the 

type of allocation cost method used to determine this cost. 
Chapter 9: Cost Estimating Procedures

199

(a) Single-Purpose Plants 

Figure 9-48 gives steam costs for single-purpose plants. These costs assume a 

performance ratio 5.17 kg/MJ (12 lb/kBtu heat input). These calculations assume a boiler 

efficiency of 85 percent for a range of fuel costs. 

(b) Dual-Purpose Plants 

The cost of steam from a dual-purpose plant has an economic advantage over a singlepurpose plant because a dual-purpose plant uses fuel more efficiently. In a dual-purpose 

plant, steam produces some electrical energy before it is extracted for use in the desalting 

plant. Therefore, less fuel is consumed than would be required by two separate facilities 

in a typical single-purpose installation. Cost savings are also realized from using shared 

facilities in following areas: 

• Seawater intake facilities 

• Concentrate disposal systems 

• Operation and maintenance labor 

• Maintenance shops and control room 

• Licensing and permitting 

Cost allocation is used to: 

• Present the costs of each product (i.e., water and electricity) 

• Determine the appropriate selling price of each product 

• Assist the planner in understanding the makeup of each production cost and, 

therefore, in understanding the cost impacts for changes in design 

• Establish the basis for comparing different alternatives 

• Two methods of cost allocation generally are in use today: the “power credit” and 

“available energy” methods. Figures 9-49 and 9-50 give the cost of steam, when 

using the power credit and available energy methods. For estimating purposes, 

the cost of electricity can be taken as that given at the start of this section. 

Power Credit Method.—In the power credit method, the power production cost is charged 

with the plant and site costs resulting from constructing and operating only the power 

plant. The water plant is charged only with the cost of additional facilities, site work, 

fuel use, and operating expenses required for water production. The energy cost is 

determined by calculating the difference between the cost of power produced by the dualpurpose plant and the single-purpose plant. In this allocation method, the difference in 

power costs that results from constructing the dual-purpose plant is charged to the water 

production plant. The difference in cost depends on the amount of steam removed from 
Desalting Handbook for Planners 

200

the power cycle for desalination production and the pressure at which steam is extracted 

from the turbine. 

Available Energy Method.—In the available energy method, costs are allocated to the 

electrical and water production, based on the energy that each use. In this method, the 

available energy is the energy that would be used for power production. (i.e., the enthalpy 

difference between the main steam pressure and the condensing pressure) and the power 

and water use. That is, the fraction of the steam cost attributed to power production is the 

ratio of the enthalpy drop across the turbine to the total enthalpy drop available for 

electrical power production in a single-purpose plant. 

The unit cost of electrical product will, in each case, be greater than that for the base case 

(power production only). 

9.3.4.4 Replacement Parts and Maintenance Materials 

Figures 9-51, 9-52, and 9-53 give the annual costs for repairs and spare parts for performance 

ratios of 3.44, 4.30, and 5.17 kg/MJ (8, 10, and 12 lb/kBtu), respectively. 

These costs assume 1 percent of the total capital costs for membrane systems for replacement 

parts and maintenance materials. 

9.3.4.5 Membrane Replacement Cost 

Figure 9-54 gives annual membrane replacement costs for SWRO, BWRO/NF, and EDR. These 

costs are calculated for the following desalting treatments: 

• SWRO, $0.032/m3 ($0.12/kgal) 

• BWRO/NF, $0.021/m3 ($0.08/kgal) 

• EDR, $0.039/m3 ($0.15/kgal) for membranes, spacers, and stack parts 

9.3.4.6 Insurance 

Insurance costs assume 0.5 percent of total capital costs. 

9.3.4.7 Annual Cost of Capital 

The annual cost of depreciating capital normally includes the owner’s cost for interest 

amortization. The annual payment necessary to repay principal and interest in a present sum of 

money is called the Capital Recovery Factor. Table 9-5 lists the Capital Recovery Factors for 

different interest rates and amortization periods. Capital Recovery Factor, multiplied by total 

capital cost, is the payment each year for a loan at the indicated interest rate (i) and amortization 

period (y). 
Chapter 9: Cost Estimating Procedures

201

CRF

i i

i

y

y = +

+ −

*( )

( )( )

1

1 1

Table 9-5.—Capital recovery factors

Interest Repayment period (years) 

rate 20 30 40 

4.0 0.07358 0.05783 0.05052 

4.5 0.07688 0.06139 0.05434 

5.0 0.08024 0.06505 0.05828 

5.5 0.08368 0.06881 0.06232 

6.0 0.08718 0.07265 0.06646 

6.5 0.09076 0.07658 0.07069 

7.0 0.09439 0.08059 0.07501 

7.5 0.09809 0.08467 0.07940 

8.0 0.10185 0.08883 0.08386 

8.5 0.10567 0.09305 0.08838 

9.0 0.10955 0.09734 0.09296 

9.5 0.11348 0.10168 0.09759 

10.0 0.11746 0.10608 0.10226 

9.3.4.8 Plant Factor 

Plant factor is the amount of time (percent) the units will operate during the year at the percent of 

design capacity (i.e., onstream percent times percent of design capacity). For all cases, it is 

assumed that the desalination plants will operate at design capacity for 310 days per year (i.e., a 

plant factor of 85 percent). This allows ample time for preventive maintenance and unforeseen 

shutdowns. 

References 

Chapman-Wilbert, M., 1998. “The Desalting and Water Treatment Membrane Manual: A 

Guide to Membranes for Municipal Water Treatment,” (2nd edition), Water Treatment 

Technology Program Report No. 29, Bureau of Reclamation, Denver, Colorado, 1998. 

Mickley and Associates, 2001. “Membrane Concentrate Disposal Practices and Regulations,” 

Bureau of Reclamation, Desalination Water Purification Report Series No. 69, Denver, 

Colorado. 
Desalting Handbook for Planners 

202

Table 9-6a.—Design criteria 

Item Requirement 

Finished water quantity, m3/d (mgd) 

Process capacity, m3/d (mgd) 

Product water quality (mg/l) 

Finished water quality (mg/l) 

Distance to supply (m) 

Number of trains 

Design feed water temperature (o

C) 

Finished water quality goals: 

 - Total hardness (as CaCO3) 

 - Alkalinity (as CaCO3) 

 - Langelier Saturation Index 

Pump types and sizes: 

 Intake or well pumps 

 Feed pumps 

 Transfer pumps 

 Housekeeping pumps (cleaning, flushing, etc.) 

 Product pumps 

 High service pumps 

Pretreatment method 

Post-treatment method 

Fresh water flush system type 

Concentrate disposal method 

Product storage tank size 

Staffing requirements 

Daily operational period 

Disinfection equipment type 
Chapter 9: Cost Estimating Procedures

203

Table 9-6b.—Economic criteria 

Item Requirement 

Cost year 

Interest rate (percent) 

Service life (years) 

Plant factor (percent) 

Electricity cost ($/kWh) 

Steam cost $/MJ ($/Btu) 

Labor rate ($/hour) 

Labor overhead rate (percent) 

Contingency (percent) 

Contractor overhead and labor (percent) 

Engineering and administration (percent) 

Freight and insurance (percent) 

Chemical costs ($/T): 

 Acid 

 Scale inhibitor 

 Caustic 

 Chlorine 

 Sodium bisulfite 

 Anti-foam 

 Lime 

 Coagulant 

 Other 

Membrane replacement (percent/year) 

Repair and spare parts (percent/year) 

Facility insurance (percent/year) 

Land cost ($/hectare) 
Desalting Handbook for Planners 

204

Table 9-7.—Supporting sheet (sheet 1 of 3) 

Project: 

Date: 

Project Description: 

Project Type: 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: Required Quality: 

Distance, Point of Delivery from Desalting Plant: 

Required Reliability: 

Facilities Required: 

WATER SUPPLY CHARACTERISTICS 

Physical Description: 

Chemical Description: 

Other: 

Facilities Required: 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: 

Replaceable Items: 

Frequency of Replacement (years): 

Concentrate Disposal Requirements: 

Service Required at Source: 

Cost of Services Purchased: 

Fuel: 

Steam: 

Electric Power: 

Auxiliary Facilities Required: 
Chapter 9: Cost Estimating Procedures

205

Table 9-7.—Computation sheet (sheet 2 of 3)

Project Description: Project: 

Date: 

Desalting Plant Capacity in m3/d 

Process Capital Costs 

Concentrate Disposal Capital Costs 

Pretreatment Capital Costs 

Intake Capital Costs 

Feed Water Supply Piping Capital Costs 

Boiler Capital Costs 

Site Development Costs 

Post-Treatment Capital Costs 

Auxiliary Capital Costs 

Building Capital Costs 

Interest During Construction 

Land Cost 

Annual Labor Costs 

Annual Chemical Costs 

Annual Steam Costs 

Annual Electric Power Costs 

Annual Repairs and Spares Costs 

Other Computations 
Desalting Handbook for Planners 

206

Table 9-7.—Cost summary (sheet 3 of 3)

Project Description: Project: 

Date: 

Price Level: 

Desalting Plant - Type: Capacity (m3/d): 

Annual Plant Factor (percent): Interest Rate (percent): 

Annual Production (m3): Plant Life (years): 

Capital Recovery Factor: 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 

2. Concentrate Disposal 

3. Pretreatment 

4. Water Intake 

5. Feed Water Pipes 

6. Steam Supply 

7. General Site Development 

8. Post-Treatment 

9. Auxiliary Equipment (product storage, product 

transmission, step-down transformers, emergency 

generators) 

 

10. Building and Structures 

 Subtotal Direct Capital Cost (DCC) 

11. Freight and Insurance 5 percent of DCC 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in years) 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 

15. Contingency 10 percent of DCC 

 Total - Depreciating Capital 

16. Land Costs 

17. Working Capital (1/6 x items 18-24) 

 Total - Nondepreciating Capital 

 Total Capital Costs 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 

19. Chemicals 

20. Steam 

21. Electric Power 

22. Supplies, Maintenance, and Insurance 

23. Insurance 

 Total Operation and Maintenance 

24. Annual Costs - Depreciating Capital 

25. Annual Costs - Nondepreciating Capital 

 Total Annual Capital Charges 

26. Annual Replacement Costs 

 Total Annual Costs 

COST OF WATER (Total Annual Costs/ Yearly Production) 
Chapter 9: Cost Estimating Procedures

207

Figure 9-1 Total construction cost—MSF process

1

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Construction Costs Include:

Desalting Plant Proper

Interstage Piping

Pumps, Motors and Controls

Seawater Supply

Pretreatment

Post Treatment

Building

Cleaning System

Electrical Distribution

Indirect Cost

Plant Type: Recycle

Operating Temperature: 110 C (235 F)

Configuration: Cross-Tube

Construction Costs do not Include:

Land Costs

Product Water Delivery

Boiler

Figure 9-2 Process construction cost—MSF 

1

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Plant Type: Recycle

Operating Temperature: 110 C (235 F)

Configuration: Cross-Tube

Costs Include:

Desalting Plant Proper

Interstage Piping & Valves

Pumps, Motors and Controls

Cleaning System

Electrical Distribution Costs do not Include:

Pretreatment

Intake System

Post Treatment

Building

Auxiliary Equipment

Storage & Delivery Systems

Land Costs

Boiler

Indirect Cost
Desalting Handbook for Planners 

208

Figure 9-3 Total construction cost—MED process 

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Construction Costs Include:

Desalting Plant Proper

Interstage Piping

Pumps, Motors and Controls

Seawater Supply

Building

Pretreatment

Post Treatment

Electrical Distribution

Cleaning System

Indirect Cost

Plant Type: Horizontal Tube

Operating Temperature: 71.1 C (160 F)

Construction Costs do not Include:

Land Costs

Product Water Delivery

Boiler

Figure 9-4 Process construction cost—MED

1

10

100

1,000

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Costs Include:

Desalting Plant Proper

Interstage Piping

Pumps, Motors and Controls

Cleaning System

Electrical Distribution

Construction Costs do not Include:

Pretreatment

Post Treatment

Intake System

Auxiliary Equipment

Land Costs

Building

Boiler

Storage & Delivery Systems

Indirect Cost
Chapter 9: Cost Estimating Procedures

209

Figure 9-6 Process construction cost - MVC

1

10

100

1,000 10,000 100,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Costs Include:

Desalting Plant Proper

Interstage Piping

Pumps, Motors and Controls

Electrical Distribution Construction Costs do not Include:

Pretreatment

Post Treatment

Auxiliary Equipment

Product Water Storage and Delivery

Land Costs

Building

Boiler

Intake System

Indirect Cost

Figure 9-5 Total constuction cost - MVC process

1

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Costs Include:

Desalting Plant Proper Interstage 

Piping

Pumps, Motors and Controls

Seawater Supply

Pretreatment

Post Treatment

Cleaning System

Electrical Distribution

Building

Indirect Cost

Plant Type: Mechanical 

Vapor Compressor

Operating Temperature: 

107.2 C (225 F)

Construction Costs do not Include:

Land Costs

Product Water Delivery

Concentrate Disposal
Desalting Handbook for Planners 

210

Figure 9-7 Total construction cost—SWRO, BWRO, and NF plants with surface water feed 

Figure 9-8 Total construction cost—SWRO, BWRO, and NF plants with well water feed 

1

10

100

1,000

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

SWRO BWRO NF

Construction Costs Include:

Desalting Plant Proper

Inplant piping

Pumps, Motors and Controls

Pretreatment

Post Treatment

Building

Cleaning System

Electrical Distribution

Indirect Cost

Construction Costs do not Include:

Land Costs

Product Water Delivery

Concentrate Disposal

1

10

100

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2,000)

SWRO BWRO NF

Construction Costs Include:

Desalting Plant Proper

Inplant Piping

Pumps, Motors and Controls

Post Treatment

Building

Cleaning System

Electrical Distribution

Indirect Costs

Costs do not Include:

Land Costs

Product Water Delivery 

Concentrate Disposal
Chapter 9: Cost Estimating Procedures

211

Figure 9-10 Construction cost—concentrate disposal pipeline for distillation processes

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day) 

Construction Cost ($1,000,000 - 2000)

Disposal Point 457 m (1,500 ft) Disposal Point 762 m (2,500 ft) Disposal Point 1,067 m (3,500 ft)

Costs Include:

Disposal Pipeline

Costs do not Include:

Indirect Costs

Figure 9-9 Total construction cost - EDR plant

1

10

100

1,000 10,000 100,000

Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

3 stage SW 4 stage SW 3 stage GW 4 stage GW

Construction Costs Include:

Desalting Plant Proper

Inplant Piping

Pumps, Motors and Controls

Brackish Water Supply

Building

Post Treatment

Electrical Distribution

Cleaning System

Indirect Cost

Construction Costs do not Include:

Land Costs

Product Water Delivery

Concentrate disposal
Desalting Handbook for Planners 

212

Figure 9-11 Construction cost—concentrate disposal pipeline for membrane processes 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day) 

Construction Cost ($1,000,000 - 2000)

SWRO 1500ft SWRO 3000ft SWRO 4500ft BW/EDR/NF 3000ft BW/EDR/NF 1500ft BW/EDR/NF 4500ft

Costs Include:

Disposal Pipeline

Costs do not Include:

Indirect Costs

Figure 9-12 Construction cost—evaporation ponds for membrane processes 

1

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Cost estimate based on:

Net Evaporation rate of 127 cm/yr. (50"/yr)

Duplicate cells

50% freeboard

85% plant factor

Cost estimate does not include:

Land cost

Indirect costs

14.17 hectares (35 acres)

141.65 hectares (350 acres)

70.82 hectares (175

354.11 hectares (875 acres)

708.22 hectares (1750 acres)
Chapter 9: Cost Estimating Procedures

213

Figure 9-13 Construction cost—-injection wells for membrane processes 

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Wells

Redundant Well

Wellhead equipment

Costs do not Include:

Indirect Costs

Figure 9-14 Construction cost - pretreatment for distillation processes

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (mgd)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Chlorine Treatment

Sodium Bisulfite Treatment

Acid Treatment

Anti-Foam Treatment

Decarbonator

Deaerator

Costs do not Include:

Indirect Costs
Desalting Handbook for Planners 

214

0.1

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Chlorine Treatment

Sodium Bisulfite Treatment

Two-stage Filtration

Sludge treatment

Costs do not Include:

Indirect Costs

Figure 9-15 Construction cost—surface water pretreatment for membrane processes

Figure 9-16 Construction cost—open intake for distillation processes 

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

MSF Open Intake Open Intake MED Open Intake MVC

Construction Costs Include:

Intake Channel

Intake Structure

Traveling Screens & Trash Racks

Pumps, Motors and Controls

Costs do not Include:

Indirect Costs
Chapter 9: Cost Estimating Procedures

215

Figure 9-17 Construction cost—open intake systems for smaller membrane processes 

Figure 9-18 Total construction cost—wellfields 

0.1

1

10

100

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

SWRO BW-400ft BW-600ft BW-800ft Costs Include:

Wells

Pumping Equipment

Electrical & Controls

Wellhouse

Lateral piping

Collector piping

Construction Costs do not

Include:

Land Costs

Well Water Delivery

Brackish wells assumed to 

have depths of 200, 400, 

600, 800ft

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

SWRO BWRO

Construction Costs Include:

Intake Channel

Intake Structure

Traveling Screens & Trash Racks

Pumps, Motors and Controls

Note: Use BWRO costs 

for EDR and NF

Costs do not Include:

Indirect Costs
Desalting Handbook for Planners 

216

 

Figure 9-20 Construction cost - seawater feed water pipeline for membrane processes 

Figure 9-19 Construction cost—seawater feed water pipeline for distillation processes 

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000) )

Pipe Length 457 m (1,500 ft) Pipe Length 762 m (2,500 ft) Pipe Length 1,067 m (3,500 ft) Pipe Length 1,372 m (4,500 ft)

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Pipe Length 457 m (1,500 ft) Pipe Length 762 m (2,500 ft)

Pipe Length 1,067 m (3,500 ft) Pipe Length 1,372 m (4,500 ft)
Chapter 9: Cost Estimating Procedures

217

Figure 9-21 Construction cost – package boilers for single-purpose plant 

0.1

1

10

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Boiler

Controls

Costs do not Include:

Indirect Costs

Figure 9-22 Construction cost - site development for distillation processes

0.01

0.1

1

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 1999)
Desalting Handbook for Planners 

218

Figure 9-24 Construction cost – Post-treatment for distillation processes 

Figure 9-23 Construction cost—site development for membrane processes 

0.01

0.10

1.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Surface SWRO Surface BW/EDR/NF GW SWRO GW BW/EDR/NF

0.1

1

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Chlorine Treatment

Caustic Treatment

Lime Treatment

Costs do not Include:

Indirect Costs
Chapter 9: Cost Estimating Procedures

219

Figure 9-26 Construction cost – product storage for distillation systems using steel 

tank construction 

0.1

1

10

1,000 10,000 100,000 1,000,000

Storage Capacity (m3

)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Product Storage Sized for Full Day Capacity

Costs do not Include:

Indirect Costs

Figure 9-25 Construction cost—post-treatment for membrane systems 

0.1

1.0

10.0

1,000 10,000 100,000 1,000,000

Plant capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Degassification 

pH adjustment 

Disinfection, NaOCl 

Corrosion Inhibitor 

Costs Do Not Include:

Indirect Costs 

Fluoride 
Desalting Handbook for Planners 

220

Figure 9-28 Construction cost – product transmission pipeline 

Figure 9-27 Construction cost—product storage for membrane systems using 

prestressed concrete tank construction 

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Storage for one day's production

Costs do not Include:

Indirect Costs

0.1

1

10

100

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

Transmission Length 3,657 m (12,000 ft) Transmission Length 7,315 m (24,000 ft)

Transmission Length 14,630 m (48,000 ft)

Costs Include:

High Service Pumping

Transmission Pipeline Length as shown

Building

Costs do not Include:

Indirect Costs

Assumes Pipeline Installation

Cost at $27.89/m ($8.50/ lineal foot)
Chapter 9: Cost Estimating Procedures

221

Figure 9-29 Construction cost – emergency generators 

Figure 9-30 Construction cost – step-down transformers 

0.01

0.1

1

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Diedel Generator

Fuel Tank

Spill Containment

Costs do not Include:

Indirect Costs

0.1

1

10

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

Costs Include:

Step-down Tranformers

Installation

Costs do not Include:

Indirect Costs
Desalting Handbook for Planners 

222

Figure 9-31 Construction cost—distillation process buildings 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Construction Cost ($1,000,000 - 2000)

$807.00/m2 ($75.00/sf) $1,076/m2 ($100.00/sf) $1,614/m2

($150.00/sf)

Figure 9-32 Construction cost—membrane process buildings 

0.1

1.0

10.0

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Construction Cost ($1,000,000 - 2000)

SWRO$807.00/m2 ($75.00/sf) SWRO$1,076/m2 ($100.00/sf) SWRO$1,614/m2 ($150.00/sf)

BW$807.00/m2 ($75.00/sf) BW$1,076/m2 ($100.00/sf) BW$1,614/m2 ($150.00/sf)
Chapter 9: Cost Estimating Procedures

223

Figure 9-33 Owner’s direct expense and COH factors 

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 10 20 30 40 50 60 70

Project Construction Cost ($1,000,000 - 2000)

Indirect Cost Adjustment Factor

Owner's Cost Factor COH Cost Factor

Base Case:

Owner's Expense -10% of Construction Cost

COH - 15% of Construction Cost

Figure 9-34 Land requirements—for distillation process plants

0.001

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Land Requirement (hectare)

Building Land Process Land Site Land Total Land Requirement
Desalting Handbook for Planners 

224

Figure 9-36 Annual cost—labor for distillation process 

0.10

1.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

All Processes Single Purpose All Processes Dual Purpose

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Land Requirement (hectares)

Surface SWRO Surface BW, NF, EDR Wellwater SWRO Groundwater BW, NF, EDR

Figure 9-35 Land requirements—for membrane process plants 

1 hectare=2.471 acres
Chapter 9: Cost Estimating Procedures

225

Figure 9-37 Annual cost – labor for membrane processes 

0.1

1

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Annual Labor Cost ($1,000,000 - 2000)

SWRO BWRO/EDR/NF

Figure 9-38 Annual cost - MSF chemicals

0.1

1

10

100

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 1999)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu)

PR=3.44 kg/MJ (8 lbs/kBtu)
Desalting Handbook for Planners 

226

Figure 9-39 Annual cost - MED chemicals

0.1

1

10

100

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 1999)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu)

PR=3.44 kg/MJ (8 lbs/kBtu)

Figure 9-40 Annual cost - MVC chemicals

0.01

0.1

1

1,000 10,000 100,000

Plant Capacity (m3

/day)

Annual Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu)

PR=3.44 kg/MJ (8 lbs/kBtu)
Chapter 9: Cost Estimating Procedures

227

Figure 9-42 Annual cost—chemicals for ground water membrane processes 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Annual Chemical Cost ($1,000,000 - 2000)

BWRO EDR NF SWRO

Figure 9-41 Annual cost—chemicals for surface water membrane processes 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Chemical Cost ($1,000,000 - 2000)

BWRO EDR NF SWRO
Desalting Handbook for Planners 

228

Figure 9-44 Annual cost – MED processes electricity 

0.1

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Annual Cost ($1,000,000 - 2000)

PR=3.44 kg/MJ PR=4.30 kg/MJ PR=5.17 kg/MJ

Power @ $0.06/kWh

Figure 9-43 Annual cost - MSF processes electricity

100

1,000

10,000

100,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Electrical Consumption (kW)

PR=3.44 kg/MJ PR=4.30 kg/MJ PR=5.17 kg/MJ
Chapter 9: Cost Estimating Procedures

229

Figure 9-45 Annual cost—MVC processes electricity 

Figure 9-46 Annual cost—SWRO processes electricity 

0.10

1.00

10.00

100.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Electrical Cost ~$1,000,000/yr

Surface Intake Beach Wells

Surface Water Power Cost includes:

Power for intake pumps

Power for pre-treated water pumps

Process power

Distribution (high service) pump power

Building services

Groundwater Cost includes:

Power for well pumps

Process power

Distribution pump power

Building services

1

10

100

1,000

1,000 10,000 100,000 1,000,000

Plant Capacity (m3

/day)

Annual Cost ($1,000,000 - 2000)

PR=3.44 kg/MJ PR=4.30 kg/MJ PR=5.17 kg/MJ

Power @ $0.06/kWh
Desalting Handbook for Planners 

230

Figure 9-48 Annual cost—steam for single-purpose plants 

0.1

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

 Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

Fuel Cost at $947.00/MJ ($1.00/MMBtu) Fuel Cost at $1,894.00/MJ ($2.00/MMBtu)

Fuel Cost at $2841.00/MJ ($3.00/MMBtu) Fuel Cost at $3788.00/MJ ($4.00/MMBtu)

Basis:

Performance Ratio=5.17 kg/MJ

(12 lbs/kBtu)

Boiler Efficiency=85%

Figure 9-47 Annual cost—BWRO, EDR, and NF plant electricity 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Electrical Cost $1,000,000/yr

BWRO-Surface EDR-Surface BWRO-Ground NF-Ground NF-Surface EDR-Ground

Surface Water Cost includes:

Power for intake pumps

Power for pre-treated water pumps

Process power

Distribution pump power

Building services

Groundwater Cost includes:

Power for well pumps

Process power

Distribution pump power

Building services
Chapter 9: Cost Estimating Procedures

231

Figure 9-49 Annual cost—steam for dual-purpose plants, power credit method 

0.0

0.1

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

Fuel Cost at $947.00/MJ ($1.00/MMBtu) Fuel Cost at $1,894.00/MJ ($2.00/MMBtu)

Fuel Cost at $2841.00/MJ ($3.00/MMBtu) Fuel Cost at $3788.00/MJ ($4.00/MMBtu)

Figure 9-50 Annual cost—steam for dual-purpose plants, available energy method 

0.0

0.1

1.0

10.0

100.0

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

Fuel Cost at $947.00/MJ ($1.00/MMBtu) Fuel Cost at $1,894.00/MJ ($2.00/MMBtu)

Fuel Cost at $2841.00/MJ ($3.00/MMBtu) Fuel Cost at $3788.00/MJ ($4.00/MMBtu)
Desalting Handbook for Planners 

232

Figure 9-51 Annual cost — MSF repairs and spares 

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000 Plant Capacity (m3

/day)

Annual Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)

Figure 9-52 Annual cost—MED repairs and spares 

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

PR=5.17 kg/MJ (12 lbs/kBtu) PR=4.30 kg/MJ (10 lbs/kBtu) PR=3.44 kg/MJ (8 lbs/kBtu)
Chapter 9: Cost Estimating Procedures

233

Figure 9-53 Annual cost—MVC repairs and spares 

Figure 9-54 Annual cost—membrane replacement 

0.01

0.10

1.00

10.00

1,000 10,000 100,000 1,000,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

SWRO BWRO/NF EDR

0.1

1

1,000 10,000 100,000

Plant Capacity (m3/day)

Annual Cost ($1,000,000 - 2000)

PR=5.17 kg/ MJ (12 lbs/ kBt u) PR=4.30 kg/ MJ (10 lbs/ kBt u) PR=3.44 kg/ MJ (8 lbs/ kBt u)

 Appendix A: Reclamation’s Desalting Program 

A-1

Appendix A: Reclamation’s Desalting Program 

A.1 Historical Background 

The U.S. Department of the Interior, Bureau of Reclamation (Reclamation) has a long history in 

desalination. Federal funding of desalination research began in 1952, with the passage of the 

Saline Water Act. Over the next 30 years, Congress appropriated over $1 billion in today’s 

dollars for research, development, and demonstration projects. The visionary goals of cost 

effectively converting saline and brackish water into usable water were advanced through the 

Department of the Interior. The Office of Saline Water (OSW) was created in 1954, and the 

subsequent Office of Water Research and Technology (OWRT) was formed in 1974. These 

programs were primarily responsible for reverse osmosis (RO) technology development and for 

many breakthroughs and improvements in thermal technologies. Today’s widespread use of 

desalination would not be possible had it not been for these programs. They were, in retrospect, 

incredibly farsighted in providing research and development stimulus to desalination at a time 

when few land-based water supply problems were even identified in the U.S. Research provided 

benefits in other technology fields, such as the advent of kidney dialysis using membranes. 

Reclamation became involved in desalination as a testing laboratory for OSW developments. In 

this role, Reclamation conducted verification testing both in-house and in the field, as well as 

supervised many pilot projects for OSW. Pilot testing conducted at field sites in Roswell, New 

Mexico, and Dalpra Farm, Colorado, evaluating membrane technology, provided Reclamation 

with pioneering in-depth capabilities in desalination. In the 1970s, Reclamation became directly 

involved when legislation for evaluating options for desalinating agricultural return flows in the 

irrigation valley in southwestern Arizona was introduced. Salinity issues in this area mandated 

action in order to conform to treaty obligations with Mexico. In time, this led to the world’s 

largest membrane desalination facility, Reclamation’s Yuma Desalting Plant. Though this plant 

has never operated for long term at full capacity, it is maintained in a standby status. The Water 

Quality Improvement Center was subsequently created at this facility and is used for research, 

technology transfer, and training activities to further desalination and water reuse technologies. 

Federal funding for desalination activities largely stopped in the early 1980s, when OWRT was 

abolished as a result of redirected congressional funding. The remaining desalination program 

was transferred to Reclamation, due to its ongoing activities in relation to the Yuma Desalting 

Plant. In the early and mid-1990s, approximately $1.3 million per year for desalination research 

funding was appropriated for Reclamation under the Water Treatment Technology Program. In 

1996, Senator Paul Simon was responsible for passage of the Water Desalination Act of 1996. 

This Act authorized program funding of $5 million per year for research and studies for 6 years, 

beginning with fiscal year 1998. In addition, $25 million was authorized over six years for 

demonstration projects. Reclamation has received appropriations as shown in table A-1, since 

the passage of the Act. It must be reauthorized by Congress in order for appropriations to be 

made after fiscal year 2002. 
Desalting Handbook for Planners 

A-2

Table A-1.—Summary of appropriations 

under the Water Desalination Act of 1996 

Fiscal year Appropriated funds 

(million $) 

1998 2.70 

1999 1.50 

2000 0.65 

2001 1.30 

2002 4.00 

Total 10.15 

 

A.2 Reclamation’s Present Program and Accomplishments 

The primary goal of Reclamation’s Desalination Research and Development Program is to 

develop more cost-effective and technologically efficient means to desalinate water. 

To accomplish this, Reclamation is forming partnerships with private industry, universities, local 

communities, non-governmental organizations, and others to address a broad range of desalting 

and water purification needs. The primary thrusts of the program, as outlined by the Water 

Desalination Act of 1996, are to: 

• Perform research on desalination technologies and related issues to push the state-of-theart forward (Research and Studies). 

• Conduct development and demonstration activities to verify advancements, to confirm 

economics, and to gain public acceptance (Demonstration). 

• Research is conducted through grants and contracts with non-Federal entities. The 

Federal share of the cost of the research, study, or demonstration project shall not exceed 

25 percent unless the Secretary of the Interior determines that the project is not feasible 

without the increased Federal contribution and then not exceed 50 percent of the total 

cost of the project. 

To attain the primary goal, the following objectives were established for Reclamation’s 

Desalination Research and Development Program: 

• Increase the ability of communities of varying sizes and financial resources to 

economically treat saline water to potable standards. 

• Increase the ability of the United States desalination industry to compete throughout the 

world, by fostering partnerships with them for the development of new and innovative 

technologies. 
 Appendix A: Reclamation’s Desalting Program 

A-3

• Develop methods to make desalting more efficient through promotion of dual-use 

facilities in which waste energy could be applied to desalting water. 

• Develop methods to ensure desalting technologies are environmentally friendly. 

• Ensure regulations are appropriate for the application by working with regulators to fully 

evaluate the effects of concentrate streams. 

• Capitalize on the recovery of by-product streams. 

• Maximize technology transfer to ensure full transfer of knowledge and commercialization 

of technology. 

Since the initial funds were appropriated for this program in fiscal year 1998, approximately 

60 assistance agreements have been awarded to private industry, universities, and research 

organizations. The following findings and accomplishments resulted: 

• Membrane bioreactors successfully treat screened and degritted sewage so that it is 

suitable for further treatment with desalting technology. This represents a ‘step-change’ 

in treatment technology by providing a smaller footprint, higher quality sludge residuals, 

and a much higher quality product. Membrane bioreactors have a cost advantage over 

conventional treatment methods, and they have less of an environmental impact. 

• Various advancements in membrane materials and technology have been made. A better 

understanding of membrane fouling has been described by several researchers, leading to 

increased membrane life, reduced chemical cleanings, reduced impact on the 

environment, and reduced capital and operation and maintenance costs. In addition, there 

is strong indication that chlorine resistance in a thin film composite membrane has been 

achieved in a project co-funded by Reclamation and the U.S. Army. 

• Several projects showed advancements in pretreatment of water for application with 

desalting membranes. In one case, specially submerged pipes on a beach were used to 

filter seawater. In another case, biological pretreatment of saline water did an excellent 

job of pretreating feedwater. A third case in progress, low-pressure membrane filtration 

(e.g., microfiltration) as pretreatment for seawater RO, has the potential to significantly 

improve performance and reliability. All of these techniques have a lower environmental 

impact than conventional pretreatment technologies. Larger scale studies are needed in 

order to estimate the cost savings associated with using these new technologies. 

• An advanced high-pressure seawater pump was developed that reduces energy 

consumption over 35 percent, compared to similar pumps using conventional energy 

recovery. Since energy consumption for seawater RO and the amortized capital 

associated with energy recovery equal about 50 percent of the total water cost, this could 

provide a significant overall cost savings. 
Desalting Handbook for Planners 

A-4

• An innovative, low-cost system using evaporation technology was developed. This can 

be constructed and operated by unskilled laborers. This system uses low-grade heat to 

produce high quality water with very little operator involvement. This technology is 

suitable for relatively low volume product water, such as rural communities. The lower 

capital and operational costs make this technology ideal for communities that cannot 

afford RO technology. 

Each of these findings is being recommended to Congress for additional funding for larger scale 

demonstration testing. 

In addition, the desalination community and potential desalination users have developed several 

significant tools for use. These include: 

• An abstract database of all the desalination literature has been completed in partnership 

with the American Water Works Association (AWWA). The database includes a fulltext database of all Federal Government desalination reports (approximately 1,200 

reports funded by OSW and OWRT written from 1954-1982) and various full-text 

conference proceedings and abstracts. 

• The first edition of a computerized desalination cost model has been developed in 

partnership with the American Membrane Technology Association (AMTA) and the 

National Institute of Standards and Technology (NIST). 

This Desalting Handbook for Planners was created to assist in the decision-making process for 

potential desalination users. 

Reclamation also wrote a comprehensive membrane manual that has been distributed around the 

world to the desalination and membrane community. In addition, this manual is used by the 

Electric Power Research Institute (EPRI) and has been sent to all of their member utilities. 

Currently, all final reports of desalination research activities funded by Reclamation are available 

to the public in hardcover, on CDROM, and are available from the program’s website 

<www.usbr.gov/water/desal.html>. 

Additionally, numerous technology transfer activities and workshops intended to promote further 

development of desalination technology have been sponsored by Reclamation. These have 

included the following: 

• “Growing the U.S. Water Supply through Purification Technologies” – A workshop held 

in Golden, Colorado, April, 2000, to discuss and develop a consensus regarding 

purification technologies. Participants included the water resources community, 

representatives from national organizations, various State/local water organizations, and 

industry, as well as congressional representatives. 
 Appendix A: Reclamation’s Desalting Program 

A-5

• Membrane Research Needs Workshop – A workshop held in July 2000, co-sponsored by 

the American Water Works Association Research Foundation (AWWARF) to develop a 

broad-based outline of the research needs of all membrane technologies. Participants 

included national and international membrane experts and municipal representatives. 

• Desalination Research and Development Workshop – A workshop held in January 2001, 

co-sponsored by the National Water Research Institute (NWRI), focused on prioritizing 

the most significant issues that need to be addressed now to speed the installation of costeffective desalination facilities. Participants included national desalination experts and 

users. 

Reclamation is also the co-founder of the Interagency Consortium for Desalination and 

Membrane Separation Research (Consortium). This Consortium was established in 1992 to 

provide a communications network for the exchange of information between Federal 

Government agencies involved in desalination and membrane research. Consortium members 

work together to gain the following benefits: 

• Prevent Federal duplication of effort 

• Pool limited Federal research funding and other resources to obtain common goals 

• Identify future research needs 

• Allow for discussion of new technologies with other experts in the field 

Federal agencies and laboratories that have participated in the yearly Consortium activities since 

its inception include: 

• U.S. Army 

• Center for Health Promotion & Preventative Medicine 

• Corps of Engineers: Cold Regions Research Engineering Lab

• Corps of Engineers: Construction Engineering Research Laboratory 

• Tank-Automotive & Armaments Command Mobility Tech Center (TACOM)

• Yuma Proving Ground

• Center for Disease Control (CDC) 

• Defense Advanced Research Projects Agency (DARPA)

• Bureau of Reclamation 

• U.S. Geological Survey 

• Department of Energy 

• Office of Fossil Energy 

• Federal Energy Technology Center 

• Office of Nuclear Energy 

• Environmental Protection Agency 

• National Aeronautic and Space Administration (NASA) - Marshall Space Flight Center

• Department of Agriculture 

• National Institute of Standards and Technology (NIST)

• U.S. Navy
Desalting Handbook for Planners 

A-6

• Command Control and Ocean Surveillance Center

• Naval Facilities Engineering Service Center

• Naval Surface Warfare Center 

• Naval Command Control and Ocean Surveillance Center 

These activities, combined with a balanced and comprehensive research studies and pilot 

projects program, have resulted in steady gains towards the program goal of more cost-effective 

and efficient means to desalinate water. The U.S. water supply is becoming more stressed as 

competing demands grow for the limited existing freshwater supplies. Desalination is one of the 

few methods by which to augment this supply and, thereby, reduce this stress. It is critical that 

the U.S. continue to create and develop desalination technologies and continue to work towards 

more cost-effective desalination technologies. Equally important, it is paramount that 

partnerships be forged between the different interest groups—those needing desalination, the 

regulators, community leaders, and those focused on environmental priorities—to achieve 

acceptable water supply solutions with which all citizens can live. 




C-1

Appendix C: ASTM Standards Applicable to 

Membrane Systems 

Published standards under the jurisdiction of ASTM Committee D19 on Water and the direct 

responsibility of Subcommittee D19.08 on Membranes and Ion Exchange Materials 

Number Date Title 

D3739-94 1998 Standard Practice for Calculation and Adjustment of the Langelier Saturation Index for 

Reverse Osmosis 

D3923-94 1998 Standard Practices for Detecting Leaks in Reverse Osmosis Devices 

D4189-95 2002 Standard Test Method for Silt Density Index (SDI) of Water 

D4194-95 2001 Standard Test Methods for Operating Characteristics of Reverse Osmosis Devices 

D4195-88 1998el Standard Guide for Water Analysis for Reverse Osmosis Application 

D4472-89 1998el Standard Guide for Record Keeping for Reverse Osmosis Systems 

D4516-00 Standard Practice for Standardizing Reverse Osmosis Performance Data 

D4582-91 2001 Standard Practice for Calculation and Adjustment of the Stiff and Davis Stability Index for 

Reverse Osmosis 

D4692-01 ND Standard Practice for Calculation and Adjustment of Sulfate Scaling Salts (CaSO4, SrSO4, 

and BaSO4) for Reverse Osmosis and Nanofiltration 

D4993-89 1998el Standard Practice for Calculation and Adjustment of Silica (SiO2) Scaling for Reverse 

Osmosis 

D5090-90 2001 Standard Practice for Standardizing Ultrafiltration Permeate Flow Performance Data 

D5091-95 2001 Standard Guide for Water Analysis for Electrodialysis/Electrodialysis Reversal Applications 

D5131-90 2001 Standard Guide for Record Keeping for Electrodialysis/Electrodialysis Reversal Systems 

D6161-98 2002 Standard Terminology Used for Crossflow Microfiltration, Ultrafiltration, Nanofiltration, and 

Reverse Osmosis Membrane Processes 

Contact: ASTM, 100 Barr Harbor Drive, West Conshohocken PA, 19428-295 <http://www.astm.org/> 
D-1

Appendix D: Worked Examples 

D-1 Multi-Stage Flash Evaporation 

D-2 Multi-Effect Distillation 

D-3 Mechanical Vapor Compression 

D-4 Surface Seawater Reverse Osmosis 

D-5 Low TDS Brackish Surface Water EDR 

D-6 Ground Water Nanofiltration 

 

Appendix D: Worked Examples

D-3

D-1 Multi-Stage Flash Evaporation 

This is an estimate for a Multi-Stage Flash Evaporation (MSF) plant to furnish 30 x 106 m3 per 

year of supplemental water to the City of Greener Pastures. The single-purpose plant will be 

located 1000 meters from the Pacific Ocean and will use seawater for feed. The City has 

sufficient water storage capacity in its present system, and product water from the desalting plant 

can be introduced into the water distribution system at the plant site. Oil is available at the site at 

an average price of $2,841 per mega joule. Electric power is available at the site at a price of 

$0.09 per kilowatthour. An oil-fired boiler will be required. The City plans to finance 

construction by a bond issue and estimates the average interest rate on bonds to be 7 percent. An 

allowance for taxes is not required, and land cost is $74,130 per hectare. 

Note: Amortization is 30 years at 7 percent; annual insurance cost is 0.5% of total depreciable 

capital; performance ratio = 5.17 kilograms distillate per mega joule (12 pounds per 1000 British 

thermal units) 
Desalting Handbook for Planners

D-4

Supporting Sheet 

Project Description: Supplemental Water Supply for City of Greener Pastures Project: SP-1 

Date: 1/2001 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 30 x 106 m3 

Required Quality: < 500 mg/l TDS 

Distance Point of Delivery from Desalting Plant: At plant 

Required Reliability: 90% at design capacity 

Facilities Required: 

Surface intake, pretreatment, desalting plant, post-treatment 

WATER SUPPLY CHARACTERISTICS 

Physical description: Seawater TDS = 34,500 mg/l 

Chemical Description: 

Other: 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: MSF 

Replaceable Items: Normal maintenance 

Frequency of Replacement (yrs): N/A 

Concentrate Disposal Requirements: Pipeline back to sea 

Performance Ratio: 5.17 kg distillate/MJ 

Service Required at Source: N/A 

Cost of Services Purchased: 

 Fuel: $2,841/MJ 

 Electric Power: $0.09/kWh 

Auxiliary Facilities Required: Step-down transformer 

OTHER NOTES 
Appendix D: Worked Examples

D-5

Computation Sheet 

Project Description: 

 Supplemental water supply for the city of Greener Pastures. Costs to include seawater intake, 

 pretreatment, desalting plant, post-treatment, and steam boiler. It is proposed to use MSF. 

Desalting Plant Capacity: 

 30 x 106 m3/yr ÷ (365 d/yr x 0.90) = 91,500 m3/d 

Process Capital Costs: 

 From Figure 9-2: $130,000,000 

Concentrate Disposal Capital Costs: 

 From Figure 9-10: $1,300,000 

Pretreatment Capital Costs: 

 From Figure 9-14: $2,600,000 

Intake Capital Costs: 

 From Figure 9-16: $5,200,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-19: For 762 m = $2.1 M; For 1000 m = $2,100,000 x 1000/762 m = $2,760,000 

Boiler Capital Costs: 

 From Figure 9-21: $1,100,000 

Site Development Costs: 

 From Figure 9-22: $470,000 

Post-Treatment Capital Costs: 

 From Figure 9-24: $230,000 

Auxiliary Equipment Capital Costs: 

 Includes Step-Down Transformer: $650,000 

Building Capital Costs: 

 From Figure 9-31: $1,000,000 @ $1,076/m2

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $145,310,000 x .07 x (1/2) 

 x 2 = $10,172,000 

Land Cost: 

 From Figure 9-34: 1.2 hectares x $74,130 / hectare = $89,000 

Annual Labor Costs: 

 From Figure 9-36: $680,000 

Annual Chemical Costs: 

 From Figure 9-38: $5,100,000 

Annual Steam Costs: 

 From Figure 9-48: $18,000,000 

Annual Electric Power Costs: 

 From Figure 9-43: $5,550,000 @ $0.09/kWh 

Annual Repairs and Spares Costs: 

 From Figure 9-51: $4,300,000 

Other Computations: 

 Step-Down Transformer: From Figure 9-30: $650,000 
Desalting Handbook for Planners

D-6

Cost Summary 

Project Description: Desalting plant facility for the City of Greener 

Pastures 

Desalting Plant - Type: MSF Capacity (m3/d): 91,500 

Annual Plant Factor (percent): 90 Interest Rate (percent): 7 

Annual Production (m3): 30 x 106 Plant Life (years): 30 

 Capital Recovery Factor: 0.0806 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 130,000,000 

2. Concentrate Disposal 1,300,000 

3. Pretreatment 2,600,000 

4. Water Intake 5,200,000 

5. Feed Water Pipes 2,760,000 

6. Steam Supply 1,100,000 

7. General Site Development 470,000 

8. Post-Treatment 230,000 

9. Auxiliary Equipment (step-down transformers) 650,000 

10. Building and Structures 1,000,000 

 Subtotal Direct Capital Cost (DCC) 145,310,000 

11. Freight and Insurance 5 percent of DCC 7,266,000 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in 

 years) 

10,172,000 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (.95) 20,707,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (.9) 13,078,000 

15. Contingency 10 percent of DCC 14,531,000 

 Total - Depreciating Capital 211,064,000 

16. Land Costs 89,000 

17. Working Capital (1/6 x items 18-23) 5,785,000 

 Total - Nondepreciating Capital 5,874,000 

 Total Capital Costs 216,938,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 680,000 

19. Chemicals 5,100,000 

20. Steam 18,000,000 

21. Electric Power 5,550,000 

22. Repairs and Spares 4,300,000 

23. Insurance (.5% of total depreciating capital) 1,077,000 

 Total Operation and Maintenance 34,707,000 

24. Annual Costs - Depreciating Capital (Total-Depreciating Capital x 0.0806) 17,012,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciation Capital x 0.0806) 473,000 

 Total Annual Capital Charges 17,485,000 

26. Annual Membrane Replacement Costs N/A 

 Total Annual Costs (Total O & M and Capital Charges) 52,192,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $1.74/m3
Appendix D: Worked Examples

D-7

D-2 Multi-Effect Distillation 

This is an estimate for a Multi-Effect Distillation (MED) desalting plant to furnish 30 x 106 m3

per year of supplemental water to the Greater Metro Utility District. The plant will be located 

1000 meters from the Pacific Ocean and will use seawater for feed. The District has sufficient 

water storage capacity in its present system. Product water will be delivered to one of the 

District’s storage reservoirs, some 3000 meters from the plant site. Plant reliability is not 

considered critical. The District plans to build this desalting plant adjacent to its new Big Rock 

power plant and utilize steam from the power plant. Fixed charges on investment are expected to 

be 7 percent interest. Fuel costs for Big Rock are expected to be $1,894 per mega joule for the 

500-megawatt unit. The available heat method is employed to determine steam costs. Electrical 

power is available at $0.03 per kilowatthour. 

Note: Amortization is 30 years at 7 percent; annual insurance cost is 0.5% of total capital costs; 

performance ratio = 5.17 kilograms distillate per mega joule (12 pounds per 1000 British 

thermal units); land cost = $74,130 per hectare 
Desalting Handbook for Planners

D-8

Supporting Sheet 

Project Description: Supplemental Water Supply for Greater Metro Utility District. It is proposed to use MED. 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 30 x 106 m3 

Required Quality: to SDWA 

Distance Point of Delivery from Desalting Plant: 3000 m 

Required Reliability: 90% at design capacity 

Facilities Required: Seawater intake, pretreatment, desalting process, steam supply, post-treatment, and delivery to distribution 

system 

WATER SUPPLY CHARACTERISTICS 

Physical Description: Seawater TDS=34,500 mg/l 

Chemical Description: 

Other: 

Facilities Required: 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: MED 

Replaceable Items: Normal preventive maintenance 

Frequency of Replacement (yrs): N/A 

Concentrate Disposal Requirements: Pipeline back to sea, 1000 m 

Performance Ratio: 5.17 kg distillate/MJ 

Service Required at Source: N/A 

Cost of Services Purchased: 

Fuel: $1,894/MJ 

Steam: Use available energy allocation method 

Electric Power: $0.03/kWh 

Auxiliary Facilities Required: Step-down transformer, product transmission 

OTHER NOTES 
Appendix D: Worked Examples

D-9

Computation Sheet 

Project Description: 

 Desalting Plant for Greater Metro Utility District 

Desalting Plant Capacity: 

 30 x 106 m3/yr ÷ (365 d/yr x 0.90) = 91,500 m3/d 

Process Capital Costs: 

 From Figure 9-4: $75,000,000 

Concentrate Disposal Capital Costs: 

 From Figure 9-10: $1,300,000 

Pretreatment Capital Costs: 

 From Figure 9-14: $2,600,000 

Intake Capital Costs: 

 From Figure 9-16: $4,700,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-19: $2,500,000 

Boiler Capital Costs: 

 N/A 

Site Development Costs: 

 From Figure 9-22: $490,000 

Post-Treatment Capital Costs: 

 From Figure 9-24: $230,000 

Auxiliary Capital Costs: 

 Includes product transmission and step-down transformer: $6,950,000 

Building Capital Costs: 

 From Figure 9-31: $1,300,000 @ $1076/m2

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $95,070,000 x .07 x (1/2) x 

 2 = $6,655,000 

Land Cost: 

 From Figure 9-34: 1.3 hectares by $74,130/hectare = $96,400 

Annual Labor Costs: 

 From Figure 9-36: $500,000 

Annual Chemical Costs: 

 From Figure 9-39: $4,900,000 

Annual Steam Costs: 

 From Figure 9-50: 3,600,000 

Annual Electric Power Costs: 

 From Figure 9-44: $1,600,000 @ $0.03/kWh 

Annual Repairs and Spares Costs: 

 From Figure 9-52: $2,800,000 

Other Computations: 

Product Transmission Capital Costs: From Figure 9-28: $6,300,000 

Step-Down Transformer Cost: From Figure 9-30: $650,000 
Desalting Handbook for Planners

D-10

Cost summary 

Project Description: Desalting Plant for Greater 

Metro Utility District 

Desalting Plant - Type: MED Capacity (m3/d): 91,500 

Annual Plant Factor (percent): 90% Interest Rate (percent): 7 

Annual Production (m3): 30 x 106 Plant Life (years): 30 

 Capital Recovery Factor: 0.0806 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 75,000,000 

2. Concentrate Disposal 1,300,000 

3. Pretreatment 2,600,000 

4. Water Intake 4,700,000 

5. Feed Water Pipes 2,500,000 

6. Steam Supply N/A 

7. General Site Development 490,000 

8. Post-Treatment 230,000 

9. Auxiliary Equipment (step-down transformer, 

 product transmission) 

 6,950,000 

10. Building and Structures 1,300,000 

 Subtotal Direct Capital Cost (DCC) 95,070,000 

11. Freight and Insurance 5 percent of DCC 4,754,000 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time 

 (Table 9.1) in years) 

6,655,000 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (.95) 13,547,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (.90) 8,556,000 

15. Contingency 10 percent of DCC 9,507,000 

 Total - Depreciating Capital 138,089,000 

16. Land Costs 96,400 

17. Working Capital (1/6 x items 18-23) 2,348,000 

 Total - Nondepreciating Capital 2,444,000 

 Total Capital Costs 140,533,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 500,000 

19. Chemicals 4,900,000 

20. Steam 3,600,000 

21. Electric Power 1,600,000 

22. Repairs and Spares 2,800,000 

23. Insurance (.5% of total depreciating capital) 690,000 

 Total Operation and Maintenance 14,090,000 

24. Annual Costs - Depreciating Capital (Total-Depreciating Capital x 0.0806) 11,130,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciating Capital x 0.0806) 197,000 

 Total Annual Capital Charges 11,327,000 

26. Annual Membrane Replacement Costs N/A 

 Total Annual Costs (Total O & M and Capital Charges) 25,417,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $ 0.85/m3
Appendix D: Worked Examples

D-11

D-3 Mechanical Vapor Compression 

This is an estimate for a Mechanical Vapor Compression (MVC) desalting plant to furnish 1.24 x 

106 m3 per year of supplemental water to the City of Great Hope. The plant is located 800 

meters from the Atlantic Ocean and will use seawater for feed. The water produced from the 

plant will be pumped to the City’s water storage tank, 7 kilometers from the water treatment 

plant site. The MVC plant will be a single-purpose facility, taking its steam from a packaged 

boiler located onsite. Fuel costs are expected to be $2,841 per mega joule for the boiler. 

Electricity will be taken from the adjacent electrical grid at a cost of $0.04 per kilowatthour. 

Note: Amortization is 30 years at 7 percent; annual insurance cost is 0.5% of total capital costs; 

performance ratio = 5.17 kilograms distillate per mega joule; land cost = $50,000 per 

hectare 
Desalting Handbook for Planners

D-12

Supporting Sheet 

Project Description: Supplemental Water Supply for the City of Great Hope 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 1.24 x 106 m3

Required Quality: <500 mg/l TDS 

Distance Point of Delivery from Desalting Plant: 7 kilometers 

Required Reliability: 90% at design capacity 

Facilities Required: Seawater intake, pretreatment, desalting plant, post-treatment, boiler, and concentrate disposal to ocean. 

WATER SUPPLY CHARACTERISTICS 

Physical Description: Seawater TDS = 34,500 mg/l 

Chemical Description: 

Other: Land Cost at $50,000/hectare 

Facilities Required: See above 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: MVC 

Replaceable items: Normal maintenance 

Frequency of Replacement (yrs): N/A 

Concentrate Disposal Requirements: Pipeline back to sea 

Performance Ratio: 5.17 kg distillate/MJ 

Service Required at Source: N/A 

Cost of Services Purchased: 

Fuel: $2,841/MJ 

Steam: 

Electric Power: $0.04/kWh 

Auxiliary Facilities Required: Emergency generator, product transmission 

OTHER NOTES: 
Appendix D: Worked Examples

D-13

Computation Sheet 

Project Description: 

 Water Supply for the City of Great Hope 

Desalting Plant Capacity: 

 1.24 x 106 ÷ (365 day/yr x 0.90) = 3,775 m3

/d 

Process Capital Costs: 

 Figure 9-6: $5,500,000 

Concentrate Disposal Capital Costs: 

 Figure 9-10: $200,000 

Pretreatment Capital Costs: 

 Figure 9-14: $290,000 

Intake Capital Costs: 

 From Figure 9-16: $380,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-19: $410,000 

Boiler Capital Costs: 

 Figure 9-21: $170,000 

Site Development Costs: 

 Figure 9-22: $30,000 

Post-Treatment Capital Costs: 

 Figure 9-24: $120,000 

Auxiliary Capital Costs: 

Includes Product Transmission, Step-Down Transformer, and Emergency Generator: $1,932,000 

Building Capital Costs: 

 Figure 9-31: $40,000 @ $807/m2

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $9,072,000 x 0.07 x (1/2) x (9/12) = 

 $238,140 

Land Cost: 

 Figure 9-34: 0.05 hectares x $50,000/hectare = $2,500 

Annual Labor Costs: 

 Figure 9-36: $360,000 

Annual Chemical Costs: 

 Figure 9-40: $24,000/yr 

Annual Steam Costs: 

 Figure 9-48 ($2,841/MJ): $750,000/yr. 

Annual Electric Power Costs: 

 Figure 9-45 @ $0.04/kWh: $800,000 

Annual Repairs and Spares: 

 Figure 9-53: $180,000 

Other Computations: 

 Product Transmission Capital Costs: Figure 9-28: $1,800,000 

 Emergency Generator Cost: Figure 9-29: $12,000 

 Step-Down Transformer Cost: Figure 9-30: $120,000 
Desalting Handbook for Planners 

D-14

Cost Summary 

Project Description: Desalting plant for the City of Great Hope 

Desalting Plant - Type: MVC Capacity (m3/d): 3,775 

Annual Plant Factor (percent): 90 Interest Rate (percent): 7 

Annual Production (m3): 1.24 x 106 Plant Life (years): 30 

 Capital Recovery Factor: 0.0806 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 5,500,000 

2. Concentrate Disposal 200,000 

3. Pretreatment 290,000 

4. Water Intake 380,000 

5. Feed Water Pipes 410,000 

6. Steam Supply 170,000 

7. General Site Development 30,000 

8. Post-Treatment 120,000 

9. Auxiliary Equipment (product transmission, step down 

 transformer, emergency generator) 

 1,932,000 

10. Building and Structures 40,000 

 Subtotal Direct Capital Cost (DCC) 9,072,000 

11. Freight and Insurance 5 percent of DCC 453,600 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in years) 238,140 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (1.01) 1,374,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (1.12) 1,016,000 

15. Contingency 10 percent of DCC 907,200 

 Total - Depreciating Capital 13,061,000 

16. Land Costs 2,500 

17. Working Capital (1/6 x items 18-23) 363,167 

 Total - Nondepreciating Capital 365,667 

 Total Capital Costs 13,427,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 360,000 

19. Chemicals 24,000 

20. Steam 750,000 

21. Electric Power 800,000 

22. Repairs and Spares 180,000 

23. Insurance 65,000 

 Total Operation and Maintenance 2,179,000 

24. Annual Costs - Depreciating Capital (Total-Depreciating Capital x 0.0806) 1,053,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciating Capital x 0.0806) 29,473 

 Total Annual Capital Charges 1,082,000 

26. Annual Membrane Replacement Costs N/A 

 Total Annual Costs (Total O & M and Capital Charges) 3,261,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $2.63/m3
Appendix D: Worked Examples

D-15

D-4 Surface Seawater Reverse Osmosis 

The coastal City of Happy Beach needs to increase its municipal water capacity to accommodate 

rapid growth in its service district. After several studies, it has been concluded that seawater 

desalination provides the highest level of reliability, at reasonable cost. The plant will be located 

within a city park approximately 1,000 meters from the ocean, will use an existing sewer outfall 

for concentrate discharge, and will have a rated capacity of 20,000 m3

 per day (5.4 million 

gallons per day). Water will be transferred by product pumps to an existing storage tank from an 

underground clearwell located on the site. Distance to the City tank is 3,000 meters. 

Note: Amortization is 20 years at 6 percent; annual insurance cost is 0.5% of total capital costs 
Desalting Handbook for Planners 

D-16

Design Criteria 

Item Requirement 

Finished water quantity, m3/d (mgd) 20,000 (5.4) 

Process capacity, m3/day (mgd) 20,000 (5.4) 

Product water quality (mg/L) <500 

Finished water quality (mg/L) <500 

Distance to supply (m) 3000 

Number of trains 3 

Design feed water temperature (oC) 22 

Finished water quality goals: 

 - Total hardness (as CaCO3) 

 - Alkalinity (as CaCO3) 

 - Langelier index 

50 

50 

±0.2 

Pump types and sizes: 

 Intake or well pumps 

 Feed pumps 

 Transfer pumps 

 Housekeeping pumps (Cleaning, flushing, etc.) 

 Product pumps 

 High service pumps 

4 vertical turbine wet pit, 556 m3

/hr 

3 horizontal multi-stage 

- 

- 

3 horizontal split can, 417 m3

/hr 

- 

Pretreatment method Conventional 

Post-treatment method CO2/Lime/Corrosion Inh./Chlorine 

Fresh water flush system type Permeate Flush 

Concentrate disposal method Existing Sewer Outfall 

Product storage tank size 20,000 m3

Staffing requirements 1 superintendent, 4 shift operators 

Daily operational period 24 hours 

Disinfection equipment type Sodium Hypochlorite 
Appendix D: Worked Examples

D-17

Economic Criteria 

Item Requirement 

Cost year 2002 

Interest rate (%) 6 

Service life (yrs) 20 

Plant factor (%) 90 

Electricity cost ($/kWh) 0.095 

Steam cost $/MJ ($/Btu) - 

Labor rate ($/yr) 25,000 

Labor overhead rate (%) Inc. 

Contingency (%) 10 

Contractor overhead and labor (%) 15 

Engineering and administration (%) 10 

Freight and insurance (%) 5 

Chemical costs ($/T): 

 Acid 

 Scale inhibitor 

 Caustic 

 Chlorine 

 Sodium bisulfite 

 Anti-foam 

 Lime 

 Coagulant 

 Other 

See Table 9-4 

Membrane replacement ($/m3) 0.032 based on 20% replacement per year 

Repair and spare parts (%/yr) 1% of direct capital 

Facility insurance (%/yr) 0.5% 

Land cost ($/hectare) N/A 
Desalting Handbook for Planners 

D-18

Supporting Sheet (Sheet 1 of 3) 

Project: 20,000 m3

/day SWRO 

Date: 1/1/02 

Project Description: Supplemental Water Supply for City of Happy 

Beach 

Project Type: Surface Water Seawater Reverse 

Osmosis 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 6.6 x 106

 m3

Required Quality: <500 mg/l TDS 

Distance, Point of Delivery from Desalting Plant: 3,000 m 

Required Reliability: 100% 

Facilities Required: Intake structure, SWRO plant, building pretreatment and post-treatment, clearwell, transfer pumps 

WATER SUPPLY CHARACTERISTICS 

Physical Description: Surface seawater, average turbidity of 12 NTU 

Chemical Description: Standard seawater analysis 

Other: N/A 

Facilities Required: Intake structure with screens, etc., low head feed pumps, coagulation/filtration. 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: SWRO 

Replaceable items: Membranes 

Frequency of Replacement: 20%/yr 

Concentrate Disposal Requirements: To existing sewer outfall 

Service Required at Source: 4160V/3ph/60Hz 480V/3ph/6Hz 

Cost of Services Purchased: 

Fuel: 

Steam: 

Electric Power: $0.095/kWh 

Auxiliary Facilities Required: Step-down transformer, product transmission, emergency generator 
Appendix D: Worked Examples

D-19

Computation Sheet (Sheet 2 of 3) 

Project Description: 

 Water Supply for the City of Happy Beach 

Desalting Plant Capacity: 

6.6 x 106 ÷ (365 day/yr x 0.90) = 20,000 m3

/d 

Process Capital Costs: 

 Figure 9-7: $27,000,000 

Concentrate Disposal Capital Costs: 

 N/A 

Pretreatment Capital Costs: 

 Included in Process 

Intake Capital Costs: 

 From Figure 9-17: $1,500,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-20: $1,500,000 

Boiler Capital Costs: 

 N/A 

Site Development Costs: 

 Figure 9-23: $92,000 

Post-Treatment Capital Costs: 

 Included in Process 

Auxiliary Capital Costs: 

Includes Product Transmission, Emergency Generator, Step-Down Transformer: $1,922,000 

Building Capital Costs: 

 Included in Process 

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $32,014,000 x .06 x (1/2) x 1 =

 $960,420 

Land Cost: 

 N/A 

Annual Labor Costs: 

 Figure 9-37: $310,000 

Annual Chemical Costs: 

 Figure 9-41: $450,000 

Annual Steam Costs: 

 N/A 

Annual Electric Power Costs: 

 Figure 9-46: $1,800,000 

Annual Repairs and Spares: 

 1.5% of Direct Capital Cost: $480,000 

Other Computations: 

 Annual Membrane Replacement Costs: $0.032 x annual capacity = $0.032 x 6.6 x 106

 = $211,200 

 Product Transmission Capital Costs: Figure 9-28: $1,600,000 

 Step-Down Transformer Cost: Figure 9-30: $280,000 

 Emergency Generator Cost: Figure 9-29: $42,000 
Desalting Handbook for Planners 

D-20

Cost Summary (Sheet 3 of 3) 

Project Description: Water Supply for the City of Happy Beach 

Desalting Plant - Type: SWRO Capacity (m3/d): 20,000 

Annual Plant Factor (percent): 90 Interest Rate (percent): 6 

Annual Production (m3): 6.6 x 106 Plant Life (years): 20 

 Capital Recovery Factor: 0.0872 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 27,000,000 

2. Concentrate Disposal N/A 

3. Pretreatment Inc. in Process 

4. Water Intake 1,500,000 

5. Feed Water Pipes 1,500,000 

6. Steam Supply N/A 

7. General Site Development 92,000 

8. Post-Treatment Inc. in Process 

9. Auxiliary Equipment 1,922,000 

10. Building and Structures Inc. in Process 

 Subtotal Direct Capital Cost (DCC) 32,014,000 

11. Freight and Insurance 5 percent of DCC 1,601,000 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in years ) 960,420 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (.97) 4,658,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (.98) 3,137,000 

15. Contingency 10 percent of DCC 3,201,000 

 Total - Depreciating Capital 45,571,000 

16. Land Costs N/A 

17. Working Capital (1/6 x items 18-23) 545,000 

 Total - Nondepreciating Capital 545,000 

 Total Capital Costs 46,116,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 310,000 

19. Chemicals 450,000 

20. Steam N/A 

21. Electric Power 1,800,000 

22. Repairs and Spares 480,000 

23. Insurance 228,000 

 Total Operation and Maintenance 3,268,000 

24. Annual Costs - Depreciating Capital [(Total-Depreciating Capital – Total Membrane Replacement Cost 

 (i.e. 5 x 211,200)) x 0.0872] 

3,882,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciating Capital x 0.0872) 47,500 

 Total Annual Capital Charges 3,930,000 

26. Annual Membrane Replacement Costs 211,200 

 Total Annual Costs (Total O & M and Capital Charges) 7,409,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $1.12/m3
Appendix D: Worked Examples

D-21

D-5 Ground Water Nanofiltration 

The City of Yellowater is going to build a new 115,000 m3 per day water treatment plant to 

replace a 50-year-old lime softening plant. The purpose of the plant is to remove natural organic 

material (NOM) from the well water and to reduce hardness. The TDS of the feed water is 

1500 mg/l. The site will include 400 ft. deep wells, a concrete ground storage tank, and a 

concentrate injection well that will be acquired by the City. 

Note: Amortization is 20 years at 6 percent; annual insurance cost is 0.5% of total capital costs; 

land cost = $40,000 per hectare
Desalting Handbook for Planners 

D-22

Design Criteria 

Item Requirement 

Finished water quantity, m3/d (mgd) 115,000 (31) 

Process capacity, m3/day (mgd) 115,000 (31) 

Product water quality (mg/l) <2 Color Units, 100 mg/l Total Hardness 

Finished water quality (mg/l) Same 

Distance to supply (m) 3,000 

Number of trains 10 

Design feed water temperature (oC) 25 

Finished water quality goals: 

 - Total hardness (as CaCO3) 

 - Alkalinity (as CaCO3) 

 - Langelier index 

<100 

>50 

±0.2 

Pump types and sizes: 

 Intake or well pumps 

 Feed pumps 

 Transfer pumps 

 Housekeeping pumps (Cleaning, flushing, etc.) 

 Product pumps 

 High service pumps 

N/A 

Pretreatment method Standard 

Post-treatment method pH adjustment, corr. inh. chlorine 

Fresh water flush system type N/A 

Concentrate disposal method Deep well injection 

Product storage tank size One day 

Staffing requirements Table 9-3 15 

Daily operational period 24 

Disinfection equipment type Chlorine gas 
Appendix D: Worked Examples

D-23

Economic Criteria 

Item Requirement 

Cost year 2002 

Interest rate (%) 6 

Service life (yrs) 20 

Plant factor (%) 90% 

Electricity cost ($/kWh) 0.06 

Steam cost $/MJ ($/Btu) - 

Labor rate ($/yr) 25,000 

Labor overhead rate (%) Inc 

Contingency (%) 10% 

Contractor overhead and labor (%) 15% 

Engineering and administration (%) 10% 

Freight and insurance (%) 5% 

Chemical costs ($/T): 

 Acid 

 Scale inhibitor 

 Caustic 

 Chlorine 

 Sodium bisulfite 

 Anti-foam 

 Lime 

 Coagulant 

 Other 

Table 9-4 

Membrane replacement ($/m3) 0.021 based on 14% replacement per year 

Repair and spare parts (%/yr) 1 

Facility insurance (%/yr) 0.5 

Land cost ($/hectare) $40,000 
Desalting Handbook for Planners 

D-24

Supporting Sheet (Sheet 1 of 3) 

Project: City of Yellowater 

Date: 10/2002 

Project Description: Nanofiltration WTP for removal of NOM and reduction 

of hardness 

Project Type: Nanofiltration 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 35.7 x 106

 m3

Required Quality: <2 Color Units > 50 Total Alkalinity <100 Total Hardness 

Distance, Point of Delivery from Desalting Plant: At plant 

Required Reliability: 90% 

Facilities Required: Well field, NF WTP, ground storage, injection wells. 

WATER SUPPLY CHARACTERISTICS 

Physical Description: Well water, turbidity <1.0, SDI <30 

Chemical Description: Hard colored ground water, TDS <500 mg/l 

Other: N/A 

Facilities Required: Well field 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: NF 

Replaceable items: Membranes 

Frequency of Replacement (yrs): 7 years 

Concentrate Disposal Requirements: Deep-well injection 

Service Required at Source: 480 V 3ph 60 Hz 

Cost of Services Purchased: 

Fuel: 

Steam: 

Electric Power: $0.06/kWh 

Auxiliary Facilities Required: Emergency generator, step-down transformer 
Appendix D: Worked Examples

D-25

Computation Sheet (Sheet 2 of 3) 

Project Description: 

 Nanofiltration to replace old lime-softening plant 

Desalting Plant Capacity: 

 35.7 x 106 m3/yr ÷ (365 d/yr x 0.90) = 115,000 m3/d 

Process Capital Costs: 

 From Figure 9-8: $35,000,000 

Concentrate Disposal Capital Costs: 

 From Figure 9-13: $11,000,000 

Pretreatment Capital Costs: 

 From Figure 9-15: $11,000,000 

Intake Capital Costs: 

 From Figure 9-18 (well field): $6,000,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-20: $4,100,000 

Boiler Capital Costs: 

 N/A 

Site Development Costs: 

 From Figure 9-23: $250,000 

Post-Treatment Capital Costs: 

 Included in Process 

Auxiliary Equipment Capital Costs: 

 Includes emergency generator, step-down transformer, product transmission, product storage: $13,875,000 

Building Capital Costs: 

 Included in Process 

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $81,225,000 x .06 x (1/2) 

 X (18/12) = $3,655,000 

Land Cost: 

 From Figure 9-35: 1.5 hectares @ $40,000 per hectare = $60,000 

Annual Labor Costs: 

 From Figure 9-37: $375,000 

Annual Chemical Costs: 

 From Figure 9-42: $800,000 

Annual Steam Costs: 

 N/A 

Annual Electric Power Costs: 

 From Figure 9-47: $800,000 

Annual Repairs and Spares Costs: 

 1% of the Direct Capital Cost: $812,000 

Annual Membrane Replacement Costs 

 $0.021 x annual capacity = $0.021 x 35.7 x 106

 = $749,700 

Other Computations: 

 Product Transmission Cost: From Figure 9-28: $8,000,000 

 Emergency Generator Cost: From Figure 9-29: $175,000 

 Step-Down Transformer Cost: From Figure 9-30: $700,000 

 Product Storage Cost: From Figure 9-27: $5,000,000 
Desalting Handbook for Planners 

D-26

Cost Summary (Sheet 3 of 3) 

Project Description: Water for the City of Yellowater 

Desalting Plant - Type: Nanofiltration Capacity (m3/d): 115,000 

Annual Plant Factor (percent): 90 Interest Rate (percent): 6 

Annual Production (m3): 35.7 x 106 Plant Life (years): 20 

Capital Recovery Factor: 0.0872 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 35,000,000 

2. Concentrate Disposal 11,000,000 

3. Pretreatment 11,000,000 

4. Water Intake 6,000,000 

5. Feed Water Pipes 4,100,000 

6. Steam Supply N/A 

7. General Site Development 250,000 

8. Post-Treatment Inc. in Plant 

9. Auxiliary Equipment 13,875,000 

10. Building and Structures Inc. in Plant 

 Subtotal Direct Capital Cost (DCC) 81,225,000 

11. Freight and Insurance 5 percent of DCC 4,061,000 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in years ) 3,655,000 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (.95) 11,575,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (.90) 7,310,000 

15. Contingency 10 percent of DCC 8,123,000 

 Total - Depreciating Capital 115,949,000 

16. Land Costs 60,000 

17. Working Capital (1/6 x items 18-23) 561,000 

 Total - Nondepreciating Capital 621,000 

 Total Capital Costs 116,570,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 375,000 

19. Chemicals 800,000 

20. Steam N/A 

21. Electric Power 800,000 

22. Repairs and Spares 812,000 

23. Insurance 580,000 

 Total Operation and Maintenance 3,367,000 

24. Annual Costs - Depreciating Capital [(Total-Depreciating Capital – Total Membrane Replacement Cost 

 (i.e. 7 x 749,700)) x 0.0872] 

9,653,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciating Capital x 0.0872) 54,000 

 Total Annual Capital Charges 9,707,000 

26. Annual Membrane Replacement Costs 749,700 

 Total Annual Costs (Total O & M and Capital Charges) 13,824,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $0.39/m3
Appendix D: Worked Examples

D-27

D-6 Brackish Surface Water EDR 

The Dry Creek Municipal Water District takes its water from a brackish creek that flows through 

the service district. The local Economic Development Agency has been successful in attracting a 

major government agency facility. However, one of the conditions of locating in the area is that 

the water supply must meet all Federal and State primary and secondary drinking water 

standards, including total dissolved solids less than 500 milligrams per liter and chloride less 

than 250 milligrams per liter. The City needs about 10,000 m3

 per day of treated water. A 

treatability study has shown that because of high silica in the brackish water supply, EDR is the 

most appropriate treatment technology. The water source is approximately 1000 m from the 

treatment facility, and has a TDS of about 3000 mg/l. Product water will be transported to an 

existing storage site approximately 3,500 m away. The land cost in the area is $25,000 per 

hectare. Concentrate will be sent to 30 hectares of evaporation ponds, requiring a 1,400-meter 

pipeline. 

Note: Amortization is 20 years at 6 percent; annual insurance cost is 0.5% of total capital costs
Desalting Handbook for Planners 

D-28

Design Criteria 

Item Requirement 

Finished water quantity, m3/d (mgd) 10,000 (2.68) 

Process capacity, m3/d (mgd) 10,000 (2.68) 

Product water quality (mg/l) <500mg/l TDS, <250mg/l Cl 

Finished water quality (mg/l) Same 

Distance to supply (m) N/A 

Number of trains 2 

Design feed water temperature (oC) 25 

Finished water quality goals: 

 - Total hardness (as CaCO3) 

 - Alkalinity (as CaCO3) 

 - Langelier index 

Pump types and sizes: 

 Intake or well pumps 

 Feed pumps (2) 

 Transfer pumps (2) 

 Housekeeping pumps (Cleaning, flushing, etc.) 

 Product pumps 

 High service pumps 

N/A 

Horizontal split case, 521 m3

/hr 

Horizontal split case, 417 m3

 /hr 

N/A 

N/A 

N/A 

Pretreatment method Existing treatment plant 

Post-treatment method pH adjustment, corr. inh., chlorine 

Fresh water flush system type N/A 

Concentrate disposal method Evaporation ponds 

Product storage tank size None 

Staffing requirements Table 9-3 - 8 persons 

Daily operational period 24 hours 

Disinfection equipment type Gas chlorination 
Appendix D: Worked Examples

D-29

Economic Criteria 

Item Requirement 

Cost year 2002 

Interest rate (%) 6 

Service life (yrs) 20 

Plant factor (%) 95 

Electricity cost ($/kWh) 0.06 

Steam cost $/MJ ($/Btu) N/A 

Labor rate ($/yr) 25,000 

Labor overhead rate (%) Inc. 

Contingency (%) 10% 

Contractor overhead and labor (%) 15% 

Engineering and administration (%) 10% 

Freight and insurance (%) 5% 

Chemical costs ($/T): 

 Acid 

 Scale inhibitor 

 Caustic 

 Chlorine 

 Sodium bisulfite 

 Anti-foam 

 Lime 

 Coagulant 

 Other 

See Table 9-4 

Membrane replacement, & stack spares $0.039/m3 based on 10% replacement per year 

Repair and spare parts (%/yr) 1% 

Facility insurance (%/yr) 0.5% 

Land cost ($/hectare) 25,000 
Desalting Handbook for Planners 

D-30

Supporting Sheet (Sheet 1 of 3) 

Project: DCMWD 

Date: 10/2002 

Project Description: Desalting addition to existing brackish surface water 

treatment 

Project Type: EDR 

PRODUCT WATER CHARACTERISTICS 

Annual Requirement: 3.5 x 106

 m3

Required Quality: <500 mg/l TDS 

Distance, Point of Delivery from Desalting Plant: Adjacent 

Required Reliability: 95% 

Facilities Required: Treatment equipment, building, concentrate pipe, evaporation ponds, land, post-treatment 

WATER SUPPLY CHARACTERISTICS 

Physical Description: Brackish surface water, high silica, acceptable turbidity, and SDI 

Chemical Description: Na, 420 mg/l; Ca, 60 mg/l; Mg, 10 mg/l; HCO3, 200 mg/l; Cl, 500 mg/l; SO4, 229 mg/l. 

Other: Silica – 72 mg/l 

Facilities Required: Feed pumps from existing plant 

DESALTING COMPLEX CHARACTERISTICS 

Process Being Considered: EDR 

Replaceable Items: Membranes and stack parts 

Frequency of Replacement (yrs): 10 years 

Concentrate Disposal Requirements: Evaporation ponds 

Service Required at Source: 480V 3ph 60 cycle electrical power 

Cost of Services Purchased: 

Fuel: 

Steam: 

Electric Power: $0.06/kWh 

Auxiliary Facilities Required: Step-down transformer, emergency generator, product transmission 
Appendix D: Worked Examples

D-31

Computation Sheet (Sheet 2 of 3) 

Project Description: 

 Desalting addition to existing brackish surface water treatment 

Desalting Plant Capacity: 

 3.5 x 106 m3/yr ÷ (365 d/yr x 0.95) = 10,000 m3/d 

Process Capital Costs: 

 From Figure 9-9, $8,100,000 

Concentrate Disposal Capital Costs: 

 Includes evaporation ponds, concentrate disposal pipeline: $2,330,000 

Pretreatment Capital Costs: 

 From Figure 9-15, $1,500,000 

Intake Capital Costs: 

 From Figure 9-17, $600,000 

Feed Water Supply Piping Capital Costs: 

 From Figure 9-20, $1,000,000 

Boiler Capital Costs: 

 N/A 

Site Development Costs: 

 From Figure 9-22, $65,000 

Post-Treatment Capital Costs: 

 Included in Process 

Auxiliary Equipment Capital Costs: 

 Includes product transmission, step-down transformer, emergency generator: $1,425,000 

Building Capital Costs: 

 Included in Process 

Interest During Construction: 

 Direct Capital Cost x Interest Rate x One Half of Construction Time (Table 9.1) in years = $15,020,000 x .06 x (1/2) 

 x 1 = $451,000 

Land Cost: 

 From Figure 9-34, 30.2 hectares x $25000 / hectare = $755,000 

Annual Labor Costs: 

 From Figure 9-37, $210,000 

Annual Chemical Costs: 

 From Figure 9-41, $180,000 

Annual Steam Costs: 

 N/A 

Annual Electric Power Costs: 

 From Figure 9-47, $280,000 

Annual Repairs and Spares Costs: 

 1% of the Direct Capital Cost = $150,000 

Annual Membrane Replacement Costs: 

 $0.039 x annual capacity = $0.039 x 3.5 x 106

 = $136,500 

Other Computations: 

 Evaporation Pond Cost, From Figure 9-12: $2,200,000 

 Concentrate Pipeline Cost, From Figure 9-11: $130,000 

 Emergency Generator Cost: From Figure 9-29: $25,000 

 Step-down Transformer Cost: From Figure 9-30: $200,000 

 Product Transmission Cost: From Figure 9-28: $1,200,000 
Desalting Handbook for Planners 

D-32

Cost Summary (Sheet 3 of 3) 

Project Description: Water for the Dry Creek Municipal Water District 

Desalting Plant - Type: EDR Capacity (m3/d): 10,000 

Annual Plant Factor (percent): 95 Interest Rate (percent): 6 

Annual Production (m3): 3.5 x 106 Plant Life (years): 20 

 Capital Recovery Factor: 0.0872 

CAPITAL COSTS 

Capital Cost Centers Est. Cost ($) 

1. Desalting Plant 8,100,000 

2. Concentrate Disposal 2,330,000 

3. Pretreatment 1,500,000 

4. Water Intake 600,000 

5. Feed Water Pipes 1,000,000 

6. Steam Supply N/A 

7. General Site Development 65,000 

8. Post-Treatment Inc. in Plant 

9. Auxiliary Equipment (Step down transformer, product transmission) 1,425,000 

10. Building and Structures Inc. in Plant 

 Subtotal Direct Capital Cost (DCC) 15,020,000 

11. Freight and Insurance 5 percent of DCC 751,000 

12. Interest During Construction (DCC x Interest Rate x One Half of Construction Time (Table 9.1) in years ) 451,000 

13. Construction Overhead 15 percent of DCC x factor from figure 9-33 (1.01) 2,278,000 

14. Owner’s Direct Expense 10 percent of DCC x factor from figure 9-33 (1.07) 1,607,000 

15. Contingency 10 percent of DCC 1,502,000 

 Total - Depreciating Capital 21,609,000 

16. Land Costs 755,000 

17. Working Capital (1/6 x items 18-24) 155,000 

 Total - Nondepreciating Capital 910,000 

 Total Capital Costs 22,519,000 

ANNUAL COSTS 

Annual Cost Centers Est. Cost ($/yr) 

18. Operation and Maintenance Labor 210,000 

19. Chemicals 180,000 

20. Steam N/A 

21. Electric Power 280,000 

22. Repairs and Spares 150,000 

23. Insurance 108,000 

 Total Operation and Maintenance 928,000 

24. Annual Costs - Depreciating Capital [(Total-Depreciating Capital – Total Membrane Replacement Cost 

 (i.e. 10 x 136,500)) x 0.0872]) 

1,765,000 

25. Annual Costs - Nondepreciating Capital (Total- Nondepreciating Capital x 0.0872) 79,000 

 Total Annual Capital Charges 1,844,000 

26. Annual Membrane Replacement Costs 136,500 

 Total Annual Costs (Total O & M and Capital Charges) 2,909,000 

COST OF WATER (Total Annual Costs/ Yearly Production) $0.83/m3
 Appendix E: Glossary

E-1

Appendix E: Glossary 

AC – Alternating electrical current. 

ACIDIFICATION – The addition of acid to the feed water of a desalting plant, usually to 

prevent alkaline calcium carbonate and magnesium hydroxide scale. 

ALKALINE – Water containing sufficient amounts of alkalinity to raise the solution pH above 

7.0. 

ALKALINITY – The ability of water to neutralize an acid because of the presence of 

bicarbonate, carbonate, and hydroxyl ions. 

AMBIENT TEMPERATURE – Temperature of surroundings, usually assumed to be 20-25 o

C. 

ANION – An ion carrying a negative charge. In an electrolytic cell, it migrates toward the 

anode. 

ANODE – The positive electrode of an electrodialysis cell. 

ANNUAL COST – The total yearly cost of owning and operating a desalting plant. This cost 

includes carrying charges on the investment, taxes, insurance, interest on working capital, 

operating and maintenance labor, energy costs, consumable supplies, repair and replacement 

costs, and the cost of concentrate disposal. 

ANTISCALANT – See Scale Inhibitor. 

ATOM – The smallest particle of an element possessing all the chemical characteristics of that 

element. 

BACK PRESSURE VALVE – A valve placed in a piping system to restrict the fluid flow, thus 

pressurizing the system. The valve is usually spring- or gas-pressure loaded so that any desired 

system pressure may be present. 

BACKWASH – Reversed flow in a filter, ion-exchange column, or membrane filter to remove or 

wash away accumulated suspended materials. 

BACTERIA – Microscopic organisms, usually consisting of a single cell. 

BAR – Unit of pressure. 1 bar = 14.5 lb/in2

. 

BASIC SOLUTION – Solution containing an excess of hydroxyl ions and with a pH greater 

than 7.0. 
 

Desalting Handbook for Planners 

E-2

BLENDING – Mixing waters of different purity and constituents to form a diluted solution. 

BLOWDOWN – See Concentrate Reject (Stream). 

BOILING POINT ELEVATION (BPE) – The difference between the boiling point of a solution 

and the boiling point of pure water at the same elevation. 

BRACKISH WATER – Saline water with a salt concentration ranging from 1,000 mg/l to about 

25,000 mg/l total dissolved solids (TDS). 

BTU – British thermal unit. The quantity of heat required to increase the temperature of 1 pound 

of water 1 o

F. 

CAPITAL COST – Total capital cost includes the indirect costs associated with owner’s costs of 

studies, engineering, licenses, interest on working capital, and insurance during the construction 

period, as well as the direct capital costs. It is the owner’s total investment up to the point that 

the plant is put into useful operation. 

CATHODE – Negative electrode of an electrodialysis cell. 

CATION – The ion in an electrolytic solution that migrates to the cathode. It carries a positive 

charge. 

CHANNELING – Unevenly distributed flow that may occur, for example, in a sand filter, an 

electrodialysis cell, or in a reverse osmosis membrane. 

CHEMICAL CLEANING – All-inclusive term for any of a number of in situ chemical cleaning 

techniques to remove fouling and scale from membranes and thermal heat transfer surfaces. 

CHEMICAL EQUATION – Notation used to describe how atoms combine to form molecules. 

CHEMICAL EQUILIBRIUM – When chemical reactions occur, not all of the reactants react to 

form products. The solution attains a chemical equilibrium. At equilibrium, there may be 

residual reactants or products. 

CHEMICAL FORMULA – Notation used to describe molecules (e.g., the formula for water is 

H2O). 

CHEMICAL REACTION – Phenomena when atoms unite, separate, or exchange places in the 

ratio of their particular atomic weights or simple multiples thereof. 

COAGULATION – The precipitation of substances in colloidal solution. 

COMPOUND – A substance that can be decomposed by chemical processes into two or more 

elements or which can be built up from two or more elements (e.g., sodium chloride, NaCl). 
 Appendix E: Glossary

E-3

CONCENTRATE – The concentrated wastewater flow from reverse osmosis, electrodialysis, 

and nanofiltration plants. 

CONCENTRATE REJECT (STREAM) – The concentrated wastewater flow from a desalting 

plant, containing most of the salts from the original feed water. Also referred to as blowdown. 

CONCENTRATE SATURATION PRESSURE – The pressure corresponding to the concentrate 

stream temperature at which boiling occurs. 

CONCENTRATION POLARIZATION – The concentration or depletion of salt ions at 

membrane surfaces. 

CONCENTRATION RATIO – The salinity of the concentrate divided by the salinity of the plant 

feed water. 

CONDENSATE – Distilled water formed by cooling and condensing water vapor. 

CONDENSER TUBES – Tubes used to heat or reheat concentrate in the distillation process 

while simultaneously condensing water vapor. 

CONDUCTIVITY – See Specific Conductance. 

CONJUNCTIVE USE – A desalting plant operating on a part-time basis to supplement a water 

supply. 

CONTAMINANT – Any undesirable substance in a water source. 

DC – Direct electrical current. 

DEAERATION – A process to remove oxygen and other noncondensable gases from the feed 

water stream. 

DECARBONATION – A process to remove carbonate alkalinity from the feed water as CO2

gas. 

DEMINERALIZATION – Any process that removes mineral substances from water. 

DESALINATION – Process of removing salts from water sources. 

DIFFERENTIAL WATER COST – In this report, the deviation from a selected water cost basis 

as the result of changes in various operating conditions. 

DIRECT CONTACT HEAT EXCHANGE – The process in which a liquid, gas, or solid to be 

heated or cooled is brought into direct contact with the heating or cooling medium. 
 

Desalting Handbook for Planners 

E-4

DISSOLVED SUBSTANCE – A substance that has gone into solution in water. See Solution. 

DISTILLATE – The final product water from a distillation plant. 

DISTILLATION – A method of desalting water that uses heat to vaporize water and to collect 

the condensed water. 

DISTRIBUTION SYSTEM – The pipes, conduits, and canals bringing water to the consumers. 

DIVALENT ION – An ion carrying a double charge, either positive or negative. Example: 

Mg+2, SO-4. 

DUAL-PURPOSE PLANT – A plant that produces both electric power and desalted water. 

EFFECT – A single evaporation or single step in a multi-effect evaporator arrangement. 

EFFLUENT – Water leaving a desalting process. May be applied to both concentrate or product 

water. 

ELECTROCHEMISTRY – Science dealing with the relation of electricity to chemical changes. 

ELECTRODIALYSIS (ED) – A process by which ions are transferred through membranes to a 

more concentrated solution as a result of using a direct current electrical potential. 

ELECTRODIALYSIS REVERSAL (EDR) – A variation of ED in which polarity and cell 

function change periodically to maintain efficient performance. 

ELECTROLYTE – A compound that disassociates into ions when dissolved in water. 

ELEMENT – A simple substance that cannot be decomposed by chemical processes into simpler 

substances (e.g., chlorine, hydrogen, oxygen, sodium). 

ENERGY RECOVERY – Possible energy saving in reverse osmosis in which the concentrate 

stream, under pressure, is used to drive a turbine that provides part of the feed pressure 

requirement. 

ENTHALPY – A measure of the total energy content of a material or system. 

ENTRAINMENT SEPARATOR – A device for separating entrained water droplets from the 

vapor produced in a distillation plant. 

EQUIVALENT PER MILLION – Calculated by dividing the concentration in parts per million 

by the equivalent weight. 
 Appendix E: Glossary

E-5

EQUIVALENT WEIGHT – The weight of an ion determined by the sum of the component 

weight of its atoms by its valence. 

EVAPORATION – The process by which water is converted to a vapor that can be condensed. 

EVAPORATOR – A process device in which water is boiled and the water vapor is collected 

and condensed to form a distilled product water. 

EXPRESSED AS CALCIUM CARBONATE (CaCO3) – Indicates that the reported quantity of a 

compound has been converted to an equivalent quantity of CaCO3. 

FEED-TO-PRODUCT CONCENTRATION RATIO – Product concentration divided by the feed 

concentration. Often expressed as percent. Sometimes referred to as “cut” in the electrodialysis 

process. 

FEED WATER – Saline water supplied to the desalting plant for processing. 

FIXED CHARGES – Charges associated with the operation of a desalting plant that continue to 

accrue, whether the plant is operating or not. This cost includes interest, amortization, taxes, and 

insurance. 

FLASHING – A physical process in which a preheated water encounters a reduced pressure that 

causes part of the water to boil rapidly or flash into steam. 

FLASHING THERMAL POTENTIAL – When a water under sufficient pressure is raised to a 

temperature higher than the saturation temperature corresponding to a subsequent system 

pressure, the water is said to have “flashing thermal potential.” 

FLOCCULATION – The gathering together of small particles in water after adding coagulant 

chemicals. 

FLUTED SURFACE TUBES; FINNED TUBES – Tubes of an evaporator or condenser in which 

the heat transfer rate is enhanced by a series of corrugations or ridges and valleys running 

parallel to or spiraling along the length of the tube. 

FLUX – Term used in reverse osmosis to indicate the rate of water permeation through the 

membrane. Usually expressed as gallons per square foot per day in the U.S., and liters per 

square meter per hour in metric. 

FOULING – The reduction in performance of process equipment (heat transfer tubing, 

membranes, etc.) that occurs as a result of scale buildup, biological growth, or the deposition of 

colloidal material. 
 

Desalting Handbook for Planners 

E-6

GENERAL AND ADMINISTRATIVE COSTS – An expense usually calculated as a percent of 

labor cost plus payroll additives to cover the costs associated with timekeeping, payroll, 

overhead, and administration. 

GROUND WATER – Water normally found underground and obtained from wells. 

HARDNESS – Usually measured in CaCO3. “Hardness” in water is the sum of calcium and 

magnesium concentrations, both expressed as calcium carbonate, CaCO3, in milligrams per liter. 

These constituents cause soaps to precipitate. 

HEAT EXCHANGER – An apparatus in which heat is transferred from one medium to another. 

HEAT OF VAPORIZATION – The quantity of heat required to vaporize 1 unit of liquid to 1 

unit of dry vapor. For water at one atmosphere, approximately 1,000 Btu/lb or 2.32 MJ/kg. 

HEAT RECOVERY SECTION – The portion of a flash distillation evaporator where the feed 

stream recovers heat from condensing vapors. 

HEAT REJECTION SECTION – The portion of a flash distillation evaporator where the heat 

released by the condensing vapors is rejected to waste with the condenser cooling water. 

HEAT TRANSFER – Physical phenomenon dealing with the flow of heat. The subject is 

particularly important in the distillation and freezing processes in which heat is transferred from 

medium to medium, usually through a heat transfer surface. 

HEAT TRANSFER COEFFICIENT – An engineering design factor defining the heat transfer 

rate in a heat exchanger. Usually given in terms of Btu/hr-ft2

-

o

F temperature difference. 

HYBRID PLANTS – Plants that combine two or more processes. 

HYDROLYSIS – A chemical process of decomposition involving splitting of a bond and 

addition of the ions of water. 

HYDROXYL ION – The (OH-

) ion present in water. 

IMMISCIBLE LIQUIDS – Liquids incapable of mixing or attaining homogeneity. 

INDIRECT CAPITAL COSTS – The owner’s costs associated with such items as studies, 

planning, engineering, construction supervision, licensing, startup, public relations, and training. 

These costs are a part of the cost of placing the plant in operation and are in addition to the direct 

capital costs associated with equipment and contracts for construction. 

INORGANIC – Substances of mineral origin, such as sand, salt, iron, and calcium salts. 

INSOLUBLE MATERIALS – Materials that do not dissolve, or dissolve only slightly, in water. 
 Appendix E: Glossary

E-7

INTAKE EQUIPMENT – The works or structures at the head of a conduit into which the feed 

water entering the desalting plant is directed. 

INVERSE SOLUBILITY – The characteristic attributed to a substance that becomes less soluble 

with increasing temperature (e.g., calcium carbonate). 

ION – An atom or group of atoms that carries a positive or negative electric charge as a result of 

having lost or gained one or more electrons. An electrically charged atom, radical, or molecule 

formed by the loss or gain of electrons. 

IONIZABLE – Compounds, such as salts, that form ions when dissolved in water. These 

materials lend themselves to separation by electrodialysis. Many organic materials are not 

ionizable, and separation can only be accomplished by physical processes such as reverse 

osmosis or distillation. See also Electrolyte. 

KWH – Kilowatthours. A measure of electrical usage. 

LANGELIER SATURATION INDEX – Measure of a water’s corrosion or scale-forming 

tendencies. 

LATENT HEAT – The amount of heat required to cause a change of state without temperature 

change, as in the melting of ice (latent heat of fusion) or the evaporation of water (latent heat of 

evaporation). See Heat of Vaporization. 

MEMBRANE – In desalting, used to describe a semipermeable film. Membranes used in 

electrodialysis are permeable to ions of either positive or negative charge. Reverse osmosis and 

nanofiltration membranes ideally allow the passage of pure water and block the passage of salts. 

MICROMHO – A unit measure of conductivity equal to one millionth of a mho. Mho is the 

reciprocal of ohm. 

MICROFILTRATION – A membrane used to treat water, with a 0.05 - 5 micron pore size. The 

membrane filters out turbidity, algae, Giarda and Cryptosporidium spores, and bacteria. The 

membrane operates by sieving. 

MICRON – A unit of length equal to one thousandth of a millimeter. 

MICROORGANISM – A plant or animal of microscopic size. 

MINERAL REDUCTION – Partial removal of dissolved salts from water. 
 

Desalting Handbook for Planners 

E-8

MODULE – The smallest packaged functional assembly of a desalting plant. A section or 

integral portion of a desalting plant that is used initially to study large-scale technology and 

critical design features in preparation for subsequent prototype construction. 

MOLE – The quantity of a chemical substance measured in mass units (e.g., grams or pounds) 

that are numerically equal to the molecular weight. For a gas, the volume occupied by such a 

weight under specified conditions. 

MOLECULE – The smallest quantity of a compound that possesses all the chemical 

characteristics of that compound. 

MONOVALENT ION – An ion that carries only a single charge, either positive or negative. 

Example: Na+

, Cl-. 

NANOFILTRATION – A membrane used to desalinate water. The membrane has a molecular 

weight cutoff of about 100, and rejects ions with greater than 100 molecular weight at about 90 

percent. The membrane operates by overcoming osmotic pressure. 

NONCONDENSABLE GASES – Gases such as air and carbon dioxide that do not condense to 

liquid with the water vapor when heat is removed. 

NONELECTROLYTE – See Electrolyte. 

ORGANIC – Substances that come from plant or animal sources and always contain carbon. 

ORGANIC COMPOUND – A compound in which the major elements are carbon and hydrogen. 

OSMOSIS – Movement of water from a dilute solution to a more concentrated solution through 

a membrane separating the two solutions. See Reverse Osmosis. 

OSMOTIC PRESSURE – The potential energy difference between two solutions of different 

concentrations separated by a permeable membrane. 

OXIDATION – The addition of oxygen, removal of hydrogen, or removal of electrons from an 

element or compound. 

PATHOGENS – Disease-causing organisms. 

PERFORMANCE RATIO (PR) – A performance rating associated with the distillation desalting 

process. It is defined as the number of pounds of distillate produced for each 1,000 Btu of heat 

input, or as kg/MJ in metric. 

PERMANENT HARDNESS – That part of the hardness that cannot be removed by boiling. 

Also known as noncarbonate hardness. 
 Appendix E: Glossary

E-9

PERMEATE – The product water from a desalting process (also called product). 

pH – A number indicating the hydrogen-ion concentration in a solution. Values greater than 7.0 

indicate a basic (alkaline) solution; values less than 7.0 indicate an acidic solution. 

PILOT PLANT – An experimental unit of small size, usually less than 400 m3

/d (0.1 mgd) 

capacity, used for early evaluation and development of new, improved processes and to obtain 

technical and engineering data. 

PLANT LOAD FACTOR – Also known as “Plant Factor” or “Load Factor.” The fraction, or 

percentage, of full-scale annual design capacity that is actually produced by a desalting plant. 

POLYAMIDE – A polymer formed by polymerization of an ester. 

POLYMER – A chemical compound formed by polymerization. 

POLYMERIZATION – A chemical reaction in which smaller molecules of the same kind (or 

sometimes of two or three different kinds) combine to form larger molecules. 

POST-TREATMENT – The processes, such as pH adjustment and chlorination, that may be 

employed on the product water from a desalting unit. 

POTABLE WATER – Water that does not contain objectionable pollution, contamination 

minerals, or infective agents and is considered suitable for drinking. In desalting, potable water 

is typically defined as having a salinity less than 500 ppm TDS (USPHS – Drinking Water 

Standards, 1962) and to be in compliance with Federal and State regulations. 

PRECIPITATE – A substance separated from a solution by chemical or physical change as an 

insoluble amorphous or crystalline solid. 

PRETREATMENT – The processes such as chlorination, clarification, coagulation, scale 

inhibition, acidification, and deaeration that may be employed on the feed water to a desalting 

unit to minimize algae growth, scaling, and corrosion. 

PRODUCT – The final desalted water, called “distillate” in distillation, “dilute” in 

electrodialysis, and “product” in the reverse osmosis and nanofiltration processes. 

PRODUCT – In a chemical equation, the result of the chemical reaction. Noted on the right side 

of the equation. 

PROTOTYPE – A full-size, first-of-kind production plant used for development, study, and 

demonstration of full-sized technology, plant operation, and process economics. 

REACTANT – A chemically reacting substance. Item on the left side of a chemical equation. 
 

Desalting Handbook for Planners 

E-10

RECOVERY RATIO – The ratio of the product flow rate to the feed water flow rate. This ratio 

is sometimes called the conversion ratio. 

RECURRING COSTS – Plant fixed costs associated with annual tax and insurance costs. 

REPLACEABLE ITEMS – Plant equipment with an estimated life less than the useful plant 

lifetime or the plant design lifetime. 

REVERSE OSMOSIS (RO) – Method of desalination which uses pressure to move water from a 

concentrated solution to a dilute solution through a membrane separating the two solutions. See 

Osmosis. 

SALINE WATER – Water with dissolved solids exceeding the limits of potability. Saline water 

may include sea water, brackish water, mineralized ground and surface water, and irrigation 

return flows. 

SALT DIFFUSION – The movement of ions or molecules under influence of a concentration 

difference. 

SALT REJECTION – A factor expressing the ability of RO membranes to reject dissolved 

solids. Usually given as: feed concentration minus product concentration divided by feed 

concentration, and expressed as percent. 

SALT TRANSPORT – Salt that transfers through reverse osmosis membranes, along with water. 

SATURATED SOLUTION – A solution that contains the maximum amount of solute that can 

be dissolved at equilibrium. See Solubility. 

SATURATION TEMPERATURE – The temperature of a liquid, under a given pressure, at 

which boiling and condensing occur. 

SCALE – Salts deposited on heat transfer or membrane surfaces that retard the rate of heat 

transfer or ion or water permeation. 

SCALE INHIBITOR – An agent that ties up and, thus, inactivates certain metal ions. It may be 

added to a feed water to extend the limits of saturation of scaling substances. The sequestering 

of calcium ions to prevent calcium sulfate precipitation is an example. Also known as 

antiscalant, sequestering agent. 

SECONDARY SEWAGE EFFLUENT – Sewage that has been processed through the 

conventional primary and secondary treatment stages. 

SEMIPERMEABLE MEMBRANE – A membrane that is permeable for certain molecules or 

ions only. RO membranes, for example, ideally will pass water but not salt. ED membranes 

pass ions with a certain charge but not water. 
 Appendix E: Glossary

E-11

SHELL – The vessel that contains the heat exchanger or condenser tubing. 

SOFT WATER – A water that has a minimal amount of calcium and magnesium ions. Such 

waters frequently contain substantial amounts of sodium. 

SOLUBILITY – A measure of the maximum amount of a certain substance that can dissolve in a 

given amount of water, or other solvent, at a given temperature. 

SOLUTE – A dissolved substance. 

SOLUTION – A homogeneous mixture of substances in which the molecules of the solute are 

uniformly distributed among the molecules of the solvent, such as water. 

SPECIFIC CONDUCTANCE (CONDUCTIVITY) – Quantitative expression for the capability 

of a particular solution to conduct electricity. It is defined as the conductance of a cube of that 

particular water that is 1 cm long and has a cross sectional area of 1 cm2

. Conductivity is usually 

expressed in micromohos per centimeter. 

SPECIFIC GRAVITY – The ratio of the density of a substance to the density of water at specific 

temperatures. 

SPECIFIC POWER – Desalting plant power or energy consumption evaluated as a function of 

water production. Usually given in kWh per 1,000 gallons or kWh per cubic meter. 

STACK – The alternating array of cation and anion permeable membranes, spacers, gaskets, and 

electrodes used in the electrodialysis desalting process. 

STAGE – A unit of desalting equipment capable of purification and separation of the feed water 

into product and concentrate. If separation is insufficient, more than one stage can be arranged 

in series. 

SUPERSATURATED SOLUTION – A solution containing more solute than its normal 

solubility would allow. This is a nonequilibrium and, therefore, temporary situation. The solute 

has a strong tendency to come out of solution as crystals until the saturated condition has been 

reached. 

SURFACE WATER – Water above the water table. 

SUSPENDED SUBSTANCE OR SUSPENDED SOLIDS – Materials that are not dissolved but 

are in suspension in the form of finely divided particles. Suspended materials may be removed 

by physical means such as filtration. 

TEMPORARY HARDNESS – Hardness that can be removed by boiling. 
 

Desalting Handbook for Planners 

E-12

THERMODYNAMIC EQUILIBRIUM – A condition in which the temperature of a liquid is in 

balance with its vapor pressure such that no boiling or condensing is occurring. 

THERMODYNAMICS – A science dealing with the mechanical action or relations of heat. 

TITRATION – Chemical method of determining the strength of a solution by measuring the 

volume of a reacting standard solution of known concentration. 

TUBE BUNDLE – A series of tubes compactly arranged in an evaporator shell to provide the 

required heat transfer surface. 

TUBE SHEET – The end plate in a tube bundle to which all of the tubes are attached. 

TUBING – Typically, flexible piping of small diameter, generally less than 5 centimeters 

(approximately 2 inches), used in distillation processes. 

TUBING AREA – Surface area of tubing. 

TURBIDITY – Opaqueness or cloudiness caused by the presence of suspended particles in 

water, usually stirred-up sediments. The turbidity of a water is measured by its capacity for 

absorbing or scattering light. 

TURBINE CYCLE – Use of turbines to generate power from high-pressure steam. Lowpressure effluent steam from such turbines can be used to drive a desalting plant. 

TURBULENT FLOW – A fluid flow condition in which the velocity at a given point varies 

erratically in magnitude and direction. 

ULTRAFILTRATION – A membrane used to treat water with about a 10,000-300,000 

molecular weight cutoff. The membrane rejects organic macromolecules, viruses, and asbestos. 

The membrane operates by sieving. 

UNIT COST – The cost per unit of product water output ($/1,000 gallons, $/cubic meter) or per 

unit energy input ($/M Btu, cents/kWh, $/MJ). 

VALENCE – Number indicating the capacity of an atom to combine with another element. 

WASTE CONCENTRATE – See Concentrate Reject (Stream). Sometimes called blowdown. 

WATER CHEMISTRY – A branch of science dealing with the chemical properties of water and 

its dissolved and suspended constituents. 

WATER PERMEABILITY – The capacity of a membrane to allow water to pass through. See 

Flux. 
 Appendix E: Glossary

E-13

WATER TABLE – The upper limit of the portion of the ground wholly saturated with water. 

This can be near the ground’s surface or many feet below it. See Surface Water. 

WATER TRANSPORT – The tendency for water to pass through a membrane. Water transport 

is desirable in reverse osmosis but undesirable in electrodialysis. 
Appendix F: Common Conversions 

From To Multiply by

Mass & Length 

Kilograms (Kg) Pounds (lbs) 2.2046 

Metric tons (tonnes) US tons (tons) 1.1023 

Meters (m) Feet (ft) 3.2808 

Centimeters (cm) Inches (in) 0.3937 

Millimeters (mm) Inches (in) 0.0394 

Area 

Square meters (m2) Square feet (ft2) 10.7639

Square centimeters (cm2) Square inches (in2) 0.155

Hectares Acres 2.4711

Volume 

Cubic meters (m3) Gallons (g) 264.1721

Cubic meters (m3) Liters (l) 1000

Cubic meters (m3) Acre-feet 8.11E-04

Cubic meters/day (m3/d) Million gallons/day (mgd) 2.64E-04 

Cubic meters/hour (m3/hr) Gallons/minute (gpm) 4.4029

Cubic meters (m3) Thousand gallons (kgal) 0.2642 

Liters per minute (lpm) Gallons per minute (gpm) 0.2642 

Liters per square meter per hour (l/m2/hr) Gallons per square foot per minute 4.09E-04 

Liters per square meter per hour (l/m2/hr) Gallons per square foot per hour 0.0245 

Liters per square meter per hour (l/m2/hr) Gallons per square foot per day 0.589 

Pressure

Bars Pounds/square inch (psi) 14.5038 

Bars Kilograms/sq. centimeter (Kg/cm2) 1.0197

Kilograms/sq. centimeter (Kg/cm2) Pounds/square inch (psi) 14.2233 

Kilopascals (kPa) Pounds/square inch (psi) 0.145 

Energy & Heat Transfer

Joules (J) British thermal units (Btu) 9.49E-04 

Joules/gram Btu/pound 0.4299

Joules/gram degree C Btu/lb degree F 0.2388 

Calories/gram degree C Btu/lb degree F 1 

Kilowatts (kW) Btu/hr 3414.43

Kilowatt-hours (kWh) Btu 3412.76

Kilowatt-hours/cubic meter Kilowatt-hours/thousand gallons 3.7854 

 (kWh/m3) (kWh/kgal) 

Micro ohms/cm Micro ohms/in 2.54 

Micromhos/cm Micromhos/in 2.54

Watts/sq. meter degree K (W/m2-K) Watts/sq. feet degree K (W/ft2-K) 0.0929

ohms/sq. centimeters ohms/sq. inches 6.4516 

Mega joules (MJ) Btu 947.8171 

Mega joules (MJ) Thousand Btu (kBtu) 0.9478 

Mega joules per cubic meter (MJ/m3) Btu per gallon (Btu/gal) 3.5879 

F-1