ufc 3-230-12a water desalination (01!16!2004)

8/14/2019 UFC 3-230-12A Water Desalination (01!16!2004)

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UNIFIED FACILITIES CRITERIA (UFC)

WATER DESALINATION

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED


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UNIFIED FACILITIES CRITERIA (UFC)

WATER DESALINATION

Any copyrighted material included in this UFC is identified at its point of use.Use of the copyrighted material apart from this UFC must have the permission of thecopyright holder.

U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)NAVAL FACILITIES ENGINEERING COMMAND

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

This UFC supersedes TM 5-813-8, dated 15 September 1986. The format of this UFC does notconform to UFC 1-300-01; however, the format will be adjusted to conform at the next revision.The body of this UFC is the previous TM 5-813-8, dated 15 September 1986.


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FOREWORD\1\The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and providesplanning, design, construction, sustainment, restoration, and modernization criteria, and appliesto the Military Departments, the Defense Agencies, and the DoD Field Activities in accordancewith USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and

work for other customers where appropriate. All construction outside of the United States isalso governed by Status of forces Agreements (SOFA), Host Nation Funded ConstructionAgreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, theSOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available tousers as part of the Services responsibility for providing technical criteria for militaryconstruction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval FacilitiesEngineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) areresponsible for administration of the UFC system. Defense agencies should contact thepreparing service for document interpretation and improvements. Technical content of UFC isthe responsibility of the cognizant DoD working group. Recommended changes with supportingrationale should be sent to the respective service proponent office by the following electronicform: Criteria Change Request (CCR). The form is also accessible from the Internet sites listedbelow.

UFC are effective upon issuance and are distributed only in electronic media from the followingsource:

Whole Building Design Guide web site http://dod.wbdg.org/.

Hard copies of UFC printed from electronic media should be checked against the currentelectronic version prior to use to ensure that they are current.

AUTHORIZED BY:

______________________________________DONALD L. BASHAM, P.E.Chief, Engineering and ConstructionU.S. Army Corps of Engineers

______________________________________DR. JAMES W WRIGHT, P.E.Chief EngineerNaval Facilities Engineering Command

______________________________________KATHLEEN I. FERGUSON, P.E.The Deputy Civil Engineer

DCS/Installations & LogisticsDepartment of the Air Force

______________________________________Dr. GET W. MOY, P.E.Director, Installations Requirements and

ManagementOffice of the Deputy Under Secretary of Defense

(Installations and Environment)
http://www.wbdg.org/pdfs/ufc_implementation.pdfhttps://65.204.17.188/projnet/cms/version2/index.cfm?WORKFLOW=CMS_CCRQAdd&Action=IDFORM&SecureTry=1http://dod.wbdg.org/http://dod.wbdg.org/https://65.204.17.188/projnet/cms/version2/index.cfm?WORKFLOW=CMS_CCRQAdd&Action=IDFORM&SecureTry=1http://www.wbdg.org/pdfs/ufc_implementation.pdf


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TECHNICAL MANUAL

WATER DESALINATION

H E A D Q U A R T E R S , D E P A R T M E N T O F T H E A R M Y

SEPTEMBER 1986


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REPRODUCTION AUTHORIZATION/RESTRICTIONS

This manual has been prepared by or for the Government and, except to the extent indicatedbelow, is public property and not subject to copyright.

Copyrighted material included in the manual has been used with the knowledge and permissionof the proprietors and is acknowledged as such at point of use. Anyone wishing to make further

use of any copyrighted material, by itself and apart from this text, should seek necessarypermission directly from the proprietors.

Reprints or republications of this manual should include a credit substantially as follows:"Department of the Army, USA, Technical Manual TM 5-813-8, Water Desalination.

If the reprint or republication includes copyrighted material, the credit should also state:"Anyone wishing to make further use of copyrighted material, byitself and apart from this text,should seek necessary permission directly from the proprietors."


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TECHNICAL MANUAL HEADQUARTERSNo. 5--813-8 DEPARTMENT OF THE ARMY

WASHINGTON, D.C. 15 September 1986

WATER DESALINATION

Paragraph PageCHAPTER 1. GENERAL

Purpose............................................................................................ 1-1 1-1General design considerations .......................................................... 1-2 1-1Special design considerations........................................................... 1-3 1-1Economic considerations ................................................................. 1-4 1-1Appendices ...................................................................................... 1-5 1-1

CHAPTER 2. SITE SELECTIONSite technical requirements .............................................................. 2-1 2-1Water storage and system modularization ........................................ 2-2 2-1

CHAPTER 3. WATER SOURCE SELECTIONGeneral............................................................................................. 3-1 3-1Quality .............................................................................................. 3-2 3-1Selection versus rejection of potential raw water sources.................. 3-3 3-1

CHAPTER 4. PROCESS SELECTION

General process selection................................................................. 4-1 4-1Process limitations............................................................................ 4-2 4-2Distil lation/condensation energy........................................................ 4-3 4-3Membrane energy............................................................................. 4-4 4-5Waste disposal ................................................................................. 4-5 4-5Preliminary process selection............................................................ 4-6 4-5Process selection.............................................................................. 4-7 4-5

CHAPTER 5. PRETREATMENT CONSIDERATIONSGeneral............................................................................................. 5-1 5-1Chemical treatment........................................................................... 5-2 5-1Degasification and deaeration........................................................... 5-3 5-5Filtration with granular media............................................................ 5-4 5-5Secondary filtration .......................................................................... 5-5 5-5

Disinfection....................................................................................... 5-6 5-6CHAPTER 6 DISTILLATION/CONDENSATION TECHNIQUES

General............................................................................................. 6-1 6-1High-temperature distil lation ............................................................. 6-2 6-1Low-temperature distil lation .............................................................. 6-3 6-1Mechanical distil lation....................................................................... 6-4 6-1Thermal discharge ............................................................................ 6-5 6-7Design analysis................................................................................. 6-6 6-7Materials of construction ................................................................... 6-7 6-7Distil lation/condensation system design ............................................ 6-8 6-7

CHAPTER 7. MEMBRANE TECHNIQUESElectrodialysis................................................................................... 7-1 7-1Electrodialysis-reversal design.......................................................... 7-2 7-1

Reverse osmosis .............................................................................. 7-3 7-3Reverse osmosis membrane staging configurations.......................... 7-4 7-7Reverse osmosis system design ....................................................... 7-5 7-11Materials of construction ................................................................... 7-6 7-13

CHAPTER 8. ION EXCHANGEGeneral............................................................................................. 8-1 8-1Pretreatment..................................................................................... 8-2 8-1Desalination...................................................................................... 8-3 8-2Demineralization............................................................................... 8-4 8-5

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CHAPTER 9. POST-TREATMENTGeneral............................................................................................. 9-1 9-1Remineralization and aeration .......................................................... 9-2 9-1Post-treatment of distilled water ....................................................... 9-3 9-1Post-treatment of reverse osmosis product water ............................. 9-4 9-1Post-treatment of electrodialysis-reversal product water.................... 9-5 9-3

CHAPTER 10. WASTE DISPOSALGeneral............................................................................................. 10-1 10-1Pretreatment wastes ........................................................................ 10-2 10-2Distillation/condensation system wastes ........................................... 10-3 10-3Membrame system wastes ............................................................... 10-4 10-5Ion-exchange system wastes ........................................................... 10-5 10-6Ultimate disposal ............................................................................. 10-6 10-6

APPENDIX A. SAMPLE PROBLEMS ...................................................................... A-1APPENDIX B. WATER TESTING REQUIRED FOR DESALINATION TREATMENT B-1APPENDIX C. REFERENCES ................................................................................. C-1BIBLIOGRAPHY ......................................................................................................... Biblio-1

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LIST OF FIGURES

FIGURE 2-1 Typical Desalination Flowsheet2-2 Typical Reverse Osmosis Desalination System4-1 Energy Consumption5-1 Pretreatment Techniques5-2 Approximate Concentration Factor Limits for Calcium Sulfate in Sea Water5-3 Disinfection Alternatives6-1 Three Methods of Vaporization6-2 Multiple-Effect Vertical-Tube Evaporation Process6-3 Multistage Flash Distillation Facility6-4 Waste Heat Recovery Evaporation Process6-5 Vapor-Compression Vertical-Tube Distil lation System7-1 Principles of Electrodialysis Desalination7-2 Reverse Osmosis Principles7-3 Reverse Osmosis Desalination System7-4 Flow-Work Exchanger Principles7-5 Construction of a Tubular Reverse Osmosis Membrane7-6 Internal Construction of a Spiral-Wound Membrane7-7 Internal Construction of a Hollow Fine-Fiber Reverse Osmosis Membrane Module7-8 Reverse Osmosis Product Staging

7-9 Reverse Osmosis Membrane Reject Staging8-1 Principles of Ion-Exchange Processes8-2 RDI Process8-3 Three-Unit Variation Desal Process8-4 Two-Unit Variation Desal Process8-5 Demineralization Two-Bed Flowsheet9-1 Break-Point Chlorination Curve

10-1 Waste Disposal Overview10-2 Pretreatment Sludge Disposal10-3 Deep-Well Disposal Unit10-4 Brine Waste DisposalA-1 Sample Use of Table 4-1A-2 Sample Use of Table 4-2

A-3 Water Analysis (Sample Problem A-1a)A-4 Sample Use of Table 4-3A-5 Water Analysis (Sample Problem A-1b)A-6 Water Analysis (Sample Problem A-1c)A-7 Low-Pressure Reverse Osmosis SystemA-8 Sample Use of Table 4-1A-9 Water Analysis (Sample Problem A-2)

A-10 Sample Use of Table 4-2A-11 Sample Use of Table 4-3A-12 Plan View of Vapor Compression SystemA-13 Water Analysis (Sample Problem A-4)A-14 Amberlite IR-120 Plus Leakage Data Regeneration-5 lbs. H2S04 (66 Be)/cu. ft.A-15 Amberlite IR-120 Plus Iso-Capacity Data Regeneration-5 lbs. H2S04

(66 Be)/cu. ft.A-16 Amberlite IR-120 Plus Capacity Correction for AlkalinityA-17 Amberlite IR-120 Plus Leakage Data Regeneration-3 lbs. H2S04

(66 Be)/cu. ft.A-18 Amberlite IR-120 Plus Iso-Capacity Data Regeneration-3 lbs. H2S04

(66 Be)/cu. ft.A-19 Amberlite IR-120 Plus Capacity Correction for AlkalinityA-20 Cation Exchangers PlanA-21 Degasifier and Clearwell Plan

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FIGURE A-22 Three Projections of the Water Blends That Satisfy TB MED 576 RequirementsA-23 Plan View of a Two-Pass Reverse Osmosis System

LIST OF TABLESTable 4-1 Preliminary Desalination Process Selection

4-2 Selecting Desalination Processes after Water Quality Data are Obtained4-3 Final Selection of a Desalination Technique from Treatability Data5-1 The Maximum Allowable Contaminants in the Feed Water Used

by Various Desalination ProcessesA-1 Water Constituents at Treatment Stages with 5 Pounds/Cubic Foot Acid

RegenerationA-2 Water Constituents at Treatment Stages with 3 Pounds/Cubic Foot Sulfuric Acid

RegenerationA-3 Cost Projections for Possible Optimum Blends

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CHAPTER 1

GENERAL

1-1. Purpose. This manual describes the guidelinesto be followed in selecting a process capable of

producing potable water supplies from brackish and seawater sources. Potable water is defined in TB MED 576.Army potable water quality requirements can be found inTB MED 576. Information on required pre- and post-treatment design, water distribution, and storage can befound in the TM 5-813 series.

1-2. General design considerations. Alldesalination facilities will be designed as simply aspossible and will produce a final product water inkeeping with operational needs. Desalination processselection will be site-specific and will include evaluationsof product quality and quantity, raw feed water sources,and available energy sources. Potable water willcontain less than 500 milligrams per liter of totaldissolved solids (TDS). (One milligram per liter is equalto one part per million.) Technological processes notcovered in this TM that are proposed for selection mustbe approved by DAENECE-G.

1-3. Special design considerations. Whenfacilities require improved water quality, such as boilerfeed water or medical facility waters, additional watertreatment systems will be installed.

1-4. Economic considerations. The selection oone particular type of design for a water desalination

system/subsystem, when two or more types of designare known to be feasible, must be based on the resultsof an economic study, in accordance with therequirements of DOD 4270.1-M and PL 95-619, 9 Nov78. The results of these economic studies are to beincluded in the design/analysis documentation for theproject. All such studies are to be conducted inaccordance with HQDA's economic-studies criteria foMilitary Construction Program (MCP) designsClarification of HQDA criteria for a particular applicationor supplementary criteria and guidelines that may berequired for special cases may be obtained by requestthrough normal channels to HQDA (DAENECE-G)Wash. DC 20314-1000.

1-5. Appendices. Appendix A contains sampleproblems illustrating use of information presented in thisdocument. Various tests and analyses required or usedin desalination treatment of water are listed in AppendixB. Appendix C contains a list of references used in thisdocument.

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CHAPTER 2

SITE SELECTION

2-1. Site technical requirements. Site technical

requirements are specific to each particular process.Generalized recommendations can be made regardinglocation, space, and access requirements. A typicaldesalination system flowsheet is shown in figure 2-1. Atypical desalination system layout, using reverseosmosis as a sample process, is shown in figure 2-2.

a. Location. Desalination facilities will be locatedas close to the raw water source as possible in order toavoid excessive pipeline or pumping costs and tominimize operation and maintenance costs for pumpingraw water (high saline content). The placement ofdesalination facilities within the perimeter of a militaryreservation will provide facility security. Topographyshould be considered in the siting of a desalinationfacility, and gravity flow should be used where possible.

b. Space requirements. The space required fordesalination facilities is determined by the process.Membrane desalination equipment needs less spacethan distillation/condensation desalination equipment.In general, space requirements are less for thedesalination equipment than for a conventional surfacewater treatment plant of the same capacity. Anexception is solar desalination systems. These systemsemploy solar collectors that require an area severaltimes greater than other types of desalination equipmentin order to achieve equal capacity.

c. Access. Access to systems must be provided to

permit routine maintenance, sludge and brine removal,and delivery of desalination equipment and supplies.The access requirements for desalination facilities aresimilar to those for conventional water treatmentfacilities.2-2. Water storage and system modularization.

a. Equipment downtime. In all distillation

condensation and many membrane desalination plantsstorage will be determined by equipment downtimewhen equipment downtime is more than 1 day. Todetermine the necessary storage, establish the longesperiod of time that could be required for planned orunplanned maintenance. Calculate the storage bymultiplying this time period by the water demand rate.

b. Peak daily demands. When maximumequipment downtime is less than 1 day, the peak dailydemands may set a larger storage demand. Whenthese peak demands set the storage requirements, refeto water storage in the TM 5-813 series.

c. Fire water storage. On a facility served by adesalination system, fire water may be saline water opotable water depending on economic analysis. Duawater distribution system will be required if saline wateris used. Hence, part of the fire protection water can beeither saline or potable water due to piping and pumpingcost. Economic evaluation of various designalternatives is usually needed to assure the optimadesign to be adopted.

d. System redundancy and modularization. Onecomplete and functional desalination module in excessof that required to supply the design flow will beprovided as redundant capacity, and all desalinationsystems will have a minimum of three independentlyfunctioning desalination modules where practicable.

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Figure 2-1. Typical desalination flowsheet.

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Figure 2-2. Typical reverse osmosis desalination system.

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CHAPTER 3

WATER SOURCE SELECTION

3-1. General. The selection of a water supply will bebased on available quantity, quality, and cost of

development. Investigate usable fresh surface waterand groundwater thoroughly, prior to consideration ofsources requiring desalination. When fresh watersources do not exist, consider saline water sources. Themost commonly used parameter to differentiate betweensaline water qualities is total dissolved solids (TDS).The total dissolved solids is defined as the sum of thedissolved organic materials and the inorganic salts.Fresh waters contain less than 1,000 milligrams per literof total dissolved solids. Brackish water contain 1,000-20,000 milligrams per liter of total dissolved solids. Seawater usually contains at least 20,000 milligrams perliter of total dissolved solids. Quantities of potable waterneeded will be determined by an analysis of the site inaccordance with TM 5-813-1. The quantity of salinefeed water necessary to produce the required quantity ofproduct water is addressed in Chapter 4.

3-2. Quality. The quality will be determined by theplanned use. Physical, chemical, and bacteriologicaltesting of source waters are required to determine thelevel of treatment to supply the necessary water quality.When the quantity withdrawn exceeds the recharge rate,quality inherently decreases; therefore, this must beconsidered during design.

a. Physical characteristics. The physicalcharacteristics of the raw water source that must beevaluated are total suspended solids (TSS) andtemperature. Turbidity and silt density index (SDI).

(1) Total suspended solids. The totalsuspended solids level of raw water sources must beevaluated to determine the level of pretreatmentprocesses required. Raw water having low totalsuspended solids levels generally requires lesspretreatment. The source with the lowest totalsuspended solids is preferred.

(2) Temperature. The temperature of the rawwater source must be matched to the specificdesalination process. In extreme cases, the watertemperature may control the desalination process

selection. A climatological survey must be made priorto finalization of process selection to determine theseasonal maximum and minimum water temperatures ofthe proposed water sources.

(3) Turbidity and silt density index. These twocharacteristics provide two different measures of the

amount of fine particulate matter in the water. Turbidityis measured in nephelometric turbidity units (a measureof the amount of light scattered by a known watesample thickness). Silt density index is a measure othe amount of 0.45-micron filter plugging caused bypassing a sample of water through the filter for 15minutes. Turbidity must be determined for aldesalination processes. Also, the silt density index musbe determined for water being considered for reverseosmosis treatment.

b. Chemical constituents. The chemicaconstituents of the raw water must be determined toprovide information for treatment selection. Appendix Bshows the water testing analyses required fodesalination treatment.

c. Bacteriological quality. The bacteriologicatesting of the raw water must include a type of acoliform indicator organism count. Procedures for filtemembrane, most probable number fermentation tubeand standard plate count, coliform organismbacteriological testing techniques can be found inStandard Methods for the Examination of Water andWastewater and TB Med 576. Manufacturersrecommendations as to the media and procedures usedto identify microbiological activity detrimental to theoperation of a particular desalination system shall befollowed.

3-3. Selection versus rejection of potential rawwater sources. After the completion of physicalchemical, and bacteriological testing, a final watesource may be selected. Extreme care must be taken inthe selection of a source where the usage rate is greatethan the recharge rate. In most cases, selection wilinvolve choosing the brackish water with the lowest leveof total dissolved solids. When brackish water is noavailable, use sea water or water as saline as sea wateras the feed water source. When the coliform indicatoorganism count of a water is greater than 10,000 mostprobable number (MPN), then the water source shouldbe rejected for sanitary reasons and a more saline water

chosen (per Standard Methods for the Examination ofWater and Wastewater and TB MED 576). If othewater is available, a water containing more than 1,000nephelometric turbidity units should be rejected on thebasis of the high cost and difficulty o

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clarification, even if the alternative water is more saline.When the total delivery pumping pressure of a lesssaline water is greater than the operating pressure of areverse osmosis system, then the desalination of themore saline water by reverse osmosis may be moreeconomical than the combined cost of delivery and

desalination of the less saline source. The finaselection of a raw water source will be based oneconomic studies. In some cases, the decision cannobe made until all systems are fully designed and lifecycle costed.

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CHAPTER 4

PROCESS SELECTION

4-1. General process selection. In selecting apotable water production system, it is important to

estimate costs of various options. The conventional unitof comparison is cost in dollars per 1,000 gallons ofproduct water. Water quality and energy sources will beestimated from simple site reconnaissance. Forexample, a sea coast site where the water sourcetemperature exceeds 95 degrees Fahrenheit indicates ahigh-salinity high-temperature combination favoringdistillation/condensation processes. Reverse osmosisrequires a feed water temperature below 95 degreesFahrenheit. If local well testing indicates salinitybetween 500 and 3,000 milligrams per liter andelectricity is inexpensive, electrodialysis reversal orhighflux reverse osmosis is indicated.

a. Desalination requirements. The design of adesalination system requires a clear understanding ofthe following: the quantity of product water desired; thequality of the desired product; and the quality of the feedwater source. This manual addresses the production ofpotable water containing less than 500 milligrams perliter of total dissolved solids. Laundries, boilers, messhalls, and hospitals may require water purer than 500milligrams per liter of total dissolved solids. Potablewater from the desalination system may be furthertreated to meet these requirements in accordance withAR 420-46.

b. Saline feed water quantity. The production ofpotable water from saline water usually requires a

significantly larger quantity of saline feed water than thequantity of potable water produced. When desalinationis necessary to produce potable water, the process splitsthe feed water into two streams. One stream is theproduct water; the other stream is the brine that containsmost of the salts originally in the feed water. In watersthat need very little desalination, high-rate reverseosmosis may only reject 5 percent of the feed stream asbrine. In reverse osmosis of sea water, more than 70percent of the intake water may be rejected as brine.Multiply the required product quantity by the reciprocalof the product water recovery fraction to find thequantity of saline water that must be processed to yield

the desired quantity of product water. In equation, form,it can be expressed as follows:

100%% recovery of product water

x water demand = saline feed water flow

In some cases, the limited quantity of an availablesaline water may require a decision to adopt a moreexpensive desalination process with a higher waterecovery rate. However, it may require choosing adifferent and more saline feed water with a greateavailability.

c. Blending of waters. Blending a highconcentration stream with a low concentration streamwastes the osmotic pressure energy between the twostreams. Therefore, it is best to match the design of thedesalination system to the product quality desiredWhen a desalination process cannot be economicallymatched to the desired product quality, then a processthat yields water with a very low dissolved materiacontent must be used. To conserve capital andequipment costs and meet the desired water demandthe high purity product water can be blended with thepretreated saline feed water to produce the requiredproduct quantity and quality. The following equation canbe used to calculate the concentration of a blendedwater stream:

( concentration i x flow i

)/total flow =

blended concentrationWhen only two streams are blended, the equation canbe rearranged to show the flow of concentrated watethat when blended with a dilute flow will result in thedesired product concentration. This rearrangedequation is as follows:

(P - H)(D) =FC-P

Where:P = Desired product water

concentrationH = High purity water concentrationD = Flow of the high purity waterC = Concentration in the impure

concentrated streamF = Flow rate of the concentrated

streamReblending for remineralization is discussed in Chapte9. The same blend equations will apply to blending

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for remineralization, which is a more commonprocedure. See Appendix A for sample problems.4-2. Process limitations. The various desalinationprocesses presently available have limitations that mustbe considered prior to selecting a desalination processfor a particular site. These limitations apply only to thedesalination processes themselves; pretreatment can beand is often used to bring a saline feed water withinlimits so that a desalination process can be used. Theraw feed water chemistry for all desalination systemsmust be evaluated thoroughly for constituents that mayprecipitate in the desalination system.

a. High-temperature distillation. High-temperaturedistillation is limited by the saturation of alkaline earthmetal salts, such as CaSO4, BaSO4, SrSO4, CaCO3,BaCO3, and SrCO3. Carbonate salt scaling can becontrolled by acid addition. The recovery of water froma hightemperature distillation plant is usually limited bycalcium sulfate solubility. When the concentration ofthe sulfate and the limiting alkaline earth metal is one-third of the saturated condition at ambient temperature,

distillation design must include pretreatment to reduceor inhibit the scaling ions. High-temperature distillationis also limited to oil and grease levels below 1 milligramper liter. All other limitations on the high-temperaturedistillation process are equipment specific and requireindividual evaluation.

b. Low-temperature and mechanical distillation.Low-temperature and mechanical distillation systemsare limited to operation below saturation of alkalineearth sulfates and carbonates. The lower operatingtemperature permits economical operation on watersthat are at or below half saturation at ambienttemperature. Oil and grease are limited to less than 1

milligram per liter. Any other limitations are equipmentspecific.

c. Reverse osmosis. The most severe limitationon reverse osmosis is the maximum limit of 50,000milligrams per liter of total dissolved solids in the feedwater. Another limitation is that there must be no iron inthe feed water. This limitation is so rigid that onlystainless steel and nonferric materials will be useddownstream of the iron removal. The solubility ofalkaline earth sulfates and carbonates limits reverseosmosis treatment. Any water containing less than4,000 milligrams per liter of total dissolved solids thatwould be saturated with an alkaline earth sulfate when

the concentration is multiplied by 1.5 should not beconsidered for reverse osmosis desalination. Reverseosmosis is limited to waters that do not have silicasaturation in the reject brine. Silica chemistry is

extremely complex. When the molybdenum reactivesilica concentration exceeds 30 milligrams per liter asSiO2 or the pH exceeds 8.3 in the brine stream, anenvironmental chemist or engineer should be consultedReverse osmosis is also limited to the treatment owaters with less than 1 milligram per liter of oil andgrease.

(1) Cellulose acetate membranes. Celluloseacetate membranes are usually limited to pH levelsbetween 4.0 and 7.5. Cellulose acetate membranesrequire some form of continuous disinfection with thefeed water to prevent microbial degradation of themembranes and can tolerate up to 1 milligram per literof free chlorine. Therefore, cellulose acetatemembranes are usually disinfected by maintaining 0.2 to0.9 milligrams per liter of free chlorine in the feed waterCellulose acetate membranes cannot be used on waterswhere the temperature exceeds 88 degrees FahrenheitCellulose acetate membranes should not be used apressures greater than the manufacturer'srecommended pressure, since they are prone to

membrane degradation by pressure compaction.(2) Polyaromatic amide membranes

Brackish water polyaromatic amide membranes aregenerally limited to operation in feed waters between pH4 and pH 11. Polyaromatic amide membranes are lesspH tolerant and should not be used outside of the rangepH 5 to pH 9. All polyaromatic amide membranes arelimited to use on feed streams that are free of residuachlorine. If chlorination is necessary or desirable as apretreatment option, complete dechlorination must beeffected. Polyaromatic amide membranes are toleranof water temperatures up to 95 degrees FahrenheitWhile polyaromatic amide membranes are not as

quickly or completely compacted as are celluloseacetate membranes, manufacturer's recommendedpressures must be followed to prevent mechanicadamage to membrane modules.

d. Electrodialysis reversal. While electrodialysisreversal has been used to treat water as saline as seawater, 4,000 milligrams per liter of total dissolved solidsis considered to be an upper limit for economicaoperation. Some electrodialysis membranes cantolerate strong oxidants, like chlorine, but most cannotThe reversal of polarity used in electrodialysis reversafor removal of scale allows operation on water that issaturated with alkaline earth carbonates. Saturation

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with an alkaline sulfate with low carbonate alkalinityshould be avoided.4-3. Distillation/condensation energy. Indistillation/condensation plants, energy is used in theform of steam and elec

tricity. Steam is used to heat the saline water toincrease its vapor pressure. Normally, electricity is usedto run the compressor in vaporcompression distillationIf excess steam is available, its use as a power sourceshould be

Figure 4-1. Energy consumption.

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Appendix B

TM 5-81

Table 4-1. Preliminary desalination process selection.

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investigated. The amount of electricity or mechanicalwork that steam will yield depends on its temperature aswell as the temperature to which it can be condensed.The energy consumption of both vapor compression andthermal distillation, as related to the total dissolvedsolids of feed water, is shown in figure 4-1.

4-4. Membrane energy. Historically, membranedesalination systems use less energy than othersystems. Brackish water disalination should beaccomplished by membrane separation processesbecause of the reduced energy requirement. Theenergy consumption of electrodialysis reversal can bemade to follow reduced or variable salinity, while theenergy consumption of reverse osmosis is set principallyby membrane water flux. Again, the energyconsumption of electrodialysis reversal and reverseosmosis as a function of the total dissolved solidscontent of the feed water is shown in figure 4-1. Asmembrane materials are developed, energyconsumption may be reduced.

4-5. Waste disposal. Waste disposal may influenceprocess selection. Since brine disposal costs can be animportant part of process economics, brine disposalalternatives must be explored while water qualityanalyses are being performed. For further information,refer to Chapter 10 on waste disposal.

4-6. Preliminary process selection. Usepreliminary site information to eliminate certaindesalination processes. A decision logic table for usewith preliminary information is shown in table 4-1Decisions based upon table 4-1 are to be consideredpreliminary only. Necessary water quality tests tofurther support the recommedations made in Column Eof table 4-1 are in Column F.

4-7. Process selection. When initial site and rawwater source selections have been made, usepreliminary water quality information with table 4-1 toassist in a preliminary process selection. As morespecific information is obtained from laboratory analysesof water quality, make an initial process selection usingthe second decision logic table, table 4-2. After atreatability investigation has been completed, select thefinal desalination process. A final decision logic tabletable 4-3, assists in the final process selection. The useof the decision logic table sequence will only providegeneralized assistance in process selection; additiona

economic, engineering, and environmental studies mayindicate that methods or combinations of methods mustbe used.

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Table 4-2. Selecting desalination processes after water quality data are obtained.

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Table 4-3. Final selection of a desalination technique from treatability data.

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CHAPTER 5

PRETREATMENT CONSIDERATIONS

5-1. General. Before raw water is desalinated, theundesirable materials will be removed or reduced to

acceptable levels. Such materials to be removedinclude solids, immiscible liquids, and sparingly solublesalts. Accumulations of these materials on desalinationprocess surfaces greatly reduce efficiencies. Withoutadequate pretreatment, desalination facilities aredestined for reduced lifetimes, shortened periods ofoperation, and high maintenance. The feedwaterlimitations for various desalination processes aresummarized in table 5-1. An overview of typicalpretreatment options is shown in figure 5-1.

a. Solids . Physical and chemical treatmentprocesses will be used to remove solid materials. Thelevel of solids removal will depend on the desalinationprocess to be used.

(1) Settleable solids. Settleable solids areeasily removed from water by gravity settling orfiltration.

(2) Non-settleable solids. Non-settleablesolids are removed by chemical treatment. Chemicaltreatment may be designed to remove not only non-settleable solids but to precipitate and remove scale-forming solids.

b. Immiscible liquids. Emulsified oil andgrease are the principle sources of immiscible liquidfouling in desalination facilities. Use gravity settling andchemical treatment processes followed by a filtration

system to maintain immiscible liquid concentrationswithin acceptable levels. To measure low levels of oiland grease, a sophisticated concentration proceduremust be performed.

c. Dissolved organic contaminants. The mostcommon organic materials, tannins and humic acids,can be removed from water by chemcial treatment.Some organic materials will carry across adistillation/condensation process with the water.Pesticides and industrial organic chemicals may bedifficult to remove by distillation/condensation. Reverseosmosis will usually remove the majority of the largepesticide molecules. If during pretreatment tests a non-

ionized organic contaminate is not reduced toacceptable levels, then a full treatability investigationwill be performed to ensure that potable water standardsare met.

d. Sparingly soluble salts. As physical andchemical conditions change during desalination, somesoluble materials become insoluble (precipitate). Thisprecipitation of solid materials can foul desalination

systems through the formation of scale. Materials thaprecipitate easily to form scale are double-charged

positive ions, such as calcium and magnesium salts. Aswater volume is reduced, the concentration of all ionsand materials is increased. When the concentrationreaches saturation, scale is formed. With alkaline earthsulfates and carbonates, the saturation concentration isreduced by increasing temperature. Ion exchange (seeChap. 8) as well as chemical treatment followed byfiltration can reduce the levels of sparingly soluble saltsbefore desalination processes. However, such atreatment system may not be cost effective, economicevaluation is needed before proceeding with detaileddesign. The concentration factor limits for calciumsulfates in sea water are shown in figure 5-2.5-2. Chemical treatment. Many materials, e.g.calcium salts, magnesium salts, oil and grease, totasuspended solids and some organic contaminates, thaare harmful to desalination processes can be reduced toacceptable levels by chemical treatment. Chemicatreatment may involve any of the following: coagulationeither with or without clarification; pH adjustment, whichmay influence fluoride removal; or scale inhibitionWhen required, degasification and secondary filtrationfollow chemical treatment.

a. Coagulation. Non-settleable solids and somesuspended materials do not precipitate because ofelectrical charges on the surface of the particles. If thecharges on the particles can be reduced, the particles

may precipitate. Chemicals that lower surface chargesare lime, alum, ferric salts, and polyelectrolytesEvaluate each chemical used in pretreatment todetermine its effect on the successive steps in thedesalination process. For example, calcium present inlime can cause an increase in scale formation indistillation/condensation processes; the iron present inferric salts can cause excessive fouling in membranedesalination systems; alum is usually the best coagulanfor desalination systems. Do not use pretreatmenchemicals without prior operational experience and anexhaustive actual application review. The nonsettleable solids and the suspended solids in conjunction

with any added coagulants will be removed by eithestandard clarification techniques or direct filtrationLime softening

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NOTES *Based upon manufacturer's guarantee

Table 5-1. The maximum allowable contaminants in the feed water used by various desalination processes

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without adequate downstream filtration and pHadjustment will lead to suspended lime particles in thefeed water. Lime softening will usually remove asignificant amount of fluoride and may adjust the pH tothe higher levels, above 7.0, where fluoride removal isoptimized. Laboratory analysis or pilot studies shall be

conducted to assure the optimal coagulation processdesign.

b. pH adjustment. The pH adjustment step ofpretreatment must result in the optimal pH level for theindividual desalination system. After coagulants have

Figure 5-1. Pretreatment techniques.

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been added, the pH is often changed significantly. Inmost cases, the pH must be returned to a neutral or aslightly acid level. Adjustment chemicals to lower thepH include carbon dioxide, sulfuric acid, andhydrochloric acid. Carbon dioxide should not be usedfor pH adjustment of lime addition systems because ofthe previously mentioned scaling problem associated

with lime pretreatment. Sulfuric acid should not be usedfor pH adjustment of systems where the resultingincrease in sulfate salts can cause scaling in thedesalination system. Fluoride removal will be hinderedif the pH is reduced below 7.0.

c. Scale inhibition. The formation of scale

Source. American Institute of Chemical Engineers

Figure 5-2. Approximate concentration factor limits for calcium sulfate in sea water.

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limits the efficiency of product water recovery duringplant operation. The precipitation of calcium carbonate,magnesium hydroxide, and calcium sulfate can beprevented through the addition of scale-inhibitingchemicals. Two scale-inhibiting chemical types arecommonly used: polyphosphates and polyelectrolytes.The most commonly used scale inhibitors arepolyphosphate chemicals. Polyphosphates lose theireffectiveness above 190 degrees Fahrenheit, whichlimits upper operating temperatures. To prevent theformation of scale within desalination systems,polyphosphate-based chemicals are the best generalpurpose additive. The most commonly usedpolyphosphate compound is sodiumhexametaphosphate. Polyelectrolytes or organicpolymers are relatively new scale inhibitors. The use ofpolyelectrolytes in potable water shall be in accordancewith paragraph 7 of AR 420-46.5-3. Degasification and deaeration. The removalof dissolved gases from a feed water can beaccomplished in one of two ways. The dissolved gas

may be stripped out with another gas, or the dissolvedgas may be stripped out with water vapor. A forceddraft degasification tower will remove nonatmosphericgases. This is done by blowing air up through a tower ofpacking. The water to be degasified is then sprinkledover the packing. When full deaeration is required toprevent the interference of oxygen and nitrogen withwater vaporizaton or flashing, steam or water vapormust be used as the stripping gas. On high-temperaturedistillation systems, a steam deaerator may be used.On most distillation desalination systems, the onlyeconomical method of deaeration the water is to lowerthe pressure with a vacuum pump or venturi eductor or

ejector to produce enough water vapor to strip away alldissolved gases. There are two basic designs for waterdistribution in deaerators. A design that sprays thewater in the deaerator vessel will usually create themost complete deaeration. The other commonly useddesign involves cascading the water over trays. Whilethe internal design of this deaerator is prone to shortcircuiting and incomplete deaeration, it is more compactand requires no pressure to operate the sprayers.

5-4 Filtration with granular media. Before thefiltration is designed, the possibility of use ofwells/infiltration galleries shall be evaluated. Filtration is

necessary when the desalination process requires a lowlevel of particulate material. Primary filtration withgranular media removes particulate materials left afterlime, coagulates, or other chemicals have been addedto the feed waters. Single-or dual-media filtration

systems are generally used. In some cases, mixedmedia filtration may be necessary. Filters may useeither pressure or gravity f low methods.

a. Single-media filtration. Single-media filtrationconsists of one media. This media is often smallgrained silica sand; however, anthracite may be usedafter lime and lime-soda softening. Some desalinationpretreatment systems use an alternate media such asgreensand to remove iron compounds. Diatomaceousearth media is not recommended for primary filtrationbecause of its characteristic high head loss and shortfun times.

b. Dual-media filtration. Dual-media filtrationconsists of two media with different specific gravitiesThe difference creates a two-layer separation effectUse silica sand or greensand for one layer; useanthracite for the other layer. The use of dual media wilallow larger quantities of material to be filtered and wilreduce head loss during operation. The use of twomedia types will provide a good coarse-of-fine filtrationprocess for desalination facilities.

c. Mixed-media filtration. When three media areused in filters, a better coarse-to-fine filtration patterncan be constructed. High-density silica sand, garnetand anthracite are commonly used to provide the filterbed. The different media do not stratify completelyInstead, there is a small amount of intermixing amongthe different layers. This gradual change in media sizeprovides a gradient from coarse to fine and creates amedia flow pattern necessary to achieve a very low siltdensity index.

5-5. Secondary filtration. Secondary filtration isnecessary to ensure that particulate material does no

pass from the pretreatment systems into a membranedesalination process. Secondary filtration is usednormally as a precautionary measure. Cartridge filtersare commonly used. Ultrafiltration systems can be usedas a process and precautionary measure.

a. Cartridge units. The necessary protection ismost easily accomplished with cartridge filters. Thecartridges are replaced easily, and the filter mesh canbe sized to remove from 20-micron to 0.05-micronparticles. Although some cartridge filters can bebackwashed, most are replaced when head loss reachesexcessive levels.

b. Ultrafiltration units. The newer process fo

secondary filtration is ultrafiltration. New

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Figure 5-3. Disinfection alternatives.

materials provide 0.005-micron filtration in a stable,backwash operational unit. Some ultrafiltration unitspossess a salt-rejection capability, which reduces boththe ultrafine particulate material and salt loading onmembrane desalination processes. The ability to ensureparticulate removal at 0.005-micron levels, as well as

backwash capabilities, makes ultrafiltration an excellentsecondary filtration technique.

5-6. Disinfection. Biological growth can seriouslyinterfere with desalination surfaces and equipmentCellulose acetate reverse osmosis membranes can beattacked by bacteria and require continuous disinfectionapplication at the membrane surface. Disinfection opotable waters is required by TB MED 576 for sanitary

reasons. Marine intake structures are particularly prone

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to biogrowths. A strong biocide is usually needed toprevent extensive marine growths from fouling intakes,piping and storage facilities. An overview of disinfectionalternatives can be found in figure 5-3. Chlorination isnot advisable in the pretreatment process when organiccontents are high.

a. Ultraviolet irradiation. One effective disinfectiontechnique is ultraviolet irradiation.Raw wafers flow through a chamber where they areexposed to ultraviolet lamps. The resulting disinfectionprevents bacterial destruction of cellulose acetate-basedmembrane systems and eliminates biofouling ofpolyaromatic-based membrane systems. Becausepolyaromatic fiber membranes are very susceptible tochemical oxidant attack, ultraviolet disinfection is thebest disinfection technique for these systems.

b. Oxidants. The addition of chemical oxidants,such as chlorine, bromine, iodine, or ozone, can providebiological disinfection before membrane processes. Theuse of oxidants must be monitored carefully to keep thechlorine below 1.0 milligrams per liter of free chlorine

residual that would even damage cellulose acetatemembrane systems. For cellulose acetate membranesystems, a carefully monitored oxidant addition is thebest technique for disinfection of biologically active feedwaters. When biological and other organic materials arechlorinated, the resulting chlorine oxidation generateshalogenated carbon compounds, such as thetrihalomethane class of compounds. While theplacement of chlorine disinfection upstream ofcoagulation of filtration will increase chlorine contact

time, which will increase the number of organismsdestroyed, it will also increase trihalomethane formationWhen an open intake requires optional disinfection asshown in figure 5-1 and chlorination is shown bypretreatment investigation to bring the trihalomethanecontent above 0.1 milligrams per liter, then no form ofchlorine shall be used as a raw water disinfectant. Thelegal limit (per National Interim Drinking WateRegulations) for trihalomethanes in potable wateeffluents for communities and installations with aneffective population of 10,000 or more is 0.1 milligramsper liter.

c. Dechlorination. When chlorine is used as adisinfectant, complete dechlorination of pretreatedwaters must occur before desalination in polyaromaticmembrane systems. Dechlorination can beaccomplished chemically through sulfite compoundaddition or passage through granular-activated carbonComplete dechlorination and destruction of the chlorineresidual by reducing compounds will ensure thachemicals do not attack these sensitive membrane

systems. When a chlorinated water source must beused and complete dechlorination below 1.0 milligramsper liter as free chlorine is not possible, usedistil lation/condensation. When dechlorination is onlyeconomically practical to between 0.2 and 1.0milligrams per liter as free chlorine, a cellulose acetate-based membrane system may be used.

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CHAPTER 6

DISTILLATION/CONDENSATION TECHNIQUES

6-1. General. Distillation/condensation is the mostcommon desalination process. More than 70 percent of

all desalination facilities in use today employ somevariation of the distillation/condensation process.6-2. High-temperature distillation. High-temperature distillation faciilties that operate attemperatures greater than 205 degrees Fahrenheit arethe most prevalent desalination facilities in the worldtoday. There are three methods of vaporizaiton:submerged tube vaporization; flash vaporization; andthin-film vaporization. These methods are illustrated infigure 6-1. Submerged tube vaporization is the leastefficient vaporization technique, but it allows for easymaintenance. This type of vaporization system is mostoften used in exhaust gas waste heat recoverydistillation systems. The flash vaporization technique ispresently the most common technique in existingdistillation units. The impact of sprayed hot brine withinthe evaporator unit causes both erosion and corrosion ofmost metals. Using a thin-film sprayvaporizationprocess, the raw water is introduced at slightly less thanatmospheric pressure through an orifice onto heatexchanger tubes for immediate vaporization. Thecorrosive environment is reduced from the flashvaporization system, but scaling can occur on the heattransfer surfaces. These vaporization techniques areused in the two major high-temperature distillationprocesses, multiple-effect (ME) evaporation, andmultistage flash (MSF) evaporation.

a. Multiple-effect evaporation units. To maximizethermal energy efficiency within adistillation/condensation system, several units or effectsare used. The heat from the condensation step of oneeffect is used to supply vaporization heat for thefollowing effect. The next effect is a slightly loweredpressure and temperature. This gradual reduction byheat transfer results in a much greater yield of productwater from a given quantity of thermal energy. A typicalmultiple-effect evaporation unit is shown in f igure 6-2.

b. Multistage flash-evaporation units. Distillationtechnology was advanced through the development ofmultistage flash evaporation units. Stages of flash

evaporation are operated using heat from an externalsource. Pressure is reduced gradually in eachsuccessive stage to continue flash operation atsuccessively lower temperatures and pressures.Because scaling is not a serious problem, this designhas become the most prevalent distillation process. Atypical multistage flash-evaporation unit is shown infigure 6-3. Although internal scaling is not a great

problem, corrosion of flash-evaporation units is ofconcern.

6-3. Low-temperature distillation. Distillationcondensation facilities that operate at temperatures lessthan 205 degrees Fahrenheit are low-temperature unitsIn situations where waste heat is plentiful, low-temperature waste-heat-recovery evaporation units areused. A waste-heat-recovery unit is shown in figure 6-4For onshore application, low-pressure waste steam frompower generation facilities can provide the necessarythermal energy for desalination systems. The mosrecent developments in distillation/condensationtechnology involve the use of waste heat or lowpressuresteam with evaporation units and a mechanical vapocompression system. Multiple stages then derive themaximum vapor and product water production from thesystem.

6-4. Mechanical distillation. The use omechanical methods for vapor production and heattransfer can result in a highly efficient desalinationsystem. These systems operate at temperatures lessthan atmospheric boiling point and use a variety omethods to vaporize raw waters. These mechanicaprocesses commonly use multiple effects to maximizethe efficiency of the applied mechanical energy.

a. Vapor compression. The technique of vapocompression uses a mechanical energy source, such asan engine of electric motor, to power a compressionturbine. This turbine draws vapor from the distillationvessel and compresses it, which raises the temperatureof the exhaust vapor. The vapor is then passed over aheat exchanging condenser, where it returns to theliquid state as product water. The heat removed duringcondensation is returned to the raw water to assist in theproduction of more vapor. The more recent vaporcompression multiple-effect units produce aconcentrated brine byproduct that has had its excessheat reduced by the multiple effects.

b. Waste heat. Adding waste heat to vapocompression systems results in a highly efficien

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Figure 6-1. Three methods of vaporization.

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Figure 6-2. Multiple-effect vertical-tube evaporation process.

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Figure 6-3. Multistage flash distillation facility.

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distillation/condensation process. These systems aredesigned to maximize the production of product waterwith minimal energy input. A typical vapor-compressionmultiple-effect system is shown in figure 6-5.

The advantages of this type of system include a loweenergy demand than high-temperature distillation, lesscorrosion due to possible use of thermoplastic materialsand lower operational temperatures.

Figure 6-4. Waste heat recovery evaporation process.

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Figure 6-5. Vapor-compression vertical-tube distillation system.

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6-5. Thermal discharge. A problem resulting fromall distillation/condensation facilities is thermaldischarge of liquids. Older high temperature facilitiesproduce brine at very high temperatures. Coolingtowers, heat exchangers, or similar equipment must bedesigned into the process to handle the thermaldischarge from distillation/condensation facilities. Moresophisticated desalination units employ a system of heatexchange devices that use the raw feed water to coolthe brine and reclaim this waste heat to help providethermal energy for system operation.

6-6. Design analysis. When it is necessary toreview several water distillation/condensation designs,standard dimensionless analysis will be used for designcomparison. If dimensionless correlations for particularaspects of design do not exist, a bench- or pilot-scalestudy should be done.

6-7. Materials of construction . The corrosivenature of high-temperature brines, acid pretreatments,and chemical scaling can cause plant failure. Presently,the only acceptable construction materials for wettedsurfaces in high-temperature systems are an austeniticstainless steel, such as AISI Type 316L of titanium.Anodized aluminum and many thermoplastic materialsare acceptable for use in low-temperature systems.

6-8. Distillation/condensation system design .Pursuant to finalized site and process selection, onedistillation/condensation system will usually be chosen.When the process selection does not yield a singleprocess, then designs must be prepared for more thanone process.

a. Identification of work. When the base site hasbeen selected and a schedule for construction has beenprepared, this information will be made available to thewater treatment engineer. The identification of thelocation and the time schedule will be considered in thedesign; this includes the date the system must be online.The minimum number and minimum size of themodules will be determined. Any restrictions thatstorage will place on maximum allowable downtime willalso be determined. With distillation/ condensationsystems, the design must address the maximumallowable total dissolved solids and, where applicable,the minimum allowable rejection of distillable material.

Distillable material is defined as nonaqueous, volatilewater contaminants.

b. Existing or planned facilities. Distillationcondensation systems design must include availabilityof energy information. See sample problem A-8Alternative steam sources considered in the design mustinclude steam temperature, steam pressure, andavailable quantity of steam. The design must showavailable electrical power including voltage, amperagephase of the available electricity, and frequency of theavailable electrical power.

c. Raw water information. One of twocircumstances will limit the quantity of raw waterconsumed. Both of these limitations must beconsidered in the design:

-The availability of raw water may place alimitation on the raw water used in theprocess.

-The maximum amount of waste brine thatcan be economically disposed of maylimit the raw water used in the process.

The principle requirement in a desalination design is anaccurate projection of the chemical makeup of the wors

quality water that will be used as raw feed water at thesite being investigated. The design must includeconsideration of the maximum total dissolved solidsindividual ions (see App. B), maximum amount of totasuspended soiled present in the feed water, maximumorganic contaminant loading, and any gas or potentiacorrosive agent that may be in the feed water. Alknown or anticipated future qualities of the feed watershall be considered in the design.

d. Process design. When adistillation/condensation process has been identified asthe most economical, then the design will be limited tothe single process. The process design for any

distillation/condensation process will include a minimumrequired input temperature and some maximumrequired heat sink temperature. Between these twotemperature criteria, the process must be capable ofproducing the required product water quality andquantity. When a particular metallurgy is required fostrategic, corrosion design, or economic reasons, thismetallurgy shall be designated for all applicable partsas well as spare parts. All required instrumentationmust be included in the design. The design must showthe required output water quality based on the worst rawwater input chemistry and quality. The system designmust be based on equipment with a history of successfu

water treatment system experience. The requiredexperience history should include a minimum of 2 yearsof operating experience meeting water quality andsystem design

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goals, treatment capacity, maximum allowable repairfrequency and duration, and a maximum allowable ratioof experienced capital cost to repair cost.

The requirement for successful experience will limit theamount of untested innovation used at a facility.

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CHAPTER 7

MEMBRANE TECHNIQUES

7-1. Electrodialysis. The ions in a water solutioncan be made to migrate by applying an electric field tothe solution. By arranging various barriers to the flow of

ions, it is possible to directly desalinate water withelectricity. Such barriers are called ion-exchangemembranes. Membranes that allow a reasonable flowof cations, but block or reduce the flow of anions, arecalled cationic-exchange membranes. Membranes thatallow a reasonable flow of anions, but block or reducethe flow of cations, are called anion-exchangemembranes. Membranes that pass both anions andcations are called neutral membranes.

a. Theory . In solutions containing dissolved ions,electric currents are carried by movement of the ions.Positive ions migrate in the direction of the current flow,and negative ions migrate against the current direction.When the anions are blocked by a cationic-exchangemembrane, they stop and form a localized charge at themembrane face. This accumulated negative charge isneutralized by the flow of cations across the cationicmembrane. This generates a concentrated solution onthe side of a cationic-exchange membrane that facesthe negative electrode. It also generates a dilutesolution on the side of the cationic membrane that facesthe positive electrode as shown in figure 7-1.

b. Electrodialysis stack. If both a cationic andanionic membrane are placed across a current flow inan electrolyte solution, the side of the cationicmembrane facing the positive electrode and the side ofthe anionic membrane facing the negative electrode will

become less saline. If the cationic membrane is closerto the negative electrode and the anionic membrane iscloser to the positive electrode, the solution between themembranes will become less saline as the ions migratein their respective directions. Any number of pairs ofcationic and anionic membranes can be placed across acurrent-carrying solution, such that the cationicmembrane is closest to the negative electrode, and thesolution between will be diluted (fig. 7-1). A battery ofseveral such membrane pairs is called an electrodialysisstack. Several variations of the standard electrodialysisstack have been developed, but none have been provensuperior to this standard stack of alternating cationic-

and anionic-exchange membranes to desalinate naturalbrackish water.

c. Electrodialysis reversal. One importanimprovement now used in electrodialysis installations isto reverse the polarity periodically and move the ions in

the opposite direction. This returns anions across theanionic membranes and helps break up scale formed onthe concentrating face of the membranes. Water wilflow osmotically across both membranes from the diluteproduct stream to the concentrated brine stream in anelectrodialysis-reversal stack. This osmotic producwater loss concentrates uncharged material, such asturbidity and bacteria. This concentration effect must beconsidered during the design to ensure meeting waterturbidity and product water bacterial count requirementsMost electrodialysis membranes are not tolerant ofchlorine. When possible, water desalinated byelectrodialysis reversal should be disinfected aftedesalination is completed. The membranes should be

protected by a 10-micron cartridge filter.

7-2. Electrodialysis-reversal design. Whenelectrodialysis reversal can be shown to be the mosteconomical process for desalination, then only anelectrodialysis-reversal system will be designed. Whenthe process selection does not yield a specific processthen designs must be prepared for more than oneprocess.

a. Identification of work. The design engineer wilbe made aware of the base site and constructionschedule. The location and time schedule will beconsidered in the design; this includes the date the

system must be online. The minimum number andminimum capacity of the modules will be determinedAny restrictions that storage will place on maximumallowable downtime will also be considered. Amaximum allowable output conductivity in the producwater will be determined, based on the worst possiblefeed water.

b. Existing on planned facilities. Whenelectrodialysis reversal is being designed, it is essentiato determine the types of available electrical powerThe voltage, phase, frequency, and available amperageof all electrical power sources, planned or existing, musbe considered in the design.

c. Raw water information. One of twocircumstances will limit the quantity of raw waterconsumed. Both of these limitations must beconsidered in the design:

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-The availability of raw water may place alimitation on the raw water used in the process.

-The maximum amount of waste brine that canbe economically disposed of may limit the rawwater used in the process.

The principle requirement in a desalination design is anaccurate projection of the chemical makeup of the worsquality water that will ever be used as a raw feed waterat the site being investigated. The designdocumentation must include maximum total dissolvedsolids, the individual ions (see App. B), the maximumamount of total suspended solids present in the feedwater, and any gas or potential corrosive agent that maybe in the feed water.

Figure 7-1. Principles of electrodialysis desalination

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d. Process specifications. When an electro-dialysis-reversal process has been identified as mosteconomical, the design will be limited to the singleprocess. The process design for any electrodialysis-reversal process will include a minimum/maximumallowed product water conductiv ity. The design mustshow the required product conductivity that must beobtainable at the required product flow, based on theworst conductivity raw water. A 10-micron cartridgefilter to be placed before the membranes must beincluded in the design. When a particular metallurgy ormaterial is required for strategic, corrosion design, orprocess economic reasons, this metallurgy or materialwill be designated for all applicable parts and spareparts and equipment. All required instrumentation,including a voltmeter and an ammeter, for eachelectrodialysis-reversal stack must be designed. Thesystem design must be based on equipment with ahistory of water treatment system experience. Therequired experience history should include a minimumof 2 years of operating experience meeting water quality

and system design goals, current operating capacity,maximum allowable repair frequency and duration, andmaximum allowable ratio of experienced capital cost torepair cost. The requirement for successful experiencewill limit the amount of untested innovation used at afacility.

7-3. Reverse osmosis. Diffusion through materialsis influenced by the nature of the diffusing material. Anumber of materials allow water to pass through withrelative ease. Some of these materials allow only aminute passage of ionized material compared to thepassage of water through them. These semipermeable

materials are used for desalination. If a thin barrier ormembrane is used, water can be forced through themembrane while ions are stopped by the membrane. Ingeneral, nonionized materials, such as some gases andmany organics, will not be removed by thesemembranes. Some larger organic molecules may notpass through the membranes.

a. Osmotic pressure. When a semipermeablemembrane that will pass solvent is placed between twosolutions of different concentrations containing the samesolvent at identical temperatures, the solvent must passfrom the less concentrated to the more concentratedsolution as shown in figure 7-2. This flow of solvent

produces a pressure head difference. The equilibriumliquid pressure head difference is called the osmoticpressure difference of the solutions (see App A for thecalculation). If these pressures are reversed, pure waterwill be forced from the more concentrated solutionthrough the membrane into the less concentrated

solution, provided that the pressure differential exceedsthe osmotic pressure. A typical reverse osmosis flowsheet is shown in figure 7-3.

b. Energy recovery. Reverse osmosis produces aconcentrated, high-pressure brine. With reverseosmosis, the energy lost in depressurizing the brine canbe returned efficiently to the feed water by mechanicamethods. In small systems, consider a flow-workexchanger; in large systems, consider an energyrecovery turbine.

(1) Flow-work exchanger. A flow-workexchanger, figure 7-4, is a simple piston driven bypressurized brine to compress the saline feed. Whenthe piston has traveled a full stroke, the valving ischanged, and the saline feed is used to expedepressurized brine. Flow-work exchangers have beenunder development since 1980.

(2) Energy recovery turbines. Several largereverse osmosis systems have been built with energyrecovery turbines. These turbines can be installed toassist directly in pumping or to drive synchronous

motors and generate electricity. While reverse osmosisis an energy efficient desalination process for highlysaline waters, energy recovery can reduce the amounof energy used by as much as one-third.

c. Mechanical strength and packing of membranesFor containment of high pressures with thin membranesin reverse osmosis, three alternative arrangements havebeen developed.

(1) Porous tubes. Porous tubes lined withsemipermeable membrane material have beendeveloped for concentration of valuable products inindustry. Such systems are no longer used for watedesalination. A packing density of less than 110 square

feet of surface area per cubic foot of volume makes thisconfiguration too expensive for water production. Seefigure 7-5.

(2) Spiral-wound membranes. By usingspacers it is possible to roll a membrane envelope ontoa slotted product water tube as shown in figure 7-6. Thisreverse osmosis membrane configuration is known asthe spiral-wound configuration. This arrangement allowsfor surface densities of greater than 250 square feet ofsurface area per cubic foot of volume. With thedevelopment of this spiral-wound configuration, waterproduction from brackish water sources by reverseosmosis became economical in many applications. See

figure 7-6.

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Figure 7-2. Revere osmosis principles.

(3) Hollow fine fibers. A large research anddevelopment effort made it possible to coat minutehollow fibers, smaller in diameter than a human hair,with semipermeable membrane material. This reverseosmosis membrane configuration is known as the hollowfine-fiber configuration. Packing densities with hollowfine fibers have exceeded 4,900 square feet of surfacearea per cubic foot of volume. See figure 7-7.

d. Membrane materials. There area number ofsuccessful reverse osmosis membrane materials.

Currently, two principal types of membrane materialsare being used: cellulose acetate and polyaromaticamide. Both materials are destroyed by dehydration ofthe membranes. To avoid dehydration, product watermust be supplied to allow osmotic water to flow backthrough the membranes in order to dilute the feed waterto approximate product water concentrations.

If product water is not supplied, then the osmoticsuction, if the feed water side of the membrane isdepressurized, will draw air back into the membranesand dehydrate them. Usually, the required volume oproduct water is supplied by a suck-back tank, whichmaintains a minimum volume and back pressure on thereverse osmosis membranes.

(1) Cellulose acetates. This material suffersfrom slow chemical decomposition through a processcalled hydrolysis. The use of acids to prevent scaling

increases the rate of this form of membrane decayCellulose acetates are also biodegradable and must beprotected from bacterial attack. One of the importanadvantages of cellulose acetate is its resistance toattack by chlorine.

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Figure 7-3. Reverse osmosis desalination system.

Most cellulose acetate membranes can be used withfeed waters containing less than 1 milligram per liter ofresidual chlorine, which will protect the membranes frombiological attack.

(2) Polyaromatic amides. These membranesare stable, biologically and chemically. Despite this

chemical stability, these membranes cannot tolerate anyresidual oxidant. If chlorination is required to reduce theamount of biological suspended solids, thendechlorination must be complete if polyaromatic amidemembranes are used.

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Figure 7-4. Flow-work exchanger principles.

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Figure 7-5. Construction of a tubular reverse osmosis membrane.

7-4. Reverse osmosis membrane stagingconfigurations . Two kinds of membrane staging arecommonly used in reverse osmosis desalination plants:product staging and reject staging. Reject staging isused to treat waters with low salinity, so that most of theraw feed water will eventually be recovered as productwater. Product staging is used to treat highly saline

waters, whose product water salinity cannot be reducedto the required concentration by a single pass throughthe membrane under consideration. Banking is the termusually used for parallel arrangement of a number ofmembrane modules operating from the discharge of asingle pump.

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Figure 7-6. Internal construction of a spiral-wound membrane.

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Figure 7-7. Internal construction of a hollow fine-fiber reverse osmosis membrane module.

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Figure 7-8. Reverse osmosis product staging.

Banking of membranes usually require a flow restrainton the brine reject of each module in the bank. A pigtail

of nylon tubing is often used for this flow equalizationheadloss. This pressure drop maintains a balanced flowof brine out of each membrane module in the bank.Unbalanced flow can shorten the useful life of

membrane modules. While higher flow rates will tend toclean membranes, excessive flow can fatigue or fray

both spiral and hollow fine-fiber membranes. Low flowrates allow the concentrated brine to stagnate, whichleads to scaling and fouling of membrane surfaces.

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a. Product staging. Product staging is true seriesoperation of two or more reverse osmosis membranesystems, as shown in figure 7-8. Product staging isused when a single pass through one reverse osmosismembrane does not bring a constituent concentrationdown to specification. The second stage alwaysrequires its own pressurizing pump, taking suction fromthe suck-back or storage tank of the first stage reverseosmosis system. When the water produced from thesecond stage is significantly lower in dissolvedconstituents than required, the product water from thefirst stage may be blended with the second stageproduct to produce the desired water quality. Whenpotable water and other waters with lower dissolvedsolids content are required, a product staging systemcan be used to supply the desired quality or qualitiesbetween that of the first and second stage product. Lifecycle costing should be used to evaluate dual- andtriple-distribution systems where product staging isrequired.

b. Reject staging. Reject staging, figure 7-9, is

used when the low salinity of the raw water permits ahigh water-recovery ratio. Most membrane modulemanufacturers have a minimum allowable brine rejectflow for any given membrane of their manufacture. Themanufacturer's recommended maximum feed water flowrate and minimum recommended brine reject flow canbe used to calculate a maximum recommended singlestage recovery fraction by use of the following equation:

F - B =RF

Where:

F = Maximum recommended feed flow permodule

B = Minimum recommended brine reject flowper module

R = Maximum recommended recovery rate

This maximum single stage water recovery is onemeans of evaluating a membrane module beingconsidered for low salinity reverse osmosis desalination.When the reject stream is still diluted enough for furtherconcentration after the maximum recommendedrecovery ratio is reached, the brine can be piped directlyinto another membrane module for further water

recovery. This is accomplished by combining the brineflow from a number of first stage modules onto a fewernumber of secondary membrane modules. It isoccasionally possible to further concentrate the brine ona third reject stage as shown in figure 7-9.

The design of reject staging, in order to balance theutilization of the membrane modules for optimumeconomical life cycle cost, is a complex activity to beperformed by the membrane manufacturer oOperations Engineering Manufacturer.

c. Combined product and reject staging. In thedesalination of highly saline waters such as seawaterproduct and reject staging can be effectively combinedThe second stage of a product staged system can bedesigned as a reject staged subsystem. Any of threefactors may limit reverse osmosis water recoveryosmotic pressure; sparingly soluble salts; or turbidityWater from a primary reverse osmosis treatment systemwill have three properties pertaining to these limitations:

-A lower osmotic pressure than the rawfeed water.

-A disaproportionately reduced concentration of divalent ions.

-No turbidity.These qualities of primary reverse osmosis producwater can allow for greater water recovery from a

secondary product staged reverse osmosis subsystemthan is allowed by the manufacturer's maximumrecommended recovery rate. When the water recoveryof the second stage of a product staged system can beincreased by reject staging, the secondary stage shalbe reject staged. When the brine from the secondarystage of a product staged system is less concentratedthan the primary stage feed water by more than 1,000milligrams of total dissolved solids per liter, the use ofdedicated desalination of this lower concentration wateshall be life cycle costed. This life cycle cost fodedicated secondary stage brine desalination shall becompared with the life cycle cost of blending the

secondary stage brine into the primary stage feed water.

7-5. Reverse osmosis system design. Whenprocess selection does not yield a specific membrane oeven a particular process, then designs must beprepared for more than one process.

a. Identification of work. When a schedule and abase site have been selected, this information will bemade available to the design engineer. Theidentification of the location and the time schedule wilbe considered in the design; this includes the date thesystem must be online. The minimum number oindependently operable membrane banks and the

minimum capacity of the banks must be determinedAny restrictions that storage will place on maximumallowable downtime will also be determined.

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Figure 7-9. Reverse osmosis membrane reject staging.

All reverse osmosis designs will consider a minimummembrane rejection for all objectionable materials in thepretreated saline water.

b. Existing or planned facilities. Reverse osmosisdesign must include information on the availablesources of mechanical energy. Electrical power is themost common energy source for conversion tomechanical pumping energy.

The phase, voltage, frequency, and amperage of alavailable or planned electrical power will be determinedThe process design, particularly with respect to pumpingand pretreatment, must be detailed clearly.

c. Raw water information. As with othedesalination systems, one of two limitations exist on thequantity of raw water to be desalinated.Both of these limitations must be considered in thedesign:

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-The maximum amount of waste brine thatcan be economically disposed of mayplace a limitation on the raw water used inthe process.

-Availability of the raw water may limit theraw water used in the process.

The principle requirement in a reverse osmosis design isan accurate projection of the chemical, biological, andphysical makeup of the poorest quality water that willever be used as raw feed water at the site beinginvestigated. The design documentation must includethe following:

-Maximum total dissolved solids.-Maximum concentration of every ion thatcould precipitate or influence the activitycoefficient of a precipitation reaction.(See App. B.)

-Maximum concentration of each ion thatmust be controlled in the product water.

-Concentration of both molybdate reactiveand molybdate nonreactive silica.

-Maximum allowable concentration ofnonionizable material.

-An oil and grease analysis to levels below10 milligrams per liter.

-Any gas or potential corrosive agent thatmay be in the feed water.

d. Process design. When a particular membranehas been identified as the most economic, the designwill be limited to the one membrane type. The processdesign for any reverse osmosis process will considerraw water quality and the required final product waterquality.

A suitable tank to meet suck-back requirements will bedesigned for all membranes that could be damaged bydehydration. The system design must be based onequipment with a history of successful water treatmenexperience. The required experience should include aminimum of 2 years of experience, treatment capacityrepair frequency and duration, and a ratio of repair costo capital cost. The requirement for successfuexperience will limit the amount of untested innovationused at a facility. When a particular metallurgy omaterial is required for strategic, corrosion design, oprocess economic reasons, this metallurgy will beincluded for all applicable parts and spare parts andequipment.

7-6. Materials of construction.Ferric ions will cause severe problems in membranesystems. For this reason, never permit carbon steel tobe in contact with the feed water being supplied to amembrane desalination plant. Use nylon or otheplastics capable of maintaining the desired pressures

whenever possible. Use 316L stainless steel for pumpimpellers and other feed-water-contact metal surfaces ifhexametaphosphate is

ufc 3-230-12a water desalination (01!16!2004)

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