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Key issues in Seawater Desalination

Nowadays, desalination* has become a very affordable solution to cope with fresh water shortage typically in t

The desalination core process is based on Reverse Osmosis

Membrane technology, but stand alone, it doesn't provide

safe drinking water, nor does it guarantee an efficient plant.

The pre-treatment includes all the necessary treatment ste

ahead of the reverse osmosis plant. t is determining for

plant life time and to minimise chemical cleaning and

membrane replacement. t has a direct impact on the plant

performance.

There are as many membrane types as applications. They

range from !high re"ection! to !ultra low energy! or ! high

boron re"ection!.

The reverse osmosis process can also be built with one or

two passes, depending on the product water re#uirements

and the seawater salinity and temperature. n most cases,

$ pass is sufficient to reach the %& drinking water standards,

specially regarding the boron content $ mg()*. To reach

+O boron guideline .mg()*, a second pass might be

necessary /oron removal process*

The energy recovery device is the key factor that determines the plant electrical costs. t must be

chosen carefully based on the local energy costs and environment policies.

0ost-treatment and(or polishing steps are re#uired to condition the water after the reverse osmosi

membrane process to make it suitable to your application.

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/rine disposal can be an environmental and economical issue in some areas where the fauna and

flora are sensitive to local seawater salinity increase. /rine disposal should be studied and

engineered case by case.

The art of desalination is to determine and combine available

technologies to optimi1e water production costs and #uality.

To adapt our 2esalination 0lants to your local needs, we offer containeri1ed mobile units from

ntake to 2istribution up to a production capacity of 3 m4(h of desalinated water.

All type of water can be produced from a desalination plant

- !"# or $% drin&ing water

- 'rrigation water

- (rocess water  boiler feed water, cooling water

- Demi or %ltrapure water

All type of natural seawater source can be treated

-ShallowSurface

-Deep

-)rac&ishriver

- )each well seawater

All essential process steps in desalination plants

Also misspelled Desaltation, Desaliniation, Desalinisation, Desalisation, Desaliation

or Desal (lant

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Water Desalination

Desalination $+pands as echnology )ecomes ore Affordable

A New Zealand army ofcer tests water quality at an army mobile desalination and purication station supplying ree water to residents in the

suburb o New Brighton on February !" #$$ in %hristchurch" New Zealand&

Cameron Spencer/Getty Images

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Updated November 08, 2013Desalination 9also spelled desalini:ation; is the process o creating resh water by remo*ing saline 9salt; rom bodies o salt water& 3here are *arying degrees o salinity in water" which a<ects the difculty and e7pense o treatment" andthe le*el o saline is typically measured in parts per million 9ppm;& 3he +&6& /eological 6ur*ey pro*ides an outline o what constitutes saline water= $"### ppm > ?"### ppm is low salinity" ?"### ppm > $#"### ppm is moderate salinity"and $#"### ppm > ?@"### ppm is high salinity&

Water that contains saline le*els less than $"### ppm is generally considered resh water" and is sae to drin' and use

or household and agricultural purposes& For a reerence point" typical ocean water contains about ?@"### ppm" the

/reat 6alt 1a'e contains *ariations o @#"### > #"### ppm" and the %aspian 6ea contains an a*erage o about$"### ppm& 3he more concentrated saline is in a body o water" the more energy and e<ort it ta'es to desalini:e it&

Desalination Processes 3here are numerous methods o desalination described below& 5e*erse osmosis is currently the most commonly oundtype o desalination" and multistage ash distillation is the method that currently produces the most amount o desalinated water& 93here are se*eral other less requent types o desalination methods and energy sources notdiscussed here&;Reerse !smosis5e*erse osmosis is a process where pressure is used to push the water solution through a membrane" with themembrane pre*enting the larger solutes 9the salt; to pass through& 5e*erse osmosis is generally considered to be theleast energy consuming o all the largeCscale processes&

 3here are se*eral setbac's o re*erse osmosis& 3he membranes are currently prone to gather too much bacteria andclog up"E although they ha*e impro*ed since they were rst used& 3he membranes deteriorate when chlorine is used

to treat the bacteria& -ther setbac's are the arguable water quality that re*erse osmosis produces" along with the

considerable preCtreatment that the salt water requires&

For"ard !smosisForward osmosis utili:es the natural osmotic process a substance mo*ing rom an area o low concentration to anarea o high concentration& )t generally requires about hal o the cost o re*erse osmosis" due to less energy beingused to complete the process& )nstead o orcing the solution through a pressure gradient" this process allows it tonaturally occur& When desalinating water" a solution o seawater mo*es across a semiCpermeable membrane to ahighly concentrated solution o ammonia salts" lea*ing the sea salts on the other side o the membrane& Aterwards"the solution is heated to e*aporate the ammonia salt" and that salt is reusable&

 3he main setbac' to orward osmosis is that it has great potential" but is still airly new to largeCscale desalination andthereore needs unding and research to e7plore the possibilities that could impro*e it and reduce energy costs&

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Electrodialysis,lectrodialysis re*ersal utili:es a membrane" li'e that in re*erse osmosis" but sends an electric charge through thesolution to draw metal ions to the positi*e plate on one side" and other ions 9li'e salt; to the negati*e plate on theother& 3he charges are periodically re*ersed to pre*ent the membrane rom becoming too contaminated" as typicallyound in regular electrodialysis& 3he ions located on both plates can be remo*ed" lea*ing pure water behind& 5ecentlyde*eloped membranes reportedly ha*e been chlorine resistant" and generally remo*e more harmul ions 9not Gust salt;than re*erse osmosis& 3he primary setbac' to electrodialysis re*ersal is the upront cost to create the acility" as well

as the energy costs&Thermal Desalination

 3hermal desalination is a method o cleaning water that can occur through many di<erent processes" and includesremo*ing salt as well as other contaminants& All thermal desalination is the process o heating the water solution andgathering pure water when the *apor cools and condensation occurs& 3wo types requently used to desalinate waterare=#ultistage Flash Distillation(ultistage ash distillation occurs when the product o the heated water is reheated multiple times" each timeunctioning on lower pressure than the last& (ultistage ash distillation plants are built alongside power plants in orderto use the wasted heat& )t requires much less energy than re*erse osmosis plants& 6e*eral large acilities in 6audiArabia use multistage ash distillation" accounting or around H@I o all desalinated water" though there are morere*erse osmosis plants than there are multistage ash distillation plants& 3he main disad*antages o multistage ashdistillation are that it requires more inta'e o salt water than re*erse osmosis and the upront and maintenance costsare considerably high&

#ultiple$E%ect Distillation(ultipleCe<ect distillation is a simple process similar to multistage ash distillation& 3he salt water solution is heatedand the pure water that is produced ows into the ne7t chamber& 3he heat energy that it carries is used to boil itagain" producing more *apor& 3he main setbac' is that it is best used or smallerCscale desalination& 3he costs are *eryhigh or large acilities&&egaties o' DesalinationA ew general setbac's or the processes o desalination also e7ist& Dumping the wasted salt solution bac' into theocean ma'es the process more difcult and has the potential to harms ocean lie& 3he energy required to start up andpower desalination plants is a huge e7pense and because most current power sources are deri*ed rom burning ossiluels" it is generally loo'ed upon as Gust a matter o choosing one en*ironmental crisis o*er another& Within the energyissue" nuclear energy is potentially the most costCe<ecti*e energy source" but remains largely untapped due to publicopinion on ha*ing a local nuclear power plant or waste acility& ) regions situated away rom the coast or in a highaltitude try to use desalinated water" it is an e*en more e7pensi*e process& 2igher altitudes and ar distances requiregreat resources to transport the water rom the ocean or body o salt water&

Geography o' Desalination/eography o Desalination Desalination is currently used by countries that ha*e an e7treme need or resh water" ha*eenough money to und it" and posses the amount o energy required to produce it& 3he (iddle ,ast holds the top spotor desalinated water" due to se*eral countriesJ large acilities" including 6audi Arabia" the +nited Arab ,mirates" and)srael& Also large producers o desalinated water are= 6pain" the +nited 6tates" Algeria" %hina" )ndia" Australia" andAruba& 3he technology is e7pected to spread increasingly" particularly in the +nited 6tates" 1ibya" %hina" and )ndia&

6audi Arabia is currently the worldJs number one producer o desalinated water& 3hey use multiCash distillation in

se*eral large plants" pro*iding water or many large cities" including the largest city" 5iyadh" situated hundreds o 

miles rom the coast&

)n the +nited 6tates" the largest desalination plant is located in 3ampa Bay" Florida" though it has a *ery small output

compared to most acilities in the (iddle ,ast& -ther states that are de*eloping plans or large desalination plants

include %aliornia and 3e7as& 3he +nited 6tates need or desalination plants is not as se*ere as many other countries"but as the population continues to e7plode in dry" coastal areas" the need increases&

Future !ptions o' DesalinationDesalination is process primarily done in de*eloped countries with enough money and resources& ) technologycontinues to produce new methods and better solutions to the issues that e7ist today" there would be a whole newwater resource or more and more countries that are acing drought" competition or water" and o*erpopulation&

 3hough there are concerns in the scientic world about replacing our current o*eruse o water with complete relianceon sea water" it would undoubtedly be at least an option or many people struggling to sur*i*e or maintain theirstandard o li*ing&

5eawater ntake 5ystems

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6i7 5urface water Open ntake

'nta&e screens are mounted at the end of the inta&e pipe, at about to / m below sea level.

 

he screen is bac&washed with air to prevent solids to clog the screen surface

Courtesy of Johnson Screens

 

/each well

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)each wells are recommended when seawater 0uality can vary over the season and1or when high turbidity pea&s can occu

#ne vertical beachwell can draw up to /22 m31h.

A radial well can ta&e up to 222 m31hour, depending on the soil layers.

 

Mobile 5urface water Open ntake

'deal for temporary plants or if civil wor&s are not possible.

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courtesy of Hydromobi 

 

Desalination (retreatment

+hy a pre treatment8

4everse #smosis hin 5ilm 6omposite membranes are sub7ect to fouling by suspended materials that are present in seawa

5ouling 6ause

)iological fouling )acteria, microorganisms, viruses, protooan

(article fouling sand, clay 8turbidity, suspended solids9

6olloidal fouling #rganic and inorganic comple+es, colloidal particles, micro-algae

#rganic fouling 9atural Organic Matter 8N#9 humic and fulvic acids, biopolymers

ineral fouling :alcium, agnesium )arium or Strontium sulfates and carbonates

#+idant fouling :hlorine, #one, Kn#:

5eawater 2esalination typical pretreatment processes;

A very cost-effective way to avoid biological fouling is seawater chlorination . %nfortunately, chlorine o+idies the membra

A common dechlorination process is the in7ection of sodium bisulfite or metabisulfite, classified as a chlorine scavenger.

N#, particles and colloids can be removed by so-called ;conventional treatment; consisting of coagulation followed by dadded in case of very turbid shallow seawater.

he non-conventional pretreatment for N# , particles and colloids is u ltrafiltration.

An antiscalant solution should be dosed before the reverse osmosis membranes to disperse calcium carbonate and sulfate

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6ine filtration 8<-micron9 is re0uired as a last step before the 4# membranes to prevent any debris, sand particles or pipi

2esalination 0retreatment; 5eawater chlorination

6hlorination

Surface water contains various microorganisms, bacteria, protooan, that would contributes to a biofilm formation on

the membrane surface. A biofilm is a bacteria colony that grows on a designated surface where a carbon source and

nutrients are available. Killing bacteria and microorganism before entering the membranes will prevent biofouling.

he typical chlorine dose is about 3 mg1= of active chlorine. 't gas be in7ected by dosing Sodium "ypochlorite 8Na#6l9

for small scale plant or 6hlorine gas 86l/9 for medium scale plants.

=arge Scale desalination plants uses electrochlorination to produce sodium hypochlorite 8Na#6l9 from the e+cess of

sodium chloride salt 8Na6l9 present in seawater at about 32 to :2 g1=

9a:l < 3O < direct current --= 9aO:l < 3

p" should be corrected around >.< to have optimied chlorine disinfection potential.

ore details on 6hlorine disinfection mechanisms, clic& here.

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chlorination can be processed by / processes

• >ddition of 5odium meta* bisulfite 5/5*

dosing rate is 3 of S)S6l/ 8active chlorine dose9. he reaction time is about / minutes.

NaHSO3 + Cl2 +H2O --> NaHSO4 + 2 HCl 

Na2S2O5 + Cl2 +H20 --> Na2SO4 +SO2 + 2HCl 

• ?ranulated Activated 6arbon filter

emisorption process where chlorine reacts with the carbon surface.

CL2 --> Cl-