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PresentationOutline

Compositionofseawater

Whatisreverseosmosis

Pretreatmentoffeedwater

Reverseosmosismembrane

Posttreatmentofpermeate

Futureaspectsofreverseosmosis

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Water 97.5%oftheearthswaterisseawaterandthe

remaining2.5%

is

freshwater

found

in

glaciers

and

underground

Figure2:IllustrationofwateronEarth

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CanWeDrinkSeawaterFigure2:ClipofHomertryingtodrinkseawater

Simpsons season 5, episode 8: Boy Scoutz n the Hood

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Compositionof

Seawater

TotalDissolvedSolids(TDS)areinorganicsaltsandsmallamountsoforganicmatterthataredissolved

inwater

TheTDSisseawaterranges10,000to45,000ppmcomparedtofreshwaterthatis1500ppm

Figure3:Tableofthechemicalcompositionanditsconcentrationofseawater

G Migliorini, et al., Desalination 165 (2004)289-298

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Desalination

Theprocessofremovingdissolvedsaltsandothermineralsfromseawatertoobtainwatersuitableforhumanandanimalconsumption,irrigationandotherindustrialuses

Twodesalinationmethods:bythermalprocess(multistageflash)orbymembrane(reverse

osmosis)

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Osmosis Osmosisisthespontaneoustransportofsolvent

fromadilutesolutiontoaconcentratedsolution

acrossan

ideal

semi

permeable

membrane

Atacertainpressure,theosmoticpressure,equilibriumisreachedandtheamountofsolventwhichpassesineachdirectionisequal

Figure4:Illustrationofosmosisandosmoticequilibrium

(N.ztrketal.,Desalination223(2008)19)

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ReverseOsmosis

Ifthepressureisincreasedabovetheosmoticpressureonthe

solution

side

of

membrane,

theflowisreversedandwaterflowsfromtheconcentratetothediluteside,thisiscalledreverse

osmosis

(RO)

ROisbasedontwovariables:highfluxofwaterandhighsaltrejection

Operatesatpressuresof110MPa

Figure5:IllustrationofRO

(N.ztrketal.,Desalination223(2008)19)

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ReverseOsmosis Itisoneofthemost

efficientsystem

based

ontheenergyspentperm3 offreshwater

produced

from

seawater

Reverseosmosisrejectsnearlyalldissolvedcompounds

Figure6:TypicalROdesalinationplant

ComptonEngineering,

Inc,

(2007)

City

of

Pascagoula

Reverse

Osmosis

Water

Plant,

http://www.comptonengineering.com/pascagoula.html

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ReverseOsmosis

Generalschematicofreverseosmosis Waterabstraction >pretreatment >membrane

separationunit

>post

treatment

>distribution

SanDiegoCountyWaterAuthority,Desalination,from:http://www.sdcwa.org/desalination

Figure7:IllustrationofRO

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WaterAbstraction

Watercanbeabstractedthroughcoastorbeachwellsorthroughopenseawaterintakesystems

Coastand

beach

wells

provide

better

quality

water

due

to

lowalgaeandTDScomparedtoopenseawater

Figure8:Illustrationofwater

abstraction

scwd2

(2008)IntakeApproaches,RetrievedNovember12,2010from:http://www.scwd2desal.org/PageProjectComponents_IntakeApproaches.php

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ParticulateScreening

Thescreengetsridofsomeoftheorganic

mattersuch

as

algae

and

other

particulates

suchaspebbles

Figure9:

Illustration

of

a

wirescreenonawaterintake

scwd2

(2008)IntakeApproaches,RetrievedNovember12,2010from:http://www.scwd2desal.org/PageProjectComponents_Intake

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Pretreatment

Variouscontaminantscanharmthemembrane,leadingtoirreversibledamageassociatedwithreducedrejection

capabilitiesand

even

destruction

of

the

membrane

Figure10:Illustrationofthepretreatmentofseawater

C.Fritzmannetal.,Desalination216(2007)176

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Chlorination Preventsfoulingofthemembrane(biologicaland

particulate)and

acts

as

adisinfectant

for

bacteria

andotherbiologics

Chlorineisaddedtotherawwaterassodium

hypochlorite(NaOCl)

or

Chlorine

gas,

(Cl2),

water

hydrolyzestohypochlorousacid

Cl2 +H2O >HOCl+HCl

NaOCl+H2O >HOCl+NaOH

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pHAdjustment

Needstobedonetopreventscaling

Scalingis

caused

by

super

saturation

of

inorganic

compounds(whenitexceedstheirsolubilitylevel)concentratedonthefeedsidethatprecipitateson

themembrane

surface

leaving

athin

layer

AcidsusedareHCl andH2SO4 Ca2++HCO3

H+ +CaCO3 As

H+ increases,

the

equilibrium

favors

the

left

hand

side

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Additionof

Coagulants

and

Flocculants

Dissolvedsolidsadsorbonhydroxidesformedandcolloidalmattertoagglomerate(forminamassgroup)

IronorAluminumsaltsareused(FeCl3

,Al2

(SO4

)3

)

2FeCI3+3Ca(HCO3)2 2Fe(OH)3 +3CaCl2+6CO2 Sedimentationandsandfiltrationremovethe

agglomerates

from

the

feed

water

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Dechloriniation DechlorinationisdonepriortoROstagebecause

residualchlorine

in

the

feed

water

to

the

reverse

osmosiselementdamagesthemembrane(polyamides)

SodiumMetabisulfite

(Na2S2O5)

is

commonly

used,

Na2S2O5 +H2O >2NaHSO32NaHSO3 +2HOCl >H2SO4 +2HCl+Na2SO4

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ReverseOsmosis

Membrane

Semipermeable

membranesare

used

to

separatesolutespresentinthesolution

ROmembranes

fall

into

twocategories:asymmetricmembranes,

and

thin

film

composite

(TFC)membranes

Figure11:Polymersusedfor

reverseosmosis

(G.Micale

(2009), SeawaterDesalinationConventionalandRenewableEnergyResources,SpringerHeidelbergDordrechtLondonNewYork

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ThinFilm

Composite

Membrane

Composedofathinpolymerlayeronaporoussupport(polysulfone)andasupportbacking

(polyestercloth)

Figure12:Illustrationofthinfilmcompositemembrane

M.E.Williams,2003,ABriefReviewofReverseOsmosisMembraneTechnology,pg19

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CrossLinked

Aromatic

Polyamides

Crosslinkedaromatic

polyamidesare

the

most

commonlyusedpolymer Morebeneficialthan

celluloseacetate

Displaysstronger

resistance

tobacterialdegradation Doesnothydrolyze

Stableunder

awider

pH

of

411comparedtoapHof4 7

Figure13:Crosslinkedaromaticpolyamide

J . Roh et at. J . Mater. Chem., 2002, 12, 23342338

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Howis

Water

Separated?

Thewaterinpolyamidemovesby"jumps"betweenweaklylocalizedsites

Wateroscillates

around

localized

sites

until

events,

suchasthermalfluctuations,enableanotherjump

SaltdiffusioninthepolymerdependsonbothNa+

andCl to

satisfy

electroneutrality

Figure 14: Number of molecules participating in solvating of Na+and Cl-

M.J . Kotelyanskii et al., J ournal of Membrane Science 139 (1998) 1-16

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WhyDechlorination isimportantfor

PAmembrane

HOCl + H2O -> H3O++ OCl-

HOCl acts as a Lewis acid and thereforeproduces Cl+

The Cl+attacks the aromatic ring

C.J.Gabelichetal., JournalofMembraneScience258(2005)6470

Figure14:Reactionofchlorinewiththepolyamidemembrane

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TDSof

Permeate

After

RO

TheTDSisgreatlyreduced

Figure 15: The concentration of ions after RO

G Migliorini, et al., Desalination 165 (2004)289-298

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PostTreatment

DuetolowTDSthedrinkingwascanbeunpalatable,corrosiveandunhealthy

Watermust

be

re

hardened

CO2 +CaCO3 +H2O >Ca(HCO3)2

Figure16:Illustrationoftheposttreatmentprocess

C.Fritzmannetal.,,Desalination216(2007)176

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Disinfectionof

Water

Producedwaterisfreeof

bacteria

and

viruses Disinfectionisnecessarytoprotectconsumersfrompollutionintroducedduring

distribution

and

storage

Chlorineisatypical

chemicalused

for

disinfection

From: http://www.ionizetohealth.com/

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Future ThefutureofROistoimprovemembranestoresist

fouling

Vancouveris

fortunate

to

have

readily

available

drinkingwater(fromsnowmeltsandrainfalls)

From: http://www.rent2010.net/i/Listing892/Olympics-2010-Ocean-View-West-Vancouver-