Seawater Desalination
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Transcript of Seawater Desalination
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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-