Biomedical Engineering Reference
In-Depth Information
desalination.process.progresses.
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.Concentration.polarization.on.the.external.surfaces.of.
the.membrane.results.from.the.concentration.imbalance.created.by.the.solution.dilution.
on.one.side.of.the.membrane.and.the.solution.concentration.on.the.other.during.separation.
processes..It.should.be.noted.that.while.progress.has.been.made.in.developing.high-lux.
membranes,
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.increasing.lux.often.leads.to.increasing.polarization.
2
.Conceptually,.it.is.
useful.to.imagine.the.concentration.polarization.region.as.a.pseudo-membrane.in.series.
with.the.physical.membrane,.impeding.low.and.adding.to.the.overall.energy.losses.in.the.
system..
Due.to.the.increasing.pressure.requirements.with.feedwater.salinity.increase,.40%.is.a.
common.product.water.recovery.rate.for.these.systems,
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.although.recent.systems.often.run.
at.50%-55%.recovery.at.the.expense.of.slightly.higher.(~0.5.Wh/L).energy.consumption..
Higher.recovery.ratios.reduce.the.size.of.the.system.needed,.as.well.as.the.amount.of.pre-
treatment.of.the.feedwater..Feedwater.quality.is.of.high.concern.in.RO.systems,.as.a.given.
membrane. material. operates. within. a. ixed. set. of. operating. parameters. relating. to. pH,.
organics,. algae,. bacteria,. particulates,. and. other. foulants. found. in. salinated. water.
12
. As. a.
consequence,.RO.systems.are.typically.characterized.by.several.pre-treatment.stages..From.
an.energy.standpoint,.RO.systems.come.closest.to.approaching.the.thermodynamic.limits,.
requiring.approximately.2.5.Wh/L.of.water.product.for.the.RO.process.alone;.however,.it.
should.be.noted.that.the.total.process,.including.pretreatment,.recirculation,.and.distribu-
tion,. requires. additional. energy.
5
. Of. concern. in. many. areas. with. RO. is. the. crossover. of.
boron.(usually.in.the.form.of.boric.acid),.which.in.concentrations.above.0.6.µg/L.(ppb).can.
be.toxic.to.plants..Thus,.for.source.waters.high.in.boron,.and.for.desired.uses.of.the.desali-
nated.waters.to.be.below.0.6.ppb,.a.second.RO.stage.is.added.to.take.approximately.20-30%.
of.the.product.water.from.the.irst.stage..The.pH.of.the.irst-stage.product.water.is.raised.
above.10.to.transform.the.boric.acid.to.borate,.which.is.more.successfully.rejected.by.the.
RO.membranes..The.two.remaining.product.water.streams.are.blended.to.bring.the.boron.
concentrations.down.to.acceptable.levels..The.overall.salt.concentration.is.also.reduced,.but.
the.usage.of.a.partial.second.stage.results.in.higher.overall.energy.usage..Recent.improve-
ments.in.the.RO.process.have.been.brought.about.by.the.use.of.more.selective.membranes.
and.the.implementation.of.energy.recovery.devices.that.concentrate.the.stream.exiting.the.
pressure.vessels.
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.The.minimum.energy.for.a.given.amount.of.product.water.is.a.compli-
cated.function.of.minimizing.the.cost.of.pre-.and.post-treatment,.the.energy.required,.and.
capital.costs..Therefore,.each.location.and.system.typically.operates.slightly.differently.
19.2.3.5 Electrodialysis (ED)
Electrodialysis. was. introduced. in. industrial. desalination. applications. in. the. 1960s. as. a.
membrane. desalination. technique. suited. to. the. removal. of. ions. from. brackish. water.
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.
The. conceptual. idea. is. based. on. the. iltration. mechanism. used. in. medical. systems. and.
the.principles.for.artiicial.kidney.operation,.with.these.systems.presenting.the.irst.evi-
dence.of.the.inluence.of.bionanotechnology.on.commercial.water.desalination.processes..
The.ED.process.shown.in.Figure 19.6.removes.electrically.charged.salt.molecules.of.saline.
water. by. employing. alternating. stacks. of. cationic. or. anionic. selective. membranes,. with.
ionic.lux.driven.by.an.applied.electric.ield.
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.As.the.cations.and.anions.are.pulled.through.
the.membranes,.the.salinity.of.the.water.drops.at.each.stage.and.freshwater.is.produced.
(Figure 19.6)..Energy.requirements.of.ED.systems.are.based.on.the.concentration.of.ions.
within. the. water;. thus,. electrodialysis. can. be. competitive. with. RO. processes. at. concen-
trations. up. to. 3,000. ppm.
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. A. full. membrane. stack. for. this. process. can. be. composed. of.
a.bank.of.membranes.comprising.hundreds.of.layers.of.alternating.anionic.and.cationic.
