Biomedical Engineering Reference
In-Depth Information
19.2 ReviewofExistingWaterDesalinationMethods
A. huge. variety. of. water. desalination. methods. exist.. In. this. section,. a. brief. review. of.
the. most. common. methods. is. presented;. therefore,. not. all. techniques. or. references. are.
included. in. this. review.. However,. the. reader. is. pointed. to. several. review. articles. and.
books,.along.with.a.broad.variety.of.literature,.to.allow.further.reading.given.the.reader's.
speciic.interests.
19.2.1 Theoretical Minimum Energy Requirement for Water Desalination
Consider.an.equilibrium.analysis.for.the.process.of.water.desalination..From.the.second.
law.of.thermodynamics,.for.a.reversible.process,.the.amount.of.energy.used.for.such.a.
process.is.independent.of.the.method.used. 7 .As.a.consequence,.given.the.starting.salinity.
of.water.and.the.target.salinity.of.desalted.water,.it.is.possible.to.calculate.a.theoretical.
minimum.amount.of.energy.required.for.water.desalination..This.exercise.may.appear.
academic,.but.is.of.value.since.the.energy-water.nexus 8,9 .is.a.major.consideration.toward.
evaluating.the.current.state.of.water.desalination.technologies.and.identifying.new.tech-
nologies.(see.Section.19.3)..Consider.an.ideal.compressor.for.moving.water.vapor.from.a.
tank.of.seawater.(typically.assumed.at.~35,000 ppm.or.mg/L.salinity).to.a.tank.of.fresh-
water. (typically. assumed. at. ~500. ppm. salinity). 7 . Using. these. conditions,. the. minimum.
energy. requirement. for. water. desalination. at. 25°C. with. a. recovery. rate. of. zero. (i.e.,. a.
negligibly.small.amount.of.water.produced.from.a.near-ininite.amount.of.seawater).is.
0.70 kWh/m 3 . 7 .The.recovery.rate.is.deined.as.the.ratio.of.freshwater.produced.to.the.inlet.
saltwater..For.a.viable.system,.recovery.ratios.must.be.maximized.in.contrast.to.the.waste.
or.brine.streams..An.increase.of.the.recovery.rates.to.25,.50,.and.75%.requires.theoreti-
cal.energy.minimums.of.0.81,.0.97,.and.1.29.kWh/m 3 .of.freshwater.product,.respectively. 2 .
Therefore,.in.principle.it.should.be.possible.to.remove.salt.from.water.in.an.eficient.man-
ner.with.low.energy.consumption..However,.several.challenges.exist.to.achieving.these.
theoretical.limits.
19.2.2 Challenges to Desalination Processes
Inherent. irreversibilties. present. in. real. systems. typically. drive. the. energy. requirements.
higher.than.the.theoretical.minima..Some.of.these.irreversibilities.relate.to.the.presence.of.
organics.and.particulates,.and.varying.pH.and.salinity.of.source.waters..Others.relate.to.
the.operation.of.mechanical.and.electrical.equipment.at.varying.energy.eficiencies..The.
varying.source.water.content.can.lead.to.scale.formation.and.deposition.or.membrane.foul-
ing.in.desalination.plants. 10 .In.order.to.mitigate.these.problems,.desalination.plants.often.
employ. extensive. pretreatment. steps. involving. intensive. chemical. treatment. processes,.
including.precipitation,.locculation,.lime.softening,.ion.exchange.columns,.or.mechanical.
processes.such.as.aeration.and.sedimentation..For.example,.in.order.to.minimize.scaling,.
pretreatment.of.feedwater.by.introducing.an.acid,.followed.by.CO 2 .degassing,.has.been.
shown. to. be. an. effective. method. for. preventing. alkaline. scale. formation. 10 . Antiscalants.
are.particularly.popular.due.to.their.effectiveness.at.low.concentrations;.the.chief.chemi-
cal. families. from. which. antiscalants. have. been. developed. are. condensed. polyphos-
phates,.organophosphonates,.and.polyelectrolytes..Of.these.three.classes.of.compounds,.
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