Environmental Engineering Reference
In-Depth Information
SilverCare™.products.
Initial.efforts.to.market.the.washing.machine.met.with.resistance.
in.Germany.and.the.washing.machine.was.taken.off.the.market.in.Sweden.for.a.brief.
time.due.to.concerns.over.the.potential.toxic.effects.of.discharging.silver.nanoparticles.
from.the.use.of.these.machines.to.wastewater.treatment.plants.[11,.12].
Chapter.4.dis-
cusses.regulatory.actions.in.the.United.States.regarding.such.washing.machines.
Attempts. to. quantify. the. discharge. of. silver. from. using. the. washing. machine.
—. and. thus. illuminate. the. potential. effects. on. a. municipal. wastewater. treatment.
plant.—.provide.a.range.of.answers.based.on.the.available.data.
In.addition.to.the.
information.provided.above.regarding.the.mass.and.potential.form.of.silver.released,.
the.following.assumptions.about.wastewater.generation.were.used.to.complete.a.con-
servative.mass.balance:
•
•
Each.wash.cycle.uses.12.68.gallons.of.water.[13].
The.typical.residence.generates.approximately.70.gallons.of.wastewater.per.
person.per.day.[14].
A.four-person.household.does.two.loads.of.laundry.per.day.on.average.
All.the.silver.generated.in.the.washing.machine.enters.the.sewage.
•
•
Further,.the.authors.measured.the.size.of.a.stick.of.gum.at.approximately.0.2.by.1.8.
by.7.2.cm,.assumed.that.the.density.of.a.silver.bar.was.10.4.g/cm
3
.[8],.and.conser-
vatively.assumed.that.the.entire.mass.of.silver.in.the.two.plates.would.be.entirely.
consumed.within.the.3000-cycle.lifetime.
As.a.irst.approximation,.the.amount.of.nanosilver.particles.that.could.enter.a.
wastewater. treatment. plant. from. the. use. of. SilverCare™. in. washing. clothes. could.
range.from.0.001.micrograms.per.liter.(µg/L).to.an.extreme.upper.bound.concentra-
tion.of.9.µg/L.
The.lowest.estimate.is.based.on.the.reported.release.of.0.05.g.silver.
per.year.and.the.assumption.that.only.25%.of.the.mass.would.comprise.nanoparticles.
(rather.than.ions).of.silver.
The.highest.estimate.is.based.on.complete.consumption.
of.the.two.silver.plates.during.the.unit.lifetime.and.the.assumption.that.75%.of.the.
silver.was.in.nanoparticulate.form.
The.actual.concentration.of.nanoparticles.would.
be.lower.than.either.of.these.estimates.due.to.adsorption.and.agglomeration.
Labora-
tory.experiments.with.solutions.of.25-nm.and.130-nm.silver.particles.showed.that.
upon.vortex.mixing,.the.silver.agglomerated.into.particles.ranging.up.to.16.µm.in.
diameter,.well.outside.the.nanoparticle.range.[15].
Further,.the.mass.balance.calcula-
tions.do.not.account.for.dilution.by.sources.of.wastewater.other.than.domestic.sew-
age.from.homes.using.SilverCare™.washing.machines.
Dilution.from.other.sources.
would.also.decrease.the.concentration.of.silver.nanoparticles.
Thus,.the.upper.bound.
estimate.of.9.µg/L.should.be.regarded.as.an.extreme.upper.bound.
What. effect. could. this. discharge. of. silver. have. on. the. microorganisms. in. a.
wastewater.treatment.plant?
As.described.previously,.silver.has.antimicrobial.prop-
erties.
At.the.time.this.topic.was.written,.the.authors.could.not.identify.published.
benchmarks.that.enabled.them.to.directly.compare.the.estimated.discharge.of.silver.
nanoparticles.to.levels.that.are.either.“safe”.or.“toxic”.to.microorganisms.at.a.sew-
age.treatment.plant.
The.acute.ambient.water.quality.criterion.for.silver,.which.was.
not. derived. speciically. for. nanoparticles,. is. 3.2. µg/L. [16].
This. concentration. is.
comparable.to.the.upper.bound.estimate.of.the.discharge.of.silver.nanoparticles.into.
