14. hazardous waste treatment
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Hazardous Waste
Treatment
Chemical TreatmentPhysical/Chemical Treatment
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Chemical Treatment It does not make the hazardous substances
disappear
It converts the hazardous substance into to
another form that is at least less toxic than
the original substance
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Chemical TechniquesNEUTRALIZATION
OXIDATION
PRECIPITATION
REDUCTION
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NEUTRALIZATION A waste is considered hazardous if,
among other things, the pH is less
than 2 or greater than 12.5
Good treatment practice requires that
the final pH should be at 6-8 toprotect the natural biota
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Sulfuric (H2SO4) or hydrochloric
(HCl) acid is added to basic solutions
to reduce the pH
Caustic (NaOH) or slaked lime
[Ca(OH)2] is added to acidicsolutions to increase pH
NEUTRALIZATION
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OXIDATION Via the use of chlorine and ozone
as oxidizing agents
Via electrolytic oxidation
Via wet air oxidation (also known
as the Zimmerman Process)
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Wet Air Oxidation It is potentially the most widely
applicable of all chemical oxidation
methods
It operates on the principle that most
organic compounds can be oxidizedby oxygen given sufficienttemperature and pressure
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It is the aqueous phase oxidation of
dissolved or suspended organic particles at
temperatures of 175 to 325C and
sufficiently high pressure to preventnt
excessive evaporation
Air is bubbled through the liquid
Wet Air OxidationWet Air Oxidation
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The process is fuel efficient
Once oxidation has begun, the
process is usually self-sustaining
In many instances, the addition ofmetal salt catalysts can increase the
destruction efficiency or allow theprocess to run at lower temperatureor pressure
Wet Air Oxidation
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PRECIPITATION The goal of precipitation is to remove
hazardous substances from solution by
reducing the solubility and precipitating the
substances
This is often applied to metals
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REDUCTION
Although most heavy metals readily
precipitate as hydroxides, hexavalentchromium (Cr+6) must be reduced to its
trivalent form (Cr+3) before it will
precipitate
Reduction is usually done with sulfur
dioxide (SO2) or sodium sulfite (NaHSO3)
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3SO2 + 2H2CrO4 Cr2(SO4)3 +2H2O
Because the reaction proceeds rapidly at
low pH, an acid is added to control the
pH
REDUCTION
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Chemical/Physical Treatment The waste is not detoxified but only
concentrated for further treatment or
recovery
Several treatments are used to separate
hazardous waste from aqueous solution.
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Chemical/Physical Techniques CARBON ADSORPTION
DISTILLATION ION EXCHANGE
ELECTRODIALYSIS
REVERSE OSMOSIS
SOLVENT EXTRACTION
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Adsorption is a mass-transfer process in
which gas vapors or chemicals in solution
are held to a sold by intermolecular forces
(e.g. hydrogen bonding and van der
Waals interactions)
It is a surface phenomenon
CARBON ADSORPTION
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Pressure vessels having a fixed bed are
used to hold the adsorbent
Common adsorbents:
o Activated carbon
o Molecular sieves
o Silica gel
o Activated alumina
CARBON ADSORPTION
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The active sites become saturated at
some point in time
If the adsorbed organic material hascommercial value the bed is
regenerated by passing stream through
it
The vapor-laden stream is condensed
and the organic fraction is separated
from water
CARBON ADSORPTION
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If the organic compounds have no
commercial value, the carbon may either be
incinerated or shipped to the manufacturer
for regeneration
CARBON ADSORPTION
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It is the separation of more volatile
materials from less volatile ones by a
process of vaporization When a liquid mixture of two or more
components is brought to the boiling
point of the mixture, a vapor phase is
created above the liquid phase
DISTILLATION
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If the vapor pressure of the purecomponents are different then the
constituents are different having thehigher vapor pressure will becomemore concentrated in the vapor phasethan the constituents having the lowervapor pressure
If the vapor phase is cooled, a partialseparation of the constituents willresult
DISTILLATION
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ION EXCHANGE In ion exchange, the waste stream
containing the ion to be removed is
passed through a bed of resin The resin is selected to remove either
cations or anions
In the exchange process, ions of likecharge are removed from the resin
surface in exchange for ions in solution
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When the bed becomes saturatedwith the exchanged ion, it is shut
down and the resin is regenerated bypassing a concentrated solutioncontaining the original ion (typicallyhydrogen or sodium) back throughthe bed
The exchanged pollutant is forcedoff the bed in a concentrated form
that can be recycled
ION EXCHANGE
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During the ion exchange, the normal
flow pattern is downward through the
bed Because the surface of the bed acts like a
filter, the regeneration is often
countercurrent, that is, the regeneration
solution is pumped into the bottom of
the column and the flow is upward
ION EXCHANGE
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The electrodialysis unit uses amembrane to selectively retain or
transmit specific molecules The membranes are thin sheets of
ion exchange resin reinforced by a
synthetic fiber backing Cation membranes are placed
alternate with anion membranes
ELECTRODIALYSIS
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An electric potential is applied across
the ion to provide the motive force for
ion migration
Cation membranes permit passage of
only positively charged ions, while
anion membranes only allownegatively charge ions to pass
ELECTRODIALYSIS
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Osmosis is the spontaneoustransport of a solvent from a dilute
solution to a concentrated solutionacross an ideal semipermeablemembrane that impedes thepassage of the solute but allows thesolvent to flow
Solvent flow can be reduced byexerting pressure on the solution
side of the membrane
REVERSE OSMOSIS
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If the pressure is increased above the
osmotic pressure on the solution
side, the flow reverses
Pure solvent will then pass from the
solution into solvent
REVERSE OSMOSIS
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Also called liquid extraction
In the solvent extraction process,
the solvent and the waste stream are
mixed to allow mass transfer of the
constituents form the waste to the
solvent
SOLVENT EXTRACTION
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The wastewater is contacted with a
solvent having a greater solubility
for the target contaminants than thewastewater
The contaminants will tend migrate
from the wastewater into the solvent
SOLVENT EXTRACTION