aop for water and wastewater
DESCRIPTION
AOPTRANSCRIPT
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(ADVANCED) OXIDATION PROCESSES
FOR WATER AND WASTEWATER TREATMENT
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Oxidation
Ultimate aim is to mineralize: convert constituents of an organic pollutant into simple, harmless and inorganic molecules:
• Carbon to carbon dioxide• Hydrogen to water• Phosphorus to phosphates or phosphoric acids• Sulfur to sulfates• Nitrogen to nitrates• Halogens to halogen acids
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Oxidation• Inorganic compounds: removal of electrons to
produce higher oxidation state (e.g. from Fe+2 to Fe+3)
• Organic compounds: combination of C-containing molecules with O2 to produce more heavily oxygenated molecules
Chemical Oxidants and Oxidizing Potentials• Strong oxidizing agents are formed from the
electronegative elements of the top right hand corner of the P.T (F, Cl, Br, O, S)
• Free radicals. Advanced oxidation processes (AOP) are based on insitu generation of free radicals, particularly the hydroxyl radical (·OH).
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Comparison of Oxidation Potentials and
Reactivities of Common Oxidants Species O-Potential
Florine 3.03 V
Hydroxyl radical 2.80 V
Atomic Oxygen 2.42 V
Ozone 2.07 V
H2O2 1.78 V
Perhydroxyl radical 1.70 V
Permanganate 1.68 V
Hypobromous acid 1.59 V
Chlorine dioxide 1.57 V
Hypoclorous acid 1.49 V
Chlorine 1.36 V
Organic Compound
Rate constant (M s-1)
Benzene 2
Toluene 14
Cl-benzene 0.75
3-Cl-ethylene 17
4-Cl-ethylene <0.1
n-Butanol 0.6
t-Butanol 0.03
ozone ·OH
7.8 x109
7.8 x109
4.0 x109
4.0 x109
1.7 x109
4.6 x109
0.4 x109
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FREE RADICALS• A free radical is not an ionic species, but is formed
from an equal cleavage of a two electron bond:
OH:OH ·OH + ·OH• Each ·OH is uncharged and two of them may
recombine to form H2O2 again.• The symbol “·” indicates the radical center and
represents a single unpaired electron.• Free radical formation may be initiated by photolysis,
ozone, hydrogen peroxide, heat (pyrolysis), γ-waves, sonolysis (ultrasound), etc.
• Once initiated, a series of simple reactions will follow.• Complexity of chemistry with FR is due to large
number of reactions that occur within very short time.• Rate of oxidation depends on concentration of
initiating species, pH, pollutant conc, temp, ions and scavengers.
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Advanced Oxidation Processes (AOPs) for Water and Wastewater Treatment
List of AOPs for Water and WW Treatment• Catalysis UV*• Electrochemical UV/H2O2
• Fenton’s Reagent** UV/H2O2/O3**• Ferrate Vacuum UV• Ionizing Radiation Wet Air Oxidation**• Microwave• Photo-Fenton’s Reagent• Photocatalysis• Pulsed Plasma• Supercritical water oxidation**• Ultrasound
* Commercial applications; *** used at full scale
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Areas of Opportunities fotr AOPs
• Groundwater Industrial wastewater
• Odor and VOC’s Industrial sludges• Surface water Municipal wastewater• Swimming Pools Leachates• Water Recyling Municipal sludges• Disinfection Ultrapure
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Amino acids MTBE
Antibiotics Tannery wastewater
Arsenic Municipal sludge
Chromium Pesticide wastewaters
Coliforms* VOCs*
Disinfection byproducts* Paper mill effluent
Distillery wastewater Landfill leachate*
Drug residues* Taste and odor causing compounds
Glass fiber wastewater Grey water
Hospital wastewater Rubber process wastewater
Insecticide Chemical specialities wastewater
Kraft bleaching wastewater Humic materials
Natural organic matter* Stilbene derivatives
Nickel plating wastewater Cyanide
Oilified wastewater Escherichia coli*
Olive mill wastewater Municipal ww treatment plant effluents
Parasites Urine
Phenolic wastewater* Pesticides
Printing wastewater Endocrine disruptors*
Seed corn wastes Phenolic resins*
X-ray contrast media Spent caustic
Trinitrotulene (TNT) Textle dyebath effluent*Co
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Greywater: Any water that has been used in the home, except water from toilets, is called greywater. Dish, shower, sink, and laundry water comprise 50-80% of residential "waste" water. This may be reused for other purposes, especially landscape irrigation.
Stilbene Derivatives: Aromatic hydrocarbons such as C14H12 used as a phosphor and in making of dyes.
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Advantages of AOP
• Effective in removing resistant organic compounds
• Capable of complete mineralization of organic compounds to CO2.
• Not susceptible to the presence of toxic chemicals
• Generally produce innocuous end products
• Can be used to pretreat toxic compounds so that they can be bio-treated
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Applications
O3/H2O2 has gained the widest acceptance
because of effectiveness and low cost.
H2O2/UV has the advantage of simplicity (only
chemical is H2O2, cheap and soluble). Suited
to small, minimum maintenance or intermittent
operation systems. Some problems if
materials in water absorb UV.
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O3/UV considered less favorable because of
high pH requirement (chemical costs) but
okay for low flows.
Least used are the TiO2 systems although
they have some advantages such as
photocatalysts made be used, natural light
may be used as a UV source, additional
radical initiators are not required.
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One of the operating issues that must be taken into account is the quantity of hydroxyl radical produced versus oxidant consumed. The following table list these theoretical amounts.
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The following tables compare the
effectiveness
of various AOP’s on the degradation of
phenol (approx 100 mg/L).
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Process pH Time (min)
%
Reduction
O3 5 – 3.4 80 80.6
O3 5 – 3.4 80 58.3
O3 6.8 80 90.4
O3/UV 6.8 80 80.9
O3/UV 9.3 80 92.5
O3/UV 9.3 80 88.8
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Process pH H2O2
(mM)
Fe(II)
(mM)
Time
(min)
% Reduction
Fenton 5-3 1.07 0.054 9 32.2
Fenton 5-3 2.45 0.054 9 58.0
Fenton 5-3 5.34 0.054 9 90.0
Fenton 5-3 10.7 0.054 9 100
Fenton 5-3 2.45 0.13 9 84.7
Fenton 5-3 2.45 0.26 9 87.2
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Process pH TiO2
(mM)
Time
(min)
%
Reduction
Photocatalysis ~6 0.05 150 42.2
Photocatalysis ~6 0.2 150 58.6
Photocatalysis ~6 0.6 150 74.4
Photocatalysis ~6 0.8 150 73.1
Photocatalysis ~6 2 150 73.4
Photocatalysis ~6 5 150 74.6
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Approximate cost per kg of phenol destruction:
Process Cost ($/kg)
O3/H2O2 2.93
O3/UV 11.7
O3 1.09
UV/H2O2 28.7
Fenton 2.61
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Because the chemistry of wastewater matrices
can be very different pilot testing is almost always
required to test the technical feasibility of a
specific AOP for a specific wastewater.
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1- UV Photolysis• Defn: Electromagnetic radiation of wavelength between 4-
400 nm.
• Region of interest in AOP applications: UVC, where both pollutants and water constituents absorb the radiation.
• Vacuum UV (VUV) is of particular interest, because water absorbs in this region
H2O + hν (λ<190 nm) H + OH
VUV UVC UVB UVA Visible Near IR
100 200 280 315 400 700 1000
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Quantum YieldFor direct UV photolysis of a pollutant P:
Determination: Chemical Actinometer (A chemical system or compound, which undergoes a well characterized photochemical reaction with a known quantum yield.
Most frequently used actinometers are:
Potassiumferrioxalate, potassiumpersulfate, uranyloxalate, iodide/iodate and urine.
Pabsorbedbynsofmolesphoto
sformedMolesPtran
)(
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Electrical Energy Per OrderDefn: the electrical energy in kWh to degrada a
contaminant C by one order of magnitude in a unit
volume of 1000 L:
Batch Operation:
Flow-through Operation:
P=rated power of the UV system (kW)
V=volume of water treated in the reactor in time t (L)
ci,cf=initial and final concentrations of pollutant (M)
t= time (h)
F=water flow rate (m3/h)
fiEO CCV
PtE
/
1000
fiEO CCF
PE
/
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Electrical Energy Per Mass
fiEM CCVM
PtE
/
1000Batch:
Flow-through: fiEM CCFM
PE
/
1000
Defn:Electrical energy (kWh) to degrade a unit mass of a contaminant C
M=molar mass of C.