AOP for Water and Wastewater

(ADVANCED) OXIDATION PROCESSES

FOR WATER AND WASTEWATER TREATMENT

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

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).

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

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.

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

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

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

nta

min

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an

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aste

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OP

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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.

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

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.

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.

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.

The following tables compare the

effectiveness

of various AOP’s on the degradation of

phenol (approx 100 mg/L).

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

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

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

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

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.

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

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

)(

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

/

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.