(PDF) REMOVAL OF MICRO-POLLUTANTS FROM DRINKING WATER … - PDFSLIDE.NET (2024)

Fabian Sander

REMOVAL OF MICRO-POLLUTANTS FROM DRINKING WATER WITH ADVANCED OXIDATION PROCESSES

Thesis CENTRAL OSTROBOTHNIA UNIVERSITY OF APPLIED SCIENCES Degree Programme in Chemistry and Environmental Technology April 2009

I

ACKNOWLEDGEMENT

This work was entirely performed at ITT Wedeco´s research and development

facility in Herford, Germany. I would like to express my gratitude to all my co-

workers who supported me during this time from June 2008 until March 2009. The

R&D department with Dr. Achim Ried, Dr. Jörg Mielcke, Mike Bromen, Arne

Wieland, Stephan Baeck, Mechthild Rediker, Gesa Kruse, Jürgen Vogt, Khalid Al

Barim and especially Jens Krüger for his willingness to support my ideas. I want to

thank Joachim Anton and Friedhelm Krüger from the computational fluid design

team for the good co-operation and interest into my “new perspectives” of gas

diffusion. Furthermore I would like to thank Niels Peters, the master of the

warehouse, for his endless support of providing me plastic pipes for the practical

part of the work. Also, what would life be without food? Therefore I would like to

thank the staff restaurant team, Doris and Doris for their cuisine.

I want to thank David Owen from O.I.Filtration Ltd, Stoke on Trent, England, who

supplied a stone diffuser sample to us. In addition I want to thank Rob Jerman

from Markel Corporation, Plymouth Meeting, USA, for his tubing samples.

This work was supervised by MSc Jana Holm and MSc Esko Johnson from the

Central Ostrobothnia University of applied Sciences; I want to thank both for their

time and feedback regarding this work. Tack! Kiitos!

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THESIS ABSTRACT

Department

Technology and Business,

Kokkola, Finland

Date

30.04.2009

Author

Fabian Sander

Degree Programme

Degree Programme for Chemistry and Environmental Technology

Name of Thesis

Removal of micro-pollutants from drinking water with advanced oxidation

processes.

Instructor

Jens Krüger and Dr. Achim Ried

Pages

96 + Appendices(2)

(57 confidential version)

Supervisor

MSc Jana Holm

This thesis determines the feasibility of ozone gas and ultraviolet (UV)

technologies to reduce pollutants from drinking water. The merging of both

techniques, ozone and UV, is known as a so called advanced oxidation process

(AOP). ITT Wedeco is developing new technologies to meet the growing concern

of water works to remove micro-pollutants from their water.

The emphasis of this work was put on the formation of the oxidation by-product

bromate and the removal of the solvent 1.4 dioxane from drinking water. Three

different ozone gas diffusing technologies were compared and evaluated. An

injector was compared to a gas diffuser and micro porous tubing. One part was the

reduction of the pollutant, but the principal item of this pilot study was to control the

formation of bromate, due to the immediate destruction of ozone gas through

ultraviolet light into hydroxyl radicals.

Key words

Advanced oxidation process, drinking water, oxidation byproducts, ozone, UV,

micro-pollutants, bromate, 1.4 dioxane.

II

TABLE OF CONTENTS

LIST OF GRAPHS I

LIST OF TABLES I

LIST OF ABBREVIATIONS III

1 INTRODUCTION 1

2 THEORY OF ADVANCED OXIDATION PROCESS 4

2.1 Introduction to water pollutants 4 2.1.1 Micro pollutants 5

2.1.2 Solvent 1.4 dioxane 7 2.1.3 Transformation and persistence 9

2.2 Ozone 10 2.2.1 Chemistry of oxidation 12

2.2.2 Oxidation potential 12 2.2.3 Applications of ozone 14

2.2.4 Ozone generation from oxygen 15 2.2.5 Reaction properties 17

2.2.6 Scavengers of the hydroxyl radical 19 2.2.7 Ozone decomposition 19 2.2.8 Diffusion into water 20

2.3 UV radiation 22 2.3.1 Introduction to ultraviolet radiation 22

2.3.2 UV spectrum 24 2.3.3 Effect on micro organism 25 2.3.4 Dose of ultraviolet light 26 2.3.5 Ultraviolet lamps 27

2.3.6 Low pressure high output amalgam lamps 29 2.3.7 Medium pressure lamps 29

2.3.8 Effects of water quality on UV efficiency 31

2.4 Advanced oxidation processes 32 2.4.1 Ozone destruction by UV 33 2.4.2 Ozone photo dissociation 34 2.4.3 Hydroxyl radical formation 35

2.4.4 Comparison of ozone to hydrogen peroxide 38 2.4.5 Low pressure and medium pressure lamp differences 39 2.4.6 UV photolysis 40 2.4.7 Example of ozone destruction through ultraviolet light 40 2.4.8 Applications for AOP 40 2.4.9 Advantages and disadvantages of AOPs 41

III

2.4.10 Parameters affecting the AOP efficiency 44

2.5 Oxidation by-products 45 2.5.1 Bromate 45

2.5.2 Formation of bromate 46 2.5.3 Formation pathways 48

3 AOP SYSTEM AND PERFORMANCE 50

3.1 Pre examinations 50 3.1.1 The initial design of the reactor 50

3.1.2 Conclusions after first test run 50 3.1.3 Injector 50

3.1.4 Diffuser 51 3.1.5 Tubing 54 3.1.6 Determining bubble size of injector, diffuser and tubing 54 3.1.7 The first performance test: co-current 55 3.1.8 The first performance test: counter-current 55

3.1.9 Bubble contact time with UV radiation 56

3.1.10 Compounds of the system 56 3.1.11 Sampling 56

3.2 Results of pre study 57

4 DEGRADATION OF 1.4 DIOXANE AND BROMATE CONTROL 57

4.1 Final pilot runs 57

4.1.1 Sampling amounts and procedure 57

4.2 Results 57

5 CONCLUSIONS AND RECOMMENDATIONS 57

REFERENCES 58

APPENDIX 1 62

APPENDIX 2 64

i

List of graphs

GRAPH 1. The distribution of the world´s water resources (USGS 1996, courtesy of the U.S. Geological Survey) 1 GRAPH 2. Spiking of water 6 GRAPH 3. Generation of ozone from oxygen feed gas (Wedeco 2008) 15

GRAPH 4. Ozone electrode and ozone purification system (Wedeco 2008) 17 GRAPH 5. The spectrum of electromagnetic radiation (Treiman 2008, courtesy of USRA Lunar and Planetary Institute) 24 GRAPH 6. Wavelengths between 100 and 780 nm (Wedeco 2008, courtesy of ITT Wedeco Corporation) 25

GRAPH 7.Effects of UV on the DNA of micro organism (NASA 2009, courtesy of NASA) 26

GRAPH 8. Emission of electrons (Wedeco 2008, courtesy of ITT Wedeco Corporation) 28 GRAPH 9. UV lamp during operation 28 GRAPH 10. Peaks of LP and MP lamps (Bolton 2008, courtesy of AWWA) 30 GRAPH 11. The molar extinction coefficient of ozone (Kenshi 2001) 35

GRAPH 12. Hydroxyl radical pathways (Ried 2005) 37 GRAPH 13. Ozone destruction by UV-C light 40

GRAPH 14. The bromate formation pathway (Yasunaga 2005, courtesy of Mitsubishi Electric Corporation) 49

GRAPH 18. Injector venturi effect 51 GRAPH 19. Injector during performance 51 GRAPH 20. Diffuser built into the rector 52

GRAPH 21. Fine bubble distribution of ceramic diffuser 53

GRAPH 22. Surface energy dependency of bubbles (Burries 1999, courtesy of Alan Burries, Alabdiffusers) 53 GRAPH 23.Tubing built into reactor 54

GRAPH 25. Diffuser during gas flow rates of 20 l/h 55 GRAPH 26. Bubbles during gas flow rates of 20 l/h 55

GRAPH 27. Co-current flow injector arrangement 55 GRAPH 28. Co-current flow diffuser arrangement 55 GRAPH 29. Counter current arrangement 55

GRAPH 30. Contact time for co-current and counter-current arrangements 56

List of tables

TABLE 1. Inorganic contaminants and their MCL (USEPA 1995) 6 TABLE 2. Organic contaminants and their MCL (USEPA 1995) 7 TABLE 3. Fact sheet of 1.4 dioxane (USEPA 1995) 9 TABLE 4. Physical properties of ozone (Perry 1997) 11 TABLE 5. Standard redox potential of some oxidant species (Perry 1997) 13

TABLE 6. Typical industrial applications of ozone (Wedeco 2008) 14 TABLE 7. Ozone conversion table (Perry 1997) 16 TABLE 8. Comparison of LP and MP lamps (Bolton 2008) 30 TABLE 9. Advanced oxidation technologies 32 TABLE 10. Advantages and disadvantages of AOPs 43

ii TABLE 11. Description of drinking water disinfection by-products 45 TABLE 12. Comparison of gas diffusing technologies 55 TABLE 14. Determined properties during performance 56

TABLE 17. Measured parameters during AOP performance 62 TABLE 18. Parameters calculated according to the measured values 64

iii

List of abbreviations

AOP Advanced oxidation process

AOT Advanced Oxidation Techniques

AOX Adsorbable Organohalogens

COD Chemical Oxygen Demand

COD Catalytic Ozone Destructor

DBD Dielectric Barrier Discharge

DOC Dissolved Organic Carbon

EDC Endocrine Disrupting Compounds

EEO Electrical Energy per Order

GAC Granulated Activated Carbon

H2O2 Hydrogen Peroxide

LOX Liquid Oxygen

LP Low Pressure UV lamp

MCL Maximum Contaminant Level

MP Medium Pressure UV lamp

MTBE Methyltertbutylether

NDMA N-nitrosodymethylamine

NOM Natural Organic Matter

NTP Normal Temperature Pressure

PPM Parts Per Million

PSA Pressure Swing Adsorption

RO Reverse Osmosis

TCE Trichloroethylene

THM Trihalomethane

TOC Total Organic Carbon

TSS Total Suspended Solids

USEPA United States Environmental Protection Agency

UV Ultraviolet

UVT Ultraviolet Transmission

WHO World Health Organisation

1

1 INTRODUCTION

No life on our planet would exist without water. Our planet’s surface is covered by

71% with water, which is equal to 1,386,000,000 billion m³. Only a fraction of that

amount can be used for consumption, as only 3% is fresh water.

GRAPH 1. The distribution of the world´s water resources (USGS 1996, courtesy

of the U.S. Geological Survey)

Fresh water is a carrier for minerals and organic substances which are essential

for flora and fauna. These substances such as salts, calcium carbonate, sodium,

nitrogen, carbon dioxide, etc. are part of a natural water matrix, but unfortunately

water can also carry undesired substances.

Only less than 5% of the commercially distributed water is consumed or used in

food preparation. This so called drinking water is of sufficient high quality which

can be consumed without risk of immediate or long term harm. Drinking water

quality is determined under two categories: physical/chemical and microbiological.

Physical/chemical parameters include heavy metals, trace organic compounds,

total suspended solids (TSS), and turbidity. Microbiological parameters include

Coliform bacteria, E. coli, and specific pathogenic species of bacteria, viruses, and

protozoan parasites. The physical/chemical parameters are associated to a more

chronic health risk than microbiological. Mostly produced by mankind, these trace

2 organic compounds such as pesticides, endocrine disrupting compounds (EDC),

by-products of industrial productions and other toxic synthetic products cause

increasing problems to natural water systems. Once released into the

environment, these elements blend into surface water and eventually end up

polluting lakes, rivers and ground water reservoirs. The amount of these

compounds in water is rather low. Concentrations of micro-pollutants range in milli

(m), micro (µ), or even nano (n) grams per liter.

Nevertheless, they are considered pollutants due to the long term toxicity on

human and animals. While short term consumption of such polluted water would

not be expected to cause problems on human, the long term consumption of

polluted drinking water is a contentious issue. The world health organisation

(WHO) and national environmental authorities like the United States

Environmental Protection Agency (USEPA) have released recommendations such

as the maximum contaminant level (MCL) which is the legal threshold limit on the

amount of a hazardous substance that is allowed in drinking water. This MCL is

applied for pesticides, solvents and disinfection by products and is usually

expressed as a concentration in milligrams or micrograms per liter of water.

To achieve such low concentrations in drinking water, water distributors need to

treat the product before it is released into the water network. As to the removal of

micro pollutants, this treatment step often requires very high efforts, due to the

complex molecular structure of most pollutants. In most cases, the typical

treatment by bacteria biodegradation is not successful, since absorption by

activated carbon does not achieve the desired results either and reverse osmosis

(RO) leaves the problem of a concentrate. The destruction of the undesired

molecule into less harmful substances would therefore be ideal.

The so called advanced oxidation process (AOP) is capable of breaking the

molecular bonds within the pollutant molecule and therefore crack it into smaller,

less harmful parts. Ideally, this would be carbon dioxide and water. In most cases

the oxidation of undesired molecules will create less harmful substances, but

unfortunately this is not always the case.

3

This work determines the degradation of a solvent called 1.4 dioxane, which is

fortunately known to break into less harmful fragments after oxidation. Such

advanced oxidation technologies (AOT) are capable of destroying pollutants due

to the generation of very reactive radicals. Hydroxyl radicals (•OH) are a

connection of oxygen and hydrogen atoms by a covalent bond, and highly reactive

with any substance present in water. The generation of such radicals can be

initiated through different pathways such as the combination of ultraviolet light

(UV) with ozone, UV with hydrogen peroxide (H2O2) or hydrogen peroxide and

ozone.

(CONFIDENTIAL)

4

2 THEORY OF ADVANCED OXIDATION PROCESS

2.1 Introduction to water pollutants

Water pollution is estimated to be the leading cause of deaths and diseases

worldwide and is expected to account for the deaths of 14,000 people per day.

Water is typically referred to be polluted when it is impaired by manmade

contaminants and does not support human use or negatively affects its constituent

part for animals and plant life. These contaminants can affect human health on a

short term basis such as micro organism pollution; but also long term

contamination with substances such as heavy metals or chemicals ranging in low

concentrations in water. These micro pollutants can cause chronic poisoning or

support the chance of cancer if consumed by humans through drinking water.

(USEPA 1994.)

This thesis work about the technology of advanced oxidation processes is

specialized to pollutions ranging in very low concentrations in water. Since the late

eighties of the last century, pesticides are found in raw water sources. The issue

has caused increasing attention during the last years, with ongoing debates about

maximum contaminant levels in the EU. They were encountered in surface water

and bank filtrate, as well as in ground water. This development triggered the water

supply companies to search for solutions in the removal / degradation of the

pesticides in order to meet the European standard. The Netherlands are affected

by this problem in particular, since most of the drinking water is obtained from

surface water bank filtrate. Surface water has the highest tendency of being

polluted. Some water works have started considering the reduction of micro

pollutants for the sake of their customers even without any mandatory reduction

level by the authorities.

5 ITT Wedeco has started pilot plant co-operations with water suppliers in the

Netherlands for the reduction of such pollutants by either ozone-hydrogen

peroxide, UV-ozone or UV-hydrogen peroxide AOPs. In general, the pesticide

concentration can be lowered by oxidation, granulated activated carbon (GAC)

and membrane filtration. Disadvantages of oxidation is the formation of smaller

degradation products and the relatively high costs if the process is applied for

pesticide control only. The advantage of oxidation processes may be the

combination of disinfection and degradation of organic micro-pollutants.

Optimization of the process conditions with respect to the desired degree of

degradation of micropollutants and by-products formation is needed. (Ijpelaar

2001.)

2.1.1 Micro pollutants

As can be referred from the word micro (µ), micro pollutants range in

concentrations of parts per million (ppm). Such pollutants are therefore hard to

detect, do not cause immediate intoxication, are often difficult to remove and,

unfortunately, are expensive to treat. The removal requires very high dosages of

treatment material. In the case of ultraviolet radiation with ozone, this would be a

high intensity of light flux (J/m²) and ozone gas (g/m³). A 99.99% (4log)

deactivation of micro-organisms from water by UV light would typically require a

dose of 400 J/m², whereas the 90% (1log) removal of a micro pollutant such as 1.4

dioxane would require 10000 J/m² and a second media such as ozone to achieve

a reduction of the desired pollutant. (Wedeco 2008.)

This problem of high treatment dosages is eventually a commercial one. The

treatment cost of one cubic meter of water will increase significantly if micro-

pollutants are being removed. Municipal water works try to keep the treatment

costs as low as possible, meaning that only obliged pollutants are being removed.

Graph 2 gives an example of spiking a pollutant into water for degradation tests.

To obtain the concentration of 80 µg/l, the content of this pipette, 1.4 dioxane, was

diluted with 2000 l of water; this is equal to 1000 beakers in the graph.

6

GRAPH 2. Spiking of water

The following table provides information about common water impurities, as well

as the related effect on humans from ingestion. The threshold value in drinking

water is usually expressed as the maximum contaminant level (MCL), with the

exception of micro organism control which is expressed in percent removal from

the initial concentration.

TABLE 1. Inorganic contaminants and their MCL (USEPA 1995)

Inorganic chemicals

Potential health effects from ingestion of water

MCL (mg/l)

Sources of contaminant

Arsenic Skin damage, problems with circulatory systems, risk of getting cancer.

0.010 Runoff from factories.

Cadmium Kidney damage. 0.005 Discharge from metal refineries.

Cyanide Nerve damage or thyroid problems.

0.2 Discharge from steel, metal, plastic factories.

Mercury Kidney damage. 0.002 Discharge from refineries and factories, runoff from landfills and croplands.

7 TABLE 2. Organic contaminants and their MCL (USEPA 1995)

Organic chemicals

Potential health effects from ingestion of water

MCL (mg/l)

Sources of contaminant

Atrazine Cardiovascular system or reproductive problems.

0.003 Runoff from herbicide used to grow crops.

Benzene Anemia; decrease in blood platelets; increased risk of cancer.

0.005 Discharge from factories; leaching from gas storage tanks and landfills.

Polychlorinated biphenyls (PCBs)

Skin changes, immune deficiencies; reproductive or nervous system difficulties, risk of cancer.

0.0005 Runoff from landfills; discharge of waste chemicals.

Tetra chloroethylene

Liver problems; increased risk of cancer.

0.005 Discharge from factories and dry cleaners.

Dioxin Reproductive difficulties; increased risk of cancer.

0.00000003

Emissions from waste incineration and other combustion; discharge from chemical factories.

1,4 dioxane Damage to liver and kidneys.

No limit

Solvent for manufacture of textile, cosmetics, shampoos, paper and cotton.

2.1.2 Solvent 1.4 dioxane

The solvent 1.4 dioxane is a manmade compound primarily used as an industrial

solvent or solvent stabilizer that prevents the breakdown of chlorinated solvents

during manufacturing processes. C4H8O2, 1.4 dioxane, also called dioxane, dioxan,

p-dioxane, diethylene dioxide, diethylene oxide, diethylene ether or glycol ethylene

ether, is a colourless, flammable, and oily liquid with the melting point of 11.8°C

and boiling point of 101°C. (Heiland 2004.)

One remarkable property is the excellent dissolving power of solvents. Due to

these properties, it is commonly used as a solvent stabilizing agent. It is an

8 ingredient in paints, varnishes, detergents, cements, stains, inks, cosmetics and is

a natural component in vine-ripped tomatoes and tomato products, fresh shrimps,

brewed coffee and fried chicken. It is also present in ordinary household products

like shampoos, liquid/ dishwashing soap, baby lotion, hair lotions, bath foam, and

other cosmetic products. (Zenker 2003.)

Other solvent applications include those for dyes, fats, greases, lacquers, mineral

oil, paints, resins and waxes. It can be blended with water and most organic

solvents in any kind of ratio. On the other hand, dioxane has a high stability and is

not reactive in chemical reactions. Therefore it can be used in chemical reactions,

in determining the optical activity and for cryoscopy and it is formed as a by-

product during the manufacture of polyester and various polyethoxylated

compounds. (Heiland 2004.)

Dioxane is commonly used to dissolve acetyl cellulose, cellulose ether and

chlorinated rubber. Furthermore it is used for extraction of animal and plant tissue.

It can form an addition reaction with several organic and inorganic substances,

whereas the bonding is formed through van der Waals forces, donor-acceptor, or

hydrogen bonds. In high concentrations, dioxane is narcotic to human and causes

severe damage to kidney and liver. In vapour, it irritates eyes and the respiratory

tract. In liquid phase, it can be absorbed by human skin. It is expected to be

human carcinogen by the USEPA and therefore classified as group B2 (probable

human carcinogen). A person must drink 2 liters per day of water over the action

level of 3 µg/l for a period of 70 years in order to generate one additional cancer

case out of a million people. (Zenker 2003.)

The long term effects on animals were examined in laboratory trials. Rats, which

were exposed to 1.4 dioxane in their drinking water, showed damage to liver and

kidneys. Short-term inhalation exposure to high levels of 1.4 dioxane has caused

vertigo, drowsiness, headache, anorexia and irritation of the eyes, nose, throat,

and lungs in humans. It may also irritate the skin. (USEPA 1995.)

9 Dioxane is highly soluble in groundwater, does not readily bind to soils, and

accidental solvent spills have resulted in the contamination of groundwater with

1.4 dioxane. It can remain in groundwater for several years and has caused

increasing attention to the public. Dioxane has been detected at concentrations of

1–220,000 µg/l in groundwater, 1-290 µg/l in surface water and 0.1-2,100 µg/l in

drinking water. The EPA has not yet established a federal drinking water standard

or maximum contaminant level for 1.4 dioxane, but some state level Environmental

Protection Agencies in the US have established state drinking water guidelines

which vary between 3 µg/l in California and 70 µg/l in Maine. (USEPA 1995.)

To treat waters containing 1.4 dioxane, volatilization and sorption are not

significant treatment mechanisms due to 1.4 dioxane's complete miscibility with

water. Advanced oxidation processes, and therefore the destruction of molecular

bonds, are the only proven technology for 1.4 dioxane treatment at the moment. In

North America, there are several known ground water remediation sites using

either UV-H2O2 or ozone-H2O2 to treat 1.4 dioxane. (USEPA 1995.)

TABLE 3. Fact sheet of 1.4 dioxane (USEPA 1995)

Property Value

Common synonyms 1.4 diethylene oxide

Molecular formula C4 H8 O2

Physical state Flammable liquid, faint pleasant odor

Molecular weight 88.10 g/mol

Melting point 11.80ºC

Boiling point 101.1ºC at 760 mm Hg

Density 1.0329 g/ml at 20ºC

Henry's law constant 4.88 x 10-6 atm m3/mol

2.1.3 Transformation and persistence

In soil, the low estimated soil-sorption coefficient will cause that 1.4 dioxane

readily leach to ground water. The estimated Henry’s Law constant suggests that

10 volatilization from moist soils will be slow; however, based on its vapor pressure,

volatilization from dry soils should be fast. 1.4 dioxane is not expected to

biodegrade in soil. (Howard 1989.)

In water, ethers in general have been classified as generally resistant to

hydrolysis. Therefore 1.4 dioxane is not expected to hydrolyze significantly. The

estimated Henry's Law constant for 1.4 dioxane and its miscibility in water suggest

that volatilization will be slow. It is not expected to significantly adsorb to

suspended sediments and is not expected to biodegrade in water. (Howard 1989.)

2.2 Ozone

Ozone is a gas consisting of three oxygen atoms which is highly reactive. It is

formed naturally in the upper zones of the atmosphere (25 km above sea level)

where it surrounds the earth and protects the surface of our planet from UV-B and

UV-C radiation which are dangerous to human. In the atmosphere close to the

surface, ozone is unfortunately a toxic gas with a maximum contaminant level of

0.1 parts per million (ppm) for an exposure of 8 h. Exposure of 0.1 to 1 ppm

produces headaches, burning eyes, and irritation to the respiratory passages.

(Carrithers 1997.)

On the other hand, due to the reactive properties of ozone, it became popular for

the treatment of water, medicine, organic chemical synthesis, etc. as an

environmental friendly treatment technology. The reactivity is due to the structure

of the molecule. Each oxygen atom has the following electronic configuration

surrounding the nucleus: 1s² 2s² 2px2 2py

1 2pz1. The valence band has two

unpaired electrons, each one occupying one 2p orbital. In order to combine three

oxygen atoms and yield the ozone molecule, the central oxygen rearranges in a

plane sp² hybridation from the 2s and two 2p atomic orbitals of the valence band.

With this rearrangement the three new sp² hybrid orbitals form a triangle. The high

11 reactivity can be attributed to the electronic configuration of the molecule.

(Carrithers 1997.)

Ozone has gained increased attention for the disinfection of water since the

discovery of trihalomethanes (THM), due to chlorination of natural substances

present in raw water, in the late 1970s. The search began for alternative oxidant

disinfectants that could play the role of chlorine without generating the problem of

THM formation. (Beltran 2004.)

Studies have shown that organohalogen compounds formed in the treatment of

surface waters with chlorine and other chlorine derived oxidant disinfectants cause

a higher disinfection by-product risk than ozone. However, chlorine is not the only

concern in water quality. Other compounds are often discharged into natural

waters or in soils and eventually leach into the aquatic system, resulting in

contamination of wells and aquifers. Ozonation or hydroxyl radical oxidant based

processes have proven to be efficient technologies for the removal of such

pollutants. (Beltran 2004.)

Table 4 illustrates the physical properties of ozone. Under normal conditions,

ozone is not present in gases, but might occur from the destruction of oxygen. This

topic will be introduced in the following section.

TABLE 4. Physical properties of ozone (Perry 1997)

Property Value

Melting point, °C -251

Boiling point, °C -112

Molar mass 47.998 g/mol

Solubility in water 0.105 g / 100ml (0 °C)

Specific gravity 1.658 higher than air

Density 2.144 g/l (0 °C), gas

12 2.2.1 Chemistry of oxidation

The term oxidation corresponds to the transfer of one or more electrons from an

electron donor (reductant) to an electron acceptor (oxidant). Both the oxidant and

the reductant are chemically transformed. Species with an odd number of valence

electrons are produced as well, which are known as radicals. Radicals are highly

unstable and highly reactive because one of their electrons is unpaired which is

represented by a dot next to the chemical structure, for example the hydroxyl

radical (•OH). Radicals that were produced in oxidation reactions tend to react

further between radical oxidants and other organic and inorganic reactants until

thermodynamically stable compounds are formed. The oxidation potential

measures the ability of an oxidant to initiate reactions. The end products of

complete oxidation (mineralization) of organic compounds such as MTBE or 1.4

dioxane are carbon dioxide (CO2) and water (H2O). (Beltran 2004.)

2.2.2 Oxidation potential

The oxidation potential, expressed in electron volts, is an indicator of how strongly

a compound is potentially forming a oxidation reaction with other molecules. The

higher the Eo value, the more reactive the species, but also the more difficult the

handling of such reactions will become. The following table presents the oxidation

potential of the most oxidative species known. (Perry 1997.)

Fluorine has the highest oxidation potential, but for commercial applications, this

compound is not suitable due to the high reactivity. The next available compound

is the hydroxyl radical, which is produced during the reaction of ozone and UV.

Ozone alone has a lower potential to oxidize species, but it is still relatively high.

13 TABLE 5. Standard redox potential of some oxidant species (Perry 1997)

Oxidant species Eo, Volts Relative potential to ozone

Fluorine 3.06 1.48

Hydroxyl radical 2.80 1.35

Atomic oxygen 2.42 1.17

Ozone 2.07 1.00

Hydrogen peroxide 1.77 0.85

Chlorine dioxide 1.50 0.72

Bromine 1.09 0.53

Oxygen 0.40 0.19

14

2.2.3 Applications of ozone

Ozone has a wide range of commercial applications. Due to its environmental

friendly properties to decompose into oxygen, ozone is widely used for water

purifications, as presented in the following table.

TABLE 6. Typical industrial applications of ozone (Wedeco 2008)

Application Usage

Drinking water Improvement of flocculation

Decoloration, taste, odor improvement

Iron / manganese elimination

Disinfection

TOC-elimination

Elimination of persistent substances

Desodoration Oxidation of H2S

Ozonolysis

Synthesis

Synthesis of new products

Modification of synthesis

Surface treatment

Laminating / coating

Bleaching

Pulp

Kaolin

Wastewater treatment

Elimination of COD

Elimination of surfactants

Elimination of phenols

Decoloration

Sludge treatment

Elimination of AOX

Elimination of endocrine and/or persistent substances

Process water

Cooling water (in power plants)

Swimming pools

Rinsing water in beverage industry

Product polishing

Stabilization of still water

Disinfection of food (salad, fish)

15 2.2.4 Ozone generation from oxygen

Because of its relatively short half-life, ozone is always generated on-site by an

ozone generator. The main principle of ozone generation is based on the

dielectric-barrier discharge (DBD). It is the electrical discharge between two

electrodes separated by an insulating dielectric barrier. The process uses high

voltage alternating current. The gap distance between the electrodes is about 1

mm in ozone generators. A multitude of random arcs form between the two

electrodes during operation, as the charges collect on the surface of the dielectric,

they discharge in microseconds, leading to their reformation elsewhere on the

surface. Oxygen gas is flowing through the electrodes and due to the discharging

arcs, the oxygen is split into two single oxygen atoms. The oxygen atoms will

eventually recombine to the formation of three oxygen atoms: ozone. (Wedeco

2008.)

GRAPH 3. Generation of ozone from oxygen feed gas (Wedeco 2008)

For the production of ozone, ambient air can be used as well as pure liquid oxygen

(LOX) or on site generated oxygen (PSA). To condition this air, dryers and dust

filters must clean the air before usage. Ozone generators require a high current

between 6-10 kV and frequencies between 300–600 Hz for operation. Ozone is

always present as a gas under normal temperature and pressure (NTP). The

applications introduced in Table 6 utilize ozone in gaseous form, it must be

16 indicated though, that ozone will never be available in pure form. The technical

generation of ozone from oxygen is limited to the following concentrations of

ozone in oxygen gas. Table 7 indicates the ozone/oxygen ratio for various

concentrations of ozone, which were utilized during the performance of this thesis.

(Wedeco 2008.)

TABLE 7. Ozone conversion table (Perry 1997)

Ozone concentration g/m³ (NTP)

O3 to O2

Wt%

O3 to O2

Vol%

kg Oxygen /

kg Ozone

m³ Oxygen /

kg Ozone

10 0.6 0.4 143.2 100

20 1.3 0.9 71.7 50

50 3.4 2.3 28.9 20

100 6.8 4.6 14.6 10

150 10 6.9 9.8 6.9

200 13 9.2 7.4 5.2

The oxygen/ozone ratio shows the limitations of ozone generation. A typical ozone

generator cannot produce a higher ozone ratio due to physical limitations. The

highest utilized ozone concentration during the experiments was 200 g/Nm³; which

would correspond for about 9% of ozone by volume. The largest part of the utilized

gas was therefore still oxygen.

Graph 4 presents a schematic view of a large ozone water purification unit. (Not

AOP). The gas supply can consist of liquid oxygen, air or on-site oxygen

generation through pressure swing adsorption (PSA). A chiller unit is required for

cooling of the ozone generator. The off-gas ozone must be converted back into

oxygen before released into the environment; this is done by a catalytic ozone

destructor (COD). Ozone is generated from oxygen, based on the dielectric barrier

discharge phenomenon, inside the ozone electrodes.

17

GRAPH 4. Ozone electrode and ozone purification system (Wedeco 2008)

2.2.5 Reaction properties

Due to its electronic configuration, ozone has different reactions in water. The

three main categories are: Oxidation-reduction, dipolar cycloaddition and

electrophilic substitution reaction. (Beltran 2004.)

In some cases, free radicals are formed from these reactions. These free radicals

propagate themselves through mechanisms of elementary steps to yield hydroxyl

radicals, which are extremely reactive with any organic matter present in water.

For this reason, ozone reactions can be divided into direct and indirect reactions.

Direct reactions undergo with any other type of chemical species, like molecular

products and free radicals. Indirect reactions are those between the hydroxyl

radical, formed from the decomposition of ozone or from other direct ozone

reactions, with compounds present in water. The direct ozone reaction is the

initiation step towards the indirect reaction. (Beltran 2004.)

Redox reactions are characterized by the transfer of electrons from one species

(redactor) to another one (oxidant). The oxidizing or reducing character of any

18 chemical species is given by the standard redox potential. Ozone has one of the

highest redox potentials, lower only than those of fluorine atom, oxygen atom and

hydroxyl radical. Because of its high standard redox potential, the ozone molecule

has a high capacity to react with numerous compounds. (Beltran 2004.)

Addition reactions are those reactions resulting from the combination of two

molecules to yield a third one. One of the molecules usually has atoms sharing

more than two electrons, unsaturated compounds with carbon double bonds, and

the other molecule has an electrophilic character. (Beltran 2004.)

In electrophilic substitution reactions one electrophilic agent (ozone) attacks one

nucleophilic position of the organic molecule (e.g. an aromatic compound),

resulting in the substitution of one part (e.g. atom, functional group) of the

molecule. This type of reaction is the base of the ozonation of aromatic

compounds. (Beltran 2004.)

Indirect reactions of ozone are due to the action of free radical species resulting

from the decomposition of ozone in water. The free radical species are formed in

the initiation or propagation reactions of the mechanisms of advanced oxidation

processes. In the ozone decomposition mechanisms, the hydroxyl radical is the

main responsible species in the indirect reactions. The reactions of ozone with the

hydroxyl radical can be considered the main initiation reactions of the ozone

decomposition mechanism in water. (Beltran 2004.)

Other initiation reactions develop when other agents, such as UV radiation are

present. The direct photolysis of ozone that yields hydrogen peroxide and then

free radicals, or the ozone adsorption and decomposition are also examples of

initiation reactions. There are also other reactions that lead to the decomposition

or stabilization of ozone in water. These substances are initiators, inhibitors and

promoters of the decomposition of ozone. The initiators are the substances that

directly react with the ozone to yield the superoxide ion radical, such as the ionic

form of hydrogen peroxide. The superoxide ion radical is important for propagating

free radical species, because it reacts immediately with ozone to yield the hydroxyl

19 radical. Promoters are those substances which propagate the radical chain to yield

the free radical through their reaction with the hydroxyl radical. Hydrogen peroxide

plays an important part in this role, it is in fact the initiating agent, but it also acts

as a promoter of ozone decomposition. (Beltran 2004.)

2.2.6 Scavengers of the hydroxyl radical

Inhibitors are also called hydroxyl-free radical scavengers because their presence

limits or inhibits the action of these radicals on the target contaminants. The

presence of carbonates for example, reduces the efficiency of ozonation to oxidize

contaminants in water. Theses carbonates, which also account for the hardness of

water, limit the degradation of the desired pollutant significantly. (Beltran 2004.)

Hydrogen carbonate: HCO3- + ●OH → CO3

- + H2O

Carbonate: CO32- + ●OH → CO3

- + HO-

Another problem is the presence of bromide ion in ozonated water. Ozone readily

oxidizes bromide ion to yield bromate ion, which is considered a highly toxic

pollutant. Formation of bromate is highly dependent on the presence of other

substances that consume ozone, for instance hydrogen peroxide. The presence of

natural organic matter (NOM) is another factor which can affect the decomposition

of ozone; it can act as promoter or inhibitor. A fraction of hydroxyl radical yields the

superoxide ion radical during reaction with NOM. Therefore, a fraction of NOM is a

promoter of the ozone decomposition. (Beltran 2004.)

2.2.7 Ozone decomposition

Once dissolved in water, ozone is unstable and decomposes. This can be

considered an advantage, but also a drawback. When ozone decomposes, free

radicals are generated and oxidation of compounds occurs. But due to its

instability, ozone cannot be used for a final disinfectant of water. There are two

20 ozone decomposition periods when treating natural waters. The first one, called

fast ozone demand, lasts from a few seconds to 1-2 minutes and is due to the

presence of substances that readily react with ozone through direct reactions.

Once these substances have disappeared or their concentrations decrease, the

longer ozone decomposition starts. During this period, instantaneous or very fast

consumption takes place. During the second or long ozone decomposition, ozone

slowly decomposes until all dissolved ozone has decomposed. (Beltran 2004.)

2.2.8 Diffusion into water

In a gas-liquid reacting system, diffusion convection and chemical reactions

proceed simultaneously. In general, when gas and liquid phases are in contact,

the components of one phase can transfer to the other phase to reach equilibrium.

If component A of a gas phase is transferred to the liquid phase, the rate of mass

transfer or absorption rate of A is:

NA= kG (PAb – Pi) = kL (CA – CAb)

kG = mass transfer coefficient for gas phase

kL = mass transfer coefficient for liquid phase

PAb = Partial pressure of A in bulk gas

Pi = Partial pressure of A at the interface

CA = Concentration of A at the interface

CAb = Concentration of A in the bulk of liquid

(Beltran 2004.)

The transfer rate is dependent on the physical properties of gas and liquid, the

difference in concentration across the surface and turbulence. (Beltran 2004.)

When two phases (gas and water) are in contact, the main resistance to mass

transfer is the layer of thickness close to the interface. This so called film layer

consists of two films in a gas-liquid system, one for each phase. In most situations

the gas is bubbled into the liquid phase, thus the interfacial surface is due to the

21 external surface of bubbles. Ozone solubility is a fundamental parameter in the

absorption rate. Ozone-water systems are characterized by a low concentration of

dissolved ozone, ambient pressure and temperature. In order to achieve good

degradation of pollutants in water, a high ozone transfer into water phase is

desired. (Beltran 2004.)

This absorption process is dependent on pressure and temperature according to

Henry’s law:

P = He * x

P = partial pressure of gas above the solution (in atm)

x = mole fraction of gas in solution

He = Henry’s law constant which accounts pressure and temperature dependency

Henry’s law can be described at a constant temperature. The amount of a given

gas dissolved in a given type and volume of liquid is directly proportional to the

partial pressure of that gas in equilibrium with that liquid. However the Bunsen

adsorption coefficient is expressed as a volume of gas at NTP which is dissolved

at equilibrium by a unit volume of liquid at a given temperature, when the partial

pressure of the gas is the unit atmosphere. The Bunsen coefficient has no

dimension. In the coefficient, the gas absorption rate into water decreases with

increasing temperature. (Beltran 2004.)

For efficient ozone transfer, it is therefore important to achieve low water

temperatures. In practice, this is not always applicable, but it is important to

monitor the temperature of water during application of ozone. In real life

applications, less gas can be absorbed into a liquid than stated in Henry’s law.

This is caused by circ*mstances which the absorption factor takes into

consideration.

Af = (Vw / Vg) * α * P

22 The absorption factor determines the amount of gas absorbed by the solving liquid

under known conditions.

Vw= Volume of solving liquid

Vg= Volume of gas

α= Bunsen coefficient

P= Total pressure

(Beltran 2004.)

The efficiency of dissolving ozone into water, which is usually carried out at

conditions close to NTP, is not ideal. A higher operating pressure would allow a

higher gas to liquid ratio, but this process is virtually impossible during continuous

operation of the AOP system. (Beltran 2004.)

As can be inferred from the absorption factor, the solubility of ozone gas is

depending on the concentration in gas and partial pressure and temperature, pH

and ion concentration in the solution. The solubility can theoretically be enhanced

by increasing the ozone concentration in the air (oxygen), increasing air pressure

(oxygen), decreasing the water temperature, decreasing the amount of solutes or

decreasing the pH. (Beltran 2004.)

In practice, these values are considered as limiting factors, because the only

economical reasonable variable is the pH factor and ozone concentration in air,

while other factors remain important to be monitored during operation.

2.3 UV radiation

2.3.1 Introduction to ultraviolet radiation

Ultraviolet radiation is energy of electromagnetic waves. Electromagnetic radiation

is classified according to the wavelength or frequency of the light wave. This

radiation includes radio waves, microwaves, terahertz radiation, infrared radiation,

23 visible light, ultraviolet (UV) radiation, X-rays and gamma rays. Of these, radio

waves have the longest wavelengths and gamma rays the shortest. Ultraviolet

radiation is defined as that portion of the electromagnetic spectrum between x rays

and visible light, between 40 and 400 nm. (Bolton 2008.)

The ultraviolet radiation spectrum is divided into UV-A (320-400 nm), UV-B (290-

320 nm), UV-C (220-290 nm), Far UV (190-220 nm) and Vacuum UV (40-190 nm),

whereas the UV-C radiation spectrum is the important frequency range for the

technical usage of light. Ultraviolet light, which travels in “packets” of energy

known as photons, can exert a variety of beneficial or deleterious effects on plants,

animals and materials in the biosphere, including processes responsible for driving

atmospheric chemistry. Other photochemical processes, utilizing the absorption of

light (either visible or ultraviolet), include photosynthesis in plants, sun

tanning/burning, snow blindness, fading of textiles and in contaminated waters the

photo degradation of organic pollutants by advanced oxidation processes. UV

radiation is also commonly used for disinfection of water or surfaces from micro

organism. (Bolton 2008.)

24 2.3.2 UV spectrum

Graph 5 presents the wavelength spectrum of electromagnetic radiation. Light is

the same form of electromagnetic radiation as other non visible wavelengths, yet

with a different wavelength. The human eye will interpret wavelengths between

400 and 700 nm as visible light. (Zeman 2008.)

GRAPH 5. The spectrum of electromagnetic radiation (Treiman 2008, courtesy of

USRA Lunar and Planetary Institute)

Visible light only accounts for a small fraction of the wavelength range. The

ultraviolet spectrum has a shorter wavelength. Therefore a shorter frequency and

carries more energy. The shorter the wavelength, the more damage such radiation

can cause to human skin. Gamma rays penetrate deep into the human cell tissue

and cause severe damage to cells. (Zeman 2008.)

25

GRAPH 6. Wavelengths between 100 and 780 nm (Wedeco 2008, courtesy of ITT

Wedeco Corporation)

Electromagnetic wavelengths between 100 and 780 nm can be considered as

light. The ultraviolet spectrum is further divided into fractions, where the UV-C

spectrum causes most damage to human skin. The low pressure lamps used in

water disinfection have their maximum power output at 254 nm. (Wedeco 2008.)

2.3.3 Effect on micro organism

Certain electromagnetic wavelengths are mutagenic to bacteria, viruses, other

micro-organisms and even humans. UVC is almost never observed in nature

because it is absorbed completely in the atmosphere, as are far UV and vacuum

UV. The wavelength of 254 nm will break the molecular bonds within micro-

organism DNA, producing thymine dimmers in their DNA and destroying them,

rendering them harmless or prohibiting growth. A similar process can be seen with

the UV-B effect on humans, including sunburn. Micro-organisms have less

protection from UV than humans and cannot survive prolonged exposure to it.

(Infralight 2008.)

26

GRAPH 7.Effects of UV on the DNA of micro organism (NASA 2009, courtesy of

NASA)

Due to the ability of UV-C light to act germicidal to damage the DNA, this

wavelength is commonly used in water purification systems. ITT Wedeco is

specialized into this environmental friendly disinfection technology, and is currently

the world leader in waste and drinking water disinfection systems.

2.3.4 Dose of ultraviolet light

In waste and drinking water applications, a certain dosage of UV light irradiation is

required to achieve a so called 4-log reduction (99.99%) of harmful organism. This

dosage is measured in J/m2, the amount of photons per area. For disinfection of

drinking water this dose usually requires about 400 J/m2. In contrast, the required

dose for AOP applications is usually around 10000 J/m². Certain factors, such as

the water transmission and turbidity, affect the calculation of such doses.

Therefore the UV dose depends greatly on the specific water property.

Furthermore the light intensity is also expressed as the amount of light applied per

volume of water. This value is expressed as watts per cubic meter of water

(W/m3). This value is used to express a cost based calculation value, since the

27 energy demand of the light source is expressed as watts. The utilized UV lamp

during this performance has the power output of 330 W. (Wedeco 2008.)

The energy of light (and any other electromagnetic energy) is given by Planck’s

equation:

hhc /

The intensity of UV light at 254nm is expressed as h

h is Planck’s constant = 6.63 x 10-34 J sec

has the unit sec-1, also called “Hertz” or Hz

is the wavelength, expressed in meter (m) or nm

c is the speed of light in (m/s)

The energy of the emitted light is inversely proportional to its wavelength () or

directly proportional to its frequency (Bolton 2008).

2.3.5 Ultraviolet lamps

The most important lamps for UV disinfection and therefore AOP as well are gas

discharge lamps. These usually have two electrodes, one at each end of a tube

containing a gas, which contains metal atoms (mercury). In such lamps, light

emission arises from atoms that have been exited to higher energy states by high

energy electrons emitted from the electrodes. The lamps used in UV disinfection

systems are mercury vapour lamps. In the lamps, electric current that flows

through the ionized hot plasma causes the mercury atoms to be raised to higher

energy states. When exited states return to their ground state, UV light is emitted

with a wavelength inversely proportional to the energy difference between the

exited state and the ground state. The lifetime of the exited mercury atoms is

limited and in microseconds they decay to lower energy levels with the excess

energy emitted as light or heat. The wavelength of the light is determined by the

difference in the energy levels. Graph 8 illustrates this process. (Wedeco 2008.)

28

GRAPH 8. Emission of electrons (Wedeco 2008, courtesy of ITT Wedeco

Corporation)

Graph 8 illustrates the exiting of mercury atoms in a low pressure lamp. The

operation principle is commonly used in luminous tubes, with one difference: UV

lamps are filled with a different gas mixture to achieve a lower wavelength output.

GRAPH 9. UV lamp during operation

Graph 9 illustrates the operation of low pressure high output amalgam UV lamps in

water. The radiation of 254 nm is interpreted as blue by human eyes. There are

four common types of mercury discharge lamps: low pressure, low pressure high

output, low pressure high output amalgam and medium pressure. However, only

29 the two most competing systems, low pressure high output amalgam and medium

pressure are presented here.

2.3.6 Low pressure high output amalgam lamps

Low pressure lamps emit only in narrow lines with no emission between the lines.

Graph 10 will show the wavelengths and relative emittances for the lines of a low

pressure mercury vapour lamp. As can be seen from the graph, the emission

spectrum is rather concentrated to 254 nm. The inner surface of the lamp

envelope can be coated with various types of phosphors, which absorb the 245

nm radiation and emit longer wavelengths. This is the basis of the popular

fluorescent lamp. The low pressure high output amalgam lamp is a modified

version with a 2 to 3 times higher output than a regular low pressure lamp. The

lamps have heavy electrodes and contain no free mercury. A solid amalgam (a

compound of mercury and another element, such as indium or gallium) spot is

placed on the inner wall of the lamp. This spot serves to control the mercury

vapour pressure. The XLR 30 low pressure high output amalgam lamp used in the

experiments has a length of 143 cm and was capable of 330 W output energy.

(Bolton 2008.)

2.3.7 Medium pressure lamps

The emission lines of a mercury lamp are only sharp when the pressure of the gas

is low (<100 Pa). If the pressure is increased, the lamp can carry much more

power, but the increased collision rate from other gas molecules causes the

emission lines to broaden. For the same length of lamp, an MP lamp (pressure of

100 KPa) can carry up to 30000 W instead of 330 W. Medium pressure lamps

operate very hot. Thus they are less efficient in comparison to low pressure lamps.

The emission spectrum is polychromatic, with several emission peaks between

186 and 600 nm. The operating bulb temperature is ranging between 600 and

900°C. Medium pressure lamps require several times more energy than LP lamps

30 for the same UV output, but for one MP lamp up to ten low pressure lamps are

required to meet the same dose of radiation per area. Generally, the much higher

energy efficiency of LP lamps compensates their low power output, but increases

their footprint of the system. (Bolton 2008.)

TABLE 8. Comparison of LP and MP lamps (Bolton 2008)

Characteristic LP high output amalgam MP

Emission Monochromatic (254 nm) Polychromatic (185-600 nm)

Operating bulb temperature

60-100°C 600-900°C

Germicidal efficiency 30-35% 12-16%

Power density (W/cm) 0.6-1.2 125-200

Graph 10 illustrates the emission curves of LP and MP lamps with relative spectral

emittance of medium pressure (MP) and low pressure (LP) lamps. The higher gas

pressure of MP lamps results in several emission peaks. The desired peak at 254

nm is only partly covered, whereas the undesired peaks between 300 and 600 nm

result in higher power consumption than LP lamps for the same light flux. (Bolton

2008.)

GRAPH 10. Peaks of LP and MP lamps (Bolton 2008, courtesy of AWWA)

31 A UV lamp must be separated by an air space from flowing water because a UV

lamp needs to operate at higher temperatures than water. This separation is

accomplished by placing the lamp inside a sleeve, which is usually made of quartz

because it permits the passage of UV in the 200-300 nm region. Normal glass

does not allow the passage of such frequencies and therefore no harm occurs if

UV is blocked by normal glass.

2.3.8 Effects of water quality on UV efficiency

Water quality can have a major effect on the efficiency and performance of a UV

reactor. Several water quality factors are important.

UVT (ultraviolet transmission) is the most important parameter because it

measures the amount UV light absorbed by the water itself, rendering it

unavailable to activate ozone gas. Furthermore, when more UV light is absorbed

(by low UVT), more lamps are needed to meet a UV dose than with a higher UVT

can be achieved. Typically, UVT at 254 nm is used as the primary water quality

design parameter. For UV systems with low pressure (LP) lamps, the most

important design parameter is the UVT at 254 nm. (Bolton 2008)

It is essential that the transmittance of the quartz sleeve between the UV lamp and

water be as high as possible, so that the maximum amount of the UV light from the

lamp can enter the water. However, in some cases the quartz sleeve can become

fouled with deposits that reduce the transmittance and adversely affect the

performance of the UV reactor. (Bolton 2008.)

Turbidity is caused by suspended particles in the water. These particles can affect

the UV light distribution in the water, by scattering the light or by absorption of light

by components in the particles. If algae are present in the water, this may reduce

the UVT. However, most algae are removed by the conventional

coagulation/sedimentation/filtration processes that exist in most water treatment

plants. (Bolton 2008.)

32 2.4 Advanced oxidation processes

The elimination of organic contaminants by advanced oxidation processes is a

destructive process. Typically a chemical or photochemical oxidant is used to

destroy a contaminant compound directly or indirectly through reaction

intermediates. In the case of AOP, this oxidant is either ozone or the hydroxyl

radical or ultraviolet radiation. Since the contaminant is physically broken up into

its elemental components, the process depends on the efficient transfer of energy

to achieve contaminant destruction. If such a process is not 100% efficient,

remnants of the contaminant may still remain. These remnants may produce

adverse health effects or reform the parent compound.

The hydroxyl radical has a higher oxidation potential and reacts immediately with

nearly all organic compounds. This enhanced reaction leads to better treatment

results regarding advanced degradation and faster kinetics (shorter contact times).

Theoretically, there are several technologies that exist for advanced oxidation, see

table 9.

TABLE 9. Advanced oxidation technologies

AOP combinations

Hydrogen peroxide + ultraviolet light

Hydrogen peroxide + ozone

Ultraviolet light + ozone

Ultraviolet light + titanium dioxide

The photo-fenton process

Oxidation in combination with electrolysis

Various catalytic processes (oxidant chemical + catalyst)

Although there are a number of processes which are defined as advanced

oxidation processes, the most widely used definition includes techniques that are

based on ozone, hydrogen peroxide and ultraviolet light. In the case of ultraviolet

33 light and ozone, an ozonised gas stream, produced by a dielectric barrier

discharge, is introduced into the aqueous medium to be treated. The ultraviolet

light radiation has a wavelength of 254 nm and is readily absorbed by ozone,

resulting in the formation of highly reactive hydroxyl radicals. These short-lived

radicals are more powerful oxidizing agents than ozone and hydrogen peroxide

alone and are capable of converting virtually all organic compounds to carbon

dioxide, water and similar species. (Sakaji 2002.)

There are a number of alternative reactions that can take place and it is known

that high concentrations of both ozone and hydrogen peroxide need to be avoided

as hydroxyl radicals are not selective in their reactivity and can react with excess

oxidants. The oxidation rates achieved with hydroxyl radicals are much higher than

those attainable from conventional oxidants such as ozone. In some cases,

reaction rates achieved using hydroxyl radicals are 106 to 109 times higher than

the rates achieved using ozone alone. (Sakaji 2002.)

2.4.1 Ozone destruction by UV

The pilot plant in this work was designed for the purification of water in which an

ultraviolet lamp simultaneously irradiates the water and ozone mixture, combining

the catalytic effect of ultraviolet radiation and the disinfecting and oxidizing effects

of ozone on bacteria, viruses and chemical compounds in the water. Photolysis

(direct absorption of UV) and photo-initiated oxidation (activation of ozone by UV

to form oxidative hydroxyl radicals) have great potential for the destruction of a

wide range of organic contaminants in water. (Taghipour 2004.)

During the advanced oxidation process of ozone with UV, ozone concentrations

are expected to decrease inside the reactor. This is due to the ozone

decomposition by UV radiation. The concentration of •OH is higher near the UV

lamp, where the UV fluence rate is higher. This is expected as •OH formation

depends on the fluence rate. The concentration of micro pollutants, such as

atrazine or 1.4 dioxane, would be reduced along the reactor due to direct

34 photolysis and oxidation with •OH radicals. The water pollutant would show lower

concentration near the UV lamp, where higher UV fluence rate and •OH radicals

are present. It is evident that the pollutant decomposition is influenced by both

reactor hydrodynamics and fluence rate. (Taghipour 2004.)

2.4.2 Ozone photo dissociation

The energy of an absorbed photon is strong enough to break molecular bonds and

therefore capable of molecular fragments. One example of photo dissociation can

be found in the stratospheric ozone. Ozone is produced in the stratosphere from

molecular oxygen through the following pair of reactions:

O2 + hν → O + O and O + O2 → O3 where hν represents the energy of a photon of

ultraviolet light with a wavelength less than 260 nm. (Taghipour 2004.)

Ozone is also dissociated by short-wavelength ultraviolet light (200-300 nm)

through the reaction: O3 + hν → O2 + O. The oxygen atom formed from this

reaction may recombine with molecular oxygen to regenerate ozone, thereby

completing the ozone cycle. The ozone layer in the stratosphere absorbs the UV-C

radiation and “filters” the sunlight radiation before it reaches the earth’s surface.

The same reaction is used in the advanced oxidation processes, with the

difference that the UV light is used to break the ozone into radical fragments.

(Taghipour 2004.)

The utilized wavelength for this process is 254 nm, because the highest ozone

absorption rate is achieved at exactly this point. The molar extinction coefficient ε,

which describes the amount of absorbed photons by the ozone molecule, is 3300

M-1 cm-1. The higher the extinction coefficient, the easier it is for ozone to absorb

the UV light. The following graph presents the absorption rate of ozone for

different wavelengths of light. The highest absorption rate is achieved at exactly

254 nm. This benefits the selection of low pressure lamps in contrast to medium

pressure lamps which would work less efficient. (Kenshi 2001.)

35

GRAPH 11. The molar extinction coefficient of ozone (Kenshi 2001)

2.4.3 Hydroxyl radical formation

The formation of hydroxyl radicals is based on a multi step mechanism. The

introduced ozone is first broken up into one oxygen (O2) molecule and one oxygen

atom (O). This reaction is very efficient at the wavelength of 254 nm with a

quantum yield of almost Ф=1. The quantum yield is the number of destroyed

molecules divided by the number of photons absorbed by the system. That means

almost all photons are absorbed by the ozone. The oxygen atom reacts with water

to form hydrogen peroxide. Hydrogen peroxide will partly be destructed directly by

the UV irradiation into hydroxyl radicals, but due to the low extinction rate of

hydrogen peroxide, most of the formed liquid will react further with ozone and HO2-

which can be seen in the following equations. There is the significant difference of

dissolved ozone and gaseous ozone in water. Ideally, all ozone should dissolve

when introduced into the water phase. (Beltran 2004.)

36 The following reactions indicate the properties of dissolved ozone in water and

hydroxyl radical formation with hydrogen peroxide formation. (Beltran 2004.)

O3 (aqueous) + h → 2 •OH

•OH + •OH → H2O2

H2O2 ↔ HO2- + H+

HO2- + O3 → O3

- + HO2

HO2 + h → •OH + O

O + H2O → 2 OH.

H2O2 + •OH → H2O + HO2

H2O2 + h → 2 •OH

In contrast, the following reactions present the reactions of gaseous ozone in

water. However these reactions can only occur on the film layer of gas and water.

(Beltran 2004.)

O3 (gas)+ h → O2 + O

O + O3 → 2 O2

O + H2O → 2 •OH

O + H2O → H2O2

O3 + H2O2 → O2 + •OH + HO2-

H2O2 ↔ H+ + HO2-

O3 + HO2- → •OH + O2

- + O2

O3 + O2- → O3

- + O2

O3-+ H2O → •OH + HO- + O2

•OH + •OH → H2O2

H2O2 + •OH → HO2 + H2O

H2O2 + h → 2•OH

HO2 + h → •OH + O

There are further ways for the generation of hydroxyl radicals which are dependent

on the generated intermediates such as excited oxygen atoms (O), hydrogen

peroxide or the conjugated base of hydrogen peroxide (HO2-). (Beltran 2004.)

37 Graph 12 shows the four main pathways for the generation of hydroxyl radicals in

advanced oxidation processes.

ozone (O3)

• OH radical

UV - h • , = 254 nm , = 19 M-1

cm-1

Water compounds

OH- , Fe , TOC

Initiation

Promotion

O1

2 • OH H2O2

H2O2 HO2-

UV h • = 254 nm,

= 3.300 M-1 cm-1

H2O2

+ h •

O3 + HO2-

O3

+ HO2-

O1

+H2O

+ H2O2

a)a) b)b) c)c)

+ H2O244

332211

GRAPH 12. Hydroxyl radical pathways (Ried 2005)

1. Ozone alone. Typical water compounds such as hydroxyl anions, iron ions or

organic compounds can initiate or promote a decomposition of dissolved ozone

and generate hydroxyl radicals. When using ozone for waste water treatment,

there is a possibility of generating the hydroxyl radical without the addition of UV or

H2O2. However, it is difficult to predict the hydroxyl radical formation in this

pathway as it is highly specific to water chemistry and the reaction kinetics of the

system. It is unlikely that one would use ozone alone as an advanced oxidation

process for water, but it is important to note that the impact of ozone alone may

achieve the desired result. (Ried 2005.)

2. Ozone in combination with UV. Different oxidized species will be generated

during the UV radiation of ozone molecules in water. Depending on the generated

intermediates, e.g. excited oxygen atoms (O), hydrogen peroxide or the

conjugated base of hydrogen peroxide (HO2-), there are different further pathways

(a, b, c in the graph) for hydroxyl radical generation. (Ried 2005.)

38 3. Ozone in combination with hydrogen peroxide. In the presence of hydrogen

peroxide, ozone reacts with the conjugated base of hydrogen peroxide (HO2-) to

form hydroxyl radicals. (Ried 2005.)

4. Ultraviolet light in combination with hydrogen peroxide. The UV radiation of

H2O2 leads directly to the formation of hydroxyl radicals. From the stoichiometric

yield (1 mol H2O2 → 2 mol •OH radicals) this process is theoretically the most

efficient, but the molar extinction coefficient for H2O2 (at 254 nm) is only 19 mol-

1cm-1. This means that this process is highly dependent on the UV transmittance,

when the UVT decreases, the low coefficient results in lower hydroxyl radical

yields. For a given UV radiation this low coefficient and low radical yield can be 20

times less than the ozone + UV process. It is possible to work with wavelengths in

the range of 180 to 250 nm to improve the molar extinction coefficient. (Ried

2005.)

Since low pressure lamps have a 99% output at 254 nm, a polychromatic, medium

pressure (MP) lamp may be more efficient at converting H2O2 to the •OH radical.

The energy input will be significantly increased in this case due to the use of MP

lamps. This may become the limiting factor in comparison to other AOP. In

addition, the depth of penetration of lower wavelengths (180–200 nm) is very low

so the positive effect on the extinction coefficient at this lower wavelength is highly

dependent on UVT. (Ried 2005.)

2.4.4 Comparison of ozone to hydrogen peroxide

While the production of ozone is relatively more expensive than hydrogen

peroxide, one major advantage is due to the photochemical properties of ozone.

The molar absorption coefficient at 254 nm is much higher for ozone than for

hydrogen peroxide with 3300 M-1cm-1 in comparison to 19 M-1cm-1, respectively.

Therefore a high efficiency of ozone absorption can be achieved at 254 nm, in

comparison to the UV-hydrogen peroxide AOP. The problems of ultraviolet - ozone

based systems are the high operating costs for both the ozone generation and UV

39 light radiation. A high intensity of UV flux is required for ozone to break. While this

is also the case in UV-hydrogen peroxide AOP, the capital costs are lower due to

the easier treatment train. The only energy consumed in these systems is the UV

light demanded to break the liquid hydrogen peroxide. In comparison, ozone-UV

AOP require the UV light unit, but also the ozone generation unit, which will cause

high initial costs and therefore a larger treatment unit in general. (Ijpelaar 2007.)

2.4.5 Low pressure and medium pressure lamp differences

Low pressure and medium pressure lamps are both used for the “destruction” of

hydrogen peroxide by UV light. The wavelength of 254 nm, which is used in LP

lamps, is excellent for H2O2 irradiation compared to the spectrum of MP lamps.

The contribution of the 254 nm emission of a typical MP lamp to the total emission

is 5-10%. For a LP lamp this is >90%. At the same input level of UV dose, LP

lamps perform comparable in high water quality or better in low water quality

regarding the fraction of light absorbed by H2O2. (Ijpelaar 2007.)

Due to the low molar extinction coefficient of the UV + H2O2 process, the percent

conversion of H2O2 to the hydroxyl radical is low. One way to overcome this fact is

to dose a higher amount of peroxide into the water. By overdosing, the required

hydroxyl radicals may be produced, but there will also be excess H2O2 in the water

after the UV process. Especially for drinking water, excess H2O2 cannot remain in

the system therefore an absorption process (GAC), quenching process

(chlorination, sodium bisulphite) or catalytic quenching with metal oxides must be

considered downstream of the UV oxidation step. UV + ozone based AOP will also

produce hydrogen peroxide during reaction, but the remaining concentrations of

peroxide are rather low and therefore do not require further removal. (Ijpelaar

2007.)

40 2.4.6 UV photolysis

The pathways mentioned in graph 12 are specifically used for the formation of the

hydroxyl radical, but certain contaminants can be treated without the generation of

radicals. For example, N-nitrosodymethylamine (NDMA) can be destroyed by

direct UV photolysis. UV photolysis takes place when UV light is absorbed by the

target contaminant and causes it to breakdown. In effect, NDMA absorbs UV light

at 228 nm, which results in cleavage of the N-N bond. No hydrogen peroxide is

required with UV photolysis since the hydroxyl radical is not needed. The emitted

photons from the UV lamp are strong enough to break some molecular bonds and

therefore capable of destroying the pollutant. This effect only works for some

pollutants, such as NDMA, while most compounds are somewhat resistant to the

ultraviolet radiation. (Ijpelaar 2007.)

Another example where the hydroxyl radical is not required is the oxidation of

endocrine disrupting compounds (EDCs). Studies have shown that ozone alone

can oxidize most EDCs and achieve removal levels greater than 90%. For

treatment of EDCs in wastewater, UV based AOP systems may also be adversely

affected by the typically low transmission of wastewater, compared to drinking

water, whereas UVT has no impact on ozone oxidation. (Ried 2005.)

2.4.7 Example of ozone destruction through ultraviolet light

(CONFIDENTAIL)

GRAPH 13. Ozone destruction by UV-C light

2.4.8 Applications for AOP

There are several areas where advanced oxidation processes can be

commercially used for the degradation of pollutants from water. Examples include

41 endocrine disrupting compounds, pharmaceuticals, pesticides (e.g. atrazine),

persistent substances, taste and odor causing substances (such as algae during

the summer months), 1.4 dioxane, NDMA (N-nitrosodimethylamine), MTBE

(methyl-tertiary-butyl-ether), chlorinated hydrocarbons (e.g. TCE).

2.4.9 Advantages and disadvantages of AOPs

The advantage of AOPs is the actual treatment technology. AOP is a destruction

process and the final products are only carbon dioxide, water and inert salts. The

process residuals do not need any further treatment. If ozone is utilized, the

oxidant can replace the disinfection properties of chlorine and therefore reduce the

toxic properties of chlorine. Because ozone must be generated on site and used

immediately, no storage area is required for the oxidant. A wide variety of

contaminants and concentrations can be treated.

Some pollutants (e.g. MTBE) can be removed by air stripping, granulated

activated carbon (GAC) adsorption and resin sorption, whereas the actual

destruction of the pollutant would require additional processes. In contrast, AOPs

destroy primary organic contaminants directly in water. Several AOP technologies

involving UV light or ozone would also have the positive side effect of disinfection

capability of the source water. Therefore no further disinfection treatment step

would be required.

The components utilized in AOP (UV light and ozone) have been used by the

water community in commercial water disinfection applications for many years

already. Consequently, even conservative treatment plant operators are already

familiar with these technologies in contrast to hydrogen peroxide, Fenton´s

reaction, TiO2 or other emerging technologies.

42 The disadvantage is that the reaction between organic contaminants and the

hydroxyl radical is fast, but does not directly result in the mineralization of these

contaminants. Instead, organic oxidation by-products are produced which can

further react with hydroxyl radicals for complete mineralization.

Ideally, AOP treatment systems are capable to completely mineralize the organic

contaminants of concern to carbon dioxide and water. In real life applications this

may require high energy and greater chemical doses and can therefore be too

costly in certain applications.

In some cases, the actual pollutant cannot be mineralized completely, and the

remaining substances might cause higher risks of concern than the initial

substance. These oxidation by-products must be fully understood and determined

according to the specific water matrix to be treated. Both organic and inorganic

oxidation by-products can be formed. When ozone is employed in AOP

applications, bromate can be formed from bromide, which is naturally present in

the source water. This will be further discussed in the following section.

The effectiveness of AOPs can be limited through the presence of nitrites, TOC

and other inorganics. This is due to the scavenging of radicals that would

otherwise react with pollutants. Operational costs will increase when

concentrations of these scavengers in source water increase. Contact times and

chemical dosages will also increase to maintain treatment goal. AOPs are hard to

plan due to the sensitivity of operational costs to changing source water.

43 TABLE 10. Advantages and disadvantages of AOPs

Tech-nology

Advantages Disadvantages

General AOPs

AOPs are destructive processes.

Several AOPs have disinfectant capabilities.

Many AOP components have been utilized in the water industry.

Potential for accumulation of oxidation by-products.

Radical scavenging by interfering compounds can reduce effectiveness of AOPs.

O3 - H2O2 Efficient in treating waters with high pollution concentrations.

Supplemental disinfectant.

Established technology for remediation applications.

Potential for bromate formation.

May require treatment of excess H2O2.

May require ozone off-gas treatment and permit.

O3 - UV Supplemental disinfectant.

More efficient at generating radicals than other processes for equal oxidant concentrations.

Energy and cost intensive process.

Potential for bromate formation.

Turbidity can interfere with UV light.

Ozone diffusion may result in mass transfer limitations.

May require ozone off-gas treatment and permit.

Interfering compounds can absorb light.

Potential increase in THM and HAA formation when combined with pre/post chlorination.

UV – H2O2

No potential for bromate formation.

UV radiation can serve as disinfectant.

No off-gas treatment required.

Not limited by mass transfer relative to ozone processes.

Turbidity can interfere with UV light.

Less stoichiometrically efficient at generating radicals than O3 – UV process.

Interfering compounds can absorb light.

Potential increase in THM and HAA formation when combined with pre chlorination.

44 2.4.10 Parameters affecting the AOP efficiency

There are several water quality parameters that influence the effectiveness of any

AOP. The most important parameters are the following:

Alkalinity: The hydroxyl radical is non selective and can be exhausted by the

presence of organic or inorganic compounds other than the contaminants of

concern. Both carbonate and bicarbonate will scavenge hydroxyl radicals to create

carbonate radicals which react with other organic or inorganic compounds present,

but at a much slower rate. Source waters with medium to high alkalinities usually

contain carbonate and bicarbonate. High concentrations are likely to react with the

hydroxyl radical and therefore scavenge it, which could otherwise react with micro

pollutants. (Hoigne 1976.)

TOC and NOM: Organic matter present in water will scavenge and therefore limit

the effectiveness of the hydroxyl radical. Total organic carbon (TOC)

measurement include all organic compounds in water; with concentrations usually

ranging from <1 mg/l to >7 mg/l and include pesticides, oil and gasoline traces and

chlorinated compounds. Non organic matter (NOM) accounts macromolecular

organic compounds, which also negatively affect the effectiveness of hydroxyl

radicals. Furthermore there are concerns about potential negative side effects of

UV irradiation in combination with chlorine addition. Such combination might lead

to the formation of harmful by-products. (Steward 1993.)

Nitrates and nitrites adsorb ultraviolet light in the range of 230 to 240 nm and 300

to 310 nm and will decrease the formation of radicals. This effect is especially

affecting the polychromatic spectral curve of medium pressure lamps, which have

several emission peaks in that region. (Steward 1993.)

Phosphates and sulfates are present in low amounts in source waters, but are

slow in reacting with hydroxyl radicals. Therefore their influence can be

diregarded.

45 Turbidity and transmission lower the transmittance of the source water for light.

The efficiency will decrease with increasing turbidity. (Wedeco 2008.)

2.5 Oxidation by-products

Minimizing the risks of disinfection by-products while maintaining adequate

protection from microbial and organic contamination is one of the dilemmas water

utilities and regulating agencies are faced with today. Disinfection by-products are

formed during water treatment when disinfectants and oxidants react with organic

and inorganic matter present in water. Several of these by-products have been

associated with various health effects, including cancer risks. The by-product

concerning the usage of ozone-UV techniques is called bromate. (Bundy 2002.)

TABLE 11. Description of drinking water disinfection by-products

Disinfection by-products

Potential health effects from ingestion of water

MCL

(mg/l)

Bromate Increased risk of cancer. 0.010

Chlorite Anemia, nervous system effects. 1.0

Haloacetic acids Increased risk of cancer. 0.060

Total Trihalomethanes

Liver, kidney or central nervous system problems; increased risk of cancer.

0.080

2.5.1 Bromate

Bromate (BrO3-) is an inorganic disinfection by-product of potable water that is

potentially carcinogenic and mutagenic (Neal 2006). It is not expected to be

present in natural water, but may be formed during treatment by oxidation.

Deafness, renal failure, coma and convulsions to humans have been attributed to

bromate ingestion (Quick 1975), which results in bromic acid formation in the

stomach in the presence hydrochloric acid. (Turkington 1994).

46

Bromate is considered a potential human carcinogen by the International Agency

Research on Cancer and a probable human carcinogen by the

disinfection/disinfection by-product rule of the USEPA (USEPA 1994).

Compounds with bromine in formal oxidation state −1 are called bromides;

therefore bromide is the ion of bromine and is present in natural soft water in

concentrations between 30 and 200 µg/l (Haag 1983.)

Bromide can originate from saltwater intrusion, natural fractionation, anthropogenic

bromide emissions, water disinfection with chloride, or agricultural applications.

Even if bromide ion in drinking water supplies has not been observed to cause

direct public health problems, it is a precursor of the formation of bromate ion.

(Myllykangas 1999.)

2.5.2 Formation of bromate

Bromate is formed in potable water mostly as a result of ozonation of raw water

containing bromide. For those processes using ozone alone the by-product that

causes most concern is the bromate ion. EU, Japanese and the US regulations

specify a bromate standard of 10 µg/l, the WHO guideline value is 25 µg/l and the

Australian regulations limit bromate to 20 µg/l. (Legube 1996.)

The mechanism for bromate formation is rather complicated and includes both

molecular ozone and •OH radical reactions. During the chemical oxidation and / or

disinfection of natural waters containing bromide with ozone, bromate is formed at

concentrations ranging from 0 to 50 µg/l under normal drinking water treatment

conditions. Fortunately, the reactions of ozone with bromide and hypobromide ions

are relatively slow. Hence, ozonation of waters during a brief period of time will

lead to a low oxidation yield of bromide ion to bromate ion. (Legube 1996.)

47 Bromate ion formation by reaction of hydroxyl radicals on bromide ion is possible

only in the presence of hypobromite ion. Moreover, hypobromite ion cannot be

formed by reaction of hydroxyl radical with bromide ion. Therefore, hydroxyl

radicals alone are not precursors of bromate ion formation. (Legube 1996.)

Several parameters can impact the conversion of bromide to bromate. Parameters

such as bromide concentration, dissolved organic carbon (DOC), temperature and

pH are important to be determined during performance. The rates of reaction

depend especially on pH because ozone reacts predominantly on OBr-. Therefore

acid waters have lower risk of bromate formation. (Neal 2006.)

It is possible to optimize the design and the operation to keep the bromate

formation down. Low residual ozone concentrations in water, the addition of

hydrogen peroxide, pH control, several injection points instead of one, the reaction

time of ozone or the direct irradiation of ozone gas by UV light are some

possibilities to use ozone even in the presence of bromide in the source water.

Furthermore, UV irradiation in the wavelength range of 180-300 nm provides

energy sufficient for producing intermolecular changes of the irradiated molecules.

The decomposition of bromate leads to the production of bromite ion. (Neal 2006.)

2 BrO3- + h → 2BrO2

- + O2

Bromite ion (BrO2-) is further decomposed to hypobromite ion (BrO-) and bromide

ion (Br-) via complex reactions which are initiated by the products generated by the

primary reactions of photolysis.

2BrO2- + h → 2BrO- + O2

2BrO- + h → 2Br- + O2

These reactions indicate that the photodecomposition of bromate leads to the

production of Br- and oxygen as end products. However, the required UV

irradiation dose cannot be applied for typical AOP applications due to the high

energy needed for this process. (Siddiqui 1996.)

48

(CONFIDENTIAL)

2.5.3 Formation pathways

Bromate can be produced through different pathways. Ozonation of water forms

two powerful oxidants: aqueous molecular (O3) and hydroxyl radical (●OH). Both

O3 and ●OH radicals react with organic and inorganic species in water. The three

major pathways are as follows:

The first pathway is the direct ozonation pathway, in which O3 sequentially

oxidizes Br- to aqueous bromine (HOBr/OBr-) and then BrO3-. Because OBr- rather

than HOBr can be oxidized to bromite (BrO2-) at an appreciable rate, BrO3

-

formation of favored at high pH upon continuous ozonation. A second pathway for

BrO3- formation is that O3 oxidizes Br- to aqueous bromine, followed by ●OH

oxidation of HOBr/OBr- to BrO●, which disproportionates to BrO2-. BrO2

- is then

oxidized by O3 to form BrO3-. The third pathway for BrO3

- formation is: Br- is first

oxidized by ●OH to form BrO●. This continues to form bromate through

disproportionation to BrO2-, followed by O3 oxidation to BrO3

-. (Yasunaga 2005.)

About 17% of bromide in solution can be transformed to bromate. Temperature of

water plays an important role in bromate formation. Generally, all the rates of

reactions involved in formation of bromate are raised with temperature and the

value of the dissociation constant of hypobromous acid (Ka of HOBr/OBr-)

diminishes as the temperature rises. Consequently, the hypobromite ion

concentration increases with the temperature thus increasing the formation of the

bromate ions. (Li 2007.)

Graph 14 presents the formation pathway of bromate from the ozonation of

bromide in water. The two main pathways are the direct ozone pathways, which is

by far more important and the radical pathway which is less likely to occur.

49

GRAPH 14. The bromate formation pathway (Yasunaga 2005, courtesy of

Mitsubishi Electric Corporation)

50

3 AOP SYSTEM AND PERFORMANCE

3.1 Pre examinations

(CONFIDENTIAL)

3.1.1 The initial design of the reactor

(CONFIDENTIAL)

3.1.2 Conclusions after first test run

(CONFIDENTIAL)

3.1.3 Injector

The injector type of gas injection system works on the principle of the venturi

effect. A high pressure is required on the inlet of the nozzle, where pressurized

water enters the injector inlet. The water stream is then constricted towards the

injection chamber and changes into a high velocity jet stream. The increase in

velocity through the injection chamber results in a decrease in pressure, thereby

enabling an additive material to be drawn through the suction port and entrained

into the motive stream. This media can be liquid or gas, depending on the type of

application. In the case of advanced oxidation process, ozone is used as the gas

media. As the jet stream is diffused toward the injector outlet, its velocity is

reduced and it is reconverted into pressure energy.

(CONFIDENTIAL)

51

Graph 18 illustrates the venture effect.

GRAPH 15. Injector venturi effect

The injector working principle is based on the venturi effect. A stream is

compressed into a smaller diameter section, whereas the velocity is increased. As

a result of the high velocity, the pressure will drop and a second media can be

sucked into the stream. Graph 19 illustrates water that is flowing with high velocity

through the suction point where the highest velocity is reached. At this point, air is

sucked into the system and a high mixing rate is achieved. The left graph shows

the injector “Körting 135-5-2556” utilized during the performance, whereas the

right graph is for illustration purpose only.

(CONFIDENTIAL)

GRAPH 16. Injector during performance

3.1.4 Diffuser

O.I.Filtration Ltd, of Stoke on Trent, England, supplied the stone diffuser sample

with a mean pore size of 30 microns (30 µm) which was small enough to fit into the

reactor. In comparison, human hair is 100 µm wide.

52 (CONFIDENTIAL)

GRAPH 17. Diffuser built into the rector

Graph 20 shows the diffuser built into the reactor bottom part. The small reactor

diameter of 80 mm was the limiting factor for most commercial diffusers. The

utilized stone diffuser was capable of creating relatively small bubbles.

For processes such as ozonation, aeration of drinking water, waste water,

aquariums, fish farms, lakes and ponds, air stripping of volatile organic

compounds, iron and sulfide removal by oxidation, the bubble creation is very

important. Especially the bubble diameter can critically affect process efficiency,

because it determines the surface area of the bubbles. For two different spherical

bubble diameters with the same total volume of gas, the ratio of the total surface

areas is inversely proportional to the ratio of the diameters. To give an example, if

the mean bubble diameter would be decreased from 2.5 mm to 0.5 mm, the

interfacial contact area between the air and water would increase by the factor 5

for the same gas flow rate.

A finer pore size is expected to offer the best solution for increasing the ozone

transfer efficiency. Ceramic diffusers with very small pores can create fine

bubbles, but are not suitable for this type of ozone diffusion due to the high

pressure drop. The ozone generator used in this AOP application, “Ozomantic

Modular 8HC”, is only capable of handling pressure up to 1 bar.

A fine ceramic diffuser with pore size of 2.0 µm (Kerafol, Membrane disc Ø 152

mm) would have delivered very fine bubbles, but the bubble point of air into water

had to be larger than 1.6 bar, which would have required a larger ozone generator.

In addition to this, the size of the diffusing disk did not fit into the reactor with a

diameter of only 80 mm. Graph 21 illustrates the bubble size of the Kerafol disk

diffuser. It created very fine bubbles, which would have been ideal for the AOP

pilot trials.

53

GRAPH 18. Fine bubble distribution of ceramic diffuser

Experiments have shown that the diffuser material property that is the primary

determinant of bubble size is not only the pore size, but also the free energy at the

surface, which is a result of intermolecular attraction. With the small contact angle,

water displaces the gas from the high energy diffuser surface pore so that the

tension holding the bubble to the surface is minimized. A smaller upward force

from the gas balloon is enough to detach it to rise up into the liquid. In contrast, the

contact angle for a low surface energy diffuser is greater so that the gas displaces

the water. The bubble must increase in dimension before the upward force

produced by the density difference between gas and liquid is sufficient to

overcome the surface tension of the larger area and rises up into the liquid. The

surface energy of materials is the primary factor for the yield of small bubbles.

(Burries 1999.)

GRAPH 19. Surface energy dependency of bubbles (Burries 1999, courtesy of

Alan Burries, Alabdiffusers)

54 3.1.5 Tubing

(CONFIDENTIAL)

GRAPH 20.Tubing built into reactor

3.1.6 Determining bubble size of injector, diffuser and tubing

The actual size of the gas bubble is the key indicator for efficient transfer of ozone

into the water phase. In order to achieve reliable results, the comparison between

the compared technologies included the optical determination of average bubble

size. This test was once again conducted with a plastic pipe and the size was

manually evaluated according to the following graphs. For safety reasons, the test

was performed with oxygen gas, not with ozone. The bubble size during ozonation

is expected to remain similar. One influencing factor is the water flow rate. During

counter current flows, the water will act against the rise of the bubbles. This leads

to a flattening of the bubble.

The average bubble size of the injector was determined to be ~2.5 mm in

diameter. This value is based on averages taken from several pictures during flow

rates of 20, 50 and 80 liters of oxygen gas, respectively.

(CONFIDENTIAL)

55

GRAPH 21. Diffuser during gas flow rates of 20 l/h

(CONFIDENTIAL)

GRAPH 22. Bubbles during gas flow rates of 20 l/h

TABLE 12. Comparison of gas diffusing technologies

(CONFIDENTIAL)

3.1.7 The first performance test: co-current

(CONFIDENTIAL)

GRAPH 23. Co-current flow injector arrangement

(CONFIDENTIAL)

Graph 28 illustrates the co-current diffuser arrangement.

(CONFIDENTIAL)

GRAPH 24. Co-current flow diffuser arrangement

(CONFIDENTIAL)

3.1.8 The first performance test: counter-current

(CONFIDENTIAL)

GRAPH 25. Counter current arrangement

56 3.1.9 Bubble contact time with UV radiation

(CONFIDENTIAL)

GRAPH 26. Contact time for co-current and counter-current arrangements

3.1.10 Compounds of the system

(CONFIDENTIAL) TABLE 13. Determined properties during performance

Analytical determination parts

Description / Type

Conductivity meter WTWConduktometer LF191

pH meter Schott Lab 850

Photometer Dr. Lange CADAS 100

Gas flow meter ABB fa*g6100 140 l/h

Water flow meter Krohne optiflux 1100 c

Ozone gas concentration BMT 961 tpc

BMT 964 c

Dissolved O3 concentration, and water temperature

Orbisphere 510

3.1.11 Sampling

(CONFIDENTIAL)

57 3.2 Results of pre study

(CONFIDENTIAL)

4 DEGRADATION OF 1.4 DIOXANE AND BROMATE CONTROL

(CONFIDENTIAL)

4.1 Final pilot runs

(CONFIDENTIAL)

4.1.1 Sampling amounts and procedure

(CONFIDENTIAL)

4.2 Results

(CONFIDENTIAL)

5 CONCLUSIONS AND RECOMMENDATIONS

(CONFIDENTIAL)

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Accessed online: January 2009

APPENDIX 1

Determined parameters

The following process parameters were measured during the AOP comparison.

TABLE 14. Measured parameters during AOP performance

Parameter Unit Description

Gas flow m3/h Volume of oxygen/ozone gas mixture flowing into the

reactor.

Water flow m3/h Volume of water flowing into the reactor.

Ozone IN g/Nm3 Concentration of ozone in oxygen/ozone gas mixture

streaming into the reactor from the generator.

Ozone OUT g/Nm3 Concentration of ozone in oxygen/ozone gas mixture

streaming out of the reactor after UV.

Ideally, no ozone should be detected here anymore.

Ozone OUT

(no UV)

g/Nm3 Concentration of ozone in oxygen/ozone gas mixture

streaming out of the reactor, when no UV light was

present. This indicates how much ozone is

“consumed” by the water matrix even without any UV

light present.

Dissolved

ozone pre UV

mg/l Amount of dissolved ozone in water matrix before the

UV irradiation. This value shows how much ozone

would have been able to be destructed by UV light.

Dissolved

ozone after UV

mg/l Amount of dissolved ozone after the UV irradiation.

Ideally no ozone should be detected here anymore,

due to the UV destruction.

Parameter Unit Description

pH The pH is one of the most important factors affecting

the efficiency of AOPs.

Conductivity µS/cm The conductivity should not change significantly during

the oxidation process.

Water

transmission

% The transmission is an important factor for UV light

efficiency.

Hydrogen

peroxide after

UV

mg/l Hydrogen peroxide is formed through the process.

This value is determining how much peroxide is still left

after the process, i.e. has not been destructed by the

UV light.

Water

temperature

°C Ambient water temperature is important for reaction

kinetics and how much ozone can be dissolved into

water phase.

APPENDIX 2

TABLE 15. Parameters calculated according to the measured values

Parameter Unit Description

Ozone dose IN g/m3 The dose of ozone applied to the water per hour. It is

calculated from (ozone IN*gas flow)*water flow.

Ozone dose

OUT

g/m3 The dose of ozone which has not been activated by

the UV. Ideally, this value should be zero.

It is calculated from (ozone OUT*gas flow)*water flow.

Reacted ozone

TOTAL

g The amount of ozone reacted inside the system. For

both gas and dissolved phases.

This value is most important, because it shows how

effective ozone has been activated.

Ideally, if ozone dose IN is 4 g, 4 g should have also

reacted in this value.

Reacted ozone

gas phase

% Amount of ozone which has reacted in the gas phase.

The higher the percentage, the better the percentage

activation.

Activated

dissolved

ozone

mg/l Amount of ozone which has reacted into radicals in

dissolved phase. It subtracts dissolved ozone pre UV

– dissolved ozone after UV.

Activated

dissolved O3

% Amount of dissolved ozone which has been activated

in percent.

Hydrogen

peroxide

concentration

(theoretical)

mg/l It is calculated how much hydrogen peroxide would

theoretically be formed during the process from the

reaction theory of ozone.

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