Assessment of synergistic effect of UV h2o2 integrated disinfection process

Especially, the Ultra violet light/hydrogen peroxide UV/H2O2 process has received particular attention for potential application in treatment of drinking water.. This phenomenon was obse

PEROXIDE INTEGRATED DISINFECTION PROCESS

SOWPATI JAYAKER

NATIONAL UNIVERSITY OF SINGAPORE

2010

PEROXIDE INTEGRATED DISINFECTION PROCESS

SOWPATI JAYAKER

B Sc (Chemical Technology), Loyola Academy (affiliated to Osmania University), India

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING

DIVISION OF ENVIRONMENTAL ENGINEERING AND SCIENCE

NATIONAL UNIVERSITY OF SINGAPORE

2010

First of all, I would like to thank the Lord, God and Almighty, whom I personally believe

is supreme and sovereign It is He that I believe has strengthened me and helped me to persevere through all the thick and thin and has placed me in the midst of all the people that I have mentioned in this acknowledgement

I would like to record my earnest gratitude to A/P Hu Jiangyong for her guidance, supervision and direction from the very early stage of this research Her wide-ranging expertise made her a constant source of ideas and her passion in drinking water treatment has exceptionally inspired and enriched my growth as a student and as a researcher Most importantly, she provided me unwavering encouragement and support in various ways especially when I was in difficult times One could not wish for a more considerate and more thoughtful supervisor I am indebted to her in many ways

I could never thank the staff of Water Science and Technology Laboratory enough, including S.G Chandrasegaran, Michael Tan, Tan Xiaolan, Lee Leng Leng, and Tan Hwee Bee for their kind assistance and technical support in ensuring the successful completion of this study I would also like to thank all my fellow post-graduate students, especially Guo Huiling, Elaine Quek, Mark Goh Voon Wei and research staff Chu Xiaona, Dr Park Se Keun for their advice and company that alleviated my work stress in

the laboratory I would not have made it this far without all of them

friends in the church and the care group members for their continual prayers, encouragement and not only moral but also practical support when I and my family was in need of help The support from the church during my daughter’s birth was extraordinary and unforgettable

I would like to thank my wife, Sheeba, who has sacrificed a lot for the sake of my studies This thesis would not have come to completion without her constant encouragement and support Thank you my love I would like to thank my sweet little daughter, Jayneeta whom God has blessed us with She has been our stress relief in the times of distress

Finally, I would like to thank everybody who was important to the successful realization

of my thesis, as well as expressing my apology that I could not mention personally one by one

SUMMARY vii

LIST OF FIGURES x

LIST OF TABLES xii

NOMENCLATURE xiii

CHAPTER 1: INTRODUCTION 1

1.1 Drinking Water Disinfection 1

1.2 Selection of Disinfection Strategy 3

1.3 Disinfectants as Oxidants: A Better Strategy 6

1.4 Research Objectives 8

1.5 Thesis Organization 8

CHAPTER 2: LITERATURE REVIEW 9

2.1 Alternative Disinfectants 9

2.2 UV Disinfection 10

2.2.1 Use of UV light in drinking water treatment 12

2.2.2 UV sources and lamp technologies 15

2.2.2.1 Mercury emission lamps 16

2.2.2.2 Low pressure lamps 18

2.2.2.3 Medium pressure lamps 19

2.3 Advanced Oxidation Processes (AOPs) 20

2.3.1 Principles of AOPs 22

2.3.1.1 Peracetic acid/UV radiation (PAA/UV) 22

2.3.1.2 Hydrogen Peroxide/UV radiation (H2O2/UV) 23

2.3.1.3 Ozone/UV radiation (O3/UV) 23

2.3.1.4 Ozone/H2O2 (O3/H2O2) 24

2.3.2 Applications of AOPs 27

2.4 Hydroxyl (*OH) Radicals 28

2.4.1 *OH radical chemistry 29

2.4.1.1 *OH radical scavenging reactions 31

2.4.2 *OH radicals and disinfection 33

2.5 Integrated UV Disinfection Systems 38

2.5.3.1 UV/H2O2 synergy 44 2.6 Effect of Water Matrix on UV/H2O2 Integrated Disinfection 45

2.6.1 Effect of organic matters 45

3.2 Chemical Reagents and UV Sources 53

3.2.1 Emission spectra of LP, MP and FMP and absorbance spectrum

3.4.1.1 Estimation of residual H2O2 concentration 69 3.4.1.2 Determination of phenol degradation rate constant 70 3.4.1.3 Determination of *OH radical concentration 72

4.1 Determination of OH Radical Rate Constant for Phenol Degradation 73

4.2 Disinfection Performance of MS2 Coliphage by UV/H2O2 Integrated System 75

4.2.1 Effect of UV dose on MS2 log inactivation 75

4.2.1.1 LPUV/H2O2 75

4.2.1.2 MPUV/H2O2 76

4.2.1.3 FMPUV/H2O2 77

4.2.2 Effect of H2O2 concentration on MS2 log inactivation 79

4.2.2.1 LPUV/H2O2 79

4.2.2.2 MPUV/H2O2 80

4.2.2.3 FMPUV/H2O2 80

4.2.3 Synergistic effect of UV/H2O2 integrated disinfection 82

4.2.3.1 LPUV/H2O2 82

4.2.3.2 MPUV/H2O2 83

4.2.3.3 FMPUV/H2O2 84

4.2.4 *OH radical concentration of UV/H2O2 integrated disinfection 84

4.2.4.1 LPUV/H2O2 84

4.2.4.2 MPUV/H2O2 89

4.2.4.3 FMPUV/H2O2 91

4.2.5 Synergy and *OH radical correlation for different lamp sources 92 4.3 Effect of Synthetic Water Matrix on MPUV/H2O2 Integrated System 94

4.3.1 Effect of organic matter 94

4.3.1.1 MS2 log inactivation 95

4.3.1.2 Synergy 96

4.3.1.3 *OH radical concentration 98

4.3.2 Effect of turbidity 99

4.3.2.1 MS2 log inactivation 99

4.3.2.2 Synergy 101

4.3.2.3 *OH radical concentration 102

4.3.3 Effect of bicarbonate ion ( − 3 HCO ) 103

4.3.3.2 Synergy 105 4.3.3.3 *OH radical concentration 106 4.3.4 Effect of chloride ion (Cl−) 107

4.3.4.1 MS2 log inactivation 107

4.3.4.3 *OH radical concentration 110 4.4 Effect of Real Water Matrix on MPUV/H2O2 IntegratedSystem 111

Advanced oxidation processes (AOPs) were one among the suggested options to tackle the problems encountered with chlorine disinfection Their ability to deal with disinfection and decontamination at one go was the key for such a consideration Especially, the Ultra violet light/hydrogen peroxide (UV/H2O2) process has received particular attention for potential application in treatment of drinking water The combined action of UV/H2O2, which is inactivating microorganisms as well as oxidizing organic matter, was successfully demonstrated by a few researches This ability to minimize the DBP precursors and to oxidize certain micro pollutants and at the same time to be able to provide disinfection are attributed to the hydroxyl (*OH) radicals produced *OH radicals are generated by photolysis of the peroxidic bond when H2O2 absorbs UV light directly Use of short wave UV wavelengths (200–280 nm) was reported to achieve the most efficient *OH radical yields Synergistic effect reported for UV/H2O2 was one of the key benefits especially for disinfection UV/H2O2 disinfection comprises of two modes of inactivation, the first being UV and the second being *OH radical mediated It is generally agreed that *OH radicals generated play a key role in synergy However, there have been reports where decreased *OH radical concentration did not show a corresponding decrease

in the log inactivation, but rather an increase in log inactivation This phenomenon was observed in UV/H2O2 treatment of surface water and it was attributed to the production of other reactive oxygen species (ROS) and also to the *OH radical generating action of the natural organic matter (NOM), which are generally considered as scavengers It was also reported that the *OH radical production was dependent on the UV lamp source that was used So the log inactivation, synergy and the *OH radical concentration are dependent on

were used, which are low pressure UV (LP), medium pressure UV (MP) and filtered (>295 nm) medium pressure UV (FMP) MP was selected for further study as this showed the highest potential to produce *OH radicals

Log inactivation of MS2 coliphage, for UV alone was as expected, the highest for MP and the lowest for FMP H2O2 addition enhanced the log inactivation differently for the three lamp sources There was no significant reduction beyond 10 mg/L H2O2 for LP, but there was generally an increasing log inactivation trend for MP and FMP with increase in H2O2 With an increase in the UV dose, synergistic effect decreased for LP, increased for FMP and there was no clear trend for MP The highest concentration of *OH radicals were produced for MP and the lowest for FMP A correlation between synergy and *OH radical concentration showed that the linear correlation for MP was the highest, around 74.8%, for LP was around 23.6% and for FMP was the lowest, around 16.4%

The impact of the synthetic water matrix and the real water matrix was assessed by comparing to the results obtained for deionised (DI) water using MPUV/H2O2 system With the increase in the TOC, there was a gradual and significant impact on the log inactivation for UV alone However, H2O2 addition helped in countering this negative effect of TOC In general TOC positively influenced the synergy and negatively influenced *OH radical production The increase in the turbidity reduced the log inactivation slightly H2O2 addition did not help much in countering the slightly negative effect of turbidity In general turbidity affected both synergy and *OH radical production

of bicarbonate ions ( −

3

HCO ) and H2O2 addition narrowed down the enhancement achieved HCO3− ions negatively influenced both synergy and *OH radical production There was an enhancement in the log reduction for UV alone in the presence of chloride ions (Cl−) and H2O2 addition narrowed down the enhancement achieved only slightly in comparison toHCO3− Cl− ions in general negatively influenced synergy with occasional positive effects and negatively influenced *OH radical production There was only a slight impact of the real water matrix on log inactivation for UV alone Log inactivation sustained and increased with H2O2 addition Real water matrix positively influenced synergy only at 30 mg/L H2O2 and negatively influenced *OH radical production

Figure 2.1 Electromagnetic spectrum 11

Figure 2.2 Energy emission when electrons return from activated state to original state 16 Figure 2.3 Typical Emission spectrum of low pressure mercury lamps 19

Figure 2.4 Typical emission spectrum of medium pressure mercury lamps 20

Figure 2.5 Reaction of *OH radical with organic substrate, M in the

presence of HCO 3 - ion 32

Figure 2.6 Composition of the CO2/(H2CO3)/ HCO3-/CO32-/H2O system as a function of pH 33

Figure 3.1 Calibration curve for HA (TOC) 56

Figure 3.2 Emission spectra of LP, MP and FMP and absorbance spectrum for H2O2 57

Figure 3.3 UV collimated beam apparatus 66

Figure 4.1 Plot of Eq 3.15 at constant Phenol to H2O2 ratio 74

Figure 4.2 Effect of LP dose on MS2 disinfection at constant H2O2 concentration 76

Figure 4.3 Effect of MP dose on MS2 disinfection at constant H2O2 concentration 77

Figure 4.4 Effect of FMP dose on MS2 disinfection at constant H2O2 concentration 78

Figure 4.5 Effect of H2O2 concentration on MS2 disinfection at constant LP dose 79

Figure 4.6 Effect of H2O2 concentration on MS2 disinfection at constant MP dose 80

Figure 4.7 Effect of H2O2 concentration on MS2 disinfection at constant FMP dose 81

Figure 4.8 Synergistic and antagonistic effects for LPUV/H2O2 82

Figure 4.9 Synergistic effects for MPUV/H2O2 83

Figure 4.10 Synergistic effects for FMPUV/H2O2 84

Figure 4.11 Concentration of *OH radical with the increase in LP dose 85

Figure 4.13 Concentration of *OH radical with the increase in FMP dose 91

Figure 4.14 Correlation of synergy and *OH radical for the three lamp sources 93

Figure 4.15 Effect of TOC on MS2 log inactivation 95

Figure 4.16 Effect of TOC on synergy 97

Figure 4.17 Effect of TOC on *OH radical concentration 98

Figure 4.18 Effect of turbidity on MS2 log inactivation 100

Figure 4.19 Effect of turbidity on synergy 101

Figure 4.20 Effect of turbidity on *OH radical concentration 103

Figure 4.21 Effect of bicarbonate ion on MS2 log inactivation 104

Figure 4.22 Effect of bicarbonate ion on synergy 106

Figure 4.23 Effect of bicarbonate ions on *OH radical concentration 107

Figure 4.24 Effect of chloride ion on MS2 log inactivation 109

Figure 4.25 Effect of chloride (Cl−) ion on synergy 110

Figure 4.26 Effect of chloride (Cl−) ions on *OH radical concentration 111

Figure 4.27 Effect of real water matrix on MS2 log inactivation 112

Figure 4.28 Effect of real water matrix on synergy 115

Figure 4.29 Effect of real water matrix on *OH radical concentration 116

Table 2.1 Oxidation potentials of a few of the strong oxidants 28 Table 3.1 Measured water quality parameters of filtered lake water 52 Table 3.2 Operational and water quality parameters 57 Table 4.1 *OH radical rate constant for phenol degradation for different R

ATCC American Type Culture Collection

CFU Colony forming units

DBPs Disinfection by-products

DNA Deoxyribonucleic acid

FMP Filtered medium pressure

HAA Haloacetic acids

HO2* Hydroperoxyl radical

MCL Maximum Contaminant Level

NOM Natural organic matter

NTU Nephelometric turbidity units

PFU Plaque forming units

RNA Ribonucleic acid

ROS Reactive oxygen species

THMs Trihalomethanes

TOC Total organic carbon

TSB Tryptic soya broth

USEPA United States Environmental Protection Agency

1.1 Drinking Water Disinfection

Water is one of the most indispensable necessities for human survival Ensuring microbiological safety of drinking water is of supreme importance Disinfection is an important step in ensuring that water is safe to drink This step in water treatment is crucial in safeguarding public health Epidemics like typhoid and cholera were common

100 years ago These epidemics were traced back to the consumption of unsafe water in the late 1800s Identification of the root cause for the epidemics very soon led to the discovery of disinfectants in order to reduce the occurrence of epidemics Disinfection is a major factor in the reduction of epidemics since then The key aspect of disinfection step

in the water treatment train is to ensure that water is free from pathogens Disinfection is generally placed towards the end of the treatment train to protect human beings from pathogens and there by safeguarding human health Disinfection of drinking water is one

of the major public health advances in the 20th century A wide range of disinfectants and disinfection technologies have emerged over the period of time ever since disinfection has been discovered to control typhoid outbreak in 1908 in Chicago

Prior to the widespread use of disinfectants, many people became ill or died because of contaminated water Disinfection reduces or eliminates illnesses acquired through

Introduction

drinking water Primary disinfection kills or inactivates bacteria, viruses, and other potentially harmful organisms in drinking water Effective disinfection of adequately filtered influent water or raw water of suitable quality can be accomplished by either chemical or physical means such as the use of chlorine, chlorine dioxide, ozone or ultraviolet light (UV) However, the disinfection processes will not be as effective on influent waters of inferior quality Some disinfectants are more effective than others at inactivating certain potentially harmful organisms The selection of an appropriate disinfection process depends upon site-specific conditions and raw water characterization that is unique to each drinking-water system Process selection decisions must consider and balance the need to inactivate human pathogens while minimizing the production of disinfection by-products Commonly accepted chemical disinfectants are free chlorine, monochloramine, chlorine dioxide and ozone UV disinfection is often used as a physical disinfection method The application of UV light is an acceptable primary disinfection process Disinfection processes may be different for different utilities based on their disinfection needs and also to meet regulatory requirements

Secondary disinfection provides long-lasting water treatment as the water passes through pipes to consumers Secondary disinfection is made possible by maintaining residual disinfectant while the water is supplied through the distribution systems Residual disinfectant maintains water quality by killing potentially harmful organisms that may get into the water as it moves through pipes Drinking water is typically treated before it passes through the pipes, however, water is not sterile and can contain low levels of microorganisms that survive through treatment and distribution and also there is a

possibility of regrowth of microorganisms due to the presence of nutrients in the water More so if UV or advanced oxidation processes (AOPs) involving UV are employed as primary disinfection barrier, there is also a possibility of repair, by which the microorganisms that were rendered inactive could be active again So some residual disinfectants are necessary to account for repair and also for regrowth Microbes can grow

on pipe surfaces forming a thin biofilm layer In some cases, biofilms have been known to harbor pathogens that cause diseases, especially in severely immunocompromised persons The presence of residual disinfectant (secondary disinfection) is also thought to help inactivate pathogens that might enter into a water distribution system through contamination, as well as prevent microbial interactions with pipe wall biofilms (Propato and Uber, 2004)

1.2 Selection of Disinfection Strategy

A decision making process has to be used in order to determine whether the current primary disinfectant can meet disinfection and disinfection by products (DBPs) requirement The first aspect of the decision making process is the microbial limits

Giardia lamblia , Legionella, HPC, total coliform turbidity, and viruses are the regulated

pathogens (EPA guidance manual, 1999) Process modification has to be considered, if the existing primary disinfectant is not able to meet the inactivation requirements To move the application point of disinfectant, increase dose, increase contact time, or adjust

pH are some of the strategies for process modification The cases in which meeting the disinfection requirements is not viable by process modification of the existing primary

disinfectant, a new disinfectant has to be considered The second aspect of the process is the DBPs limits Under varying water quality conditions, 80% of the MCL is set as an action level to determine the change of treatment practices, to meet the established limits

on a consistent basis Pretreatment optimization (i.e., coagulation, filtration, etc) or process modifications such as moving the disinfection point can possibly reduce the DBPs In cases where the optimized processes do not meet the microbial and DBPs requirements, a new disinfectant is needed

If the decision making process reveals that a new primary disinfectant is required or desired because of better public health protection, this decision requires the knowledge of the keys components such as TOC concentration, bromide concentration and whether the treatment comprises of filtered or non-filtered system A high TOC concentration indicates a high potential for DBP formation In such scenarios, the decision will favor those disinfectants that will not produce DBPs or will produce the least amount of DBPs Precursor removal and enhanced coagulation are used to reduce TOC during treatment optimization “High TOC” quantifies the potential to produce DBPs and is defined as one

of the conditions, TOC exceeds 2 mg/L or TTHM exceeds MCL (0.08 mg/L under Stage

1 disinfection byproduct rule, DBPR) or HAA5 exceeds MCL (0.06 mg/L under Stage 1 DBPR) Formation of hypobromous acid and bromate ion discourages the use of strong oxidants such as ozone and ozone/peroxide (peroxone) as a primary disinfectant for waters with high bromide concentration (0.1 mg/L) In the unfiltered systems wherein there is no benefit of biofiltration to reduce ozonation byproducts or biodegradable organic matter (BOM)Zred, the use of ozone or peroxone is strongly discouraged

The selection of secondary disinfection is primarily driven by primary disinfection

However, assimilable organic carbon (AOC) concentration, DBP formation potential

(DBPFP) and distribution system retention time are the three important aspects that has to

be considered in selecting a secondary disinfection strategy AOC generation is possible if

a strong oxidant like ozone is used as a primary disinfectant in waters rich in TOC and AOC concentration of higher than 0.1 mg/L after filtration is considered high To prevent the regrowth in the distribution systems, the water containing high AOC has to be additionally treated with biological or GAC treatment to stabilize the finished water Disinfection by-product formation potential (DBPFP) serves as an indication of the amount of organic byproducts that could be expected to form in the distribution system if chlorine is used Because DBP formation continues in the distribution system, the DBP content at the plant effluent should be limited A high DBPFP is defined as a water meeting one of the two criteria which are total trihalomethanes (TTHM) seven-day formation exceeds the MCL (0.08 mg/L under Stage 1 DBPR); or HAA5 seven-day formation exceeds the MCL (0.06 mg/L under the Stage 1 DBPR) In a large distribution system, booster stations may be required to maintain the disinfection residual Since chlorine dioxide has an upper limit for application, its usage may not be feasible if relatively high doses are required to maintain a residual in the distribution system A distribution system retention time is considered high if it exceeds 48 hours (EPA guidance manual, 1999)

1.3 Disinfectants as Oxidants: A Better Strategy

Most disinfectants are strong oxidants and/or generate oxidants as byproducts (such as hydroxyl free radicals, *OH) that react with organic and inorganic compounds in water While disinfection is the primary focus of the disinfectants, many of the disinfectants like ozone are also used for other purposes in drinking water treatment, such as taste and odor control, improved flocculation, and nuisance control Because DBPs are produced irrespective of the intended purpose of the oxidant, it is important to also address uses of disinfectants as oxidants in water treatment Two approaches are commonly used to reduce the formation of DBPs during drinking water treatment The first approach consists of using a non-chlorine-based disinfection process as a ‘primary’ disinfectant prior to addition of chlorine based ‘secondary’ disinfectant The overall disinfection efficiency remains the same, however the amount of chlorine needed is significantly reduced Therefore, the quantity of DBPs formed is comparatively lower UV treatment unlike other chlorine based disinfectants and ozone, produces no known DBPs, because it does not involve the use of any chemicals The second approach consists of reducing the amount of NOM in the raw water prior to chlorination using a combination of chemical and physical processes (e.g coagulation/flocculation and filtration) An alternative group

of technologies that can potentially be used to minimize the formation of DBPs are advanced oxidation processes (AOPs) (Zhou and Smith, 2001; Oppenlander, 2003) In AOPs, *OH are formed These radicals are extremely reactive and are capable of oxidizing some of the NOM present in raw water sources (Langlais et al., 1991; Gottschalk et al.,2000) As a result, AOPs have been documented to reduce the total

organic carbon (TOC) concentration and the trihalomethane formation potential (THMFP)

of raw source water (Amirsardari et al., 2001; Kusakabe et al., 1990; Sierka and Amy, 1985; Glaze et al., 1982; Peyton et al., 1982) The most common processes used to generate *OH is through the use of combined catalytic oxidants such as ozone-ultraviolet (O3/UV), hydrogen peroxide-ultraviolet (H2O2/UV) and hydrogen peroxide-ozone (H2O2/O3) (Gottschalk et al., 2000) Although all three processes can produce *OH, the O3based oxidants produce ozonation DBPs Therefore the UV/H2O2 process is preferred over the other two processes UV/H2O2 treatment for organic contaminant control offers

an enormous disinfection potential Based on the results reported, UV/H2O2 can generally

be applied as a disinfection barrier against all microorganisms (Kruithof et al., 2002) Lubello et al (2002) reported only slight synergistic benefits for UV/H2O2 treatment of wastewater, while Koivunen and Tanski (2005) observed no synergies but some antagonism in peptone water for MS2 coliphage The potential of UV/H2O2 process for virucidal (MS2) inactivation was evaluated and *OH exposure (CT) was calculated to present a relationship between the *OH dose and microbial inactivation (Mamane et al, 2007) However, the relationship between synergy and *OH concentration was not made clear This relationship is very crucial to acquire a better understanding concerning the synergy and the contribution of *OH to the synergy Thus, in UV/H2O2 integrated disinfection system, synergism could be understood better, and the contribution of *OH and UV to the synergy can be clearly demonstrated by establishing the relationship between the synergy and the *OH concentration

1.4 Research Objectives

The primary objective of this study was to assess the synergistic effect of the AOP, UV/H2O2 integrated disinfection system, using MS2 coliphage as the target microorganism The significance of the *OH radicals in the UV/H2O2 synergism was also investigated The effect of the water matrix such as TOC, turbidity and the presence of certain ions on *OH radical concentration as well as UV/H2O2 synergism were studied

1.5 Thesis Organization

Detailed literature review about background of UV disinfection and AOPs and some details about the integrated UV disinfection systems are provided in Chapter 2 Materials and methods used in this study are described in Chapter 3 The experimental results are discussed in Chapter 4 Final conclusions that are drawn based on the experimental results are presented in Chapter 5

2.1 Alternative Disinfectants

Chlorine has been the most commonly used disinfectant since its first use in 1908 However in 1970s it was first reported that chlorine can form by-products which may be carcinogenic in nature Ever since then there has been an extensive research conducted and many studies indicated that chlorine can react with naturally-occurring materials in the water to form unintended DBPs which may pose health risks (Fallon and Fliermans, 1980; Cheh et al., 1980; Maruoka and Yamanaka, 1980; Ringhand et al., 1987) Some pilot-plant studies also confirmed the same (de Greef et al., 1980; Zoetemanet al., 1982; Kruithof et al., 1985; Miller et al., 1986) Stricter health regulations with regards to the levels of DBPs have been implemented which impacted the use of chlorine as a disinfectant Apart from this, there are specific microbial pathogens, such as

Cryptosporidium, that are resistant to traditional disinfection practices In 1993,

Cryptosporidium caused 400,000 people in Milwaukee to experience intestinal illness More than 4,000 were hospitalized, and at least 50 deaths have been attributed to the outbreak There have also been Cryptosporidiosis outbreaks in Nevada, Oregon, and Georgia over the past several years (USEPA, 1998) Formation of mutagenic and carcinogenic agents in the water treated with chlorine and its inability to deal with certain pathogens had prompted research to seek alternative disinfection methods that would

Literature Review

minimize environmental and public health impacts Wide arrays of disinfectants and disinfection technologies have emerged over the period of time ever since the discovery

of DBPs Use of chloramine, Ozone, UV light and combination of these disinfectants with

a weaker disinfectant H2O2 are some of the strategies adopted to overcome the problems that surfaced with chlorine disinfection

2.2 UV Disinfection

UV disinfection involves the use of UV radiation for disinfection UV radiation is part of the electromagnetic spectrum that lies between the x-rays and the visible light regions, and spans the wavelengths from 100 to 400 nm (Figure 2.1) Within the short wavelength range for UV radiation, the spectrum is further divided into four sub-regions (USEPA, 1999) as described The first is UV-A region comprising of wavelengths from 314 to 400

nm The wavelengths between 300 and 400 nm are sometimes called near UV UV-B is the second in line ranging from wavelengths 280 to 315 nm, also called medium UV The wavelengths 200 to 280 nm categorized as third region called UV-C region This region is

of great importance as far as UV disinfection is concerned The fourth region is called vacuum UV which comprises of wavelengths 100 to 200 nm This part of UV has wavelengths that are strongly absorbed by water and air This part of the UV is capable of directly causing a cleavage of the water molecule leading to the production of *OH radicals

Figure 2.1 Electromagnetic Spectrum (Wright and Cairns, 1998)

The whole range of UV light wavelengths is called actinic waves, also known as chemical waves, in opposition to the thermic waves of a higher frequency Actinic wavelengths involve energies that are able to provoke direct chemical changes on the irradiated molecules (activation, ionization, dissociation, etc.), and to promote biological changes in the systems accordingly The region with the longest wavelengths is known as the UV-A region UV-A radiation is associated with skin ageing (Yin et al., 2001), is responsible for the production of melanin in skin to cause tanning, and is the least harmful category of

UV radiation (Bolton, 1999) Wavelengths in the UV-B region are shorter than that of those in the UV-A region and have higher energy levels It has been found that exposure

to UV-B radiation can lead to skin cancer, since DNA damage can occur which result in cell mutations and cancerous growths (Gies et al., 1986; Abarca and Casiccia, 2002) UV-

C radiation consists of the shortest wavelengths present in the atmosphere (since vacuum

UV is strongly absorbed by compounds in the atmosphere) and have the highest level of energy, allowing the radiation to penetrate deeply into the cells to cause maximum

damage Of the various wavelength regions, the wavelength region that is of interest in

UV disinfection is the UV-C range, which is also known as the germicidal range This is because the wavelengths in the UV-C region are known to be strongly absorbed by biomolecules (Tyrell, 1996), which is the main mechanism of inactivation by UV radiation It also has the highest energy levels among the various categories (other than vacuum UV) of UV radiation, and is hence able to produce the most lethal and significant amount of damage to inactivate pathogens

2.2.1 Use of UV light in drinking water treatment

UV radiation can be and has been used for the improvement of drinking water quality Presently, disinfection is the primary purpose of applying UV irradiation in water treatment The technical method was introduced by drinking water facilities in the beginning of the 20th century The bactericidal effect of sunlight radiant energy was first reported by Downes and Blunt (1877) However, the UV part of the sunlight that reaches the earth surface is merely confined to wavelengths higher than 290 nm The so-called

“Boston sunlight on a cloudy day”, has a total intensity of 340 W/m2 However, the instant irradiation that depends on the height of the sun can vary by a factor of 2 to 100

At 30o the total intensity is about 50% higher in the high mountains than on the flat lands

at sea level (Keifer, 1977) In addition, only less than 10% of the total sunlight intensity that reaches the surface of the earth is UV light, while little active radiation for water disinfection available from this percentage Therefore, UV disinfection is essentially a technological process for use in the water treatment

The first large scale application of UV light, at 200 m3/day for drinking water disinfection was in Marseille, France from 1906 to 1909 (Clemence, 1911) This application was followed by UV disinfection of ground water for the city of Rouen, France However, considerable discussions and controversy occurred on the comparative benefits of UV vs filtration The applications of UV for water sanitation were delayed in Europe during World War I In the United States, the first full-scale application of UV light in 1916 was reported for 12,000 inhabitants of Henderson, Kentucky Other applications began in Berea, Ohio (1923); Horton, Kansas (1923); and Perrysburg, Ohio (1928) The application

of UV in the United States are referenced in early publications of Walden and Powell (1911) and Fair (1920)

All these applications were abandoned in the late 1930s The reasons were unknown but presumably costs, maintenance of equipment and aging of the lamps (which at that time were not fully assessed) were determinants Disinfection with chlorine probably was preferred for easier operation and for lower cost at that time During 1950s, the UV technique moved into full development again Kawabata and Harada (1959) reported on necessary disinfecting UV doses In Belgium, the first full-scale application was installed and operated in Spontin for the village of Sovet in 1957 and 1958 It is currently still in operation New applications and technologies are continuously examined and developed Most of the applications in Europe were concerned drinking water or clear water systems, including ultrapure water for pharmaceutical and medical applications Contrary to those

in the United States and Canada, the application to wastewater remained rare, but innovations are underway As far as drinking water is concerned, upto 1980 the

information on the use of UV in the United States was anecdotal (Malley, 1999) The EPA Surface Water Treatment Rule (SWTR) of 1989 did not indicate UV as the best

available technology for inactivation of Giardia lamblia The proposed Groundwater

Disinfection Rule (GWDR) (U.S EPA, 2000), however included UV as a possible technology Since 1990, joint research efforts have been made by American Water Works Association (AWWA) and the AWWA Research Foundation (AWWARF) In 1998, it

was demonstrated that the UV could be appropriate for the inactivation of oocysts of Cryptosporidium parvum Other researchers also confirmed and made this finding definite (Linden et al., 2002; Mofidi et al., 2002a; Craik et al., 2000; McGuigan et al., 2006; Li et al., 2007; Campbell et al., 2002) These findings have further aided for the development

of UV disinfection U.S Environmental Protection Agency (USEPA) has put in place guidelines to help water authorities design, monitor and manage UV disinfection systems for drinking water treatment, as set out in the UV Disinfection Guidance Manual (USEPA, 2006) As of 2002, there were over 3000 drinking water facilities using UV disinfection in Europe In the United States, many water treatment plants are changing to

UV disinfection and the number of installation of such systems is expected to continue to increase In 1986 and 1996, the European Committee of the International Ozone Association (IOA) organized a symposium (Masschelein, 1986, 1996) on the use of ozone, UV and also potential synergisms of ozone and UV for water sanitation The same topics were on agenda of the IOA Conference at Wasser, Berlin in 2000 At present, the use of UV is a major development, perhaps more in the field of wastewater treatment than directly for drinking water, although direct treatment of raw water sources become attractive Following the developments of ozone-UV, the possibilities of UV in

conjunction with Hydrogen Peroxide (H2O2) and catalysts with UV are actively under examination Although the applications of these new technologies still remain limited as far as the drinking water is concerned, their areas of development include removal of refractory micropollutants (such as herbicides, organochlorine compounds and polycyclic aromatic hydrocarbons), disinfection and less formation of by-products

2.2.2 UV sources and lamp technologies

For years, solar radiant energy has been the only known and available source of UV light

on earth The radiant energy received by earth is estimated at 1400 J/m2, with so-called solar constant of 1374 W/m2 Most of the emitted light is UV, about 98%, but only a small part of emitted UV is received on earth Two basic mechanisms occur: diffusion (scattering) and absorption The diffusion of Rayleigh is concerned more with short wavelengths because it is proportional to λ-4 Absorption by nitrogen and oxygen eliminates all vacuum ultraviolet (VUV) Wavelengths under 200 nm when absorbed by oxygen, generate ozone, whereas ozone itself undergoes photolysis when absorbing in the range of 220 to 300 nm As a consequence, UV-A and a little UV-B are UV components that reach the surface of the earth

Light can also be generated by activating electrons to a higher orbital state of an element; the return of that activated species to lower energy states is accompanied by the emission

of light The energy difference between the activated state, E1 and the original state, E0

can quantitatively be expressed as the emission energy, hν In other words, wave-lengths obtained depend upon the energy difference between the activated state and return state

Figure 2.2 Energy emission when electrons return to the original state after activation

2.2.2.1 Mercury emission lamps

Activation (or ionization) of mercury atoms by electrons (i.e., electrical discharges) at present is by far the most important technology in generating UV light as applicable to water disinfection The reason for the prevalence of mercury is that it is the most volatile metal element for which activation in the gas phase can be obtained at temperatures compatible with the structures of the lamps Moreover, it has an ionization energy low enough to enable the so-called “avalanche effect”, which is a chain reaction underlying the electrical discharge Activation-ionisation by collision with electrons and return to a lower energy state (e.g., the ground state) is the principle for the production of light There is a whole series of return levels from ionized or activated metastable states appropriate for emitting in the UV range Natural mercury is composed of five isotopes at

approximately equal weight proportions, thus small differences in the line emissions exist, particularly at higher vapour pressures, and give band emission spectra instead of line emissions

The most used filler gas is argon, followed by other inert gases These gases have completed outer electron shells and high ionization energies In most technologies, argon

is used as filler gas The ionization energy of argon is 15.8 eV, but the lowest activated metastable state is at 11.6 eV The energy of metastable state can be lost by collision If it

is collision with a mercury atom, ionization of the latter can take place and this can be followed by emission of light When the energy of the metastable state is higher than the ionization energy of mercury, the whole constitutes a penning mixture Consequently penning mixtures are possible with mercury, argon, neon, helium but not with krypton and xenon By modifying the composition of the penning gas, the output yield can be modified and sometimes improved, but also the spectrum of the emitted light can be changed Incorporating neon together with argon provides easier starting and can produce increased liner output Doping the carrier gas with indium, deuterium and some metal halides are examples of such modifications of penning gas These modifications are made

to get not only stable temperature independent outputs, but also emit a required spectrum

of lights according to composition of the penning mixture There are more lamp technologies available However, only low pressure (LP) and medium pressure (MP) lamps are discussed in detail as they were used in this study

2.2.2.2 Low pressure lamps

Mercury lamps are operated at different mercury-gas pressures The low-pressure (LP) mercury lamp for the generation of UV normally is operated at a nominal total gas pressure in the range of 102 to 103 Pa, the carrier gas is in excess in a proportion of 10 to

100 The most usual LP lamp emission spectrum is illustrated in Figure 2.2 The spectrum

is of the line or ray type; the emission is concentrated at a limited number of well-defined lines and the source is called monochromatic source The resonance line at 253.7 nm is by far most important The 253.7 nm line represents around 85% of the total UV intensity and is directly useful for disinfection

For LP UV lamps, the overall UV-C proportion of the UV light wavelengths emitted are

in the range of 80 to 90% of the total UV power as emitted These data determine the ratio

of the useful UV light in disinfection vs the lamp emission capabilities Increasing the linear (UV-C) output is a challenge for upgrading the LP UV lamp technologies as applicable to water treatment to reduce the number of lamps to be installed The linear total UV output of the discharge length for lamps appropriate for use in disinfection is in the range of 0.2 to 0.3 W (UV)/cm

An efficient life time of 4000 h is presently possible and efforts are ongoing to improve lamp life During the first 100 to 200 h of lamp operation an initial drop in emission yield occurs After that period the emission is stable for months Under normal conditions, LP

UV lamps are fully operational for at least one year

Figure 2.3 Emission spectrum of low pressure mercury lamps, germicidal lamps

(Sharpless and Linden, 2001)

2.2.2.3 Medium pressure lamps

Medium-pressure (MP) UV lamps operate at a total gas pressure in the range of 104 to 106

Pa At nominal operating temperature of 6000K (possible range is 5000 to 7000K), all the mercury within the lamp enclosure is gaseous The emission of MP UV lamps (Figure 2.3) is a result of series of emissions in the UV region and also in the visible and IR region The source is called polychromatic source To optimize the emission in the UV-C range, and consequently the reaction and disinfection capabilities, broadband and multiwave MP UV lamps have been developed by Berson The total intensity of the irradiance yield is about 65% However, only a part of the intensity is in the specific UV range and potentially useful for disinfection The UV output is approximately directly proportional to the input voltage that also determines (the high voltage) the average power input to the lamps The correlation holds between 160 and 250 V (voltage of the mains)

Figure 2.4 Typical emission spectrum of medium pressure mercury lamps

(Sharpless and Linden, 2001)

In the most recent developments, optimization of electrical parameter enables the production of lamps emitting upto 30% of the light in the UV-C range These lamps are operated at an electrical load of 120 to 180 W/cm A classical lifetime to maintain at least 80% of emission of germicidal wavelengths is generally 4000 h of operation In recent technologies, lifetimes from 8,000 to 10,000 h have been reached

2.3 Advanced Oxidation Processes (AOPs)

Advanced oxidation processes (AOPs) have been known since the 1970's The widely accepted definition for AOPs came from Glaze et al., 1987: “Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals (a short lived, potent oxidizer) in sufficient quantity to affect water purification” As a result, AOPs have been documented to reduce the total organic carbon (TOC) concentration and

the trihalomethane formation potential (THMFP) of raw source water (Amirsardari et al., 2001; Kusakabe et al.,1990; Sierka and Amy, 1985; Glaze et al., 1982; Peyton et al., 1982) There are many processes able to generate the highly reactive *OH radical species The most common ones are combinations of ozone (O3), hydrogen peroxide (H2O2) and ultraviolet (UV) radiation (Peyton et al., 1990) Photocatalysis (UV/catalyst (TiO2), ozonation at high pH and combination of ozone and ultrasound (US), Fenton’s reaction and Photo Fenton reaction are some other examples of AOPs AOPs are generally used for the destruction of synthetic organic chemicals (SOCs) in water, apart from disinfection and deactivation of pathogenic microorganisms that are difficult to degrade biologically Both catalysed and uncatalysed processes are used for the effective removal of such compounds The uncatalysed processes are comprised of direct photolysis in aqueous solution (Boule et al., 1982, Krijgsheld et al., 1986) or UV- photooxodation using ozone

or H2O2 as the photoactivated oxidant (Sundstorm et al, 1986) In addition to what was discussed in section 1.3, AOPs can also effectively mineralize many organic contaminants and have become attractive for the control of synthetic organic compounds in wastewater treatments (Kang and Lee 1997) AOP have many advantages in water treatment processes and have been proposed as an alternative for the control of DBP precursors (Symons and Worley, 1995; Eggins et al., 1997) Among the proposed methods for DBPs control AOPs were documented to deserve more study for potential applications in drinking water treatment (Wang et al., 2000)

2.3.1 Principles of AOPs

Among those AOPs applied in the water treatment, the most frequently used techniques are the combinations of O3, H2O2 and UV AOPs involving PAA are also considered for disinfection applications of wastewater in combination with UV Generation of *OH radicals is the key which is common for all these AOPs The principles of the mechanisms

of the generation of *OH radicals of some of the AOPs applicable in water treatment are discussed below The details of the AOPs involving UV are discussed in the later section However, O3/H2O2 system is discussed in detail in the following sub section, as this is not covered under integrated UV systems

2.3.1.1 Peracetic acid/UV radiation (PAA/UV)

PAA (CH3COOOH) is an unstable organic peracid marketed as an equilibrium mixture of acetic acid (CH3COOH), H2O2, peracetic acid, stabilizers and water PAA breaks down in water according to the following equations:

O H COOOH CH

O H COOH

2 3

3COOOH CH COOH ½O

2 4

2 2

2

According to some authors (Liberti et al., 1998) PAA’s disinfectant action is based on an active oxygen release, according to others (Ausimont, 1999) the killing effect primarily comes from *OH radical However, PAA resultant microorganisms deactivation involves

an alteration, or destruction of proteins, cytoplasmatic membrane, some metabolic enzymes and DNA

2.3.1.2 Hydrogen peroxide/UV radiation (H2O2/UV)

In the hydrogen peroxide/UV system, the production of *OH radicals is due to the direct photolysis of H2O2

In this system, the photolysis of O3 by UV radiation initiates the production of *OH radical UV radiation at 254 nm wavelength decomposes O3 and directly yields H2O2 (Eq

2.7) Similar mechanism as shown in the O3/H2O2 system leads to *OH radicals formation

It is to note that at 254 nm, the extinction coefficient for O3 (eO3_254nm = 3300 M-1cm-1) is much higher than that for H2O2 (eH2O2_254nm = 18.6 M-1cm-1), thus the photodecomposition

of O3 is much higher than that of H2O2 at 254 nm (about 1000 times higher) and is the main initiator for *OH radical production

2 2 2 2

It is to note also that the O3/UV system is the most complete O3-based AOP since it involves up to three possible initiation reactions for *OH radical generation (i.e O3/H2O2system, photolysis of the formed H2O2 (H2O2/UV), and O3/OH-)

2.3.1.4 Ozone/Hydrogen peroxide (O3/H2O2)

The combination of O3 with H2O2 is also known as the peroxone system In aqueous solutions, H2O2 is found in the acid-base equilibrium as shown by Eq 2.8 The hydroperoxide ion (HO2-, the conjugate base of H2O2) reacts rapidly with ozone (Eq 2.9)

to initiate a radical chain mechanism that leads to the formation of *OH radicals The overall reaction equation is shown by Eq 2.10

)8.11(

2 2

2O ⇐⇒HO−+H+ pK a =

)10

2.2

* 3

* 2 3

→

* 2

2

Among the above four AOPs, the only AOP which does not involve UV but still capable

of producing *OH radicals is the AOP comprising of O3 and H2O2 Several methods have been used to increase O3 decomposition and produce high concentrations of *OH radicals One of the most common of these processes involves adding H2O2 to ozonated water, a process commonly referred to as peroxone By accelerating the O3 decomposition rate, the

*

OH radical concentration is elevated, which increases the oxidation rate This procedure increases the contribution of indirect oxidation over direct O3 oxidation The Metropolitan Water District of Southern California (MWDSC) conducted extensive research into the use of peroxone to control organics and oxidize taste and odor compounds (e.g., geosmin and 2-methylisoborneol [MIB]) while providing sufficient levels of molecular O3 to guarantee CT values and primary disinfection A key issue with the use of peroxone as a disinfection process is that the process does not provide a measurable disinfectant residual It is therefore not possible to calculate CT similar to the use of other disinfectants While no credit can be given for *OH free radicals because it cannot be measured directly, some credit may be considered for any detected O3 in peroxone systems

Both peroxone and other advanced oxidation processes have been proven to be equal or more effective than O3 for pathogen inactivation Disinfection credits are typically described in terms of CT requirements Because peroxone leaves no measurable, sustainable residual, calculation of an equivalent CT for disinfection credit is not possible, unless, there is measurable O3 residual The mode of action of O3 on microorganisms is poorly understood Some studies on bacteria suggest that O3 alters proteins and unsaturated bonds of fatty acids