Disinfection and Sterilization Using Ozone



Disinfection and Sterilization Using Ozone


Linda K. Weavers

G. B. Wickramanayake

Mikhail Shifrin

Gerald McDonnell



Ozone (O3), an unstable three-atom allotrope of oxygen, is formed by the excitation of molecular oxygen (O2) into atomic oxygen (O) in an energizing environment that allows the recombination of atoms into O3, which is a powerful oxidizing agent with a standard reduction potential of +2.07 V (O3/O2).1 Pure O3 melts at a temperature of -192.5°C ± 0.4°C and boils at -111.9°C ± 0.3°C.2 At ambient temperatures, O3 is a blue-colored gas, but this color is not noticeable at the low concentrations typically used for disinfection purposes.3 The O3 was first introduced as a chemical disinfectant in drinking water treatment in 1893 at Oudshoorn, Netherlands,4 but has subsequently been described and used for a variety of disinfection and sterilization applications. Ozone is used in its gas form, as well as when dissolved in liquids such as water, for antimicrobial purposes.


BENEFITS OF AND CHALLENGES WITH OZONE IMPLEMENTATION

For practical applications, it is important to note that ozone is potentially the strongest and fastest acting oxidizer/disinfectant commercially available today based on its chemical properties, depending on how it is applied. That puts ozone in the position to be theoretically applied in any applications where other widely used oxidizing biocides such as chlorine or chlorine-releasing products (see chapter 15), chlorine dioxide (see chapter 27), hydrogen peroxide, and peracetic acid (see chapter 18) are used but typically more effectively and economically. In addition to ozone being considered more ecologically safer unlike many other biocides, it is produced from a raw product (air, or specifically the oxygen in air) that is free, and this makes ozone in most applications more economically attractive than other alternative oxidizers that are not typically produced on site. There are few applications where ozone cannot be practically used for antimicrobial purposes, and at worst, these are only considered to be slower in practice due to some external factors. Examples include the presence of extraneous materials (eg, organic soils or in the presence of biofilms; see chapter 67) that can react with to neutralize ozone and make it less available for antimicrobial activity, other restrictions in the access of ozone to target microorganisms, and other factors such as temperature. At lower temperatures in water, ozone was effective in inactivating Escherichia coli at lower temperatures and in the absence of contaminating (in this case, rumen content or feces-contaminated water).5 Ozone was found to inactivate E coli in water at an estimated concentration of 22 to 24 ppm ozone at 5°C, but efficacy was dramatically reduced in the presence of soil. Others found that the antimicrobial activity of ozone in water was greater at lower pH (pH 6 compared to 8) and at higher temperatures (35°C compared to 15°C), suggesting that optimal conditions can be applied depending on the exposure conditions and concentration of ozone applied.6 But equally, the stability of ozone will increase as the temperature increases. For water applications, typical ozone concentrations used are in the 0.2 to 0.4 mg/L range at a pH of 6 to 7, but higher concentrations (eg, 5 mg/L) are recommended in the presence of organic materials (eg, in wastewater) due to the increased neutralization of ozone. The process does not necessarily become more expensive even at lower temperature conditions because half-life of ozone in cold water is significantly longer and less ozone is therefore needed to be dissolved to achieve the desired antimicrobial concentration.

A further economic advantage, with the installation of a good quality ozone generating system, is limited operating costs other than a small electric consumption. In many cases, ozone is generated directly for air (approximately 20% oxygen), from which it is naturally made on exposure to ultraviolet (UV) light in the atmosphere. In general, no other consumables are regularly required, such as in the case of the use of chlorine systems or UV
light installations. But higher capacity ozone generators (eg, at 50-400mg/L) have also been developed that can generate ozone from oxygen (oxygen concentrators, liquid oxygen (LOX) or bottled oxygen), such as by passing through corona discharge or dielectric barrier discharge. Other methods such as with UV light and electrolytic generating cells are used for smaller applications, as they often have lower efficiency and higher associated costs.

Although ozone itself can be toxic to health at antimicrobial concentrations, the implementation of ozone is often considered a safer option than the use of most chemical oxidizers. Ozone has been reported to be mutagenic, causes reproductive damage, and inhalation leads to irritation of the lungs and upper respiratory tract.7,8 Higher exposure can cause headache, vomiting, and pain. Inhalation can cause tightness of the chest, including coughing, shortness of breath, and sensitivity to allergens. Repeated high exposure can cause lead to further lung damage and a fluid buildup in the lungs (pulmonary edema). At the same time, ozone has a relatively sharp odor that can be detected at low concentrations (approximately 0.01 to 0.005 ppm). The recommended occupational exposure limits in air (based on an 8-hour time weighted average) is at 0.1 ppm (at 25°C and 760 mm Hg) and 5 ppm as a ceiling or immediately dangerous to life or health.7,8 The rapid and reactive nature of ozone with contaminants allows it to be used effectively at often low concentrations and in a safe manner while limiting significant negative effects on human health. In many cases, such as in drinking water disinfection, swimming pool water treatment, life support systems, etc, that require higher concentrations of chemicals for proper disinfection, there is often direct and indirect harmful effect to people and other species from the use of chemical oxidizers, including by-products of their use such as trihalomethanes in the case of chlorine that are considered potentially harmful.8,9,10 Water disinfection by-products may also formed with ozone, but the safety in the use of ozone in comparison to chlorine continues to be a matter of debate.11,12 Ozone may also reduce problems associated with the risks of using other chemicals, such as chlorine. Ozone can be easily produced on site eliminating any need for chemical storage, transportation, and stringent personal training where injuries and death from accidents related to chemical handling have been reported.

The practical implementation of ozone in disinfection and sterilization applications is not without its challenges, where many reports in the literature can be misleading on the successful application, advantages, and disadvantages of the technologies. But there has been an increased focus in the area to understand ozone technology and demonstrate successful implementation. These include



  • Drinking water and waste water disinfection13,14


  • Water handling system sanitization, such as heat and cooling water and high purity water distribution systems15,16,17,18


  • Marine aquaria and ponds19,20


  • Food surface (eg, fruit, vegetables, and fish), packaging, and food-handling surface disinfection21,22


  • Laundry disinfection23,24


  • Air, air-handling systems, and area deodorization and disinfection15,25


  • Medical and dental device disinfection21,26,27 and sterilization.28,29 Over the last 20 to 30 years, there have been such systems patented and developed, but all have had little commercial success. This has been particularly due to material compatibility with devices, including metals, polymers, and adhesives that have limited repeated applications with ozone at higher concentrations to be able to meet disinfection and sterilization expectations for regulatory approval, such as sporicidal activity in the presence of high levels of organic and inorganic contaminants.29,30,31 Systems that claim to combine hydrogen peroxide and ozone gas for sterilization claim to have much greater material compatibility profiles than ozone alone, primarily due to the lower concentrations of ozone used in these sterilization systems.


  • In addition, there are reports on the therapeutic use of ozone, such as with skin diseases, wound healing, dentistry, periodontics, and other applications.32,33,34,35,36 This is an area of active research and further evidence is required to substantiate the safe and effective use of ozone for many of these applications in infection prevention and control.


OZONE GENERATION

Ozone generation is more efficient at low temperatures because of thermal decomposition at high temperatures and it is therefore important to remove as much heat from the discharge chamber (ozone cell) as possible. Ozone is typically generated for commercial applications by the same essential methods for 30 to 40 years, although advances have been made in the efficiencies of these systems (Figure 33.1). The most widely used methods use dielectric barrier discharge cells, also known as corona discharge cells (where a metallic mesh is used as an electrode material and plasma created around the wires of the mesh appears like a corona).

Ozone-generating cells are nothing more than an electric capacitor with two metal electrodes separated by quartz glass or ceramic dielectric material, through which dry air or oxygen is passed through to generate ozone (Figure 33.2). Just like any electric capacitor ozone-generating cell could be flat or cylindrical in geometry. Modern developments have included improvements in the density of electric discharge that creates more ozone per unit surface of an electrode, to the gas gap/electrode geometry to optimize and assist in improvement of power density, and to improvements in the materials, configurations, or cooling systems that facilitate efficient heat control inside the ozone cell created from the generation process.

The ozone produced can be used directly for airbased systems or in liquids bubbled or otherwise directly
injected into the water or other liquid. In these generation systems, ozone concentrations ranging from 1% to 3% can be produced if the feed gas is air and higher concentrations (eg, 3% to 24%) if the feed gas is oxygen. For water applications, due to low dissolved efficiency of ozone in water at low concentrations, dry air fed systems are considerably more expensive per gram of ozone dissolved than oxygen fed ones in water treatment applications. Although now less common than these discharge methods due to high power consumption and high operating cost, ozone may be generated by photochemical (eg, use of UV lights), electrolytic, and radiochemical means. The electrolytic methods can be cost-effective and convenient for certain low-volume applications, such as in laboratory or research work, where ozone can be produced already dissolved in the water. More specific details on ozone generation have been reviewed in detail by others.37,38,39 The focus on recent research has been on the design of more efficient ozone generators, to include the generation of higher ozone concentrations, high-pressure delivery systems for dissolution, decreased energy demand, and less oxygen consumption (when applicable).






FIGURE 33.1 Examples of ozone generation systems. Courtesy of Absolute Ozone®, Edmonton, Canada.






FIGURE 33.2 A representation of an ozone-generating cell.


STABILITY OF OZONE

The instability of gaseous and aqueous O3 prevents the prior generation and storage of ozone for later applications. Consequently, O3 needs to be generated on-site as needed for disinfection and sterilization purposes. Ozone is more stable in the gas phase than in the aqueous phase. For example, the half-life of gaseous O3 in ambient atmosphere is approximately 12 hours, but this can also vary depending on the other variables such as the temperature, quality of air, and presence of various contact surfaces.40,41 As shown in Table 33.1 the half-life of aqueous O3 varies
from hours to seconds, depending on water conditions, such as temperature, pH, water contaminants, O3 concentration, and concentration of promoters (that can speed up the decay rate) or radical scavengers (eg, carbonate ions that can inhibit the degradation rate).42,43,44








TABLE 33.1 Estimated half-lives (t½) of ozone in air and different water qualitiesa















































Conditions


Estimated Half-life (t½)


Air at −25°C


8 d


Air at 20°C


3 d


Air at 120°C


1.5 h


Water at pH 7 and 20°C


20 min


Water at pH 7 and 30°C


12 min


Water at pH 7 and 40°C


<4 min


Water at pH 7.5 and 15°C


15 min


Water at pH 8.5 and 15°C


10 min


Water at pH 9.5 and 15°C


5 min


Water at pH 10.5 and 15°C


<30 s


Distilled water


6 h


Tap water


30 min


Filtered lake water


10 min


aFrom McClurkin and Maier DE,40 Weschler,41 Staehelin and Hoigne,42 Batakliev et al,43 and Gardoni et al.44


Gurol and Singer45 developed the following empirical equation to incorporate some of these parameters to express the rate of decrease of O3 concentration in the pH range of 2 to 9.5:


where [O3] and [OH] are O3 and hydroxide ion concentrations, respectively, and k is the rate constant, which is a function of temperature.


ANALYTIC METHODS

Determination of residual O3 concentrations in aqueous solutions is rather difficult because of its rapid decomposition rates, its volatility from solution, and its reactivity with many organic and inorganic chemicals. It is important to note that most methods of measuring O3, which are often modifications of chlorine residual methods, are based on the determination of total oxidants in solution. Most of the O3 disinfection and sterilization data reported in the literature are collected using analytic techniques such as iodometric methods, which do not necessarily measure the O3 alone. For example, ozonation of olefins produces oxidizing species such as peroxides, hydro alkyl peroxides, and peroxy acids that interfere with iodometric and many other analytic techniques. Therefore, O3 disinfection data should be interpreted carefully, especially when different analytical techniques are used during the generation of inactivation data.

There are a wide variety of analytical methods used to determine dissolved ozone concentrations (Table 33.2).46 Overall, the most widely used, practical, and specific method is by UV adsorption, but other specific methods include the indigo method, amperometric method, and the stripping and gas phase detection method.47 The UV method is based on the absorption at 254 nm wavelength (provided from a mercury lamp at this wavelength in the UV spectrum), which is preferably absorbed by ozone.47 Although this method is more straightforward in most water applications, the effects of temperature and pressure in the sample chamber should be considered for air applications. The indigo method is the recommended standard method for measuring ozone residuals.48,49 In the indigo method, ozone adds across the carbon-carbon double bond of sulfonated indigo dye and decolorizes it. The change in absorbance is determined spectrophotometrically.49,50 The indigo method is generally considered to be subject to fewer interferences than most of the alternative colorimetric methods and all iodometric procedures that may be used.46,47 Overall, for dissolved ozone concentrations, the most accurate and widely used is the UV-based method, but these can often be more costly than alternative electrochemical methods.

Accurate measurement of gas-phase ozone is important in the determination of cost, efficiency, safety, and improvement in the design and construction of O3 generators that produce ozone at much higher concentrations.46,51 Accurate measurement of dissolved ozone is important for determining cost, efficiency, and effectiveness of water-dissolving devices as Venturi injectors or diffusers for water and wastewater disinfection as well as for establishing ozone demand and ozone system sizing. Therefore, precise and accurate analytic methods are necessary to measure O3 in the gas phase and dissolved in water. The most commonly used gas-phase analytic methods include iodometry, UV absorption, and chemiluminescence.46,51 Another method that can be used for O3 analysis includes gas-phase titration with excess nitric oxide and O3.52 The most widely used gas-phase O3 analytic techniques are listed in Table 33.3. Based on available information, the UV spectrophotometric method remains to be the most recommended for accurate determination of gasphase ozone.46,47,51 Overall, most industrial applications use ozone analyzers for measuring ozone concentration in gases and dissolved in liquids, and the most common are based on photometric UV absorption or alternative electrochemical measurement methods. Recent focus has been placed on the use of small, potable, and personal monitoring technologies for better monitoring of personal safety exposures to ozone and other gases.53,54









TABLE 33.2 Analytical methods for ozone in aqueous solutionsa










































































































































































































































Analytical Method


Detection Limit (mg/L)


Working Range (mg/L)


Expected Accuracy (%)


Expected Precision (%)


Interferences


pH Range


Current Status


Ideal


0.01


0.01-10


0.5


0.1


None


Independent


Recommended


Iodometric


0.002


0.5-100


1-35


1-2


All ozone byproducts and oxidants


<2


Not widely used


Arsenic back titration


0.002


0.5-65


1-5


1-2


Oxidizing species


6.8


Continued study


FACTS syringaldazine


0.02


0.5-5


5-20


1-5


Oxidizing species


6.6


NR


DPD


0.1


0.2-2


5-20


5


Oxidizing species


6.4


NR


Indigo spectrophotometric


0.001


0.01-0.1


1


0.5


Cl2, Mn ions, Br2, I2


2


Recommended


0.006


0.05-0.5


1


0.5


Cl2, Mn ions, Br2, I2


2


Recommended


0.1


>0.3


1


0.5


Cl2, Mn ions, Br2, I2


2


Recommended


0.01


0.01-0.1


5


5


Cl2, Mn ions, Br2, I2


2


Recommended



>0.1


5


5


Cl2, Mn ions, Br2, I2


2


Recommended


GDFIA


0.03


0.03-0.4 Other ranges possible


1


0.5


Cl2 at >1 mg/L


2


Comparison studies needed


LCV


0.005


NR


NR


NR


S2-, SO3-, Cr4-


2


Continued study


ACVK


0.25


0.05-1


NR


NR


Mn >1 mg/L Cl2 >10 mg/L


2


Continued study


o-Tolidine


Not quantitative


NR


NR


Metal ions, NO2-


2


Abandon


Bisterpyridine


0.004


0.05-20


2.7


2.1


Cl2


<7


Recommended (lab test)


Carmine indigo


<0.5


NR


NR


NR


NR


2


Continued study


Electrochemical amperometric


˜1


NR


5


5


Oxidizing species


2


Relative monitoring


Amperometric iodometric


˜0.5


NR


5


5


Oxidizing species


4-4.5


NR


Bare electrode


0.2


NF


5


5


NR


NR


Continued study


Membrane electrode


0.062


NF


5


5


NR


NR


Continued study


Differential pulse dropping mercury


NR


NR


NR


NR


NR


NR


Research lab


Differential pulse polarography


0.003


NR


NR


NR


NR


4


Continued study


Potentiometric


NR


NR


NR


NR


NR


NR


Continued study


Ultraviolet


0.02


>0.02


0.5


0.5


Other absorber


Independent


Establish molar absorptivity


Isothermal pressure change


4 × 10-3


4 × 10-3 to 10


0.5


0.5


None


Independent


Comparison study


Abbreviations: ACVK, acid chrome violet potassium; DPD, N-N-diethyl-p-phenylenediamine; FACTS, free available chlorine test with syringaldazine; GDFIA, gas diffusion flow injection analysis; LCV, leuco crystal violet; NR, not reported; NF, not found.


aFrom Gordon et al.46 Copyright © 1988 American Water Works Association. Reprinted by permission of John Wiley & Sons, Inc.










TABLE 33.3 Analytical methods for ozone in gas phasea













































































Analytical Method


Detection Limit (mg/L)


Working Range (mg/L)


Expected Accuracy (%)


Expected Precision (%)


Interferences


pH Range


Current Status


Ideal


1


1-50 000


1


1


None


Independent


Recommended


Ultraviolet


0.5


0.5-50 000


2


2.5


None


NA


Recommended


Stripping absorption iodometry


0.002


0.5-100


1-35


1-2


SO2, NO2


NA


Not widely used


Chemiluminescence


0.005


0.005-1


7


5


None


NA


Recommended


Gas-phase titration


0.005


0.005-30


8


8.5


None


NA


NR


Rhodamine B gallic acid


0.001


NR


NR


5


NR


NA


NR


Amperometry


NR


NR


NR


NR


NR


NA


NR


Abbreviations: NA, not applicable; NR, not reported.


aBased on a previous report46 but some of the indicated ranges are debated.


From Gordon et al.46 Reprinted by permission of John Wiley & Sons, Inc.



IMPLEMENTING OZONE IN WATER TREATMENT APPLICATIONS

One of the biggest problems that stands on the way of successful ozone implementation in water treatment is often a misunderstanding of Henry’s law of dissolving gases in liquids; a popular misconception often promoted in the literature suggests the larger the surface contact between source gas (bubbles) and target liquid, the more gas will be dissolved. Henry’s law states that, at a given temperature, the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas to the liquid. Therefore, in considering the efficiency of ozone dissolution in water applications, the efficiency of dissolution is actually relatively low, with typical reports of <10% dissolved ozone efficiency with microbubble diffusers and about 5% dissolved ozone efficiency when regular diffusers are used. Ozone dissolution can be increased by increasing the concentration of ozone in the gas, increasing the gas (oxygen pressure), and reducing the temperature and pH of the water. As an alternative to simpler diffusers, an inexpensive Venturi-type injector (using the reduction in pressure that results from the flow through a constricted section of a pipe) can deliver 70% to 90% or higher dissolved ozone efficiency when the ozone concentration injected is 6% weight per volume and higher. An example of dissolved ozone concentrations in shallow water is given in Figure 33.3.

Reports suggest that dissolved ozone efficiency will improve somewhat in other deeper water applications, but overall, such applications of the gas are considered more efficient than the use of bubble diffusers.55,56 This is supported by Henry’s law as there is no consideration of the size of the interface surface, only pressure, temperature, and gas concentration. In diffuser systems, for available ozone to dissolve in water, it needs energy and most of this comes from the speed of the bubble moving up in the liquid; the application of smaller bubbles to offer greater contact surface will actually move much slower and there is therefore less energy to allow for dissolution. Inside Venturi injectors, the gas is injected in to a vacuum area behind the restrictor and from that moment onward the pressure of the system grows to allow for greater dissolution until the liquid pressure is equalized with the system pressure. Overall, the greater the ozone dissolution achieved, the greater the disinfection efficacy can be for most practical applications.


INACTIVATION OF MICROORGANISMS


Bacteria

Ozone, as a potent oxidizing agent, has been well established as an effective disinfectant for bacteria.57 Reports in the literature can vary considerably due to various different test systems, including ozone generation methods (including inaccurate methods of determining ozone concentrations over exposure time), presence of interfering substances (which can rapidly react with ozone to neutralize its activity), and other factors previously discussed, such as concentration, temperature, and pH. Although such studies are rare, a useful comparison of the antimicrobial efficacy of ozone against a range of bacteria, viruses, and yeasts was useful to determine the response and relative resistance of these organisms to ozonation.58 The organisms tested include Mycobacterium fortuitum, E coli, Salmonella Typhimurium, poliovirus type 1 (Mahoney), and the yeast Candida parapsilosis. The experiments were conducted on two different test systems at pH 7.0 and 24°C. The first experiment was performed in deionized
water with O3 residual levels maintained between 0.23 and 0.26 mg/L. The second experiment was performed on an activated sludge effluent with the O3 residual levels maintained between 0.29 and 0.36 mg/L. Contact time for both systems was approximately 1.7 minutes. The resistance of these organisms to inactivation was in the following order for both systems: M fortuitum > poliovirus type 1 > C parapsilosis > E coli > Salmonella Typhi (Tables 33.4, 33.5 and 33.6). This profile is roughly as expected in comparison to other physical and chemical antimicrobial methods (see chapter 3).57 Table 33.4 compares the results from these studies with others against bacteria. For example, ozone concentrations of 0.1 to 0.63 mg/L were studied with Legionella pneumophila and demonstrated 4.6 to 5.2 log10 reductions at a contact time of 20 minutes.59 A similar study with E coli and fecal streptococci studied contact times ranging from 2.7 to 19 minutes with applied O3 concentrations from 0.45 to 4 mg/L. Under the conditions tested, the most effective levels of bactericidal activity in wastewater ranged from 2.2 to 4.0 mg/L for a 19-minute contact time. This gave an approximately 3 log10 reduction (99.9%) and leaving residual ozone concentrations of 0.06 to 0.35 mg/L.

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