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Special Case of Ozone (Physicochemical Properties, Onsite Generation Technology)
C H A P T E R
2.3 Special Case of Ozone (Physicochemical Properties, Onsite Generation Technology) Mar Perez-Calvo Technical Direction, Cosemar Ozono, Madrid, Spain
2.3.1 INTRODUCTION
ozone decomposes rapidly into O2 and CO2, leaving no toxic byproducts.
The term “ozone” is well known. Everyone is aware of the importance of the ozone layer, and how the hole that certain compounds (CFCs) were causing in this layer of the stratosphere was endangering our health because the ozone layer protects us from damaging ultraviolet radiation from the sun. The term is also known—and in this case perceived negatively—by the warnings that start to be issued every summer in cities when ozone concentrations in the lower troposphere increase because of pollution. What is less known about ozone is its high efficacy as a biocide due to its oxidizing power, which also allows it to be used as a deodorant and in the elimination of all types of chemical contamination, both in water and air. This strongly oxidizing nature of ozone makes it a particularly useful tool in the agrofood industry because added to its high efficiency in the elimination of microorganisms and chemical pollutants is the great advantage of its zero residuality:
Gases in Agro-food Processes https://doi.org/10.1016/B978-0-12-812465-9.00006-2
2.3.2 CHARACTERIZATION OF OZONE Ozone (O3) is a substance whose molecule is made up of three atoms of oxygen, formed by dissociating the two atoms that normally make up oxygen gas. When the oxygen in the air is subjected to a pulse of high energy, such as lightning, the double bond O]O of the oxygen is broken. This delivers two atoms, which then recombine with other molecules of this element. These recombined molecules contain three oxygen atoms instead of two, which creates ozone. Because the electron is being shared between three atoms instead of two, the resulting molecule is very unstable and tends to pick up electrons from any approaching compound to regain its stability; in other words, it is a strong oxidant. This gives it its extraordinary biocidal, deodorizing, and chemical compound
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# 2019 Elsevier Inc. All rights reserved.
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
destruction properties. By capturing electrons from other molecules and oxidizing them, ozone destabilizes them to the point of destroying them if the concentration of ozone and/or contact time is sufficient. At ambient temperature and pressure, ozone is an acrid-smelling, generally colorless gas; however, at high concentrations it may become slightly bluish. If it is breathed in large quantities, it can cause irritation to the eyes or throat, which usually happens after breathing fresh air for some minutes.
Boiling point
110.5°C
Melting point
251.4°C
Critical temperature
12°C
Critical pressure
54 atms.
Relative density in air
1.3 times heavier than air
Unstable and likely to explode easily
Liquid–112°C
Equivalence
1 ppm ¼ 2 mg/m3
Solid–192°C
2.3.2.1 Data Sheet
Identification Chemical name
Ozone
Relative molecular mass
48 g/L 3
Molar volume
22.4 m PTN/Kmol
Empirical formula
O3
CAS registration number
10028-15-6
EINECS reference
233-069-2
Density (gas)
2.144 g/L at 0°C
Density (liquid)
1.574 g/cm3 at 183°C
Condensation temperature at 100 kPa
112°C
Melting temperature
196°C
FIG. 2.3.1
Identification
Ozone is a gas that at room temperature is characterized by a pungent odor often perceptible during electrical storms as well as in the proximity of electrical equipment, as evidenced by Dutch philosopher Van Marun in 1785. At a temperature of 112°C, it condenses to an intense blue liquid. Under normal pressure and temperature conditions, ozone is 13 times more soluble in water than oxygen, but because of the higher concentration of oxygen in air, it is dissolved in water to a greater extent than ozone. The molecule has an angular structure with ˚ ; it an oxygen-oxygen bond length of 1.28 A can be represented according to Fig. 2.3.1.
Ozone structure.
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3.4 OZONE OF ANTHROPIC ORIGIN
Due to the instability of the compound, in this type of application ozone must be produced by generators on the site where it is to be applied (generation in situ). The fact that ozone cannot be stored makes it impossible to have a large amount of potentially dangerous oxidant, as in the case of chlorine or hypochlorite. This results in greater safety and eliminates the problems of storage of toxic substances and waste management, thus minimizing risks.
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in the chemical reactions that occur in the troposphere itself from other pollutants (precursor compounds), which react under the action of sunlight. This is why ozone is often referred to as a secondary pollutant (not emitted directly as a result of a particular activity) of photochemical origin. These photochemical processes occur naturally (from emissions from plants and other living things), so there is always a certain concentration of ozone at surface levels.
2.3.2.2 Half Life Ozone, as we mentioned, is an unstable molecule that reverts rapidly to biatomic oxygen. The half-life (the time in which half the ozone in the air decomposes) is 20–60 min, depending on the temperature and humidity of the ambient air. The half-life in water is approximately the same, although it depends on the temperature, pH, and quality of the water.
2.3.3 OZONE OF NATURAL ORIGIN Ozone is present in nature in two levels of the atmosphere: in the vicinity of the ground, in the lower troposphere (a layer that can reach up to 12 km in height), and at high levels in the stratosphere (with typical thicknesses of between 12 and 50 km): this is the so-called “Ozone layer”. In all cases, its formation and destruction are photochemical phenomena. The so-called “ozone layer” with the higer concentrations of this gas at 12–40 km from the ground, filters out ultraviolet rays harmful to humans (erythematous radiation), preventing the rays from reaching the Earth’s surface. At these levels, ozone is formed mainly by the action of solar radiation on atmospheric oxygen (the oxygen molecules break in their atoms— radiative dissociation—which are then recombined in the form of ozone molecules). On the contrary, the ozone that is present in the vicinity of the ground has its origin mainly
2.3.4 OZONE OF ANTHROPIC ORIGIN Any substance or anything produced or modified by human activity is called “anthropic.” Thus, anthropic ozone is generated by human activity, purposely for biocidal purposes or inadvertently as a secondary pollutant.
2.3.4.1 Ozone as a Result of Pollution Tropospheric ozone (not to be confused with stratospheric, the layer that protects the Earth from solar radiation) is a secondary pollutant, as we have indicated. This means that it is formed from complex photochemical reactions with intense sunlight between primary pollutants such as nitrogen oxides (NO, NO2), carbon monoxide (CO) and methane (CH4), and volatile organic compounds (VOCs). Nitrogen oxides are generated in combustion processes and especially by road traffic. Volatile organic compounds are generated from a number of varied sources: road transport, refineries, paint, dry cleaning, and other activities involving the use of solvents. As we can see, these are oxidizing gases from the outside, from the street, with no control of any kind in their generation, and whose concentrations are considered to be worth reporting from 180 mg/m3 onward. This concentrations are much highertham those of residual concentrations that etc. concentrations much higher than those of
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
residual concentrations that can be detected in indoor environments treated with ozone generators, which never exceed 100 mg/m3, as will be seen later.
2.3.4.2 Artificially Generated Ozone This is the one emitted by ozone-generating equipment mainly for disinfection and deodorization, although ozone is also used for other purposes such as the removal of chemical contaminants or as a bleaching agent, for example. The use of ozone in environmental decontamination is safe, contrary to how it may seem in principle, due to the perfect control over the residual levels of ozone in the breathable air. This allows the use of a highly effective disinfectant without undesirable effects on the people who are in the communal areas of the sites treated, avoiding to a great extent the risk of contagion and improving air quality. This is done not only in terms of microbiological levels but also in terms of unpleasant odors and heavy atmospheres, providing air that is healthy, clean, and fresh.
2.3.5 TECHNOLOGIES FOR GENERATION IN SITU Ozone cannot be stored or transported like other industrial gases, as it is much less stable than biatomic oxygen. The reason for this is that it quickly converts to oxygen, and therefore must be produced in the place where it will be used: this is called generation in situ. As we have already mentioned, in nature ozone is formed from complex photochemical reactions. This is an endothermic reaction, and therefore an input of energy is required for it to take place. Based on photochemical reactions, it has been possible to generate ozone artificially in two ways, equivalent to the natural ways: providing the energy necessary for the process by means of ultraviolet light
(in nature this energy comes from the rays of the sun), or by means of electrical energy, which in nature is brought about by the electrical phenomena of storms. • Ultraviolet light: Ultraviolet lamps have been used for decades to generate ozone. These lamps emit UV light at 185 nanometers (nm). Light is measured on a scale called the electromagnetic spectrum and its increments are called nm. Air (usually ambient) is passed over an ultraviolet lamp, which divides the oxygen (O2) molecules in the gas. The resulting oxygen atoms (O-), seeking stability, bind to other oxygen molecules (O2), forming ozone (O3). Ozone is injected into the water, or air stream, where it inactivates the pollutants by breaking the cell wall of the organisms or the bonds of chemical compounds. • Corona discharge: The “corona effect” is an electrical phenomenon that is produced by the ionization of the fluid surrounding a charged conductor. The technologies involved in ozone generation by corona discharge are varied, but all operate essentially by passing dry gas, which contains oxygen, through an electrical field. The electric current causes the “division” in the oxygen molecules as described in the section on the generation of ozone by ultraviolet light. Beyond this common feature, there are many variations, but generally accepted technologies can be divided into three types: low frequency (50–100 Hz), medium frequency (100–1000 Hz), and high frequency (>1000 Hz). In Fig. 2.3.2, you can see the schema of a corona discharge: Because 85%–95% of the electrical energy supplied to a corona discharge ozone generator produces heat, some type of method is required for the removal of this. In addition, adequate cooling significantly affects the energy efficiency of the ozone generator, so most of the corona discharge systems use air, water, or both as cooling methods.
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3.5 TECHNOLOGIES FOR GENERATION IN SITU
FIG. 2.3.2
FIG. 2.3.3
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Corona discharge.
“Corona effect” diagram.
At the heart of a corona discharge is the dielectric. The electrical charge diffuses on this dielectric surface, creating an electrical field, or “corona.” A corona effect diagram can be seen in Fig. 2.3.3. The most important thing for ozone systems by corona discharge is appropriate preparation of the air. The gas that supplies the ozone generator must be very dry (minimum 62°C dew point, the temperature at which the water vapor contained in the air starts to condense, producing dew, mist, or any type of cloud), because the presence of moisture affects the production of ozone and leads to the formation of nitric acid. Nitric acid is highly corrosive to critical internal parts of a corona discharge ozone generator, and can cause premature failure and significantly increase maintenance frequency. The relative production of ozone decreases as the moisture content of the air increases. Of the above-mentioned ozone technologies, none has a clear advantage. However, to help reduce the field for a particular application, we should consider the amount of ozone required in that application. For many years now, thanks to the fact that it is possible to work at high frequency as well as the progress made in electronics, there has been a change from
using large transformers at low frequency and valves to smaller ozone equipment. That smaller equipment consumes less energy and has very low dissipation of heat at <60°C, making water cooling of the equipment unnecessary, with even higher outputs of 100 g/h and a considerably lower price. On the other hand, faced with the disadvantages of ozone generation by UV, corona discharge offers certain advantages that make this the method most used for ozone generation:
2.3.5.1 Generation Through UV 1. The maximum rate of ozone production is two grams/h per UV lamp, depending on the size. 2. The highest concentration of ozone that a 185 nm UV lamp can produce is 0.2% by volume, about 10% of the average available concentration per corona discharge. 3. More electrical energy is required to produce a given amount of ozone by UV radiation than by corona discharge. 4. The low concentrations in the gas phase of ozone generated by UV radiation result in the management of much higher gas volumes than in the case of ozone generated by corona discharge. 5. UV lamps solarize over time, requiring periodic replacement.
2.3.5.2 Generation by Corona discharge 1. Corona discharge ozone generators can use prepared oxygen, thus doubling the production of ozone by volume given compared to dry air.
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
2. These are small constructions that allow the generator to be installed practically anywhere. 3. It can generate a purer form of ozone without giving rise to other harmful or irritating gases if dry air or oxygen is used as the feed gas. 4. The cell life of the corona can exceed 10 years. 5. It can generate large amounts of ozone. 6. It is more cost-effective than UV ozone generation.
2.3.6 OZONE MECHANISM OF ACTION Ozone is a gas with substantial oxidizing power. Its high capacity to destroy microorganisms such as bacteria, viruses, and contaminating chemical compounds (among them the molecules responsible for odors), together with its innocuousness, make it a valuable tool for reaching the highest levels in environmental quality and food safety. Unlike other chemical products, ozone, after performing its disinfecting function, reverts to oxygen in a relatively short space of time, guaranteeing the absence of any chemical residue on the surface of the food or water treated through this process.
2.3.6.1 How Ozone Depletes Microorganisms This gas can exert its oxidizing power through two mechanisms of action: 1. Direct oxidation of compounds by molecular ozone. 2. Oxidation by hydroxyl free radicals. Hydroxyl free radicals, (OH), are generated according to the reactions shown in Fig. 2.3.4. The free radicals thus generated constitute one of the most potent oxidants in water, with a potential of 2.80 V. However, they have the disadvantage that their half-life is on the order of microseconds, although the oxidation they carry out is much faster than direct oxidation by ozone molecules.
FIG. 2.3.4
Hydroxyl free radicals generation.
Thus, depending on the conditions of the medium, either oxidation route may predominate: • Under low pH conditions, molecular oxidation predominates. • Under conditions that favor the production of hydroxyl radicals, such as high pH, exposure to ultraviolet radiation, or the addition of hydrogen peroxide, oxidation by hydroxyl starts to predominate (EPA Guidance Manual, 1999). As you can see in Fig. 2.3.5, hydroxyl radicals (OH-) and ozone show the higher ORP. It can be said that ozone has no limits in terms of the number and species of microorganisms that it can eliminate because it acts on them on several levels. Direct oxidation of the cell wall is its main mode of action. This oxidation causes the rupture of said wall, thus causing the cellular components to emerge outside the cell. Furthermore, the production of hydroxyl radicals as a consequence of disintegration of the ozone causes an effect similar to that explained. The damage caused to microorganisms is not limited to the oxidation of their wall: ozone also causes damage to the components of nucleic acids (DNA and RNA), causing the rupture of carbon-nitrogen bonds and resulting in depolymerization. The microorganisms are therefore unable to develop immunity to ozone as they face other compounds. As an example, we can say that the human body also protects itself through oxidation processes. In fact, white blood cells locate microbes in the bloodstream and are phagocytized by them. Once inside the cell wall of the
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3.7 STANDARDS AND DOSAGE
FIG. 2.3.5
ORP of common disinfectants.
microorganism, the white blood cell will metabolize water into oxidants such as hydroxyl (OH-) and hydrogen peroxide (H2O2) to destroy the cell from the inside out. Ozone is therefore effective in the elimination of bacteria, viruses, protozoa, nematodes, fungi, cell aggregates, spores, and cysts. Meanwhile, it acts at a lower concentration and with less contact time than other disinfectants such as chlorine, chlorine dioxide, and monochloramines. The effect of ozone below a certain critical concentration value is insignificant or zero. Above this level, all pathogens are ultimately destroyed. This effect is known as an “all-ornothing” response, and the critical level is called a “threshold value.” According to the WHO, ozone is the most efficient disinfectant for all types of microorganisms.1 In the WHO document to which we refer, it is stated that at ozone concentrations of 0.1–0.2 mg/L.min, a 99% inactivation of rotavirus and poliovirus is achieved, among other pathogens studied. 1
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2.3.7 STANDARDS AND DOSAGE Because ozone is a compound classified as an irritant when inhaled, and above all due to its appearance as a secondary pollutant, which we have already mentioned, its presence in air for breathing is regulated by various standards from European and American organizations. Of course, they all agree when it comes to stipulating the maximum levels allowed based on the exposure time, as we will see below.
2.3.7.1 European Standards In order to regulate the use of toxic substances, whatever their purpose, the various countries of the European Union first independently issued regulations and standards so that the use of these substances would cause the least possible harm to health and the environment. With progress in toxicity studies and the globalization of trade in the European community, a directive was finally drawn up to govern this issue and made valid for all EU countries.
http://www.who.int/agua_sanitation_health/dwq/S04.pdf
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
Thus, Regulation (EU) 528/2012 of the European Parliament and of the Council of May 22, 2012, on the use and marketing of biocidal products (BPR) came into force on Sept. 1, 2013. Ozone was declared an active substance of biocidal products in the BPR and the dossiers submitted by the European Ozone Trade Association (EUOTA) and EurO3zon for its approval as an active substance have been accepted; the second part is currently being prepared, the authorization for ozone as a biocide, a product normally generated in situ. This allows the additional use of ozone in treatment processes in accordance with EU legislation.
2.3.7.2 North American Standards Ozone has been given GRAS approval by the US Department of Agriculture (USDA) and the US Food and Drug Administration (USFDA) for direct contact with food products, including all meat and poultry products. While good manufacturing procedures must be in place, no regulation exists on levels of ozone in food-processing applications. The final rule from the FDA providing GRAS approval was given in 2001; the USDA followed with the final rule granting GRAS approval for ozone in 2002. • FDA Federal Register Vol. 66 No. 123: The USFDA is amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry (FDA, 2001). • USDA final rule on ozone dated 12/17/2002, FSIS Directive 7120.1, states: Ozone can be used on all meat and poultry products. Ozone can be used in accordance with current industry standards of good manufacturing practice. No other guidelines are given on levels or dosages of ozone. • USDA Guidance on ingredients and sources of radiation used to reduce microorganisms
on carcasses, ground beef, and beef trimmings: Ozone is classified as a secondary direct food additive/processing aid allowable for all meat and poultry products. • USDA Reference 21 CFR 173.368: Ozone (CAS Reg. 10028–15–6) may be safely used in the treatment, storage, and processing of foods, including meat and poultry (unless such use is precluded by standards of identity in 9 CFR part 319), in accordance with the following prescribed conditions: (a) The additive is an unstable, colorless gas with a pungent, characteristic odor, which occurs freely in nature. It is produced commercially by passing electrical discharges or ionizing radiation through air or oxygen. (b) The additive is used as an antimicrobial agent as defined in § 170.3(o)(2) of this chapter. (c) The additive meets the specifications for ozone in the Food Chemicals Codex, 4th ed. (1996), p. 277, which is incorporated by reference. The director of the Office of the Federal Register approves this incorporation by reference in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies are available from the National Academy Press, 2101 Constitution Ave. NW., Washington, DC 20055, or may be examined at the Office of Premarket Approval (HFS–200), Center for Food Safety and Applied Nutrition, Food and Drug Administration, 200C St. SW., Washington, DC, and the Office of the Federal Register, 800 North Capitol St. NW., suite 700, Washington, DC. (d) The additive is used in contact with food, including meat and poultry (unless such use is precluded by standards of identity in 9 CFR part 319 or 9 CFR part 381, subpart P), in the gaseous or aqueous
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3.7 STANDARDS AND DOSAGE
phase in accordance with current industry standards of good manufacturing practice. (e) When used on raw agricultural commodities, the use is consistent with section 201(q)(1)(B)(i) of the Federal Food, Drug, and Cosmetic Act (the act) and not applied for use under section 201(q)(1)(B)(i) (I), (q)(1)(B)(i)(II), or (q)(1)(B)(i)(III) of the act.
2.3.7.3 Regulatory Dosage As for its toxicological data sheet, ozone is classified only as an Xi IRRITANT AGENT in air, and is not classified as carcinogenic. This classification as an irritant refers exclusively to its concentrations in air, that is, to problems arising from its inhalation, which depend on the concentration to which people are exposed as well as the time of said exposure. As we will see below, exposure to ozone, whether due to its presence as a pollutant or to the treatment of air for biocidal purposes, is fully regulated, with all standards coinciding in this respect on the maximum exposure values, taking into account the dose/exposure time ratio. In fact, ozone is one of six common pollutants limited by the US Environmental Protection Agency and other environmental regulatory agencies due, as stated at the outset, to its presence as a secondary pollutant in large cities. The Occupational Safety and Health Administration and its counterparts around the world control exposure to ozone in the workplace.
INHALATION TOXICITY DATA • TLV: 0.1 ppm. • Safety recommendations of the UNE 400-20194 standard: <100 mg/m3 (equivalent to 0.05 ppm). • The threshold limit values (TLV) establish limits for exposure to ozone according to the activity carried out, with the most restrictive
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value being 0.05 ppm (exposure times of 8 h) and 0.2 ppm for periods of <2 h. The EPA sets a standard of 0.12 ppm for 1 h exposure and the WHO proposes a reference value of 120 mg/m3 or 0.06 ppm for a maximum period of 8 h.
In the case of treatments performed without any people present, the doses can be as high as necessary in each application. Given the short halflife of ozone, the safety time prior to access to the treated site will not be very long (from 20 to 60 min), and active carbon filters, if necessary, can also easily destroy ozone excess. As for the use of ozone in water, this is regulated by its corresponding standard, its usual application being in making water safe for drinking. In the case of water treatment for uses other than human consumption, the doses vary according to the characteristics of the water to be treated and the purpose for which the water is intended. Because ozone is completely harmless when dissolved in water, there is no limit on the dose other than that which establishes the necessary efficiency in each case (recovery of waste water for irrigation, recreational or ornamental uses, removal of chemical compounds from waste water in the textile industry, bleaching of fibers, washing of foodstuffs, etc.) Standard UNE-EN 1278 Chemicals used in the treatment of water intended for human consumption. Ozone. In the words of this standard, “Ozone is used in the treatment of water for disinfection, improvement of the general organoleptic quality of water, removal of iron and manganese, elimination of color, advanced oxidation of persistent pollutants, and as a reagent to promote coagulation.” The treatment dose established, taking into account that this varies depending on the quality of the water and the objective of the treatment, is the following: • For disinfection, including inactivation of viruses and parasites, a dose is applied such
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
that a residual concentration of 0.4 mg/L is obtained after a contact time of 4–6 min. With this treatment, other general acceptability parameters such as color, taste, and odor can be complied with. In treated water, the required dose of ozone is generally within the range of 2–4 mg/L. • For the initiation of biological treatment, a preliminary rule is to consider a dosage of 0.2– 0.3 mg of ozone per mg of total organic carbon. • For the oxidation of iron and manganese, the required dose can be determined from the stoichiometry of the reactions, which must be completed with the consumption of ozone due to other components of the water. • To promote coagulation, the required dose does not generally exceed 1 mg/L and excessive dosage should be avoided. Regarding the possible need to eliminate surplus ozone, the standard indicates that: “Ozone is self-decomposing in water. Therefore, at the commonly applied doses, no elimination process is generally required. [...]”
2.3.8 CONCLUSION In accordance with the mood that marks the 21st century, the generation of ozone with disinfectant purposes is presented, as opposed to the traditional chemical disinfectants, as the most effective and innovative treatment in hygiene. This is due to the peculiar characteristics of this compound that, thanks to its high oxidation potential (much higher than that of chlorine and other biocides), make ozone a powerful disinfectant with no residual value and is respectful of the environment.
References EPA, 1999. 815-R- 99-014. Alternative Disinfectants and Oxidants. Guidance Manual. https://nepis.epa.gov/Exe/ ZyPDF.cgi?Dockey¼2000229L.txt. FDA, 2001. Federal Register Vol. 66 No. 123. Secondary Direct Food Additives Permitted in Food for Human Consumption. https://www.fda.gov/OHRMS/Dockets/ 98fr/062601a.htm.
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3 Special Case of Ozone (Physicochemical Properties, Onsite Generation Technology) Mar Perez-Calvo Technical Direction, Cosemar Ozono, Madrid, Spain
2.3.1 INTRODUCTION
ozone decomposes rapidly into O2 and CO2, leaving no toxic byproducts.
The term “ozone” is well known. Everyone is aware of the importance of the ozone layer, and how the hole that certain compounds (CFCs) were causing in this layer of the stratosphere was endangering our health because the ozone layer protects us from damaging ultraviolet radiation from the sun. The term is also known—and in this case perceived negatively—by the warnings that start to be issued every summer in cities when ozone concentrations in the lower troposphere increase because of pollution. What is less known about ozone is its high efficacy as a biocide due to its oxidizing power, which also allows it to be used as a deodorant and in the elimination of all types of chemical contamination, both in water and air. This strongly oxidizing nature of ozone makes it a particularly useful tool in the agrofood industry because added to its high efficiency in the elimination of microorganisms and chemical pollutants is the great advantage of its zero residuality:
Gases in Agro-food Processes https://doi.org/10.1016/B978-0-12-812465-9.00006-2
2.3.2 CHARACTERIZATION OF OZONE Ozone (O3) is a substance whose molecule is made up of three atoms of oxygen, formed by dissociating the two atoms that normally make up oxygen gas. When the oxygen in the air is subjected to a pulse of high energy, such as lightning, the double bond O]O of the oxygen is broken. This delivers two atoms, which then recombine with other molecules of this element. These recombined molecules contain three oxygen atoms instead of two, which creates ozone. Because the electron is being shared between three atoms instead of two, the resulting molecule is very unstable and tends to pick up electrons from any approaching compound to regain its stability; in other words, it is a strong oxidant. This gives it its extraordinary biocidal, deodorizing, and chemical compound
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# 2019 Elsevier Inc. All rights reserved.
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2.3. SPECIAL CASE OF OZONE (PHYSICOCHEMICAL PROPERTIES, ONSITE GENERATION TECHNOLOGY)
destruction properties. By capturing electrons from other molecules and oxidizing them, ozone destabilizes them to the point of destroying them if the concentration of ozone and/or contact time is sufficient. At ambient temperature and pressure, ozone is an acrid-smelling, generally colorless gas; however, at high concentrations it may become slightly bluish. If it is breathed in large quantities, it can cause irritation to the eyes or throat, which usually happens after breathing fresh air for some minutes.
Boiling point
110.5°C
Melting point
251.4°C
Critical temperature
12°C
Critical pressure
54 atms.
Relative density in air
1.3 times heavier than air
Unstable and likely to explode easily
Liquid–112°C
Equivalence
1 ppm ¼ 2 mg/m3
Solid–192°C
2.3.2.1 Data Sheet
Identification Chemical name
Ozone
Relative molecular mass
48 g/L 3
Molar volume
22.4 m PTN/Kmol
Empirical formula
O3
CAS registration number
10028-15-6
EINECS reference
233-069-2
Density (gas)
2.144 g/L at 0°C
Density (liquid)
1.574 g/cm3 at 183°C
Condensation temperature at 100 kPa
112°C
Melting temperature
196°C
FIG. 2.3.1
Identification
Ozone is a gas that at room temperature is characterized by a pungent odor often perceptible during electrical storms as well as in the proximity of electrical equipment, as evidenced by Dutch philosopher Van Marun in 1785. At a temperature of 112°C, it condenses to an intense blue liquid. Under normal pressure and temperature conditions, ozone is 13 times more soluble in water than oxygen, but because of the higher concentration of oxygen in air, it is dissolved in water to a greater extent than ozone. The molecule has an angular structure with ˚ ; it an oxygen-oxygen bond length of 1.28 A can be represented according to Fig. 2.3.1.
Ozone structure.
2. CHEMICAL AND PHYSICAL GASES PROPERTIES, GASES PRODUCTION PROCESS, UNITS
2.3.4 OZONE OF ANTHROPIC ORIGIN
Due to the instability of the compound, in this type of application ozone must be produced by generators on the site where it is to be applied (generation in situ). The fact that ozone cannot be stored makes it impossible to have a large amount of potentially dangerous oxidant, as in the case of chlorine or hypochlorite. This results in greater safety and eliminates the problems of storage of toxic substances and waste management, thus minimizing risks.
67
in the chemical reactions that occur in the troposphere itself from other pollutants (precursor compounds), which react under the action of sunlight. This is why ozone is often referred to as a secondary pollutant (not emitted directly as a result of a particular activity) of photochemical origin. These photochemical processes occur naturally (from emissions from plants and other living things), so there is always a certain concentration of ozone at surface levels.
2.3.2.2 Half Life Ozone, as we mentioned, is an unstable molecule that reverts rapidly to biatomic oxygen. The half-life (the time in which half the ozone in the air decomposes) is 20–60 min, depending on the temperature and humidity of the ambient air. The half-life in water is approximately the same, although it depends on the temperature, pH, and quality of the water.
2.3.3 OZONE OF NATURAL ORIGIN Ozone is present in nature in two levels of the atmosphere: in the vicinity of the ground, in the lower troposphere (a layer that can reach up to 12 km in height), and at high levels in the stratosphere (with typical thicknesses of between 12 and 50 km): this is the so-called “Ozone layer”. In all cases, its formation and destruction are photochemical phenomena. The so-called “ozone layer” with the higer concentrations of this gas at 12–40 km from the ground, filters out ultraviolet rays harmful to humans (erythematous radiation), preventing the rays from reaching the Earth’s surface. At these levels, ozone is formed mainly by the action of solar radiation on atmospheric oxygen (the oxygen molecules break in their atoms— radiative dissociation—which are then recombined in the form of ozone molecules). On the contrary, the ozone that is present in the vicinity of the ground has its origin mainly
2.3.4 OZONE OF ANTHROPIC ORIGIN Any substance or anything produced or modified by human activity is called “anthropic.” Thus, anthropic ozone is generated by human activity, purposely for biocidal purposes or inadvertently as a secondary pollutant.
2.3.4.1 Ozone as a Result of Pollution Tropospheric ozone (not to be confused with stratospheric, the layer that protects the Earth from solar radiation) is a secondary pollutant, as we have indicated. This means that it is formed from complex photochemical reactions with intense sunlight between primary pollutants such as nitrogen oxides (NO, NO2), carbon monoxide (CO) and methane (CH4), and volatile organic compounds (VOCs). Nitrogen oxides are generated in combustion processes and especially by road traffic. Volatile organic compounds are generated from a number of varied sources: road transport, refineries, paint, dry cleaning, and other activities involving the use of solvents. As we can see, these are oxidizing gases from the outside, from the street, with no control of any kind in their generation, and whose concentrations are considered to be worth reporting from 180 mg/m3 onward. This concentrations are much highertham those of residual concentrations that etc. concentrations much higher than those of
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residual concentrations that can be detected in indoor environments treated with ozone generators, which never exceed 100 mg/m3, as will be seen later.
2.3.4.2 Artificially Generated Ozone This is the one emitted by ozone-generating equipment mainly for disinfection and deodorization, although ozone is also used for other purposes such as the removal of chemical contaminants or as a bleaching agent, for example. The use of ozone in environmental decontamination is safe, contrary to how it may seem in principle, due to the perfect control over the residual levels of ozone in the breathable air. This allows the use of a highly effective disinfectant without undesirable effects on the people who are in the communal areas of the sites treated, avoiding to a great extent the risk of contagion and improving air quality. This is done not only in terms of microbiological levels but also in terms of unpleasant odors and heavy atmospheres, providing air that is healthy, clean, and fresh.
2.3.5 TECHNOLOGIES FOR GENERATION IN SITU Ozone cannot be stored or transported like other industrial gases, as it is much less stable than biatomic oxygen. The reason for this is that it quickly converts to oxygen, and therefore must be produced in the place where it will be used: this is called generation in situ. As we have already mentioned, in nature ozone is formed from complex photochemical reactions. This is an endothermic reaction, and therefore an input of energy is required for it to take place. Based on photochemical reactions, it has been possible to generate ozone artificially in two ways, equivalent to the natural ways: providing the energy necessary for the process by means of ultraviolet light
(in nature this energy comes from the rays of the sun), or by means of electrical energy, which in nature is brought about by the electrical phenomena of storms. • Ultraviolet light: Ultraviolet lamps have been used for decades to generate ozone. These lamps emit UV light at 185 nanometers (nm). Light is measured on a scale called the electromagnetic spectrum and its increments are called nm. Air (usually ambient) is passed over an ultraviolet lamp, which divides the oxygen (O2) molecules in the gas. The resulting oxygen atoms (O-), seeking stability, bind to other oxygen molecules (O2), forming ozone (O3). Ozone is injected into the water, or air stream, where it inactivates the pollutants by breaking the cell wall of the organisms or the bonds of chemical compounds. • Corona discharge: The “corona effect” is an electrical phenomenon that is produced by the ionization of the fluid surrounding a charged conductor. The technologies involved in ozone generation by corona discharge are varied, but all operate essentially by passing dry gas, which contains oxygen, through an electrical field. The electric current causes the “division” in the oxygen molecules as described in the section on the generation of ozone by ultraviolet light. Beyond this common feature, there are many variations, but generally accepted technologies can be divided into three types: low frequency (50–100 Hz), medium frequency (100–1000 Hz), and high frequency (>1000 Hz). In Fig. 2.3.2, you can see the schema of a corona discharge: Because 85%–95% of the electrical energy supplied to a corona discharge ozone generator produces heat, some type of method is required for the removal of this. In addition, adequate cooling significantly affects the energy efficiency of the ozone generator, so most of the corona discharge systems use air, water, or both as cooling methods.
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FIG. 2.3.2
FIG. 2.3.3
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Corona discharge.
“Corona effect” diagram.
At the heart of a corona discharge is the dielectric. The electrical charge diffuses on this dielectric surface, creating an electrical field, or “corona.” A corona effect diagram can be seen in Fig. 2.3.3. The most important thing for ozone systems by corona discharge is appropriate preparation of the air. The gas that supplies the ozone generator must be very dry (minimum 62°C dew point, the temperature at which the water vapor contained in the air starts to condense, producing dew, mist, or any type of cloud), because the presence of moisture affects the production of ozone and leads to the formation of nitric acid. Nitric acid is highly corrosive to critical internal parts of a corona discharge ozone generator, and can cause premature failure and significantly increase maintenance frequency. The relative production of ozone decreases as the moisture content of the air increases. Of the above-mentioned ozone technologies, none has a clear advantage. However, to help reduce the field for a particular application, we should consider the amount of ozone required in that application. For many years now, thanks to the fact that it is possible to work at high frequency as well as the progress made in electronics, there has been a change from
using large transformers at low frequency and valves to smaller ozone equipment. That smaller equipment consumes less energy and has very low dissipation of heat at <60°C, making water cooling of the equipment unnecessary, with even higher outputs of 100 g/h and a considerably lower price. On the other hand, faced with the disadvantages of ozone generation by UV, corona discharge offers certain advantages that make this the method most used for ozone generation:
2.3.5.1 Generation Through UV 1. The maximum rate of ozone production is two grams/h per UV lamp, depending on the size. 2. The highest concentration of ozone that a 185 nm UV lamp can produce is 0.2% by volume, about 10% of the average available concentration per corona discharge. 3. More electrical energy is required to produce a given amount of ozone by UV radiation than by corona discharge. 4. The low concentrations in the gas phase of ozone generated by UV radiation result in the management of much higher gas volumes than in the case of ozone generated by corona discharge. 5. UV lamps solarize over time, requiring periodic replacement.
2.3.5.2 Generation by Corona discharge 1. Corona discharge ozone generators can use prepared oxygen, thus doubling the production of ozone by volume given compared to dry air.
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2. These are small constructions that allow the generator to be installed practically anywhere. 3. It can generate a purer form of ozone without giving rise to other harmful or irritating gases if dry air or oxygen is used as the feed gas. 4. The cell life of the corona can exceed 10 years. 5. It can generate large amounts of ozone. 6. It is more cost-effective than UV ozone generation.
2.3.6 OZONE MECHANISM OF ACTION Ozone is a gas with substantial oxidizing power. Its high capacity to destroy microorganisms such as bacteria, viruses, and contaminating chemical compounds (among them the molecules responsible for odors), together with its innocuousness, make it a valuable tool for reaching the highest levels in environmental quality and food safety. Unlike other chemical products, ozone, after performing its disinfecting function, reverts to oxygen in a relatively short space of time, guaranteeing the absence of any chemical residue on the surface of the food or water treated through this process.
2.3.6.1 How Ozone Depletes Microorganisms This gas can exert its oxidizing power through two mechanisms of action: 1. Direct oxidation of compounds by molecular ozone. 2. Oxidation by hydroxyl free radicals. Hydroxyl free radicals, (OH), are generated according to the reactions shown in Fig. 2.3.4. The free radicals thus generated constitute one of the most potent oxidants in water, with a potential of 2.80 V. However, they have the disadvantage that their half-life is on the order of microseconds, although the oxidation they carry out is much faster than direct oxidation by ozone molecules.
FIG. 2.3.4
Hydroxyl free radicals generation.
Thus, depending on the conditions of the medium, either oxidation route may predominate: • Under low pH conditions, molecular oxidation predominates. • Under conditions that favor the production of hydroxyl radicals, such as high pH, exposure to ultraviolet radiation, or the addition of hydrogen peroxide, oxidation by hydroxyl starts to predominate (EPA Guidance Manual, 1999). As you can see in Fig. 2.3.5, hydroxyl radicals (OH-) and ozone show the higher ORP. It can be said that ozone has no limits in terms of the number and species of microorganisms that it can eliminate because it acts on them on several levels. Direct oxidation of the cell wall is its main mode of action. This oxidation causes the rupture of said wall, thus causing the cellular components to emerge outside the cell. Furthermore, the production of hydroxyl radicals as a consequence of disintegration of the ozone causes an effect similar to that explained. The damage caused to microorganisms is not limited to the oxidation of their wall: ozone also causes damage to the components of nucleic acids (DNA and RNA), causing the rupture of carbon-nitrogen bonds and resulting in depolymerization. The microorganisms are therefore unable to develop immunity to ozone as they face other compounds. As an example, we can say that the human body also protects itself through oxidation processes. In fact, white blood cells locate microbes in the bloodstream and are phagocytized by them. Once inside the cell wall of the
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FIG. 2.3.5
ORP of common disinfectants.
microorganism, the white blood cell will metabolize water into oxidants such as hydroxyl (OH-) and hydrogen peroxide (H2O2) to destroy the cell from the inside out. Ozone is therefore effective in the elimination of bacteria, viruses, protozoa, nematodes, fungi, cell aggregates, spores, and cysts. Meanwhile, it acts at a lower concentration and with less contact time than other disinfectants such as chlorine, chlorine dioxide, and monochloramines. The effect of ozone below a certain critical concentration value is insignificant or zero. Above this level, all pathogens are ultimately destroyed. This effect is known as an “all-ornothing” response, and the critical level is called a “threshold value.” According to the WHO, ozone is the most efficient disinfectant for all types of microorganisms.1 In the WHO document to which we refer, it is stated that at ozone concentrations of 0.1–0.2 mg/L.min, a 99% inactivation of rotavirus and poliovirus is achieved, among other pathogens studied. 1
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2.3.7 STANDARDS AND DOSAGE Because ozone is a compound classified as an irritant when inhaled, and above all due to its appearance as a secondary pollutant, which we have already mentioned, its presence in air for breathing is regulated by various standards from European and American organizations. Of course, they all agree when it comes to stipulating the maximum levels allowed based on the exposure time, as we will see below.
2.3.7.1 European Standards In order to regulate the use of toxic substances, whatever their purpose, the various countries of the European Union first independently issued regulations and standards so that the use of these substances would cause the least possible harm to health and the environment. With progress in toxicity studies and the globalization of trade in the European community, a directive was finally drawn up to govern this issue and made valid for all EU countries.
http://www.who.int/agua_sanitation_health/dwq/S04.pdf
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Thus, Regulation (EU) 528/2012 of the European Parliament and of the Council of May 22, 2012, on the use and marketing of biocidal products (BPR) came into force on Sept. 1, 2013. Ozone was declared an active substance of biocidal products in the BPR and the dossiers submitted by the European Ozone Trade Association (EUOTA) and EurO3zon for its approval as an active substance have been accepted; the second part is currently being prepared, the authorization for ozone as a biocide, a product normally generated in situ. This allows the additional use of ozone in treatment processes in accordance with EU legislation.
2.3.7.2 North American Standards Ozone has been given GRAS approval by the US Department of Agriculture (USDA) and the US Food and Drug Administration (USFDA) for direct contact with food products, including all meat and poultry products. While good manufacturing procedures must be in place, no regulation exists on levels of ozone in food-processing applications. The final rule from the FDA providing GRAS approval was given in 2001; the USDA followed with the final rule granting GRAS approval for ozone in 2002. • FDA Federal Register Vol. 66 No. 123: The USFDA is amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry (FDA, 2001). • USDA final rule on ozone dated 12/17/2002, FSIS Directive 7120.1, states: Ozone can be used on all meat and poultry products. Ozone can be used in accordance with current industry standards of good manufacturing practice. No other guidelines are given on levels or dosages of ozone. • USDA Guidance on ingredients and sources of radiation used to reduce microorganisms
on carcasses, ground beef, and beef trimmings: Ozone is classified as a secondary direct food additive/processing aid allowable for all meat and poultry products. • USDA Reference 21 CFR 173.368: Ozone (CAS Reg. 10028–15–6) may be safely used in the treatment, storage, and processing of foods, including meat and poultry (unless such use is precluded by standards of identity in 9 CFR part 319), in accordance with the following prescribed conditions: (a) The additive is an unstable, colorless gas with a pungent, characteristic odor, which occurs freely in nature. It is produced commercially by passing electrical discharges or ionizing radiation through air or oxygen. (b) The additive is used as an antimicrobial agent as defined in § 170.3(o)(2) of this chapter. (c) The additive meets the specifications for ozone in the Food Chemicals Codex, 4th ed. (1996), p. 277, which is incorporated by reference. The director of the Office of the Federal Register approves this incorporation by reference in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies are available from the National Academy Press, 2101 Constitution Ave. NW., Washington, DC 20055, or may be examined at the Office of Premarket Approval (HFS–200), Center for Food Safety and Applied Nutrition, Food and Drug Administration, 200C St. SW., Washington, DC, and the Office of the Federal Register, 800 North Capitol St. NW., suite 700, Washington, DC. (d) The additive is used in contact with food, including meat and poultry (unless such use is precluded by standards of identity in 9 CFR part 319 or 9 CFR part 381, subpart P), in the gaseous or aqueous
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2.3.7 STANDARDS AND DOSAGE
phase in accordance with current industry standards of good manufacturing practice. (e) When used on raw agricultural commodities, the use is consistent with section 201(q)(1)(B)(i) of the Federal Food, Drug, and Cosmetic Act (the act) and not applied for use under section 201(q)(1)(B)(i) (I), (q)(1)(B)(i)(II), or (q)(1)(B)(i)(III) of the act.
2.3.7.3 Regulatory Dosage As for its toxicological data sheet, ozone is classified only as an Xi IRRITANT AGENT in air, and is not classified as carcinogenic. This classification as an irritant refers exclusively to its concentrations in air, that is, to problems arising from its inhalation, which depend on the concentration to which people are exposed as well as the time of said exposure. As we will see below, exposure to ozone, whether due to its presence as a pollutant or to the treatment of air for biocidal purposes, is fully regulated, with all standards coinciding in this respect on the maximum exposure values, taking into account the dose/exposure time ratio. In fact, ozone is one of six common pollutants limited by the US Environmental Protection Agency and other environmental regulatory agencies due, as stated at the outset, to its presence as a secondary pollutant in large cities. The Occupational Safety and Health Administration and its counterparts around the world control exposure to ozone in the workplace.
INHALATION TOXICITY DATA • TLV: 0.1 ppm. • Safety recommendations of the UNE 400-20194 standard: <100 mg/m3 (equivalent to 0.05 ppm). • The threshold limit values (TLV) establish limits for exposure to ozone according to the activity carried out, with the most restrictive
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value being 0.05 ppm (exposure times of 8 h) and 0.2 ppm for periods of <2 h. The EPA sets a standard of 0.12 ppm for 1 h exposure and the WHO proposes a reference value of 120 mg/m3 or 0.06 ppm for a maximum period of 8 h.
In the case of treatments performed without any people present, the doses can be as high as necessary in each application. Given the short halflife of ozone, the safety time prior to access to the treated site will not be very long (from 20 to 60 min), and active carbon filters, if necessary, can also easily destroy ozone excess. As for the use of ozone in water, this is regulated by its corresponding standard, its usual application being in making water safe for drinking. In the case of water treatment for uses other than human consumption, the doses vary according to the characteristics of the water to be treated and the purpose for which the water is intended. Because ozone is completely harmless when dissolved in water, there is no limit on the dose other than that which establishes the necessary efficiency in each case (recovery of waste water for irrigation, recreational or ornamental uses, removal of chemical compounds from waste water in the textile industry, bleaching of fibers, washing of foodstuffs, etc.) Standard UNE-EN 1278 Chemicals used in the treatment of water intended for human consumption. Ozone. In the words of this standard, “Ozone is used in the treatment of water for disinfection, improvement of the general organoleptic quality of water, removal of iron and manganese, elimination of color, advanced oxidation of persistent pollutants, and as a reagent to promote coagulation.” The treatment dose established, taking into account that this varies depending on the quality of the water and the objective of the treatment, is the following: • For disinfection, including inactivation of viruses and parasites, a dose is applied such
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that a residual concentration of 0.4 mg/L is obtained after a contact time of 4–6 min. With this treatment, other general acceptability parameters such as color, taste, and odor can be complied with. In treated water, the required dose of ozone is generally within the range of 2–4 mg/L. • For the initiation of biological treatment, a preliminary rule is to consider a dosage of 0.2– 0.3 mg of ozone per mg of total organic carbon. • For the oxidation of iron and manganese, the required dose can be determined from the stoichiometry of the reactions, which must be completed with the consumption of ozone due to other components of the water. • To promote coagulation, the required dose does not generally exceed 1 mg/L and excessive dosage should be avoided. Regarding the possible need to eliminate surplus ozone, the standard indicates that: “Ozone is self-decomposing in water. Therefore, at the commonly applied doses, no elimination process is generally required. [...]”
2.3.8 CONCLUSION In accordance with the mood that marks the 21st century, the generation of ozone with disinfectant purposes is presented, as opposed to the traditional chemical disinfectants, as the most effective and innovative treatment in hygiene. This is due to the peculiar characteristics of this compound that, thanks to its high oxidation potential (much higher than that of chlorine and other biocides), make ozone a powerful disinfectant with no residual value and is respectful of the environment.
References EPA, 1999. 815-R- 99-014. Alternative Disinfectants and Oxidants. Guidance Manual. https://nepis.epa.gov/Exe/ ZyPDF.cgi?Dockey¼2000229L.txt. FDA, 2001. Federal Register Vol. 66 No. 123. Secondary Direct Food Additives Permitted in Food for Human Consumption. https://www.fda.gov/OHRMS/Dockets/ 98fr/062601a.htm.
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