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Bob Smith-McCollum is vice president of marketing for Pacific Ozone. Smith McCollum can be reached at 707.747.9600 or by e-mail at [email protected].
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Updated 10/12/20
In the March 2008 issue of Water Quality Products, the IOA column discussed mastering the fundamentals of ozone and reviewed the four basic elements required to form fully functional ozone systems: oxygen/feed gas preparation, ozone generation, mass transfer and monitoring and control.
The series continues this month with a closer look at the second basic element: ozone generation.
Ozone (O3) is created when diatomic oxygen (O2) is exposed to an electrical field or ultraviolet (UV) light. Exposure to these high levels of energy causes a portion of the diatomic oxygen molecules to split into individual oxygen atoms. These free oxygen atoms combine with diatomic oxygen molecules to form ozone.
Ozone is an unstable molecule due to the weak bonds holding the third oxygen atom, making ozone a naturally powerful oxidizing and disinfecting agent. Ozone delivers oxidizing power either directly or through the generation of hydroxyl-free radicals in the decomposition of dissolved ozone into water.
This reaction yields three simultaneous processes: oxidation, disinfection and decomposition. During the oxidation process, ozone—directly and through the highly reactive hydroxyl-free radicals—can break chemical bonds of organic compounds. For example, the components of the cell walls of microorganisms can be oxidized and broken down by ozone. This process facilitates disinfection by disrupting and lysing cell walls, exposing the contents of the cell to further oxidation and inactivation.
All common bacteria, viruses, molds, cysts and parasites can be destroyed by ozone in this manner. In the end, ozone decomposes to diatomic oxygen, leaving no unwanted residual taste or odor.
There are two principal technologies used to generate ozone: UV and corona discharge for commercial and industrial applications.
UV ozone generators utilize UV light at 185-nanometer wavelength that shine on feed gas (air or concentrated oxygen) flowing through a suitable tube chamber. Fundamentally, UV generators mimic the natural process responsible for the production of the ozone layer in the earth’s outer atmosphere. In the stratosphere, high-energy UV radiation from the sun splits diatomic oxygen that results in ozone formation. Ozone generators based on UV radiation typically produce relatively small amounts of ozone at a very low concentration.
For most commercial-industrial processes, ozone is generated by corona discharge reactor cells. A corona discharge is a diffuse and continuous luminous electrical discharge that occurs when a high-voltage electric field is produced between conductive and dielectric surfaces.
A dielectric is a substance that is a poor conductor of electricity (i.e., an insulator) but an efficient supporter of an electric field. In a corona discharge ozone reactor cell, the dielectric facilitates the formation of a broad and continuous corona across its surface to maximize the effective area of the corona in the reactor cell. As shown in the figure below, the dielectric material is bonded to one of the conductive electrode surfaces.
The space between the dielectric surface and the other conductive electrode surface forms an air gap called the dielectric gap. It is in this gap that the corona discharge forms when sufficient voltage is applied across the two electrodes. Corona discharge can produce moderate-to-high concentrations of ozone (typically 1 to 3% by weight from clean, dry air and up to 15% weight from concentrated oxygen) over broad ranges of output.
The characteristics of the dielectric materials and the size of the dielectric gap largely determine the behavior of the reactor cell. The greater the effective dielectric gap, the greater the power that must be applied to form and sustain the corona.
Power is applied to a given reactor cell at a specific frequency. This frequency is typically optimized based on the characteristics of the reactor cell and is set by the manufacturer of the ozone generator. Ozone generators generally fall into three frequency classes: low frequency (50 to 60 Hz), medium frequency (500 to 1,000 Hz) and high frequency (5,000 to 20,000+ Hz).
The ozone concentration and total ozone output from an ozone reactor cell or generator is a function of three user-controllable parameters: the power applied to the reactor cell, the oxygen concentration and the flow rate of the feed gas through the reactor cell. Ozone concentration and output (grams per hour or pounds per day) may be optimized by adjusting these parameters.
Ozone concentration and output from the reactor cell increase as more power is applied to the reactor cell. As with the frequency of the power, the range of power that can be applied to the ozone reactor cell and the duration of each pulse is typically optimized and set by the manufacturer of the generator.
Corona discharge ozone reactors dissipate approximately 85% of the energy supplied to the cell as heat. Unfortunately, ozone becomes less stable at higher temperatures. As more power is applied, the reactor cell may increase in temperature, which can limit ozone production. All commercial corona discharge reactors incorporate some form of cooling to counteract this phenomenon.
Oxygen concentration and flow rate of the feed gas also impact the ozone output of the reactor. Generally, ozone concentration can be increased by increasing the oxygen concentration of the feed gas. As with any chemical reaction, increasing the concentration of a reactant increases the concentration of the product of the reaction. Reducing the feed gas flow rate through the generator also increases ozone concentration by increasing the amount of time the oxygen spends in the reactor.
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The next article in this series will discuss mass transfer.
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Owing to ozone instability, it needs to be generated only when needed and used at once. Generation of ozone can be achieved by: (1) UV radiation, (2) an electrochemical process and (3) by corona discharge but it must be noted that the first two methods yield a low ozone output and a poor regulation of production The ozonetherapist must have an ozone generator that is safe, atoxic, and reproducible .
THE OXYGEN FLOW: this is expressed as a volume of litres per minute (l/min) and normally can be regulated from 1 up to about 10 l/min. The final ozone concentration is inversely proportional to the oxygen flow; hence, per time unit, the higher the oxygen flow, the lower the ozone concentration and vice versa.
The medical ozone generator consists of 2–4 high-voltage tubes connected in series to an electronic programme able to set up voltage differences between 4,000 and 13,000 V. In the system defined as the corona discharge ozonator, the ozone is formed when oxygen passes through a gap between high voltage and ground electrodes to create an energy field, denominated corona . The energy from the electric discharge allows the breakdown of oxygen molecules into oxygen atoms which, in the presence of an excess of oxygen molecules, form the three-atom ozone molecule. The generator is fed with pure medical oxygen and, at the supply nozzle, a gas mixture composed of no more than 5% ozone and 95% oxygen can be collected at a slightly positive pressure. The synthesis of ozone is allowed by the energy released by the electric discharge while the decomposition of ozone is accompanied by energy release. For medical purposes, air cannot be used because, by containing 78% nitrogen, the final gas mixture will contain, beside oxygen and ozone, a variable amount of highly toxic NOx
Unused ozone cannot be dispersed into the environment and it must be decomposed to oxygen by a catalytic reaction inside the indispensable destructor that contains heavy metal oxides maintained at about +70°C by an electric thermostat.
Owing to ozone instability, it needs to be generated only when needed and used at once. Generation of ozone can be achieved by: (1) UV radiation, (2) an electrochemical process and (3) by corona discharge but it must be noted that the first two methods yield a low ozone output and a poor regulation of production The ozonetherapist must have an ozone generator that is safe, atoxic, and reproducible . The instrument (Fig. ) must be built with the best ozone-resistant materials, such as Inox 316 L stainless steel, pure titanium grade 2, Pyrex glass, Teflon, Viton and polyurethane avoiding any material that could be released due to ozone oxidation. It is strongly suggested to purchase only a generator that allows to measure in real time the ozone concentration by mean of a reliable photometer.
As an example, for a volume of 100 ml blood, we use an equivalent volume of gas (1:1 ratio) with an ozone concentration of 40 mcg/ml, the total ozone dose is: 100 × 40 = 4,000 mcg or 4.0 mg.
A good ozonetherapist with an unreliable ozone generator cannot deliver an efficacious ozonetherapy.
Therefore, it is indispensable that the generator undergoes periodic maintenance including a control by iodometric titration of the photometer, to insure delivery of a precise ozone concentration.
The normal medical generators deliver ozone concentrations from 1 up to 70–100 mcg/ml. As the use of ozonetherapy will soon expand, I envisage the usefulness of a small, precise and handy generator able to produce ozone concentrations equivalent to 2, 5, 10, 20 and 30 mcg/ml. This range is suitable for treatments such as rectal insufflation, topical treatments, quasi-total body exposure, preparation of ozonated water and oil for patients to use at home under the supervision of an ozonetherapist. For several practical reasons, this new device will allow the use of ozone to many chronic patients that, otherwise, find impossible, or time-consuming, or too expensive to be treated, in a clinic.
Even today there are a number of obsolete or unchecked instruments in use so that clinical results remain questionable and often are not reproducible. If we really want ozonetherapy to progress, we need precise and reliable ozone generators. This is so because ozonetherapy is not based on a homeopatic concept that even a trace amount will be active, but on the firm pharmacological basis that ozone is and acts as a real drug and, as such, it must be quantitatively precise. Luckily European generators give the ozone concentration directly in terms of mcg/ml and the range 1–100 is sufficient for medical use. Modern generators allow to asses the ozone concentration by photometric determination. This is possible because there is a pronounced absorption of ozone within the Hartley band with a peak at 253.7 nm. At this wavelength, UV radiation (mercury vapour lamp) is linearly absorbed in a concentration-dependent fashion (in agreement with Lambert-Beer’s law) on being passed through a tube containing ozone. This system is quite sensitive and precise but the ozonetherapist must know that tends to decay due to lamp ageing. There is also the possibility of measuring ozone concentration at 600 nm (Chappuis band) that, although less sensitive, is more stable. The great advantage of the photometer is the possibility of checking on a digital display the ozone concentration in the gas mixture flowing into the syringe during withdrawal. However the photometer must be checked from time to time and possibly adjusted on the basis of ozone concentrations measured by the iodometric method considered the gold standard.
When ozone reacts with the KI solution, iodine is generated and the solution immediately acquires an amber colour which, upon reduction with a titrated solution of Na2S2O3 and a starch indicator allows the determination of the ozone concentration:
The concentration of ozone in gram per liter equals 24 × volume of thiosulphate in l × normality of thiosulphate divided by the inlet volume of gas flow, accurately measured in liters with a precise flowmeter. Both the entities of the gas and of the solution volumes per min were precisely determined at 21°C and normal atmospheric pressure. The detection limit of this analytical procedure was 0.1 mg/l and the reproducibility was ±2% of the measured ozone concentration.
The method has been approved by the IOA and the details have been reported by Masschelein in 1996.
When ozone reacts with buffered potassium iodide, iodine is generated and the colourless solution suddenly acquires an amber colour which, upon reduction with a titrated solution of sodium thiosulphate and a starch indicator allows the determination of the ozone concentration in gram/liter with a reproducibility of about 2% of the measured ozone concentration.
While the need of having a precise instrument is a must, in daily practice a couple of tips can be useful. Firstly, I learnt that what is important is the immediate use of the gas and not so much small changes (±1%) of concentration. Secondly, polypropylene, silicon-coated syringes must be used only once. Finally the generator must be regularly checked because the efficacy of the treatment depends upon the required ozone concentration.
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