By Deborah Kon
Introduction
Ozone has been used in many applications for over 100 years. As its benefits have become better known and well-established, the use of ozone in different applications has increased and continues to do so. It is an excellent alternative to chlorination in some applications and is used in a variety of industrial and residential applications. Some well-established applications are water treatment in potable water plants, treatment of industrial waste water, drinking water treatment, bottled water treatment, well water treatment and cooling towers. Some newer applications are in the food preparation and agricultural industries. Well-known residential applications include the treatment of water in swimming pools and spas.
This article provides a general overview of ozone properties, explains how ozone is produced, discusses two different types of ozone generators and provides a brief overview of some uses of ozone in residential and industrial water treatment.
What is ozone?
Oxygen is the most abundant element on our planet. It makes up about half of the earth’s crust and is present in both air and water. Oxygen atoms can exist in different forms, with one of the common forms being ozone (O3), which consists of three oxygen atoms. Ozone is a very reactive gas that causes the pungent odor sometimes noticed during electrical storms, as lightning can produce ozone. Although ozone is a pollutant in the lower atmosphere and is an easily measured byproduct of air pollution, its presence in the upper atmosphere, in a region known as the ozone layer, is extremely important because it absorbs ultraviolet light coming from the sun.
How is ozone produced and destroyed in nature?
Under the influence of sunlight, oxygen (O2) is continually being changed into ozone and ozone is likewise being converted back to oxygen. Each day, 350,000 tons of ozone are made and destroyed in the upper atmosphere. The principle reactions involved in the formation and destruction of ozone are known as the Chapman Mechanism.
Sunlight drives both ozone formation and destruction, but different wavelengths of light are involved. The absorption of the ultraviolet (UV) light and the consequent reactions are responsible for shielding the Earth’s surface from the harmful UV rays. In the formation of ozone, an oxygen molecule (O2) is split by UV light, whose wavelength is less than 200 nm, into oxygen atoms (O). The electron deficient and thus chemically unstable oxygen atoms quickly combine with other oxygen molecules (O2) to form ozone (O3). The ozone molecule absorbs the UV radiation whose wavelength is around 300 nm. This causes the ozone molecule to split into oxygen atoms (O) and oxygen molecules (O2). The shielding property of ozone is due to its ability to absorb and use the UV radiation from the sun to convert itself back to the oxygen molecule and thus oxygen is not lost in the process.
Lightning also has the ability to split the oxygen molecules thus creating oxygen atoms which combine with oxygen molecules to make ozone. In this case, however, it is the electrical energy discharge rather than UV light that splits an oxygen molecule.
What are the most common ways of producing ozone?
Ozone is a very reactive molecule that must be produced onsite using an ozone generator when employed for both commercial, residential and industrial uses. There are two main types of ozone generators: UV and corona discharge (CD).
UV ozone generation
In a similar manner to the UV rays from the sun splitting the oxygen molecules, ozone can be produced by passing ambient air over a UV lamp. This is how a UV ozone generator works.
The portion of the UV light from a typical germicidal lamp whose wavelength is 185 nm will split the oxygen molecules (O2) into oxygen atoms (O). The split oxygen atoms combine with other oxygen molecules from the air stream to form ozone (O3). The special quartz lamp used to emit the UV light is started and powered by a transformer. This method of ozone generation produces ozone outputs in the ranges of 0.01 percent by weight (100 ppm) to 0.1 percent by weight (1,000 ppm). The method does not require any feed gas air preparation systems. UV systems are suitable for use in residential applications such as in residential swimming pools and spas. They produce enough ozone to be effective in such applications and are much more economical than CD systems because special air preparation and cooling systems are not necessary.
CD ozone generation
The CD method mimics how ozone is produced by lightning in the lower atmosphere. In this method, ozone is produced with a high voltage electrical discharge. A controlled spark (corona) splits the oxygen molecules as a prepared feed gas is passed across an air gap. The spark is usually generated between two electrodes. As the spark splits the oxygen molecules, nitrogen molecules (N2) present in the air stream are also split. The generated species of nitrogen oxides (NOx) can combine with the moisture in air to produce a highly corrosive nitric acid (HNO3). Nitric acid can damage the internal components of the ozone generator, restrict the flow of ozone and even cause premature failure of the ozone generator. The presence of corrosive nitric acid poses a health and safety issue, as the internal components of units contaminated with nitric acid should not be handled by anyone, including repair and service personnel. Due to problems associated with the production of nitric acid, the feed gas must be modified or prepared before it reaches the ozone generator.
One way of preparing the feed gas is to remove the moisture from it; without the moisture, split nitrogen molecules or nitrogen oxides cannot form nitric acid. In this case, the feed gas must be dried to a dew point of at least -60F to reduce the formation of nitric acid. Beside a variety of air driers that can be used, other air preparation systems include oxygen concentrators or pressurized oxygen tanks. Oxygen tanks provide the purest possible feed gas but the tanks can be expensive, may be a safety hazard, are inconvenient due to the need for frequent change and delivery, need maintenance and have electrical requirements. In addition, approximately 85 – 95 percent of the electrical energy used by the CD ozone generator is wasted as heat. Consequently, cooling methods are required. Cooling methods can include air, water with oil, freon and water alone. The advantage of CD ozone generators is that ozone outputs are higher and can range from 0.5 – 1.7 percent by weight (5,000 – 17,000 ppm) for systems using dried air and 1.0 – 6.0 percent by weight (10,000 – 60,000 ppm) for systems using oxygen-enhanced feed gas.
A good CD ozone generating system consists of two or three parts: the ozone generator, an air preparation system and a cooling system. The air preparation and cooling systems add to the cost of the CD ozone generator, require regular maintenance, can be mechanical and may require expertise to operate and maintain. Due to high ozone output, complicated and costly air preparation and cooling systems, CD ozone generators are best suited for larger water treatment applications, such as in large industrial applications.
Properties of ozone
As an oxidizing agent, ozone is 1.5 times stronger than chlorine and it is effective over a much wider spectrum of microorganisms than chlorine and other disinfectants. Ozone is known to be effective against all known bacteria, fungi, yeast and protozoa (including parasites and amoebae). Ozone is a high energy molecule, having a half life of 20 minutes in water. After 20 minutes, half of ozone molecules will have decomposed into oxygen, leaving no undesired residues in the water. If the concentration of ozone is sufficient and if ozone is continually being injected into the water, ozone is very effective in controlling the growth of bacteria. If a lower concentration of ozone is used and if it is not being continually produced and injected into the water, ozone can still be used as an effective oxidizer. At these concentrations, ozone is able to break down non-living organic contaminants.
Residential uses of ozone
In residential swimming pools and spas, ozone is usually not being continuously introduced into the water. In a typical swimming pool application, ozone is usually introduced into the water for 12 hours per day. In spas, ozone is usually introduced into the water for about four to six hours per day. For this reason, ozone cannot be considered as a reliable disinfectant or sanitizer. However, at the concentrations used in pools and spas, ozone is an excellent oxidizer and this provides many advantages to pool and spa owners and helps to improve the quality of the water.
No matter which water treatment system is used on a pool or a spa, there is a need to eliminate materials introduced by bathers (non-living bather waste).
The safety residual in the water, which provides continual disinfection, is legislated. It must be chlorine, bromine, or biguanides. Many pool and spa owners use chlorine for oxidation, disinfection and for a safety residual. However, this more traditional approach to water treatment is not the most efficient. For example, in outdoor swimming pools, approximately 15 percent of chlorine is used for disinfection, 70 percent is used for partial oxidation, five percent to maintain a safety residual and UV light from the sun destroys the remaining 10 percent of the chlorine. If chlorine is used as a stand alone sanitizer in spa water, approximately five percent is used for disinfection, 85 percent for partial oxidation, five percent to maintain a safety residual and five percent is destroyed by the hot water. Since spa water temperature is above body temperature, bathers will tend to sweat more. This causes the amount of non-living bather waste (bather load) to be as much as 20-fold greater than in cooler pool water. Therefore, a larger percentage of chlorine is used to deal with this load. Another reason why chlorine consumption is higher in a spa is the small volume of water. Smaller volume of water makes the bather load more concentrated.
Chlorine is an excellent disinfectant and provides a residual in the water. Some problems, however, are encountered when only chlorine is used for the elimination of non-living bather waste. When chlorine contacts a non-living organic compound, it may be chemically combined with that compound and the new chlorinated compound produced cannot be broken down any further. Chlorine combines with body oils, sun tan lotions, cosmetics, perspiration and urine. The combining of chlorine uses up the chlorine and it can no longer function as a disinfectant and residual in the water. The chlorinated organic compounds form scum lines, greases that clog filters and result in the formation of soft-scale. Chlorinated compounds that were formed from sweat and urine (termed chloramines) are responsible for the obnoxious ‘chlorine odor’ and the eye and skin irritations often experienced by bathers. As these chloramines form and use up the available chlorine, more and more free chlorine is needed to establish a sufficient free chlorine residual in the water.
Ozone is the strongest oxidizer commercially available for pool and spa water treatment. Because ozone is a more powerful oxidizer than chlorine, it reacts with non-living bather waste more quickly than chlorine does. Ozone does not combine with the compounds; instead it causes them to break apart. These degradive products are more water-soluble and some can even gas-off and harmlessly dissipate into the air. Compounds from urine and perspiration are altered so that they cannot form chloramines that cause eye and skin irritations.
In pool and spa water, ozone is the primary oxidizer. The disinfection is achieved by maintaining a free-available chlorine or bromine residual in the water at all times. Ozone’s role is to remove or alter non-living bather wastes. In the water, ozone provides a continual effective high level non-chlorine shock. This means that the non-living bather waste is continually being altered so that obnoxious chlorine compounds are not being formed. When ozone is used together with chlorine, ozone increases chlorine’s effectiveness as a disinfectant and residual. Without ozone, the pool and spa owner needs to counteract the formation of chlorinated compounds by using more and more chlorine to keep a free available residual and requires ‘shocking’ compounds and other expensive specialty chemicals to treat the problems caused by chlorine compounds.
A new industrial use of ozone in the US
There is a growing concern over the safety of food; the food processing industry has been looking at ozone as a viable alternative to improve the safety and quality of pre-packaged, processed and unprocessed foods. Because this is a newer application of ozone, let us discuss how ozone is used in such an application.
Ozone has, in fact, been used by the European food industry as a standard for decades and as a sanitizer for public water for over a century. In June 2001, the FDA’s final ruling in the Federal Register approved ozone as an additive to kill food-borne pathogens, “as an antimicrobial agent on food, including meat and poultry”. This includes the use of ozone in the treatment, storage and processing of foods and even the preparing, packing, or holding of raw agricultural commodities for commercial purposes.
Ozone is becoming more widely used as a sanitizing agent for fresh produce. It has been found that chlorine has limited effect in killing bacteria in fruits and vegetables and in addition, there have been concerns about residual by-products. One such group of unwanted by-products are trihalomethanes (THMs), which are formed in the waste water and then flushed into the environment.
Another reason prompting researchers and producers to look for more effective disinfectants is that there are significant losses of produce due to microbial spoilage between the time of harvest and consumption. These losses can be as high as 30 percent. Large amounts of pesticides are used annually to combat these losses. Pesticide residues are difficult to destroy and some are left on the produce. If the produce is treated with ozone, the ozone can destroy the pesticide residues left on the produce after the harvest. This makes the produce inherently more safe for consumption.
There are two main uses of ozone in the produce industry. Ozone is used for process water disinfection and for fruit and vegetable washing. In process water disinfection, ozonated water is used to wash the fresh produce. The wash water is then treated with ozone and filtration. This significantly reduces bacteria, color, organic loads and suspended solids. The treated water can then be recycled to reduce water usage. If this water is to be released into the environment, the water would not have unwanted chlorine byproducts such as THMs and would not be full of organic load.
Pre-ozonated water is also used to wash fruits and vegetables. This reduces the amount of bacteria on the surface of the produce. Gaseous ozone can be used in cold storage to destroy the mold and bacteria on the surface of the produce. This can considerably extend its shelf life. Ozone can likewise be used during shipping to minimize bacteria, mold and yeast from growing on the surface of the produce. Ozone can eliminate undesirable flavor produced by bacteria and it can slow down the ripening process by removing ethylene gas that is produced by some fruits and vegetables. Ozone breaks down ethylene into carbon dioxide and water. This also extends the shelf life of produce.
Another advantage of using ozone in a food processing plant is that gaseous ozone can kill yeast and mold spores that float in the air. Ozone can be sprayed on all the equipment and surfaces where food is being packaged and processed. This reduces bacteria and organic matter.
Additional benefits of using ozone in the food industry are the reduction of the use of other chemicals and the minimization of any harmful byproducts. In terms of cost comparison, depending on the particular use and application, chlorine is generally less expensive; however, ozone has superior qualities. Ozone is generated on site. This eliminates the need to transport, store and handle chlorine, a known hazardous substance.
Conclusion
Ozone has found its way into many applications, from industrial to residential uses. Ozone has so many benefits that it is not surprising that more and more applications are taking advantage of it. The uses are so varied, from industrial water treatment, to food and agricultural industry, to treatment of residential swimming pools and spas. Considering ozone’s many benefits and advantages, the use of ozone will continue and ozone will find its way into more and more imaginative applications in the future.
About the author
Deborah Kon is the R&D/Operations Manager at UltraPure Water Quality Inc., a manufacturer of ozone generators for residential pools and spas. She has been with the company since 2001. Kon holds a B.Sc. (Hons.) degree, majoring in chemistry from the University of Guelph, in Guelph, Ontario, Canada. UltraPure Water Quality Inc. can be contacted at 877-281-7603, fax: 905-335-1483, or web: www.waterquality.net.