OZONECARBON TECHNOLOGIES PTE LTD.
70 Anson Road, #26-03, Apex Tower, Singapore 079905
E-mail: pmmenon@ozonecarbon.com.sg
Abstract
This paper examines the use of ozone technology, often called a “green” technology, for oxidation and disinfection of air, water and environmental surfaces.
Based on the review of the available literature and the extensive industry experience gained by the author, it is concluded that if ozone treatment is used properly, it can serve as an environmental friendly and cost effective method for disinfection and oxidation of objectionable compounds.
Introduction
Ozone has received much attention in the last few decades among the environmental health professionals, as its by-products do not cause pollution problems unlike other disinfectants such as chlorine.
The use of ozone in the air, water and hygiene industry is however limited presently, due to the lack of awareness among the users. In certain projects, ozone systems are installed initially, as the consultants recommend the use of a clean technology. However, subsequently the ozone systems are not used at their potential and the end user may have to switch to the traditional technology.
Unfortunately, such incidents are not due to the inability of ozone to disinfect or oxidise objectionable compounds, but because of failure to understand the chemistry of ozone, its applications and the underlying process engineering.
(Some industry experts also highlight reasons such as – poorly designed systems, which are either under/over sized; concerns regarding safety; and failure to service the systems since the systems are sometimes installed by “traders” who lack the knowledge and expertise to maintain them.) In order to understand how ozone works, it is necessary to take a look at its properties and the way it performs disinfection and oxidation.
What is ozone?
Ozone is an allotropic form of oxygen having three atoms of oxygen. Two of the atoms are with double bonds whereas the third atom is loosely held by a single bond. The third, loosely held atom tends to break away and attach itself to a host oxidizing it with the original ozone molecule reverting back to oxygen. It is this property of ozone which makes it a very powerful oxidizing and disinfecting agent for both the air and water applications.
O3 <=> O2 + O………..Eq. (1)
Ozone is the most powerful oxidizing agent next only to fluorine. It is present in small proportions in the atmosphere, which can increase up to 0.3 to 0.5 PPM after a thunderstorm. It is present in the earth’s stratosphere filtering the UV rays from the sun, having a wavelength of 200 to 300 nm.
| Oxidizing Agent | Oxidation potential in V (ORP) | ORP, relative to chlorine | |
| Fluorine | 3.06 | 2.25 | |
| Hydroxyl-radical | 2.80 | 2.05 | |
| Atomic Oxygen | 2.42 | 1.78 | |
| Ozone | 2.08 | 1.52 | |
| Hydrogen peroxide | 1.78 | 1.30 | |
| Hypochlorite | 1.49 | 1.10 | |
| Chlorine | 1.36 | 1.00 | |
| Chlorine dioxide | 1.27 | 0.93 | |
| Molecular Oxygen | 1.23 | 0.90 |
The Table 1 above compares the electro-chemical oxidation potential of ozone with other oxidizing agents.
Although ozone is a very powerful oxidizing agent, it is an unstable gas that breaks down to oxygen within minutes. It must therefore be produced at site for immediate use. It cannot be produced at some other site, bottled and transported to the point of use. The other important point about ozone is that it is only partially soluble in water. As a result, great importance is paid in the mixing and mass transfer of ozone from the gas phase to the liquid phase.
Production of Ozone Ozone is produced primarily by two processes:
- Corona Discharge (CD)
- Ultraviolet light (UV)
Both the processes use air or oxygen as the feed gas. In the CD process, the dry feed gas is passed between two electrically charged plates separated by a dielectric medium and a narrow discharge gap. Under these conditions part of the oxygen in the feed gas is converted into ozone:
3O2 + Energy <=> 2O3 + Heat + Light….. Eq. (2) The heat generated due to the above reaction has to be removed. The ozone generators are therefore either air-cooled or water-cooled to maximize the yield of ozone by removal of this heat.
In the UV system, the dry feed gas is passed through a high-energy radiation source (185 nm UV bulb). The energy from the UV is used to convert part of the oxygen in the feed gas to ozone. The equation (2) holds good except for the fact that ozone produced by UV process is extremely low in concentration compared to CD process.
Ozone concentrations of 1 – 4.5% by weight can be produced by CD process using air as the feed gas whereas only 0.001 – 0.1% by weight can be produced by UV process. (This is about 10 to 1000 times less than that generated by the CD process.)
Since very low concentration of ozone is produced by the UV process, the solubility of ozone produced by the CD process is much higher under the same conditions.
| Concentration | Water temperature | Solubility |
| UV process 0.1% | 25 deg C | 0.35 mg/l |
| CD process 1.5% | 25 deg C | 5.29 mg/l |
Also, higher the concentration of ozone-demanding materials with which ozone can react, the faster is the rate of ozone transfer into the water. Safe Ozone levels
Safety of the workers and the general public, which are likely to be exposed to ozone, should also be considered during design, installation and operational stages of the ozone generating systems.
Ozone systems should be designed with safety features so that the concentration of ozone in air never exceeds the guideline standards. (Most people are able to detect ozone in air at levels of about 0.02 ppm.) Such guideline values for safe levels of ozone in air are as follow.
(1) OSHA (Occupational Safety and Health Administration), USA
OSHA’s guideline values for ozone (eg. in a factory setting) are:
- Permissible Exposure Level (PEL): 0.1 ppm (by weight), time-weighted average over an 8-hr day, 5-days per week.
- Short Term Exposure Level (STEL): 0.3 ppm (by weight) averaged over 15 minutes, not to be exceeded more than twice daily.
(2) U.S. FDA (Food and Drug Administration)
In the early 1970s, FDA determined that ozone levels in air should not be above 0.05 ppm (by weight) when exposures are 24 hours/day, 7 days/week for sensitive individuals – who are very young, elderly, infirm, immuno-compromised, or confined to rooms, hospitals, nursing homes, etc.
(3) U.S. EPA (Environmental Protection Agency) EPA has set an ambient ozone level in air of 0.08 ppm (previously 0.12 ppm), not to be exceeded more than three times per year.
(4) Under the Singapore guidelines on indoor air quality in office premises, a maximum value of 0.05 ppm is recommended. Same value is also recommended by ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) for indoor air quality.
Disinfection with ozone
Ozone acts directly on the cellular walls of the microorganisms. It oxidizes organic matter in bacterial membranes, which in turn, weakens the cell wall and leads to the cell rupture. The internal cellular material/plasma is released into the external environment, which causes immediate death of the cell.
In contrast, other oxidizing and non-oxidizing biocides must be transported across the cellular membrane where they act on the nuclear reproductive mechanism or on enzymes essential for the various cell metabolisms. Such mechanism of disinfection is not as rapid and efficient as ozone, since these biocides need to be used in higher concentrations or much longer contact times.
During commercial applications, however, the disinfection process should also be viewed in terms of the exposure to materials that will come in contact with ozone. It is recommended that those materials be used, which are compatible with ozone at the concentration levels used in the disinfection process.
Disinfection and purification of air using ozone
For treatment of air, ozone should be produced in low concentrations since it is easier to mix gases of lower concentrations than high concentration. Also care should be taken to ensure that the residual ozone in air never exceeds the standard (0.05 ppm in this case).
The half-life of ozone in dry air is of the order of a few hours, but this drastically changes in the presence of humid air. The ambient air in Singapore is humid with relative humidity (RH) ranging from 80% to 100% during heavy downpours.
In humid air, the half-life of ozone reduces substantially due to the formation of short-lived hydroxyl (OH*) radical species, which have a half-life of milliseconds. These hydroxyl radical species are much more powerful oxidizing agents than ozone. Hence, at high RH, ozone disappears very quickly. It should also be noted that the stability of ozone is also a function of temperature. Ozone breaks down faster to oxygen at higher temperatures. The tropical weather of Singapore – high temperature and high RH – makes ozone an extremely short-lived species in ambient conditions.
Usually, low residual concentration of ozone – 0.02 ppm – is required to disinfect air when the RH is greater than 50%. This concentration of ozone in ambient air is below the maximum guideline value of 0.05 ppm, which is recommended for office premises.
When the RH is lower than 50%, higher concentrations of ozone (1-2 ppm) are required to get the same log reduction of microorganisms. This ozone in air is much higher in concentration for human exposure. Disinfection for such applications should be done while the place is unoccupied.
The effect of temperature on the ozone disinfection has been studied at RH >50 (with low concentration of ozone, within safe human exposure limit). Ozone was found to be most lethal to the microorganisms at 25 degree C, followed by 15 degree C.
Usually, central air-conditioning systems in Singapore operate at temperature 23-25 degree C and 50-60% RH. With low levels of ozone required at these operating conditions, ozone can be an ideal candidate for oxidation and disinfecting the air at the work place, shopping areas, hospitals and other areas where indoor air quality is a problem.
Some of the applications of ozone for air treatment are as follow:
- Ventilation and air-conditioning system for air disinfection, odour control and improved indoor air quality in various building premises
- Kitchen and food odour control • Cigarette odour control in bars and restaurants
- Sewage odour control in pump stations
- Rubbish bin centre odour (volatile organic compounds) control
- Toilet odour control
- Cold room air treatment for microbial control, odour control and extension of shelf life of fresh produce.
(Odour control using ozone is often achieved due to the oxidation of volatile organic compounds – VOCs – or inorganic substances.)
Use of ozone for treatment of water Ozone’s use for water application is much more diverse and complex than for air applications, since ozone should first be dissolved in the water.
The gas – liquid mixing principle follows Henry’s Law, which states that the solubility of a gas in liquid is a function of its partial pressure in the gas phase. Hence, ozone has to be produced at higher concentrations for a better solubility in the liquid medium.
To get higher solubility, ozone gas should be produced in high concentrations. Using air as the feed gas, ozone can be produced up to 4.5% by weight. Since air has only 21% oxygen, for higher ozone concentration, oxygen is used as the feed gas. With oxygen as the feed gas, ozone up to 15% by weight can be produced.
For disinfection in the liquid/water phase, ozone residual must be maintained for a fixed amount of time to get the required log reduction of the microorganisms.
The USEPA has suggested the concept of CT values (USEPA 1989) for the disinfection of drinking water. CT is the product of the concentration of dissolved disinfectant by the contact time during which the disinfectant residual is maintained. EPA has defined a CT value for each disinfectant used in drinking water (chlorine, ozone, chlorine dioxide) over the pH range 6-9, and for water temperatures from 0.5oC to 25oC.
For disinfection with ozone, EPA recommends attaining a CT value of 2.9 mg/L – min at < 1o C, and 0.48 mg/L – min at > 25o C. Attaining these values with ozone will yield 99.9% (3 logs) inactivation of Giardia cysts and 99.999% (5 logs) inactivation of enteric viruses.
It is important to understand that an ozone residual is produced in water only when all the ozone demand of the water is met. The stability of ozone in water is thus also a function of the water quality. Higher the pollution due to ozone demanding species, more amount of ozone is required to maintain the level of residuals. (It is this residual which is to be maintained for the required amount of time as per the recommended CT values.)
From water quality point of view, ozone is more stable under acidic conditions (low pH) than under alkaline conditions (high pH) due to the formation of short- lived Hydroxyl radicals. Hydroxyl radicals are excellent oxidizing agents but poor disinfectants in the liquid phase because of their extremely short half-life.
Ozone auto decomposes in warmer waters. It is more stable and soluble in cold waters. Thus, the temperature of water also determines the solubility and stability of ozone in water. This property of ozone makes it an excellent biocide for cold sterilization without the use of hot water.
Some of the applications of ozone for water treatment are:
- Drinking water treatment for disinfection and oxidation of impurities
- Water storage tank disinfection
- Cooling tower water treatment for Legionella control
- Swimming Pool and spa water treatment
- Treatment of water features and fountains
- Removal of dye colour in the textile industry
- Washing/disinfection of fresh produce (fruits, vegetables, sea-food etc.) in food industry • Disinfection of kitchen and food outlets
- In advanced oxidation process in combination with UV and hydrogen peroxide.
Conclusion
In order to use ozone successfully for commercial applications, the principles and chemistry of ozone must be well understood by the respective stakeholders.
References/citations
- Ozone in Water Treatment, Application and Engineering, Lewis Publication.
- Federal Technology Alert – Ozone Treatment for Cooling Towers; The U.S. Department of Energy; http://www.npl.gov/fta/6_ozone.htm
- P M Menon and Dr. A Appan (2003); OZONATION – The Answer to High Energy and Water Savings in Air-conditioning Systems; International Conference on Building Systems and Facilities Management (ICBSFM), 8th – 10th October 2003, Marina Mandarin Hotel, Singapore; Session Code PS5-1220
- P M Menon (1996) An Introduction to Ozone and it’s applications in Water and Waste water Treatment; Pollution & Environment Technology Indonesia Conference
- Dr. Rip Rice (2003) OZONE FOR AIR TREATMENT – Basic Principles; International Ozone Association; 16th. World Congress; Las Vegas; Nevada; USA.
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