By Marianne Metzger

Ozone is a triatomic molecule consisting of three oxygen atoms. Ozone is a powerful oxidizer that it is used as a disinfectant in many applications, including municipal water supplies, bottled water and wastewater. It is also used in many other industrial applications, including the following:

  • To disinfectant laundry in hospitals and other health care facilities
  • Deodorizes air and objects after fires
  • Used extensively in fabric restoration
  • Kills any bacteria on food or other surfaces in which food may be in contact
  • Sanitization of pools and spas
  • Beginning to replace chemical fumigant to kill insects ingrains in storage facilities
  • Used to kill yeast and mold spores in food processing facilities
  • Used in washing of fruit and vegetables
  • Used in water treatment to aid in the removal of contaminants, which cause taste, odor and color (such asiron, manganese and hydrogen sulfide)
  • Used to control algae in aqua features and/or fountains
  • Used as a sanitizer in pharmaceutical applications

Ozone is a gas that cannot be transported because of its short half-life, so it must be generated onsite. Ozone has a half-life of about 30 minutes in water, which means every half hour the initial ozone concentration will be reduced by one half. There are many other considerations, such as pH, temperature, the presence of organics or the presence of inorganics (such as iron) that can further shorten the half-life, so the concentration of ozone can diminish quickly.

Ozone can be generated using either corona discharge or UV lamp generators. It is gaining in popularity due to its benefits over other traditional chemicals used for disinfection, such as chlorine.

As a disinfectant in potable water, it doesn’t produce as many disinfection byproducts (DBPs) as chlorine and still provides some residual protection regardless of the brief half-life. It can produce DBPs if there are naturally occurring precursors in the water.

Specifically, if there is bromide present in raw water, using ozone can produce a byproduct called bromate, which is regulated by the US EPA as a known carcinogen. The US EPA has established a maximum contaminant level (MCL) for bromate at 0.01 mg/L and the FDA has also adopted this standard for bottled water.

Bottled water use
The Food and Drug Administration (FDA), an agency within the US Department of Health and Human Services, is responsible for ensuring public health by overseeing the safety, efficacy and security of pharmaceutical drugs, biological products, medical devices, food supply (including bottled water), cosmetics and products that emit radiation. In 1982, the FDA classified ozone as generally recognized as safe (GRAS) for use as a disinfectant in bottled water with a residual concentration of up to 0.4 mg/L of dissolved ozone.

Ozone is used almost exclusively in the bottled water industry as a primary disinfectant due to the FDA approval as GRAS. The FDA has further recognized ozone treatment to be good manufacturing practice (GMP) for the bottled water industry.

The FDA recommends using a minimum treatment of 0.1 mg/L of ozone in water in a closed system with a contact time of at least five minutes. Additionally, in June 2001, the FDA issued a final rule regarding the use of ozone as an antimicrobial agent for food, including meat and poultry, so it is becoming a more common disinfectant in the entire food industry.

Safe usage
While ozone provides excellent disinfection and oxidizing properties to many industries, at high concentrations it is harmful to human health when inhaled. When people are exposed to high levels of ozone they can experience certain health symptoms, which include dryness of the mouth and throat, coughing, headaches and chest pains. The Occupational Safety and Health Agency (OSHA) has proposed maximum acceptable concentrations (MAC) for ozone in air.

The MAC value is the concentration which a human is allowed to be exposed to over a certain period of time. For ozone, the MAC value established is 0.06 parts per million for eight hours a day, five days per week, with a maximum of 15 minutes of exposure to levels of 0.3 parts per million of ozone.

Monitoring for ozone is important in many applications. First, when using ozone that is being released into the air, it is important not to exceed levels that may present a danger to people working in that area.

Secondly, ozone that is used as a disinfectant in drinking water can produce the formation of bromate, which is a known carcinogen. The level at which ozone is applied becomes very important in controlling bromate formation.

Ozone measurement
There are many ways in which to measure residual ozone concentration depending upon application, accuracy needed, budget and available resources. This article will focus on commonly used methods that are designed to measure ozone levels in water. Some methods are better than others, depending on the application in which you need to measure.

One way to measure ozone concentrations is to utilize a probe or electrode that measures oxidation-reduction potential, commonly referred to as ORP or Redox. ORP is not a measure- ment specifically for ozone but rather all oxidizing agents, including other disinfectants such as chlorine, chlorine dioxide and peroxide. It is commonly used to measure the disinfection of pools and spas.

The disadvantage of this method is the interference of high turbidity in water. This can cause ORP readings to be below what is expected (potentially, even a reducing or negative value). Using ORP to measure ozone can only be done accurately if measuring clean water systems that have low turbidity levels.

Another way ozone is measured is the DPD (N,N-diethyl- p-phenylenediamine) method, which is more commonly used to measure chlorine. It is a US EPA-approved method for measuring chlorine in municipal water supplies.

As with ORP, the DPD method also measures other disinfectants that may be present, including chlorine, bromine and iodine. This can cause higher readings if these substances are present in the water being tested.

A colorimetric test is used in which DPD reacts with oxidizers in water (mostly used for measuring chlorine) causing a red color change that is proportional to the amount of oxidizer present. This is an inexpensive way to measure ozone, but is only useful if it is known that no other disinfectants are present. There are other more reliable methods if more accurate results are desired or if other oxidizers are known to be present.

Additional processes
Another ozone test is based upon the DPD method and is referred to as the DDPD Method (a proprietary method developed by CHEmetric, Inc.). This method can also be used to measure chlorine and other disinfectants; if other disinfectants are known to be present, this may not be the best for determining ozone levels.

In applications where ozone is the only disinfectants being used, the DDPD Method can be an accurate and cost-effective option for monitoring. The sample is initially exposed to an excess of potassium iodide, with which ozone will react.

Ozone oxidizes iodide into iodine. Iodine in turn oxidizes DDPD (a methyl-substituted form of DPD) that forms a purple colored species, which is in direct proportion to ozone concentration. The colored sample can then be compared to color comparator ampoules or analyzed in a colorimeter.

The DDPD reagent was developed because it is less susceptible to low-level chromate interferences that can affect the DPD Method. This was especially important in treatment of cooling tower water, where chromate has been used to control corrosion. Since it was found that chromate presents other health problems, it has been banned from use in comfort cooling towers in the 1990s.

A widely accepted method for testing ozone concentration is the indigo blue method, which is based upon Standard Methods 4500-O3 B (APHA, AWWA, WEF, 1995). This method uses an indigo trisulfonate reagent that reacts with the ozone present. Ozone actually bleaches the blue color in direct proportion to the amount of ozone. The color change is then measured using a colorimeter.

There is some interference with this method; for instance, if hydrogen peroxide (H2O2) is present, it will slowly bleach the indigo blue. This interference can be alleviated if samples are analyzed less than six hours after reagents are added.

Chlorine can interfere, but can also be corrected for by using malonic acid. The presence of bromine in samples can cause interferences that cannot be fully corrected by the use of malonic acid, so an accurate measure of ozone with this method cannot be made if bromine is known to be present.

In-line monitoring

Ozone can also be monitored inline by use of an electrochemical cell. This method works by passing ozone through a membrane that converts it into oxygen. This level of oxygen is then measured by an oxygen-sensitive cell.

This a common method for monitoring ozone levels within a process, so it can provide constant measurements of the ozone level. The drawback to this method is that it is fairly costly to maintain, as the probes need to be cleaned and recalibrated on a frequent basis.

Ultraviolet absorption can also be used to measure ozone levels (primarily in air) but can be used to measure directly in water if a high level of ozone is used in clean-water applications, where there is low turbidity and dissolved-solid content. Otherwise, ozone can be stripped from the water and analyzed using UV absorption.

This is a highly accurate method for detecting ozone and it is capable of measuring varying concentrations. The equipment, however, may be somewhat costly for small systems or applications.

Application selection
Ozone is being used in more applications every day, so effective monitoring is important. Ozone being used as a disinfectant requires monitoring to ensure a residual in present, just like other common disinfectants, including chlorine, chloramine and chlorine dioxide. Precise measurement of ozone in water can be problematic because of potential interferences, unpredictable half-life in different waters and time of sample collection in relation to time of analysis.

It is important, therefore, to choose a sampling and monitoring method that will give you the accuracy levels needed. This will help meet task requirements and stay within the operating budget.


  1. APHA, AWWA & WEF, “Standard methods for the examination of water and wastewater,” 19th edition, Method 45000-O3 B (1998).
  2. Bader, H. and Hoigne, J. “Determination of Ozone in Water by the Indigo Method,” Water Research Vol. 15, 449-456. 1981.
  3. Delimpasis, Konstantinos J., Ozone and Color Removal,
  4. International Bottled Water Association, Plant Technical Manual, Jan 2005, Revision 6.0.
  5. Kilham, Lawrence, “Measuring Dissolved Ozone”, Water Quality Products, October 2003.
  6. Steeves, Susan, January 20, 2003, Purdue News, “Ozone may provide environmentally safe protection for grains.”
  7. Technical Data Sheet, Ozone DDPD Method, CHEMtric, Version 5, December 2008.
  8. US Department of Health and Human Services, Food and Drug Administration Center for Food Safety and Applied Nutrition. Bottled Water: Residual Disinfection and Disinfection Bypr oducts Small Entity Compliance Guide, html, May 2009.
  9. US EPA, Office of Water April 1999, Alternative Disinfectants and Oxidants Guidance Manual.
  10. What is ORP? , Ozone Solutions, Inc. http://
  11. WQA Ozone Task Force, “Ozone Treatment,” Water Conditioning & Purification, September 2008.

About the author

Marianne R. Metzger is National Testing Laboratories, Ltd.’s GPG Business Manager. She handles various market segments including water treatment, well drilling, public water supplies and homeowners. Metzger has a degree in environmental geology and political science from Case Western Reserve University of Cleveland and spent a couple of years with Accent Control Systems as a Sales Engineer, working with water treatment equipment on commercial and industrial applications. She previously spent over 10 years with NTL in a variety of positions including customer service, technical support and business Group Manager. Metzger can be reached by phone at 1 (800) 458-3330 EXT 223 or via email at




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