By Brian Oram

When trying to build a foundation in water treatment, it is important to not only understand the various treatment processes, but to have a solid understanding of the technical history, chemistry, and historic applications of the water treatment process or chemical.

Ozone, i.e., trioxygen, is normally a colorless to pale blue gas that is made up of three oxygen atoms which breaks down into oxygen (O₂) and a free oxygen atom. The oxygen atom is a free radical that is highly reactive and unstable with a half-life that is measured in minutes to hours. The rate of degradation is a function of the water chemistry, pH, and water temperature. As the ozone decomposes, free radicals of hydroperoxy or hydrogen superoxide (HO₂) and hydroxyl (OH) are formed.[5]

Ozone is a strong oxidant that is partially soluble in water and can interact with both inorganic and organic compounds and other gases. In some cases, this interaction creates a benefit, but ozone may react negatively and cause health issues or damage equipment. For example, ozone can react with dissolved iron, manganese, and arsenic to create oxidized particles that can be removed from the water, but can also negatively react with piping, fixtures, and components of a system if it is made from cast iron, copper, brass, polypropylene, rubber, or nitrile.[3]

Unlike other treatment processes, ozone is difficult to see or touch, but this gas has a very distinctive odor with a threshold odor concentration of 0.01 to 0.04 ppm. Ozone is produced naturally when air is exposed to an electrical discharge, such as during lightning storms, through a reaction with nitrogen oxides and volatile organics in the atmosphere, or UV irradiation that produces ground level ozone. You can also detect the smell of ozone near electrical generators, electric motors, some air purification devices, and even copy machines/laser printers.[1] Therefore, the interaction of oxygen in the atmosphere with UV radiation or an electrical discharge can create ozone. In this process, the oxygen molecule (O₂) is split. The free oxygen (O) that is released is not stable, and it combines with another oxygen molecule to create ozone (O₃).

The Origin of Ozone
Scientists accidently discovered manmade ozone while experi­menting with electricity. In 1785 Dutch chemist Martinus Van Marum was experimenting with electricity and he observed a light blue gas that smelled. He called this gas “the odor of electrical matter,” but it was up to Christian Friedrich Schonbein, a German-Swiss chemist, in 1840 to give this gas a name. He named the gas ozone after the Greek word for “to smell” (ozein) and by 1865 Thomas Andrews, an Irish chemist and physicist, proved that ozone was a form of oxygen.

Through trial-and-error analysis and experimentation, these scientists and inventors developed ozone generators that mimic natural processes.[2] In the late 1850s and 1860s, ozone was being used to disinfect operating rooms and drinking water, but also treating taste and odor issues with drinking water. Because of the classic smell, ozone exposure was initially considered a health benefit, but with time it was determined that overexposure could cause negative health consequences, such as: chest pains, coughing, irritation, and damage to lungs, and in extreme cases, death.[4] Currently, OSHA regulations have set a maximum exposure limit for ozone gas in air of 0.1 ppm during an 8-hour work period.

The first ozone generator was designed by Werner Von Siemens. Siemens designed the “Siemens Type” ozone generator in 1857, and Nikola Tesla patented a corona discharge generator in 1896. By the 1900s ozone was shown to be effective as a germicide, bactericide, and algaecide.

Ozone can be generated by passing dried air thorough a corona discharge chamber, passing dried air through UV irradiation with wavelengths shorter than 240 nm, or by electrolytic ozone production. Electrolytic ozone production does not require a feed source of air or gas, but produces ozone from the water being treated. In the U.S., ozone has been utilized by the municipal drinking water industry since the 1940s and the most common method of ozone generation for the municipal applications is the electrical discharge method. The corona discharge systems can produce ozone at a higher rate than the UV light ozone generators.

Advantages and Disadvantages of Ozone
One of the unique aspects of ozone is the potentially wide variety of applications and uses for this technology over many platforms. In addition to combining ozone with other treatment techniques, ozone is developing into a go-to technology as we move to more direct/indirect water reuse, enhancing existing treatment system performance, and managing issues with PFOS/PFOA through the implementation of conventional and hybrid ozonation system, such as ozofactionation.

The biggest problems with developing or implementing an ozonation approach are:
1. Not adequately understanding all the ozone demands of the water with respect to organic or inorganic matter content and not completely documenting the chemistry and the biological quality of the untreated water.
2. Not checking the potential for negative interaction of the ozone with the components of the system, such as piping, pump seals, and tanks.
3. The need to put in place a mechanism to prevent ozone exposure to critical components and the public/user, and properly monitoring the system.

The primary advantages to the use of ozonation are as follows:
1. The ozone is generated on-site and does not persist in the water. In many cases, a system does not need additional chemical treatment, but it may require additional filtration systems to remove oxidized particles.
2. Ozone is a strong oxidant so it readily oxidizes inorganic material like sulfur-compounds, iron, and manganese, some organic materials like synthetic organic compounds, and can be very effective against bacterial agents.
3. Ozone is effective over a wide pH range.
4. Simple instruments such as an oxidation reduction potential (ORP) sensor or real-time ozone sensor can be used to monitor system performance.
5. Ozone applications have been developed to improve aeration systems and efficiency of backwashing systems for existing conventional residential and commercial water treatment systems.
6. Ozone can be used to inhibit the formation of biofilms in water treatment processing equipment and rather small distribution systems.
7. Ozone can act to inactivate bacteria, viruses, protozoans, and some algae, and in general the electrochemical oxidation potential for ozone is higher than most other common disinfecting agents.[6]

The primary disadvantages to the use of ozonation are as follows:
1. Typically the system installation has a higher equipment and installation cost compared to other technologies. Ozonation commonly requires clean, dry air or oxygen, and trained professionals are not always readily available to troubleshoot these systems.
2. The ozonation approach does not provide a long-term dis­infection residual and does not prevent or inhibit regrowth within a larger distribution system.
3. Ozonation normally requires a very skilled technician and more frequent maintenance.
4. Ozonation requires a more detailed understanding of the ozone demand and water chemistry, such as the alkalinity, bromide, and hardness content of the water, total organic carbon content and bacterial quality of the water to be treated.
5. The use of ozonation requires a thorough understanding of the downstream components and elements within an existing or proposed treatment system or infrastructural components of the facility, such as the type of piping, fixtures, and tankage.
6. The process for producing ozone can produce a gas that can cause short-term and long-term health effects. High doses of ozone are dangerous. For example, municipal ozonation requires ozone systems be isolated in a special room with monitors to test for ozone leaks and have ozone evacuation fans and ozone destruct systems before the O₃ is vented to the environment.
7. Ozone is not flammable, but it can increase the intensity of a fire, and is a fire hazard.
8. Some ozone disinfection by-products, such as bromate, aldehydes, ketones, carboxylic acids, brominated trihalo­methanes, and peroxides can be formed, but these could be addressed via pretreatment, adjusting dosage, and post-treatment of the water.[7]

Ozone is not a magic box, but a tool that if used wisely and properly, will be a benefit the end user. Ozone technology has developed into a treatment approach that will facilitate the development and implementation of more sustainable water and wastewater management solutions. Ozone is a tool that needs to be in the toolbox.

References
1 Smith, D., (March 7, 2021). Indoor Ozone: 3 Common Sources Explained. Kaiterra.com. https://learn.kaiterra.com/en/resources/
indoor-ozone-common-sources
2 Leher, J. et.al., “Domestic Water Treatment”, McGraw-Hill Book Company, New York, New York, 1980.
3 Leusink, J., (November 30, 2018). Materials Ozone Resistance Chart. Oxidation Technologies. LLC. https://www.oxidationtech.com/blog/materials-ozone-resistance-chart/
4 Rubin, M., “The History of Ozone. The Schonbein Period, 1839 – 1868”, Technion-Israel Institute for Technology, Bull. Hist. Chem., Volume 26, Number 1 (2001), page 40 – 56.
5 US EPA, Wastewater Technology Fact Sheet: Ozone Disinfection, EPA 832-F099-063, September 1999. https://www3.epa.gov/npdes/pubs/ozon.pdf
6 Leusink, J., (January 26, 2016). Oxidation Potential of Ozone. Oxidation Technologies. LLC. https://www.oxidationtech.com/blog/oxidation-potential-of-ozone-2/
7 Oram, B., (January 1, 2022). Ozonation in Water Treatment. B.F. Environmental Consultants, Inc., https://www.knowyourh2o.com/indoor-4/ozonation-in-water-treatment

About the author
Brian Oram is the Founder of B.F. Environmental Consultants, Inc. and the Know Your H20 Program. He is a licensed professional geologist and soil scientist with over 30 years of experience in applied earth and environmental science, education, and water quality. Learn more about the Know Your H20 Program at https://www.knowyourh2o.com.

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