What Is Ozonation?

Ozonation is a water treatment process that uses ozone (O₃) — a highly reactive gas composed of three oxygen atoms — to oxidize and destroy contaminants in water. Discovered in the mid-19th century, ozone was first used for drinking water disinfection in Nice, France, in 1906. Since then, the technology has evolved into a widely adopted method for municipal water treatment, bottled water production, and industrial process water, prized for its potent oxidizing power and minimal chemical residue.

Ozone is generated on-site by passing dry air or pure oxygen through a high-voltage electrical discharge (corona discharge method) or by exposing oxygen to ultraviolet light. The resulting gas is then injected into the water stream, where it rapidly reacts with organic and inorganic substances. Unlike chlorine, ozone leaves no long-lasting disinfection byproducts (except when it reacts with bromide to form bromate, which is regulated). Its half-life in water is only minutes, making it an environmentally friendly choice that breaks down into harmless oxygen.

How Ozonation Works

The mechanism of ozonation relies on direct oxidation by molecular ozone and indirect oxidation via hydroxyl radicals formed when ozone decomposes in water. These radicals are among the most powerful oxidants known, second only to fluorine. They non-selectively attack the cell walls of bacteria, viruses, and protozoa, rupturing them and preventing reproduction. The same oxidizing action also degrades dissolved organic compounds, including taste- and odor-causing geosmin and 2-methylisoborneol (MIB), pesticides, and pharmaceutical residues.

“Ozone is the strongest disinfectant approved for drinking water treatment, with a disinfection efficacy up to 3,000 times faster than chlorine.” — Water Research Foundation

The contact time required for effective disinfection is short — typically 1 to 10 minutes depending on water temperature and ozone dose. A typical dose for drinking water ranges from 0.5 to 2.0 mg/L, with higher doses used when treating heavily contaminated water. Because ozone is unstable, it must be generated on demand, and residual ozone in the treated water is quickly converted to oxygen or consumed by any remaining oxidizable substances.

How Ozonation Enhances Water Safety

Pathogen Inactivation

Ozone is exceptionally effective against a broad spectrum of waterborne pathogens. It inactivates Escherichia coli, Salmonella, Legionella, Cryptosporidium parvum oocysts, Giardia lamblia cysts, and viruses such as norovirus and adenovirus. This is critical because Cryptosporidium and Giardia are highly resistant to chlorine at typical doses and contact times, but ozone can achieve >99.9% inactivation in minutes.

Unlike chlorination, which requires residual disinfectant levels maintained through distribution systems, ozone does not leave a persistent residual. This is both an advantage and a limitation: it reduces chemical exposure but means that a secondary disinfectant (often chlorine or chloramine) must be added to maintain water quality in pipes. Nevertheless, the primary disinfection step with ozone greatly reduces the pathogen load, making the overall treatment more robust.

Reduction of Chemical Hazards

Ozone oxidizes many organic micropollutants that are of increasing concern in drinking water, including endocrine-disrupting compounds (e.g., bisphenol A, nonylphenol), pharmaceuticals (e.g., antibiotics, analgesics), and personal care products. It also breaks down pesticides and herbicides that can leach into groundwater sources. By removing these contaminants at the source, ozonation reduces long-term health risks associated with chronic exposure.

Additionally, ozone can transform inorganic contaminants. For instance, it oxidizes iron and manganese into insoluble forms that can be filtered out, improving water clarity and preventing staining. It also oxidizes hydrogen sulfide, eliminating the “rotten egg” odor.

Comparison with Chlorination

  • Disinfection byproducts: Chlorine reacts with natural organic matter to produce trihalomethanes (THMs) and haloacetic acids (HAAs), which are regulated carcinogens. Ozone does not form THMs or HAAs, but it can form bromate if bromide is present — a challenge that can be managed through pH adjustment or ammonia addition.
  • Disinfection speed: Ozone is faster at inactivating pathogens, reducing the required contact time and allowing for smaller treatment plants.
  • Residual protection: Chlorine provides a lasting residual that protects water as it travels through pipes. Ozone provides none, so it is always combined with a secondary disinfectant.
  • Taste and odor: Ozone imparts a fresher taste by removing organics, whereas chlorine can leave a chemical aftertaste.

Benefits of Ozonation for Water Quality

Improved Taste and Odor

One of the most noticeable benefits for consumers is the elimination of musty, earthy, or chemical tastes and smells. These are often caused by naturally occurring compounds like geosmin and MIB, produced by algae and bacteria in source waters. Ozone rapidly oxidizes these compounds, even at low concentrations, leaving water tasting clean and refreshing. Many bottled water brands rely on ozonation as a final polishing step for this very reason.

Oxidation of Organic Contaminants

Ozone breaks down complex organic molecules into smaller, more biodegradable compounds. This not only removes the original contaminant but also makes the water more amenable to subsequent biological filtration (e.g., granular activated carbon). The synergy between ozonation and biofiltration, known as the “ozone–BAC” process, is a powerful strategy for removing dissolved organic carbon, reducing disinfection byproduct formation, and enhancing biological stability in distribution systems.

Reduction of Disinfection Byproducts

By oxidizing precursors before chlorination, ozonation reduces the formation of THMs and HAAs. Water utilities that switch from pre-chlorination to ozonation typically see a 30–60% reduction in these byproducts. This is a major compliance advantage as regulations become stricter worldwide. For systems that use chloramines as a secondary disinfectant, ozone can also lower the concentration of total organic carbon, further improving water quality.

Extended Shelf Life

For bottled water and packaged beverages, ozonation provides a “cold sterilization” that does not leave a residual taste. Because ozone rapidly returns to oxygen, the water’s dissolved oxygen content increases, which can inhibit anaerobic bacterial growth. This extends the shelf life of bottled water without the need for preservatives or high levels of chlorination. Many bottled water plants use ozonation as the primary disinfection step, followed by filtration and filling under sterile conditions.

Reduction of Iron, Manganese, and Sulfur

Ozone is highly effective at oxidizing dissolved iron (Fe²⁺) and manganese (Mn²⁺) to their insoluble forms (Fe³⁺ and Mn⁴⁺), which can then be removed by filtration. This prevents reddish-brown or black staining of plumbing fixtures and laundry. Similarly, hydrogen sulfide (H₂S) is oxidized to sulfate or elemental sulfur, eliminating the rotten egg smell. The process is generally simpler than chemical oxidation with potassium permanganate or chlorine, as no chemical storage or dosing equipment is needed.

Implementation and Safety Considerations

System Components

A typical ozonation system includes an ozone generator (corona discharge or UV), a contactor (where ozone gas is mixed with water, often in a bubble-diffuser or injector configuration), a destruction unit (to convert residual ozone to oxygen before venting to the atmosphere), and monitors for ozone concentration in both gas and liquid phases. The contactor must provide sufficient contact time and mixing to meet disinfection targets. Common designs include deep-U-tube contactors, pressurized side-stream injection, and fine bubble diffusion.

Because ozone is a powerful oxidant, all materials in contact with the gas or dissolved ozone must be ozone-resistant. Stainless steel (types 304L or 316L), Teflon, PVDF, and certain ceramics are standard. The use of rubber or most plastics is avoided as they degrade quickly.

Safety Measures

Ozone gas is toxic to humans at concentrations above 0.1 ppm, and exposure can cause respiratory irritation, headaches, and lung damage. For this reason, treatment plants must be equipped with:

  • Ozone gas detectors with alarms in the generator room and contactor areas.
  • Ventilation systems that exhaust air to the outside.
  • Ozone destruction units (catalytic or thermal) to treat off-gas before release.
  • Automatic shutoff valves and interlocks to stop ozone production if a leak is detected.
  • Personal protective equipment (PPE) for operators, including respirators and chemically resistant gloves.

Proper training is essential. Operators must understand the hazards of ozone as well as the specific characteristics of their system, such as the maximum ozone concentration and flow rates. Regular maintenance of seals, gaskets, and sensors is required to prevent leaks.

Operational Challenges

The main operational costs of ozonation are electrical energy (about 10–20 kWh/kg of ozone produced) and the need for clean, dry feed gas (air or oxygen). Plants using oxygen must manage liquid oxygen storage or install oxygen concentrators. Ozone generators also require periodic cleaning of dielectric tubes and electrodes. Additionally, the formation of bromate in waters containing bromide must be controlled — typically by lowering pH (adding acid or CO₂) or by using ammonia to form bromamines before ozonation. Some plants use concurrent hydrogen peroxide addition to shift the reaction pathway and minimize bromate.

Applications Beyond Drinking Water

While ozonation is most commonly associated with municipal drinking water treatment, it is also widely used in:

  • Wastewater treatment: For disinfection of tertiary effluent, oxidation of trace organic contaminants, and color removal. Ozone is increasingly used in water reuse schemes.
  • Bottled water and beverages: As a primary disinfectant that leaves no taste, often combined with UV and microfiltration.
  • Swimming pools: To reduce chlorine consumption, kill waterborne pathogens (including Cryptosporidium), and improve water clarity.
  • Cooling towers and industrial process water: For biofouling control and reduction of chemical treatment.
  • Aquaculture: To maintain water quality without toxic residues that harm fish.

Regulatory and Environmental Considerations

In the United States, ozonation is recognized by the Environmental Protection Agency (EPA) as an effective disinfection method under the Safe Drinking Water Act. The EPA has established a maximum contaminant level (MCL) for bromate of 10 µg/L, requiring careful control when ozone is applied to bromide-containing waters. The World Health Organization (WHO) also provides guidelines for ozone in drinking water, recommending a residual of ≤0.4 mg/L in distribution systems where ozone is used as a secondary disinfectant (though ozone residual is rarely used for that purpose).

From an environmental perspective, ozonation is considered green because the only residual is oxygen. No chemical transportation, storage of hazardous disinfectants, or disposal of chemical containers is needed on the same scale as with chlorine. However, the energy consumption and the potential for bromate formation are environmental trade-offs. Research into advanced ozone processes, such as catalytic ozonation and the use of ozone with hydrogen peroxide (peroxone), aims to increase efficiency and reduce byproducts.

Internationally, ozonation is widely adopted in Europe, Japan, and parts of Canada. It is also gaining traction in developing countries where water quality challenges are acute, particularly for treating surface water contaminated with agricultural runoff and pathogens. Small-scale, solar-powered ozone generators are being deployed in off-grid communities for point-of-use treatment.

Conclusion

Ozonation remains one of the most powerful and versatile tools available for enhancing drinking water safety and quality. By providing rapid, thorough disinfection without the persistent chemical residues associated with chlorination, it meets the demands of modern water treatment standards. Its ability to remove taste, odor, and a wide range of organic contaminants makes it particularly valuable for improving consumer acceptance. While the higher capital and operational costs — along with the need for careful control of byproducts like bromate — require expertise and investment, the benefits often outweigh the drawbacks, especially for systems facing challenging source water conditions. As technology advances and costs decline, ozonation is likely to play an even greater role in the global effort to deliver safe, pleasant-tasting drinking water to every tap.

For more information, refer to the EPA’s guidance on ozone in drinking water and the WHO Guidelines for Drinking-water Quality. Practical design guidance is available from the American Water Works Association and the Water Research Foundation.