What Is Ozonation?

Water treatment plants around the world face the challenge of delivering safe drinking water while managing the formation of harmful disinfection byproducts (DBPs). Ozonation has emerged as a powerful disinfection and oxidation technology that directly addresses this challenge. Ozone (O3) is a highly reactive molecule composed of three oxygen atoms. It is generated on-site by passing dry air or pure oxygen through a high-voltage electrical discharge — similar to the mechanism that produces the sharp smell after a lightning storm. Because ozone cannot be stored and must be generated immediately before use, treatment plants require dedicated ozone generation and injection systems.

Once dissolved in water, ozone reacts rapidly with a broad spectrum of contaminants. It destroys bacteria, viruses, protozoa such as Cryptosporidium and Giardia, and oxidises organic and inorganic compounds. The reaction time is typically measured in seconds to minutes, making ozone a very efficient disinfectant. Unlike chlorine, which leaves a persistent residual in the distribution system, ozone decomposes back into oxygen, leaving no chemical taste or odour in the finished water.

Ozonation is not a standalone solution — it is most effective when integrated into a multi-barrier treatment train. The technology has been used in European municipal water systems for decades and is increasingly adopted in North America and Asia as regulations tighten around DBP limits and as water sources face more complex contamination from agricultural runoff, wastewater effluent, and natural organic matter.

The Problem of Disinfection Byproducts

Traditional chlorination has been the backbone of drinking water disinfection for over a century. Chlorine is cheap, effective, and provides a measurable residual that protects water as it travels through pipes. But chlorine reacts with natural organic matter (NOM) — humic and fulvic acids from decaying vegetation — to form a suite of disinfection byproducts. The most well-known are trihalomethanes (THMs) and haloacetic acids (HAAs), but more than 600 DBPs have been identified in treated water.

Long-term exposure to THMs and HAAs is linked to an increased risk of bladder cancer, colorectal cancer, and adverse reproductive outcomes. Regulatory agencies such as the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) set strict maximum contaminant levels for total THMs (typically 80 µg/L) and five haloacetic acids (60 µg/L). Many utilities struggle to meet these limits, especially during warmer months when NOM levels spike and chlorine demand is highest.

Beyond THMs and HAAs, chlorination can produce other DBPs of concern: chlorite, chlorate, haloacetonitriles, haloketones, and chloropicrin. Some of these compounds are genotoxic or carcinogenic even at low concentrations. The challenge is to maintain adequate disinfection — achieving a required log reduction of pathogens — while keeping DBP concentrations within legal limits. This is where ozonation offers a distinct advantage.

How Ozonation Minimises DBPs

Ozonation reduces DBP formation through several mechanisms:

Direct Oxidation of Precursors

Ozone is a much stronger oxidant than chlorine. It partially oxidises large organic molecules, breaking them into smaller, less reactive fragments. Many of these fragments are less likely to react with chlorine to form THMs and HAAs. In effect, ozonation “pre-treats” the organic matter, reducing the pool of DBP precursors before chlorine is added. Studies have shown that pre-ozonation can reduce THM formation potential by 30–50% and HAA formation potential by 20–40%, depending on water quality and ozone dose.

Reduced Chlorine Demand

Because ozone destroys a portion of the organic matter and imparts its own disinfection credit, the amount of chlorine needed for the final disinfection step can be lowered. Less chlorine means less reactant available to form DBPs. A well-designed ozone system can cut the chlorine dose by 25–50% while still meeting disinfection targets. This directly reduces the DBP load entering the distribution system.

Enhanced Biodegradation in Downstream Processes

Ozone partially oxidises NOM into smaller, biodegradable molecules. These can be removed by subsequent biological filtration (e.g., biologically active carbon or slow sand filters) that hosts microorganisms adapted to consume the low-molecular-weight compounds. By removing biodegradable organic matter before chlorination, the DBP precursor concentration is further reduced. This synergy between ozonation and biofiltration is a critical advantage for facilities that need to achieve very low THM and HAA levels.

The Bromate Concern

Ozonation is not without its own byproduct issue: bromate (BrO3−) can form when ozone oxidises naturally occurring bromide ions in source water. Bromate is classified as a probable human carcinogen, and the EPA has set a maximum contaminant level of 10 µg/L. This requires careful control of ozonation conditions — adjusting ozone dose, pH, and temperature and sometimes adding ammonium or hydrogen peroxide to suppress bromate formation. For many utilities, especially those with low bromide levels, the DBP reduction benefits of ozonation far outweigh the bromate risk, which can be managed with proper process design.

Ozonation in a Multi-Barrier Treatment Framework

One of the most effective strategies for minimising DBPs while ensuring robust disinfection is to place ozonation within a multi-stage treatment train. The typical arrangement is:

  1. Pre-ozonation applied to raw water for primary disinfection and oxidation of metals (iron, manganese) and taste/odour compounds.
  2. Coagulation-flocculation-sedimentation to remove suspended solids and a portion of the organic matter; pre-ozonation can improve coagulation efficiency by modifying particle surfaces.
  3. Intermediate ozonation (optional) for further oxidation and DBP precursor destruction.
  4. Biological filtration using granular activated carbon (GAC) or anthracite-sand media on which a biofilm develops. This step assimilates the biodegradable organic matter created by ozonation.
  5. Final disinfection with chlorine or chloramines at a lower dose, achieving a stable residual while DBP formation is minimised because precursors have been drastically reduced.

This integrated approach, sometimes called “ozone-biofiltration,” has been adopted by many progressive utilities. For example, the EPA’s guidance on ozone for drinking water treatment highlights case studies where THM levels dropped by over 50% after ozone-biofiltration upgrades. Another well-documented case is the Metropolitan Water District of Southern California’s Weymouth Plant, which uses ozone followed by biologically active carbon to maintain DBP compliance while treating high-bromide imported water.

When bromide is a concern, some facilities use ozone in combination with chloramination (adding ammonia to form chloramines) instead of free chlorine for the residual. Chloramines form fewer THMs and HAAs, and the lower DBP precursor load after ozonation makes chloramination even more effective.

Practical Considerations for Implementing Ozonation

Capital and Operating Costs

Ozone systems require a significant initial investment. Equipment includes ozone generators, power supplies, air preparation or oxygen feed systems, contact chambers (where ozone is mixed with water), off-gas destruct units (to destroy unused ozone in exhaust air), and monitoring instrumentation. Operating costs are dominated by electricity — ozone generation consumes 8–15 kWh per kilogram of ozone produced, depending on the efficiency of the generator and the feed gas. For a medium-sized plant treating 50 million litres per day, annual electricity costs for ozonation may be on the order of $100,000–$300,000.

However, these costs can be offset by reductions in chlorine and chemical usage, lower DBP monitoring and compliance expenses, and improved taste and odour that may reduce customer complaints and the need for additional treatment. Many utilities find that the overall cost per cubic metre of water treated is competitive with alternative advanced oxidation processes.

Safety and Maintenance

Ozone is toxic to breathe and requires rigorous safety protocols: sealed contact chambers, ambient ozone monitors, automatic shut-offs, and proper ventilation. Personnel need training in handling ozone and responding to leaks. The generation equipment also demands regular maintenance of dielectric tubes, power supplies, and air dryers. Despite these requirements, ozone has a strong safety record when operated under established standards (e.g., OSHA guidelines).

Water Quality Fluctuations

The effectiveness of ozonation depends on water temperature, pH, NOM concentration, alkalinity, and bromide level. These parameters can vary seasonally and even daily. Plants must therefore have real-time monitoring and adjustable ozone dosing systems to maintain optimum performance. Advanced control systems using online TOC (total organic carbon), UV absorbance, and ozone residual analysers can automate dose adjustments.

Future Directions: Ozonation and Advanced Oxidation

Ozonation is a mature technology, but ongoing research continues to refine its application for DBP control. One active area is the use of ozone combined with hydrogen peroxide (O3/H2O2) or UV light (O3/UV) to generate hydroxyl radicals. These advanced oxidation processes (AOPs) can mineralise organic contaminants and break down DBP precursors more completely than ozone alone. For water sources with high bromide, combining ozone with peroxide can also suppress bromate formation because the hydroxyl radicals react quickly with organic matter, leaving less ozone available to oxidise bromide.

Another promising development is the use of catalytic ozonation with metal oxides or supported catalysts to enhance contaminant removal and further reduce byproduct precursors. Meanwhile, online sensor technology and machine learning algorithms are starting to allow real-time optimisation of ozone dose to simultaneously manage disinfection credit, DBP precursor removal, and bromate formation.

For small and rural water systems, modular, skid-mounted ozone units are becoming more common, lowering the implementation barrier. New generator designs (e.g., using high-frequency discharge or cold plasma) may reduce energy consumption and maintenance needs.

The WHO Guidelines for Drinking-water Quality and the EPA National Primary Drinking Water Regulations continue to drive demand for technologies that reduce DBP formation while maintaining robust pathogen inactivation. As these regulations become stricter, and as water utilities face pressure from source water degradation and climate change, ozonation will likely become a standard tool in the water treatment toolbox.

Conclusion

Ozonation is not a complete silver bullet — no single unit process can address every water quality challenge. But when applied thoughtfully within a multi-barrier treatment train, it offers one of the most effective ways to simultaneously achieve high-level disinfection and significantly reduce the formation of disinfection byproducts. By oxidising DBP precursors, lowering chlorine demand, and enabling biological removal of biodegradable organic matter, ozone helps utilities meet stringent THM and HAA limits without compromising microbial safety.

As water treatment professionals continue to seek cost-effective, sustainable solutions for an evolving regulatory landscape, ozonation stands out as a proven, versatile technology. With careful design to manage bromate formation and rigorous attention to safety and operational controls, ozonation can play an essential role in delivering clean, safe drinking water to communities — now and in the future.