Introduction: The Growing Need for Advanced Desalination Treatment

Desalination has become a cornerstone of water supply strategy for coastal cities, island nations, and arid regions worldwide. As freshwater sources dwindle and populations grow, plants converting seawater or brackish water into potable supplies are expanding rapidly. However, the quality of the final product depends heavily on how effectively pretreatment processes remove organic matter, microorganisms, and dissolved contaminants. Among the most powerful tools in the modern desalination engineer’s arsenal is ozonation—a process that uses ozone gas (O3) to oxidize and disinfect water. This article examines the role of ozonation in desalination plants, exploring its mechanisms, benefits, limitations, and integration with other treatment stages.

What Is Ozonation? A Brief Overview of Ozone Chemistry

Ozonation is a water treatment method in which ozone is generated on-site and bubbled through water to destroy impurities. Ozone is an allotropic form of oxygen: three oxygen atoms bonded together in an unstable, highly reactive molecule. Its strong oxidation potential—2.08 V, compared to 1.36 V for chlorine—allows it to react rapidly with a broad spectrum of organic and inorganic compounds. In desalination, ozone is typically produced by corona discharge or ultraviolet light, using dry air or pure oxygen as feed gas.

When ozone dissolves in water, it decomposes into hydroxyl radicals (·OH) via a chain reaction that is accelerated by elevated pH. These hydroxyl radicals are even more reactive than ozone itself, enabling the breakdown of refractory pollutants. This dual-action (direct ozone oxidation plus radical-mediated oxidation) makes ozonation effective for disinfection, color removal, taste and odor control, and the destruction of micro-pollutants such as pharmaceuticals and personal care products.

How Ozonation Improves Water Quality in Desalination Plants

Desalination plants—particularly reverse osmosis (RO) facilities—face a constant battle against fouling of membranes by organic matter, biofilms, and colloidal particles. Ozonation addresses these issues at multiple points in the treatment train. Below we examine the three primary roles ozone plays in a desalination context: pre-treatment, disinfection, and oxidation of contaminants.

1. Pre‑treatment: Reducing Fouling and Extending Membrane Life

Seawater or brackish feed water contains dissolved organic carbon (DOC), algae, and other biopolymers that can quickly foul RO membranes if not removed. Adding ozone before the filtration or flotation stage accomplishes several objectives. Ozone oxidizes high-molecular-weight organic molecules into smaller, more biodegradable forms that can be removed by downstream biological filters or dissolved air flotation (DAF). This reduces the organic loading on the RO membranes, lowering the frequency of chemical cleaning and prolonging membrane service life.

Ozone also inactivates microorganisms that might attach to membrane surfaces and form biofilms. A 2021 study published in Desalination reported that integrating ozonation into the pretreatment of a seawater RO plant reduced the silt density index (SDI) by 35% and decreased biofouling rates by nearly 50%. Properly dosed ozone breaks down extracellular polymeric substances (EPS) produced by bacteria, making biofilms less adhesive.

2. Disinfection: A Powerful Alternative to Chlorine

Disinfection is essential to ensure that desalinated water meets microbiological safety standards. Traditionally, chlorination has been used, but chlorine can react with natural organic matter to form carcinogenic disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). Ozone avoids this problem while offering superior inactivation kinetics for most pathogens, including protozoa like Cryptosporidium and Giardia that are resistant to chlorine.

Because ozone decays rapidly (half-life on the order of minutes in water), it leaves no residual disinfectant in the finished water. Many desalination plants therefore follow ozonation with a small dose of chloramines or chlorine to maintain a residual in the distribution system. This combination—ozone as the primary disinfectant, followed by a stable secondary disinfectant—provides both safety and regulatory compliance.

3. Oxidation of Contaminants: Breaking Down Micro‑pollutants and Toxins

Beyond disinfection and fouling control, ozone oxidizes a wide range of chemical contaminants that may be present in seawater, such as pesticides, industrial chemicals, algal toxins, and pharmaceutical residues. The oxidative degradation often yields smaller, less toxic compounds that can be removed by subsequent biological or physical processes. For example, microcystins released by harmful algal blooms are effectively cleaved by ozone, protecting downstream membranes and ensuring the finished water is safe.

Ozone also oxidizes iron, manganese, and sulfide compounds, converting them to insoluble forms that can be filtered out. In desalination plants drawing from brackish groundwater, this reduces scaling potential and helps maintain consistent product water quality.

Advantages of Ozonation in Desalination

The benefits of implementing ozonation extend beyond simply cleaner water. Below we summarize the key advantages that make ozone attractive for desalination operators.

  • No persistent chemical residuals: Unlike chlorine, ozone decomposes to oxygen, leaving no taste, odor, or harmful DBP precursors in the distribution system. This simplifies compliance with drinking water standards.
  • Enhanced membrane performance: By reducing organic and biological fouling, ozone allows RO systems to operate at lower pressures, saving energy and reducing operational costs.
  • Broad-spectrum disinfection: Ozone inactivates bacteria, viruses, and protozoan cysts more rapidly and completely than most alternative oxidants.
  • Oxidation of refractory compounds: Ozone can break molecules that are resistant to biological or conventional chemical treatment, expanding the range of pollutants that can be removed.
  • Reduction of chemical consumption: With effective ozonation, the need for other chemicals (e.g., coagulants, antiscalants, biocides) can often be reduced, lowering chemical storage and handling risks.

Challenges and Considerations for Ozonation in Desalination

Despite its many strengths, ozonation is not a universal solution. Engineers must navigate several technical and economic challenges to deploy it successfully.

Energy and Equipment Costs

Ozone generation is energy-intensive, requiring 10–20 kWh per kilogram of ozone produced, depending on the technology and feed gas purity. For a large desalination plant, this can represent a significant operating expense. Additionally, ozone systems require careful design: the contactor must provide sufficient mass transfer (typically using fine bubble diffusers or injectors), and the off‑gas must be destroyed to prevent ozone emissions, adding to capital costs.

Bromate Formation

Perhaps the most important chemical challenge is the formation of bromate (BrO3–), a potential human carcinogen regulated at <10 µg/L by the US EPA and EU. Seawater naturally contains bromide ions (approximately 65 mg/L). When ozone oxidizes bromide, it can produce bromate via a complex reaction pathway. Controlling bromate formation requires careful optimization of ozone dose, pH (lowering pH reduces bromate yield), and contact time. Many plants use pH depression before ozonation or add ammonia to inhibit bromate formation. The 2023 WHO Guidelines for Drinking‑water Quality note that bromate management is a “key consideration” for desalination plants using ozone.

Corrosion and Material Compatibility

Ozone is a strong oxidant that can attack certain materials. Stainless steel grades like 316L are generally acceptable, but rubber gaskets, elastomers, and some plastics may degrade. Proper material selection and system design are essential to avoid leakage and ensure long-term reliability.

Safety Concerns

Ozone is toxic to humans at concentrations above 0.1 ppm by volume. Plant operators must install ozone monitors, ventilation, and safety interlocks. Personal protective equipment and training are mandatory. While these measures are standard in modern plants, they add to the overall cost and complexity.

Integration of Ozonation with Other Desalination Processes

Ozonation rarely works in isolation. In a typical seawater RO plant, ozone may be applied at the beginning of the pretreatment chain, followed by coagulation, flocculation, and DAF. Some advanced designs place ozonation after microfiltration but before the RO membranes. In thermal desalination plants (multi‑stage flash distillation or multi‑effect distillation), ozone is less common but can be used to treat the distillate or to control biofouling in the cooling water system.

Ozonation + Biological Activated Carbon (BAC)

A particularly effective combination is ozone followed by biological activated carbon filtration. Ozone partially oxidizes organic matter, making it more biodegradable; the BAC filter then removes the assimilable organic carbon (AOC) and residual biodegradable material. This dual treatment can reduce membrane biofouling potential and improve overall organic removal efficiency. Many European water reuse schemes and some desalination plants in the Middle East and Australia now employ this configuration.

Ozonation as a Pre‑treatment for UV and Advanced Oxidation

For removal of refractory micro‑pollutants, ozone can be coupled with ultraviolet (UV) light to generate advanced oxidation processes (AOPs). The UV‑ozone combination produces more hydroxyl radicals, accelerating the degradation of compounds like NDMA (N‑nitrosodimethylamine) and 1,4‑dioxane. While such systems are more common in wastewater reuse, they are increasingly considered for seawater desalination where source water quality is impacted by industrial or agricultural runoff.

As desalination expands, so does the drive to improve efficiency and sustainability. Several trends will shape the role of ozonation in coming years.

  • Improved ozone generation technology: Advances in dielectric barrier discharge, pulsed power, and ozone‑water contactors promise higher energy efficiency and smaller footprints. On‑site oxygen generation also reduces dependence on purchased oxygen.
  • Real‑time process control: Sensors that measure ozone residual, UV absorbance, and fluorescence can enable dose‑optimization algorithms that minimize bromate formation while maximizing disinfection. AI‑driven control systems are being tested in pilot plants.
  • Hybrid processes: Integrating ozone with membrane bioreactors (MBRs) or direct filtration may allow higher water recoveries and smaller systems.
  • Regulatory drivers: Stricter limits on DBPs and emerging contaminants will likely increase demand for ozone‑based treatment as an alternative to chlorine.

Conclusion

Ozonation has established itself as a versatile and effective process for improving water quality in desalination plants. Its ability to simultaneously disinfect, oxidize contaminants, and reduce membrane fouling makes it a valuable component of both pretreatment and advanced treatment trains. The challenges—particularly energy consumption and bromate formation—can be managed through careful design, proper pH control, and real‑time monitoring. As desalination continues to play a critical role in global water security, ozonation will remain a key technology for producing clean, safe, and palatable water from the sea.

For further reading, refer to the WHO Guidelines for Drinking-water Quality (2022 edition) for bromate and ozone residual guidelines, and consult the review on ozonation in seawater desalination published in Desalination and Water Treatment (2021). Additionally, the American Water Works Association offers practical design guidance for ozone systems in water treatment.