Ozone (O₃) is a powerful oxidant widely adopted in drinking water and wastewater treatment for its ability to inactivate pathogens, remove organic contaminants, and control taste and odor without leaving persistent chemical residuals. However, the very reactivity that makes ozone effective also creates environmental challenges when excess ozone and its byproducts are discharged into the air or water. Over the past decade, the rapid expansion of ozone-based treatment systems—driven by stricter disinfection standards and the phase-out of chlorine—has elevated concerns about the ecological and human health consequences of these discharges. Evaluating the full environmental footprint of ozone discharges is therefore essential for designing treatment plants that balance public health protection with ecosystem stewardship.

This article provides a comprehensive examination of ozone discharges from treatment plants, covering the underlying chemistry, environmental pathways, ecological and human health risks, regulatory frameworks, and advanced mitigation strategies. By integrating current research and best practices, it aims to support water professionals, regulators, and environmental managers in making informed decisions that minimize unintended harms while preserving the benefits of ozone technology.

Background: Ozone in Water Treatment

Ozone has been used for water disinfection since the early 1900s in Europe and has become increasingly common worldwide. It is generated on‑site by passing oxygen through a high‑voltage corona discharge or using ultraviolet light at 185 nm. Once dissolved in water, ozone rapidly decomposes into hydroxyl radicals (•OH) that oxidize a broad spectrum of pollutants, including bacteria, viruses, protozoa, pharmaceuticals, and micropollutants. Unlike chlorine, ozone leaves no residual disinfectant in the distribution system, which is an advantage for taste but also means a secondary disinfectant is usually added to maintain water quality through the network.

Ozone treatment systems typically consist of an ozone generator, a contact tank where the gas is bubbled into water, and a destructor unit that captures excess ozone from the off‑gas before it is released into the atmosphere. Despite these controls, complete destruction is rarely achieved, and dilute ozone concentrations may still escape. Furthermore, the reaction of ozone with bromide ions present in source water can produce bromate (BrO₃⁻), a regulated carcinogen. The formation of other brominated and chlorinated byproducts (e.g., bromoform, MX) further complicates the environmental profile of ozone discharges.

Understanding Ozone Discharges: Pathways and Fate

Atmospheric Releases

During water treatment, a fraction of the ozone gas injected into the contact tank does not dissolve or decomposes incompletely. This undissolved ozone is vented as off‑gas and, despite passing through a destructor (thermal or catalytic), small amounts—typically 0.1–1 ppm by volume—can be emitted to the atmosphere. In large‑scale plants, these releases may contribute to ground‑level ozone formation, especially in urban and industrial areas with high background nitrogen oxide (NOx) concentrations. Ozone at ground level is a respiratory irritant and a major component of photochemical smog, and even low‑level chronic exposures have been linked to decreased lung function and aggravation of asthma.

Aqueous Discharges

Ozone treatment also results in aqueous discharges: the treated water itself (effluent), backwash from filters, and periodic cleaning solutions. While ozone decays relatively quickly in water (half‑life minutes to hours depending on pH and organic content), its reaction byproducts persist. The most significant aqueous byproduct is bromate, formed when ozone oxidizes naturally occurring bromide. Bromate is classified as a probable human carcinogen (Group 2B by IARC) and is regulated in drinking water at a maximum contaminant level (MCL) of 10 µg/L in the US and 10 µg/L by the WHO. Other byproducts include aldehydes, ketones, brominated acetic acids, and assimilable organic carbon (AOC) that can promote microbial regrowth in distribution systems and receiving waters.

When effluent containing these byproducts is discharged into rivers, lakes, or coastal waters, the ecological consequences depend on dilution, background water chemistry, and the sensitivity of local biota. Bromate concentrations in the receiving environment may accumulate in areas with low flow or high bromide inputs, posing risks to aquatic organisms. Moreover, the oxygen depletion caused by the decomposition of residual ozone and the increase in AOC can alter nutrient cycles and support harmful algal blooms.

Environmental Impacts in Detail

Air Quality and Human Health

Ground‑level ozone is a criteria pollutant under the US Clean Air Act, with a primary standard of 0.070 ppm averaged over 8 hours. Even minor contributions from ozone treatment plants—when aggregated with vehicular and industrial emissions—can push local concentrations above the standard, especially in photochemically active regions. Epidemiological studies have consistently associated short‑term ozone exposure with increased hospital admissions for respiratory conditions, while long‑term exposure may contribute to cardiovascular mortality. For communities located near water treatment facilities, the chronic release of ozone off‑gas represents an added health burden that should be accounted for in environmental impact assessments.

Water Quality and Aquatic Life

When ozone‑treated effluent enters surface waters, the residual ozone itself can cause acute toxicity to aquatic organisms, although its rapid decomposition limits the spatial extent of damage. Far more concerning are the persistent byproducts. Bromate, in particular, has been shown to inhibit growth in algae and induce oxidative stress in fish embryos at concentrations as low as 50 µg/L. A 2019 study published in Environmental Science & Technology found that waters receiving ozonated wastewater exhibited elevated bromate levels and reduced abundance of sensitive macroinvertebrate species down to 5 km downstream. The formation of disinfection byproducts is not limited to bromide; iodide and organic matter also react to form iodo‑compounds that can be even more toxic. Furthermore, the increase in AOC following ozonation can stimulate bacterial growth, potentially leading to eutrophication in nutrient‑limited systems.

Terrestrial and Ecosystem Effects

Although less studied, the deposition of ozone and its byproducts onto soil and vegetation near treatment plants can impact terrestrial ecosystems. Ozone entering leaf stomata causes oxidative cell damage, reduced photosynthesis, and decreased crop yields. Sensitive plant species, such as white clover and certain conifers, exhibit visible injury at ambient levels as low as 40 ppb. For treatment plants with substantial atmospheric emissions, fumigation of adjacent vegetation can create zones of stress that alter local biodiversity. Similarly, irrigation with ozonated effluent—a practice gaining traction in water‑scarce regions—may inadvertently expose crops and soil microbes to elevated bromate and organic byproducts, with implications for food safety and soil health.

Regulatory Frameworks and Guidelines

Governments and international bodies have established guidelines to limit environmental harm from ozone discharges, though coverage remains fragmented. In the United States, the Safe Drinking Water Act sets an MCL of 10 µg/L for bromate in finished drinking water, but this standard does not apply to effluent discharged to surface waters. Wastewater treatment plants that use ozone must comply with National Pollutant Discharge Elimination System (NPDES) permits, which may include limits on total residual chlorine, pH, and specific pollutants, but bromate is not a routinely monitored parameter. The EPA’s aquatic life criteria do not yet include bromate, though some states (e.g., California) have derived their own water quality objectives.

Internationally, the WHO Guidelines for Drinking‑water Quality recommend a provisional guideline value of 10 µg/L for bromate in drinking water. The European Union’s Drinking Water Directive sets a parametric value of 10 µg/L as well. For ambient waters, the EU’s Water Framework Directive requires member states to monitor and control priority substances, but bromate is not currently listed. Japan’s water quality standards for bromate in drinking water are 10 µg/L, and China’s latest GB 5749‑2022 sets the same limit. These gaps highlight the need for more comprehensive regulation of bromate and other ozone byproducts in both drinking water and environmental discharges.

Comparative Analysis with Alternative Disinfectants

To contextualize the risks of ozone discharges, it is useful to compare its environmental profile with that of conventional disinfectants—chlorine, chloramine, and ultraviolet (UV) radiation.

Disinfectant Primary Discharge Concerns Persistence in the Environment Byproduct Hazards Mitigation Complexity
Chlorine Residual chlorine, trihalomethanes (THMs), haloacetic acids (HAAs) Moderate (hours to days) Carcinogenic THMs/HAAs; acute toxicity to aquatic life Dechlorination required; easy to monitor
Chloramine Residual chloramine, NDMA formation Moderate to high (days to weeks) NDMA (carcinogen); toxicity to fish Difficult to control NDMA; needs additional treatment
UV None (no chemical residual) N/A None (but mercury lamps have environmental risk) Low; but requires energy and lamp disposal
Ozone Bromate, AOC, aldehydes, atmospheric ozone Low (minutes) for ozone; high for byproducts Bromate (carcinogen); AOC promotes regrowth Complex; requires multiple control measures

Ozone’s main advantages—strong pathogen inactivation without persistent residual—are partially offset by the formation of bromate and other byproducts that require additional treatment steps. The comparison underscores that no disinfectant is environmentally benign; the choice must be guided by source water quality, local sensitivity, and the capacity to implement best‑available control technologies. For plants in bromide‑rich coastal or groundwater environments, alternative pre‑treatment (e.g., reverse osmosis) or switching to UV may be more sustainable if ozone cannot be controlled.

Mitigation Strategies and Best Available Technologies

Optimizing Ozone Dosage and Contact Conditions

The most effective way to reduce environmental discharges is to minimize the amount of ozone used while still achieving treatment goals. This requires accurate control of the ozone‑to‑dissolved organic carbon (DOC) ratio, pH, and temperature. Advanced process control using real‑time monitoring of residual ozone and oxidation‑reduction potential (ORP) can adjust dose on a sub‑minute timescale. Maintaining a slightly acidic pH (6.0‑7.0) reduces bromate formation, because hydroxide ions catalyze the reaction of ozone with bromide. Some plants employ a “multi‑point” injection strategy to optimize mass transfer and reduce off‑gas volume.

Off‑Gas Destruction and Capture

Modern ozone destructors use either thermal (heating off‑gas to >350 °C) or catalytic (manganese dioxide or activated carbon) methods to achieve ozone destruction efficiencies exceeding 99.9%. However, these systems require regular maintenance and energy input. For plants that discharge significant off‑gas, installing a secondary destructor or a recirculation loop can reduce atmospheric release to near zero. Additionally, capturing the ozone‑rich off‑gas and recycling it back to the contact tank can improve utilization efficiency and lower the chemical demand for generation.

Byproduct Removal Technologies

When aqueous byproducts already pose a risk, post‑treatment polishing is necessary. Activated carbon filtration (granular or powdered) effectively adsorbs bromate and many organic byproducts, especially when the carbon is fresh or biologically activated. Alternative methods include:

  • Biological activated carbon (BAC): Microbes colonize the carbon media and metabolize AOC and some aldehydes, while also reducing bromate through denitrification in anoxic zones.
  • Ion exchange: Selective resins can remove bromide before ozonation, preventing bromate formation. Combined with ozone, this approach has been proven effective at pilot scale.
  • Advanced oxidation processes (AOPs): Using UV/H₂O₂ or O₃/H₂O₂ can degrade recalcitrant byproducts, though they may also produce new byproducts such as bromate in bromide‑containing waters.
  • Reverse osmosis (RO): RO removes nearly all byproducts but is energy‑intensive and produces a concentrated brine stream that requires careful management.

Integrated Management for Low‑Impact Operation

Beyond unit processes, an integrated approach that considers the entire treatment train is crucial. For example, pre‑removal of bromide by ion exchange or RO allows ozone to be used without bromate risk. Combining ozone with downstream biological filtration (BAC) leverages the AOC produced by ozone as a carbon source for biofilm, resulting in lower regrowth potential and reduced than when using ozone alone. Operator training, routine monitoring of bromate and other indicator byproducts, and an environmental management system that tracks discharges to air and water are essential components of sustainable operation.

Case Studies in Sustainable Ozone Discharge Management

Wulpen Wastewater Treatment Plant, Belgium

The Wulpen plant treats municipal wastewater to very high standards, including the removal of micropollutants via ozone. To control bromate, the plant uses a two‑stage ozonation with intermediate pH adjustment. In the first stage, ozone is applied at low dose (0.3 mg O₃/mg DOC) without pH control; in the second stage, lime is added to raise pH to 7.2, and a higher ozone dose is used. The resulting bromate levels in the effluent are consistently below 5 µg/L. The plant also uses a catalytic destructor for off‑gas and quarterly monitoring of aquatic invertebrate communities downstream.

Los Angeles Aqueduct Filtration Plant, USA

This plant uses ozone as a primary disinfectant for surface water from the Owens River, which contains elevated bromide (0.2‑0.4 mg/L). To meet the bromate MCL, the plant combines ozonation with ammonia addition (to form chloramine residual) and operates at a reduced pH (6.5). The off‑gas is thermally destroyed, and an air monitoring station tracks ambient ozone levels within the plant perimeter. The plant’s compliance record demonstrates that with careful operation, atmospheric and aqueous discharges can be kept well below regulatory limits.

Emerging Research and Future Directions

Several promising developments may further reduce the environmental impacts of ozone discharges. Electrochemical ozone generation coupled with membrane degassing is being explored to achieve higher dissolution efficiencies and virtually eliminate off‑gas. Advances in non‑thermal plasma reactors could generate ozone at the point of use with precise control. The use of biological pre‑treatment to remove bromide before ozonation (via denitrification of bromate) is under investigation, though it remains at the laboratory scale. Finally, integrating real‑time sensors for bromate and other byproducts with automated dose control algorithms could allow treatment plants to operate at the edge of compliance without safety margins, minimizing both chemical use and discharge.

Regulatory agencies are also moving toward more comprehensive evaluation. The EPA’s Unregulated Contaminant Monitoring Rule (UCMR) includes bromate and several iodinated byproducts, which may lead to future MCLs for ambient water. Similarly, the European Commission is considering listing bromate as a priority substance under the Water Framework Directive. These developments will likely drive investment in advanced removal and monitoring technologies across the water sector.

Conclusion

Ozone discharges from water treatment plants present a complex environmental challenge. While ozone offers significant benefits for disinfection and contaminant removal, the potential for ground‑level air pollution, aquatic byproduct toxicity, and terrestrial ecosystem effects cannot be overlooked. A thorough understanding of the chemistry, pathways, and regulatory context is essential for responsible management. The most effective mitigations—such as optimizing ozone dose, implementing advanced destructors, and removing bromide before ozonation—can reduce discharges well below levels of concern. Continued innovation in process control, byproduct removal, and real‑time monitoring will further align the environmental performance of ozone treatment with sustainability goals.

Water utilities, regulators, and researchers must work together to close the gaps in monitoring, regulation, and public awareness. By adopting a proactive, integrated approach to ozone discharge management, treatment plants can continue to protect public health while minimizing their footprint on the surrounding environment. The path forward lies not in abandoning ozone—a highly valuable tool—but in refining its use so that the benefits it provides are not overshadowed by unintended consequences.