Emerging Contaminants: An Unseen Threat to Drinking Water

Modern water treatment has long focused on removing pathogens, turbidity, and conventional chemical pollutants. However, a new class of substances—often called emerging contaminants—now challenges the safety of our drinking water supplies. These compounds, including pharmaceuticals, personal care products, industrial chemicals, and hormones, are not routinely monitored but are increasingly detected in source waters at trace concentrations. Chronic exposure, even at low levels, raises concerns about endocrine disruption, antibiotic resistance, and other long-term health effects. Traditional treatment methods such as sedimentation, filtration, and chlorination are often ineffective at removing these recalcitrant molecules. Ozonation has emerged as a powerful and versatile oxidation technology capable of breaking down a wide spectrum of emerging contaminants, offering a robust barrier between these pollutants and the tap.

What Are Emerging Contaminants?

The term "emerging contaminants" refers to chemicals and microorganisms that have not historically been regulated or routinely monitored but are now being recognized as potential risks to human health and the environment. They enter waterways through multiple pathways: wastewater effluent, agricultural runoff, industrial discharges, and even household disposal of unused medications. Because conventional treatment plants were not designed to remove them, many pass through into drinking water sources.

Common categories include:

  • Pharmaceuticals and personal care products (PPCPs): Antibiotics, analgesics, antidepressants, contraceptives, and synthetic musks from soaps and lotions.
  • Endocrine-disrupting chemicals (EDCs): Natural and synthetic hormones like estradiol and bisphenol A (BPA) that interfere with hormonal systems.
  • Pesticides and herbicides: Atrazine, glyphosate, and other agricultural chemicals that persist in the environment.
  • Industrial chemicals: Per- and polyfluoroalkyl substances (PFAS), plasticizers like phthalates, and flame retardants.
  • Antimicrobial resistance genes and pathogens: Genetic material that can spread resistance among bacteria.

Though present at parts-per-trillion or parts-per-billion levels, the cumulative load and bioactivity of these contaminants have prompted agencies like the U.S. Environmental Protection Agency (EPA) to list them as contaminants of emerging concern (CECs) and to investigate treatment strategies.

The Process of Ozonation

Ozonation harnesses the strong oxidizing power of ozone gas (O3) to transform and degrade organic and inorganic contaminants in water. Ozone is an unstable, pale blue gas that must be generated on-site, typically by passing dry air or pure oxygen through a high-voltage electrical discharge (corona discharge method). The ozone-enriched gas is then injected into the water stream through fine bubble diffusers, static mixers, or venturi injectors inside a contact chamber.

Once dissolved, ozone reacts with contaminants via two primary pathways: direct molecular ozonation and indirect radical oxidation. Direct ozonation selectively attacks unsaturated bonds, aromatic rings, and nucleophilic sites (e.g., phenols, amines). Indirect oxidation occurs when ozone decomposes to form highly reactive hydroxyl radicals (•OH), which non-selectively oxidize virtually any organic compound. The ratio of these pathways depends on water pH, temperature, and the presence of radical scavengers like natural organic matter (NOM) or alkalinity. At higher pH, ozone decomposition accelerates, favoring hydroxyl radical formation—advantageous for breaking down persistent pollutants.

Typical ozone doses for drinking water treatment range from 1 to 4 mg/L, with contact times of 10 to 30 minutes. The process is controlled to ensure sufficient disinfection and contaminant removal while minimizing the formation of unwanted byproducts.

Effectiveness Against Specific Emerging Contaminants

Extensive research demonstrates that ozonation effectively removes a broad array of emerging contaminants. The degree of removal depends on the compound’s chemical structure, ozone dose, contact time, and background water matrix. Below are key classes and their treatability.

Pharmaceuticals and Personal Care Products

Ozone readily oxidizes many active pharmaceutical ingredients (APIs). Antibiotics such as sulfamethoxazole and ciprofloxacin achieve >95% reduction at moderate ozone doses. Analgesics like ibuprofen and acetaminophen are also highly amenable to ozonation. However, some compounds (e.g., the anticonvulsant carbamazepine) are ozone-resistant and require enhanced oxidation with hydroxyl radicals. Personal care product ingredients like triclosan and synthetic musks degrade efficiently, though byproduct toxicity must be assessed.

Pesticides and Herbicides

Atrazine, a widely used herbicide, is slowly oxidized by molecular ozone but is rapidly destroyed by hydroxyl radicals. A typical ozone dose of 2–3 mg/L can remove 80–90% of atrazine. Organochlorine pesticides (e.g., lindane) are more recalcitrant, often requiring advanced oxidation processes (AOPs) that combine ozone with hydrogen peroxide or UV light.

Endocrine-Disrupting Chemicals (EDCs)

Natural and synthetic hormones such as 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) are highly susceptible to ozonation. At typical doses, >99% removal of estrogenic activity is achievable. Bisphenol A (BPA) also reacts rapidly with ozone, breaking down into less active intermediates. Monitoring estrogenic activity via bioassays confirms substantial reduction of endocrine-disrupting potential.

Per- and Polyfluoroalkyl Substances (PFAS)

PFAS, known for their extreme chemical stability (carbon-fluorine bonds), pose a significant challenge. Conventional ozonation alone is largely ineffective for full mineralization of PFAS. However, ozone-based advanced oxidation processes (e.g., O3/H2O2, O3/UV) can partially degrade short-chain PFAS, while longer-chain compounds require additional treatment such as granular activated carbon (GAC) or ion exchange. Research continues on combining ozonation with catalytic or electrochemical processes to improve PFAS removal.

Advantages of Ozonation in Water Treatment

  • Broad spectrum oxidation: Ozone reacts with a wider range of contaminants than chlorine or chloramine, including many emerging contaminants.
  • Rapid reaction kinetics: Most contaminant transformations occur within minutes, allowing for smaller contact basins.
  • Disinfection bonus: Ozone is a powerful disinfectant against bacteria, viruses, and protozoa like Giardia and Cryptosporidium, providing a twin benefit.
  • Reduced halogenated byproducts: Unlike chlorination, ozonation does not form trihalomethanes (THMs) or haloacetic acids (HAAs), though it can produce bromate in bromide-containing waters.
  • Enhanced biodegradability: Ozonation breaks down large organic molecules into smaller, more biodegradable compounds, which improves subsequent biological filtration (e.g., biologically activated carbon).
  • Minimal sludge production: Unlike adsorption processes, ozonation does not generate significant waste residuals; contaminants are transformed rather than transferred.
  • On-site generation reduces transport hazards: Ozone is produced and used immediately, eliminating the need for storing hazardous chemicals like chlorine gas.

Challenges and Considerations

Despite its advantages, ozonation presents operational and environmental challenges that require careful management.

Byproduct Formation: Bromate

When source water contains bromide (Br−), ozone can oxidize it to bromate (BrO3−), a potential human carcinogen regulated by the EPA at 10 μg/L. Utilities must monitor bromide levels, control ozone dose and pH, or use scavengers (e.g., ammonia) to minimize bromate formation. Alternative strategies include switching to chlorine-based disinfection for bromide-rich waters or combining ozone with hydrogen peroxide to shift reaction pathways.

Other Transformation Byproducts

Ozonation does not always fully mineralize contaminants; it can produce transformation products (TPs) that may be more toxic or persistent than the parent compound. For instance, certain pharmaceuticals form nitrosamines (e.g., NDMA) under ozone. Comprehensive toxicity screening is essential to ensure that treatment does not create new hazards. Many TPs are biodegradable and can be removed in subsequent biological filtration steps.

High Energy and Capital Costs

Ozone generation requires significant electrical energy—typically 10–20 kWh per kilogram of ozone produced—along with air preparation (drying, compression) and ozone destruction equipment for off-gas treatment. Capital costs for ozone systems are higher than for chlorination or UV, though they are often offset by reduced chemical consumption and improved water quality. Operational expertise is needed to maintain optimal performance.

Health and Safety

Ozone is toxic if inhaled. Enclosed spaces must have continuous monitoring and ventilation to ensure worker safety. Leak detection and emergency shutdown protocols are mandatory. Additionally, ozone off-gas must be destroyed before release to the atmosphere, typically using thermal or catalytic destruct units, to prevent fugitive emissions.

Ozonation in Combination with Other Treatment Processes

Because few treatment technologies achieve complete removal of all contaminants alone, ozonation is often integrated into a multi-barrier treatment train.

  • Ozone/Biological Activated Carbon (O3/BAC): Ozonation increases the biodegradability of organic matter, which is then removed by BAC filters. This combination effectively removes both parent contaminants and their ozonation byproducts, reduces disinfection byproduct precursors, and extends GAC bed life.
  • Advanced Oxidation Processes (AOPs): Adding hydrogen peroxide (O3/H2O2) or UV light (O3/UV) accelerates hydroxyl radical generation, enhancing the oxidation of ozone-resistant contaminants like carbamazepine and NDMA. These AOPs are widely used in water reuse applications.
  • Ozone/Granular Activated Carbon (O3/GAC): GAC adsorbs contaminants not fully oxidized by ozone, while ozone pre-treatment reduces organic loading and competes for adsorption sites.
  • Membrane Filtration (O3/UF or O3/RO): Ozone can be used as a pre-treatment to control biofouling on membranes, while reverse osmosis provides a barrier to essentially all contaminants, including those unaffected by ozone.

The World Health Organization (WHO) recognizes ozone as an effective treatment for microbial and chemical contaminants when applied in properly designed systems, often recommending it as part of a broader treatment strategy.

Research and Real-World Applications

Numerous utilities worldwide have adopted ozonation to address emerging contaminants. For example, the Swiss Federal Office for the Environment mandated ozone-based treatment upgrades at many wastewater treatment plants to reduce micropollutant loads. In the United States, the City of Dallas and the Santa Clara Valley Water District employ ozone for drinking water and water reuse, respectively, achieving high removals of pharmaceuticals and EDCs.

Ongoing research focuses on improving ozone dosing control through real-time monitoring of organic matter and contaminant surrogates (e.g., UV absorbance at 254 nm, fluorescence). Catalytic ozonation using metal oxides or carbon-based catalysts is being studied to enhance reaction rates and selectivity. Additionally, the integration of ozone with electrochemical or sonochemical processes shows promise for tackling the most persistent compounds, including PFAS.

A comprehensive review by the journal Water Research highlights that ozonation, when coupled with biological post-treatment, can achieve >90% removal of most emerging contaminants with careful control of byproducts. Such studies underscore the need for site-specific optimization based on source water characteristics.

Conclusion

Ozonation is a powerful tool in the fight against emerging contaminants in drinking water. Its ability to oxidize a wide range of pharmaceuticals, pesticides, and endocrine-disrupting chemicals—often with rapid kinetics and minimal halogenated byproducts—makes it an attractive upgrade for conventional treatment plants. However, challenges such as bromate formation, energy costs, and the production of unknown transformation products demand careful engineering and monitoring. When combined with biological filtration or advanced oxidation processes, ozonation becomes part of a resilient multi-barrier system that provides safe, high-quality drinking water. As regulatory frameworks evolve and analytical methods improve, ozonation will likely play an increasingly central role in protecting public health from the invisible threat of emerging contaminants.