Water scarcity and drought conditions are intensifying across the globe, driven by climate change, population growth, and over-extraction of freshwater resources. In many regions, traditional water supplies are becoming insufficient, and communities are forced to turn to advanced treatment technologies to stretch every drop. Among these technologies, ozonation has emerged as a powerful and versatile tool. By harnessing the oxidizing power of ozone gas, ozonation can disinfect, decontaminate, and improve the quality of water from both conventional and alternative sources. This article explores the fundamentals of ozonation, its key benefits for water management, and its specific role in mitigating the impacts of drought—while also examining the challenges and future directions that will determine its adoption at scale.

Understanding Ozonation: How It Works and Why It Matters

Ozonation is a chemical water treatment process that involves introducing ozone (O3) gas into water. Ozone is a highly reactive molecule composed of three oxygen atoms; it is a powerful oxidant that rapidly reacts with a wide range of contaminants. When ozone dissolves in water, it breaks down into hydroxyl radicals (·OH) and other reactive species that attack and destroy pollutants through oxidation. This direct and indirect oxidation pathway makes ozonation effective against bacteria, viruses, protozoa, organic compounds, and even some inorganic substances like iron and manganese.

Ozone can be generated on-site using corona discharge, ultraviolet (UV) light, or electrolytic methods. Corona discharge is the most common approach, where a high-voltage electrical field is passed through a stream of oxygen or air, splitting O2 molecules into atoms that recombine into O3. The ozone gas is then injected into a contact chamber where it mixes thoroughly with the water. Contact time and ozone dose are carefully controlled to achieve the desired level of treatment without wasting energy or ozone. Unlike many chemical disinfectants, ozone leaves no lasting residual—after a short period it decomposes back into harmless oxygen, making it an environmentally friendly option.

This unique chemistry gives ozonation several advantages over traditional methods. For example, chlorine can react with natural organic matter to form disinfection byproducts (DBPs) such as trihalomethanes and haloacetic acids, which are regulated carcinogens. Ozone, on the other hand, produces fewer DBPs—primarily bromate in waters with high bromide levels—and actually helps remove precursors that would otherwise form DBPs during subsequent chlorination. Additionally, ozone is far more effective at inactivating chlorine-resistant pathogens like Cryptosporidium and Giardia, which are a major concern in surface waters and recycled wastewater.

Key Benefits of Ozonation in Water Management

The advantages of ozonation extend well beyond disinfection. When applied as part of a comprehensive water treatment train, ozone contributes to multiple water quality objectives. Below are the most impactful benefits for managing water resources, especially under scarcity conditions.

Superior Disinfection and Pathogen Control

Ozone kills microorganisms faster and at lower concentrations compared to chlorine, chlorine dioxide, or chloramines. It is effective against a broad spectrum of bacteria (e.g., E. coli, Legionella), viruses (e.g., norovirus, adenovirus), and protozoan cysts (Cryptosporidium). For example, a typical ozone dose of 1–2 mg/L with a contact time of 5–10 minutes can achieve 3–4 log reduction of Cryptosporidium, whereas chlorine requires much higher doses and longer contact times. This potency is particularly valuable during drought when water sources may be compromised by low flows and increased pathogen concentrations.

Reduced Dependence on Chemical Disinfectants

By using ozone as a primary disinfectant or as an oxidant ahead of membrane filtration, utilities can significantly lower the amount of chlorine and other chemical additives required. This reduces the formation of chemical residuals in the finished water, decreasing taste and odor complaints as well as the risk of producing harmful DBPs. In recycled water applications, minimizing chemical usage also improves the suitability of the water for sensitive uses like agricultural irrigation, where chlorine residuals can damage crops.

Improved Aesthetic Water Quality

Ozone is exceptionally effective at removing substances that cause unpleasant tastes, odors, and color. Compounds like geosmin and 2-methylisoborneol (MIB)—responsible for musty or earthy odors in drinking water—are rapidly oxidized by ozone. Similarly, natural organic matter that imparts a yellow or brown color is broken down, producing water that is clearer and more appealing to consumers. These improvements are especially important when utilities must draw from reservoirs or rivers that are often under stress during droughts, when algae blooms and organic loading increase.

Removal of Micropollutants and Emerging Contaminants

In addition to pathogens and bulk organic matter, ozone can oxidize trace organic compounds including pharmaceuticals, personal care products, pesticides, and endocrine-disrupting chemicals. While complete mineralization is rare, ozonation often transforms these contaminants into less harmful or more biodegradable forms. This makes ozone a key technology for potable water reuse projects, where the removal of such micropollutants is mandatory. Research published by the Water Research Foundation has demonstrated that ozone combined with biological activated carbon (BAC) can achieve over 80% removal of many emerging contaminants.

Environmental Sustainability

Because ozone decomposes to oxygen, it leaves no persistent chemical residues in the treated water. This is a critical advantage for applications where the released water may return to natural aquatic ecosystems, such as in groundwater recharge or river augmentation. Ozonation also reduces the need for transporting and storing hazardous chemicals like chlorine gas or sodium hypochlorite, lowering the risk of accidental releases. When paired with on-site oxygen generation and renewable energy, the overall carbon footprint of ozonation can be minimized.

The Role of Ozonation in Drought Management and Water Scarcity

Drought conditions force water managers to exploit every available source, often turning to lower-quality waters that would normally be avoided. Surface waters become stagnant, shallow, and prone to contamination; groundwater levels drop, sometimes causing saline intrusion or mobilizing naturally occurring contaminants. Meanwhile, the demand for water does not decline, leaving utilities with no choice but to implement advanced treatment. Ozonation fits naturally into these strategies, especially in the context of water recycling and reservoir management.

Enabling Water Recycling and Reuse

Wastewater recycling is one of the most effective ways to augment water supplies during drought. Ozonation is a cornerstone of many advanced water treatment trains, particularly in the framework of indirect potable reuse (IPR) and direct potable reuse (DPR). Typically, the treatment sequence after secondary wastewater treatment includes microfiltration (MF) or ultrafiltration (UF), reverse osmosis (RO), and an advanced oxidation process (AOP) that often uses ozone combined with hydrogen peroxide (O3/H2O2). This combination generates hydroxyl radicals that oxidize any residual pollutants not removed by RO.

For non-potable reuse, such as landscape irrigation, industrial cooling, or agricultural watering, ozonation alone or with biofiltration can provide sufficient quality while being more cost-effective than RO. The Orange County Water District's Groundwater Replenishment System in California, one of the world's largest water reuse facilities, uses ozone as part of its advanced treatment process, demonstrating that ozonation can reliably produce water that meets or exceeds drinking water standards. Learn more about the Orange County Groundwater Replenishment System.

Maintaining Reservoir and Surface Water Quality

Drought often leads to longer water residence times in reservoirs, which encourages algal blooms and the buildup of organic matter. Ozonation can be applied at the intake to reservoirs or within the treatment plant to break down these compounds. For example, the city of Wichita, Kansas, uses ozone to treat water from Cheney Reservoir, particularly during warm months when taste and odor issues are recurrent. Ozone not only destroys taste-and-odor compounds but also helps control the growth of zebra mussels and other invasive species by damaging their larvae. This dual benefit is invaluable for maintaining the quality of stored water in drought-prone regions.

Improving Groundwater Quality

During drought, aquifers may be drawn down to unprecedented levels, and changes in redox conditions can mobilize arsenic, manganese, or iron. Ozone can oxidize dissolved iron and manganese, converting them to insoluble forms that are easily removed by filtration. It can also assist in the removal of arsenic by oxidizing arsenite (As3+) to arsenate (As5+), which is more readily adsorbed onto iron-based media. This represents a low-chemical method to reclaim groundwater sources that might otherwise be considered impaired.

Integration with Other Treatment Technologies

Ozone rarely works alone. It is most effective when integrated with other treatment processes in a multi-barrier approach. Below are common synergies.

Ozonation Followed by Biological Activated Carbon (BAC)

Ozone partially oxidizes large organic molecules, making them smaller and more biodegradable. This "pre-oxidation" step sets the stage for BAC, where microorganisms attached to the carbon surface consume the biodegradable organic matter. The combination—often called O3/BAC—achieves very high removal of natural organic matter, organic micropollutants, and precursors for disinfection byproducts. Many water reuse schemes rely on O3/BAC as a cost-effective alternative to RO for certain quality targets. The US EPA has published studies showing the efficacy of O3/BAC for removing pharmaceuticals from wastewater.

Ozone with Membrane Filtration

Ozone can be applied ahead of microfiltration, ultrafiltration, or nanofiltration membranes to reduce biofouling and organic fouling. By oxidizing foulants, ozone helps maintain membrane permeability, which translates to lower energy consumption and longer membrane life. However, careful dosing is required because high ozone concentrations can degrade some polymeric membranes. Ceramic membranes are more ozone-tolerant and are increasingly used in combination with ozonation for direct treatment of surface waters or wastewater effluent.

Ozone in Advanced Oxidation Processes (AOPs)

When ozone is combined with hydrogen peroxide or UV light, the rate of hydroxyl radical formation increases dramatically. These AOPs are capable of destroying even the most recalcitrant contaminants, such as 1,4-dioxane and NDMA. In potable reuse applications, O3/H2O2 AOP is often the final polishing step before water is sent to the distribution system. The World Health Organization has noted that ozone-based AOPs are a preferred technology for treating water from heavily contaminated sources. View WHO guidelines on water treatment.

Challenges and Future Prospects

Despite its many benefits, ozonation is not a silver bullet. Widespread adoption faces several hurdles that must be overcome to fully leverage its potential in managing water scarcity.

High Energy Consumption and Operational Costs

Generating ozone requires significant electrical energy, particularly for corona discharge systems. The energy cost can range from 10 to 30 kWh per kilogram of ozone produced, depending on the oxygen source and generator efficiency. For large plants, this can add substantially to operating expenses. However, innovations in high-efficiency ozone generators, advanced oxygen concentrators, and the integration of renewable energy (solar, wind) are reducing the energy footprint. For smaller, decentralized systems, UV-based ozone generators are becoming more energy-efficient and affordable.

Bromate Formation in Bromide-Containing Waters

When treating waters with elevated bromide levels (common in coastal or brackish sources), ozone can oxidize bromide to bromate, a potential human carcinogen regulated at very low levels (typically 10 µg/L). Mitigation strategies include careful control of pH (lowering pH reduces bromate formation), adding ammonia or chlorine dioxide, or using a combination of ozone and hydrogen peroxide which favors hydroxyl radical pathways that produce less bromate. Advanced control systems using real-time monitoring can optimize ozone dose to minimize bromate while still achieving disinfection goals.

Lack of Residual Disinfection Capacity

Unlike chlorine, ozone does not provide a lasting residual to protect water quality in distribution systems. Therefore, ozonation is almost always followed by the addition of a secondary disinfectant, typically chloramine or chlorine, to maintain a residual throughout the pipes. This means utilities must still manage chemical disinfectant residuals and the associated DBPs, though at lower levels than without ozonation.

Future Directions in Research and Deployment

Several promising developments are on the horizon. Electrochemical ozone generation is becoming more compact and energy-dense, suitable for point-of-use or small community systems. IoT-enabled sensors and machine learning algorithms can now optimize ozone dosing in real time based on water quality fluctuations, reducing energy waste and improving reliability. Pilot projects are testing ozone in combination with UV-LEDs and photocatalysis for complete contaminant destruction. In drought-stricken regions like the Colorado River basin, utilities are exploring ozone as part of mobile treatment units that can be rapidly deployed during emergencies.

Furthermore, the cost of ozone generation has dropped by nearly 40% in the last decade due to improvements in power electronics and oxygen concentrators. As water scarcity pressures intensify, the value proposition for ozonation—particularly in reuse and drought contingency plans—becomes stronger. The American Water Works Association has identified advanced oxidation, including ozonation, as a critical technology for building drought resilience in municipal water systems.

Conclusion

Ozonation is not merely a disinfection method; it is a versatile and environmentally sound tool for addressing the multifaceted challenges of water scarcity and drought. By enabling safe water recycling, improving the quality of impaired sources, and reducing reliance on chemical additives, ozone helps stretch limited water supplies further. While energy costs and bromate formation are legitimate concerns, ongoing technological advances and operational experience continue to push these barriers aside. For communities facing chronic water shortages, investing in ozonation—especially as part of an integrated treatment train—is a forward-looking strategy that delivers both immediate and long-term benefits.