Biofilms represent one of the most persistent challenges in maintaining water quality within distribution systems. These structured communities of microorganisms adhere to pipe surfaces, forming a protective matrix that resists conventional disinfection and facilitates corrosion, taste and odor problems, and the proliferation of opportunistic pathogens. Effective biofilm management is essential for water utilities seeking to meet regulatory standards and ensure public health. Among the advanced treatment options available, ozonation has emerged as a particularly potent strategy—not only for primary disinfection but also for disrupting and controlling established biofilms.

Understanding Biofilms in Water Distribution Systems

Biofilms develop when planktonic bacteria attach to submerged surfaces and begin excreting extracellular polymeric substances (EPS)—a slimy, gel-like matrix composed of polysaccharides, proteins, nucleic acids, and lipids. This EPS anchors the cells to the pipe wall and provides structural integrity, while also creating microenvironments that protect the community from disinfectants, shear stress, and fluctuations in water chemistry. Over time, biofilms can harbor a diverse microbial consortium, including Legionella, Pseudomonas, Mycobacterium avium, and other pathogens that can cause waterborne disease in immunocompromised individuals.

The formation process typically follows five stages: reversible attachment, irreversible attachment, microcolony formation, maturation, and dispersion. Biofilm thickness can range from a few micrometers to several millimeters, and even thin biofilms can dramatically increase pipe roughness, reduce hydraulic capacity, and accelerate corrosion through microbially induced corrosion (MIC). In drinking water systems, biofilms also act as a reservoir for regrowth, continuously seeding downstream water even when the bulk water meets regulatory disinfection targets.

Health and Operational Impacts

Biofilms represent a critical failure point in water safety. They shield pathogens from disinfectant residuals, promote the survival of antibiotic-resistant bacteria, and contribute to the formation of disinfection byproducts when chlorine reacts with organic matter within the matrix. Operationally, biofilms increase energy consumption by raising head loss, reduce pipe lifespan through corrosion, and complicate sampling and monitoring efforts. The problem is exacerbated in low-flow or dead-end sections of distribution networks where disinfectant residual is difficult to maintain.

The Role of Ozonation in Biofilm Control

Ozonation is a chemical oxidation process in which ozone gas (O3) is dissolved into water. Ozone is one of the strongest oxidants used in water treatment, with a redox potential of 2.07 V, significantly higher than chlorine (1.36 V) and chloramine (1.18 V). This high oxidation power enables ozone to rapidly react with a wide range of organic and inorganic compounds, making it highly effective for inactivating microorganisms and breaking down organic matter. Since the early 1900s, ozone has been used for disinfection in Europe, and its application has expanded globally for taste and odor control, color removal, and iron and manganese oxidation.

For biofilm control, ozonation offers a dual benefit: it directly kills microorganisms within the biofilm and, perhaps more importantly, degrades the EPS matrix that protects them. This degradation weakens the structural integrity of the biofilm, allowing shear forces from water flow to physically remove it. Moreover, by oxidizing dissolved organic carbon and other nutrients, ozone reduces the substrate available for microbial growth downstream, thereby limiting regrowth potential.

Mechanisms of Action

Ozone attacks biofilms through multiple pathways. First, ozone molecules penetrate the EPS and react with double bonds and functional groups in polysaccharides and proteins, causing depolymerization and fragmentation of the matrix. This process is referred to as "biofilm disruption." Second, ozone reacts with bacterial cell walls and membranes, causing lysis through direct oxidation of unsaturated lipids and damage to intracellular components. The breakdown of EPS exposes embedded cells to higher ozone concentrations, leading to more effective inactivation.

The rate of ozone reaction with biofilm components depends on ozone concentration, contact time, pH, and temperature. At typical drinking water application doses (0.5 to 2.0 mg/L), ozone reacts within seconds to minutes, rapidly oxidizing the outer layers of the biofilm. However, thicker biofilms may require repeated or higher-dose applications to reach the base of the matrix. The oxidation of EPS also produces biodegradable byproducts that must be managed to avoid promoting downstream regrowth—often accomplished by following ozonation with biological filtration.

Benefits of Ozonation

  • High efficacy against biofilm EPS: Ozone degrades the structural matrix, making biofilms more susceptible to physical removal and further disinfection.
  • Broad-spectrum disinfection: Ozone inactivates bacteria, viruses, protozoa, and fungal spores within biofilms, including chlorine-resistant organisms like Cryptosporidium and Giardia.
  • Reduction of chemical disinfectant demand: By controlling biofilm growth, ozonation allows utilities to maintain lower chlorine or chloramine residuals, reducing byproduct formation and corrosion risk.
  • Rapid reaction kinetics: Ozone disinfection occurs in seconds to minutes, requiring relatively short contact times compared to chloramines.
  • Improvement of water quality parameters: Ozone oxidizes iron, manganese, and taste- and odor-causing compounds, enhancing aesthetic quality.
  • Minimal persistent residuals: Ozone decomposes to oxygen, leaving no long-term chemical footprint, which can be an advantage in sensitive environments.

Implementation of Ozonation in Water Distribution Systems

Successful ozonation for biofilm control requires careful system design. Ozone is typically generated on-site by corona discharge or electrolytic cells using oxygen or air feedstock. The gas is then injected into a sidestream and dissolved through contact chambers such as venturi injectors or fine-bubble diffusers. Contact time is critical: typical contact times range from 5 to 20 minutes for disinfection, but biofilm penetration may require longer exposure or higher residual concentrations. Ozone demand decreases as the water oxidizes organic matter, so adequate dosing to satisfy the initial demand and achieve a measurable residual is necessary.

For existing distribution systems, ozonation is often applied at the treatment plant just before or after filtration. However, the application of ozone directly into the distribution network is less common due to toxic gas safety and decay kinetics. Instead, utilities use ozonation to produce biologically stable water that limits biofilm regrowth in the network. Integrating ozonation with post-treatment biological filtration (e.g., granular activated carbon) further removes the biodegradable organic matter generated by ozone oxidation.

Ozone Generation and Delivery Systems

Modern ozone generators are designed for high efficiency and reliability. Corona discharge generators pass oxygen or air through a high-voltage field, creating ozone. Electrolytic generators, though less common, can produce high-concentration ozone from water. Key design parameters include gas flow rate, power input, cooling, and gas preparation. Feed gas quality is critical: moisture, particulates, and nitrogen oxides can reduce yield and increase maintenance. Most municipal systems use pure oxygen feedstocks to achieve higher ozone concentrations (up to 10–14% by weight) and lower energy consumption.

Ozone transfer into water is typically accomplished through injected sidestream systems, static mixers, or column contactors. Transfer efficiency depends on gas bubble size, contact time, and water chemistry. Dissolved ozone concentrations are monitored in real time using electrochemical or UV absorbance sensors, allowing automated control of dosing.

Integration with Existing Water Treatment

Ozonation is often used as an intermediate treatment step. It can be placed before sand or membrane filtration (pre-ozonation) to improve flocculation and reduce membrane fouling, or after filtration (post-ozonation) as a final disinfection step. For biofilm control, the most effective location is after filtration but before clearwell storage, ensuring the water entering the distribution system has a low organic load and minimal viable biofilm seed. Because ozone decays rapidly, a residual can be maintained only for minutes; therefore, a secondary disinfectant (chlorine or chloramine) is added before the system to provide lasting protection. The combination of ozone with biological filtration reduces assimilable organic carbon (AOC) by up to 50–70%, starving biofilms in the network.

Comparison with Alternative Biofilm Control Methods

Several disinfection technologies are used to manage biofilms, each with advantages and limitations. Chlorine is the most common disinfectant but has limited ability to penetrate EPS at typical residuals (0.2–1.0 mg/L). It reacts with organic matter to form trihalomethanes (THMs) and haloacetic acids (HAAs), regulated by the US EPA. Chloramine, while more persistent and less reactive with organic matter, has a slower action against biofilms and may promote nitrification in some systems. Ultraviolet (UV) light provides rapid disinfection but leaves no residual and has no effect on EPS unless combined with hydrogen peroxide (advanced oxidation).

In contrast, ozone’s high oxidation power allows it to degrade EPS and kill microorganisms in a single step. Studies have shown that ozone treatment reduces biofilm mass by 80–95% on pipe surfaces, whereas chlorine reduces it by only 30–50% under comparable conditions. However, ozone does not provide a long-lasting residual, so it must be paired with a secondary disinfectant. Ozone also generates fewer chlorinated byproducts than chlorine, though it can create bromate in waters containing bromide ions.

Efficacy Against Biofilms

A 2019 study published in Water Research compared ozonation to chlorination for biofilm control on PVC and iron pipes. Ozone at 1.0 mg/L for 10 minutes achieved a 3-log reduction in viable biofilm cells and reduced EPS polysaccharide content by 70%, while chlorine at 2.0 mg/L for 30 minutes achieved only a 1.5-log reduction and 30% EPS reduction. These findings underscore ozone’s superior ability to disrupt biofilm structure. Another field study in a full-scale distribution system reported that switching from chlorine to ozone–UV treatment reduced biofilm gene copies from Legionella spp. to below detection limits over a 12-month period.

Byproduct Formation

The main byproduct concern with ozonation is bromate, a probable human carcinogen regulated at 10 µg/L in the US and 10 µg/L in the EU. Bromate forms when ozone oxidizes naturally occurring bromide ions. The risk is highest in waters with high bromide concentrations (>0.1 mg/L). Utilities can minimize bromate formation by lowering pH, adding ammonia, or using less ozone in bromide-rich waters. In comparison, chlorine and chloramine produce THMs and HAAs, which are more universally present in treated waters and also regulated. Ozone’s byproducts are generally fewer and more easily managed through operational adjustments.

Challenges and Considerations

Implementing ozonation for biofilm control is not without obstacles. High capital and operating costs are significant barriers. Ozone generation requires electricity for operation and oxygen for feed gas. Energy consumption can range from 10 to 25 kWh per kg of ozone produced. Maintenance of generators, contactors, and destruct units adds to life-cycle costs. Furthermore, ozone gas is toxic and requires careful handling with monitoring for leaks, worker training, and safety equipment.

Operational complexity is another factor. Ozone demand varies with water quality, temperature, and organic load, requiring skilled operators to adjust dosing in real time. Overdosing can lead to excessive byproducts, while underdosing leaves biofilm control ineffective. Also, the rapid decay of ozone means that its effect is primarily at the treatment plant; maintaining biofilm control throughout distant parts of the network depends on reducing the AOC content and applying an appropriate secondary residual.

Compatibility with materials must also be considered. Ozone can accelerate corrosion of certain metals (e.g., copper, steel) and degrade elastomers and plastics used in gaskets and seals. Pipe materials resistant to oxidation—such as stainless steel, cement-lined ductile iron, and PVC—are preferred in ozonated systems. In older distribution networks with mixed materials, ozone may cause localized corrosion at joints or repair points.

Research and Case Studies

Numerous studies have demonstrated the efficacy of ozonation for biofilm control. At the University of Illinois Pilot-scale Water Distribution System, researchers applied ozone (1.5 mg/L) continuously for six months to a loop simulating a distribution network. Biofilm coverage on pipe coupons decreased by 88% compared to a control loop dosed with 1.0 mg/L chlorine. The ozone-treated loop also showed significantly lower total organic carbon (TOC) in the bulk water and reduced corrosion rates.

In a real-world application, the city of Zurich, Switzerland, has used ozone since the 1960s as part of a multiple-barrier treatment approach. Studies of their distribution system found that ozone–biofiltration produced water with less than 10 µg/L assimilable organic carbon, effectively limiting biofilm growth even in large-diameter mains. In contrast, systems using only chlorination had biofilm densities two to three times higher.

A more recent pilot in the United States (Tampa Bay Water, Florida) evaluated ozone and chlorine dioxide for biofilm control in PVC and iron pipes. Ozone provided the highest reduction in heterotrophic plate counts (HPC) and eliminated Pseudomonas aeruginosa in the biofilm within two weeks. However, the study noted that hydrogen peroxide generated as an intermediate byproduct of ozone decomposition provided additional oxidative benefit.

Future Directions

The role of ozonation in biofilm management is likely to grow as water utilities face stricter regulations on disinfection byproducts and emerging pathogens. Advanced oxidation processes (AOPs) that combine ozone with hydrogen peroxide (O3/H2O2) or UV (O3/UV) enhance hydroxyl radical generation, which can attack biofilms even more aggressively than ozone alone. Research is ongoing to optimize these AOPs for distribution system applications, particularly for controlling Legionella in building plumbing.

Another promising development is the use of ozone in smart, sensor-driven systems. Real-time monitoring of biofilm growth using optical or electrochemical sensors can trigger short, high-dose ozonation pulses—a strategy that minimizes chemical use and byproduct formation while maintaining effective control. Machine learning algorithms could predict biofilm accumulation based on historical water quality data and adjust ozone dosing accordingly.

Finally, integration with biological filtration is becoming standard practice. As EPA guidelines and WHO recommendations evolve, ozonation combined with biofiltration is recognized as a best practice for producing biologically stable water. Studies continue to refine the understanding of how to balance ozone dose with subsequent biological activity to achieve maximum biofilm reduction.

Conclusion

Ozonation offers a powerful and versatile tool for controlling biofilms in water distribution systems. By oxidizing the extracellular matrix and directly inactivating embedded microorganisms, ozone reduces biofilm accumulation more effectively than traditional disinfectants. While challenges remain—including cost, byproduct management, and operational complexity—the benefits of improved water quality, reduced corrosion potential, and lower reliance on chemical disinfectants make ozonation an attractive option, especially when combined with biological filtration or advanced oxidation. As technology improves and experience accumulates, ozonation will play an increasingly central role in the quest to deliver safe, high-quality water through distribution networks.