Ozonation has become a cornerstone of advanced water treatment, prized for its ability to inactivate a broad spectrum of pathogens and degrade recalcitrant organic compounds without leaving behind disinfection by-products as persistent as those from chlorination. Yet the full consequences of this powerful oxidative process extend far beyond immediate pathogen kill counts. Ozone fundamentally reshapes the microbial ecosystem that inhabits water treatment infrastructure—from the distribution pipes to the filter media—with implications for system stability, treatment efficiency, and long-term water quality. Understanding these microbial ecology shifts is not merely an academic pursuit; it is essential for designing robust, sustainable treatment trains that can resist the emergence of resistant organisms and maintain biological stability from source to tap.

The Chemistry and Application of Ozone in Water Treatment

Ozone (O₃) is a highly reactive allotrope of oxygen, generated on-site by passing dry air or oxygen through a high-voltage corona discharge or via ultraviolet light. When injected into water, ozone rapidly decomposes, generating a cascade of reactive oxygen species, most notably hydroxyl radicals (·OH). These radicals are among the strongest oxidants known, capable of attacking cell walls, nucleic acids, and enzymes in microorganisms, as well as oxidizing dissolved organic matter, iron, manganese, and micropollutants such as pharmaceuticals and pesticides.

The operational parameters of ozonation—ozone dose, contact time, residual concentration, and water chemistry (pH, alkalinity, temperature, natural organic matter content)—all influence both the immediate disinfection efficiency and the subsequent ecological impacts. Typical ozone doses for disinfection range from 1 to 5 mg/L, with contact times of 5 to 20 minutes. Because ozone decomposes relatively quickly, it does not maintain a residual in the distribution system, often requiring a secondary disinfectant (e.g., chlorine or chloramine). This dual-barrier approach is common in large municipal plants, but the initial oxidative shock has enduring effects on the microbial communities that survive and recolonize downstream.

The Microbial Community Before Ozonation: Who Lives There?

Raw water sources—whether surface water (rivers, lakes, reservoirs) or groundwater—harbor complex microbial communities comprising bacteria, archaea, fungi, protozoa, and viruses. In surface waters, common bacterial phyla include Proteobacteria (often dominated by Alphaproteobacteria and Betaproteobacteria), Actinobacteria, Bacteroidetes, and Cyanobacteria. These communities vary seasonally, with nutrient inputs, temperature, and sunlight driving shifts in abundance and composition. Groundwater tends to support lower biomass and less diverse communities, often dominated by chemolithoautotrophic bacteria adapted to oligotrophic conditions.

Before ozonation, the microbial community in the treatment plant's raw water intake already reflects the ecological state of the source. This community will be the baseline against which the impacts of ozonation are measured. Importantly, some of these organisms are harmless environmental strains, while others include pathogens such as Giardia lamblia cysts, Cryptosporidium parvum oocysts, Escherichia coli O157:H7, Legionella pneumophila, and enteric viruses. Ozone is among the few disinfectants that can rapidly inactivate both Cryptosporidium and Giardia, which are notoriously resistant to chlorine.

Immediate Effects on Microbial Viability and Diversity

Oxidative Shock and Cell Lysis

When ozone contacts microorganisms, the oxidative burst causes immediate damage. Cell membranes lose integrity, DNA is fragmented, and essential enzymes are denatured. For vegetative bacteria, the kill rate is extraordinarily fast—typically achieving 2–6 log reductions within seconds to a few minutes at appropriate doses. Viruses are also rapidly inactivated because ozone damages their protein capsids and RNA/DNA. Spore-forming bacteria (e.g., Bacillus spp.) and fungal conidia require higher doses but are still susceptible.

The consequence of this mass cell lysis is a sudden release of intracellular organic matter (IOM) into the water. This IOM includes nucleotides, amino acids, carbohydrates, and lipids, which become a pulse of assimilable organic carbon (AOC). This phenomenon, known as "cell lysis release," can paradoxically fuel the growth of surviving microorganisms in downstream processes—a key ecological consequence often overlooked in routine disinfection monitoring.

Reduction of Microbial Diversity

Ozonation does not sterilize water, but it drastically reduces both biomass and species richness. The surviving community is typically a subset of the original, composed of organisms that either possess intrinsic resistance mechanisms (e.g., robust cell envelopes, efficient DNA repair, high antioxidant enzyme activity) or occupy physical niches (e.g., aggregates, biofilm interiors) that shielded them from full ozone exposure. Numerous studies employing 16S rRNA gene amplicon sequencing have reported significant reductions in Shannon diversity index and Chao1 richness following ozonation, with the community shifting toward fewer dominant taxa.

For example, research on full-scale drinking water treatment plants shows that Gammaproteobacteria (including Pseudomonas spp.) and certain Firmicutes often become more abundant after ozonation, while Alphaproteobacteria and Actinobacteria may decline disproportionately. This selective pressure is not random—it reflects fundamental differences in cellular resistance to oxidative stress. Over time, repeated ozone exposure may lead to an engineered selection for ozone-resistant strains, with implications for system resilience and pathogen control.

Selective Pressure and the Rise of Ozone-Resistant Microbes

Mechanisms of Resistance

Microorganisms have evolved multiple strategies to withstand oxidative stress. In the context of ozonation, some of the most important include:

  • Enhanced antioxidant systems: Elevated levels of superoxide dismutase, catalase, and peroxidases that neutralize reactive oxygen species.
  • DNA repair pathways: Induction of SOS response, base excision repair, and recombination pathways to fix oxidative DNA damage.
  • Membrane modifications: Increased saturation of fatty acids or production of extracellular polymeric substances (EPS) that scavenge ozone before it reaches the cell.
  • Spore or cyst formation: Dormant structures with multiple protective layers that are inherently resistant to oxidation.
  • Biofilm phenotype: Cells embedded in EPS matrices benefit from diffusion limitations and cooperative detoxification by neighboring cells.

The presence of these resistance mechanisms means that a small but resilient fraction of the community survives ozonation. If that fraction includes pathogenic species—or if resistance genes are horizontally transferred to previously susceptible strains—the potential for public health risk increases. Notably, research has shown that some opportunistic pathogens such as Pseudomonas aeruginosa and Mycobacterium avium can exhibit greater tolerance to ozone compared to E. coli or Salmonella, raising questions about their regrowth potential in distribution systems after ozonation.

Evidence from Full-Scale Systems

A study by Zhang et al. (2019) examined the microbial community dynamics across a train that included pre-ozonation, coagulation, sedimentation, and biofiltration. They observed that while total bacterial 16S rRNA gene copy numbers decreased by three orders of magnitude after ozonation, the relative abundance of Pseudomonas and Stenotrophomonas increased from less than 1% to over 30% in the ozonated effluent. Moreover, the regrowth potential in the subsequent biofilter was significantly higher than in a parallel train without ozonation, indicating that the surviving community was better adapted to exploit the AOC released from lysed cells.

Another investigation by Huang et al. (2020) tracked the dynamics of antibiotic resistance genes (ARGs) across an ozone-biofiltration system. They found that ozonation reduced the absolute abundance of most ARGs, but the relative abundance of certain ARGs (particularly those encoding multidrug efflux pumps) increased in the post-ozone biomass. This suggests that the selective pressure of ozone may inadvertently enrich for organisms that also exhibit multidrug resistance, a concerning collateral effect for public health.

Disruption of Biofilms: Surface Ecology Changes

Biofilms are structured communities of microorganisms attached to surfaces—pipes, filter media, tank walls—embedded in a self-produced matrix of EPS. In water treatment systems, biofilms play a dual role: they contribute to biological removal of nutrients and organic matter (beneficial in biofilters), but they can also harbor pathogens and contribute to pipe corrosion and chlorine demand (detrimental in distribution systems).

Ozone, when applied to water, can penetrate and disrupt biofilms to a limited extent. The oxidative species degrade EPS components (polysaccharides, proteins, nucleic acids), weakening the biofilm matrix and releasing cells into the bulk water. This can temporarily reduce biofilm thickness and viable cell counts. However, ozone's effect on biofilms is strongly influenced by mass transfer limitations: the surface layers are oxidized relatively quickly, but deeper layers near the substratum may remain intact, especially if the biofilm is thick or contains inorganic precipitates.

Furthermore, the disruption of one biofilm community can create ecological niches for new colonizers. If the ozonation is intermittent (e.g., applied only at certain times of day or seasonally), the biofilm may recover rapidly, often dominated by fast-growing, ozone-tolerant organisms. Over months or years, the mature biofilm community in an ozonated system may differ substantially from that in a non-ozonated system, with potential effects on biostability, corrosion rates, and the release of microbial products into the water.

Impact on Biological Filtration

Many treatment plants use biological filters (granular activated carbon or sand) after ozonation. The purpose is to remove biodegradable organic matter (BOM) produced by ozone and to stabilize the water biologically, reducing regrowth potential in the distribution system. The bacteria that colonize these biofilms are key to BOM removal.

Ozonation influences the biofilter microbial community in two ways: (1) it inoculates the filter with ozone-surviving bacteria, which may have different metabolic capacities than the community that would have colonized from raw water; and (2) it alters the type and concentration of nutrients (higher AOC, different composition of biodegradable compounds). Studies have reported that biofilters fed ozonated water develop a higher relative abundance of Sphingomonadales, Rhizobiales, and Burkholderiales, taxa known for their ability to degrade a wide range of organic compounds and their relatively high resistance to oxidative stress. These shifts can improve removal of specific organic contaminants but may also reduce the filter's resilience to sudden changes in water quality.

Implications for Water Quality and Public Health

Enhanced Pathogen Removal

The most direct benefit of ozonation is improved microbiological safety. Ozone is highly effective against Cryptosporidium and Giardia, parasites that cause severe waterborne disease and are chlorine-resistant. In many countries, ozonation is mandated for surface water supplies that face risks from these protozoa. The immediate reduction in pathogen load is well documented and saves lives.

Regrowth of Opportunistic Pathogens

However, the selective effects described above can give rise to post-ozone regrowth of opportunistic pathogens, particularly in premise plumbing or low-flow parts of the distribution system. For example, Legionella pneumophila can survive in amoebae and biofilms, and some studies suggest that ozonation may increase the relative abundance of Mycobacterium spp. in distributed water if the disinfection residual is insufficient. A notable example from the literature is a survey by Haack et al. (2017), which found that drinking water systems using ozone and chloramine had higher proportions of Mycobacterium avium compared to chlorine-only systems. The authors hypothesized that ozone-resistant mycobacteria thrived in the low-nutrient, low-chloramine environment created after ozonation.

Emergence of Antibiotic Resistance

The enrichment of multi-drug resistant bacteria following ozonation is an area of active concern. While ozone itself does not create resistance genes, the oxidative stress it imposes can co-select for strains carrying efflux pumps or other mechanisms that also confer antibiotic resistance. Moreover, the release of DNA from lysed cells may facilitate horizontal gene transfer via natural transformation, potentially spreading resistance genes to ozone-surviving bacteria. Full-scale monitoring studies in Europe and Asia have detected increased relative abundance of genes conferring resistance to beta-lactams, tetracyclines, and sulfonamides in biofilms downstream of ozonation.

Managing the Microbial Ecology Impacts of Ozonation

Optimizing Ozone Dose and Contact Time

Minimizing the negative ecological consequences of ozonation begins with treatment process design. Overdosing ozone can increase AOC release and cell lysis, promoting excessive regrowth in downstream filters and distribution. Conversely, underdosing may leave pathogens viable. Optimizing the dose to achieve target inactivation (e.g., 2 log removal of Cryptosporidium while minimizing lysis of beneficial organisms) requires site-specific research and online monitoring of ozone residual and assimilable organic carbon.

Implementing Biological Stability Post-Ozone

The most effective strategy to prevent the uncontrolled proliferation of ozone-resistant microbes is to combine ozonation with a robust biological treatment step—biological activated carbon (BAC) filtration is the gold standard. The active biofilm in a well-maintained BAC filter consumes the AOC produced by ozonation, reducing the energy available for regrowth in the distribution system. Moreover, the BAC community itself can outcompete pathogenic strains for resources, provided the filter is operated at appropriate empty bed contact times (EBCTs, typically 10–20 minutes) and with adequate backwashing to prevent excessive biomass sloughing.

Using a Secondary Disinfectant

Because ozone does not persist, a secondary disinfectant (chlorine, chloramine, or chlorine dioxide) is necessary to maintain a residual in the distribution system. The choice of secondary disinfectant can modulate microbial community composition. Chloramine, for instance, is more stable than free chlorine but less biocidal, allowing some ozone-resistant bacteria (like certain Mycobacterium spp.) to survive. In some cases, switching to free chlorine or implementing zone-specific booster chlorination can help control specific regrowth problems.

Monitoring Microbial Ecology

Advances in molecular microbiology—especially 16S rRNA amplicon sequencing, metagenomics, and quantitative PCR for specific pathogens and resistance genes—are increasingly being applied in water treatment plants for operational insights. Regular monitoring of the microbial community (both bulk water and biofilm) before and after ozonation can detect early shifts toward undesirable dominance (e.g., a rise in Mycobacterium or ARG markers). This information allows operators to adjust ozone dose, contact time, or downstream processes before a full-blown regrowth event occurs.

Future Directions and Research Needs

Despite decades of successful use, the microbial ecology of ozonated water systems is not fully understood. Key research gaps include:

  • Long-term community succession: Most studies focus on short-term (days to weeks) effects. How do microbial communities in distribution systems evolve over months or years of repeated ozone exposure? Do they reach a stable, potentially harmful climax community?
  • Interactions with other treatment steps: The combined effects of ozonation with UV, membrane filtration, or advanced oxidation processes (e.g., O₃/H₂O₂) on microbial ecology are only beginning to be explored.
  • Viable but non-culturable (VBNC) state: Ozone can induce the VBNC state in some pathogens, including E. coli and Vibrio spp. These cells are not detected by standard culture methods but can resuscitate under favorable conditions. Their role in distribution system regrowth is poorly quantified.
  • Ecological modeling: Integrating kinetic models of ozone disinfection with models of microbial growth and competition could help predict the outcome of different ozone dosing strategies on community structure.

As water utilities face increasing threats from climate change, source water degradation, and emerging contaminants, ozonation will continue to be an essential tool. The challenge lies in harnessing its disinfection power without inadvertently engineering a microbial community that undermines its own benefits. A nuanced, ecologically informed approach to ozonation—one that respects the complexity of the microbial world in pipes and filters—will be key to sustainable water management in the 21st century.

Conclusion

Ozonation exerts a profound influence on the microbial ecology of water treatment systems, extending well beyond simple pathogen inactivation. It reduces microbial diversity, imposes selective pressure that can favor ozone-resistant strains, and releases pulses of organic carbon that fuel regrowth in downstream processes. While these effects can enhance overall water quality when properly managed, they also introduce risks: the potential emergence of resistant pathogens, enrichment of antibiotic resistance genes, and destabilization of beneficial biofilm communities. Fortunately, these risks can be mitigated through careful optimization of ozone dose, integration with biological filtration, appropriate secondary disinfection, and routine molecular monitoring. The most forward-thinking utilities view ozonation not as a stand-alone barrier, but as part of an integrated ecological strategy that recognizes the living nature of water treatment systems.

For further reading on best practices for applying ozone in drinking water treatment, see the U.S. Environmental Protection Agency's ozone guidance page and the WHO Guidelines for Drinking-water Quality, which include a comprehensive discussion of ozone disinfection and its ecological implications.