What Is Ozonation?

Ozonation is a water and wastewater treatment process that uses ozone (O3), a highly reactive allotrope of oxygen, to oxidize organic and inorganic contaminants and to inactivate microorganisms. First applied to drinking water disinfection in Europe in the early twentieth century, ozonation acts through direct reaction of the ozone molecule with target compounds and through the generation of hydroxyl radicals (OH·), which are among the most reactive oxidizing species known and react non-selectively with nearly all organic molecules. The technique is used in municipal water treatment, industrial effluent management, and advanced water reuse applications to remove trace organic pollutants, reduce color and odor, and achieve microbial inactivation.

Ozone is generated on-site by passing oxygen or air through a corona discharge or UV irradiation system, since its instability makes storage and transport impractical. The ozone-containing gas is then dissolved into the water stream through diffusers, packed columns, or venturi injectors. Ozone's effectiveness as an oxidant, combined with the absence of chlorinated disinfection byproducts, makes it an attractive alternative or complement to chlorine in water treatment.

Ozone Chemistry and Reaction Mechanisms

Ozone reacts with organic compounds through two primary pathways. Direct oxidation involves the ozone molecule attacking electron-rich sites, such as double bonds in aromatic rings and alkenes, with relatively high selectivity. Indirect oxidation proceeds through hydroxyl radicals generated when ozone decomposes in water, a process accelerated by alkaline pH, UV light, and added hydrogen peroxide. Hydroxyl radicals react with organic molecules at near-diffusion-limited rates, without the selectivity of direct ozone reactions, making the indirect pathway particularly effective for recalcitrant micropollutants such as pharmaceuticals and pesticides. The rate constants for direct ozone reactions with common water contaminants span many orders of magnitude, from very fast for phenols to nearly negligible for certain halogenated compounds. Research on ozonation kinetics of organic compounds published in Water Research by von Sonntag and collaborators provides a systematic treatment of these reaction pathways.

Disinfection Byproduct Formation

Despite its advantages over chlorination, ozonation can generate its own set of disinfection byproducts (DBPs). The most regulated is bromate (BrO3-), formed when bromide ions naturally present in source water are oxidized by ozone; the WHO and US EPA set a maximum contaminant level of 10 micrograms per liter for bromate in drinking water. Ozonation also converts natural organic matter into more biodegradable fragments, including aldehydes, ketones, and carboxylic acids, which can support microbial regrowth in distribution systems if not removed by subsequent biological filtration. Research published in Environmental Science & Technology on bromate formation control during ozone disinfection in water reuse shows how operational parameters, including ozone dose, contact time, and pH adjustment, can be optimized to minimize bromate while maintaining disinfection efficacy.

Advanced Oxidation Processes

Ozone is frequently combined with hydrogen peroxide (O3/H2O2) or ultraviolet irradiation (O3/UV) to produce enhanced concentrations of hydroxyl radicals. These combinations, called advanced oxidation processes (AOPs), are used to treat micropollutants that resist direct ozonation, such as NDMA (N-nitrosodimethylamine), certain pharmaceuticals, and taste-and-odor compounds like geosmin and 2-methylisoborneol. AOPs are applied in indirect potable reuse systems, where highly treated wastewater is returned to drinking water supplies after passing through multiple treatment barriers. A PMC review of advanced oxidation processes for water and wastewater treatment documents the state of AOP research, comparing ozone-based systems to other oxidant combinations such as UV/H2O2 and Fenton chemistry.

Applications

Ozonation has applications in a range of fields, including:

• Municipal drinking water disinfection and taste and odor removal
• Advanced wastewater treatment and water reuse
• Industrial effluent treatment for pulp, paper, and textile industries
• Swimming pool and recreational water disinfection
• Air purification and indoor ozone treatment systems
• Food processing and surface sanitation