Ozone in wastewater treatment: when, why and how?
Ozone (O₃) is an extremely powerful oxidising agent that is applied at wastewater treatment plants as an advanced, non-conventional treatment step. Its primary purpose is to complement conventional biological organic matter removal by eliminating micropollutants, pathogens, odorous compounds and colour-causing substances.
Ozonation effectively degrades organic micropollutants such as pharmaceutical residues, hormone-active compounds and certain industrial chemicals. Ozone treatment can achieve micropollutant removal efficiencies of over 80%, while also providing a disinfection effect.
Ozone vs. Activated Carbon vs. UV
In addition to ozonation, activated carbon and UV technologies are also widely used in advanced wastewater treatment. Each has different strengths and limitations:
1. Ozone (O₃)
Effect: molecular breakdown through strong oxidation (micropollutant degradation combined with disinfection).
Advantage: direct chemical oxidation of organic compounds (can be combined with other technologies).
Disadvantage: may form oxidation by-products (e.g. bromate); energy-intensive; secondary reactions require careful management.
2. Activated carbon (GAC / PAC)
Effect: adsorption – micropollutants are physically bound to the carbon surface.
Advantage: capable of removing a wide range of micropollutants without chemical transformation; does not form oxidation by-products.
Disadvantage: regeneration or replacement of carbon is costly; does not provide disinfection on its own.
(For this reason, ozone and activated carbon are often combined: ozone oxidises and partially degrades difficult-to-remove compounds, while activated carbon adsorbs the remaining substances and oxidation by-products.)
3. UV (ultraviolet) treatment
Effect: UV radiation inactivates microorganisms via photochemical reactions; in some cases it can also degrade organic micropollutants, but typically with lower efficiency than ozone or activated carbon.
Advantage: fast, chemical-free disinfection.
Disadvantage: less effective for micropollutant removal; certain compounds require very high UV doses.
Considering these factors, UV is primarily suited for disinfection and limited compound degradation, while ozone and activated carbon provide broader-spectrum micropollutant removal. At many wastewater treatment plants, only a combination of these technologies delivers a satisfactory solution.
EU regulations and trends
Updated EU regulation (2025)
The amendment to the EU Urban Wastewater Treatment Directive (UWWTD), which entered into force on 1 January 2025, is expected to bring significant changes to wastewater treatment:
Requirements will be extended to more agglomerations and a wider range of pollutants, including micropollutants.
Member States must ensure the removal of biodegradable substances by 2035 and gradually introduce a fourth (quaternary) treatment stage for micropollutant removal.
A yet-to-be-adopted European Parliament proposal already signals a trend whereby quaternary treatment may become mandatory at all wastewater treatment plants serving more than 150,000 population equivalents (PE) to protect against micropollutants.
Practical trends
The removal of micropollutants (pharmaceuticals, hormones, chemicals) is increasingly becoming an expectation across EU Member States, leading to more frequent implementation of ozone and activated carbon systems.
Optimisation of combined technologies (ozone + activated carbon or ozone + UV) is gaining importance in order to balance environmental risk reduction with operational costs.
