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Author/Compiled by
Feilicien Mazille (Aquasis, cewas, international centre for water management services)
Dorothee Spuhler (seecon international gmbh)
Executive Summary

Ozonation (also referred to as ozonisation) is a chemical water treatment technique based on the infusion of ozone into water. Ozone is a gas composed of three oxygen atoms (O3), which is one of the most powerful oxidants. Ozonation is a type of advanced oxidation process, involving the production of very reactive oxygen species able to attack a wide range of organic compounds and all microorganisms. The treatment of water with ozone has a wide range of applications, as it is efficient for disinfection as well as for the degradation of organic and inorganic pollutants. Ozone is produced with the use of energy by subjecting oxygen (O2) to high electric voltage or to UV radiation. The required amounts of ozone can be produced at the point of use but the production requires a lot of energy and is therefore costly.

Rapidly reacts with bacteria, viruses and protozoa over a wide pH range
Stronger germicidal properties than chlorination
No chemicals are added to water
Also efficient for organics degradation and inorganics removal
Removes colour, taste and odour
Relatively high equipment costs
Requires large amounts of energy
Qualified professionals required for design and system maintenance
Formation of potentially harmful disinfection by-products (DBPs) in the case of brome existence in water
No residual effect is present in the distribution system
Potential fire hazard and toxicity associated with ozone generation
In Out

Freshwater, Drinking Water, Blackwater, Greywater, Non-biodegradable Wastewater, Treated Water, Energy

Drinking Water, Treated Water


Ozone (O3) has been used in water treatment since the late 19th century. Today it is applied for the disinfection of drinking water, for the removal of effluents from wastewater treatment plants in a process called ozonation (or ozonisation) as well as for the degradation of organic and inorganic pollutants in wastewater (see also advanced oxidation process).

Disinfection with Ozone

Ozone is an excellent disinfectant and can even be used to inactivate microorganisms such as protozoa, which are very resistant to conventional disinfectants (VON GUNTEN 2003). However, ozone is an unstable gas that transforms to oxygen hence no residual disinfection effect takes place with ozonation.

Degradation of Organic Pollutants with Ozone

Ozonation is an efficient treatment to reduce the amounts of micropollutants released in the aquatic systems by wastewater treatment plants (MARGOT et al. 2011). Although no residual by-products are generated by ozone itself, some concerns are raised regarding oxidation by-products when water containing both organics and ions, such as bromide, iodide and chlorine ions, are treated with ozonation. A typical ozonation system consists of an ozone generator and a reactor where ozone is bubbled into the water to be treated.

Scheme of a typical ozonation process

Scheme of a typical ozonation process. Source: THE OZMOTICS INSIDER (2008) 

The Effect of Ozone

The effectiveness of ozone results from its powerful oxidising effect on chemicals and microorganisms caused by the generation of reactive oxygen species during ozone transformation to oxygen. Ozone directly attacks the surface of microorganisms and destroys their cell walls. The cells thus loose their cytoplasm and can no longer reactivate themselves (STUCKI et al. 2005). Ozone can induce an oxidative degradation of many organics and leaves more biodegradable compounds (DERCO et al. 2001). Besides, ozone can oxidise metallic ions such as Fe(II), Mn(II) or As(III) producing insoluble solid oxides that can be easily separated from water by filtration or sedimentation.

 Type of contaminants removed by ozonation. Source: MAZILLE (2011)

Type of contaminants removed by ozonation. Source: F. MAZILLE (2011) 

Production of Ozone

Because of its relatively short half-life, ozone is generated on-site by an ozone generator. The conventional ways to produce ozone are UV-light and corona-discharge. Ozone generation by corona-discharge is most common nowadays and has many advantages such as longer lifespan of the unit, higher ozone production and higher cost efficiency. Production with UV-light is an option where only small amounts of ozone are required. Other ozone generators that are available involve electrolysis of water and the use of membranes. With this method, the ozone is dissolved in the process water as soon as it is formed resulting in ozonation using minimum equipment.

Technical Aspects Relative to Disinfection

Adapted from WHO (2009)

Whilst ozone is the most effective disinfectant overall and is more effective than chlorine in inactivating viral agents, there are significant disadvantages to its use. Ozone does not provide residual protection against recontamination during distribution and as ozone affects biological stability, it may encourage re-growth of bacteria. However, given the concerns about the use of chlorine in many countries because of the formation of toxic disinfection by-products (DBP), the use of ozone is increasingly investigated and the lack of residual may be dealt with by employing regular booster ozonation during distribution.

Cost Considerations

The cost of ozone disinfection systems depends on the manufacturer, the site, the capacity of the plant, and the characteristics of the wastewater to be disinfected. Ozonation costs are generally high in comparison to other disinfection techniques (EPA 1999). For the removal of micropollutants in wastewater, the additional operation costs for ozonation combined with sand filtration are around 3-4 Swiss cents/m3 (MARGOT et al. 2011).

Operation and Maintenance

Adapted from EPA (1999)

Ozone generation uses a significant amount of electrical power. Constant attention must thus be given to the system to ensure that power is available. Moreover, ozone should not be released from the system and connections in or surrounding the ozone generator should not be leaking. The operator must monitor the appropriate subunits on a regular basis to ensure that they are not overheated. Therefore, the operator must check for leaks routinely since a very small leak can cause unacceptable ambient ozone concentrations.

Health Aspects

There are significant health and safety concerns for operators regarding the production and application of ozone. However, far less is known about ozonation and the effect of ozone on human health (WHO 2009). There are also concerns regarding by-product formation during the disinfection of drinking water containing bromide ions (VON GUNTEN 2003).

At a Glance

Working Principle

Infusion of ozone, a gas produced by subjecting oxygen molecules to high electrical voltage, which reacts with microorganisms and pollutants


High tech equipment required


High efficiency


Relatively high operation costs

Self-help Compatibility

Engineers are required for the design


Continuous input of electrical power required


Reliable if operating conditions are scaled taking into account wastewater content

Main strength

Very efficient and fast method for disinfection and as a AOP

Main weakness

Requires complicated equipment as well as large amounts of energy and qualified operators


Ozonation has been successfully applied for water disinfection and can kill most bacteria viruses and protozoa. However, there is no residual disinfection effect and ozonation is more expensive than chlorination. Ozonation is a suitable process to degrade organic pollutants (e.g. for micropollutants removal and landfill leachate pre-treatment) and oxidise metallic ions (e.g. iron manganese).

Design and construction needs skilled staff and high-tech equipment. Sophisticated generators consuming high-amounts of electricity are required to produce ozone. Although operation and maintenance costs are relatively low, precise monitoring and dosing adjustment of ozone is needed to ensure the efficiency of the treatment (NABI BIDHENDI et al. 2006).

Library references
Further Readings


TECH BRIEF (1999) In: National Drinking Water Clearinghouse. . URL [Accessed: 03.10.2011]
Case Studies

Ozone Planet

This study describes the ozonation facilities installed in Yorkshire for pesticide removal and algae control to enhance the slow sand filters without the need for granular activated carbon.

OZONIA (2000): Loftsome Bridge Yorkshire Water Services URL [Accessed: 03.10.2011]
Training Material
Awareness Raising Material

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