Ozonation

Ozone (O₃) has been used in water treatment since the late 19^th 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).

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.

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.

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.

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.

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.

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).

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.

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).