Ozone, when properly applied and dissolved at the required residual concentration in water, is a very powerful and effective oxidizing and disinfecting agent. Its properties destroy microbiological organisms and degrade many organic contaminants present in the water.

Ozone is preferred over conventional chemical agents because it is generated on site with limited storage or handling concerns. Additionally, residual dissolved ozone ultimately decomposes into oxygen, making it both process and environmentally friendly.

Ozonation is typically only one step in a chain of treatment processes. As such, it is often removed prior to subsequent unit processes for a variety of reasons.

Unlike bottled water, where a dissolved ozone residual reaching the bottle is desired (or even mandated), production of other beverages may require removal of dissolved ozone to prevent reactions with sweeteners, concentrates or other ingredients.

Similarly, in ultrapure water (UPW) applications, dissolved ozone is removed the majority of the time to prevent contact with ingredients and is periodically allowed to flow through process loops for CIP sanitization.

Municipal water treatment plants use ozone for a variety of reasons, such as, THM precursor control, cryptosporidium and giardia removal, taste, odor and color removal, general disinfection, etc. However, even here the dissolved ozone must be removed prior to the addition of chlorine used for water protection in the distribution system.

Destruction of residual ozone is therefore essential in these and other applications before the ozonated water can be utilized or continue in the treatment process.

The following are some of the ways ozone destruction can be effectively accomplished:

• In purified water systems (UPW for example), strategic placement of properly designed and sized medium pressure ultraviolet (UV) ozone destruction equipment simply and effectively reduces residual ozone to below detectable levels with the additional benefit of TOC destruction. Positioning a UV ozone destruction unit directly before the water treatment components requiring protection from ozone (i.e. DI polishing) maximizes the sanitizing benefits provided by residual ozone up to that point. When loop sanitizing is desired, the UV is simply turned off and any sensitive process step is bypassed for a brief period of time. Medium pressure UV systems in UPW also offer the added benefits of TOC destruction and act as "back-up sanitizers" to ozone.
• In municipal drinking water applications, removal of residual ozone in a contact system can be accomplished with the addition of hydrogen peroxide (H202). Known as ozone quenching, this step is important during plant start-up, performance testing or in general when ozone equipment has to be operated to meet CT criteria and then followed by addition of chlorine or other chemicals.

Other methods used to remove ozone from water include:

• Aeration by cascade, packed column or air diffusion. This method strips off the ozone to the atmosphere, so proper controls and monitoring must be employed for safety and environmental reasons.
• Filtering to water through activated carbon adsorbs the dissolved ozone on the carbon and adds the removal benefit of carbon in general. This technique can be employed where water quality is less than ultrapure and the carbon can actually contaminate the water.

For more information on this or other ozone and UV related topics, contact your nearest Ozonia office.