Ozone is an allotropic form of oxygen; each molecule contains three atoms of oxygen instead of the usual two. Ozone is a powerful disinfectant and has been used commercially for the treatment of potable water since 1904. It is produced on site in the same way that ozone is formed naturally, by the discharge of electricity during a thunderstorm. A high voltage is passed across a gas stream containing oxygen. This method of ozone generation is called corona discharge. The energy of the high voltage splits an oxygen molecule (02) into two oxygen atoms (0) - which recombine with ordinary molecules of oxygen (02) to form ozone (03).
Ozone is also a very powerful and effective oxidizing agent, mainly used to degrade organic contaminants. The same chemical properties that allow ozone to react with organic material give it the ability to react with organic material within the human body and potentially cause harmful health consequences. When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections.
In ‘clean’ processes or ultrapure water applications, ozone is mainly used for CIP sanitization purposes in the process water loops. However, residual ozone represents a potential health hazard and it can be damaging to finished goods, as well as to processing and fabrication equipment. Breakdown of residual ozone is therefore essential in these and other applications before the ozonated water can be used. The process of de-ozonation has been performed in the past by passing through a filter bed of granular activated carbon (GAC). However, the same porous structure of GAC that enables GAC to remove ozone, allows GAC filters to become prime incubators for micro-organisms. Other methods used are dosing with hydrogen peroxide H2O2 (ozone quenching) or aeration by cascade, in a packed column or by air diffusion.
In purified water systems, strategic placement of properly designed and sized medium or low pressure UV systems simply and effectively reduces residual ozone to below detectable levels with the additional benefit of TOC destruction. Positioning a UV system directly before the water treatment components requiring protection from ozone (e.g. DI polishing) maximizes the sanitizing benefits provided by residual ozone up to that point. When loop sanitizing is required, the UV is simply turned off and any sensitive process step is by-passed for a brief period of time. Using UV to remove ozone has an advantage because UV does not require additives that may leave behind residuals. In addition, dissociation of ozone with UV may also help to oxidize Total Organic carbon (TOC).
The mechanism for removing ozone is through UV energy severing one of the oxygen bonds in an ozone molecule. Ozone strongly absorbs UV in the 210 - 300 nm range. As a result of this reaction, each ozone molecule is converted into one oxygen radical and one oxygen molecule. Free oxygen radicals will then combine with each other to form oxygen molecules:
O3 + hn → O2 + O·
O· + O· → O2
Although ozone is readily destroyed by UV, it requires a higher UV dose than used for disinfection (inactivation of microorganisms) – approximately three to four times the UV disinfection dose is required.