UV and ozone: Companions in managing pool water contamination

UV and ozone as chlorine’s companion

UV and ozone have been used in pool water treatment for decades, but only recently has the interest level in these technologies escalated.

The renewed interest is primarily a result of the clear identification of chlorine resistant micro-organisms, such as Cryptosporidium, and to a lesser extent, Giardia, as the causal agents in most RWIs.

UV and ozone are also more widely accepted for their role in chloramine destruction. Further, recent developments within the drinking water and pool industries has pointed to the synergistic oxidation and sanitizing benefits achieved when UV and ozone are used in combination.

Chlorine’s effectiveness as a sanitizer is measured by its CT value, where ‘C’ stands for concentration and ‘T’ for time. As logic would dictate, the higher the concentration of chlorine or the longer exposure time it has to a micro-organism, the more effective it is at eradicating it. Established CT values of chlorine for some common pool water microbes are shown in Table 2.

For instance, Cryptosporidium requires 15,300 ppm of chlorine acting on it for one minute in order to effect a 99.9 per cent reduction in the organism. Alternatively, a lower concentration of chlorine can be used, but the time required for inactivating the organism will be lengthened (e.g. one ppm chlorine acting for 15,300 minutes has the same CT value as 15,300 ppm for one minute).

In any event, it should be clear that destroying Cryptosporidium with chlorine requires impractical chlorine levels or impractical periods of time. While chlorine is a very good sanitizer, and has been a mainstay in the pool industry for decades, it is not the perfect end-all solution to pool water microbial treatment. Additionally, as has been stated previously, chlorine combines with other chemistries in the pool water to form chloramines—also known as combined chlorines. These chlorine forms are irritants to bathers and advanced treatment methods are needed to reduce their levels.

Using UV and ozone together as part of an auxiliary water maintenance program can have many benefits. UV, as a standalone supplement to chlorine, for example, provides tremendous protection against a wide range of micro-organisms. Similar to chlorine, the effectiveness of UV can be observed by its IT value, where ‘I’ stands for intensity and ‘T’ for time. For micro-organisms of interest to the pool industry, the IT value has been established (see Table 3) and, fortuitously, UV easily destroys Cryptosporidium. The recognition of UV’s value to Cryptosporidium inactivation occurred back in the ’90s after a drinking water issue in Milwaukee, Wis., left 69 people dead from Cryptosporidium contamination. UV light was soon determined to be very effective, requiring only small doses of light energy to destroy it.