CT Values

Commentary on Chlorine Dioxide and CT against Cryptosporidium from Solarwise

I have been asked to comment on the CT value for chlorine dioxide against Cryptosporidium.

Zychem’s chlorine dioxide product, now branded as Zydox, was widely used during the major outbreak of cryptosporidium in Australian Public Swimming pools in 1998.

The experience during that outbreak provided many of the State Health Department guidelines in place today.

For Example, the Queensland Health Pool Guidelines state the following:

Shock dose chlorine dioxide

Research has identified chlorine dioxide as an effective Cryptosporidium and Giardia disinfectant agent. This research suggests a C.t value of 78 (exposure to 1.3 mg/l of chlorine dioxide for 1 hour) and results in inactivation of >90% for both parasites at normal swimming pool temperatures and pH levels.

The effectiveness of the shock dosing method as a Cryptosporidium and Giardia

control option is not only reliant on the concentration and contact time of the dosing, but on the regularity of these doses. This shock dosing method should be carried out overnight due to degradation by sunlight and is recommended on a weekly basis during peak season for commercial operations. A 0.25 mg/l concentrated dose for six hours is suggested for this option.

Zychem’s own Swimming Pool brochure quotes the following:

Effectiveness against Cryptosporidium and Giardia

Super Chlorination

Chlorination at traditional levels will not kill Crypto. Experiments conducted by Korich in 1990 determined that 1.3 mg/L of ClO2 for 60 minutes achieved greater than 90% inactivation of Crypto, whereas 80 mg/L of chlorine for 90 minutes was required to achieve the same degree of inactivation. The general consensus is that, to be effective, superchlorination needs to be at a minimum of 20 ppm for 6 to 8 hours, after which sodium thiosulphate is required to rebalance the pool.

Why Zydox® is the most cost effective alternative

Apart from the Korich research in 1990, many other trials have shown the superiority of chlorine dioxide in Crypto elimination. Brussels Intercommunal Waterboard demonstrated in 1989 that ClO2, at a dose of 0.31 mg/L achieved a 97% oocyst reduction in 15 minutes, compared with a 99% result from 1.11 mg/L of ozone in 4 minutes. Chlorine at normal doses was ineffective after 18 hours.

In March and April 1998 over forty pools in Eastern Australia, with positive Crypto counts from 6 to 1140 oocysts per 10 litres of water, were treated by Zychem with our chlorine dioxide. All returned non-detectable counts the morning following treatment.

The dose used was 1.25 mg/L, that is a CT value of 75 which has never failed to be effective. I understand that NSW Health is seeking to achieve a CT of 100, that is 1.66 mg/L. This level will provide a faster kill, however, given that pools are invariably vacated and dosed overnight, there is generally a window of around 12 hours for the product to work without additional demand and thus the lower dose is considered adequate and has indeed proven so in all cases over the past 14 years.

Chlorine Dioxide is not intended as a chlorine replacement, but as an adjunct to destroy chloramines and guard against crypto and giardia infections when used on a regular (weekly) basis and as a shock treatment when there is a suspected crypto infection.

It does not affect the existing free chlorine levels in swimming pools, other than to prevent and remediate combined chlorine levels. To the best of our knowledge and consistent with our own experience in Australia and New Zealand, a CT value of between 75 and 90, seems to be realistic for Cl02 against cryptosporidium.

However, Zydox chlorine dioxide will achieve a CT value of 100 at 1.66 mg/L.

It is important to note that not all products which are marketed as chlorine dioxide perform equally. There is a considerable variance between the quality of products that have emerged on the Australian market and consumers need to be cautious in their choice of suppliers.

Warren Thomas
Managing Director
Solarwise NSW

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