About ZYDOX®
What is stabilised chlorine dioxide?


Stabilised chlorine dioxide, ClO2, (not to be confused with on-site generated ClO2) has been used in America and Europe for over twenty years to treat drinking water and swimming pools for Cryptosporidium and Giardia contamination, and as a food safe sanitiser and bleach.

Zydox® is a complex mixture of oxychlorine compounds which are able to release chlorine dioxide in solution. It is a colourless, minimally corrosive product with a very mild ozone/chlorine like odour which, being biodegradable, has many safety attributes over traditional chlorine disinfectants.

How does it work?

It works by destroying microorganisms, aerobic and anaerobic bacteria, viruses, algae, fungi and mould. Chlorine dioxide is particularly reactive with aromatic amines and aliphatic tertiary amines present in structural proteins.

Inactivation of key enzymes or structural/surface proteins explains the highly effective germicidal properties of chlorine dioxide. Because chlorine dioxide interferes with the metabolism of microorganisms, it is not possible for them to mutate or build a resistance to it.

Benefits of chlorine dioxide

Chlorine dioxide, the active agent in Zydox®, is an oxidant with 250% higher oxidising capacity than chlorine. Unlike chlorine, chlorine dioxide does not react with organics to produce chlorinated compounds. Also, the presence of ammonia will place a demand on chlorine and reduce its effectiveness as well as forming chloramines. Chlorine dioxide does not react with ammonia and, unlike chlorine which is pH dependent, Zydox® is effective over a wide range of pH conditions, as well as temperature range, water hardness and organic load.

Unlike chlorine, chlorine dioxide does not combine with organic compounds to form haloforms or toxic chlorinated compounds (absorbable organic halogenated compounds), hence its widespread use in the treatment of drinking water.

Application for Swimming Pools for Chloramine Odour Control

High use, highly loaded swimming pools, particularly heated indoor pools, are very susceptible to chloramine odour. This is caused by the reactivity to ammonia in the pool water and the resultant formation of mono, di and tri chloramine – the latter two being the cause of sore eyes and a typical unpleasant “chlorine” odour.

It is normal for these pools to be shock dosed with calcium hypochlorite to at least 10 ppm on a regular (weekly) basis to oxidise those chloramines.

The use of Zydox® in place of a chlorine based product to oxidise chloramines has many advantages. Because of its stronger oxidising capacity (at least 2.5 times that of chlorine), you use lower doses yet achieve better clarity and healthier pool water. Zydox® is the best way to burn out chloramines!

Support of Independent Third Party Trials

Aqua Science Consultants trialed the product over a 17 week period in a highly loaded learn to swim pool in 1999.

These were their significant findings:

  • Zydox additions reduced combined chlorine odour. Combined chlorine odour was not detectable during applications of weekly Zydox treatments, regardless of dose rate or activation method.

  • Presence of chloramine odour was consistently recorded when no Zydox was applied and conventional superchlorination methods only were used.

  • Water clarity improved during Zydox applications.

  • Free chlorine level remained generally stable throughout the trial. No increase or significant fluctuation in this parameter was observed during Zydox applications.


Aqua Science Consultants Recommendations:

  • “Zydox can be incorporated into a weekly pool maintenance program,to improve water clarity.

  • “Zydox can be used as an alternative to routine superchlorination with chlorine, to reduce combined chlorine odour.

  • “There is potential for Zydox to minimise the formation of combined chlorine species in heavy bather load conditions.” 

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.

Corona Discharge Ozone

Corona Discharge Ozone is a very effective treatment for Crypto. However, its major drawback is the high cost of installation and the fact that ozone only kills Crypto in the plant room. Ozone must be removed from the water prior to its return to the pool. Therefore, Chlorine or Bromine is still required at residual levels in the pool but at these levels they are ineffective against Crypto. It also takes at least four complete filtration cycles to treat 98% of pool water.

Micro Filtration

Because of the size of the oocysts, most filter systems in commercial pools will not eliminate Crypto from the system. The retro fitting of micro filtration and/or DE filters to commercial pools, whilst potentially effective, is very expensive. The constant replacement and cleaning of filter medium is time consuming and there is always the risk of contamination returning to the pool during the process.

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 chlorine dioxide. All returned non-detectable counts the morning following treatment.