Chlorine dioxide is a well-known disinfectant for drinking water with many advantages over chlorination. Today there are estimated to be over 900 public water systems using chlorine dioxide for water disinfection world-wide.
The acceptance of chlorine dioxide in Australia as a disinfecting agent for drinking water has been hampered by the crude methods available for generating chlorine dioxide – methods that require relatively complex equipment and/or produce impure chlorine dioxide with many undesirable impurities. CleanOxide overcomes these problems. CleanOxide is a method of generating chlorine dioxide free from undesirable by-products as and when required by mixing two precursor solutions.
The chemistry of chlorine dioxide is quite different to that of chlorine gas or sodium hypochlorite the most commonly used water disinfecting agents. These compounds react with water to form hypochlorous acid (HClO) which is the primary sanitising agent in conventional methods of “chlorine disinfection”.
Chlorine dioxide (ClO2) is a gas that dissolves readily in water but does not react with water. Unlike chlorine, chlorine dioxide cannot be supplied in compressed gas cylinders. It must be generated in situ and handled as dilute aqueous solutions.
CleanOxide 75 is a 0.75% (7500mg/L) solution of chlorine dioxide in water generated by mixing two precursor solutions. The concentration of this solution is the maximum practicable concentration of chlorine dioxide that can safely be handled.
Aqueous solutions of chlorine dioxide are extremely powerful disinfectants even at very low concentrations. Chlorine dioxide is effective against all microorganisms including algae, bacteria, bacterial spores, fungi, protozoa and viruses. Chlorine dioxide works by disrupting the functioning of the cell membrane.
The mechanism is such that microorganisms cannot build up resistance to chlorine dioxide making it an extremely important compound in a world where many organisms are becoming resistant to conventional biocides and antibiotics.
Chlorine dioxide remains effective over a wide pH range from about 4 to 10. This is in sharp contrast to chlorine which is only effective over a narrow pH range below about 7.5.
The use of chlorine can result in the formation of many undesirable chlorinated compounds including chloroform, chloramines and organochlorine compounds including dioxins. Substituting CleanOxide 75 for chlorine eliminates the formation of these pollutants.
CleanOxide 75 has all the beneﬁts of chlorine with none of its downside. Furthermore, because chlorine dioxide is a more powerful disinfectant than chlorine, CleanOxide 75 can be used at much lower dose rates (typically about one ﬁfth to one tenth of the rate of chlorine) resulting in reduced chemical usage and lower environmental impact.
An important property of chlorine dioxide is its ability to destroy bioﬁlm. Bioﬁlm is a ﬁlm of slime produced by microorganisms on surfaces in contact with water. It consists of polymeric substances produced by aerobic and anaerobic bacteria, fungi and algae (if light is present).
Bioﬁlm acts as a reservoir of microorganisms that is not removed or controlled by normal levels of chlorination. Even heavy doses of chlorine (super chlorination) often do not remove bioﬁlm.
The reason that chlorine Dioxide is so effective against biofilm is because it exists in solution as a dissolved gas; it does not react with water. The dissolved gas is able to diffuse into the bioﬁlm where it can destroy the microorganisms responsible for the bioﬁlm.
Conventional biocides including chlorine are only able to affect the surface layers of biofilm. Chlorine dioxide is more effective than chlorine and chloramines at inactivating viruses and controlling Cryptosporidium and Giardia.
Chlorine dioxide is very effective at oxidising common water pollutants such as sulphides, iron and manganese Taste and odours resulting from algae and decaying vegetable matter and phenolic compounds are all controlled by chlorine dioxide. Chlorine dioxide provides residual disinfection (for up to 72 hours) in drinking water depending on water quality.
The Australian Drinking Water Guidelines (the ADWG) published by the National Health and Medical Research Council (NHMRC) and Natural Resource Management Ministerial Council (NRMMC) in 2004 are not mandatory standards but provide a basis for determining drinking water quality.
Chlorine dioxide has been approved by NHMRC for use in disinfecting drinking water since 2005. The ADWG recommends a maximum of 1mg/L for chlorine dioxide on health grounds and a maximum of 0.4mg/L on aesthetic grounds (taste). According to the ADWG, drinking water may need to be dosed at 1 to 1.25mg/L chlorine dioxide to achieve a residual concentration of 0.4mg/L.
The main impurities associated with chlorine dioxide are chlorite and chlorate. The ADWG recommends a maximum value of 0.3mg/L chlorite on health grounds but currently does not have any recommendation in relation to chlorate due to insufficient data being available. New guidelines are expected to be published in the near future which will recommend a maximum value for chlorite of 0.8mg/L and 0.3mg /L for chlorate.
These guidelines have been developed by reference to conventional methods of generating chlorine dioxide that are associated with high levels of both chlorite and chlorate. CleanOxide 75 is a solution of 7500mg/L chlorine dioxide with a maximum chlorite concentration of 75mg/L. At the maximum dose rate of 1.25mg/L chlorine dioxide suggested by ADWG, chlorite concentration from CleanOxide 75 would be less than 0.0125mg/L.
Conventional methods of generating chlorine dioxide also generate free chlorine as a by-product. Other methods of generating chlorine dioxide such as “stabilised” chlorine dioxide rely on acidiﬁcation of sodium chlorite. These methods of generating chlorine dioxide are associated with high rates of corrosion.
However, it is not the chlorine dioxide that causes corrosion; it is the by-products that are responsible. CleanOxide 75 is practically pure chlorine dioxide. Its formation is not accompanied by any free chlorine and it is not acidic at normal rates of use. Thus CleanOxide does not contribute to increased levels of corrosion.
It is well known that chlorination of water produces a number of undesirable by-products including trihalomethanes, haloacetic acids, chloral hydrate, chlorinated phenols and other chlorinated organic compounds.
Together with chloramines, these compounds contribute signiﬁcantly to undesirable taste and odour associated with chlorinated drinking water. CleanOxide can be added to drinking water treated with chlorine as the primary disinfectant to remove many of these undesirable compounds thus improving the aesthetic quality of the water.