Chlorine dioxide is an effective and popular water treatment solution. Dissolved into water, it creates a powerful, yet selective oxidant, biocide, and disinfectant used for both treating drinking water and commercial treatment applications.
However, because of the traditional chemicals used, generation systems, and storage requirements of chlorine dioxide, it can mean that the benefits are sometimes overshadowed by logistical concerns for businesses.
To understand the potential, we asked our experts to answer the 17 most frequently asked questions they get on the topic of chlorine dioxide.
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Chlorine dioxide (ClO2) is used in water treatment because it is an effective oxidant, biocide, and disinfectant at relatively low concentrations. It also has minimal reactivity with organic matter and minimal byproducts are formed during the treatment process.
Chlorine dioxide can kill bacteria, viruses, and other microbes in water that will help prevent the spread of waterborne diseases without hydrolyzing. Chlorine dioxide will stay as a dissolved gas in a solution, is around ten times more soluble than chlorine, and can be removed by aeration. This potency, combined with safety, makes it an appealing option for water treatment use[i]
ClO2 is a disinfectant, best utilized in systems with surface germs and biofilms. Chlorine dioxide removes biofilms and inactivates germs by destroying cell walls. Furthermore, the oxidation and disinfection capabilities of ClO2 are independent of the pH of the water across a wide range. ClO2 can be used effectively in water with a pH 6 to 9.[ii]
Chlorine dioxide is a safe form of water treatment. It is used in various applications, including the treatment of drinking water for human consumption.
The Environmental Protection Agency (EPA) set the maximum allowed concentration of chlorine dioxide in drinking water at no more than 0.8 parts per million (ppm). The Occupational Safety and Health Administration (OSHA), an agency of the United States Department of Labor, has an 8-hour exposure limit of 0.1 ppm in air (0.3 mg/m3) for people in contact with chlorine dioxide. [iii]
The EU DWGL 98/83 sets the limit for chlorine dioxide application as low as 0.2 ppm. Due to this low addition ratio, the DBP levels are 0.2 ppm for chlorite and 0.25 ppm for chlorate being on a very low level.
Chlorine dioxide (ClO2) is a compound, composed of a chlorine atom and two oxygen atoms. Chlorine and chlorine dioxide are both oxidizing agents, meaning they remove electrons from other compounds during chemical reactions. But whereas chlorine has the capacity to take in two electrons, chlorine dioxide has the capacity to absorb five in highly acidic environments. In a neutral ambient, as in swimming pools and drinking water, ClO2 takes only one electron.
In addition, chlorine dioxide does not react with many organic compounds, meaning it does not create chlorinated organics, which can be environmentally dangerous.
When used at the concentrations required for disinfection without accumulation e.g., in loops, chlorine dioxide is minimally corrosive. Chlorine dioxide is approximately ten times more soluble in water than chlorine[iv] and safe methods for chlorine dioxide production have been developed, meaning the corrosive effects of using chlorine dioxide for treatment are minimal.
ClO2 removes biofilms and keeps them under control. Through this, it helps to reduce steel corrosion in an indirect way.
Chlorine dioxide is known as an effective water treatment method because of its oxidation capacity at low levels. The predominant effect is the enrichment of dissolved gaseous chlorine dioxide at boundary where microorganisms find their ideal conditions for survival.
The high efficiency of chlorine dioxide at low concentrations versus other disinfectants can be seen when comparing its c x t – numbers with other chlorine species.
For water treatment purposes, chlorine dioxide is usually prepared on-site, using methods that produce solutions without a gaseous stage. There are four common methods of production:
The concentration of chlorine dioxide produced by these methods varies between 1 and 3 g/L. Whereas methods a., b., and c. are used for the disinfection of municipal drinking water, method d. is limited to the disinfection of mains and tanks as it requires highly extended exposure time for complete conversion of the precursors into chlorine dioxide.[v]
Depending on the cost of the precursors that are used to produce chlorine dioxide, for example the Acid Chlorite vs. Chlorine Chlorite process when compared to chlorine, it’s between five and ten times more expensive. The efficacy of ClO2 for biofilm control makes it attractive for operators.
Chlorine dioxide cannot be stored as a gas because at concentrations over 10%, or under pressure, there is a risk of explosion by self-decomposition.[vi]
If stored for extended periods of time, chlorine dioxide will mainly form the intermediate byproduct chlorite, finally ending in chlorate. A dissociation into chlorine and oxygen only occurs in the gas phase. For these reasons, chlorine dioxide is stored as a solution at concentrations of around 0.3 % ClO2 (3 g/L) and kept away from light and heat. Under these conditions, chlorine dioxide is stable and soluble.
Chlorine dioxide is usually made on site using one of the production methods mentioned in Question 8, or by using a generation system provided by suppliers.
Chlorine dioxide is considered hazardous to the environment. However, chlorine dioxide does not last long in air, water, or soil environments - up to minutes in air and up to hours in water or soil.
Due to its reactivity, chlorine dioxide photolyzes rapidly in the atmosphere. It has a tropospheric half-life of just a few seconds and will breakdown rapidly in natural waters or waters that contain a moderate level of organic matter.
Chlorine dioxide is converted to the intermediate product chlorite and the final product chlorate. Chlorate levels can be minimized with proper use of ClO2 – refer to Question 15.
The most common applications of CIO2 in water treatment are potable water, swimming pools & water parks, food and beverage processing, cooling towers, utility water and wastewater.
ClO2 is a relatively mild oxidant with a low ORP value in comparison to hypochlorous acid. It oxidizes to generate chlorinated DBP, including THM, as chlorine and hypochlorites do.
In addition, it has an excellent residual effect in purified water whereas ozone is depleted by self-decomposition. Once ozone has vanished, the regrowth of germs accelerates, which doesn’t happen with chlorine dioxide.
When ClO2 interacts with water ingredients and decomposes in feedstock solutions, chlorate is generated. The chlorate formation can be minimized by accurate selection of the generation process (batch or inline) and design of the system — the size of the generator, size of the dosing system, and size of the batch tank.
Chlorine dioxide has been proven as a powerful disinfectant and oxidizing agent that works at lower residual levels than other common disinfectants. With a low reactivity to organic compounds, CLO2 also minimizes chlorinated organics in the treatment process, meaning it may be the optimum choice for cooling tower treatment.
The chlorine-chlorite process is 20 % more effective than the chlorite/acid process when considering the chemistry of generation - see Question 8 reaction A and B. Furthermore, sodium chlorite and hydrochloric acid are expensive chemicals. It is recommended to use the most economic chlorite - chlorine generation process for large-scale applications such as cooling water loops and large-scale drinking water treatment systems.
There is no system available that provides ClO2 in a definite concentration and quantity from one (generally the precursor is sodium chlorite and sometimes sodium chlorate). But each of these precursors requires being activated and transferred to form ClO2 by acidification or by oxidation usually with chlorine.
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[i] Chlorine Dioxide Disinfection in the Use of Individual Water Purification Devices, USACCHPM, TIP Technical Information Paper #31-007-0306 [iii] Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBPR) 63 FR 69390, December 16, 1998, Vol. 63, No. 241 Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) 71 FR 388, January 4, 2006, Vol. 71 [v] Bohner, Bradley (1993): Corrosivity of Chlorine Dioxide used as Sanitizer in Ultrafiltration Systems, pp. 3348-3352Journal of Diary Science, Vol. 74, No. 10, 1991[vi] New Jersey Department of Health and Senior Services (2005) Hazardous Substance Fact Sheet: Chlorine Dioxide
