What is it?Chlorine Dioxide in Water TreatmentChlorine Dioxide GenerationPotential Disadvantages of using Chlorine Dioxide in Water TreatmentThe Future of Chlorine DioxideChlorine dioxide is a yellow gas first discovered in 1811. The chemical oxidant has many varying applications, including water disinfection and treatment. Chlorine dioxide was first used in the treatment of water in the early 1940s. At the time, the water municipalities had too many difficulties finding an economical method to generate the chemical and its use was not widely adopted. It has received new interest because of its strong disinfectant strength and fewer disinfection by-products than the more commonly used chlorine.
Historically, the most widespread use of the chemical is in the paper and pulping industry. In this application, chlorine dioxide is used as a bleach for paper pulp. The reason chlorine dioxide is so widely used in the paper industry is because it does not react with the organic lignin in the wastewater to form chlorinated by-products such as the trihalomethanes (THMs). Chlorine, on the other hand, does react with organic matter. The result is potentially harmful chlorinated organics in the mill's effluent wastewater.
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Chlorine dioxide (ClO2) is used during the treatment of drinking water consumed by an estimated 12.4 million people in the United States. While this oxidant is either as good or better than chlorine as a disinfectant, its most common uses are to oxidize reduced iron, manganese, sulfur compounds, and certain odor-causing organic substances in raw water. Its popularity as a preoxidant, however, is primarily because it, unlike chlorine, will not chlorinate organic compounds and, therefore, will not react with natural organic matter (NOM) in raw water to form trihalomethanes (THMs) or other chlorinated byproducts. Dietrich et al. (1992) reported that most utilities they surveyed used ClO2 primarily to reduce THM levels. Some utilities favor ClO2 because their raw water contains significant levels of bromide ion (Br-) and ClO2 will not oxidize Br- to either bromate ion (BrO3), which will be regulated at a very low level (0.010 mg/L) by the Disinfectant-Disinfection By-Product (D-DBP) Rule, or hypobromous acid (HOBr), which can form brominated DBPs in reactions with NOM.
The following graph compares the various reasons that treatment plants utilize chlorine dioxide in water treatment.
As a disinfectant, ClO2 is either as good or better than chlorine for the inactivation of Giardia and is better than either chlorine or chloramines for the inactivation of Cryptosporidium. Reported CT values for chlorine, chloramines, ClO2, and O3 are shown below.
| Chloramines | Chlorine | Chlorine Dioxide | Ozone | |
|---|---|---|---|---|
| Giardia 0.5 log inactivation pH 6-9, 5°C | 365 | 16 - 47 | 4.3 | 0.3 |
| Viruses 2 log inactivation pH 6-9, 5°C | 857 | 4 | 5.6 | 0.6 |
| Cryptosporidium pH 7, 25°C | 7200 2 log inactivation | 7200 1 log inactivation | 78 1 log inactivation | 5 - 10 2 log inactivation |
During the past two decades, the typical ClO2 dosages applied to drinking water in the United Sates were less than 1.5 mg/L because the U.S. Environmental Protection Agency (USEPA) recommended that the combined, finished-water concentrations of ClO2 and its by-products be restricted to < 1.0 mg/L, and most state regulatory agencies adopted that recommendation. Both of ClO2 by-products, chlorite ion (ClO2-) and chlorate ion (ClO3-), have been linked to potential adverse health effects involving red blood cells and blood chemistry. Chlorine dioxide and ClO2- were considered during recent negotiated regulations, and maximum contaminant levels (MCLs) for these two substances appeared in the recently proposed Stage 1 Disinfectant/Disinfection By-Products (D/DBP) Rule. The proposed MCL for ClO2 is 0.8 mg/L and for ClO2- is 1.0 mg/L. No ClO3- MCL has been proposed, and the matter is currently under review by USEPA.
Chlorine dioxide applied during drinking water treatment is typically
generated by reacting a 25% sodium chlorite (NaClO2)
solution with either gaseous chlorine or strong chlorine solutions. The reactions are:
If chlorine is used in excess (e.g. 200-300% of the stoichiometric requirements), ClO2 yields > 95% can be obtained but the pH will be depressed and ClO3- concentrations may increase. Maintenance of appropriate ratios of chlorine gas and NaClO2 in this type reactor, as well as proper flowrates of water through the generator, is requisite to producing high ClO2 yields.
The generation of chlorine dioxide for the purpose of water treatment can be achieved by on-site equipment. The following slide shows typical chlorine dioxide generation equipment.
A dosage of 1.0 mg/L ClO2 to raw water will most often result in finished-water ClO2- concentrations ranging from 0.6 mg/L to 0.8 mg/L19, both of which are below the proposed MCL of 1.0 mg/L specified in the Stage 1 D-DBP Rule. The Stage 2 ClO2- MCL may be lower than 1.0 mg/L and, if so, removal of excess ClO2- during treatment will be required.
Chlorite ion removal from drinking water can be accomplished by the addition of activated carbon, ferrous iron, and reduced-sulfur compounds, each of which reduces ClO2- to chloride ion (Cl-). Addition of these substances obviously should be at a point in the treatment train following an adequate ClO2 contact time.
The currently proposed Stage 1 MCLs do not include an MCL for ClO3-; however, utilities using either ClO2 or hypochlorite ion will be required to monitor ClO3- concentrations in finished water under provisions of the Stage 1 D-DBP Rule regulations.
In producing potable water, one of the main goals is to produce an aesthetically pleasing supply, one without any taste or odor. In some municipal treatment facilities using the chemical disinfectant chlorine dioxide, complaints regarding taste and odor threaten the continued use of the disinfectant. In a recent study, (Hoehn et al. 1990) these consumer complaints were found to have been caused by gaseous reactions between tap water and certain organics in the air in the consumer's household. Unfortunately, this would produce smells that were described as 'kerosene-like' and 'cat urine-like'.
This unpleasant side-effect was found in homes that had installed new carpets. A unknown chemical used in the preparation of carpet material reacted with gaseous chlorine dioxide from tap water to form the unpleasant odors. Further research into this area is needed.
As of yet, chlorine dioxide is not the first choice of municipal water facilities as a chemical disinfectant. The higher costs of the equipment associated with generating the chemical on-site appears to be the main stalling point. In the future, the large amount of research into the use of chlorine dioxide as well as its strong advantages over chlorine may increase its use in the future.
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