Chlorine Dioxide

from Lenntech

What are the advantages of the use of chlorine dioxide?

The interest in the use of chlorine dioxide as an alternative for or addition to chlorine for the disinfection of water has increased in the last few years. Chlorine dioxide is a very effective bacterial disinfectant and it is even more effective than chlorine for the disinfection of water that contains viruses. Chlorine dioxide has regained attention because it is effectively deactivates the chlorine-resistant pathogens Giardia and Cryptosporidium. Chlorine dioxide removes and prevents bio film.
Disinfection with chlorine dioxide does not cause odor nuisance. It destroys phenols, which can cause odor and taste problems. Chlorine dioxide is more effective for the removal of iron and manganese than chlorine, especially when these are found in complex substances.

Does chlorine dioxide form chlorinated disinfection byproducts?
The use of chlorine dioxide instead of chlorine prevents the formation of harmful halogenated disinfection byproducts, for example trihalomethanes and halogenated acidic acids. Chlorine dioxide does not react with ammonia nitrogen, amines or other oxidizable organic matter. Chlorine dioxide removes substances that can form trihalomethanes and improves coagulation. It does not oxidize bromide into bromine. When bromide containing water is treated with chlorine or ozone, bromide is oxidized into bromine and hypobromous acid. After that these react with organic material to form brominated disinfection byproducts, for example bromoform.

Is the chlorine dioxide concentration needed for sufficient disinfection high?
The use of chlorine dioxide reduces the health risk of microbial pollutions in water and at the same time decreases the risk of chemical pollutions and byproducts. Chlorine dioxide is a more effective disinfectant than chlorine, causing the required concentration to kill microorganisms to be much lower. The required contact time is also very low.

Does the pH value influence chlorine dioxide efficiency?
Contrary to chlorine, chlorine dioxide is effective at a pH of between 5 and 10. The efficiency increases at high pH values, while the active forms of chlorine are greatly influenced by pH. Under normal circumstances chlorine dioxide does not hydrolyze. This is why the oxidation potential is high and the disinfection capacity is not influenced by pH. Both temperature and alkalinity of the water do not influence the efficiency. At the concentrations required for disinfection, chlorine dioxide is not corrosive. Chlorine dioxide is more water-soluble than chlorine. In the last few years better and safer methods for chlorine dioxide production have been developed.

Figure 3: the influence of pH on efficiency is larger for chlorine than for chlorine dioxide

Can chlorine dioxide be used combined with other disinfectants?
Chlorine dioxide can be used to reduce the amount of trihalomethanes and halogenated acidic acids, formed by the reaction of chlorine with organic matter in water. Before the water is chlorinated, chlorine dioxide is added. The amount of ammonium in the water decreases. The chlorine that is added afterwards, oxidizes chlorite into chlorine dioxide or chlorate. Ozone can also be used to oxidize chlorite ions into chlorate ions.
By the use of chloramines, nitrification can take place in the distribution network. To regulate this, chlorine dioxide is added.
Byproducts control by chlorine dioxide can take place in combination with adequate disinfection, especially the reduction of bromine containing trihalomethanes and halogenated acidic acids that originate from the reaction of bromine containing water with natural organic matter. Chlorine dioxide itself combined with bromine does not form hypobromous acid or bromate, while chlorine and ozone do. Chlorine dioxide has excellent anti-microbiological qualities without the non-specific oxidation of ozone.

What are the disadvantages of the use of chlorine dioxide?

Is chlorine dioxide explosive?
When producing chlorine dioxide with sodium chlorite and chlorine gas, safety measures must be taken with regard to the transport and use of chlorine gas. Sufficient ventilation an gas masks are required. Chlorine dioxide gas is explosive.
Chlorine dioxide is a very unstable substance; when it comes in contact with sunlight, it decomposes.
During chlorine dioxide production processes, large amounts of chlorine are formed. This is a disadvantage. Free chlorine reacts with organic matter to form halogenated disinfection byproducts.

Does chlorine dioxide form byproducts?
Chlorine dioxide and its disinfection byproducts chlorite and chlorate can create problems for dialysis patients.

Is chlorine dioxide effective?
Chlorine dioxide is generally effective for the deactivation of pathogenic microorganisms. It is less effective for the deactivation of rotaviruses and E. coli bacteria.

What are the costs of chlorine dioxide use?
Chlorine dioxide is about 5 to 10 times more expensive than chlorine. Chlorine dioxide is usually made on site. The costs of chlorine dioxide depend upon the price of the chemicals that are used to produce chlorine dioxide. Chlorine dioxide is less expensive than other disinfection methods, such as ozone.

What are the health effects of chlorine dioxide?

Chlorine dioxide gas
While using chlorine dioxide as a disinfectant, one has to keep in mind that chlorine dioxide gas can escape from a watery solution containing chlorine dioxide. Especially when disinfection takes place in a sealed space, this can be dangerous. When chlorine dioxide concentrations reach 10% or more in air, chlorine dioxide becomes explosive.
Acute exposure of the skin to chlorine that originates from the decomposition of chlorine dioxide, causes irritations and burns. Eye exposure eyes to chlorine dioxide causes irritations, watering eyes and a blurry sight. Chlorine dioxide gas can be absorbed by the skin, where it damages tissue and blood cells. Inhalation of chlorine dioxide gas causes coughing, a sore throat, severe headaches, lung oedema and bronchio spasma. The symptoms can begin to show long after the exposure has taken place and can remain for a long time. Chronical exposure to chlorine dioxide causes bronchitis. The health standard for chlorine dioxide is 0,1 ppm.

Development and reproduction
Chlorine dioxide is thought to have effects on reproduction and development. However, there is too little evidence to ground this thesis. Further research is required.

The Ames test is used to determine the mutagenity of a substance. The Ames test uses Salmonella bacteria that are genetically modified. No bacterial colonies are formed, unless they come in contact with a mutagenic substance that alters genetic material. Tests show that the presence of 5-15 mg/L ClO2 increases the mutagenity of water. It is difficult to prove the mutagenity of chlorine dioxide and chlorine dioxide byproducts, because the substances are biocides. Biocides usually kill the indicator organisms that are used to determine mutagenity.

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