What is Chlorine Dioxide?
Chlorine Dioxide has the chemical formula ClO₂ and is a yellow to brown coloured gas at room temperature and pressure. It is a highly reactive oxidant and for all practical areas of water disinfection, it must be generated on site using proprietary reaction and dosing equipment.

Chlorine Dioxide Solubility
Chlorine Dioxide ClO₂ is approximately 5 times more soluble than chlorine and 50 times more soluble than ozone. Even though Chlorine Dioxide is soluble, it is still a gas and the solubility of the gas is governed by Henry’s Law. In closed pipelines, virtually no loss out of water into the gas phase can be expected. In open tanks, Chlorine Dioxide ClO₂ in solution will slowly decrease until equilibrium is established between ClO₂(g) and ClO₂(aq). According to Le Chatelier’s Principle, if Chlorine Dioxide ClO₂ is continually removed from the gas phase above an open tank, the concentration in solution will continue to decrease until it reaches zero.

Chlorine Dioxide Generation Reactions
There are a large number of Chlorine Dioxide generation reactions. However, not all of these are commercially suitable for water treatment or water disinfection. The following four are the most common. Please click on the links for information on systems we provide

Electrochemical ① Anode (oxidation):   ClO2- → ClO2 + e- ② Cathode (reduction):  2H2O + 2e- → H2 + 2OH- ① + ② (combined)  2ClO2- + 2H2O → 2ClO2 + H2 + 2OH- Acid-Chlorite 5NaClO2 + 4HCl → 4ClO2 + 5NaCl + 2H2O Chlorine-Chlorite Cl2 + H2O →  HOCl + HCl Then refer three chemical reaction below   Three Chemical 2NaClO2 + HOCl + HCl →  2ClO2 + 2NaCl + H2O

In the acid-chlorite reaction, excess acid is used to drive the reaction to completion . In the chlorine-chlorite reaction, excess chlorine is used. The excess reactant is important as this continues through into the water when chlorine dioxide is dosed. In the acid-chlorite reaction, acid and chloride will be added with the chlorine dioxide which may necessitate pH correction afterwards. The positive aspect of this reaction is that the chlorine dioxide produced is chlorine free. In the chlorine-chlorite reaction, chlorine will be present with chlorine dioxide in the treated water. The presence of chlorine will produce chlorinated organic reaction by-products which are undesirable. The electrochemical reaction only requires one chemical and electrical power and has by-products of caustic and hydrogen.

Chlorine Dioxide Generation Yield & Running Cost
There is a difference between chlorite conversion and overall conversion. Typical chlorite yield for an acid-chlorite generator varies between 65-68%. Overall conversion efficiency is much lower than this as much of the acid remains unreacted. For a discussion on the yield of acid-chlorite generators, please see attached white paper.
For low capacity generation, acid-chlorite is a simple process which can be installed and operated at low cost. The low conversion efficiency of this process becomes unacceptable when the output of ClO2 is higher (>150 g/hr). The greater conversion of the electrochemical generator and three chemical generator come into their own. Electrochemical production of chlorine dioxide using the Electricide CDE generator can produce chlorine dioxide at 95-99% purity and greater than 80% chlorite conversion. Overall conversion is the same as chlorite as there is only one chemical precursor. The chlorine-chlorite process can get as high as 95% depending on the pH of the reaction mixture and the concentrations of reactants present. 

Chlorine Dioxide Generators - Continuous or Contiguous Generation
The Electricide CDE electro-chemical generator produces chlorine dioxide in a contiguous manner. The resultant chlorine dioxide solution is made and stored in a solution storage tank. Chlorine dioxide solution is dosed from this tank to the dosing points.

The Electricide CD2D and CD2C acid-chlorite generators have continuous generation and dosing of chlorine dioxide into the water stream. The dosing pumps add chemical into the reaction chamber and this chamber doses directly into the water stream by-pass.

Most chlorine-chlorite generators operate on a contiguous basis. An intermediate storage tank of approx. 200 - 500 L contains the chlorine dioxide solution at a concentration of around 5 g/L. This tank is level controlled and the low level turns on the generation process at a fixed rate. The tank then fills up and stops at the high level. Metering pumps dose the chlorine dioxide solution from the storage tank into the water to be treated.

Chlorine Dioxide Reaction By-Products
Pure chlorine dioxide will react with NOM (Naturally Occurring Organic Matter) such as humic and Fulvic acids to form a number of oxidised organic compounds such as carboxylic acids and aldehydes in the ppb concentration range. No formation of chlorinated organic by-products will occur unless chlorine is present in the reaction mixture. THM’s will only be formed with the chlorine-chlorite process.

Chlorite is the major inorganic by-product of the reaction of chlorine dioxide in water. Usually, the amount of chlorite formed will be 40-60% of the amount of chlorine dioxide which has reacted. For example, if 1.00 ppm of chlorine dioxide is added to water and 10 minutes later, 0.60 ppm remains as a residual, 0.40 ppm has therefore reacted. We can expect the chlorite to be 0.16 - 0.24 ppm.

Chlorine Dioxide Safety
Chlorine Dioxide gas can explode if the concentration in air exceeds the explosive threshold of 5%. Acid-chlorite generators are designed so that vacuum cannot be present where high concentration Chlorine Dioxide is stored.

Measurement of Residual
Chlorine Dioxide ClO₂ can be measured with a comparator or photometer using DPD1 as reagent. Measurement is easy with the electrochemical or acid-chlorite process as no chlorine will be present with the chlorine dioxide. However, the chlorine-chlorite process will mean that treated water will contain both chlorine and chlorine dioxide. Both of these species will react with DPD1 so differentiation of just ClO₂ will not be easy.

Chlorine Dioxide Reaction with Inorganic Compounds
Ammonia Nitrogen: No reaction. This can be a good thing if Chlorine Dioxide ClO₂ is being utilized for disinfection of water where ammonia is present. This is typically the case in some cooling towers where control of TPC and Legionella is required. Waste water or recycled water can contain high ammonia concentration so chlorine dioxide is a great alternative to chlorine to achieve disinfection where chlorine would not. .

Iron: Iron is often present in ground water and various industrial waste waters as either ferrous ion or compounds containing ferrous ion.  In the case of potable water, it is important to remove this soluble iron so that contamination of the reticulation does not occur by the precipitation of ferric oxide.

If Chlorine Dioxide is dosed at a rate of 0.24 parts of ClO₂ per part of iron, oxidation of ferrous to ferric will occur, causing rapid precipitation of ferric oxide. This reaction is essentially pH independent and is very quick. Chlorine Dioxide ClO₂ can be dosed at the front end of a water treatment plant i.e. before clarification or sand filters and the ferric oxide will either settle out or be captured in the sand filter bed. Thus, it is removed and problems such as brown staining of clothes and bacterial regrowth will be avoided.

Reaction of Chlorine Dioxide with ferrous ion will cause ClO₂ to undergo a two stage reaction, first to chlorite ion which is very fast. The second stage is the reaction of chlorite with ferrous ion which is slower and results in chloride ion as the by-product. Hence, it is possible for ferrous ion to be oxidised by Chlorine Dioxide without increasing the chlorite concentration of the treated water.

Ferrous ion can also be bound in humic complexes. In this case, Chlorine Dioxide will break these complexes and oxidize the ferrous ion.

Manganese: Manganese is often present in ground water as Mn2+ ion. Chlorine Dioxide can be utilized to remove manganese by oxidation of the Mn2+ to MnO2 which will precipitate out. The advantage of using ClO2 over other oxidants is firstly speed: ClO2 reacts with Mn2+ very quickly so the reaction will be complete by the time the water reaches filters or settling tanks. If Chlorine is used, the reaction is slower so some MnO2 may precipitate out in the reticulation causing black staining of clothes. Secondly, the possibility of forming permanganate is avoided with Chlorine Dioxide ClO2. Oxidation of Mn2+ using ozone is possible but overdosing will produce permanganate ion which will impart a pink color to the water. It is not possible to overdose with ClO2 as the oxidation reaction cannot proceed all the way to permanganate and excess ClO2 will be employed for disinfection.

Reaction in neutral or alkaline conditions will result in Chlorine Dioxide forming chlorite ion as by-product. As the concentration of chlorite is regulated in most water supplies throughout the world, the maximum concentration of Mn2+ which can be oxidized is therefore limited by the chlorite regulatory limit and the stoichiometry of the reaction.

Manganese is effectively oxidized by Chlorine Dioxide when humically bound in complexes. Chlorine is not effective for this purpose .

Sulfur Compounds: Under the appropriate conditions, it is possible to utilize all the oxidizing power of ClO₂ to convert sulfides, H₂S and Mercaptans to sulfate ion. With chlorine and ozone, colloidal sulfur will be formed which may or may no be desired.

Cyanide: It is only possible to oxidize cyanide to cyanate ion. Thus, chlorine is preferred over ClO₂ as chlorine can oxidize cyanide first to cyanate and then to nitrogen gas and carbonate ion.

Oil and Gas. Chlorine Dioxide is the disinfectant chemical of choice in the oil and gas industry. One major area of interest is the treatment of frac water.

Chlorine Dioxide Disinfection
ClO₂ is an effective and powerful disinfectant. It is capable of inactivating bacteria and viruses, spores and moulds. Inactivation of Giardia is possible with low doses and Cryptosporidium Parvuum with a CT value of 78.

Table 1 1

Bacterial Reduction Using Chlorine Dioxide
Micro-organisms	ppm of ClO2	Contact Time (s)	Inactivation in %
Staphylococcus aureus	1	60	99.999
Eschericia Coli	0.15	300	99.9
Eschericia Coli	0.25	60	>99.999
Streptococcus	1	15	>99.999
Lactobacillus Brevis	0.15	300	99.9
Lactobacillus Brevis	1	300	>99.999
Pseudomonas aeruginosa	1	60	>99.999
Fungicidal Activity of Chlorine Dioxide
Micro-organisms	ppm of ClO2	Contact Time (min)	Inactivation in %
Saccharomyces diastaticus (yeast)	0.15	10	99.9
Saccharomyces diastaticus (yeast)	1	1	>99.999
Saccharomyces diastaticus (yeast)	0.5	10	>99.999
Saccharomyces diastaticus (yeast)	1	1	>99.999
Penicillum expansum (mould)	0.5	60	99.99
Penicillum expansum (mould)	2	20	99.999
Pediococcus Damnosus (yeast)	0.15	20	99.99
Pediococcus Damnosus (yeast)	0.3	5	99.99
Pediococcus Damnosus (yeast)	1	5	99.999
Pectinatus cervisiiphilus (yeast)	0.1	5	99.9

pH Independence. The main advantage of using Chlorine Dioxide  for disinfection is the pH independence of the reaction. Unlike chlorine, Chlorine Dioxide ClO₂ will inactivate pathogenic micro-organisms at the same rate between pH 5 and 9. This makes it ideal for disinfection of potable water and process water where the pH is up around 8.0. Chlorine hydrolyzes to hypochlorite ion significantly around pH 8.0 which renders it quite ineffective for disinfection.

Chlorine Dioxide produced by the electrochemical or acid-chlorite processes will not produce any THM’s upon reaction with organic matter. THM’s are regulated in most water supplies so this is an advantage for ClO₂.

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1. C. Ruzic. Chlorine Dioxide based water treatment in the food industry. 1996.