Chlorine Dioxide Facts

What is Chlorine Dioxide?

Chlorine Dioxide has the chemical formula ClO2 and is a yellow to brown colored 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 ClO2 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.

Electrochemical (Pureline Generator) ClO2 + e ClO2
Acid-Chlorite 5NaClO2 + 4HCl 4ClO2 + 5NaCl + 2H2O
Chlorine-Chlorite Cl2 + H2O HOCl + HCl
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.

Chlorine Dioxide Generation Yield & Running Cost

It is clear from the above reactions that the acid-chlorite reaction can only be maximum 80% efficient in terms of conversion of chlorite to chlorine dioxide. Electrochemical production of chlorine dioxide using the Pureline Chlorine Dioxide generator can produce chlorine dioxide at 99.5% efficiency. This means that the overall running cost of the electrochemical and chlorine-chlorite processes will be lower than the acid-chlorite process.

Chlorine Dioxide Generators – Continuous or Contiguous Generation

The Pureline Chlorine Dioxide generator produces chlorine dioxide in a contiguous manner. Level control in the absorber tank is utilised for the production of chlorine dioxide. When a low level point is achieved, dilution water and chlorine dioxide generation occur together. The resultant chlorine dioxide solution is made and stored in the absorber tank and this stops at high level. Chlorine dioxide solution is dosed from this tank to the dosing points.

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. 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 utilised 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 and the ammonia can be ignored. However, if the aim is to remove ammonia then chlorine should be dosed to achieve breakpoint.

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 ClO2 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 ClO2 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 oxidise the ferrous ion.

Manganese: Manganese is often present in ground water as Mn2+ ion. Chlorine Dioxide ClO2 can be utilised 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 colour 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 ClO2 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 oxidised is therefore limited by the chlorite regulatory limit and the stoichiometry of the reaction.

Manganese is effectively oxidised 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 utilise all the oxidising 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 oxidise cyanide to cyanate ion. Thus, chlorine is preferred over ClO2 as chlorine can oxidise cyanide first to cyanate and then to nitrogen gas and carbonate ion.

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 pm 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 pm 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
Pediococcus Damnosus (yeast) 0.1 5 99.9

The main advantage of using Chlorine Dioxide ClO2 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 hydrolyses 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.