Chlorine Dioxide and its Production

Helmut Knapp
（Alldos Dosiertechnik）

Abstract The negative effects that are produced for the disinfection of drinking water in chlorine are illuminated tersely in this paper. As a more effective disinfectant, the use of Chlorine dioxide has microbiologic advantages, ecological advantages and advantages in use compared with active chlorine products, so it is a excellent substitute for elementary chlorine. Otherwise, this thesis is involved in the theory of the chlorine dioxide-production from the NaClO2 and Cl2, and the material balance of this production theory is calculated in this paper.
Keyword　Chlorine dioxide Disinfectant Substitute for chlorine Theory of production

General

Drinking water is not only H₂O. According to its origin, it contains different quantities of dissolved salts as well as organic compounds, Which are produced during the microbial in the earth. Viruses and bacteria like water, too: the World Health Organization WHO estimates, that microbiologically contaminated drinking water is responsible for around 80% of all illnesses and epidemics in the Third World today.
The disinfection of drinking water consumes less than 0.1% of the chlorine production per year.
Dangerous illness like typhus and cholera have nearly completely disappeared in the industrialized countries. The reason is the consequent disinfection of drinking water: since 1900. The germicidal effect of chlorine is used for this purpose.
The annual chlorine consumption for the drinking water treatment in the Federal Republic is estimated at about 2.000t - not even 0.1% of the total produced quantity - by the Union of Chemical Industry (VCI).
Despite its hydienical success, the chlorination of drinking water has its negative aspects: in an aqueous solution, chorine becomes the instable, reactive hypochlorite (ClO, active chlorine) , which transforms the organic compounds into chlorinated products. About 10% of the added chlorine react in this non-desired way.
Above all, chlorinated an bromed methane derivates are produced, which were first discovered in drinking water in 1974; since the, they have become the subjects of intensive toxicologic studies under the name "trihalomethanes".
Due to these side effects, the search for a disinfection agent for the drinking water treatment, being hygienically as effective as chlorine, but without side products, has become important. One of the most important substitutes for elementary chlorine is chlorine dioxide (ClO₂) - also a gaseous compound, which can explode in a pure form or in high concentrations. Chlorine dioxide is therefore not stored, bus produced before us from sodium chlorite (NaClO₂), and then used as a weak concentrated aqueous solution.
The most important adventage of chlorine dioxide: the formation of trihalomethanes is nearly completely stopped. Not the chlorine, but the oxygene is the most effective part of chlorine "dioxide", explains Oswald Helmling from the Department of Technique of use of Active Oxygene of the Degussa in Hanau. Therefore the gas, which is soluble in water, does not react with ammonia or amino compounds. At the usual chlorination, chloramines are produced, which consume so much active chlorine that the legal content of 0.1mg residual chlorine/l in water cannot always be kept.
Moreover, chlorine dioxide is a more effective disinfectant: Its good bactericide characteristics remain unchanged, also for higher pH-values. Chlorine dioxide suppresses the appearance of smell and taste intensive chlorine phenoles. However, compounds like trichlorchinon, which cannot be registrated by sensors and the risk potential of which is not yet clarified, appear.
An Inconvenient of the chlorine dioxide is its price.
However, the more effective chlorine dioxide can be used in much smaller quantities, so that the total procedure is only 2 or 3 times more expensive.
Chlorine dioxide is a very strong and quick-acting disinfection, oxidation and deodorizing agent. The fields where it is used today are the disinfection of drinking, service, cooling and waste water, treatment of exhaust air, up to the use of chlorine dioxide in brewing water and water for the beverage industry to wash the bottles, container and tank cleaning and disinfection, disinfection of recoolers, pasteurizers.
The use of chlorine dioxide has many adventages compared with other disinfection agents, especially compared with active chlorine products.

1. Microbiologic advantages
-the microbiocid effect is 2 to 3 times stronger than of active chlorine
-the microbiocid efficacy is nearly independent from the pH-value
-the treated water has a better smell, taste and colour
-oxidation of organic combined iron and manganese compounds
-better sporicide, algicide and virucide characteristics than chlorine
-long-lasting bactericide and bacteriostatic net protection.

2. Ecologigal advantages
-no formation of trihalomethan (haloforms) compared with active chlorine or ozone
-reduced formation of higher molecular organ halogen compounds
-no formation of chlorophenole
-no reaction with ammonia and amino compounds
-low waste water pollution due to low concentration of use

3. Advantages in use
-very effective in wide pH-ranges
-independence of the redox potential from the pH-valve
-independence of the redox potential from the presence of ammonia ions and amines in the water
-long-lasting of chlorine dioxide in the water

Theory of the chlorine dioxide-production

1.Adventage of chlorine dioxide vs. Chlorine
-oxidation of chlorine dioxide is 2.6 times higher than chlorine
-avoids the build up of halogens and chlorinamine, which results in the oderless and tastless distraction of microorganism, alga and other organic contamination
-odor-and taste level appr.4 times higher as chlorine
-sterilization time of bacteria/germ is quicker as pH-level increases
2.The production of chlorine dioxide the following method is used:
　　Production of chlorine dioxide form the components of NaCl₂ and chlorine gas (Cl₂) as of formula
2NaClO₂ + Cl₂ = 2ClO₂ + 2NaCl
Sodiumchlorite + Chlorine = Chlordioxide + Sodiumchloride
NaClO₂ is used as an undiluted 24% - solution with a content of 300g NaClO2/l.
The chlorine gas is mixed and injected in a proportion of 3.5g Cl2 per 1 liter of water thru a chlorine gas dosing system where it reacts as follows:
Cl₂ + H₂O HOCl + HCl + H₂O
Gas water chloricacid hydrochloric water
(excess capecity) (low level) acid
designated as chlorine solution
Both out going solutions, NaClO₂ and the chlorine solution are mixed in the proportion of 0.55g Cl to 1g NaClO₂ in the chlorine dioxide reactor which produces the ClO₂-solution. Again diluted it reaches the level controlled storage tank.
The quantitative determination of the reaction shows as follows:
2NaClO₂ + Cl₂ 2ClO₂ + 2NaCl
2＊90.5 = 181g + 71g 135g
Because we mix NaClO2 and Cl2 in the proportion 1:0.55 it shows as:
181g NaClO₂ + 100gCl₂ 135gClO₂ + 29gCl₂ (unused) + NaCl
As a rule of thumb we use the equation in regards to 1 liter discharge solution NaClO₂ (300g NaClO₂)
300gNaClO₂ + 166g Cl₂ 233.7g ClO₂　+　48.1g Cl₂　+　NaCl
135＊300 　29＊300 　48.1
　　　　=223.7 =48.1
　　　 181 181
　　For the oxidation level meaning:
　　233.7＊2.6= 581.6 ClO2 calculated as chlorine
　　+ 48.1 unused chlorine
-----------------------------------------
629.7
that means:
1 liter NaClO₂ with a concentration of 300g NaClO₂/1 and 47.5l chlorine solution with a concentration of 3.5g Cl₂/l results in:
48.5l chlorine-chlorine dioxide solution is able to oxidize the same amount as 629.7g calculated in chlorine.
1 liter of this solution has the reaction power of 13g calculated in chlorine
The 2/3 of the reactor is filled with RASCHIGring.
The size selection of the reactor should allow the solution to stay in the reactor a minimum of 5 minutes.
The ClO₂-solution has a concentration of appr. 4.7 CLO₂/l at the discharge and has to be deluted to 3g ClO₂/l.
This solution reaches a level controlled storage tank and is metered as needed into the system via metering pump.