Chlorine Dioxide
Chlorine Dioxide Duty2 – an inorganic chemical compound, chlorine oxide in the IV oxidation state. It is a strong oxidant with great stability. In its pure state or in high concentrations, it decomposes explosively into its elements under the influence of light, compression or after slight heating.
- Physical properties
- Solubility and stability
- Disinfection properties
- Chlorine dioxide - Legionella
- Chlorine dioxide - E.coli bacteria
- Chlorine dioxide durability
- Dose and requirement
- Chlorine dioxide in water treatment
- Reactivity with iron and manganese
- Surface water disinfection
- Final water disinfection
- Tank disinfection
- Sewage disinfection
- Disinfection of hospital wastewater
- Disinfection in the food industry
- Cooling water treatment, biofilm
- Chlorine dioxide production
- Chlorine dioxide generators
- Hydrochloric acid
- Sodium chlorite
- Security and storage
- Chlorine dioxide toxicity
At room temperature, chlorine dioxide (ClO2) is a gas denser than air, yellow-gray in color, highly soluble in water.
Chlorine dioxide (ClO2) is highly soluble in water, more so than chlorine or ozone. The solubility of a gas is expressed by the concentration of the dissolved gas in equilibrium, i.e. between its gaseous and dissolved state. The graphs below show the concentration values of CIO2 at different temperatures and partial pressures. The solubility of CIO2 the potential presence of chlorine in the water has no effect. In the pH range characteristic of drinking water (i.e. 6-8), chlorine dioxide does not hydrolyze, but remains in solution as a dissolved gas.
The presence of hypochlorite ions (CIO) causes chlorine dioxide to decompose at moderately alkaline pH environments. The presence of chlorite ions (CIO2-) has little effect, but in combination with hypochlorite it accelerates the loss of chlorine dioxide. From a practical point of view, these observations are important because the stability of aqueous solutions of chlorine dioxide is related to its purity. Furthermore, the decomposition of chlorine dioxide in aqueous solution is intensified by light and in this case the decomposition products are mainly chloride ions (Cl-) and chlorate ions (CIO3-). Blue and UV light are effective in photodecomposition of chlorine dioxide, where the reaction rate is dependent on the light intensity. Therefore, to maintain the required solution stability, aqueous chlorine dioxide must be stored in closed containers, at low temperature and away from light. Moderate acidification (pH = 6) can facilitate stabilization and prevent decomposition of chlorine dioxide.
Solubility and stability in aqueous solutions - download pdf.
Chlorine dioxide has excellent bactericidal, virucidal, sporicidal and algicidal properties, which is why it is widely used for water disinfection. Unlike chlorine or sodium hypochlorite, the oxidizing and disinfecting properties of chlorine dioxide remain practically unchanged in a wide pH range (from 4 to 10).
The evaluation of the effectiveness of a disinfectant is usually based on the concept of "concentration and duration" [Sxt]. This indicator refers to the concentration of the dosed disinfectant and the contact time needed to destroy selected strains of bacteria or viruses using a given disinfectant. The relationship between the concentration [S] and the contact time [t] is expressed by the following empirical equation:
k = Sn • t
Where:
S - is the concentration of the disinfectant,
n - is the dilution factor,
t - is the contact time required to achieve a specific % deactivation,
k - is a specific constant for each microbial population.
Based on this indicator and acting under specific conditions and on selected microorganisms, it is possible to compare the effectiveness of different disinfectants. Therefore, the lower the value of this indicator in relation to a specific group of bacteria, the more active the given disinfectant is. The table below gives the values of the [Sxt] indicator in relation to the effectiveness of some disinfectants in neutralizing 99% different microorganisms.
The biocidal efficiency of chlorine and sodium hypochlorite decreases rapidly, moving from pH 7, where the dominant form is hydrochloric acid (HClO) to pH 9, where the dominant form is hypochlorite ion (ClO-). In the same pH range, the biocidal efficiency of chlorine dioxide remains constant. Due to its stability and durability over time, chlorine dioxide is particularly used for final disinfection of treated water, i.e. before it is introduced into retention tanks or networks. The main advantage in this case is the longest duration of action of this agent, allowing for disinfection of network ends and not impairing the taste and smell of disinfected water. Such disinfection can be achieved thanks to the sequential action of chlorine dioxide (an effective bactericidal agent) and chlorite (bacteriostatic and slightly biocidal). The table below presents the biocidal efficiency, stability and the effect of pH on the efficiency of some disinfectants.
The mechanism by which chlorine dioxide inactivates microorganisms is not yet fully understood, but has been and is the subject of two types of research. On the one hand, the chemical reactions of CIO are studied2 with the molecular components of microbial cells, and on the other hand the influence of CIO is investigated2 on their physiological functions. Studies have shown rapid reactivity of CIO2 with some amino acids (such as cysteine, tryptophan, and tyrosine) but not with ribonucleic acid (RNA) in viruses. The conclusions of these studies are that inactivation of viruses by CIO2 is caused by a change in the protein content of the viral capsids. Other studies, however, indicate a CIO reaction2 from the Poliovirus RNA to such an extent as to damage the synthesis of the RNA itself. Other researchers, however, found that CIO2 is reactive with fatty acids in the cytoplasmic membrane. Studies have not yet clarified whether the primary action of CIO2 takes place at the level of peripheral structures (cell membranes) or internal structures (nucleus, mitochondria). However, it is reasonable to assume that both of these actions contribute to the inactivation of microorganisms. In any case, the action at the level of peripheral structures (changes in proteins and lipids of the cell membrane) would cause an increase in the permeability of the membrane itself, while the action at the level of internal structures would lead to changes in protein synthesis and/or respiratory activity. In any case, the actions listed above ultimately lead to cell death.
Disinfection properties of chlorine dioxide - download pdf.
Chlorine dioxide is the only disinfectant that effectively eliminates Legionella bacteria. Legionella can occur anywhere, but its largest concentrations are found in hot water installations in public buildings (including hospitals, nursing homes, hotels, etc.). In addition, 90% of Legionella bacteria is found in the biofilm mucus, and only 10% in water. Biofilm protects microorganisms from the action of disinfectants. In order to completely eliminate Legionella germs, the formation of biofilm covering pipelines and technological devices must be removed and prevented, and chlorine dioxide is excellent at this. In addition, constant dosing of the appropriate amount of chlorine dioxide to water protects water installations from the development of Legionella bacteria and its secondary contamination.
Disinfection properties of chlorine dioxide - Legionella - download pdf.
The graph below shows the effect of some disinfectants on the destruction of 99% E. coli strains at a water temperature of 150C. From the graphs it can be seen that chlorine and sodium hypochlorite seem to be more effective than chlorine dioxide in inactivating E. coli but this is only true when the water being disinfected has a p lower than 7. At pH above 7, when chlorine and sodium hypochlorite are present as hypochlorite ion (ClO-) and chloramines (NHCl2, NH2Cl), the disinfection capacity of chlorine dioxide is higher.
Disinfecting properties of chlorine dioxide - E.coli bacteria - download pdf.
Because chlorine dioxide is a relatively unstable gas that becomes explosive when its concentration in air is greater than 10% by volume, the gas cannot be compressed and liquefied and therefore must be generated "in situ" at the dosing site and then dissolved in water.
Chlorine dioxide solutions at concentrations of 30 g/l or more are dangerous. Solutions diluted to 3 g ClO2/l are completely safe and stable and can therefore be used in the disinfection process. Factors that can cause a change in the stability of these solutions are pH, contaminants present, heat and light.
Whenever chlorine dioxide is used, as well as when treating with chlorine or sodium hypochlorite, it is important to determine the "water requirement for a given disinfectant" before starting dosing.
The demand for chlorine dioxide means nothing more than the amount of chlorine dioxide that reacts with the tested water in a specified time (from 5 to 60 minutes, depending on the contact time in the installation). This value is commonly referred to as the so-called "breakthrough point" and is a reference area for further applications of chlorine dioxide, because it is, in a sense, an assessment of the quality of the disinfected water.
In relation to the dosing point, a distinction is made between lower, equal and slightly higher dosages than the demand itself. Lower dosage is used for sewage, where 20-30% of the demand is usually sufficient to achieve the required bacteriological level. Dosage equal to the breakpoint is used in drinking water treatment during pre-oxidation. Dosage with excess is used for final disinfection of drinking water.
It should be remembered that this demand refers to the total chlorine dioxide demand of the water intended for treatment and therefore includes the amount of CIO2 consumed in reaction with the microorganisms present and the amount of CIO2 consumed in reaction with previous chemicals. For the same reason, in order to achieve the correct use of CIO2 and achieve the intended goal, it is necessary to carry out other complementary analyses (for example, some microbiological analyses or actual tests carried out directly on a given object).
Chlorine dioxide can be used in drinking water treatment as a disinfectant or as an oxidizer. As a disinfectant, it can be used both in the preliminary and final phase.
In pre-oxidation of surface water, chlorine dioxide is used to prevent the growth of bacteria and algae in subsequent treatment phases. The use of chlorine dioxide in this phase, in contrast to chlorine or sodium hypochlorite, has the advantage that it significantly reduces the formation of halogenated organic compounds, including trihalomethanes (THM). The possibility of THM formation is higher in the case of surface water containing high levels of organic precursors. In addition, in the pre-oxidation phase, chlorine dioxide oxidizes colloidal substances, thus improving the coagulation process that reduces water turbidity. The effect of chlorine dioxide in the pre-oxidation process is primarily related to organic matter, present in both dissolved and colloidal form. However, in the case of chlorine dioxide, pH and temperature are of no importance, which can strongly affect other pre-oxidation processes (a classic case is sodium hypochlorite). Any residual chlorine dioxide can be easily removed by successive passes through granular activated carbon filters.
Chlorine dioxide as an oxidant in water treatment is used for: removing iron and manganese from water, reducing turbidity and color of water, removing odors and tastes, controlling algae growth and removing some pesticides from water. The mechanism of action of chlorine dioxide in this area is as follows:
- iron and manganese are present in water in reduced form or in complexes with organic substances (especially humic and fulvic acid) and are oxidized to hydroxides, which are retained on the filter media by physical precipitation. In this case, chlorine dioxide is significantly more effective than chlorine or sodium hypochlorite, because at pH > 7 the reaction rate of chlorine dioxide with respect to manganese is very fast. Moreover, in contrast to chlorine and sodium hypochlorite, the oxidation reaction is not associated with a significant reduction in water alkalinity and a resulting change in the calcium-carbonate balance in the treated water.
- water turbidity is related to the presence of colloidal particles in the suspension, the elimination of which requires the administration of coagulants. In this phase of treatment, the action of chlorine dioxide is helpful through its oxidizing effect on the substances covering the colloids that keep them in suspension and after the addition of chlorine dioxide, the process of supporting the formation of flocs takes place.
- the presence of odours and tastes in water is caused by numerous compounds: metabolites of organisms (algae, actinomyocytes, etc.) present in surface waters; phenols, originating from industrial pollution or resulting from the decomposition of algae, or occurring with chloramines formed in cases where water was pre-oxidised with chlorine or sodium hypochlorite; chlorides and bromides present in groundwater contaminated with e.g. seawater; hydrocarbons.
- oxidizing, bactericidal, fungicidal and algicidal action of CIO2 makes it used to improve the organoleptic properties of water without the formation of chlorophenols and chloramines. However, it should be remembered that it is non-reactive with some hydrocarbons, which may make it ineffective in removing some odors.
- the presence of algae gives the water an unpleasant smell, taste and colour, makes it difficult to remove turbidity and can block or contaminate the distribution system or sand filters. Chlorine dioxide is effective in this case too, because it has the ability to pyroliferously attack the chain rings of chlorophyll. Therefore, when using chlorine dioxide for this purpose, a dose of 0.5 ÷ 1.0 mg/l is used when adding to water stored in the tank (preferably at night to avoid its degradation under the influence of sunlight).
- pesticides that can be removed by CIO2 are DMDT (dimethoxodichlor) and aldrin. Herbicides such as paraquat and diquat are eliminated within minutes, but at pH above 8. For pre-oxidation and reduction of organic pollutants, the required doses are from 0.5 to 2.0 mg/l, with contact time depending on water quality usually from 15 to 30 minutes
As indicated by the redox potentials of Fe3+ / Fe2+ and Me4+ / Mn2+, Fe ions2+ and Me4+ are oxidized by chlorine dioxide to form iron hydroxide and manganese dioxide, respectively, which, being poorly soluble or precipitated, will settle on the solid elements of a given system.
Fe3+ + e- ↔ Fe2+ E0 = 0.77V
Mn3+ + 2e- ↔ Mn2+ E0 = 0.37V
Oxidation-reduction reactions at pH 7 are as follows:
Duty2 + Fe2+ → Fe3+ +ClO2-
Fe3+ +3OH- → Fe(OH)3
2ClO2 + Me2+ +2H2O → 2ClO2- + MnO2 +4H+
At pH greater than 7, full reduction to Cl occurs.-
Theoretical ClO2 consumption is:
1.2 mg ClO2 per mg Fe2+
1.45 mg ClO2 per mg Mn2+
The advantage of chlorine dioxide is that its degree of oxidation is higher than that of chlorine or sodium hypochlorite. The ability of chlorine dioxide to oxidize iron and manganese, and thus to extract them from water by precipitation, is used in particular in the treatment of water intended for human consumption.
Reactivity of chlorine dioxide with iron and manganese - download pdf.
At the water treatment plant, which takes surface water from a mountain river, tests were carried out to assess the possibility of replacing the current method of disinfection with sodium hypochlorite in favor of chlorine dioxide. Current disinfection is carried out both in the process of pre-oxidation and in the final disinfection.
Problems associated with disinfection using sodium hypochlorite were mainly caused by excessive formation of chlorinated organic compounds, which exceeded the permissible limits of 50 µg AOX/I, partly due to pre-oxidation (dosage of 3.0 mgCl2/l) and partly due to final disinfection (dose 1.2 mgCl2 /l).
The chemical and microbiological properties of water at the station inlet are as follows:
Chemical and microbiological properties of water treated with sodium hypochlorite in pre-oxidation and final disinfection:
Water disinfection with chlorine dioxide was carried out on raw water with an hour's contact time and at doses of 2.05, 1.74, 1.43 and 1.02 mg/l, respectively, corresponding to 100, 85, 70 and 50% of water demand for CIO.2. The results are presented below:
Comparing the results of final water disinfection obtained using sodium hypochlorite and chlorine dioxide, the following conclusions can be drawn:
- after disinfection with hypochlorite, there is an increased formation of AOX, with values significantly exceeding the permissible values;
- Disinfection with chlorine dioxide is more effective than sodium hypochlorite because it reduces microbiological parameters more effectively and at the same time does not create excessive disinfection by-products.
- in the analyzed case, the dose of chlorine dioxide should be: about 1.5 mg ClO2/l during pre-oxidation and 0.4 mg ClO2/l for final disinfection.
Chlorine dioxide in the process of disinfection of surface (river) water - download pdf,
Chlorine dioxide can be used in drinking water treatment as a disinfectant or as an oxidizer. As a disinfectant, it can be used both in the preliminary and final phase.
In final disinfection, chlorine dioxide has a dual effect: bactericidal and virucidal in the form of CIO2 and bactericidal and weakly bactericidal in the form of chlorite (CIO2-). As a bactericidal agent, it can remain active in water for at least 48 hours, and its effectiveness is guaranteed for a much longer period than chlorine or sodium hypochlorite. In this way, the use of chlorine dioxide in this phase can guarantee the inhibition of bacterial regrowth in the event of secondary contamination in the water supply network. In addition, in relation to potential viral contamination, the virucidal and sporicidal properties of chlorine dioxide are much better than those of chlorine or sodium hypochlorite. In the final disinfection, a dose of 0.20 ÷ 0.40 mgCIO is used2/l, because it does not cause the sum of chlorites and chlorates to exceed the normative values. These values are closely related to the nature of the water supply network, which in some cases may require further chlorination.
Chlorine dioxide in final water disinfection - download pdf.,
Chlorine dioxide has much better properties for disinfecting drinking water reservoirs than sodium hypochlorite or chlorine gas. In the case of retention reservoirs, it is important to determine the required dose of disinfectant, taking into account the quality of the retained water and the theoretical time of water retention in the reservoir. For this purpose, water tests were performed to find the optimal dose of CIO2 in relation to the chemical and microbiological parameters of water related to the disinfection process. The tests were conducted for chlorine dioxide and sodium hypochlorite (chlorine). The tests were not conducted for ozone due to its short-term effectiveness period (up to 15 minutes) in relation to the variable and long-term water retention period (even up to 48 hours) combined with the typical lack of uniform water retention time (formation of so-called distribution channels and water stagnation (dead zones)). Three different doses of chlorine dioxide were tested, in relation to the percentage demand and after a 15-minute contact time. The results were compared with 1 mg of chlorine / sodium hypochlorite.
The relevant data is included in the table below:
It is noteworthy that AOX values increased significantly (55 % increase) after disinfection with chlorine/sodium hypochlorite, whereas after disinfection with chlorine dioxide the AOX content increased only by 10 %. Similar trends were also observed for THMs.
Chlorine dioxide in the process of tank disinfection - download pdf,
The main applications of chlorine dioxide in this field are:
- disinfection of sewage before discharge or sending to water recycling,
- removal of odors produced in anaerobic conditions,
- improving the sedimentation rate of sludge in activated sludge processes,
- removal of contaminants such as tetraethyl lead, cyanides, nitrites, sulphides, aromatic hydrocarbons, phenols and the like.
Wastewater disinfection is a stage of the treatment process that will become increasingly important, especially in the case of reuse of treated wastewater. Traditional wastewater treatment processes do not completely eliminate the risk of microbiological contamination of water supplied by sewage discharged from treatment plants.
In the case of discharges of sewage into bathing waters, the European Community Regulation (EC Directive No. 76/169) provides for a maximum of 2,000 colonies of coliform bacteria per 100 ml and a corresponding 100 units of fecal coliform bacteria per 100 ml. In the case of the method based on chlorine and sodium hypochlorite, the presence of significant amounts of ammonia and organic substances in the sewage causes a significant consumption of the disinfectant with the simultaneous formation of chloramines (which have 80 times less bactericidal effect compared to free chlorine). In addition, as a result of the reaction with organic substances, chlorine and sodium hypochlorite produce halogenated organic compounds (AOX), which can accumulate in the environment and pollute water. This situation can have very serious consequences in cases where the purified water is reused for irrigation purposes. Chlorine dioxide does not react with ammonia, forms limited amounts of organic halogenated compounds in the presence of organic substances, oxidizes phenols and is active over a wide pH range. It forms an exchangeable residue that can be used for automatic dosing and usually does not require a subsequent dechlorination phase (e.g. with sodium bisulfite).
Studies conducted by the US Environmental Protection Agency (US-EPA) on wastewater from secondary settling tanks (filtered and unfiltered at the tertiary stage) have shown that after a 60-minute contact time, the doses of chlorine dioxide required to achieve the same degree of destruction of all coliform bacteria are 2 to 70 times lower than those required for chlorine or sodium hypochlorite.
Moreover, laboratory tests conducted by the US Environmental Protection Agency (US-EPA) on municipal wastewater show that wastewater disinfected with chlorine dioxide does not contain measurable amounts of trichloromethanes and the content of chlorinated organic compounds is 10 to 20 times lower compared to the same dose of chlorine or sodium hypochlorite.
Disinfection of sewage with chlorine dioxide - download pdf,
The bactericidal and virucidal effectiveness of chlorine dioxide recommends it for disinfection of hospital sewage. Laboratory tests conducted on sewage samples from the infectious diseases department of the city hospital confirm the very high effectiveness of chlorine dioxide, with the simultaneous occurrence of very small formations of halogenated organic by-products (AOX). Virucidal activity was assessed by contaminating untreated sewage with a poliovirus vaccine type 1 (viremia 200,000 TCID50) and exposing it to increasingly larger doses of chlorine dioxide (5, 10 and 15 mg/l) for 30 minutes. A high percentage of virus inactivation was recorded already at a dose of 5 mgCIO2/la complete annihilation occurred at a dose of 10 mgCIO2/I. Due to the high chemical and biological variability of sewage and the different treatment options that sewage can undergo, the dose of chlorine dioxide can vary significantly. The basic parameters that differentiate the dose include: suspended solids, bacterial content, organic carbon content, temperature and pH. In sewage that has undergone a tertiary treatment process and has a dissolved organic carbon and suspended solids content below 10 mg/l, with appropriate mixing, a dose of 1.5 - 2.0 mgCIO is generally necessary2/l with a contact time even shorter than 15 minutes. In these conditions, the operating dose is about 20-30% of the total CIO requirement2. In the case of hospital sewage, which may contain a large number of microorganisms responsible for serious infections, the dose should be about 10 mgClO2/l.
Disinfection of hospital wastewater with chlorine dioxide - download pdf,
Chlorine dioxide is successfully used in many applications in the food industry, and in particular in the following areas of activity and operations:
- washing and transporting fruit and vegetables and processing fish and meat;
- disinfection of cooling water;
- washing food and beverage containers;
- frozen food production;
- beer production.
Washing and transporting fruits and vegetables and meat production
In the storage and processing of fruits and vegetables, products are transported from the unloading point to the processing plant by a water jet. Chlorine dioxide is dosed into the transport water to control microorganisms present on the surface of the products, reduce potential contamination in the plant, and reduce the time and costs associated with cleaning the transport channels. Treating the transport water with chlorine dioxide instead of chlorine or sodium hypochlorite has the advantage that it does not create chloramines or other undesirable by-products that may be hazardous to human health. In addition, chloramines can cause organoleptic changes in the product (unpleasant tastes and odors).
For this type of application, the dose of CIO2 depends largely on the type of food and its associated level of microbiological contamination. It generally ranges from 2 ppm (to ensure a residual chlorine dioxide level of around 0.5 ppm) to 5 ppm, e.g. for shrimp processing and up to 9 ppm for chicken processing. At a much higher dose, chlorine dioxide can also be used to treat water used for washing, transporting, processing and sorting fresh produce before it is sold (100 - 150 mgClO2/l) and for initial regeneration (200 - 250 mgClO2/l), which is intended to slow down the processes of maturation/growth and development of pathogens (especially fungi).
Cooling water disinfection
Satisfactory results have also been obtained with the use of chlorine dioxide for disinfecting cooling water for food containers. In fact, it is important that this water is used without microorganisms in order to avoid possible further bacterial contamination of the food and, consequently, a deterioration in the quality of the product and possible microbiological poisoning and/or infection.
Moreover, it is useful in this case to use cooling water free from microorganisms. For this application, a dosage of 1.2-1.5 ppm of chlorine dioxide is recommended, or one that will leave a residue of between 0.2 and 0.5 ppm in any case.
Washing food and beverage containers
The quality of a food product (liquid or solid) can be affected by improper washing and disinfection of the containers in which the product will be stored. Washing containers is particularly important in cases where they are to contain products that are easily biodegradable (such as milk, beer, fruit juices and soft drinks). For example, in the washing process of glass bottles, a second rinse is provided, which takes place in ideal conditions for the development of bacteria (temperature around 30 - 40 ° C, high humidity and pH around 9). For this reason, microbiological control must be ensured by using disinfection of the packaging. Disinfection with chlorine dioxide is better than disinfection with hypochlorite, because chlorine dioxide is still active at high pH and does not form chlorophenols or organic compounds with halogen action. Containers are disinfected with chlorine dioxide at a concentration of 0.5 to 2 ppm, depending on the properties of the water, but in any case it is important to have about 0.2 ppm of its residue. When disinfecting with chlorine dioxide, a very short contact time is sufficient (even less than one minute).
Frozen food production
In addition to purifying water used for transport and washing, chlorine dioxide can be used in the frozen food production process to disinfect water used, for example, for freezing. This process is used after processing the final product (e.g. washing, slicing, chopping, etc.). In fact, if a given water circuit is not disinfected, microbiological contamination can occur, most often by bacteria of the type Listeria, because they survive even at low temperatures. To avoid this problem, the water in the freezer is disinfected with chlorine dioxide to ensure 0.2 - 0.5 ppm of its residue.
Beer production
Brewing requires 6-10 liters of water per liter of beer produced. The water that is part of the final product must of course be free of microorganisms and cannot give the beer any smells or flavors that change its taste. Chlorine dioxide-based treatment also has several advantages in this case, because it does not produce chlorophenols and only a negligible amount of chlorinated organic compounds, moreover, it does not affect the taste of the beer and guarantees the elimination of microorganisms even in a wide pH range (6÷9). For this application, the dose can vary from 0.05 to 0.5 ppm.
In the food industry, integrated use of chlorine dioxide as well as its convection is possible in all operations where water is used.
- water that is feedwater (primary water) and/or intended for human consumption (if it is taken from a well and not distributed through the water supply system);
- general washing water (washing or transport water);
- cooling water (in the tomato preservation industry (evaporators, towers), in breweries, cheese factories and dairies, in the cooked meat processing industry, in canning factories where food is preserved after thermal sterilization);
- water used for processing (for example in breweries, shrimp and chicken processing);
- sewage (for disinfection before discharge into the environment).
Disinfection with chlorine dioxide in the food industry - download pdf,
Complex fouling deposits, which may contain microbiological and macrobiological substances, inorganic particles and corrosion products, pose a significant problem in relation to the performance of cooling water systems, heat exchangers and piping systems. The first stage of fouling is the uncontrolled growth of microorganisms on surfaces, with the initial formation of a biofilm as a result of the vital processes of living cells and their metabolic residues, resulting in the formation of mucus.
The mechanism of biofilm (membrane of biological origin) formation can be summarized as follows:
- surfaces are covered with primary colonizing bacteria and other organisms;
- In the transitional stage, multilayered cells develop, settling into their polymeric material;
- In the final stage, the population density increases rapidly, leading to the formation of a biofilm.
Biofilm is a substrate leading to the deposition and adhesion of other biological and inorganic materials. However, the term "fouling" is used to refer to the final mixture of biofilm, suspension of solids, corrosion products and macroorganisms that then adhere and grow on surfaces. Fouling reaches its maximum development after the attachment of marine or marine-derived organisms. In general, we distinguish between macrofouling and microfouling. Macrofouling refers to the growth of crustaceans (Barnacles), molluscs (Mussels and Clams) and coelenterates (Hydroids), while micro-fouling involves algae and bacteria.
Fouling is harmful and causes:
- higher operating costs, because lower heat transfer causes production losses and increased flow resistance requires more energy for pumping;
- higher maintenance costs in the case of cleaning operations or replacement of pipes damaged due to corrosion under growth or overheating,
- shorter operating times because longer downtimes are required to clean or repair equipment.
The following water parameters are also important in the fouling process:
- temperature - the growth rate of microorganisms depends on seasonal temperature fluctuations;
- dissolved gas - oxygen content affects the growth of many species of aquatic organisms;
- availability of nutrients (phosphorus and nitrogen) - these are the basic elements necessary for biosynthesis;
- pH and suspended solids.
The growth of biofouling can be partially prevented in the design phase by using suitable materials (e.g. copper, AISI 316 stainless steel or surface treatment with special polymers) and by dimensioning the pipes so that a flow velocity of more than 1 m/s is achieved, which will make it difficult for organisms to adhere. In addition, there are physical and chemical methods for preventing fouling in cooling water systems. Physical treatments are mainly used during downtimes. Chemical treatment is based on non-oxidizing biocides or on oxidizing biocides, such as chlorine gas, sodium hypochlorite or chlorine dioxide. Compliance with regulations on the quality of discharged water and the need to use safe biocides led to the selection of chlorine dioxide as the biocide for cooling systems in large plants. Sodium hypochlorite or chlorine is most effective in combating fouling in waters with a high organic content, but it leads to the formation of halogenated organic compounds (in particular trichloromethanes), which are released into the environment. Sometimes the amount of chlorine or hypochlorite required to keep the system clean is so great that it requires the use of an additional reducing agent (to lower the residual chlorine value at discharge). This situation is much less severe when using chlorine dioxide.
Case 1
In a steel mill, 120,000 m3 is taken3/h of eutrophic seawater from an area with limited water exchange with the open sea. After use, the water is discharged to another, larger seawater reservoir, where mussels are bred. The input water is highly biologically polluted. It has a pH in the range of 8.1 - 8.4 at an average temperature of 27°C in summer and 15°C in winter. Systematic measurements are used to determine the biofouling index, which shows that the average fouling value is 65 kg/m2 per year, but in spring and summer this value reaches 10-15 kg/m2 monthly. Feed water demand for CIO2 ranges from 1.2 to 1.8 mg/l. After the tests began, continuous dosing of CIO was used2 at 0.5 ppm, with careful monitoring of biomass and visual inspection by cameras and divers. Chlorine dioxide dosing limited biomass growth to such an extent that the dosage could be reduced to 0.5 mgClO2/l for 10 hours a day in winter and 14 hours a day in summer. This treatment, apart from being effective, has proven to be friendly to materials and the environment.
Case 2
Chlorine dioxide was used to control the development of macro-growth (equivalent to 25 kg/m2 per year) in the cooling water of one of the power plants with the capacity of 1,260 MWe. This power plant requires 120,000 m3/h of seawater. Water disinfection with chlorine dioxide was used continuously at doses from 0.1 to 0.2 mgClO2/l CIO2, which corresponds to an average of 0.13 mgClO2/l per year. In this facility, experiments were conducted to obtain approximate indications for the use of CIO doses2. The growth of fouling species was inhibited by continuous dosing of 0.07 ppm CIO2 in the winter months (December - April) and 0.18 ppm in the summer months (May - November). Due to the low formation of CIO degradation by-products2, which were controlled in all phases of the experiment, chlorine dioxide proved to be the most effective in combating these pollutants.
Cooling water treatment with chlorine dioxide, biofilm - download pdf.
Chlorine dioxide can be produced in generators, directly at the place of its dosing. The disinfectant production process is not complicated and consists of proportional mixing of sodium chlorite (7.5%) and hydrochloric acid (9%) solutions. This process is most often used in the water treatment industry due to its reliability and the availability of products from which chlorine dioxide is produced. It meets the most diverse requirements in terms of use, safety, reliability and ease of dosing into water.
The preparation of chlorine dioxide is done by acidifying chlorite in the following reaction:
5ClO2- +4H3O+ → 4ClO2 +Cl + 6H2ABOUT
Cl ion introduction- to the system using hydrochloric acid is easier than with other acids. The most common method of producing chlorine dioxide is to use sodium chlorite and hydrochloric acid in the following reaction:
5 NaClO2 + 4 HCl → 4 ClO2 + 5 NaCl + 2 H2ABOUT
The resulting chlorine dioxide solution may therefore contain chlorine and chlorates, in addition to the expected chlorides. Appropriate operating conditions enable the production of CIO solutions2 without Cl and in small amounts (less than 5 %) CIO3-.
When the appropriate conditions are met, it is possible to obtain high efficiency of chlorine dioxide production (close to the theoretical - 4 moles of CIO2 per 5 moles of NaCIO2). The main factors include:
- the use of an excess of HCl, which in practice means using almost the same amount by weight of sodium chlorite and hydrochloric acid (hydrochloric acid in an amount exceeding 300% in relation to the stoichiometric amount);
- maximum reduction in ClO concentration2 in the produced solution;
- control of pH, which must be kept below 0.5 (at pH = 1 the reaction is very slow and chlorite is only partially converted);
- using softened water to dilute the chlorite (avoid the precipitation of carbonates, which slow down the reaction);
- ensuring adequate mixing is essential for good production efficiency.
In addition, the reaction rate of chlorine dioxide production is also affected by temperature. In the case of CIO production2 At low concentrations, the reaction rate can be significantly reduced (by a factor of 3) if the system temperature is reduced from 20°C to 10°C.
Production of chlorine dioxide from sodium chlorite and hydrochloric acid - download pdf.
The chlorine dioxide generation process begins with the mixing of sodium chlorite and hydrochloric acid solutions at two different concentrations. The choice of the concentration of reagents to be used is a function of the hourly throughput required. Dilute reagents are usually used to obtain throughputs of less than 400 gClO2/h, while concentrated reagents are used to obtain higher efficiency (using diluted reagents would require larger quantities and would involve high storage and transportation costs). In the case of concentrated reagents, 25% sodium chlorite and 33% hydrochloric acid solutions are used, while in the case of diluted reagents, 7.5% sodium chlorite and 9% hydrochloric acid solutions are used. It should be remembered that the concentration of the reagents in the reaction (production) phase is the same in both cases, because each time concentrated reagents are used, the generator is equipped with a device for their dilution.
Due to the greater commonness of the diluted method, the following description of operation will refer to this type of generator. The optimal production process takes place when the following technical conditions are met:
- the generation of chlorine dioxide takes place inside a reaction chamber designed to withstand a pressure of 10 bar, of sufficient volume to ensure a minimum reaction time of 10 minutes;
- reagents are sucked in by independent dosing pumps;
- the chlorine dioxide solution emerging from the reaction chamber at a concentration of approximately 20 g/l is then diluted to a concentration below 2 g/l by an additional flow of water. The dilution process of the produced chlorine dioxide can take place both inside and outside the chamber;
- the amount of chlorine dioxide produced depends on the amount of reagents used in the process, and therefore on the pump pulsation frequency;
- the frequency or efficiency of the chlorine dioxide production process depends on the level of the produced ClO solution in the tank2 or the signal from the flowmeter (a rigid system with small performance differences caused by the required response time);
- generators must be equipped with operational fault detection systems that shut down chlorine dioxide production when safety conditions are not met;
- Generators should absolutely be equipped with flow meters, not flow sensors. The flow sensor only ensures the presence of flow, not the magnitude of its intensity.
The "pump" system is often used in small and medium-sized generators, where the use of metering pumps provides more precise measurement. This system is more stable over time and is less susceptible to temperature fluctuations and to solid contaminants that may be present in the reagents. Changes in the temperature of the reagents determine changes in their density and viscosity. Given the fact that metering takes place in a constant volume, the production of ClO2 there may be changes in performance due to the fact that the flow meter reading is affected by temperature. The error of indication for a 10°C temperature change is approximately 7.5% (flow meters are usually calibrated at 20°C). These changes can be corrected by manual changes to the settings.
Chlorine dioxide generators - main parameters - download pdf.
Hydrochloric acid is a liquid that evaporates when present in concentrations above 20% by volume. It is a strong acid that attacks most metals with the release of hydrogen. In addition to the individual protection of those handling it - appropriate gloves, footwear and masks - a shower should be provided for washing in the event of a spill or overflow near the storage tank.
Hydrochloric acid, supplied in bulk, is usually discharged by means of a centrifugal pump made of plastic with a simple external mechanism or a pendulum magnet. Particular attention should be paid to securing the pump against "dry" running (level gauges, pressure gauges and ammeters with minimum level contact) and against operation with a closed outlet. Hydrochloric acid pipes may be made of plastic, as may valves. It is preferable that pressure-critical or impact-hazardous pipes, as well as connecting pipes, are made of glass-fibre reinforced plastic, or, better, of plastic-lined steel.
HCl added to sodium chlorite in concentrated solution causes immediate release of chlorine dioxide, which if not vented can cause tank failure. The risk of such accidents is usually related to improper exchange of nozzles in storage tanks during unloading of reagents. Therefore, it is recommended to use two different diameters of unloading nozzles and control devices such as a pH meter with an alarm that switches off the delivery pump.
Storage tanks can be made of bisphenol or polyvinyl chloride (PRFV) type, which is an aging material and should be replaced every 10 years.
For small storage facilities, polyethylene (PE) can be used with better results, while for large tanks with a capacity of more than 20 m33 Polyester Reinforced PVC (PVC + PRFV) and Rubber Lined Steel are recommended. The storage tank must be equipped with an overflow pipe that also acts as a vent and level gauge to safely handle filling operations. The HCI saturated gas must be first flushed before unloading.
The HCl tank must be placed in a protective tank with a volume equal to the volume of the tank itself plus 10 % and lined with an acid-resistant material (rubber-based bitumen, tiles or polyester).
This product is usually sold in the form of an aqueous solution in the form of a light yellow and clear liquid with a slight odor of chlorine. In smaller quantities, sodium chlorite can be in solid form in the form of white crystals. Solid sodium chlorite is an oxidizer and therefore must not come into contact with organic materials that are easily flammable, such as rubber, paper, straw or wood. When sodium chlorite is mixed with acids, CIO is formed2, therefore, the required safety aspects of use and storage should be taken into account. For safety reasons, it is recommended to use sodium chlorite solutions instead of its solid form. Sodium chlorite solution is easier to use, does not emit irritating dust and is free from the risk of errors in preparation for use. In addition, chlorite solutions are not classified as "oxidizers". On the other hand, such solutions can crystallize at low temperatures, e.g. below -3°C.
Materials that are compatible with sodium chlorite in solution are: PVC, polyethylene, bisphenol polyester, vinyl ester, AISI 316 stainless steel or preferably 316L, as chlorite can cause corrosion at welds.
Storage tanks may be made of all of the above materials, with the exception that for large tanks it is more economical to use bisphenol polyester, painted externally to avoid quality deterioration. When using sodium chlorite, contact with iron, copper and its alloys, aluminum and natural or synthetic rubber should be strictly avoided. The design properties of the protective tray and storage tank are the same as for hydrochloric acid, with the exception of the acid-resistant lining.
Remember to wear protective equipment, such as gloves, goggles, and appropriate protective clothing. Sodium chlorite application areas must be equipped with running water for immediate cleaning in case of accidental contact with the solution.
Chlorine dioxide is produced and used in the form of an aqueous solution. The safety problems are actually related to the explosive properties of chlorine dioxide when its concentration in air is greater than 10% by volume. For this reason, every effort must be made to prevent the formation of any gas pockets. The graph below shows the correlation between the concentration of the chlorine dioxide solution and the percentage of chlorine dioxide in air at different temperatures.
Therefore, for safety reasons, generating equipment is designed to operate in conditions below the risk level. CIO concentration2 in the solution at the generator outlet cannot exceed 28 ÷ 30 g/l, which means that generators using sodium chlorite and hydrochloric acid must be equipped with an appropriate dilution circuit at the reactor inlet so that the CIO concentration at the generator outlet2 does not exceed 20 g/l. Water for diluting sodium chlorite must be demineralized or softened to avoid precipitation of calcium carbonate. All generators must be equipped with flow meters for measuring reagents and their dilution water. These devices must be capable of switching off the generator in the event of a fault (e.g. lack of one of the reagents).
The gas produced in aqueous solution has a strong chlorine-like odor and can be easily evaporated and is detectable in air at concentrations of 1.4 to 1.7 ppm.
CIO concentrations2 in the air around 4.5 ppm can cause respiratory irritation, while prolonged inhalation can cause pulmonary edema. The occupational exposure safety limit is 0.1 ppm. Chlorine dioxide generation and distribution devices must be installed only in places equipped with adequate ventilation. Additionally, it is recommended to use a ClO analyzer2 detecting excessive levels of this gas in the air.
As a safeguard in the event of a leak, a mask equipped with an activated carbon filter to protect the respiratory tract must be available near the generator.
Materials that are to be in contact with concentrated chlorine dioxide solutions should be made of Teflon, polyvinylidene fluoride (PVDF) and polyvinyl chloride (PVCu).
The oxidizing action of chlorine dioxide leads to the formation of chlorites, chlorides and small amounts of chlorates. The amount of chlorites produced usually reaches a value of about 60-70% of the chlorine dioxide consumed, i.e. 0.6 - 0.7 mg CIO2- per mg of CIO used2. Under appropriate conditions of use of chlorine dioxide, the predominant reaction is partial reduction of CIO2 to chlorine, an intermediate step in the reduction of chlorine dioxide. Chlorates can be formed by the oxidation of hypochlorous acid (HClO) from chlorite, which in turn is the result of the ClO reaction2 with some organic substances, according to the following reaction:
HClO + ClO2- +OH- → ClO3- +Cl- H2ABOUT
In addition, small amounts of hypochlorous acid in the presence of organic substances naturally occurring in water (e.g. humic acids) cause the formation of very small amounts of total organic halides (TOX). The presence of chlorates is related to the efficiency and method of chlorine dioxide production and potential photolysis upon exposure to sunlight.
Currently available toxicological studies indicate that doses of chlorine dioxide, chlorites (CIO2-) and chlorates (CIO3-) used in water treatment do not pose any health hazard. The results of clinical and biochemical studies conducted in the United States on the effects of chlorine dioxide consumed through regular water consumption indicate that the threshold of chlorite concentration above which there may be some effect on health is equal to 24 ppm for healthy individuals and 5 ppm for individuals affected by G6PD (glucose-6-phosphate dehydrogenase) enzyme deficiency. Toxicological studies conducted on animals have shown that the concentration of chlorite at which hemolytic stress begins to occur is 250 ppm. In Poland, the Regulation of the Minister of Health "On the quality of water intended for human consumption" does not specify the maximum concentration of chlorine dioxide in water, but the maximum value of the sum of chlorites and chlorates is specified, which is 0.7 mg/l (measured at the consumer's point of intake).
When disinfecting water with chlorine dioxide, attention should be paid to the potential formation of organic compounds containing halogens as by-products of the oxidation of soluble organic fractions (natural organic matter). In the current state of technology and knowledge, trihalomethanes (THM) are an effective indicator of the total content of halogenated organic compounds and are commonly used in the assessment of the disinfection process (in relation to the formation of such by-products). Therefore, the assessment of THM is logically justified, because it is the fraction that is potentially most toxic to human health.
A comparison of THM values for chlorine and chlorine dioxide is given in the table below: