Sodium hypochlorite, chlorine dioxide and ozone comparison
At present, from the type of water disinfection, there are chlorine, sodium hypochlorite, bleach, trichloroisocyanuric acid (sodium dichloroisocyanurate), chlorine dioxide, hydrogen peroxide, ozone and other agents and methods, in addition to ultraviolet disinfection and some other means.
Because of the chlorine in the transportation, storage, there are security risks; in the quantitative dosage, because of chlorine in the water solubility is low, chlorine is easy to dissipate, making it difficult to stay in the water to meet the standards; at the same time, chlorine bottles are constantly changing the air pressure, there is a lack of accuracy in the measurement of the dosage of the problem; chlorine has a strong diffusion of the environment there is a poisonous effect; free chlorine is easy to form a lot of high activity, like carbon tetrachloride, a class of carcinogenicity. Like carbon tetrachloride a class of carcinogenic substances, therefore, in the field of conventional disinfection, the abolition of liquid chlorine is more and more advocate, but also increasingly concerned about.
Take the safety of chlorine, it is always a problem that people are always on the alert. In our country, there are safety accidents of chlorine tank leakage almost every year. Chlorine as a hazardous material by the security organs of the strict control. Some years ago, occurred in Fujian Sanming Railway Station chlorine cylinder transportation in the run chlorine incident, resulting in the emergency evacuation of thousands of people; in Beijing, some swimming pools due to the operator is not careful, three minutes of the run chlorine, there are 37 children hospitalized. March 29, 2005, 18:50, Huai'an City, Jiangsu Province, the territory of a Shandong Province, RuH-00099 loaded with liquid chlorine hazardous materials transport vehicles, line to the Beijing-Shanghai 103KM+300M on the Beijing-Shanghai Expressway, collided with a Lu QA0938 truck, causing Lu H-00099 to overturn and leak liquid chlorine. As of 8 o'clock on March 31st, the accident has caused 28 people to die of poisoning, 285 people were sent to the hospital for treatment. Immediately after the accident, the relevant departments organized the evacuation of nearly 10,000 villagers, resulting in the Beijing-Shanghai Expressway Suqian to Baoying section closed for 20 hours. China's Tianjin region has clearly stipulated that public **** entertainment venues are prohibited from using chlorine for disinfection.
In many developed countries abroad, such as the United States, Germany, Japan and other strict restrictions on the use of chlorine, chlorine is mainly used for sewage treatment. And public places and small and medium-sized waterworks generally no longer use liquid chlorine, and more use of sodium hypochlorite liquid disinfection. Of course, according to the situation of water consumption, other disinfection methods can also be used. Such as small amounts of drinking water disinfection can be used such as ultraviolet light, ozone, hydrogen peroxide and other means of sterilization.
Chlorine, sodium hypochlorite, chlorine dioxide and ozone [1] are agricultural and industrial production and daily life are relatively easy to see several strong oxidizing agent, in addition to ozone, they are not naturally occurring chemical substances. Generally, they can be used as aquatic biocides. They not only have the function of killing bacteria and viruses, but also can be used to bleach paper, fiber and chemical synthesis. They are widely used in tap water disinfection, swimming pool water disinfection, sewage treatment, circulating water de-algae, paper industry, chemical synthesis industry, as well as medicine and health and epidemic prevention and other fields.
But different chemicals have different performance and characteristics, just as different manufacturers of products have not the same quality. Chlorine, sodium hypochlorite, chlorine dioxide and ozone are very different in physical and chemical properties, as well as practical use. In terms of the application of these disinfectants, sodium hypochlorite is the most safe and effective, easy to store and most convenient to use.
We are familiar with the performance and use of chlorine. Liquid chlorine sterilization effect is very good, and easy to obtain, economical and cheap, and easy to add, occupies a small place, but its safety is relatively low, the management is easy to neglect. Here, no more detailed analysis of the situation of liquid chlorine, specifically to explore and compare the performance of sodium hypochlorite, chlorine dioxide and ozone three disinfectants, as well as the use of related equipment characteristics.
Sodium hypochlorite
Sodium hypochlorite molecular formula is NaOCl, belongs to the strong alkali weak acid salt, it is clear and transparent, is a kind of liquid can be completely dissolved in water. But because of the sodium hypochlorite liquid is not easy to store for a long time, sodium hypochlorite is mostly electrolyzed low concentration of salt water site preparation.
Sodium hypochlorite liquid can be prepared by electrolysis of brine, this equipment is called sodium hypochlorite generator. The generation process of sodium hypochlorite can be expressed by the chemical equation as follows:
The total reaction is expressed as follows:
NaCl + H2O → NaOCl + H2↑
Electrode reaction:
Anode: 2Cl- - 2e → Cl2
Cathode: 2H+ + 2e → H2
Solution reaction. 2NaOH + Cl2 → NaCl + NaOCl + H2O
Of course, sodium hypochlorite disinfectant liquid to sodium hypochlorite generator production is the best. Because, the liquid sodium hypochlorite produced by it is more stable, single, and easy to store, and does not contain those complex and even harmful ingredients produced by chlorine plants.
On the sodium hypochlorite generator, China has been released on January 12, 1990 GB 12176-90 national standard. It is a recognized, reliable, very stable, and authoritative information can be found in the product. Sodium hypochlorite generator has more than one hundred years of history, has proved to be a very low operating costs, drug dosage accuracy, excellent disinfection effect of the equipment.
As far as disinfection is concerned, sodium hypochlorite has obvious advantages. As a truly efficient, broad-spectrum, safe and powerful sterilization, virucidal agents, its affinity with water is very good, can be dissolved in water than any arbitrary, it does not exist in liquid chlorine, chlorine dioxide and other agents of security risks, and its disinfection effect has been recognized and chlorine gas is equivalent to the addition of accurate dosage, safe operation, easy to use, easy to store, non-toxic to the environment, no leakage of running gas, so it can be in any environmental conditions. The product can be used in any working condition.
In fact, sodium hypochlorite is widely used for disinfection of various water bodies including tap water, water, industrial circulating water, swimming pool water, hospital sewage and so on. Sodium hypochlorite is also capable of destroying cyanide ions and is used to treat wastewater containing cyanide. The high concentration of sodium hypochlorite liquid can also be used to strip the mud attached to the equipment and pipeline[2].
Sodium hypochlorite sterilization principle is mainly through its hydrolysis to form hypochlorous acid, hypochlorous acid and then further decomposition of the formation of new ecological oxygen [O], the new ecological oxygen of the strong oxidizing bacteria and viruses protein denaturation, so that the pathogenic microorganisms lethal. (The principle of chlorine disinfection is also the same).
According to the chemical determination, the hydrolysis of sodium hypochlorite is affected by the PH value, when the PH exceeds 9.5, it will be unfavorable to the generation of hypochlorous acid. However, most of the water quality PH value is 6-8.5, and for the PPM level concentration of sodium hypochlorite in the water is almost completely hydrolyzed to hypochlorous acid, its efficiency is higher than 99.99%. The process can be expressed by chemical equation as follows:
NaOCl + H2O → HOCl + NaOH
HClO → HCl + [O]
Hypochlorous acid can not only act on the cell wall and viral shell in the process of sterilizing and killing viruses, but also because hypochlorous acid molecules are small and uncharged, it can be infiltrated into the body of the bacterial (viral) and bacterial (viral) proteins, nucleic acid, and the body of the bacterial (viral), and the bacterial (viral) protein, nucleic acid and the body of the bacterial (viral), and the bacterial (viral) protein and nucleic acid. It can penetrate into the body of bacteria (virus) and oxidize the protein, nucleic acid, and enzyme of bacteria (virus), thus killing the pathogenic microorganisms.
R-NH-R + HOCl → R2NCl + H2O
At the same time, chlorine ions can significantly change the osmotic pressure of bacteria and viruses, so that they lose their activity and die.
In terms of disinfection, it is worthwhile to confirm that, due to the disinfectant produced by the sodium hypochlorite generator is not like chlorine gas, chlorine dioxide and other disinfectants in the water to produce free molecular chlorine, so it is generally difficult to form due to the presence of molecular chlorine chlorine chlorination reaction occurs, resulting in the generation of toxic and hazardous substances that are not conducive to human health. Moreover, sodium hypochlorite will not cause serious corrosion to metal pipes as chlorine reacts with water to form hydrochloric acid. However, it can react with ammonia to produce trace amounts of chlorine-ammonia compounds in the water with an odor, but this substance is also a safe biocide, but far less than the biocidal ability of sodium hypochlorite.
NH3 + HOCl → NH2Cl + H2O
NH2Cl + HOCl → NHCl2 + H2O
NHCl2 + HOCl → NCl3 + H2O
Operating costs, the use of sodium hypochlorite disinfection of the operating costs of the cost is very low, a little more than chlorine. According to a set of statistics from the United Kingdom, the cost of sodium hypochlorite compared to chlorine is about 1.05 :1[3].
The use of sodium hypochlorite for disinfection is optimized by the use of sodium hypochlorite generators. Previously, sodium hypochlorite generator can not be widely spread in our country is mainly because of the past in the anode anti-corrosion material is not good, followed by our country's economic development is lagging behind and the lack of attention to the water treatment technology, and again is the sodium hypochlorite generator than the chlorine of the one-time investment of a slightly higher factors caused by the factors.
In practice, there are still some units of water disinfection using sodium hypochlorite produced from chlor-alkali plant. In fact, the chlor-alkali factory production of sodium hypochlorite liquid and sodium hypochlorite generator site preparation of sodium hypochlorite liquid is still a certain difference. Sodium hypochlorite is a by-product of the chlor-alkali plant production process, which is generated by absorbing excess chlorine gas through the lye. It is a process that must be set up to ensure safety. For most chlor-alkali plants, sodium hypochlorite, as a by-product, has a complex composition and decomposes easily. According to some reports, some factories use graphite as electrode to produce a lot of dioxin from economic benefits.
2OH- + Cl2 → Cl- + ClO- + H2O
Generally speaking, the reaction is usually carried out at low temperatures, because at low temperatures a molecule of chlorine can still be combined with eight molecules of water to form a temporary chlorine hydrate, which is free chlorine in water. Thus, when the temperature is slightly higher, it will be released from the water naturally, and it cannot be stored for a long time, and it is easy to evaporate and become ineffective, and some chlorine gas escapes from the dosage. In addition, it needs large plastic drums for storage, which takes up some space, and it is still troublesome in transportation, storage and management. Therefore, this kind of sodium hypochlorite solution containing some free molecular chlorine for water disinfection, of course, is not as good as the field use of sodium hypochlorite generator. But it is still safer and more reliable than the use of liquid chlorine disinfection.
In addition, it must be noted that the use of sodium hypochlorite disinfection, it is inevitable that the presence of some salt in the water. However, due to the dosage is a few grams per ton of water standards, such as tap water and other flowing water there is no problem of accumulation, more likely to produce the feeling of salt. For swimming pool water, a short period of time may have some accumulation, but because the swimming pool itself will regularly backwash the purification equipment, and therefore need to replenish a portion of the fresh water, coupled with a very small amount of dosing, about a few parts per million of the amount of the pool in the long term, the pool will not have the accumulation of salts, the pool water more likely to become salty salt. Through our investigation and visit, we also did not find which user has the use of sodium hypochlorite generator equipment and caused the pool water salty cases.
Chlorine dioxide
Chlorine dioxide molecular formula is ClO2, in higher than 11oC, chlorine dioxide boil, become a kind of yellow-green gas. It is an extremely active compound, a little heat, will be rapidly and explosively decomposed into chlorine and oxygen. Chlorine dioxide is more irritating and toxic than chlorine. Because it is a gas, easy to diffuse, heat and easy to decompose, in the surface of the fiber to stay for a short time, and with the reaction of water can also generate a strong bleaching ability of HClO2, can not degrade and damage the fiber, so in the paper, printing and dyeing and other industries have been very good application. Chlorine dioxide as a strong oxidizing agent, also has and chlorine similar to the ability to kill.
Chlorine dioxide is extremely unstable, can not be transported like sodium hypochlorite, the transportation is very easy to explode, so only rely on the site preparation. Generally, it is prepared by the reaction between sodium chlorate and acid. However, the reaction between sodium chlorate and sulfuric acid is very violent, and the resulting chlorine dioxide is almost explosively decomposed into chlorine and oxygen, which is of course related to the sulfuric acid in the reaction of a large amount of heat. The chemical equation is as follows:
3NaClO3 + 3H2SO4 → 3NaHSO4 + 3HClO3
3HClO3 → 2ClO2↑ + HClO4 + H2O
2ClO2 → Cl2↑ + 2O2↑
The most benign method is the reaction between oxalic acid and sodium chlorate to produce chlorine dioxide. Gas:
2NaClO3 + 2H2C2O4 → Na2C2O4 + 2H2O + 2CO2↑ + 2ClO2↑
Some domestic manufacturers use hydrochloric acid to generate chlorine dioxide by quantitatively controlling the addition of sodium chlorate drip method, some of this equipment can be obtained up to no more than 50% chlorine dioxide and more than 50% of chlorine gas.
Generally speaking, the reaction between sodium chlorate and hydrochloric acid is more complicated. If the reaction with dilute hydrochloric acid, the product can be obtained as a mixture of chlorine dioxide and chlorine gas [4], but the scale of preparation must be equipped with explosion-proof devices, the operation must be very careful, because chlorine dioxide is very easy to explosive decomposition of heat:
NaClO3 + HCl (dilute) → NaCl + Cl2↑ + 2ClO2↑ + 2H2O
Actually, the reaction is also divided into two steps.
In fact, this reaction is also divided into two steps, sodium chlorate reacts with hydrochloric acid to form chloric acid and sodium chloride, and the chloric acid then decomposes into chlorine dioxide, chlorine gas and water.
When concentrated hydrochloric acid is used to react with sodium chlorate, only chlorine gas is emitted from the product, and there is no chlorine dioxide gas[4]:
NaClO3 + 6HCl(concentrated) → NaCl + 3Cl2↑ + 3H2O
It is clear that in an intermediate range of hydrochloric acid concentrations, both of the above reactions take place, and the two equations can be added together. can be expressed as [4]:
ClO3- + 7Cl- + H+ → 4Cl2↑ + 2ClO2↑ + 5H2O
From the above equation, hydrochloric acid reacts with sodium chlorate to produce chlorine dioxide content is very unstable, and the gas produced is mainly chlorine gas, a small amount of chlorine dioxide.
Because the production of chlorine dioxide requires the use of sodium chlorate or potassium chlorate, so the operating cost is very high, about five times more than the operating cost of sodium hypochlorite [2]. In addition, because hydrochloric acid is volatile and strongly corrosive, it is relatively troublesome to manage and requires more safety containers for storage.
In industry, there is a preparation of chlorine dioxide aqueous solution of the process [1], the process is more complex, the specific method is: let the nitrogen dioxide from the bottom upward through a filled tower, while the sodium chlorate solution from the top to the bottom of the flow, the reaction equation is expressed as follows:
ClO3- + NO2 → NO3- + ClO2
This kind of aqueous solution is not very high in concentration, and it is safer to handle. It is safer to handle (improper handling of chlorine dioxide in aqueous solution over 30% can cause an explosion), and dissolution is actually a physical process. When placed in the sun, the solution will slowly decompose into a mixture of acids. However, this method is more expensive to run and is not generally used to disinfect drinking water.
According to the data, pure chlorine dioxide used for water disinfection is also similar to chlorine, but slightly different. It has two characteristics that chlorine does not have: first, it is used in a wide range of PH, in PH6-10 can effectively kill most of the microorganisms; second, it does not react with ammonia to produce an unpleasant taste. However, its decomposition in the water will produce chlorite as a by-product, such as for swimming pool disinfection, chlorite long time accumulation will make the water yellow, there will be skin and eye irritation, generally used to add a certain amount of chlorine to eliminate the method [3].
Some information on the chlorine dioxide can kill spores, but the specific mechanism and the actual effect is not known. At present, the use of chlorine dioxide for disinfection of tap water, water and other very successful examples are few. Because all gas disinfectants dissolved in water, there are very low capacity, are very unstable, unsafe, volatile factors, it is difficult to make the water body to achieve the amount of chlorine residual detection should be, therefore, the tap water, swimming pools, etc. need to maintain a certain amount of disinfectant, chlorine dioxide disinfection is more difficult to meet the standard, the water body of the chlorine residual detection is more difficult to be guaranteed. What's more, chlorine dioxide is not yet as high as chlorine pressure can be formed through the chlorinator with the water body temporary hydrate ability, so, technically speaking, the large-scale use of chlorine dioxide dosing is also very unrealistic.
It is generally believed that the disinfection principle of chlorine dioxide is the same as chlorine, a small amount of chlorine dioxide reacts with water to produce chlorite HClO2, chlorite is a weak weak acid with oxidative bleaching effect.
2ClO2 + H2O → HClO2 + HClO3
Industry generally does not use chlorine dioxide directly, but the application of sodium chlorite solution for bleaching. A single sodium chlorite can be obtained by mixing the aqueous chlorine dioxide solution produced at the same time with sodium peroxide.
2ClO2 + Na2O2 → 2NaClO2 + O2
Sodium chlorite is a soft bleaching agent that gradually releases chlorite through hydrolysis and can bleach many natural and synthetic fibers without degrading them, as well as oils, paints, and beeswax [1]. This technology has not been around for a long time. Admittedly, the cost of using this technology, from investment in equipment to operating costs, is very high and difficult for small-scale enterprises to afford.
Domestic production of chlorine dioxide generator enterprises rarely mastered the production of chlorine dioxide aqueous solution of this higher security technology, most of the sodium chlorate and hydrochloric acid with quantitative titration, control the reaction of the amount of generation of the method to achieve. The cost of such equipment is very low, but the safety is very poor, a little carelessness will cause accidents, the management needs to be particularly careful. The safety of such equipment is being questioned and evaluated by the state through the technical department, and experts in the field are calling for its technical specification or banning and elimination.
For example, chlorine dioxide is used to disinfect water in Peking University's swimming pool, Beijing's Tiantan Hospital, and Erlong Road Hospital, all of which were forced to stop using it because of successive safety incidents. Because, the heat of chlorine dioxide is very easy to explosive decomposition, directly resulting in the leakage of toxic gases and pollution of the environment:
2ClO2 → 2O2 + Cl2
In addition, there is now on the market with the industrial use of saturated saline production of chlorine gas is exactly the same way, the production of a known as chlorine dioxide can be prepared by the equipment. In fact, through the diaphragm to isolate the cathode and anode, which more than 98% of the chlorine gas is still produced. In principle, the electrolysis of saturated brine is the first chlorine ions get electrons to generate chlorine gas, part of the chlorine gas reacts with water to generate hypochlorite ions, hypochlorite can be further oxidized in the electrolysis to generate chlorite, chlorate ions, which are thermally decomposed to produce chlorine monoxide, chlorine dioxide and other gases. However, in this electrolysis method, the efficiency of generating chlorite and chlorate ions is very low. In other words, the efficiency of converting chlorine dioxide by electrolysis is not only low, but also unnecessary, wasteful of electricity, and uneconomical.
And, as chlorine and chlorine dioxide are heavier-than-air gases, they also leak easily and spread along the ground. Once contamination is established, these toxic gases cannot be eliminated in a short period of time. Because chlorine gas is highly toxic and corrosive, and was used by Hitler to poison Jews during World War II, it is produced by specialized chlor-alkali industry manufacturers and is packaged and transported in special, dry chlorine gas cylinders. Chlorine gas is also scrutinized by the national security authorities.
In fact, this equipment was not very successful in practice, and many problems arose. Chlorine leakage is serious, the diaphragm is usually damaged in half a year or so, maintenance is frequent, the drug dosage can not reach the water quality set requirements. Such as Dongdan swimming pool, Beijing Sports University swimming pool, diving center of the National Sports Commission Training Center and some hospitals since the installation can not be used normally, have to be converted to use sodium hypochlorite for disinfection.
Chlorine dioxide's strong chemical corrosiveness is almost the same as chlorine, and its toxicity is still forty times that of chlorine.
Take chlorine, modern medical research has proved that, because chlorine can be chlorinated with many organic substances in the water, generating a lot of chlorinated organic matter, and most of the chlorinated organic matter is extremely harmful to health, such as the generation of trichloromethane, tetrachloromethane, dioxin and other chlorinated substances. Cancer-causing chlorinated substances such as trichloromethane, tetrachloromethane and dioxin are often detected in tap water disinfected with chlorine. According to the American Medical Association, the incidence of bladder, rectal and colon cancers in people who have been drinking chlorinated tap water for a long period of time is dozens of times higher than that of the control group, or even hundreds of times higher [5].
In 1979, the U.S. Environmental Protection Agency developed the first chlorinated drinking water by-products in the content of the regulations, restrictions for more than 10,000 people drinking water supply source of all trihalomethanes (of which chloroform is the most prevalent) shall not exceed 100 micrograms per liter. November 1998, the U.S. Environmental Protection Agency adopted a more stringent water source standards, will be the limits of trihalomethanes standard to 80 micrograms per liter, while also setting limits for other potentially hazardous byproducts such as bromate and haloacetic acid, and mandating that water companies remove reactive organic compounds from water before disinfecting it with chlorine [5].
Recently, the relevant professional journals have also published a research article on disinfection by-products in drinking water in Australia [6], which is worthy of reference for interested colleagues.
Ozone
Ozone, whose molecular expression is O3, is a light blue gas in its normal state and is highly toxic. It gets its unflattering name because it has a fishy odor. It condenses to a dark blue liquid at -112OC and to a blackish purple solid at -192OC. Ozone has little solubility in water, but is more soluble than oxygen. Liquid ozone is not miscible with liquid oxygen. Contrary to oxygen, ozone is very unstable and decomposes slowly at room temperature, more rapidly above 200OC, and releases a large amount of heat when it decomposes. Pure ozone is also explosive [1,7].
2O3 → 3O2
Chemically, ozone is more oxidizing than oxygen in acidic, neutral, and alkaline media. Because of this, ozone can be used as a biocide and can be applied to disinfect and treat various bodies of water. It can also be used to bleach cotton, linen, paper, and to deodorize hides and skins.
Ozone is normally found in the upper atmosphere and is produced by the activation of oxygen molecules by ultraviolet light from the sun. Ozone in the upper atmosphere protects life on Earth, including humans, by absorbing ultraviolet rays and thus protecting life from their harmful effects.
Small amounts of ozone are produced when oxygen and air are discharged, so it can be found near working motors and high-voltage appliances. For example, some ozone is emitted from a photocopier that has been in operation for a long time. During the summer thunderstorm season, some ozone is also produced when lightning strikes the air.
The production of ozone begins with the dissociation of oxygen O2, forming highly activated atomic oxygen, most of which quickly recombines to form oxygen O2, but a few oxygen atoms react with oxygen O2 to produce ozone O3 :
O2 + O → O3
In addition, some household appliances equipped with high-voltage devices, as well as automobiles, can also produce a small amount of ozone. Since ozone decomposes, the amount of ozone in the air is usually very small. However, because people need to breathe the oxygen in the air, if the ozone content in some ambient air exceeds 1μg/m3, it will directly harm human health, and the higher the concentration, the greater the harm, therefore, some countries have set environmental standards for the ambient air ozone content is also a strong limit.
Ozone can be obtained by electrolyzing sulfuric acid at high current strength, and at low temperatures, up to 30% of the ozone can be contained in the oxygen emitted from the anode [1]. However, this can only be done in small quantities in the laboratory. Because sulfuric acid is a very strong acid, the electrodes are not only corroded and consumed rapidly in strong acids, but also the materials are not easily available.
In nearly 20,000V voltage, through the oxygen discharge mode, the world's most advanced equipment through the pure oxygen can produce the ozonization of oxygen can contain 10% of the ozone. Now can be used for small-scale treatment of water disinfection ozone generator, mostly through the compressed air discharge to obtain ozone. The concentration of ozone produced by this equipment is not very high, generally 1-2% [8].
Because of the strong oxidizing ability of ozone, coupled with the discharge will produce a lot of heat, so the discharge electrodes are easy to wear and tear, so must be replaced regularly discharge electrodes. Discharge electrodes are composed of very special materials, generally high oxidation-resistant precious metals (such as platinum) or alloy composite materials. The surface of the electrodes of the equipment that performs the silent discharge to excite the air into odorous oxidized air also has a dielectric layer. Therefore, a set of high-quality ozone equipment, the price is not expensive, the output of a little larger equipment more than a million dollars.
Ozone's oxidation potential is very high, in terms of oxidative disinfection capacity than chlorine is even better, second only to fluorine. Therefore, in the food industry as well as a very few swimming pools also have the use of ozone generator equipment to disinfect. Recently, some experts also advocate that it can be used on a small scale for sewage treatment, but after a trial, the treatment effect is not good.
We know that ozone is often in an unstable state, especially in the decomposition of water with the increase in water temperature and enhancement. When ozone decomposes in water, it directly releases monatomic oxygen [O], which has a powerful oxidizing and disinfecting effect. Due to the small molecules, ozone can quickly diffuse and penetrate into the water of bacteria, spores, viruses, strong and effective oxidation and decomposition of bacteria, viruses, algae and various tissue substances. In addition, it does not generate any substances with special smell, which can improve the water quality in terms of taste, smell and color [3]. This characteristic determines its application in the food industry is very promising. For example, large-scale pure water, mineral water production plants are using ozone generators to disinfect.
But for the treatment of swimming pool water, because ozone is not easily dissolved in water, in the water in any case is not stable, only the body of water and ozone fully mixed with the contact may be effective in disinfection, therefore, ozone disinfection of swimming pool water must be installed in a large-scale scrubber to be feasible, and the larger the amount of water in the pool scrubber should be correspondingly larger. Generally speaking, the amount of water in the special storage scrubber should be at least one-third of the pool water, occupying a large place. Swimming pools in developed countries abroad are more self-built, smaller pools, scrubbers are not big.
At the same time, due to the toxicity of ozone, the maximum permissible concentration of ozone in swimming pool water can not exceed 0.01 ml / L, the maximum permissible concentration of ozone in the air can not exceed 0.001mg / m3 (1μg / m3). Therefore, before the ozone sterilized water enters the swimming pool, activated carbon must be used to absorb the excess ozone. Therefore, there is no residual effective disinfection of pool water in this type of disinfection, and a set of auxiliary dosing system such as hypochlorite is required to maintain a residual chlorine level of 0.5-1.0mg/L[3]. In this, sodium hypochlorite can also consume part of the excess ozone into the water:
O3 + HOCl → HClO2 + O2
We believe that, for swimming pool disinfection using ozone generating equipment, these related auxiliary measures and equipment is not missing. Because, ozone disinfection does not reach a certain concentration will also affect the disinfection effect, but more than the prescribed concentration will make people headache, dizziness, nausea and even vomiting symptoms of poisoning. According to medical information, long-term exposure to ozone causes a hundred times higher chance of cancer. This is for the original purpose of exercise for the purpose of the swimming pool, the management of ozone sterilization complexity is imaginable.
It is clear that although ozone is more effective in disinfecting water, it is only in the area where the water is exposed to ozone. Moreover, because of the complexity of the equipment used to disinfect swimming pools, including air dryers, ozone generators, ozone scrubbers, activated carbon adsorbers and other major accessories, so the investment is expensive, the management level is very high, and can afford the unit is not much.
China's basic national conditions is a large population, a vast area, the lack of water resources, but also the world's largest economic developing countries. This also determines the construction of swimming pools in our country mainly tends to public, can only build more large swimming pools to meet the needs of the people's lives. Swimming pools are relatively large, the pool water volume is more, can solve the swimming flow is more concentrated, the problem of lower levels of consumption, in this way, not only to achieve the purpose of reducing costs, but also able to provide residents with a convenient place to exercise. However, the reality of this large swimming pool disinfection, the use of ozone generators is often more than worth it. According to our survey, many large swimming pools using ozone disinfection is not good, the summer also occurs serious water phenomenon, most less than a year on the discontinued.
In addition, the ozone of the water PH range of requirements and chlorine similar to the alkaline water can not adapt to alkaline water, alkaline water will only accelerate the decomposition of ozone. In addition, ozone disinfection of swimming pool water on the iron pipe corrosion is also relatively large, highly reactive oxygen atoms can easily generate iron oxide with iron.
Again, from the practical point of view, the ozone generator power consumption is very large, and the service life is relatively short, that is, the normal use of a year or so, maintenance and replacement is more frequent, each time the cost of replacement is also very large. In this regard, we found that some of the use of ozone generators units (including the Asian Games for the construction of the Beijing Yingdong Swimming Pool) in the later had to carry out disinfection equipment to change the face. It may be that these factors greatly limit the promotion and use of ozone sterilization equipment. In terms of ozone production rate increase, the current ozone generator there are many urgent need to improve the place.
Conclusion
In summary, from the development trend of disinfection equipment, the choice of a better non-toxic, non-polluting, inexpensive, safe to use disinfection is of course an ideal. But each disinfectant can not be no pros and cons. From the disinfection ability, ozone and sodium hypochlorite are very good, but ozone is slightly faster than sodium hypochlorite sterilization, water quality is not too much odor generated, but as a liquid sodium hypochlorite is better than ozone management, water quality can be fully guaranteed to meet the standards. In terms of equipment space occupied by the size of the two are similar, only sodium hypochlorite generator than the ozone generator needs more salt storage area of one or two or three square meters. From the investment to the use of equipment to consider, sodium hypochlorite generator disinfection than ozone has the advantage of much less investment than the ozone, only one-fifth of the ozone, the equipment is more stable, the service life of up to 20 years or more. Running costs, sodium hypochlorite generator less power consumption, maintenance is also convenient, but also more advantageous.
Whether it is chlorine, ozone or chlorine dioxide gas, not only diffusion and run leakage problems, but also their solubility is relatively poor, in the drug dosage to do like sodium hypochlorite and other liquid disinfectants as convenient and accurate is more difficult. Generally speaking, they are formed through the generator gas pressure is roughly atmospheric pressure, will not be much higher than atmospheric pressure, much less likely to be similar to the pressure of liquid chlorine, there is no vacuum chlorinator such as dosing equipment, which is difficult to dosage to meet the requirements of the main factors of the water quality standards.
Chlorine dioxide used as a water disinfection through the popularization of the use of recent years, there are many unsatisfactory places, the actual use of the effect and laboratory results vary greatly, drug dosing can not make the water quality to meet the required indicators, safety is also very poor. These defects are closely related to the nature of chlorine dioxide and the use of technology, but also with the demand for the unit's existing equipment management personnel's knowledge level. At present, the country has not been for chlorine dioxide to develop a more complete and unified national standards for the product, but also can not determine a set of the most mature and feasible effective solutions. Although in the country held several meetings about chlorine dioxide, but, chlorine dioxide shows the characteristics of the lack of stability, indicating that the chlorine dioxide involved in bleaching and disinfection of the mechanism is still to be studied in depth, chlorine dioxide in the field of disinfection and ozone is still in the same exploration and product improvement stage.
Sodium hypochlorite is still a relatively stable and reliable biocide. Sodium hypochlorite generator after many years of development and improvement, the establishment of a strict national standards, has become a quite perfect practical equipment, worth a wide range of promotion.
References
[1] [English] C. Chambers A. K. Holliday Inorganic Chemistry Peking University Press 1987, 9, first edition
[2] Zhou Bensheng, "industrial cooling water system in the corrosion and protection of metals," the Chemical Industry Press 1993, 10, first edition
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