First of all, we once again emphasize that there is no way to do without chlorine or chlorine-forming compounds, if water is distributed to consumers through the water supply system, since only chlorine has an aftereffect or prolonged action, thanks to which bacteriological safety of water is guaranteed at any point of the distributing network all the way to each consumer’s tap.
All of us consume table salt (NaCl), which after getting into the human body, interacts with the water, forming pure chlorine, namely: 2NaCl+2H₂O= Cl₂+2NaOH+ H₂
Since water in the human body has pH=7.0 -7.5, the chlorine gets in the metastable zone and gives the solution the highest antibacterial activity. Hence it appears that chlorine protects our body and other organisms against the destructive effects of microorganisms. Now the phrase: «The daily need of an adult human for chlorine (2-4 g) is covered by food» becomes clear. Namely the «daily need», since chlorine is spent to destroy microorganisms dangerous for humans and, accordingly, must be replenished.
Studies of recent decades have found that all higher multi-cellular organisms, including humans, synthesize hypochlorous acid and highly active chlorine-oxygen and hydroperoxide compounds in special cellular structures to control microorganisms. This mechanism of antibacterial protection, created by Nature, has functioned in the internal environment of the body of animals and humans for millions of years.
The Ministry of Health allows the use of more than 200 products for disinfection and sterilization. Let us elaborate only on the main disinfectants used in the CIS countries and abroad.
All technological schemes for water purification and disinfection (old and new ones) should be based on the key criteria for drinking water quality:
Drinking water is water, suitable for consumption, complying with the established quality standards.
According to SNPiN 214 1074-01 3.1 Drinking water must be safe with epidemic and radiological regards, harmless in chemical composition and have favorable organoleptic properties.
3.2 The quality of drinking water must comply with the hygiene standards before it enters the distribution network, as well as at the water intake points of the external and internal water supply network.
Besides, these documents take into account the fact that the danger of human diseases due to microbiological water pollution is many thousands of times higher than due contamination of water with various chemical compounds.
In the current drinking water purification practice, chlorination is used most often as the most economical and effective method in comparison with any other known methods.
In the USA, 98.6% of water (the vast majority) is chlorinated.
This popularity of chlorination is also results from the fact that this is the only way to provide microbiological water safety at any point of the distribution network at any time due to the aftereffect. This effect consists in the fact that after introduction of chlorine molecules into water («aftereffect»), they retain their activity as related to microbes and inhibit their enzyme systems on the route of water along the water supply networks from the water treatment facility to each consumer. It should be highlighted that the aftereffect is typical only for chlorine. Taking into account the condition of our water supply networks, to forget about the presence of microbes in them is self-defeating.
In addition to the main disinfection function, due to the unique oxidation properties and the aftereffect, chlorine serves other purposes as well — control of water taste and odor, prevention of algae growth, keeping filters clean, removal of iron and manganese, destroying hydrogen sulphide, water discolouring, etc. In this respect, none of the chlorine alternatives can be compared with it in terms of universality and ease of use.
Let’s consider the use of ozonization and UV irradiation in the context of safety.
Despite the wide experience of the use of ozone in water treatment technology, there are still many unsolved problems. Very often ozonization is called an environmentally friendly method of disinfection. It is not clear what the basis of such a definition was. Recent researches have shown that the opinion about ozonation as a more harmless method of water disinfection is mistaken.
Thus, the ozone reaction products with organic substances contained in water are aldehydes (formaldehyde, acetaldehyde, glyoxal, methylglyoxal), ketones, carboxylic acids and other compounds, the presence of which creates a number of additional problems in the water treatment process, including aldehydes increase the danger of formation of organochlorine by-products.
The use of another alternative disinfectant — UV irradiation allows you to get rid of the by-products of disinfection, which is its undoubted advantage. But for today its industrial application is complicated by the lack of the possibility to control the effectiveness of water disinfection operatively. The relevant methodological guidelines highlight the possibility of using UV irradiation at the stage of primary water disinfection, provided that technological researches are carried out at the water water-supply source. At the same time, methodological instructions point out that UV irradiation provides a desired bactericidal and virucidal effect only when all the established operating conditions are met. One of the most important issues of the application of this method is the creation of guarantees for the passage of all disinfected water through the plant, i.e., the capacity of the plant shall be equal to the capacity of the water supply station. One of the most important questions of the application of this method is the creation of guarantees for its reliability. For this purpose, the system should be equipped with sensors for measuring the intensity of UV radiation in the disinfection chamber, an automation system providing the sound and light signals when the minimum dose is reduced, the lamp accumulated hour meters and the indicators of their serviceability for timely cleaning when there is encrustation or replacement.
It is well known that the quality of water disinfection with chlorine-containing reagents depends on the hydrogen index pH, since it is the pH value of the water that determines the forms of the chlorine compounds in the water and their activity. When pH values are low (from 0 to 3), the molecular chlorine Cl2 predominates and in the upper half of this range Hypochlorous acid HClO begins to form, increasing quantitatively so that even in the pH range of 3 to 6, there is only hypochlorous acid HC1O in water.
And then (pH> 6) hypochlorous acid decomposes into H⁺ and ClO ions. Thus, for example, when pH is 6, the persantage of HClO is 97%, and the percentage of ClO⁻ is 3%. When pH is 7, the persentage of HClO is 78%, and ClO⁻ is 22%, when pH is 8, the percentage of HClO is 24%, ClO⁻-76%. When pH> 9 HSlO it reacts to form the ClO⁻ hypochlorite ion.
Thus, the diagram in the Fig. 1 ensures that, depending on the water pH, there are zones of stability of chlorine reagents in water: the Cl₂ zone, the HClO zone, the ClO zone, in which their activity is not present, and the instability zones: the Cl₂-HClO zone (pH = 1.5 -3.5), the HClO-ClO zone (pH = 6-9). Since the water pH of the surface sources is 6.5-8.5, the second instability zone should be the subject of our attention, since it is the zone in which high bactericidal activity is present, with the highest bactericidal activity of oxygen chlorine compounds appearing in the pH range of from 7.0 to 7.5, where the concentrations of hypochlorite ions and hypochlorous acid are comparable.
This is explained by the fact that these compounds, being conjugated by the acid and the base (HClO + H₂O → H 3 O⁺ + ClO⁻; ClO⁻ + H₂O → HClO + OH⁻), form in the indicated range of of pH values to a greater degree are determined by the well-known Le Chatelier principle, i.e. hypochlorite ions all this time will delay to turn into an active form, which will have a detrimental effect on the water disinfection.
And what about chlorine? The technology introduction of chlorine into drinking water is as follows: first, on drinking water taken from a water conduit, chlorine water is prepared as a basis by introduction of gaseous chlorine into it, and then chlorine water is introduced into the same water conduit. Virtually no disturbance of chemical equilibrium and pH values occur, which means that the quality of water disinfection in its application is guaranteed.
This is the first advantage of chlorine
Sodium hypochlorite has a significantly lower bactericidal activity than hypochlorous acid, the concentration of which is maximal when chlorine is dissolved in water. From the data given in the diagrams.
It can be seen that it takes hypochlorous acid, sodium hypochlorite and chloramine at the same active chlorine concentration, for example, 0.1 mg/l, less than 2 minutes, more than 100 minutes and about 500 minutes respectively to achieve the same affect of drinking water disinfection.
This is the second advantage of chlorine.
Sanitary and microbiological studies carried out in 2002 by the Institute of Medical and Environmental Problems and Health Risk Assessment (St. Petersburg) revealed deficiencies of hypochlorite from the perspective of the functional efficiency and environmental purity. It turned out that the chlorine solution in water is several dozen times more effective than hypochlorite in the residual quantity of bacteria. In addition, hypochlorite is ineffective against cysts, which limits its use on long length water networks.
This is the third advantage of chlorine
Discussions about the disinfecting ability of hypochlorite completed by epidemiologists long ago, and their conclusions are set forth in the practical guidance that states that sodium hypochlorite obtained chemically and electrochemically (no matter how) is ineffective against spore forms of microorganisms.
This is the fourth advantage of chlorine.
In addition, it is well known that sodium hypochlorite is not able to provide the removal of biofilms from the surface of pipelines that are favorable for the development of microorganisms and secondary water pollution.
This is the fifth advantage of chlorine.
It is also important that the replacement of gaseous chlorine with sodium or calcium hypochlorite for water disinfection instead of molecular chlorine does not reduce, but significantly increases the probability of formation of trihalomethanes (THM), which worsens the quality of water due to the fact that hypochlorite increases the pH and the THM formation process prolongs to several hours, and the greater their number, the greater the pH, all other conditions being equal. This is due to the fact that low-activity hypochlorite ions are not able to oxidize the most reactive parts of molecules of humic substances rapidly and therefore they react with them to form trihalomethanes.
This is the sixth advantage of chlorine
Comparison of the operating costs of disinfection systems by chlorine and hypochlorite, as well as the costs of their implementation, does not count in favor of hypochlorite.
(This is the seventh one)
When chlorine is replaced with hypochlorite, on the one hand, water quality worsens in terms of chemical composition and the bacteriological characteristics of water deteriorate, and on the other hand, the cost of water treatment increases.
This is the eighth advantage of chlorine.
There are no statistics for the CIS countries, only some data are available, but due to the increase in the use of hypochlorite, the picture of the accident rate will not differ much from countries having extensive experience in using this reagent.
This statistics of incidents with hypochlorite is quite explanable. On the one hand, the fact that the potential danger of hypochlorite is its complete incompatibility with acids, because when pH <5 NaClO hydrolysis reaction equilibrium shifts toward the formation of molecular Cl₂. Therefore, the largest accidents occur when hypochlorite is mixed with acids, which results in the emission of a gaseous cloud of chlorine. It should be noted that in such cases, moist chlorine is released, which, when penetrated into the lungs, does not cause pain, so it is most dangerous and leads to great sacrifices. On the other hand, these are constant gas releases during the natural decomposition of hypochlorite (see Figure 5).
Therefore, in cases where hypochlorite turned out to be between two closed locking devices, explosions of ball valves, filters, and other devices were observed. Moreover, chlorine was also a part of the released gas, therefore it was necessary to equip the rooms of pumping stations, tunnels, filter plants and other similar spaces with air purification systems, such as those that provide the elimination of the released chlorine, i.e. in accordance with the «Safety Rules during production, storage, transportation and use of chlorine.» PB 09-594-03, clause 5.11″ Premises where chlorine can be released should be equipped with automatic chlorine content detection and control systems.
If the maximum permissible concentration of chlorine (MPC) equal to 1 mg / m3 is exceeded, light and sound signaling and emergency ventilation interlocked with the emergency absorption system should be activated. When using an absorption method for catching accidental emission by a chlorine sensor signal, pumps to supply a neutralizing solution for the sanitary column irrigation, and then emergency ventilation must be turned on with a delay by the time required to supply the irrigating solution into the sanitary column. When using a two-threshold chlorine gas analyzer, if the concentration of chlorine exceeds 1 MPC, light and sound alarms should be triggered, and when 20 MPC is exceeded, emergency ventilation interlocked with the emergency absorption system must be activated.
There are problems with the selection of equipment, and with its operation in the environment of hypochlorite solutions, which have very high corrosive activity.
When sodium hypochlorite is used instead of gaseous chlorine, during the introduction of this reagent into the piping system for its dilution, a residue is formed there consisting of magnesium hydroxide and silicon dioxide, which clogs the water channels; therefore, additional measures are required to prevent calcination of the fittings, especially injection points – injectors and diffusers.
There are many such examples. And from all that has been said above, it follows that the use of hypochlorite solution, regardless of method of its production (industrial or on local plants) in comparison with chlorine does not only reduce the risk of accidents at industrial water treatment facilities, but also contributes to an intensive destructive impact on technological equipment, encouraging its premature breakdown.
This is the tenth advantage of chlorine
The decision timely made at a meeting of Rostekhnadzor on the topic: «The state and prospects for the development of chlorine-using facilities for the water treatment system of the housing and public utilities» in April 2008 noted that the facilities using imported or produced sodium hypochlorite, chlorine dioxide and ozone were dangerous and the Federal Law No. 116-FZ «On Industrial Safety of Hazardous Production Facilities» shall be applied to them, which was reflected in the new version of the Law dated 30.12. 2008 No. 313-FZ.
Thus, the transition to sodium hypochlorite according to the safety principle is faulty. This applies both to concentrated sodium hypochlorite of A grade with an active part content of 190 g/l produced by the industrial method and to a low concentration hypochlorite of E grade with an active part content of about 6 g/l produced at the site of its use. According to the UN classification, sodium hypochlorite is classified as corrosive — grade 8, No. UN — 1791, storage hazard group — PG II or PG III depending on concentration and according to existing «Hazardous Goods Regulations», storage of sodium hypochlorite in amounts of more than 250 liters requires registration of a license (licensing for corrosive substances and poisons). Safety requirements during production of chlorine by electrolysis method are set out in the chapter III of PB 09-594-03. The technology of chlorine production should exclude the possibility of formation of explosive chlorine-hydrogen mixtures in technological equipment and communications under standard operating procedures. However, considering the operation scheme of the electrolyzer producing a low concentration sodium hypochlorite solution from a salt solution in a flow-type electrochemical reactor, it should be noted that not pure hydrogen but an explosive mixture consisting of hydrogen, oxygen and chlorine is formed in the tank with the resulting hypochlorite solution. Only the ventilation of explosive electrolysis gases results in uncontrolled dispersion of chlorine in the atmosphere, which is not permissible and therefore the use of electrolyzers at the facility should include a device for chlorine emissions neutralization. Thus, the facilities on which sodium hypochlorite is used, stored, processed, etc. are classified as chemically hazardous objects, which are subject to registration in the State register of hazardous production facilities in the established order.
At this tenth position, chlorine and hypochlorite are equal