In chlorination method of disinfection chlorine (or its compounds) is used as disinfectant. Chlorine is an element, having the symbol CI with an atomic weight of 35.45, melting point -101.5°C and boiling point 34.5°C. Gaseous chlorine is greenish yellow in colour and is approximately 2.5 times heavier than air. Under pressure, it is a liquid with an amber colour and oily nature approximately 15 times as heavy as water. Liquefaction of chlorine gas is accomplished by drying, cleaning and compressing the gas to 35 kg/cm2.
It is soluble in water to the extent of 4.61 volumes to 1 volume at 0°C and 2.26 volumes to 1 volume at 20°C, and the solution is called chlorine-water. Chlorine- water is unstable and its decomposition is particularly rapid on exposure to sunlight. Chlorine gas has a pungent odour which causes irritation when inhaled.
It causes serious damage to lungs and other tissues even if present in the atmosphere in more than the minutest traces and may result in death of the persons inhaling the gas. Severe coughing may be caused by the presence of 1 volume of the gas in 10000 volumes of air. Chlorine is non-combustible, but it supports combustion. In the presence of moisture, it is very active-chemically and corrosive to metals.
Action of Chlorine:
When chlorine is added to water, the following reaction takes place:
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Cl2 + H2O ↔ HOCl + H+ + CI- (Hydrolysis)
This hydrolysis reaction is reversible.
The hypochlorous acid (HOCl) dissociates into hydrogen ions (H+) and hypochlorite ions (OCl–) as indicated below:
HOCl ↔ H+ + OCl– (Ionization)
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This reaction is also reversible.
It is the hypochlorous acid (HOCl) and the hypochlorite ions (OCl–) which accomplish disinfection of water. The undissociated HOCl is about 80 to 100 times more powerful as disinfectant than the OCl ion. Further the chlorine existing in water as hypochlorous acid, hypochlorite ions and molecular chlorine is defined as free available chlorine.
Both the above noted reactions depend on the pH value of water. When the pH value of the chlorinated water is above 3, which is normally the case, the hydrolysis reaction is almost complete and the chlorine exists entirely in the form of HOCl. As the pH value of water increases, more and more HOCl dissociates to form OCl ions.
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At pH values of 5.5 and, below it is practically 100% unionised HOCl, while at pH values above 9.5 it is all OCl ions. For pH values between 6.0 to 8.0, there occurs a very sharp change from undissociated to completely dissociated hypochlorous acid with 96% to 10% of HOCl, with equal amounts of HOCl and OCl ions being present at pH value 7.5 as shown in Fig. 9.34. The addition of chlorine does not produce any significant change in the pH value of the natural waters because of their buffering capacity.
Relative effectiveness of the components of free available chlorine the relative effectiveness of the components HOCl and OCl– of free available chlorine can be expressed in terms of mathematical expressions.
In hydrolysis reaction the hydrolysis constant Kh, (or stability coefficient) is expressed as-
The value of Ki depends on temperature of water as indicated in the following Table:
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Combined Available Chlorine:
The free chlorine can react with compounds such as ammonia, proteins, amino acids and phenol that may be present in water to form chloramines and chloro-derivatives which constitute the combined chlorine. This combined available chlorine possesses some disinfecting properties though to a much lower degree than the free available chlorine. Since these reactions are usually not 100% complete, some free available chlorine exists along with combined available chlorine.
The reaction between free available chlorine and ammonia is indicated by the following equations:
The monochloramine (NH2Cl) and the dichloramine (NHCl2) have disinfectant properties but about twenty five times less than that of free chlorine. The trichloramine has no disinfectant properties at all.
The trichloramine is found only at pH value below 4.4. Between pH values 4.4 to 5.5, only dichloramine exists and in the range of 5.5 to 8.4 both mono and dichloramines exist in a ratio fixed by the pH value. At pH value 7.0, equal quantities of mono and dichloramines are found and at pH value above 8.4 only monochloramines are found.
Chlorine Demand:
Chlorine and chlorine compounds by virtue of their oxidising power first react with organic and inorganic materials present in water before any disinfection is achieved. The amount of chlorine consumed in the oxidation of these materials present in water is known as chlorine demand of water. After the chlorine demand is fulfilled, the amount of unreacted chlorine left in water as a residual chlorine will serve as a disinfectant to kill the pathogenic organisms present in water.
It is, therefore, essential to provide sufficient time and sufficient dose of chlorine to satisfy the various chemical reactions and leave some amount of unreacted chlorine as residual for the disinfection of water. Chlorine demand is, therefore, the difference between the amount of chlorine added to water and the amount of residual chlorine after a specified contact period.
Dosage of Chlorine:
The amount of chlorine required to be added to the water sample can be determined in the laboratory by adding varying doses of chlorine to equal portions of the water sample and finding the amount of residual chlorine after a period of contact of 10 minutes.
The dose of chlorine which leaves a residual chlorine of about 0.2 mg/litre at the end of 10 minutes contact period is selected which gives the optimum dose of chlorine for the given water sample. This total dose of chlorine in mg/litre minus the residual chlorine (i.e., 0.2 mg/litre) thus represents the chlorine demand of the water sample.
Forms of Application of Chlorine:
Chlorine may be applied to water for disinfection in one of the following ways:
1. Bleaching Powder:
Bleaching powder or calcium hypochlorite Ca(OCl)2 is a chlorinated lime. When bleaching powder is added to water it dissociates into calcium Ca++ and hypochlorite OCl– ions as indicated below –
Ca (OCl)2 ↔ Ca++ + 2OCl–
The hypochlorite ions (OCl–) combine with hydrogen ions present in water to form hypochlorous acid (HOCl) as indicated below:
H+ + OCl– ↔ HOCl
This process of chlorination is known as hypo-chlorination.
The bleaching powder contains about 30 to 35 percent of available chlorine. It is a very unstable compound and it goes on losing its chlorine content when exposed to atmosphere, and hence it requires very careful storing.
The usual quantity of bleaching powder required for normal water is about 0.50 to 2.50 kg per million litres of water. The required quantity of bleaching powder is taken and dissolved in water to form a concentrated solution of it. This solution is then added in required quantity to water to be disinfected.
At present the process of hypo-chlorination is carried out by using commercial compounds such as HTH (high test hypochlorite), Pittchlor, Pittcide, Hoodchlor, Perchloron, etc., instead of bleaching powder. These compounds have a chlorine content of about 65 to 70 percent, are stable, easily soluble and non-hygroscopic.
The process of hypo-chlorination is generally not adopted for large public water supply schemes. It may, however, be adopted for small installations such as small colonies, swimming pools, etc.
2. Chloramines:
Chloramines are the compounds formed by the reactions between ammonia and chlorine. These compounds are quite stable in water and remain in water as residuals for a sufficient time, contrary to the unstable chlorine which evaporates after some time. Further chloramines do not cause bad taste and odour when left as residuals, as is caused by chlorine alone. However, chloramines are much weaker disinfectants as compared to free chlorine.
For producing chloramines, ammonia is added to water generally in the ratio of one-half to one-fourth of the amount of chlorine. Ammonia dissolves quickly in water, but it does not diffuse easily in water. Hence it is necessary to mix it with the help of mechanical means, at least 20 minutes to 1 hour earlier than the application of chlorine.
Ammonia may be used in the form of gas or as solution or as ammonium sulphate or as ammonium chloride. Since the disinfecting reactions are much slower in the case of chloramines, the water after treatment with chloramines should be supplied to consumers after an interval of about 20 minutes to 1 hour.
The following are the advantages of treatment for disinfection with chloramines:
(i) It is more effective than chlorine alone. Also its bactericidal effect persists for a longer duration. The residual chlorine may be up to 60 to 90 percent of the chlorine added even after 20 hours.
(ii) The quantity of chlorine required becomes less, especially if large amount of organic matter is present.
(iii) It does not cause bad taste and odour.
(iv) Water treated with chloramines causes less irritation to nose and eyes. Hence it is more useful for treating water for swimming pools.
(v) There is no danger of overdose of the disinfectant.
3. Chlorine Gas or Liquid Chlorine:
There are two methods of application of chlorine to water to be disinfected:
(i) Chlorine gas may be fed directly to the point of application to the water supply, or
(ii) Chlorine gas may first be dissolved in a small flow of water and the chlorine-water solution is fed to the point of application to the water supply.
The first method of application of chlorine is less expensive but it is less satisfactory because of poor diffusion of chlorine in water. Further it is found that at low temperatures (<10°C) crystalline hydrates of chlorine are formed, and hence when chlorine is directly fed through pipelines and if temperature falls down, choking of pipes leading chlorine may take place. There is also a possibility of corrosion in pipes and valves resulting from accumulation of undissolved chlorine gas. As such only the second method of application of chlorine is used.
In order to feed a regulated quantity of chlorine to the water to be disinfected an apparatus (or device) known as chlorinator is used.
The chlorinators are of two types viz.:
(a) Pressure type gravity feed chlorinator, and
(b) Vacuum type chlorinators.
(a) Pressure Type Gravity Feed Chlorinator:
In the pressure type gravity feed chlorinator dry gas at slight pressure is introduced into a cylindrical tower made of corrosion resistant material. Water is introduced at the top of tower. As water flows down slowly, it gradually absorbs the chlorine. The resultant solution flows out of the tower by gravity to the point of application.
(b) Vacuum Type Chlorinator:
In a vacuum type chlorinator chlorine gas is maintained under vacuum throughout the system. This type of chlorinator is most common because of safe operation.
An automatic vacuum chlorinator consists of a glass bell jar (A) placed in tray (B) in which a seal is maintained with some constant water level. Chlorine gas is supplied to the bell jar from the chlorine cylinder through a float controlled needle valve (C). The chlorine cylinder contains liquid chlorine which evaporates and enters the bell jar as gas. In the bell jar the chlorine gas mixes with water and chlorine-water solution is formed. When water flows through ejector (F) vacuum is created, due to which the chlorine-water solution present in the bell jar enters the glass orifice tube (E) which encloses a tube (D) leading the chlorine-water solution to the ejector. A scale is provided on or near the glass orifice tube (E) to measure the quantity of chlorine being fed.
The water level in the glass orifice tube above the water level in the bell jar gives a measure of the differential head on the orifice. The chlorine-water solution from the ejector is fed directly to the main flow of water. A vacuum relief valve (or vacuum pressure breaker) is provided to prevent the possibility of water being drawn into the apparatus.
Advantages of Using Free Chlorine as a Disinfectant:
Chlorine in gaseous or liquid form is nowadays universally adopted as disinfectant for public water supplies owing to its following advantages:
(i) It can be stored for a long period without any risk of its deterioration.
(ii) It occupies less space for storage.
(iii) The chlorine dosage can be precisely controlled and there are no chances of overdose or under dose.
(iv) The initial cost of installation of chlorination plant as well as the cost of disinfection is comparatively less.
(v) It is relatively cheap and easily available.
(vi) It is a powerful disinfectant and may remain in water as residual for sufficient time.
(vii) It can be uniformly applied to the entire body of water.
(viii) It can be easily and cheaply transported.
(ix) The operation of the chlorination plant does not require much skilled supervision. However, the storage and handling of chlorine, especially liquid chlorine requires careful handling.
(x) No sludge is formed in its application, as may be formed by using hypochlorite or chloramines.
4. Chlorine Dioxide Gas (ClO2):
Chlorine dioxide gas is produced by passing chlorine gas through sodium chlorite solution as indicated by the following equation –
2NaClO2 + Cl2 ↔ 2NaCl + 2ClO2
Since chlorine dioxide gas is unstable it is commonly generated at the point of its use by the introduction of sodium chlorite solution into the chlorinator discharge line. However, the aqueous solution of chlorine dioxide is stable.
Chlorine dioxide has an oxidizing capacity 2½ times that of chlorine. Further it is most effective in the removal of tastes and odours, particularly those which are caused by phenolic substances and algal growths. It is also reported to be more effective than chlorine as a bactericide and as a sporicide, and its action is relatively unaffected by pH values between 6 and 10. It, however, does not combine with ammonia and most organic impurities before oxidizing them.
Although chlorine dioxide is a very effective and powerful disinfectant but due to its high cost of production and application its use solely for the purpose of disinfection is not economical. However, it can be most economically applied at any point after oxidation of the organic matter by chlorine.
As such in the use of chlorine dioxide the procedure adopted may comprise chlorination for disinfection followed by the application of chlorine dioxide to remove tastes and odours. The dose of chlorine dioxide required for effective removal of taste and odour varies from 0.5 to 1.5 ppm.
Determining the Amount of Residual Chlorine:
In order to ensure satisfactory disinfection of water by chlorination it is necessary to determine the amount of residual chlorine (both free and combined) in water.
Several tests have been developed for determining the amount of residual chlorine (both free and combined) in water, but the following three tests are usually employed:
1. Orthotolidine Test:
The orthotolidine test is most commonly used to determine the amount of residual chlorine in water. Orthotolidine is an organic compound which is oxidized by chlorine into a yellow coloured compound called holoquinone, i.e.-
Orthotolidine + Chlorine → Holoquinone
Thus when orthotolidine is added to water containing chlorine a yellow colour develops, the intensity of which is proportional to the amount of residual chlorine present in water. In other words greater is the amount of residual chlorine present in water, deeper will be the yellow colour produced.
In this test, 10 ml (milliliter) of chlorinated sample of water is taken after the required contact period, in a glass tube. To this 0.1 ml of orthotolidine solution is added and the yellow colour produced is noted. The amount of residual chlorine present in the water sample is ascertained by comparing the colour developed in the glass tube with the coloured glass standards tinted for each concentration of residual chlorine.
The following points are to be noted in the case of orthotolidine test:
(i) For potable water a lemon yellow colour is considered to be safe.
(ii) When water is highly alkaline, a blue tinge is formed instead of yellow. In such a case the quantity of orthotolidine to be added should be doubled.
(iii) If chlorine has been used as disinfecting material, colour of water should be observed 5 minutes after orthotolidine solution is added, and if chloramines are used as disinfecting material, colour of water should be observed 15 minutes after orthotolidine solution is added. In each case the deepest colour of water developed should be recorded for the analysis of chlorine content in water.
(iv) It is possible to distinguish between free residual chlorine and combined residual chlorine in water. The reaction of orthotolidine with free residual chlorine is instantaneous and it is completed within about 5 seconds of the addition of orthotolidine. On the other hand the reaction of orthotolidine with combined residual chlorine is very slow and it begins only after about 5 to 10 seconds of the addition of orthotolidine, and it continues for about 5 minutes.
As such the colour produced after 5 seconds of the addition of orthotolidine when compared with the coloured glass standards will give the amount of free residual chlorine in water, and that produced after 5 minutes of the addition of orthotolidine when compared with the coloured glass standards will give the amount of total residual chlorine, (i.e., the sum of free and combined residual chlorine) in water. The amount of free residual chlorine when subtracted from the amount of total residual chlorine will give the amount of combined residual chlorine in water.
However, the orthotolidine test is not able to correctly discriminate between free and combined residual chlorine in water. Further the orthotolidine test is not fool-proof, because the presence of certain substances in water may interfere with the results obtained by the test.
The interfering substances include nitrite, ferric compounds, manganese compounds, organic iron compounds, lignocelluloses and micro-organisms. The effect of these substances is to produce yellow colour with orthotolidine thereby indicating incorrect value of the residual chlorine. In order to overcome these difficulties orthotolidine-arsenite test has been developed.
2. Orthitolidine-Arsenite Test:
In this test in addition to orthotolidine, sodium arsenite (NaAs02) is used which acts as a dechlorinating agent. The principle of the test is that chlorine both free and combined is removed by addition of sodium arsenite whereas the colour produced by the reaction of orthotolidine with chlorine as well as with interfering substances remains unaffected.
The test is carried out as follows:
(i) 10 ml of water sample is taken in a glass tube and to this 0.5 ml of sodium arsenite solution is added, which dechlorinates the water sample. Then 0.5 ml of orthotolidine solution is added and the colour produced is noted. Since chlorine has been eliminated from the water sample, the colour produced will be entirely due to the interfering substances.
The colour produced is compared with the coloured glass standards and the corresponding chlorine concentration is recorded. Let the intensity of the colour produced in this case correspond to a chlorine concentration of R1.
(ii) Another 10 ml of water sample is taken in a glass tube and to this 0.5 ml of orthotolidine solution is first added and just after 5 seconds, 0.5 ml of sodium arsenite solution is added. The sodium arsenite will arrest the colour produced by the combined residual chlorine. Hence in this case the colour produced will be due to the free residual chlorine plus that due to the interfering substances.
The colour produced is compared with the coloured glass standards and the corresponding chlorine concentration is recorded. Let the intensity of the colour produced in this case correspond to a chlorine concentration of R2.
(iii) A third 10 ml of water sample is taken in a glass tube and to this 0.5 ml of orthotolidine solution is added, and 5 minutes after the addition of orthotolidine solution the colour produced is noted. In this case the colour produced will be due to the free and combined residual chlorine plus that due to the interfering substances.
The colour produced is compared with the coloured glass standards and the corresponding chlorine concentration is recorded. Let the intensity of colour produced in this case correspond to a chlorine concentration of R3.
The amounts of free and combined residual chlorine in the water sample may be obtained as follows:
Amount of free residual chlorine = (R2 – R1)
Amount of combined residual chlorine = (R3 – R2)
If water does not contain any of the interfering substances, step (i) will not be necessary.
Both orthotolidine as well as orthotolidine-arsenite tests should be carried out under well controlled and understood conditions in order to avoid error in the interpretation of the results.
3. Starch-Iodide Test:
Starch-iodide test (also called iodometric method) is more precise than the orthotolidine test, particularly when residual chlorine is greater than 1 p.p.m.
In starch-iodide test the following procedure is adopted to determine the amount of residual chlorine in water:
(i) One litre of water sample is taken in a casserole which is a heat-proof earthenware vessel.
(ii) 10 ml of potassium-iodide solution is added to the above water sample and thoroughly mixed with it.
(iii) 5 ml of starch solution is then added to the above mixture which produces blue colour.
(iv) This blue colour is removed by titration with N/100 sodium thiosulphate solution.
(v) The amount of chlorine is then ascertained by using the following relation:
