When water contains disease-carrying pathogenic bacteria, these bacteria have got to be killed before the water is supplied to the consumer, otherwise, it would lead to the outbreak of epidemics like typhoid, dysentery, cholera, etc. Sterilization of water means the killing of these bacteria. Common disinfection processes include the addition of chlorine or chlorine and ammonia, ozone treatment and ultraviolet ray treatment.
i. Chlorination
Chlorine is almost universally used for disinfecting water. It is cheap and reliable,and has a more lasting effect on the disinfected water. When chlorine is added to water, the following reactions take place:
Cl 2 + H 2 O HOCl + HCl
HOCl H + + OCl -
The hypochlorous acid HOCl, breaks up into H + ions and OCl - hypochlorite ions.It is the hypochlorous acid and the hypochlorite ions which carry out the disinfection.The amount of the hypochlorous acid and the hypochlorite ions in water is called free available chlorine.When chlorine is added to water, some of it is wasted in reacting with the organic and inorganic matter present in the water. Organic and inorganic matters react with the chlorine to neutralize it, and thereby they prevent the formation of hypochlorous acid. Chlorine also reacts with ammonia to form chloramines. This chlorine in water, in chemical combination with ammonia or other nitrogenous compounds, is known as the combined available chlorine.
The chlorine demand of a water is the difference between the amount of chlorine added, and the amount of chlorine left after the reactions are over. About ten minutes are sufficient to carry out disinfection after adding chlorine.The application of chlorine to an alkaline solution of water with high pH is more effective than the application of chlorine to acid solution with a low pH. If there are two samples of water, one with a pH of 4 and another with a pH of 8, and if other conditions are the same for both samples, then for the more acidic water, the amount of chlorine required will be about 150 times the amount of chlorine required for the alkaline water, to produce the same sterilizing effect. That shows that the effect of chlorine is more pronounced with an alkaline water with pH more than 7.
Chlorine is used mainly for sterilizing the water, but it can also be used in the removal of iron and manganese, or for the sterilization of water mains.Chlorine may be applied to water as a gas or in solution, or in the form of bleaching powder. The method of application does not affect the results. The factors that affect the results is whether the water is acidic or alkaline; and the amount of organic and inorganic matter present in the water.
Chlorine Dosing : The chlorine gas may be applied directly to the water coming out of the filters. Direct feeding of chlorine as gas is less satisfactory. Instead, it is preferable to dissolve chlorine gas in a small amount of water, and then to feed this solution to the water to be disinfected. For this, liquid chlorine stored in a steel cylinder is allowed to expand to become chlorine gas by passing it through a number of pressure-reducing valves. The chlorine gas is then mixed with water to form a solution with the help of an apparatus known as a solutionizer.
Bleaching Powder: Chlorine and quicklime, when reacted, produce chlorinated lime or bleaching powder (Ca(OCl) 2 ). Addition of bleaching powder to water yields hypochlorous acid. which kill the bacteria. Due to easy availability of gas and liquid chlorine, bleaching powder is already obsolete.
Amount of Chlorine Required: The amount of chlorine required depends upon the hydrogen-ion-concentration of the water, organic impurities present in it, the temperature of the water, and other factors. After mixing with other compounds, and after killing bacteria, whatever chlorine still remains in water is known as residual chlorine. Chlorine is applied in such a dose that the residual chlorine, found in the disinfected water ready for the consumer, is between 0.1 and 0.2 ppm. The dosage of chlorine required is generally between 0.2 and 1 ppm. The presence of residual chlorine in the water is considered desirable to ensure against future contamination of the water while it is on its way to the consumer through the distribution network.
ii. Chloramination
Chlorine alone, when fed to water, produces tastes and odours which are objectionable. As a remedy, ammonia is fed together with chlorine. The feeding of ammonia before adding chlorine to the water is known as chloramination. When chloramination is carried out, tastes and odours due to chlorine are removed.Chlorine with ammonia forms monochloramine (NH 2 Cl) and dichloramine (NHCl 2 ) compounds which remain stable in water and are disinfectants, though to a lesser extent than HOCl.
Chlorine first reacts with water to form hypochlorous acid, and hypochlorous acid then reacts with ammonia. The reactions are:
HOCl + NH 3 = NH 2 Cl + H 2 O
2HOCl + NH 3 = NHCl 2 + 2H 2 O
Ammonia and chlorine are generally added in the ratio of one part of ammonia to four parts of chlorine.
iii. Ozone Treatment
One single atom of oxygen, called nascent oxygen, is very active. It reacts with other substances immediately. Ozone (O 3 ) is a molecule containing three atoms of oxygen. It is highly unstable. It breaks down into oxygen (O 2 ) and an atom of nascent oxygen. The nascent oxygen oxidises the organic matter of bacteria and kills them. In chlorination, residual chlorine remains in water When chlorine is used, and causes taste and odours. In the case of ozone, tastes and odours are absent.But no disinfecting chemical remains in the water after sterilization (like residual chlorine), and so there is no proper safeguard against recontamination after the treatment. When compared to chlorination, ozone treatment costs more. Due to its highly unstable nature, ozone cannot be manufactured or stored or transported. It has to be produced just before use right at the place of treatment.
iv. Ultra Violet Ray Treatment
Ultra-violet rays can also be used for disinfecting water. These invisible light rays beyond the violet of the spectrum denature the Deoxy ribo Nucleic Acid (DNA) in microorganisms and there by kill them. These are very effective in killing all types of bacteria. These rays can be generated by passing an electric current through mercury enclosed in quartz-bulbs.The water to be disinfected is passed round the bulbs through a spherical container, so that the ultra-violet rays fall on the water several times, through a distance of not more than four inches. The rays are effective only over a short distance. The water should be free from turbidity and colour, otherwise the rays cannot pierce through, and cannot kill bacteria. For this reason, the turbidity of the water should not be more than 15 ppm.
No tastes or odours are produced by this process, but the cost is greater. The process can be used for small quantities of water. It may be used in pharmaceutical works or in industrial houses.
i. Chlorination
Chlorine is almost universally used for disinfecting water. It is cheap and reliable,and has a more lasting effect on the disinfected water. When chlorine is added to water, the following reactions take place:
Cl 2 + H 2 O HOCl + HCl
HOCl H + + OCl -
The hypochlorous acid HOCl, breaks up into H + ions and OCl - hypochlorite ions.It is the hypochlorous acid and the hypochlorite ions which carry out the disinfection.The amount of the hypochlorous acid and the hypochlorite ions in water is called free available chlorine.When chlorine is added to water, some of it is wasted in reacting with the organic and inorganic matter present in the water. Organic and inorganic matters react with the chlorine to neutralize it, and thereby they prevent the formation of hypochlorous acid. Chlorine also reacts with ammonia to form chloramines. This chlorine in water, in chemical combination with ammonia or other nitrogenous compounds, is known as the combined available chlorine.
The chlorine demand of a water is the difference between the amount of chlorine added, and the amount of chlorine left after the reactions are over. About ten minutes are sufficient to carry out disinfection after adding chlorine.The application of chlorine to an alkaline solution of water with high pH is more effective than the application of chlorine to acid solution with a low pH. If there are two samples of water, one with a pH of 4 and another with a pH of 8, and if other conditions are the same for both samples, then for the more acidic water, the amount of chlorine required will be about 150 times the amount of chlorine required for the alkaline water, to produce the same sterilizing effect. That shows that the effect of chlorine is more pronounced with an alkaline water with pH more than 7.
Chlorine is used mainly for sterilizing the water, but it can also be used in the removal of iron and manganese, or for the sterilization of water mains.Chlorine may be applied to water as a gas or in solution, or in the form of bleaching powder. The method of application does not affect the results. The factors that affect the results is whether the water is acidic or alkaline; and the amount of organic and inorganic matter present in the water.
Chlorine Dosing : The chlorine gas may be applied directly to the water coming out of the filters. Direct feeding of chlorine as gas is less satisfactory. Instead, it is preferable to dissolve chlorine gas in a small amount of water, and then to feed this solution to the water to be disinfected. For this, liquid chlorine stored in a steel cylinder is allowed to expand to become chlorine gas by passing it through a number of pressure-reducing valves. The chlorine gas is then mixed with water to form a solution with the help of an apparatus known as a solutionizer.
Bleaching Powder: Chlorine and quicklime, when reacted, produce chlorinated lime or bleaching powder (Ca(OCl) 2 ). Addition of bleaching powder to water yields hypochlorous acid. which kill the bacteria. Due to easy availability of gas and liquid chlorine, bleaching powder is already obsolete.
Amount of Chlorine Required: The amount of chlorine required depends upon the hydrogen-ion-concentration of the water, organic impurities present in it, the temperature of the water, and other factors. After mixing with other compounds, and after killing bacteria, whatever chlorine still remains in water is known as residual chlorine. Chlorine is applied in such a dose that the residual chlorine, found in the disinfected water ready for the consumer, is between 0.1 and 0.2 ppm. The dosage of chlorine required is generally between 0.2 and 1 ppm. The presence of residual chlorine in the water is considered desirable to ensure against future contamination of the water while it is on its way to the consumer through the distribution network.
ii. Chloramination
Chlorine alone, when fed to water, produces tastes and odours which are objectionable. As a remedy, ammonia is fed together with chlorine. The feeding of ammonia before adding chlorine to the water is known as chloramination. When chloramination is carried out, tastes and odours due to chlorine are removed.Chlorine with ammonia forms monochloramine (NH 2 Cl) and dichloramine (NHCl 2 ) compounds which remain stable in water and are disinfectants, though to a lesser extent than HOCl.
Chlorine first reacts with water to form hypochlorous acid, and hypochlorous acid then reacts with ammonia. The reactions are:
HOCl + NH 3 = NH 2 Cl + H 2 O
2HOCl + NH 3 = NHCl 2 + 2H 2 O
Ammonia and chlorine are generally added in the ratio of one part of ammonia to four parts of chlorine.
iii. Ozone Treatment
One single atom of oxygen, called nascent oxygen, is very active. It reacts with other substances immediately. Ozone (O 3 ) is a molecule containing three atoms of oxygen. It is highly unstable. It breaks down into oxygen (O 2 ) and an atom of nascent oxygen. The nascent oxygen oxidises the organic matter of bacteria and kills them. In chlorination, residual chlorine remains in water When chlorine is used, and causes taste and odours. In the case of ozone, tastes and odours are absent.But no disinfecting chemical remains in the water after sterilization (like residual chlorine), and so there is no proper safeguard against recontamination after the treatment. When compared to chlorination, ozone treatment costs more. Due to its highly unstable nature, ozone cannot be manufactured or stored or transported. It has to be produced just before use right at the place of treatment.
iv. Ultra Violet Ray Treatment
Ultra-violet rays can also be used for disinfecting water. These invisible light rays beyond the violet of the spectrum denature the Deoxy ribo Nucleic Acid (DNA) in microorganisms and there by kill them. These are very effective in killing all types of bacteria. These rays can be generated by passing an electric current through mercury enclosed in quartz-bulbs.The water to be disinfected is passed round the bulbs through a spherical container, so that the ultra-violet rays fall on the water several times, through a distance of not more than four inches. The rays are effective only over a short distance. The water should be free from turbidity and colour, otherwise the rays cannot pierce through, and cannot kill bacteria. For this reason, the turbidity of the water should not be more than 15 ppm.
No tastes or odours are produced by this process, but the cost is greater. The process can be used for small quantities of water. It may be used in pharmaceutical works or in industrial houses.
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