THE METHODS OF DISINFECTION AND THE TYPES OF CHLORINATION

METHODS OF DISINFECTION AND TYPES OF CHLORINATION

1. DISINFECTION UNIT 5

2. Disinfection The filtered water may normally contain some harmful disease producing bacteria in it. These bacteria must be killed in order to make the water safe for drinking. The process of killing these bacteria is known as Disinfection or Sterilization. Disinfection Kinetics When a single unit of microorganisms is exposed to a single unit of disinfectant, the reduction in microorganisms follows a first-order reaction. dN / dt =- kN              N=N e -kt This equation is known as Chick’s Law:-     N = number of microorganism (N 0  is initial number)  k = disinfection constant t = contact time

3. Minor methods of disinfection Boiling of water Treatment with excess lime Treatment with ozone Treatment with iodine & bromine Treatment with ultra-violet rays Treatment with potassium permanganate Treatment with silver (Electro- katadyn process)

4. Methods of Disinfection Boiling :   The bacteria present in water can be destroyed by boiling it for a long time. However it is not practically possible to boil huge amounts of water. Moreover it cannot take care of future possible contaminations. Treatment with Excess Lime:   Lime is used in water treatment plant for softening. But if excess lime is added to the water, it can in addition, kill the bacteria also. Lime when added raises the pH value o water making it extremely alkaline. This extreme alkalinity has been found detrimental to the survival of bacteria. This method needs the removal of excess lime from the water before it can be supplied to the general public. Treatment like recarbonation for lime removal should be used after disinfection. It cannot protect water from recontamination.

5. Treatment with Ozone:   O zone gas is faintly blue gas of pungent smell and an excellent disinfectant. Can be produced by passing high tension electric current through a stream of air in a closed chamber. under high electric voltage Ozone readily breaks down into normal oxygen, and releases nascent oxygen. The nascent oxygen is a powerful oxidising agent and removes the organic matter as well as the bacteria from the water. + O About 2- 3 ppm of ozone is consumed to obtain a residual chlorine of 0.1 ppm for a contact period of 10 min. Ozone and chlorine are quite competitive disinfectants each having merits and demerits over each other. However chlorine is universally adopted.  

6. Treatment with iodine & bromine Addition of iodine & bromine to water helps in killing pathogenic bacteria. Quantity of this disinfectant may be limited to about 8 ppm and a contact period of 5 min. Now-a-days available in the form of pills and very handy. Not used for treating large scale water supplies, but may be used for treating small supplies for army troops, private plants and swimming pools

7. Treatment with ultra-violet rays Ultra violet rays are invisible light rays of wave lengths of 1000-4000 mµ Basically found in sun light, but can also be produced by passing electric current through mercury enclosed in quartz bulbs. Highly effective in killing all types of bacteria snd yields truly sterilized water. Water to be treated should be less turbid and color. Sterilization with UV rays does not impart any additional taste or odor to water. Very costly and needs technical support and may gets interrupted due to electricity failure. However treats small quantities of water in hospitals and dispensaries for surgical use and drinking purposes. Developed countries use this to treat swimming pools

8. Popular disinfectant in treating well water supplies in villages which gets contaminated with less amounts of bacteria. Kills bacteria and also oxidizes taste producing organic matter. Used as algicide and for removing color and iron from water Produces pink color and well should not be used for at least 48 hrs Normal dose is 1-2 mg/L and contact period is 4-6 hrs Cannot guarantee 100 % removal only 98% Treatment with potassium permanganate

9. Treatment with silver ( Electro- katadyn process) Metallic silver ions are introduced into water by passing it through a tube containing solid silver electrode connected to a D.C. supply of about 1.5 volts. Silver ions have strong germicidal action and acts as disinfectant. Dose may vary between 0.05- 0.1 mg/L for 15 min to 3 hrs Neither imparts taste or odor to water Does not produce any harmful effect to human body But very expensive and suspended organic matter and hydrogen sulphide should be removed before treating with silver

10. Chlorination :  The germicidal action of chlorine is explained by the recent theory of  Enzymatic hypothesis , according to which the chlorine enters the cell walls of bacteria and kill the enzymes which are essential for the metabolic processes of living organisms.

11. Chlorination – a process of disinfection Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water . Chlorine in its various forms is invariable and almost universally used for disinfecting public water supplies. It’s cheap, reliable, easy to handle, easily measurable and above all it is capable of providing residual disinfecting effects for long periods . It provides complete protection against further contamination of water in the distribution system . It’s only disadvantage is that when used in greater amounts, it imparts bitter and bad taste to waters . It is an effective method in water emergency situations as it can eliminate pathogens relatively quickly.

12. History of Chlorination Chlorine was first discovered in Sweden in 1744. At that time, people believed that odours from the water were responsible for transmitting diseases . In 1835, chlorine was used to remove odours from the water. After 1890, chlorine became a way to reduce the amount of disease transmitted through water Chlorination began in Great Britain and then expanded to the United States in 1908 and Canada by 1917 . Earlier used to kill Bactrian responsible for cholera, typhoid, dysentery, and some viruses like polio virus and hepatitis-A followed by parasitic protozoans

13. Chlorine Chemistry Chlorine is added to the water supply in two ways. It is most often added as a gas, Cl 2 (g). However, it also can be added as a salt, such as sodium hypochlorite ( NaOCl ) or bleach. Chlorine gas dissolves in water following Henry's Law.                   Cl 2(g )      Cl 2( aq )       Once dissolved, the following reaction occurs forming hypochlorous acid ( HOCl ):                 Cl 2( aq ) + H 2 O     HOCl + H +  + Cl - Hypochlorous acid is a weak acid that dissociates to form hypochlorite ion ( OCl -).                   HOCl   OCl -  + H +     All forms of chlorine are measured as mg/L of Cl 2  (MW = 2 x 35.45 = 70.9 g/ mol )

14. Hypochlorous acid and hypochlorite ion compose what is called the free chlorine residual. These free chlorine compounds can react with many organic and inorganic compounds to form chlorinated compounds . If the products of these reactions posses oxidizing potential, they are considered the combined chlorine residual. A common compound in drinking water systems that reacts with chlorine to form combined residual is ammonia. Reactions between ammonia and chlorine form chloramines, which is mainly monochloramine (NH 2 Cl), although some dichloramine (NHCl 2 ) and trichloramine (NCl 3 ) also can form. Many drinking water utilities use monochloramine as a disinfectant. If excess free chlorine exits once all ammonia nitrogen has been converted to monochloramine , chloramine species are oxidized through what is termed the breakpoint reactions.

15. The overall reactions of free chlorine and nitrogen can be represented by two simplified reactions as follows: Monochloramine Formation Reaction: This reaction occurs rapidly when ammonia nitrogen is combined with free chlorine up to a molar ratio of 1:1.                         HOCl  +NH 3    NH 2 Cl + HOCl Breakpoint Reaction: When excess free chlorine is added beyond the 1:1 initial molar ratio, monochloramine is removed as follows:                  2NH 2 Cl + HOCl   N 2(g ) + 3H + + 3Cl - + H 2 O

16. If ammonia is present in water, chlorine reacts with ammonia to form chloramines: HOCl + + Cl ( monochloramine ) HOCl + Cl + ( dichloramine ) HOCl + + ( trichloramine ) Chlorine existing in chemical combination with ammonia (chloramines) is called combined available chlorine  

17. Types of Chlorine Used in Water Treatment “ Pure chlorine” is seldom used for water treatment. The three most common chlorine-containing substances used in water treatment are chlorine gas, sodium hypochlorite, and calcium hypochlorite. The choice of the chlorine type to be used often depends on cost, on the available storage options and on the pH conditions required. Chlorination affects pH and pH affects results—a fact that is commonly overlooked in residential water treatment. Chlorine Gas Calcium Hypochlorite Sodium Hypochlorite

18. CHLORINE GAS: Chlorine gas is greenish yellow in color and heavier than air. Its high toxicity makes it an excellent disinfectant for water but also a hazard to humans who handle it. Chlorine gas, of course, is a deadly weapon when used in chemical warfare. It is a respiratory irritant and can irritate skin and mucous membranes and can cause death with sufficient exposure. Because of chemical changes that occur when it is introduced into water, chlorine gas is no more toxic to humans when used to treat drinking water than other forms of chlorine. Chlorine gas, which is actually sold as an amber- colored compressed liquid, is the least expensive form of chlorine and is, consequently, the preferred type for municipal water systems.

19. CALCIUM HYPOCHLORITE Calcium hypochlorite is manufactured from chlorine gas. It is best known as chlorine pellets and granules in residential water treatment. It is a white solid with a very pungent odor and it can create enough heat to explode, so it must not be stored near wood, cloth or petroleum products. Calcium hypochlorite increases the pH of the water being treated.

20. SODIUM HYPOCHLORITE Sodium hypochlorite is a chlorine-containing compound most easily recognized as household bleach . It is a light yellow liquid that has a relatively short shelf life. It is the easiest to handle of all the types of chlorine. Sodium hypochlorite also increases the pH of the water being treated. A lower concentration of chlorine in this form is needed to treat water than with calcium hypochlorite or chlorine gas.

21. TYPES OF CHLORINATION PLAIN CHLORINATION PRE –CHLORINATION POST CHLORINATION DOUBLE CHLORINATION SUPER CHLORINATION BREAK POINT CHLORINATION DECHLORINATION

22. Plain-Chlorination This term is used to indicate that only the chlorine treatment and no other treatment has been given to the raw water . Under plain chlorination, therefore, raw water is fed into the distribution system after giving chlorine treatment only. This helps in removing Bactria, organic matter and colour from the raw water . This technique may be used for treating relatively clearer waters (with turbidity less than 20 to 30 mg/L) obtained from lakes, reservoirs, etc . It may also be used during emergencies, when full-fledged treatment cannot given, such as for supplying water to army troop during the war times . Amount of chlorine required for plan chlorination is about 0.5 mg/L or more.

23. Pre-Chlorination It is the process of applying chlorine to the water before filtration or rather before sedimentation coagulation . It helps in improving coagulation , and reduces the loads on the filters. It also reduces the taste, odour, algae and other organisms . The chlorine dose should be such that about 0.1 to 0.5mg/L of residual chlorine comes to the filter plant. The normal doses required are as high as 5 to 10mg/L . Pre-chlorination is, however is always followed by post chlorination, so as to ensure the final safety of water.

24. Post-Chlorination Post chlorination sometimes called as Chlorination is the normal standard process of applying chlorine in the end, when all other treatment have been completed . While treating normal public supplies, post chlorination is adapted after filtration and before the water enters the distribution system . The dosage of chlorine should be such as to have a residual chlorine of about 0.1 to 0.2mg/l, having a contact period of about 30 min . The residual chlorine will ensure the disinfection of water, it will handle any further recontamination occurs in the distribution system.

25. Double Chlorination / Re - chlorination Double chlorination indicates that the water had been chlorinated twice. Chlorination and post-chlorination both are used in this process . Post-chlorination is generally always used but pre-chlorination is used when there is high turbidity and contamination . The water get both the benefits of pre chlorination and post chlorination . This is done when the distribution system is large and complex as the contamination can occur at the joints . Residual chlorine concentration level is 1mg/l.

26. Super chlorination It refers to the excess amount of chlorine (i.e. 5 to 15 mg/l). This may be required in special cases like highly polluted waters or during the epidemic of the water borne diseases. The quantity of chlorine added in this process is such as to give about 1 to 2mg/l of residue beyond the breakdown point, in the treated waters. Sometimes higher dosage of chlorine may be used and the resultant waters to be de chlorinated by using de-chlorinating agents like activated carbon, sodium thyosulphate , sulphur dioxide etc. This ensures the removal of the bad taste and odour causes by presence of excess chlorine. In such cases, desirable amount of chlorine is added to the waters of about 0.1 to 0.2 mg/l. This process in generally done when emergency times for providing immediate water supply (having a very less contact period

27. Break-point chlorination It is a term which gives an idea of the extent of chlorine added to the water. It represents, that much dose of chlorination, beyond which any further addition of chlorine will appear as free residual chlorine . When chlorine is added in waters, first it generally reacts with ammonia present in waters to form chloramines. If the residual chlorine is tested at this stage then it can be seen that the amount of residual chlorine keeps on increasing. It is the first stage of breakpoint chlorination (denoted by curve AB).

28. If the chlorine is further added beyond point B, the organic matter present in the waters gets oxidised and hence, the residual chlorine content suddenly falls down as show by the curve BC. At point B where the oxidation of organic matter starts it gives a bad smell and taste which disappears at Break point (c). This is the second stage of the break point chlorination. The point “C” is the point beyond which any further addition of chlorine will appear equally as free chlorine, nothing of it will be utilized. This Point “C” is called as breakpoint as any amount of chlorine added beyond this point will appear as residual chlorine. The addition of chlorine be this point is known as break-point chlorination . Generally the amount of residual chlorine beyond point “C” is 0.2 to 0.3mg/l of free chlorine.

29. The disinfection process by chlorine responds to a curve known as the curve of the Breakpoint in which the formation of chloramine compounds, apart from others, occurs. During the initial phase, all the chlorine added is used to be combined with organic matter so that, as a result, the residual chlorine level is zero. Arriving at the stage BB', the level of chlorine residuals increases, but all this chlorine is combined in the form of chloramines, which are products that have a low disinfectant power hence producing an unpleasant smell. These compounds are the cause of the so-called pool smell. During BC phase the added chlorine is used to destroy chloramines by which the measured residual chlorine decreases up to a minimum in C called Breakpoint. From this point on, all the added chlorine is used to increase the residual chlorine that would be as free chlorine and with more sanitizing power rather than combined chlorine forming chloramines. Therefore, this Breakpoint must be overcome so that the combined chlorine be the minimum possible.

30. De - c hlori n a tion It indicates “Removal of the chlorine from the waters” which is generally done after super chlorination process. This process can be taken place in 2 ways:- Either by removing the chlorine to such a extent that sufficient amount of residual chlorine remains in water. Or by completely removing the chlorine from waters and again adding the chlorine to prevent it from further contamination . This process of dechlorination is done by dechlorinating agents like Activated carbon, sodiumthyosulphate , sulphur dioxide gas and ammonia. Note:- Ammonia is generally used as dechlorinating agent as is economical and easily reacts chlorine to form chloramines

31. 10 Chlorination - A way to disinfect domestic water supplies 8 / 9 / 201 6

32. Chlorine demand and its measurement Chlorine demand is the difference between total chlorine added in the water and residual chlorine. It is the amount which reacts with the substances in water, leaving behind an inactive form of chlorine. Chlorine demand can be caused in a water body due to rain containing ammonia or the addition of fertilizers which can be oxidized by chlorine. To purify water supplies and make them suitable for purposes like drinking, cooking and swimming, chlorine is added. Chlorine demand from the total chlorine added can be explained using the following equation: Chlorine demand = Total chlorine – Chlorine residual

33. The purity of water can be determined by monitoring the value of chlorine demand. If the value is zero, the water is already free of pathogenic microorganisms. If the value is less than the total chlorine, it shows that the amount of chlorine added initially to the water was sufficient . Residual chlorine is the amount of chlorine that remains in the water after a certain period or contact time. There are three forms of residual chlorine in water treatment: Free - Residual chlorine composed of dissolved hypochlorite ions, hypochlorous acid and chlorine gas Combined - Composed of chloramines that can kill bacteria and oxidize organic matter Total - The sum of free and combined residual chlorine.