Disinfection
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Disinfection of water involves chemical treatments to eliminate bacteria, with methods including chlorination, ozonization, and UV ray application. Chlorination is the most common technique, utilizing chlorine in various forms, while other methods like ozonization and UV disinfection offer alternatives with specific advantages and drawbacks. Understanding chlorine dosage and application methods, including pre-chlorination and breakpoint chlorination, is crucial for effective water treatment.
Disinfection
- 1. Disinfection – Raw water treatment method By Mr. Ravi Kumar GARRE
- 2. Disinfection • The treatment of water with chemicals to kill bacteria is called as disinfection. • Sterilization, is boiling of water before using for domestic purposes. • Boiling kills disease germs of cholera and typhoid within few minutes. • Chlorination, ozonization, ultra-violet ray method and application of silver, Iodine of Bromine method are the principal methods used for disinfection of water.
- 3. Disinfecting materials/ disinfectants • The disinfecting material should be harmless and unobjectionable to the consumer. • It should be able to retain residual disinfecting effect for a long period. • The common materials for disinfection are: Chlorine Ozone Lime Silver, Iodine and Bromine Ultraviolet rays
- 4. Chlorination • Chlorination is the application of small quantities of chlorine or chlorine compounds to water. • The dose applied is generally less than 1 mg/lit. • The amount of chlorine required to be added depends upon the chlorine demand of water. • Chlorine demand is difference between the amount of chlorine added and the amount of chlorine remaining at the end of a contact period of a 10 – 20 minutes. • Chlorination possesses great disinfecting powers. • It is universally accepted method for public water supplies
- 5. Theory of chlorination • Chlorine hydrolyses in water to form hypochlorous acid (HOCl) and further produce hypochlorite ion (OCl). The HOCl and Ocl together are known as free available chlorine. • If Ammonia is also present in water, chlorine reacts to form chloramines (monochloromine, dichloramine and trichloramine). The chlorine existing in the form of chloramines is called as combined available chlorine. • These resulting chlorine compounds in water interfere with certain enzymes in the bacterial wall forming a toxic chloro-compounds thus destroying bacteria completely.
- 6. Theory of chlorination • The effect of chlorine as disinfectant depends on the contact period and the concentration of chlorine in water. • The killing power of disinfectant is proportional to the product of contact period and chlorine concentration. • The factors affecting chlorination are: 1. pH value of water 2. Water temperature 3. Residual chlorine in the form of free available or combined available chlorine
- 7. Characteristics of chlorine In the application of chlorine to water, certain important characteristics of chlorine should be understood. • Greenish-yellow gas • 2.5 times heavier than air • When compressed chlorine gas liquifies • When liquid chlorine drawn from cylinder, it changes into gas and the temperature inside the cylinder falls.
- 8. Application of chlorine • Chlorine can be applied by any of the following methods: 1. As dry chlorine gas 2. As chlorine liquid/ solution 3. In powder form (bleaching powder/ sodium hypochlorites)
- 9. Method-1 (as dry chlorine gas) • As dry chlorine gas from the liquid chlorine cylinder, it is applied directly to water. • By supplying through submerged diffusers. • This method is unsatisfactory because of improper and • It caused corrosion to pipes.
- 10. Method-2 (as liquid chlorine solution) • Liquid chlorine solution is prepared in a solution feed chlorinator, by mixing chlorine gas with a small quantity water. • The chlorine solution is applied to the water supply by using a water injector through a discharge pipe. • This method is commonly used in water works practice.
- 11. Method-3 (in powder form) • In powder form chlorine available as two forms as hypochlorites: 1. Bleaching powder – Ca(Ocl)2 2. Sodium hypochlorite – NaOCl • Hypochlorites are applied to water by using hypochlorite feeding apparatus. • Bleaching powder is not as stable as high strength hypochlorites. It loses strength on long storage or exposure.
- 12. Chlorine dosage of water • Too little chlorine is ineffective and too much chlorine cause taste and odours. • So the amount of chlorine required to be added should be determined. • It can be determined in the laboratory: by adding various doses of chlorine to equal proportions of water sample and finding the amount of residual after a period of contact of 10-20 minutes. • Chlorine demand is difference between the amount of chlorine added and the amount of chlorine remaining at the end of a contact period of a 10 – 20 minutes.
- 13. Chlorine dosage of water • The minimum dose giving a residual of 0.05 – 0.20 mg/l is generally selected. • Chlorine dosage can also be calculated with help of following formula: Dosage in kg. of chlorine = (volume of water in lit. X dosage in mg/l)/1,000,000
- 14. Special methods of chlorination • Chlorine is generally applied after all other treatments have been given to the water supply. This may be termed as post chlorination. • There are other special methods of chlorination, depend upon the particular purpose: 1. Pre-chlorination 2. Double chlorination 3. Super chlorination 4. Break point chlorination
- 15. Pre-chlorination • Pre-chlorination is the application of chlorine preceding filtration. • Either added in to pipe lines or to water as it enters in the mixing basin. • Pre-chlorination reduces bacterial load on filters and oxidising excessive organic matter thus removing taste and odour.
- 16. Double chlorination • Double chlorination is the application of chlorine at two points in the treatment process. • It includes pre-chlorination before filtration and post chlorination after filtration also. • Advantage of double chlorination: 1. Decrease the load on filters 2. Greater efficiency in removal of bacteria 3. Control of algae and slimy growths in clarifiers and filters
- 17. Break point chlorination • Also called as free-residual chlorination • It involves the addition of sufficient chlorine to oxidize all the organic matter. • It reduces substances and free ammonia in raw water. • It leaves free residual chlorine which possesses strong disinfecting action against pathogens.
- 18. Break point chlorination • It is observed that, the applied chlorine to water, the reactions are marked as follows: A – destruction chlorine by reducing compounds B – formation of chloro- organic compounds and chloramines C – destruction of chloro- organic compounds and chloramines D – formation of free available chlorine
- 19. Break point chlorination • The addition of chlorine at the break ( or dip) is termed as break point chlorination. • Because of highly persistent and powerful disinfection possessed by free available chlorine, any type of pathogens present in water destroyed making disinfection highly effective.
- 20. Ozonization • Ozone contains high oxidizing power. So it is used for disinfection. • Ozone contains one Oxygen molecule (O2) and one nascent oxygen (O). • The nascent oxygen reduce organic matter present water. • The ozone dose is 2 to 3 mg/l. • Need contact period as 10 minutes. • Advantages: It will not produce objectionable tastes and odours like chlorine. • Disadvantages: it is costly to manufacture and not quite suitable to highly turbid waters.
- 21. Ultra-violet rays • UV rays is an effective method for disinfecting of clear water. • The rays are generated by passing electric current thorough mercury-vapour lamp enclosed in quartz bulb. • water requiring disinfection is passed over the lamp. • Advantages: no taste, no odour in water and no danger of over dose. • Disadvantages: high cost and not suitable for small installations.
- 22. Excess lime • Sufficient lime kills bacteria. • Coliform reduction may be high as 99%. • Dose to be given between 10 to 20 mg/l. • Advantage: combined objectives of softening and disinfection • Disadvantage: it is necessary to remove excess lime after disinfection through recarbonation.
- 23. Silver • This process also called as electro-katadyn process. • Tubes of silver electrodes contained in hollow cylinders are used. • Allow water to inside of these hollow tubes, thus water gets energized, thus bacteria will be killed. • But it is ineffective in killing bacterial spores and algae. • High cost.
- 24. Iodine and Bromine • Iodine and Bromine possess disinfecting power. • Iodine and Bromine are cheaply available in the form of pellets. These are restricted to small quantities of waters like swimming pools. • Dosage is 8 to 10 mg/l. • Only objection is it results medicinal taste.
- 25. Potassium permangane • It is commonly known as ‘pinki’ or ‘lalpani’. • Its action is principally oxidizing capacity on organic matter. • Dosage is 0.5 mg/l. • Effective in killing of cholera vibrio only. Not effective for other types of germs. • Disadvantage: it produces coating on glass and porcelain vessels which are difficult to remove.