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Engineerin Chemist, , OF WATER FOR DOMESTIC, ) MUNICIPAL WATER TREATMENT, , Rivers and lakes are the most common sources Of Water, , sed by municipalities. These water should be free from, , used jal impurities. domestic sewages, industrial effluents ang, , nae producing bacterias. Hence domestic supply of water, involves the following stages in the purification processes., , , , 1.4 TREATMENT, SUPPLY (or, , , , , , , , , , Screening Aeration, , Coagulation ke Sedimentation |, , Sources of Water, , , , , , Filtration, , , , , , Sterilisation, (or), Disinfection, , , , 1. Primary Treatment, , 1. Screening, , It is a process of removing the floating materials like,, leaves, wood pieces, etc. from water. The raw water is allowed, to pass through a screen, having large number of holes, which, retains the floating materials and allows the water to pass., , 2. Aeration, The process of mixing water with air is known as, aeration. The main purpose of aeration is, (i) to remove gases like CO>, H5S and other volatile, impurities causing bad taste and odour to water., , (ii) to temove ferrous and manganous salts 2, insoluble ferric and manganic salts.

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rand its Treatment, , walt saeenaisniateshietenees Ld, , Sedimentation, , It is a process of removing suspended impurities by, , %, allowing the water to stand undisturbed for 2-6 hours in a big, , tank. Most of the suspended particles settle down at the butions, , due to forces of gravity, and they are removed. Sedimentation, , y P t, pemoves only 75% of the suspended impurities,, , ed, Coagulation, , Finely divided clay, silica, etc, do not settle down easily, and hence cannot be removed by sedimentation, Such, impurities are removed by coagulation method,, , In this method certain chemicals, called coagulants, like, alum, Al,(SO4)3 etc., are added to water, When the, , Al,(SOq)3 is added to water, it gets hydrolysed to form a, gelatinous precipitate of Al(OH);. The gelatinous precipitate of, A\(OH)3 entraps the finely divided and colloidal impurities,, , settles to the bottom and can be removed easily., , 5. Filtration, It is the process of removing bacteria, colour, taste, odour, , and suspended particles, etc., by passing the water through, filter beds containing fine sand, coarse sand and gravel. A, , typical sand filter is shown in the figure 1, , of a tank containing a thick top, layer of fine sand followed by coarse sand, fine gravel and, coarse gravel. When the water passes through the filtering, medium, it flows through the various beds slowly, The rate ot, filtration decreases slowly due to the clogging of impurities In, the pores of the sand bed. When the rate of filtration becomes, very slow, the filtration is stopped and the thick top layer of, fine sand is scrapped off and replaced with clean sand., Bacterias are also partly removed by this process., , The sand filter consists, , Ba

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Engineering Chemisp, 1.14, , ----- t——— Water, , , , inlet, , , , , , , , >, >, , %, ¥, Y, y, S:, », >, >, >, >, %, >, >, >, , Coarse sand, , >, >, >, >, », »y, >, yy, yy, %, y, ?, , cooo ltt hy, 0000, , >, >, , coool tse:, ooo0o0, , >, imsiniace hy., , A, , t+— Fine gravel, , tee ew eh)., wee ef, ==), eee eo oh),, : §, , - >, , . =f>, waehy, , >, , » wersreeht, , , , oooo|"**, coco |tttee:, coool tts:, , Coarse gravel, , 1] Opo° |°, |, , , , , , , , =—=> Water outlet, , , , Fig 1.1 Sand filter, , Il. Sterilisation (or) Disinfection, , The process of destroying the harmful bacterias is known, as sterilisation or disinfection. The chemicals used for this, purpose are called disinfectants. This process can be carried, out by the following methods., , 1. By using ozone, , Ozone is a powerful disinfectant and is readily absorbed, by water. Ozone is highly unstable and breaks down to give, nascent oxygen., , 0; —>0, + [0], , __ The nascent oxygen is a powerful oxidising agent and, kills the bacterias., , Disadvantages, , (a) es process is costly and cannot be used in large, scale,, , (b) Ozone #s unstable and cannot be stored for long, , time,

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____ Engineering Chomistry |, , 1.16, ation, , ‘hlorin, Break point Ch —_ —_, following, impurities, , Water contains the, (i) Bacterias., Organic impurities., , , , (ii), ‘| at 1 tC, (iii) Reducing substances (Fe , HS, etc.)., (iv) Free ammonia., § i ‘Destruction of, ‘2S ' Formation of {chloramine &, ge ' chloramine & ‘chloro , f is, 2 oe ' chloro compoun is o, & 3 B! compounds 1 S, Bal oe! Ke, = q 6} 1, 3|| 3 =, a Ss 2 | !, a 33 | |, ae i, , , , , , Applied chlorine a, , Fig 1.2 Break point chlorination, , Chlorine may be added to water directly as a gas or in, the form of bleaching powder. When chlorine is applied to, water, the results obtained can be depicted graphically in the, following Fig. 1.2. The graph shows the relationship between, the amount of chlorine added to water and the residual, , chlorine., It is observed from the graph that initially the applied, chlorine is used to kill the bacterias and oxidises all the, , reducing substances present in the water and there is no free, , residual chlorine., , As the amount of applied chlorine increases, the amount, of combined residual chlorine also increases. This is due lo, , the formation of chloramine and other chloro compounds: