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Purification of Water

Purification of Water

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Purification of Water

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  1. Purification of Water

  2. Index: • Purification of water - on a large scale - on a small scale

  3. the purpose of water treatment is to produce water that is • safe and wholesome • LARGE SCALE • Storage • Filtration • Disinfection • SMALL SCALE

  4. PURIFICATION OF WATER ON A LARGE SCALE

  5. STORAGE • water is drawn from the source and impounded in natural • or artificial reservoirs • storage → considerable amount of purification 1. Physical:- • about 90% of suspended impurities settle down in 24 • hours by gravity • - water becomes clearer, allows penetration of light

  6. 2. Chemical:- • aerobic bacteria oxidize organic matter present in • water with the aid of dissolved oxygen 3. Biological:- - a tremendous drop in bacterial count

  7. - pathogenic organisms gradually die out • optimum storage period of river water is about 10-14 days - longer storage periods leads to development of vegetable growths → bad smell and color

  8. FILTRATION - an important stage - 98 - 99% of the bacteria are removed - two types of filters in use - Biological / Slow sand filters - Mechanical / Rapid sand filters

  9. SLOW SAND OR BIOLOGICAL FILTERS • first used for water treatment in 1804 in Scotland and then • in London ELEMENTS OF A SLOW SAND FILTER: • supernatant (raw) water • a bed of graded sand • an under-drainage system • a system of filter control valves

  10. 1. SUPERNATANT WATER: - 1 → 1.5 meter depth - provides constant head of water and a waiting period of some hours (3 – 12 hours), for the raw water to undergo partial purification by sedimentation, oxidation and particle agglomeration - the water level is always kept constant

  11. 2. SAND BED: - thickness ~1 meter - sand grains --- 0.2 - 0.3 mm diameter • supported by a layer of graded gravel (30 - 40 cm deep) --- prevents the fine grains from being carried into the drainage pipes - water percolates very slowly through the sand bed - rate of filtration -- 0.1 and 0.4 mm3/hour/m2 of sand bed surface

  12. VITAL LAYER: • a newly laid filter acts merely as a mechanical strainer • but very soon, the surface of the sand bed get covered • with a slimy growth known as vital / zoogleal / • biological layer • this slimy, gelatinous layer consists of threadlike algae, • plankton, diatoms and bacteria • - formation of vital layer --- “RIPENING” ofthe filter

  13. several days required for the full formation of vital layer • removes organic matter, holds back bacteria, oxidizes • ammoniacal nitrogen into nitrates, yields bacteria-free • water • until vital layer is fully formed, the first few days • filtrate is usually run to waste

  14. 3. UNDER-DRAINAGE SYSTEM: • at bottom of filter bed • consists of porous / perforated pipes -- provide an outlet for • filtered water & support filter medium above • cannot be seen once the filter bed has been laid 4. FILTER CONTROL • filter is equipped with certain valves to maintain a • constant rate of filtration

  15. an important component of the regulation system → • VENTURI METER -- measures the bed resistance or “LOSS • OF HEAD”. When the resistance builds up, the regulating • valve is opened to maintain a steady rate of filtration. • time to clean filter bed -- when bed resistance increases to • such an extent that the regulating valve has to be kept fully • open → since any further increase in resistance reduces • filtration rate

  16. - the sand bed is cleaned by “scraping” off the top portion of the sand layer to depth of 1 or 2 cm - after ~ 20 or 30 scrapings, the thickness of the sand bed will have reduced to about 0.5 - 0.8 meter → plant is closed down

  17. RAPID SAND OR MECHANICAL FILTERS • first installed in USA in 1885 STEPS: • Coagulation • Rapid mixing • Flocculation • Sedimentation • Filtration

  18. 1. COAGULATION: - water is treated with a chemical coagulant such as alum 2. RAPID MIXING: - treated water is subjected to violent agitation in a “mixing chamber” for a few minutes

  19. 3.FLOCCULATION: - slow and gentle stirring of the treated water in a FLOCCULATION CHAMBER --- 30 mins - results in the formation of a thick, copious, white flocculent precipitate of aluminium hydroxide

  20. 4. SEDIMENTATION: - water is then led into sedimentation tanks and detained for 2-6 hours --- bacteria and flocculent precipitate settle down in the tank 5. FILTRATION: - Rapid sand filtration - size of sand particles →0.4 - 0.7 mm (depth is 1 meter) - below is a layer of graded gravel - filtration rate – 5 - 15 cubic mm3/hour/m2

  21. - as filtration proceeds, suspended impurities and bacteria clog the filters - filters become dirty and begin to lose efficiency - when theLOSS OF HEAD approaches 7 - 8 feet, filtration is stopped and the filters are subjected toBACK-WASHING

  22. BACK-WASHING - reversing the flow of water through the sand bed - dislodges impurities and cleans up the sand bed - washing is stopped when clear sand is visible and the wash water is sufficiently clear -a15-minute process

  23. Advantages of Rapid sand filter over Slow sand filter: • deals with raw water directly, preliminary storage not required • less space for filter beds • rapid filtration (40 - 50 times faster) • washing of filter easy • more flexibility in operation

  24. DISINFECTION • The chemical used as a disinfectant • should be capable of destroying the pathogenic organisms • present • should not leave products of reaction which render the • water toxic or impart colour to water • economical, readily available, safe • leave a residual concentration • be amenable to detection by practical, rapid and simple • analytical techniques

  25. - when chlorine is added to water, there is formation of • HCl and hypochlorous acid (HOCl) • HCl is neutralized by the alkalinity of the water; HOCl • ionizes to form hydrogen ions and hypochlorite ions (OCl) - the disinfecting action of chlorine is mainly due to the hypochlorous acid and hypochlorite ions

  26. PRINCIPLES OF CHLORINATION Mere addition of chlorine to water is not chlorination. 1. Water to be chlorinated should be clear and free from turbidity 2.The “chlorine demand” of the water should beestimated The point at which the chlorine demand of the water is met is called theBREAK-POINT. If further chlorine is added beyond the break point, free chlorine begins to appear in the water

  27. 3. Contact period - at least one hour is essential to kill bacteria & viruses 4. The minimum recommended concentration of free chlorine is 0.5 mg/L for 1 hour 5. Correct dose of chlorine to be applied = chlorine demand of the specific water + free residual chlorine of 0.5 mg/L

  28. Methods of Chlorination: • Chlorine gas • Chloramine • Perchloron (H.T.H.)→ 60 - 70% of available chlorine Tests for Chlorination: • Orthotolidine (OT) Test • Orthotolidine - Arsenite (OTA) Test

  29. Other agents: • Ozonation • Ultraviolet irradiation

  30. PURIFICATION OF WATER ON A SMALL SCALE

  31. 1. HOUSEHOLD PURIFICATION OF WATER (A) Boiling • “rolling boil” for 5 - 10 minutes • removes temporary hardness • no “residual protection” (B) Chemical disinfection - Bleaching powder or chlorinated lime [CaOCl2] -- 33% of “available chlorine” -- unstable -- with excess of lime → “stabilized bleach”

  32. - Chlorine solution -- prepared from bleaching powder -- 5% solution -- unstable - High test hypochlorite (H.T.H.) / Perchloron -- more stable -- solutions of H.T.H. also used in water disinfection

  33. - Chlorine tablets -- NEERI tablet → 15 times better than ordinary halogen tablets -- a 0.5 g tablet can disinfect 20 litres of water - Iodine -- 2 drops of 2% ethanol solution of iodine sufficient for 1 litre of clear water -- contact time → 20 - 30mins -- high cost, physiological activity (thyroid)

  34. - Potassium permanganate -- powerful oxidizing agent -- limited anti-microbial activity -- alters colour, smell & taste of water

  35. (C)Filtration Types of filters: • Pasteur Chamberlain – porcelain candle • Berkefeld – kieselgurh / infusorial earth candle • “Katadyn” – candle coated with silver catalyst

  36. 2.DISINFECTION OF WELLS • by using bleaching powder – most effective, most economical “THEDOUBLE POT METHOD” • NEERI, Nagpur • to be used during an emergency

  37. Thank You