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Water filtration plant

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Water filtration plant

  1. 1. WATER PURIFICATION ON LARGE SCALE
  2. 2. Slow sand filter • First used in 1804 in Scotland & subsequently in London. • During 19th Century used through out the world. • consists of concrete rectangular basin containing carefully selected graded sand supported on gravel and stones.
  3. 3. Filter box 1-1.5 m 1m 0.3 m
  4. 4. Slow sand filter 1. Supernatant water above the sand bed Depth 1-1.5 mtr
  5. 5. Slow sand filter 2. A bed of graded sand – Most important part of the filter – Thickness – about 1 mtr – Sand : preferably rounded with effective diameter of 0.2-0.3 mm – Supported by graded gravel (30-40 cm deep) – Water percolates through the bed and gets filtered by • Mechanical straining, sedimentation, adsorption, oxidation and bacterial action – Rate of filtration: 0.1-0.4 m3/hour/m2
  6. 6. Vital Layer – The Heart of Slow Sand Filter Surface of the sand bed gets covered with a slimy growth known as ‘Schmutzdecke’, vital layer or zoological layer or biological layer It is slimy geletinuos, consists of algae, plankton, diatoms and bacteria. The formation of this layer is known as RIPENING of the filter, which may take several days to fully develop (2-3 cm) Removes organic matter, holds back bacteria, oxidizes ammonical nitrogen to nitrates and yield a bacteria free water Until the vital layer is fully formed, filter only worked as a mechanical strainer & so for the first few days filter water run to
  7. 7. Slow sand filter 3. An under-drainage system porous of perforated pipes at the bottom of filter – Provides an outlet to the filtered water and – supports the filter medium
  8. 8. Slow sand filter 4. A system of filter control valves Venturi meter -regulation system – To maintain constant rate of filtration – measures the resistance of the sand bad or “loss of head”
  9. 9. Slow sand filter • Filter cleaning – When the bad resistance increases to such an extent that the regulating valve has to be kept fully open, filter bed needs to be cleaned. – Cleaning is done by • Draining of supernatant water • Scrapping of the top portion of the sand bed to a depth of 1-2 cm – After 20-30 scrapings , new bed (filter) is required to be constructed
  10. 10. Slow sand filter • Advantages : √ Simple to construct and operate √ Cheaper √ Physical, Chemical & Bacteriological quality of water is very high √ Reduces bacterial count by 99.9% & E. coli by 99%
  11. 11. Disadvantages × Old fashioned and outdated method of water purification (but still in use) × Initial cost is low but maintenance cost is much more than rapid sand filter × These filters need a lot of space
  12. 12. Rapid sand filter • In 1885, the first rapid sand filters were installed 1885 in USA • 2 types – Open/gravity type— – Closed/presure type— PATERSON FILTER Candy’s Filter • Steps of filtration Coagulation Rapid Mixing Flocculation Sedimentation 
  13. 13. STEPS • Coagulation – Alum is added – 5-40 mg/liter depending upon turbidity, color, pH & temperature • Rapid mixing of Alum – By violent agitation in a mixing chamber for few minutes • Flocculation – Slow & gentle stirring of the treated water for about 30 minutes – Results in formation of thick, copious, white flocculent precipitate of aluminium hydroxide • Sedimentation – For 2-6 hours – Flocculent precipitate together with impurities and bacteria – 95% of the flocculent needs to be removed before the water enters the rapid sand filter
  14. 14. Filter beds – Each unit has a surface of 80-90 m2 Supernatant water – 1 to 1.5 mtr Depth of sand bed – 1 mtr Effective size of the sand particle is 0.4-0.7 mm Supported graded gravel – 30-40 cm Filtration rate – 5-15 m3 / m2/ hr
  15. 15. Rapid sand filter Filtration – – Retained alum- flocs is held back on sand bed • Forms a slimy layer [comparable to the zoological layer in slow sand filter] • adsorbs bacteria from water – Oxidation of ammonia also takes place during the passage of water
  16. 16. Rapid sand filter • Cleaning & Maintenance –When the loss of head reaches 7-8 ft, it needs to be cleaned –cleaned by Backwashing. •Backwashing is done by reversing the flow of water through the sand bed. dislodges the impurities and cleans up sand bed. •Compressed air can be used in back washing
  17. 17. Rapid sand filter • Advantages – deal with raw water directly Occupies less space Filtration is rapid Washing of filter is easy More flexibility in operation • Disadvantages × Relatively high skill operation × Costlier × Removes bacteria by 98-99%
  18. 18. Slow sand filter Space Rate filtration Rapid sand filter Occupies large area Small area 0.1-0.4 m3/m2/ hour 5-15 m3 / m2/ hour Effective size of sand 0.2-0.3 mm 0.4-0.7 mm Preliminary treatment Plain sedimentation Chemical coagulation & sedimentation Washing By scraping the sand bed Backwashing Operation Less skilled Highly skilled Loss of head allowed 4 feet 6-8 feet Removal of turbidity Good Good Removal of colour fair Good Removal of bacteria 99.9-99.99 % 98-99%
  19. 19. THANK YOU

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