Water Treatment

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Water Treatment
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The 7 stages of water treatment

• Screening
• Flocculation
• Settlement (Sedimentation)
• Filtration
• Chlorination
• Fluoridation
• pH adjustment

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Step one- Screening

• Water taken from river, lake or resevoir
• Passed through a wire mesh
• Removes floating debris such as twigs plastic
  bags etc

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Step two- Flocculation

• The word means coagulation or precipitation
• Chemical agents called flocculating agents added
  to clump together small suspended solids into
  larger particles
• Al2(SO4)3 (Aluminium sulfate) most commonly used
• Particles then allowed settle out in next stage

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Step Three-Settlement

• Water is passed into settlement tanks
• Water allowed to settle, clear water removed from
  top
• Over 90 of particles removed, water loses its
  cloudy appearance

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Step Four- Filtration

• Water is then passed through large beds of sand,
  these beds remove any remaining susended solids
• The sand is supported on a layer of gravel,
  layers of sand are cleaned regularly
• Water is now completely clear but still contains
  bacteria

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Step Five- Chlorination

• Chlorine added to sterilise water
• A careful amount of chlorine gas added to water
  (0.2-0.5 ppm)
• Too much chlorine gives water an upleasant taste
  and smell

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Step Six- Fluoridation

• Flourine compounds added to reduce tooth decay
  e.g. NaF
• Only small amounts 1 p.p.m

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Step 7- pH adjustment

• Ph adjusted, if water is slightly acidic it
  corrodes pipes-ph would be raised above 7 using
  calcium hydroxide
• Sodium carbonate used for softening water can
  cause the ph to be too high so carbon dioxide or
  dilute sulphuric acid used
• Optimum pH level for homes and factories is 7-9

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