The Drinking Water Treatment Process

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The Drinking Water Treatment Process
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Why do we need to treat water?

• There are many impurities in the raw water
• These impurities can be grouped into three
  categories
• Physical
• materials that do not dissolve in water and make
  the water appear "dirty"
• Chemical substances dissolved in the water from
  both natural and man-made processes
• Biological viruses, bacteria, algae, and other
  small living organisms.

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Is the drinking water that comes out of our tap
"pure"?

• No "Chemically pure" water, entirely free from
  any other materials, does not exist in nature.
• Distilled water, is usually flat and tasteless
  and few people enjoy drinking it.
• It would be prohibitively expensive and possibly
  unhealthy to purify our entire water supply to
  that level.
• "Natural water", free from any man-made
  additives (if it still exists) contains
  concentrations of minerals such as calcium,
  magnesium, and iron which are beneficial to human
  health in small quantities.

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Drinking water treatmentLarge particles and
debris are removed from the raw water by
travelling screens just as the water enters the
treatment plants.There are five different
processes our water goes through
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1. Coagulation, Flocculation, and Sedimentation

• Rapid mixing of chemicals known as coagulants
• Coagulant make the small physical particles in
  the water clump together (coagulation),
• gentle mixing to form larger groups of particles
  known as floc (flocculation).
• Thicker, denser floc floats down and settles out
  of the water in large tanks (sedimentation) or is
  removed during the next stage, filtration

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Coagulants

• Alum (aluminum sulphate), polyaluminum chloride
  and a group of chemicals known as
  polyelectrolytes
• .Large ve Charge attracts -ve charged clay
  particles
• Zeta potential
• Large charge on small ion Al Fe

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Settlement
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2. Filtration

• removal of the remaining floc,
• other chemical and physical impurities,
• and most of the biological impurities (bacteria,
  etc.)
• . Dual media filters are layers of sand and
  anthracite,

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3. Disinfection

• The addition of the chemical chlorine,
• The chemical is added to our water at different
  points in the treatment process.
• When chlorine is added at beginning
  pre-chlorination.
• After the filtration stage it is known as
  post-chlorination.
• Superchlorination when the levels of bacteria are
  high.
• Sulphur dioxide is then added to remove excess
  chlorine

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Break Point Chlorination
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4. Fluoridation

• Add additional fluoride after the filtration
  stage to raise the level to 1.2 mg/l.

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5. Ammoniation

• Ammonia is added at the end of the treatment
  process and combines with the remaining chlorine.
  
• This stabilizes the chlorine so that it remains
  dissolved in the treated water for longer
  Ammoniation also prevents chlorine from
  evaporating out of your drinking water causing
  smells and associated tastes.

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Ground Water Treatment

• groundwater is significantly easier to treat than
  surface water. .

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THE TREATMENT PROCESS

• Aeration. Raw water pumped from the well is
  mixed with air.
• The mixing releases carbon dioxide and hydrogen
  sulfide gases present in the water.
• Aeration also oxidizes any iron,
• causing it to "precipitate" (or settle out)
• removed by precipitation and filtration.

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Lime (calcium hydroxide)

• Added to remove the calcium and magnesium salts.
• The pH of the water is raised from approximately
  7.6 to a range of 10.4 to 10.6.
• Converts the calcium and magnesium from a soluble
  to insoluble form,
• CaCO3 Ca (OH)2 --gt Ca (HCO3 )2 H20
• causing the insoluble material to precipitate
  out.
• Lime "sludge" on the bottom of the basins ,
  re-use in agriculture.

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Recarbonation.

• Liquid carbon dioxide is mixed with the water.
• The liquid carbon dioxide converts insoluble
  salts back to soluble salts.
• Ca (HCO3 )2 CO2 --gt Ca CO3 H20

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Filters

• Composed of layers of filter sand and graded
  gravel.
• These are washed cleaned from the filter bed
  approximately every 50 hours through a process
  known as "backwashing"..

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The End