CE 370

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CE 370

• Softening

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What is Hardness

• Hardness is
• the ability of the water to consume excessive
  amounts of soap before foaming
• OR the ability of the water to produce scale in
  water heaters and boilers where water temperature
  is increased dramatically

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Causes of Hardness

• Water hardness is principally caused by
• calcium ions
• magnesium ions
• Source of calcium and magnesium ions
• geological formations

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Types of Hardness

• Carbonate
• CO3, HCO3
• Non-carbonate
• SO4, Cl, NO3

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Water Hardness

• The maximum level of hardness considered for
  public supply is 300 to 500 mg/l, though many
  customers object to water harder than 150 mg/l.
• Disadvantages of hardness
• excessive soap consumption during laundering
• scale-formation in hot water heaters and pipes.
• The use of synthetic detergents and pipe linings
  can overcome those problems.

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Hardness Ranges
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Softening

• In precipitation softening, lime (CaO) and soda
  ash (Na2CO3) are used to precipitate calcium and
  magnesium form water. Lime treatment can also
• kill bacteria
• remove iron
• help in clarification of surface water
  (coagulant)
• Lime treatment will raise the pH value, so
  recarbonation, by carbon dioxide, is used to
  lower the pH by converting the hydroxide and
  carbonate ions to bicarbonate ion.

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Lime

• Lime is commercially available in the forms of
• quicklime
• hydrated lime
• Quicklime
• available in granular form
• contains minimum of 90 CaO
• magnesium oxide is the primary impurity
• Hydrated Lime
• contains about 68 CaO
• Slurry lime is written as Ca(OH)2.

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CO2

• Carbon dioxide is
• gas
• colorless
• clear
• used to recarbonate lime-softened water
• produced by burning fuel such as coal, oil, or
  gas.
• applied through diffusers immersed in the
  treatment tank

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Reactions
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Reactions

• From the reaction equations it can be seen that
• lime reacts first with free carbon dioxide (eq.
  1)
• next, lime reacts with calcium bicrarbonate (eq.
  2)
• lime also reacts with magnesium carbonate and
  bicrabonate (eqs. 3 and 4)
• noncarbonate hardness (magnesium sulphate and
  chloride) requires the addition of soda ash for
  precipitation (eq. 5)
• noncarbonate hardness (calcium sulphate and
  chloride) requires the addition of soda ash only
  for precipitation (eq. 6)

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Pros and cons of Softening

• Advantage of precipitation softening
• the lime added is removed along with the hardness
  taken out of solution.
• TDS of the water are reduced
• the chemical reactions can be used to estimate
  the quantity of sludge produced.
• Disadvantage of precipitation softening
• sodium ions, from the addition of soda ash,
  remain in the finished water

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Recarbonation

• Recrabonation is used to stabilize lime-treated
  water, thus reducing its scale-forming potential.
• Carbon dioxide is used for the recarbonation
  process. It converts lime to calcium carbonate.
  Further recarbonation will convert carbonate to
  bicarbonate.

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Recarbonation Reactions
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Excess Lime Softening

• Uses of Excess Lime Softening
• to remove Ca and Mg to the practical limit of 40
  mg/l
• excess lime addition is needed to remove
  magnesium
• In Excess Lime Softening
• after excess lime addition, the water is
  flocculated and settled to remove CaCO3 and
  Mg(OH)2 precipitates
• After that, recarbonation is carried out in two
  stages
• in the first stage, CO2 is added to lower the pH
  to 10.3 and converts excess lime to CaCO3.
• water is then flocculated and settled
• if needed, soda ash is added at this stage to
  remove noncarbonate hardness
• In the second stage, CO2 is added to further
  lower the pH to the range of 8.5 to 9.5 to
  convert most of the remaining carbonate ion to
  bicarbonate ion in order to stabilize the water
  against scale formation.

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Selective Calcium Carbonate Removal

• If the water to be treated contains low
  concentration of magnesium (lt40 mg/l as caCO3),
  selective calcium carbonate removal can be used.
• Magnesium hardness of more than 40 mg/l as caCO3
  is not recommended due to the possible formation
  of hard magnesium silicate in high temperature
  waters (180? F)
• enough lime is added but not in excess
• soda ash may be used depending on the extent of
  noncarbonate hardness
• if precipitation of CaCO3 is not satisfactory,
  alum or a polymer can be used to aid flocculation
• recarbonation is used to reduce scale formation
  on the filter and to produce stable water

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Split-Treatment Softening

• Split-treatment softening is done by splitting
  the flow of the raw water into two lines, of
  different portions, for softening in a two-stage
  system
• the larger portion is given excess lime treatment
  in the first stage
• water is flocculated and settled
• treated water is mixed with split flow
• excess lime form the first stage reacts with
  calcium hardness of the split water
• soda ash is added to the second stage
• in this case, excess lime is used and not wasted,
  so recarbonation might not be necessary.
• Recarbonation is recommended to produce stable
  water
• Advantages
• lime and recarbonation costs are lower than
  excess lime treatment
• possibility of reducing magnesium hardness to
  less than 40 mg/l

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Examples
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