CE 547 Softening by 5i32VE

VIEWS: 10 PAGES: 28

									 CE 547


Softening
              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
         Causes of Hardness
Water hardness is principally caused by:
    Calcium ions
    Magnesium ions
    Strontium
    Iron ions
    Manganese ions


Source of calcium and magnesium ions
    geological formations
Cations Causing Harness Anions Associated with them
Ca2+                    HCO3-
Mg2+                    SO4-
Sr2+                    CL-
Fe2+                    NO3-
Mn2+                    SiO32-
         Types of Hardness
Carbonate
    CO3, HCO3
Non-carbonate
    SO4, Cl, NO3
               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.
         Hardness Ranges
Degree of Hardness   Hardness Concentration
                             (mg/l)
 Moderately Hard            60 – 120


      Hard                 120 – 180


    Very Hard            180 and Over
                       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.
                          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.
                       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
                       Reactions

Ca (OH ) 2  CO2  CaCO3   H 2O....( )
                                     1
Ca (OH ) 2  Ca ( HCO3 ) 2  2CaCO3  2 H 2O.....(2)
Ca (OH ) 2  Mg( HCO3 ) 2  MgCO3  CaCO3  2 H 2O.....( )
                                                        3
Ca (OH ) 2  MgCO3  CaCO3   Mg(OH ) 2  .....(4)
Ca (OH ) 2  MgSO4  CaSO4  Mg(OH ) 2  .....( )
                                              5
Na2CO3  CaSO4  Na2 SO4  CaCO3  .....( )
                                        6
                   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)
    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
            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.
     Recarbonation Reactions


CO2  Ca (OH ) 2  CaCO3   H 2 O
CO2  Mg(OH ) 2  MgCO3  H 2 O
CO2  CaCO3  H 2 O  Ca ( HCO3 ) 2
         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.
Selective Calcium Carbonate Removal
 If the water to be treated contains low concentration of
 magnesium (<40 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
      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
Examples

								
To top