CE 547CE 547

SofteningSoftening

What is HardnessWhat is Hardness

Hardness is Hardness is – the ability of the water to consume excessive the ability of the water to consume excessive

amounts of soap before foamingamounts of soap before foaming– OR the ability of the water to produce scale in OR the ability of the water to produce scale in

water heaters and boilers where water temperature water heaters and boilers where water temperature is increased dramaticallyis increased dramatically

Causes of HardnessCauses of Hardness

Water hardness is principally caused by:Water hardness is principally caused by:Calcium ionsCalcium ions

Magnesium ionsMagnesium ions

StrontiumStrontium

Iron ionsIron ions

Manganese ionsManganese ions

Source of calcium and magnesium ionsSource of calcium and magnesium ionsgeological formationsgeological formations

Cations Causing HarnessCations Causing HarnessAnions Associated with themAnions Associated with them

CaCa2+2+HCOHCO33--

MgMg2+2+SOSO44--

SrSr2+2+CLCL--

FeFe2+2+NONO33--

MnMn2+2+SiOSiO332-2-

Types of HardnessTypes of Hardness

CarbonateCarbonateCOCO33, HCO, HCO33

Non-carbonateNon-carbonateSOSO44, Cl, NO, Cl, NO33

Water HardnessWater Hardness

The maximum level of hardness considered for public The maximum level of hardness considered for public supply is 300 to 500 mg/l, though many customers supply is 300 to 500 mg/l, though many customers object to water harder than 150 mg/l.object to water harder than 150 mg/l.

Disadvantages of hardnessDisadvantages of hardnessexcessive soap consumption during launderingexcessive soap consumption during launderingscale-formation in hot water heaters and pipes.scale-formation in hot water heaters and pipes.

The use of synthetic detergents and pipe linings can The use of synthetic detergents and pipe linings can overcome those problems.overcome those problems.

Hardness RangesHardness Ranges

Degree of HardnessDegree of HardnessHardness Concentration Hardness Concentration (mg/l)(mg/l)

Moderately HardModerately Hard60 – 12060 – 120

HardHard120 – 180120 – 180

Very HardVery Hard180 and Over180 and Over

SofteningSoftening

In precipitation softening, lime (CaO) and soda ash In precipitation softening, lime (CaO) and soda ash (Na(Na22COCO33) are used to precipitate calcium and ) are used to precipitate calcium and

magnesium form water. Lime treatment can also:magnesium form water. Lime treatment can also:kill bacteriakill bacteria

remove ironremove iron

help in clarification of surface water (coagulant)help in clarification of surface water (coagulant)

Lime treatment will raise the pH value, so Lime treatment will raise the pH value, so recarbonation, by carbon dioxide, is used to lower the recarbonation, by carbon dioxide, is used to lower the pH by converting the hydroxide and carbonate ions to pH by converting the hydroxide and carbonate ions to bicarbonate ion.bicarbonate ion.

LimeLime

Lime is commercially available in the forms of:Lime is commercially available in the forms of:quicklimequicklimehydrated limehydrated lime

QuicklimeQuicklimeavailable in granular formavailable in granular formcontains minimum of 90% CaOcontains minimum of 90% CaOmagnesium oxide is the primary impuritymagnesium oxide is the primary impurity

Hydrated LimeHydrated Limecontains about 68% CaOcontains about 68% CaO

Slurry lime is written as Ca(OH)Slurry lime is written as Ca(OH)22..

COCO22

Carbon dioxide is:Carbon dioxide is:gasgas

colorlesscolorless

clearclear

used to recarbonate lime-softened waterused to recarbonate lime-softened water

produced by burning fuel such as coal, oil, or gas.produced by burning fuel such as coal, oil, or gas.

applied through diffusers immersed in the treatment applied through diffusers immersed in the treatment tanktank

ReactionsReactions

)6.....(

)5.....()()(

)4.....()()(

)3.....(2)()(

)2.....(22)()(

)1....()(

342432

2442

2332

233232

23232

2322

CaCOSONaCaSOCONa

OHMgCaSOMgSOOHCa

OHMgCaCOMgCOOHCa

OHCaCOMgCOHCOMgOHCa

OHCaCOHCOCaOHCa

OHCaCOCOOHCa

ReactionsReactions

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

Pros and cons of SofteningPros and cons of Softening

Advantage of precipitation softening:Advantage of precipitation softening:the lime added is removed along with the hardness the lime added is removed along with the hardness taken out of solution.taken out of solution.TDS of the water are reducedTDS of the water are reducedthe chemical reactions can be used to estimate the the chemical reactions can be used to estimate the quantity of sludge produced.quantity of sludge produced.

Disadvantage of precipitation softening:Disadvantage of precipitation softening:sodium ions, from the addition of soda ash, remain in sodium ions, from the addition of soda ash, remain in the finished waterthe finished water

RecarbonationRecarbonation

Recrabonation is used to stabilize lime-treated Recrabonation is used to stabilize lime-treated water, thus reducing its scale-forming water, thus reducing its scale-forming potential.potential.

Carbon dioxide is used for the recarbonation Carbon dioxide is used for the recarbonation process. It converts lime to calcium carbonate. process. It converts lime to calcium carbonate. Further recarbonation will convert carbonate Further recarbonation will convert carbonate to bicarbonate. to bicarbonate.

Recarbonation ReactionsRecarbonation Reactions

23232

2322

2322

)(

)(

)(

HCOCaOHCaCOCO

OHMgCOOHMgCO

OHCaCOOHCaCO

Excess Lime SofteningExcess Lime Softening

Uses of Excess Lime Softening:Uses of Excess Lime Softening:to remove Ca and Mg to the practical limit of 40 mg/lto remove Ca and Mg to the practical limit of 40 mg/lexcess lime addition is needed to remove magnesiumexcess lime addition is needed to remove magnesium

In Excess Lime Softening:In Excess Lime Softening:after excess lime addition, the water is flocculated and settled to remove after excess lime addition, the water is flocculated and settled to remove CaCOCaCO33 and Mg(OH) and Mg(OH)22 precipitates precipitatesAfter that, recarbonation is carried out in two stagesAfter that, recarbonation is carried out in two stagesin the first stage, COin the first stage, CO22 is added to lower the pH to 10.3 and converts excess is added to lower the pH to 10.3 and converts excess lime to CaCOlime to CaCO33..water is then flocculated and settledwater is then flocculated and settledif needed, soda ash is added at this stage to remove noncarbonate hardnessif needed, soda ash is added at this stage to remove noncarbonate hardnessIn the second stage, COIn the second stage, CO22 is added to further lower the pH to the range of 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 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. ion in order to stabilize the water against scale formation.

Selective Calcium Carbonate RemovalSelective Calcium Carbonate Removal

If the water to be treated contains low concentration of If the water to be treated contains low concentration of magnesium (<40 mg/l as caCOmagnesium (<40 mg/l as caCO33), selective calcium carbonate ), selective calcium carbonate removal can be used.removal can be used.Magnesium hardness of more than 40 mg/l as caCOMagnesium hardness of more than 40 mg/l as caCO33 is not is not recommended due to the possible formation of hard recommended due to the possible formation of hard magnesium silicate in high temperature waters (180magnesium silicate in high temperature waters (180 F) F)enough lime is added but not in excessenough lime is added but not in excesssoda ash may be used depending on the extent of noncarbonate soda ash may be used depending on the extent of noncarbonate hardnesshardnessif precipitation of CaCOif precipitation of CaCO33 is not satisfactory, alum or a polymer is not satisfactory, alum or a polymer can be used to aid flocculationcan be used to aid flocculationrecarbonation is used to reduce scale formation on the filter recarbonation is used to reduce scale formation on the filter and to produce stable waterand to produce stable water

Split-Treatment SofteningSplit-Treatment SofteningSplit-treatment softening is done by splitting the flow of the Split-treatment softening is done by splitting the flow of the raw water into two lines, of different portions, for softening in raw water into two lines, of different portions, for softening in a two-stage system a two-stage system the larger portion is given excess lime treatment in the first the larger portion is given excess lime treatment in the first stagestagewater is flocculated and settledwater is flocculated and settledtreated water is mixed with split flowtreated water is mixed with split flowexcess lime form the first stage reacts with calcium hardness excess lime form the first stage reacts with calcium hardness of the split waterof the split watersoda ash is added to the second stagesoda ash is added to the second stagein this case, excess lime is used and not wasted, so in this case, excess lime is used and not wasted, so recarbonation might not be necessary.recarbonation might not be necessary.Recarbonation is recommended to produce stable waterRecarbonation is recommended to produce stable waterAdvantages:Advantages:

lime and recarbonation costs are lower than excess lime treatmentlime and recarbonation costs are lower than excess lime treatmentpossibility of reducing magnesium hardness to less than 40 mg/l possibility of reducing magnesium hardness to less than 40 mg/l

ExamplesExamples