Water Technology Part 2

Calgon Conditioning: Sodium hexametaphosphate (Na2[Na4(PO3)6]

When calgon was added to boiled water, it forms soluble complex with CaSO4 - hence this prevents scale and sludge formation

Na2[Na4(PO3)6] 2Na+ + [Na4P6O18]2-

2 CaSO4 + [Na4P6O18]2- [Ca2P6O18]2- + 2Na2SO4

Treatment with sodium Aluminate: (NaAlO2)

NaAlO2 + 2 H2O NaOH + Al(OH)3

MgCl2 + NaOH Mg(OH)2 + 2 NaCl

Gelatinus precipitate

When sodium aluminate was added to hard water, it hydrolyzed to give NaOH.

Released sodium hydroxide was reacted with MgCl2 to precipitateMg(OH)2

Hence Al(OH)3 and Mg(OH)2 produce flocculent precipitate insidethe boiler, entraps finely suspended and colloidal impurities,including oily drops and silica.

These loose precipitate can be removed by blow-down operation

Electrical conditioning:

Sealed glass bulbs, containing mercury connected to a battery, are set rotating in the boiler. When water boils, mercury bulbs emit electrical discharges which prevents scale forming particle to stick together to form a scale

Radioactive conditioning:

Tablets containing radioactive salts are placed in inside boilerwater at few points. Energy radiations emitted by these salts prevent scale formation

Complexometric method:

1.5 % of alkaline (pH=8.5) solution of EDTA to boiler feed water

EDTA binds with scale forming cations to form stable and soluble complex

Scale and sludge formation is prevented

This treatment also:

Prevents iron deposits of iron oxide in the boilerReduces carry over oxides with streamProtects the boiler unit from corrosion by wet steam

Caustic Embrittlement

It is type of boiler corrosion caused by using highly alkaline water in the boiler

During water softening process (by lime soda process), free Na2CO3is usually present in small proportion in the softened water.

In high pressure boilers, this Na2CO3 decomposes to NaOH and CO2and makes boiler water so caustic

Na2CO3 + H2O 2NaOH + CO2

NaOH containing water gets into the minutes hair-cracks by capillary action. Eventually water evaporates and the dissolved NaOH concentration increases.

Caustic soda attacks surrounding area and dissolving iron of boiler as Sodium ferroate. This causes embrittlement of boiler parts

Conc. NaOHSolution

Dilute NaOHSolution

Iron atPlan surfaces

Iron at Rivets,bends, joints etc

+ -

Caustic cracking can be explained by considering the following concentration cell

Iron surrounded by dilute NaOH solution becomes cathodic side

Iron part in contact with concentrated NaOH becomes anodic side which is consequently dissolved or corroded

Prevention of caustic Embrittlement

1) Usage of sodium phosphate as water softening agent in stead of sodium carbonate

2) By adding tannin or lignin to boiler water which blocks boiler hair-cracks. So NaOH can not flow in to cracks

3) By adding sodium sulphate to boiler water. This also blocks hair-cracks.

Caustic cracking can be prevented if sodium sulphate is used in boiler water in the following ratios at particular pressure

[NaOH Concentration]

[Na2SO4 Concentration] 1: 1 at 10 atm. Pressure2:1 at 20 atm. Pressure3:1 at above 20 atm. pressure

Boiler Corrosion

It is decay of boiler material by a chemical or electrochemical attack by its environment

It is mainly due to

1. Dissolved oxygen

2. Dissolved CO2

3. Acids from dissolved salts

Boiler Corrosion

Dissolved oxygen:

Water has 8mL of dissolved O2 per liter at RT

This oxygen attacks boiler material at High Temperature

2Fe + 2H2O + O2 2 Fe(OH)2

4 Fe(OH)2 + O2 2[Fe2O3.2H2O]

Rust

Boiler Corrosion

Removal of oxygen: 1) By adding calculated quantity of sodium sulphite

or hydrazine or sodium sulphide

2Na2SO3 + O2 2Na2SO4

N2 + 2H2OH2NNH2 + H2O

Na2S + 2O2 Na2SO4

Hydrazine is the ideal treatment for removal of oxygenAs it gives harmless nitrogen gas evolution without any increasing salt concentration

Boiler Corrosion

Removal of oxygen: 2) Mechanical de-aeration

Water sprayed in a perforated plate-fittedTower

Heated from sides and connected Vacuum pump

High temperature, low pressureAnd large exposed surface reduced the dissolved O2 in water

Boiler Corrosion

2. Dissolved CO2 : It is carbonic acid which has slow corrosive effects on the boiler material

CO2 + H2O H2CO3

CO2 is also released inside the boiler if bicarbonate containing water used for steam generation

MgCO3 + H2O + CO2Mg(HCO3 )2Heat

Removal of CO2 in water:

Mechanical de-aeration along with O2

Boiler Corrosion

3. Acids from dissolved salts:

Water containing dissolved Mg salts liberates acid upon hydrolysis

Mg(OH)2 + 2HClMgCl2 + 2 H2O

Liberated acid reacts with iron (Fe of the boiler) in chain likereactions to produce more HCl again

2HCl + Fe FeCl2 + H2

FeCl2 + 2H2O Fe(OH)2 + 2HCl

Consequently, presence of even small amount of MgCl2 will causecorrosion of iron to a large extend

Priming and Foaming

Priming :

When boiler is steaming rapidly, some particle of the liquidwater are carried along with steam. This process of wet steam formation is called Priming

Priming is caused by:1. Presence of large amount of solid2. High steam velocities3. Sudden boiling4. Improper boiler design5. Sudden increase in steam production rate

Priming and Foaming

Foaming :

The production of persistent foam or bupples, which do notbreak easily

This formation is due to the presence of oily substance(which greatly reduces the surface tension of the water)

Priming and Foaming

Priming and Foaming usually occur together:

This is problematic because:

1. Wet steam carries dissolved salt content to super heater and turbine blades where salts get deposited as water evaporatesThis deposit reduces their efficiency

2. Same way dissolved salts enter other pats of machinery wheresteam is used.Hence lifetime of machinery is decreased

3. Actual height of water column can not be judged properlyMaintaining boiler pressure is difficult

Priming and Foaming

Priming can be avoided by:

1. Fitting mechanical steam purifiers

2. Avoiding rapid change in steaming rate

3. Maintaining low water level in boilers

4. Efficient softening and filtration of the boiler feed water

Foaming can be avoided by:

1. Adding anti-foaming chemicals like castor oil

2. Removing oil from boiler water adding chemicals likesodium aluminate

Hard and soft waterUnits of Hardness Disadvantages of hard waterScale and SludgeCaustic embrittlementCorrosionPriming and Foaming

Estimation of Hardness:

EDTA method

Alkali Titration method

Determination of Hardness of water

1. EDTA – Method

2. O. Hehner’s Method

EDTA - Method

NCH2CH2NCH2COOH

CH2COOH

HOOCH2C

HOOCH2C

H2C CH2

NNH2C CH2

C CO O

MO O

-OOCH2C CH2COO-

EDTA

EDTA with Metal ion

M = Ca2+, Mg2+

To determine equivalence point (just completion of metal ioncomplex formation), indicator Eriochrome Black T (EBT in EtOH)is used

EBT in alcohol : Blue colour

EBT could form weak complex with Ca2+ and Mg2+

EBT-Metal complex: Wine Red colour

This complex formation is effective at pH 10Hence buffer NH4OH-NH4Cl is employed

M2+ + EBTpH 10

[M-EBT]

Wine Red colour

M2+ + EBTpH 10

[M-EBT]

[M-EBT] + EDTA [M-EDTA] + EBT

M2+ + EBTpH 10

[M-EBT]

Wine Red

Wine Red Blue

Blue Wine RedIf Ca2+, Mg2+

still present

End point for titration: colour change wine red to blue

When nearly all M2+ have formed stable M-EDTA complex,next drop of EDTA frees EBT from M-EBT complex, wine redcolour changes to blue.

d

EBT in presenceof Ca2+, Mg2+

EBT (when no Metal ion present)

Preparation for Titration

1. Preparation of standard hard water1g of pure dry CaCO3 was dissolved in minimum quantity of dilute HCl and evaporated to dryness. This dry residuewas re-dissolved in distilled water 1L1 mL contains 1mg of CaCO3 equivalent hardness

2. Preparation of EDTA solutionDissolve 4g of pure EDTA and 0.1g of MgCl2 in 1L of distilled water

3. Preparation of Indicator0.5g of Eriochrome black T was taken in 100 mL alcohol

4. Preparation of buffer solution67.5 g of NH4Cl to 570 mL of conc, NH4OH and mixturewas diluted upto 1L using distilled water

Titration

1. Standardization of EDTA solutionRinse and fill the Burette with EDTA solution. Pipette 50 mL ofstandard hard water in a conical flask. Add 10-15 ml of Buffer and 4-5 drops of EBT indicator. Titrate with EDTA solution till wine-red changes to blue. Let EDTA volume used to obtain end point is V1 mL

2. Titration of unknown hard watreTitrate 50 ml of unknown hard water sample in a similar fashion in step 1.Let EDTA volume used to obtain end point (using unknown hard water) is V2 mL

3.Titration of permanent hardnessTake 250 ml of unknown hard water in beaker and boil it tillThe volume gets reduced 50 mLAll MgHCO3 and CaCO3 are decomposed to give insolubleCaCO3 and Mg(OH)2Filter, wash the precipitate with distilled water and collect filtrate, washings in a standard flask (250 mL).Make the solution up to 250 ml using distilled waterFrom this boiled water, titrate 50 ml of sample just as beforeLet EDTA volume used to obtain end point (using unknown boiled hard water) is V3 mL

Calculation50 mL of std hard water = V1 mL of EDTA50X1mg of CaCO3 = V1 mL of EDTA1mL of EDTA = 50/V1 mg of CaCO3 equ.

Now,

50 mL of unknown Hard water = V2 mL EDTA= V2X50/V1 mg of CaCO3 equ.

1L of (1000 mL) of unknown = [V2X50/V1]X1000/50hard water

= [V2/V1]X1000 mg of CaCO3 equ.

Total hardness of water = [V2/V1]X1000 ppm

Calculation

50 mL of unknown boiled Hard water = V3 mL EDTA= V3X50/V1 mg of CaCO3 equ.

1L of (1000 mL) of unknown = [V3X50/V1]X1000/50Boiled hard water

= [V3/V1]X1000 mg of CaCO3 equ.

Permanent hardness of water = [V3/V1]X1000 ppm

Temporary hardness = Total – Permanent= V2/V1X1000 – V3/V1X1000

= [1000(V2-V3)]/V1 ppm

Advantage

1. Greated accuracy

2. Convenience

3. More rapid procedure

50 mL of a sample water consumed 15mL of 0.01M EDTA before boilingAnd 5mL of the same EDTA after boiling. Calculate total hardness, temporaryhardness and permanent hardness

50mL water = 15mL of 0.01M EDTAFor 1L of water = [15/50]X1000 = 300 mL of 0.01 M EDTAMolarity of EDTA = 2 X Normality of EDTA

= 300X2= 600 ml (0.6L) of 0.01N EDTA= 0.6X0.01X50 g of CaCO3 equ. = 0.3 g CaCO3 equ.

Total hardness = 300 mg CaCO3 equ. = 300 ppm

50 mL of boiled water = 5mL of 0.01 M EDTA= 5/50X1000 of 0.01M EDTA= 100 mL of 0.01M EDTA= 200 mL of 0.01 N EDTA= 0.20X0.01X50 g CaCO3 equi.= 0.1g or 100mg CaCO3 equ

Permanent hardness = 100 ppm

Temporary hardness = 300 -100= 200 ppm

0.5g of CaCO3 was dissolved in HCl and the solution is made upto 500 mLwith distilled water. 50mL of the solution required 48 mL of EDTA for titration50mL hard water required 15 mL of EDTA and after boiling 10 ml of EDTARequired for 50 ml of boiled water. Calculate the hardness

0.5g (500mg) of CaCO3 in 500 mL :1mL containg1mg of CaCO350 mL of Water requires = 48 mL of EDTA

1mL EDTA = 50/48 mg of CaCO3 equi.

50mL hard water = 15 mL EDTA= [50/48]X15 mg of CaCO3 equi.

1L of hard ware = [(50/48)X15X1000]/50Total Hardness = 312.5 mg of CaCO3 equ

50 ml of boiled water = 10 mL EDTA= [50/48]X10 mg CaCO3 equ

1000L = [(50/48)X10X1000]50 mg CaCO3 equ= 208.3mg per liter or ppm

Temporary hardness = 312.5 - 208.3 = 104.2 ppm

20 mL standard hard water (containing 15g of CaCO3 per lit) requires25 mL EDTA. 100 ml of water sample requires18mL EDTA. After boilingrequired 12 mL EDTA solution. Calculate the hardness. What is the buffer and pH

O. Hehner’s MethodTemporary hardness:

Pipette out 20 ml hard water in conical flaskAdd 2-3 drops of methyl orange indicatorTitrate against N/50 Normal HClVolume of acid used to get end point is V1 mL

Take 100 ml hard water in beaker and evaporate to drynessAdd 50 ml of distilled water in warm condition and stir it till all the solublematerial gets in solution. Filter the solution and wash the residue with distilled water. Make up the volume to 100 mL using distilled waterFrom this 50 ml is titrated against N/50 HCl using methyl orange indicatorVolume of acid used to get end point is V2 mL

CaCO3 + H2O + CO2

Mg(OH)2 + 2CO2Mg(HCO3)2

Boiling

Boiling

Ca(HCO3)2In soluble

In soluble

Calculation

Volume of N/50 HCl used by temporaryhardness present in 50 mL water = V1-V2 mL

Alkalinity due to temporaryhardness present in 50 mL water = (V1-V2) ml of N/50 HCl

Alkalinity due to temporaryhardness in terms of CaCO3 equi = [(V1-V2)X1X50]/ 50X50 g per liter

= (V1-V2)/50 g of CaCO3 per lit

= [(V1-V2)X106]/50X103

=20X(V1-V2) ppm

O. Hehner’s MethodPermanent hardness: This can be removed by boiling with known sodium carbonate base. The chlorides and sulphates of calcium and magnesium formInsoluble carbonates. The residual sodium carbonate is then determined by titrating against a standard acid.The reduction in the amount of soda added is equivalent to permanent hardness

Pipette out 50 ml hard water in beaker and add it to 50 mL of N/50 Na2CO3Solution. Boil for 15 minutes. Cool the solution. evaporate to dryness. Filter the precipitate, collecting the filtrate in a conical flask. Titrate the unused sodium carbonate present in the flask against N/50 HCl using Methyl Orange as indicator.

Volume of acid used to get end point is V mL

CaCl2 + Na2CO3

MgSO4 + Na2CO3

CaCO3 + 2NaClMgCO3 + Na2SO4In soluble

In soluble

Calculation

Volume of N/50 Na2CO3 used for removingpermanent hardness in 50 mL water = (50-V) mL

Strength of permanent hardnesserin terms of CaCO3 equi = (50-V)X50]/ 50X50

= (50-V)/50 g of CaCO3 equ.

Permanent hardness = [(50-V)X106]/50X103

= 20(50-V) ppm

Methylorange Indicator

NN

N

SO

O-O

In alkaline solution: Yellow > pH 4.4Acidic solution: < pH 3.2 Red

End point: First appearance of Orange

100 mL of hard water neutralizes exactly 12 mL of0.12 N HCl using methyl orange as indicator What kind of hard nessExpress in terms of CaCO3 equivalent

720 ppm

25 ml of 1/50N Na2CO3 solution was added to 100ml hard water. After Completion of precipitation of insoluble carbonate, the unreacted Na2CO3was titrated against N/50 H2SO4 solution when 10 ml of acid requiredCalculate the hardness

150 ppm

20 ml of 0.1 N sodium carbonate was added to 100 mL of water sample. The Filterate from the above required 30 mL of 0.05 N H2SO4 required for Complete Neutralization. Calculate the hardness

250 ppm

Desalination of Brackish Water

Electrodialysis

Reverse Osmosis

Desalination: Process of removing common salt (NaCl)

Water containing dissolved with a peculiar salty taste ( brakish) is called Brakish water

Sea water contains 3.5 % salts

Electrodialysis

Ions are pulled out of salt water by passing direct current using electrode and thin rigid plastic membrane

When direct electric current passed through the saline water Na+ ion started movingTowards negative pole (cathode), Cl- ions start moving towards positive anode pole Through the membrane. As a result, concentration of NaCl decreases in central compartment while NaCl increases in both two left and right side compartments

Pure water (desalinated) in central compartment is removed time to time whileconcentrated brine (NaCl solution) is replaced in side compartments by fresh brineor sea water

For more efficient separation, ion selective membranes are employed

Ion selective membrane has permeability only one kind of ions with specific chargeCation (Positive ions like Na+) selective membrane is permeable only to cationsbecause of the presence of charged functional groups (RSO3-, RCOO-)in membranerejects anions (negative ions like Cl-)

Similarly, Anion selective membranes is permeable only for anions like Cl-because of the positive charged functional groups ( R4N+ present in the membrane

S O 3-

S O 3-

S O 3-

S O 3-

N R 3

N R 3

N R 3

N R 3

N a + C l-

N a + C l-

N a + C l-

N a + C l-

A n o d eC a th o d e

S O 3-N a +

S O 3-N a +

S O 3-N a +

S O 3-N a +

N R 3 C l-

N R 3 C l-

N R 3 C l-

N R 3 C l-


S O 3-

S O 3-

S O 3-

S O 3-

N R 3

N R 3

N R 3

N R 3

N a +

N a +

N a +

N a +


N a + C l-

C l -

C l -

C l -

C l -

C

C

C

A

A

A

C: cation selective membraneA: anion selective membrane

Electrodialysis cell

Cell contains several number of paired sets of rigid plastic membraneSaline water passed under pressure (5-6 kg m-2) between membrane pairsElectric field is applied perpendicular to the direction of water flowPositive charge present in the membrane repel + charge but permeates negative chargeNegative charge present in the membrane repel negative charge but permeates positive chargeHence water in one compartment in cell is deprived of salts while the salt concentration in adjacent compartment is increased

A = anion selective membraneC = cation selective membrane

Advantage:

Most compactInstallation and operation cost economicalIf electricity is easily available it is best used

Reverse Osmosis

Osmosis: When two solutions of unequal Concentrations are separated by semi-permeable membrane, flow of solventtakes place dilute to concentration sideMembrane does not allow the passage of ions(dissolved particles, or molecules or ions)

Reverse Osmosis:

If hydrostatic pressure applied above osmotic pressure on the high concentration side the solvent flow reverses (flow of solvent takes place from higher concentrationto lower concentration)

Hence this method, pure water is separated from its contaminates rather thanremoving contaminants from the water.

Sometimes called as Super filtration or hyper filteration

Pressure: 15-40 kg cm-2 is applied toSea water/impure water (to be treatedto force its pure water out through thesemi permeable membrane; leaving dissolved solids ( ionic and non ionic)

Membrane: cellulose acetate or polymethacrylate or polyamide

Advantages

Removes ionic as well as non ionic, colloidal, high molecular weight organics

Removes colloidal silica which is not removed by demineralization

Maintenance cost is almost entirely on the replacement of the membrane

Life time of membrane is quite high (@ 2 years)

Membrane can replaced within few minutes thereby providing uninterrupted water supply

Due to low capital cost, simplicity, low operating cost, and high reliability,

The reverse osmosis is gaining ground at present for converting sea water intodrinking and high pressure boilers water

Water Softening

The process of removing hardness producing saltsfrom water is known as softening of water

There are three methods of water softening

1. Lime-Soda Process2. Zeolite or Permutit process3. Ion exchange or de-ionization or de-mineralization process

Lime-Soda Process

In this method, soluble calcium and magnesium salts are converted to insoluble

salts by adding calculated amounts of Lime (Ca(OH)2 and Soda (Na2CO3)

Insoluble material CaCO3 and Mg(OH)2 precipitates are filtered off

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(CO3)2 + 2H2O

Mg(CO3) + Ca(OH)2 CaCO3 + Mg(OH)2

For carbonate or temporary hardness

Calcium bicarbonate requires one mole of LimeMagnesium bicarbonate requires two moles of LimeTemporary hardness does not require Soda

Lime-Soda ProcessFor non-carbonate or permanent hardness

Ca(SO4) + Na2CO3 CaCO3 + Na2SO4

CaCl2 + Na2CO3 CaCO3 + 2NaCl

Mg(SO4) + Ca(OH)2 Mg(OH)2 + CaSO4

Ca(SO4) + Na2CO3 2CaCO3 + Na2SO4

MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2CaCl2 + Na2CO3 CaCO3 + 2 NaCl

Calcium salt containing hard water requires only soda for softening. No lime is required

Magnesium salt containing hard water requires both Lime and Soda

Cold Lime Soda process

Required amount of Lime and Soda are added with the hard water in the chemical tankand thoroughly mixed at room temperature

CaCO3 precipitate formed are finely divided so that they do not settle down easily and Can not be filtered easily

It is necessary to add small amount of coagulants (like alum, aluminium sulphate, sodium aluminate etc.) which hydrolyze to give flocculent, gelatinous precipitate of Aluminiumhydroxide. This precipitates entraps the fine CaCO3 precipitates

Use of sodium aluminate as coagulant also helps the removal of silica as well as oil

Cold Lime-Soda process provides water containing residual hardness of 50-60 ppm

Continuous Cold Lime-Soda Softener

Raw water and calculated quantities of Chemicals (Lime+Soda+Coagulant) are fed from the top into the inner verticalCircular chamber, fitted with a rotating shaft carrying a number of paddles

As raw water and chemicals flow down,There is vigorous stirring and continuousmixing whereby softening water takes Place

As softened water comesint the outer coaxial chamber, it raises upward. The heavy Sludge settles down in the outer chamber by the time the softened water raise up.Softened water proceed through the filter (made by wood fiber) to ensure complete removal of sludge.Sludge settled down at the bottom is drawn off occasionally

Hot Lime Soda process

This process involves in treating water with softening chemicals at hightemperature of 80-150oC

Since hot process is operated at close to boiling point of solution

1. Reaction proceeds faster2. Softening capacity of hot process is increased to several fold3. Precipitate and sludge settle down rapidly and no coagulant is

required4. Dissolved gases (CO2 and air) driven out of water5. Viscocity of softened water is lower hence filtration of water becomes

easierand increases the filtering capacity of filters

Hot Lime-Soda process provides water of comparatively lower residual hardness of 15-30 ppm

Continuous Hot Lime-Soda Softener

This plant consists of three parts

1. A reaction tank in which raw water, chemicals, steam are thoroughly mixed2. Conical sedimentation vessel in which sludge settles down 3. Sand filter which ensures complete removal of sludge from the softened water

Advantages of Lime-Soda Process

1. It is economical2. If this process is combined with sedimentation with coagulation, lesser amount

of coagulants are required3. The process increases the pH of value of the treated water thereby, corrosion of

the distribution pipes are reduced4. Besides the removal of hardness the quantity of minerals in the water are reduced5. To certain extend iron, manganese are also removed from the wter6. Due to alkaline nature of treated water, amount of pathogenic bacterias in water

is considerably reduced

Dis-advantages of Lime-Soda Process

1. For efficient and economical softening, careful operation and skilled supervision is required

2. Disposal of large amount of sludge poses problem

3. This can remove hardness only up to 15ppm, which is not good for boilers

Solving problems on water treatment by Lime Soda process

1. To remove temporary hardness salts, lime and soda requirementCa(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2OOne equivalent Lime required

Two equivalent Lime required

2. To remove permanent hardness salts, lime and soda requirement

Ca(SO4) + Na2CO3 CaCO3 + Na2SO4

CaCl2 + Na2CO3 CaCO3 + 2NaCl

One equivalent Soda requiredMg(SO4) + Ca(OH)2 Mg(OH)2 + CaSO4

Ca(SO4) + Na2CO3 2CaCO3 + Na2SO4

One equivalent Soda and one equivalent Lime requiredMgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2CaCl2 + Na2CO3 CaCO3 + 2 NaCl

One equivalent Soda and one equivalent Lime required


3. To remove permanent hardness related to H+ (HCl, H2SO4), and FeSO4, Al2(SO4)3 salts, lime and soda requirement

2HCl + Ca(OH)2 CaCl2 + 2H2O

H2SO4 + Ca(OH)2 CaSO4 + 2 H2O

FeSO4 + Ca(OH)2 CaSO4 + Fe(OH)2

2Fe(OH)2 + H2O + O2 1Fe2(OH)3

Al2(SO4)3 + 3Ca(OH)2 2Al(OH)3 + 3CaSO4

CaX2 + Na2CO3 CaCO3 + 2 NaCl

X = SO4 or Cl

One equivalent Soda and one equivalent Lime required

***: Lime while reacting with H+ (HCl, H2SO4), MgCl2, MgSO4, FeSO4, Al2(SO4)3salts produces calcium permanent hardness salt. This requires additional one equivalent treatment of Soda


4. To remove hardness related to HCO3- (other than Ca and Mg),

lime and soda requirement For ex. NaHCO3

If the bicarbonate is sodium bicarbonate, this on reaction with Lime producessodium carbonate (Soda). Lime soda requirement for HCO3

- is

*** One equivalent lime minus one equivalent soda (L-S)

2 HCO3- + Ca(OH)2 CaCO3 + H2O + CO3

2-

5 To remove hardness related to CO2 and H2S,lime and soda requirementCO2 + Ca(OH)2 CaCO3 + H2O

H2S + Ca(OH)2 CaS + 2H2O

Both requires one equivalent of only Lime6. If hard water contains sodium aluminate, lime and soda requirement

NaAlO2 + H2O Al(OH)3 + NaOH

2 NaOH is equal to one Ca(OH)2

Lime requirement is one equivalent less (-L)

Solving numerical problems on Lime-Soda requirement for softening of hard water

7. If the analytical report show the quantities of Ca2+ and Mg2+, 1 equivant of sodais required for Ca2+ whereas one quivalent of soda, one quivalent of lime is required for Mg2+

8. If the lime and soda used are impure, and % of the purity is given, then the actual requirement should be calculated accordingly.For example given lime purity 90% and soda purity is 95%, the value obtained in Step 6 should be multiplied by 100/90 and 100/95 respectively

= 106/100 Perm.( Mg2+ + Ca2+ + Fe2+ + Al3+ ) + H+ (HCl or H2SO4) - HCO3

-

All in terms of CaCO3 equivalent

X water volume X 100/95

Soda required for softening

X water volume X 100/90= 74/100Temp. Ca2+ + 2 X Temp. Mg2+ + Perm.( Mg2+ + Fe2+ + Al3+ ) + CO2 + H+ (HCl or H2SO4) + HCO3

- - NaAlO2


Lime required for softening


Following steps may be followed

1. The units in which impurities are expressed ie. ppm or mg/liter, grains per gallonetc. are to be noted

2. Substance which do not contribute towards hardness should be ignored whilecalculating lime and soda requirement(KCl, NaCl, SiO2, Na2SO4, Fe2O3, K2SO4 etc.)

3. Convert ion of all the hardness causing in to respectiveCaCO3 equivalents

4. If the impurities are given as CaCO3 and MgCO3, these should be considered dueto Ca(HCO3)2 and Mg(HCO3)2 respectively. They are only expressed in terms of CaCO3 and MgCO3

5. The amount expressed in terms of CaCO3, it does not require further convertion.However the amount expressed in MgCO3 should be converted to its CaCO3 equivalent by multiplying with 100/84


6. The amount expressed in terms of CaCO3, it does not require further convertion.However the amount expressed in MgCO3 should be converted to its CaCO3 equivalent by multiplying with 100/84

7. Calculate the Lime soda requirement as follows


= 74/100Temp. Ca2+ + 2 X Temp. Mg2+ + Perm.( Mg2+ + Fe2+ + Al3+ ) + CO2 + H+ (HCl or H2SO4) + HCO3

- - NaAlO2



= 106/100 Perm.( Mg2+ + Ca2+ + Fe2+ + Al3+ ) + H+ (HCl or H2SO4) - HCO3-


X Volumeof water

X Volumeof water


8. If the analytical report show the quantities of Ca2+ and Mg2+, 1 equivant of sodais required for Ca2+ whereas one quivalent of soda, one quivalent of lime is required for Mg2+

9. If the lime and soda used are impure, and % of the purity is given, then the actual requirement should be calculated accordingly.For example given lime purity 90% and soda purity is 95%, the value obtained in Step 6 should be multiplied by 100/90 and 100/95 respectively

= 106/100 Perm.( Mg2+ + Ca2+ + Fe2+ + Al3+ ) + H+ (HCl or H2SO4) - HCO3

-


X water volume X 100/95


X water volume X 100/90= 74/100Temp. Ca2+ + 2 X Temp. Mg2+ + Perm.( Mg2+ + Fe2+ + Al3+ ) + CO2 + H+ (HCl or H2SO4) + HCO3

- - NaAlO2



Documents

Water Technology Part 2