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UNIT IWater TechnologyRev.Ed. 2013-14WATER TECHNOLOGYSyllabus: Hard Water – Estimation of hardness by EDTA method – Potable water- Sterilization andDisinfection – Boiler feed water – Boiler troubles – Printing and foaming , scale formation, corrosion,caustic embrittlement, turbine deposits – Softening of water – Lime soda, Zeolite processes – Reverseosmosis – Electro Dialysis, Icon exchange processObjectives: For prospective engineers knowledge about water used in industries (boilers etc.) and fordrinking purposes is useful; hence chemistry of hard water, boiler troubles and modern methods ofsoftening hard water is introduced.OUTLINES Introduction Hardness of water Estimation of total hardness of water by EDTA method Scale and sludge formation in boilers Caustic embrittlement Boiler corrosion Priming and foaming Softening methods Potable water Desalination of brackish waterEngineering ChemistryPage 1

UNIT I1.Water TechnologyRev.Ed. 2013-14INTRODUCTIONWater is the nature’s most wonderful, abundant and useful compound. Of the many essentialelements for the existence of human beings, animals and plants, water is rated to be of greatestimportance. Without food human being can survive for a number of days, but water is such an essentialthing without it one cannot survive.Water is not only essential for the lives of animals and plants but also occupies unique position inindustries. Probably its most important use as an engineering material is in the ‘steam generation’.Water is also used as a coolant, in power, and chemical plants. In addition to it, water can also be used inthe production of steel, rayon, paper, textiles, chemicals, irrigation, drinking fire fighting, etc.2. Hardness of waterHardness in water is that character, which “prevents the lathering with soap”. This is due to thepresence of certain salts of calcium, magnesium and other heavy metals dissolved in water. A sample ofhard water when treated with soap, (sodium or potassium salt of higher fatty acid such as oleic, palmaticor stearic) does not produce lather, but on the other hand forms a white scum or precipitate. Thisprecipitate is resulted due to the formation of insoluble salts of calcium and magnesium. Typical reactionsof soap with calcium chloride and magnesium sulphate are depicted below.2C17H35COONa CaCl2Sodium stearate(Hardness)(Sodium soap)(C17H35COO)2Ca 2NaCl2C17H35COONa MgSO4(C17H35COO)2Mg Na2SO4Calcium stearate(Insoluble)Magnesium stearate(Insoluble)Thus, water which does not form lather readily with a solution of soap, but forms a white scum, is hardwater. On the other hand water which lathers easily on shaking with soap solution is called soft water.Such water, consequently, does not contain dissolved calcium and magnesium salts in it.2.1 Temporary or carbonate hardnessTemporary hardness is caused due to the presence of dissolved bicarbonates of calcium, magnesiumand other heavy metals and the carbonate of iron. It is eliminated merely by boiling,when bicarbonatesare decomposed yielding insoluble carbonates or hydroxides, which are deposited as a crust at the bottomand sides of the vessel.Ca(HCO3)2CalciumBicarbonateEngineering ChemistryHeatCaCO3 H2O CO2CalciumCarbonatePage 2

UNIT IWater TechnologyHeatMg(HCO3)2MagnesiumBicarbonateRev.Ed. 2013-14Mg(OH)2 H2O 2CO2MagnesiumHydroxide2.2 Permanent or non-carbonate hardnessThis is due to the presence of chlorides and sulphates of calcium, magnesium iron and other heavymetals. Unlike temporary hardness, permanent hardness is not eliminated by simple boiling.2.3 EQUIVALENTS OF CALCIUM CARBONATEThe concentration of hardness as well as non-hardness causing ions is usually expressed in termsof equivalents of CaCO3, since this mode of representation permits the multiplication and division ofconcentration, when required. The choice of CaCO3 in particular is due to its molecular weight which is100 and moreover, it is most insoluble salt that can be precipitated in water treatment.Table.1 Calculation of equivalents of calcium carbonateDissolved O4)3FeSO4.7H2OH HClMolar 6.5The equivalents of CaCO3 8257139136.5Multiplication factor converting intoequivalents of 0/164100/114100/278100/2100/73Mass of hardnessproducing substance xEquivalent Weightof CaCO3Equivalent Weight of hardness producing substance Engineering ChemistryMass of hardness producing substance x 50Equivalent Weight of hardness producing substancePage 3

UNIT IWater TechnologyRev.Ed. 2013-142.4 UNITS OF HARDNESS1. Parts per million (ppm) is the parts of calcium carbonate equivalent hardness per 106 parts ofwater i.e., 1 ppm 1 part of CaCO3 eq. hardness in 106 parts of water.2. Milligram per liter (mg/L) is the number of milligrams of CaCO3 equivalent hardness presentper liter of water,Thus; 1mg/L 1mg of CaCO3 eq. hardness of 1 L of waterBut I liter of water weighs 106 mg1mg/L 1mg of CaCO3 eq. hardness in 106 parts of water 1 part of CaCO3 eq. hardness in 106 parts of water1mg/L 1 ppm3. Clarke’s degree is the number of grains (1/7000lb) of CaCO3 equivalent hardness per gallon(10lb) of water or its parts of CaCO3 equivalent hardness per 70,000 parts of water. Thus:1o Clarke 1 grain of CaCO3 eq. hardness in gallon of water1o Cl 1 part of CaCO3 eq. hardness in 70,000 parts of water4. Degree French is the parts of CaCO3 equivalent hardness per 105 parts of water.Thus: 1oFr 1 part of CaCO3 eq. hardness in 105 parts of water5. Milli equivalents per liter (meq/L) is the number of Milliequivalents of hardness present per literThus:1 meq/L 1 meq of CaCO3 per liter of water 10-3 x 50g of CaCO3 eq. per liter of water 50 mg of CaCO3 eq. per liter 50 mg/L of CaCO3 50ppm6. Relation between various units of hardness:1 ppm 1 mg/L1 mg/L 1 ppmoo 0.1o Fro 0.1 Fr 0.07o Clo 0.07 Cl 0.02 meq/L 0.02 meq/L1 Cl 1.433 Fr 14.3 ppm 14.3 mg/L 0.286 meq/L0.1o Fr 10 ppm 10 mg/L 0.7o Cl 0.2 meq/L 50 ppm 5o Fr 0.35 oCl1 meq/L 50mg/L3. Estimation of total hardness of water by EDTA methodThis is a complexometric method. Ethylene diamine tetra acetic acid (EDTA)HOOC.CH2HOOC.CH2Engineering ChemistryN-CH2-CH2-NCH2.COOHCH2.COOHPage 4

UNIT IRev.Ed. 2013-14Water TechnologyIn the form of its salt yields the is forms complex ions with Ca2 and Mg2 O C-OO-C OMH2C-OOC.CH2NNCH2CH3CH2CH2.COO-Where M Ca or Mg. It may be pointed that the EDTA is employed as its disodium In order to determine the equivalence point, EBT (Eriochrome Black- T) indicator is employed,which forms unstable wine red complex with Ca2 and Mg2 ions. However, this indicator is effective at apH of about 10. When EBT is added to hard water buffered to a pH of about 10, a wine red unstablecomplex is formed. Thus:pH 10M2 EBT 2 2 (Ca or Mg of hard water)M-EBT Complexwine-redSo initially a wine red coloured is obtained. During the course of titration against EDTA solution,EDTA combines with Ca2 and Mg2 ions and form stable complex M-EDTA and releasing free EBT,which instantaneously combines with M2 ions still present in the solution, thereby wine red colour isretained thus:[M-EBT] Complex EDTAwine-redTitration[M-EDTA] Complex EBT(blue)pH 10M2 EBT(Ca2 or Mg2 still present) (blue)Engineering ChemistryM-EBT Complexwine-redPage 5

UNIT IWater TechnologyRev.Ed. 2013-14However, When nearly all M2 (Ca2 or Mg2 ions)ions have formed[M-EDTA] complex, thenext drop of EDTA added displaces the EBT indicator form [M-EBT] complex and the wine –red colourchanges to blue colour. Thus, at the equivalence point,Titration[M-EBT] Complex EDTAwine-red[M-EDTA] Complex EBT(blue)Thus change of wine red colour to a distinct blue marks the end of the titration.1. Standard hard water: 1 gm of dry CaCO3 is dissolved in minimum quantity of HCl and evaporate thesolution to dryness on a water bath, and then diluted to 1 lit with water. Each mlof this solution then contains 1 mg of CaCO3 hardness.2. EDTA solution: 4 gm of EDTA crystals 0.1 gm MgCl2 in 1lit3. Indicator: 0.5 gm of EBT in100 ml of alcohol.4. Buffer solution: 67.5 gm NH4Cl 570 ml of Con. Ammonia solution diluted with distilled waterto 1 lit.5. Titration of permanent hardness of water: Take 250 ml of the water sample in a large beaker .Boil tillthe volume is reduced to 50 ml. Filter, wash the precipitate with distilled water collecting filtrate and .Finally make the volume to 250 ml with distilled water. Then titrate 50 ml of the boiled water samplejust as in step (5). Let volume used by V3 mlCalculations: 50 ml of standard hard water V1 ml of EDTA:. 50 x1 mg of CaCO3 V1 ml of EDTA:.1 ml of EDTA 50/V1 mg of CaCO3 eq.Now 50 ml. of given hard water V2 ml EDTA V2 x 50/V1 mg of CaCO3 eq.:. 1 L (1,000 mL) of given hard water 1000 V2/V1 mg of CaCO3 eq.:. Total hardness of water 1000 V2/V1 mg/L 1000 V2/V1 ppmNow 50 ml of boiled water V3 ml of EDTA V3 x 50 mg of CaCO eq3V1. . 1000 ml ( 1 L) of boiled water 1000 V3/V1 mg of CaCO3 eq.Permenent hardness 1000 V3/V1 ppmAndEngineering ChemistryTemporary hardness Total hardness – Permanent hardnessPage 6

UNIT IWater Technology 1000 V2V3V1V1Rev.Ed. 2013-14ppm1000 (V2 - V3) ppmV13.1 Advantages of EDTA method: This method is definitely preferable to the other methods, becauseof the (i) Larger accuracy; (ii) Convenience; (iii) Rapid procedure3.2 Problems :1. 50 ml of sample consumed 15 ml of 0.01 M-EDTA before boiling and 5 ml of the same EDTAafter boiling .Calculate the degree of total hardness, permanent hardness and temporary hardness.Solution. 50ml of water sample 15 ml of 0.01M-EDTA 15 X 1000 ml of 0.01 M-EDTA 300ml of 0.01 M-EDTA50 2 X 300 ml of 0.01 N-EDTA (Since Molarity of EDTA 2 x Normality of EDTA) 600ml (or 0.6 L) of 0.01 eq. CaCO3 0.6 x 0.01 x 50 g eq. CaCO3Hence, total hardness 0.30 g or 300 mg of CaCO3 eq. 300 mg/L or 300 ppm.Now 50 ml of boiled water 5 ml of 0.01 M-EDTA5 X 1000 ml of 0.01 M-EDTA. . 1000 ml of boiled water 50 100 ml of 0.01M-EDTA 200 ml (or 0.2 L) of 0.01 N-EDTA 0.2 x 0.01 x 50 g of CaCO3 eq. 0.1 g or 100mg of CaCO3 eq.Hence, permanent hardness 100 mg/L or ppmTemporary hardness (300-100) ppm 200 ppm.2. Calculate the total hardness of a sample of water containing Mg(HCO3)2 73 mg/L; Ca(HCO3)2 162mg/L, MgCl2 95 mg/L., CaSO4 136 mg/L.Ans. Total hardness of water: 350 ppm4. SCALE AND SLUDGE FORMATION IN BOILERSIn boilers, water evaporates continuously and the concentration of the dissolved salts increasesprogressively. When their concentrations reaches saturation point, they are thrown out of water in theform of precipitates which stick to the inner walls of the boiler. If the precipitation takes place in theEngineering ChemistryPage 7

UNIT IWater TechnologyRev.Ed. 2013-14form of loose or slimy precipitate it is called sludge. On the other hand, if the precipitated matter forms ahard adhering crust/ coating on the inner walls of the boiler, it is a scale.Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can be easilyscrapped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects inareas of the system, where the flow rate is slow at bends. Sludges are formed by substances which havegreater solubilities in hot water than in cold water. Examples are MgCO3, MgCl2, CaC

UNIT I Water Technology Rev.Ed. 2013-14 Engineering Chemistry Page 4 2.4 UNITS OF HARDNESS 1. Parts per million (ppm) is the parts of calcium carbonate equivalent hardness per 106 parts of water i.e., 1 ppm 1 part of CaCO 3 6eq. hardness in 10 parts of water. 2.

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