Water Hardness & Softening. Water Hardness & Softening. Water Hardness & Softening. Water Hardness and Softening

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1 Introduction Water Hardness and Softening Hardness is an important water quality parameter in determining the suitability of water for domestic and industrial uses Hard waters require considerable amounts of soap to produce foam Hard waters produce scale in hotwater pipers, heaters and boilers Ca + + HCO 3 CaCO 3 (s) + CO (g) + H O Groundwater is generally harder than surface water Principal cations causing hardness and the major anions associated with them (in decreasing order of abundance in natural waters) Cations: Ca +, Mg +, Sr +, Fe +, Mn + Anions: HCO 3, SO 4, Cl, NO 3, SiO 3 King Saud University 1 King Saud University Introduction Hardness Definition Total Hardness Technically the sum of all polyvalent cations Practically the amount of calcium and magnesium ions (the predominant minerals in natural waters) It is divided into carbonate and noncarbonate hardness. It is divided into temporary and permanent hardness. It is divided into calcium and magnesium hardness. Hardness Units milligrams per liter (mg/l) as calcium carbonate (most common) parts per million (ppm) as calcium carbonate equivalents/liter (eq/l) milliequivalents/liter (meq/l) King Saud University 3 King Saud University 4

2 Hardness Definition Hardness Definition Hard water water that requires considerable amounts of soap to produce foam or lather; also produces scale in hot water pipes, etc. Caused by the presence of multivalent cations, mostly Ca + and Mg + ; (Fe +, Mn +, Sr +, Al 3+ may be present in much smaller amounts). Not a health concern, but $$$ concern Hardness Salt Common Name Carbonate Hardness Calcium Bicarbonate Magnesium Bicarbonate Calcium Carbonate Magnesium Carbonate Noncarbonate Hardness Hardness Salt Chemical Formula Ca(HCO 3 ) Mg(HCO 3 ) CaCO 3 MgCO 3 Solubility at Indicated Temperature (ppm CaCO 3 ) 1,60 (at 3 F) 37,100 (at 3 F) 13 (at 1 F) 75 (at 1 F) Calcium Sulphate CaSO 4 1,46 (at 1 F) Calcium Chloride CaCl 554,000 (at 1 F) Magnesium Sulphate MgSO 4 356,000 (at 1 F) Magnesium Chloride MgCl 443,000 (at 1 F) King Saud University 5 King Saud University 6 Hardness Level Hardness expressed in mg/l as CaCO 3 Methods of determination Calculation: Hardness (mg/l) as CaCO 3 = [Ca ++ ]+ [Mg ++ ] EDTA (Ethylenediaminetertraacetic) titrimetric method Water softening is needed when hardness is above mg/l; Hardness 5080 is acceptable in treated water Carbonate and Noncarbonate Hardness Water hardness is principally caused by: calcium ions magnesium ions Total hardness = Carbonate hardness + Noncarbonate hardness Carbonate hardness = (temporary hardness); eliminated at elevated temperatures in boilers Ca + + HCO 3 Ca + + HCO 3 CaCO 3 + CO + H O + Ca(OH) CaCO 3 + H O Noncarbonate hardness = permanent hardness; can not be removed or precipitated by boiling. Noncarbonate hardness cations are associated with SO 4, Cl and NO 3. King Saud University 7 King Saud University 8

3 Carbonate and Noncarbonate Hardness Total Hardness (TH) for practical purposes, this is the sum of the calcium hardness and the magnesium hardness. Total Hardness = Mg + hardness + Ca + hardness UNITS mg/l as CaCO 3 Carbonate hardness the portion of total hardness that is chemically equivalent to the CO 3 and HCO 3 alkalinity present in the water. Noncarbonate hardness that hardness which is in excess of carbonate hardness; will only occur in water where TH > alkalinity Calculate the hardness of a water sample with the following analysis: Cations mg/l Anions mg/l Na + Ca + Mg + Sr Cl SO 4 NO 3 Alkalinity King Saud University 9 King Saud University 10 A sample of water having a ph of 7. has the following concentrations of ions Ca + 40 mg/l Mg + 10 mg/l Na mg/l K mg/l HCO mg/l SO mg/l Cl 11 mg/l Calculate the TH, CH, NCH, Alkalinity Ion mg/l M.W. mg/mmol Ca Mg Na K HCO SO Cl n Eq. Wt. mg/meq meq/l mg/l as CaCO 3 King Saud University 11 King Saud University 1

4 Periodical Table Ion mg/l M.W. mg/mmol n Eq. Wt. mg/meq Ca Mg Na K HCO SO Cl meq/l mg/l as CaCO 3 Sample Calculation: Equivalent Weight of Ca + = M.W. / n = 40.1/ = 0.05 King Saud University 13 King Saud University 14 Ion mg/l M.W. mg/mmol n Eq. Wt. mg/meq meq/l Ca Mg Na K HCO SO Cl mg/l as CaCO 3 Sample Calculation: Concentration of Ca + = (Concentration in mg/l) / (Equivalent Weight in mg/meq) = (40.0 mg/l) / (0.05 mg/meq) = meq/l Ion mg/l M.W. mg/mmol n Eq. Wt. mg/meq meq/l mg/l as CaCO 3 Ca Mg Na K HCO 3 SO Cl Sample Calculation: (Ca + in meq/l)* (EW of CaCO 3 ) = Ca + in mg/l as CaCO 3 (1.995 meq/l)*(50 mg/meq) = 99.8 mg/l as CaCO 3 King Saud University 15 King Saud University 16

5 Check Solution Σ(cations) = Σ(anions) to within ± 10% = mg/l as CaCO 3 (Can check using concentrations in meq/l or mg/l as CaCO 3 ) Total Hardness = Σ of multivalent cations = (Ca + ) + (Mg + ) = = 141 mg/l as CaCO 3 Alkalinity = (HCO 3 ) + (CO 3 ) + (OH ) (H + ) Since ph = 7. [H + ] = M (H + ) = eq/l [OH ] = M. (OH ) = eq/l From before we see that (HCO 3 ) = 1.80 meq/l, since n = 1 then [HCO 3 ] = 1.80 x 10 3 M Using the equilibrium expression for HCO 3 /CO 3, we find that [CO 3 ] = [HCO 3 ]/[H + ] = 1.33 x 10 6 M. Since n =, then (CO 3 ) =.66 x 10 6 eq/l Alkalinity = (1.80 x 10 3 ) + (.55 x 10 6 ) = x 10 3 eq/l * Alkalinity = x 10 3 x 1000 x 50 =90.1 mg/l as CaCO 3 King Saud University 17 King Saud University 18 Carbonate Hardness (the portion of the hardness associated with carbonate or bicarbonate) Alkalinity = 90.1 mg/l as CaCO 3 TH = 141 mg/l as CaCO 3 CH = 90.1 mg/l as CaCO 3 (Note: if TH < Alk then CH = Alkalinity; and NCH = 0 ) Hardness Measurement titration method Take approx. 100 ml of the EDTA solution. EDTA solution in the burette Dissolve the indicator tablet fully before starting the titration. Titrate water with EDTA until colour changes from red to blue. Noncarbonate Hardness NCH = TH CH = = 50.9 mg/l as CaCO 3 Water sample in the conical flask EDTA: Ethylenediaminetetraacetic acid King Saud University 19 King Saud University 0

6 Hardness Measurement titration method Eriochrome Black Indicator Hardness Measurement titration method example 50.0 ml tap water sample ml EDTA used in the titration. a) Calculate the moles of EDTA used. b) Calculate the molarity of metal ion present in the tap water sample (Remember: EDTA binds to metal ions on a 1:1 molar ratio). c) Find the ppm calcium ion concentration in the sample based on your results from question b). King Saud University 1 King Saud University Hardness Measurement titration method example a) Calculate the moles of EDTA used Molarity of EDTA = mol/l Moles of EDTA = (Molarity) (Liters of EDTA used) b) Calculate the molarity of metal ion present (Remember EDTA binds to metal ions on a 1:1 ratio) Molarity of metal ion = moles of EDTA / liters of water sample c) Calculate concentration of CaCO 3 in ppm CaCO 3 (ppm) = (Molarity of metal ion)(mw of Ca + )(1000 mg/g) CaCO 3 (ppm) = Water Softening Methods Objective: to reduce the hardness and thus prevent scaling Ion exchange Zeolite or Permutit process Mixed bed deionizezer Chemical process Lime Soda process: was patented in st municipal softening plant was installed in 1854 in England. The first limesoda softening plant was in ml tap water sample ml EDTA used in the titration. King Saud University 3 King Saud University 4

7 Calcium & Magnesium Salts Calcium Salts Calcium Bicarbonate Calcium Carbonate Calcium Chloride Calcium Sulphate Calcium Hydroxide Magnesium Salts Magnesium Bicarbonate Magnesium Carbonate Magnesium Chloride Magnesium Sulphate Magnesium Hydroxide Chemical Formula Ca(HCO 3 ) CaCO 3 CaCl CaSO 4 Ca(OH) Mg(HCO 3 ) MgCO 3 MgCl MgSO 4 Mg(OH) Solubility at 0 o C (ppm CaCO 3 ) 1, ,000 1,90,390 37, , , King Saud University 5 King Saud University 6 Calcium Bicarbonate HCO3 Carbonate CO3 Hydroxide OH Solubility Very Soluble Insoluble Somewhat Soluble Magnesium Bicarbonate HCO 3 Carbonate CO 3 Hydroxide OH Solubility Very soluble Soluble Insoluble Pretreatment unit Coagulation /flocculation (most commonly used): Ca +, Mg + CaCO 3 (s), Mg(OH) (s) limeonly process: when Ca + is present primarily as bicarbonate hardness limesoda [Ca(OH) Na CO 3 ] process: when bicarbonate is not present at substantial level King Saud University 7 King Saud University 8

8 Common Chemical Names Baking Soda Caustic Soda Soda Ash Washing Soda Quick Lime Active Lime Hydrated Lime Lime Stone NaHCO 3, Sodium Bicarbonate NaOH, Sodium Hydroxide Na CO 3, Sodium Hydroxide Na CO 3, Sodium Hydroxide CaO, Calcium Oxide CaO, Calcium Oxide Ca(OH), calcium Hydroxide CaCO 3, calcium carbonate Schematic Flow Diagram of Salbukh Water treatment plant King Saud University 9 King Saud University 30 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). Lime to remove Ca + in the form of natural alkalinity Ca(HCO 3 ) + Ca(OH) CaCO 3 + H O Lime to remove Mg + in the form of natural alkalinity Mg(HCO 3 ) + Ca(OH) MgCO 3 (soluble) + CaCO 3 + H O additional lime must be added to remove MgCO 3 MgCO 3 + Ca(OH) CaCO 3 + Mg(OH) Mg + hardness in the form of a sulfate requires both lime and soda ash MgSO 4 + Ca(OH) CaSO 4 + Mg(OH) CaSO 4 + Na CO 3 CaCO 3 + Na SO 4 CO in the water will also consume lime CO + Ca(OH) CaCO 3 + H O King Saud University 31 King Saud University 3

9 Photo of a Precipitator King Saud University 33 King Saud University 34 Rules of Thumb Lime Requirements, Ca(OH) Add 1 meq/l per meq/l CO Add 1 meq/l per meq/l alkalinity associated with hardness Add 1 meq/l per meq/l Mg++ that is to be removed (Eq. d and e) In practice, when it is desired to remove Mg++, one meq/l Ca(OH) is added in excess of the stoichiometric amounts designated above to insure adequate and rapid precipitation of the Mg(OH). Soda Ash Requirements, Na CO 3 Add 1 meq/l per meq/l of noncarbonate hardness Schematic Diagram of a Precipitator King Saud University 35 King Saud University 36

10 Selective Calcium Carbonate Removal If the water to be treated contains low concentration of magnesium (<40 mg/l as CaCO 3 ), selective calcium carbonate removal can be used. Magnesium hardness of more than 40 mg/l as CaCO 3 is not recommended due to the possible formation of hard magnesium silicate in high temperature waters (85 C) enough lime is added but not in excess soda ash may be used depending on the extent of noncarbonate hardness if precipitation of CaCO 3 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 Alkalinity and Hardness Condition CH, lime NC, lime+soda TH < alkalinity TH 0 TH = alkalinity TH 0 TH > alkalinity Alk THAlk TH: total hardness, CH: carbonate hardness, NC: noncarbonate hardness King Saud University 37 King Saud University 38 Alkalinity and Hardness Alkalinity and Hardness Solution King Saud University 39 King Saud University 40

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12 Recarbonation by bubbling CO after softening Recarbonation is usually required after limesoda process Why? To prevent supersaturated CaCO 3 (s) and Mg(OH) (s) from forming harmful deposits or undesirable cloudiness in water at a later time CaCO 3 (s) + CO + H O Ca + + HCO 3 MgCO 3 (s) + CO + H O Ca + + HCO 3 To neutralize excessively high ph caused by Na CO3 OH + CO HCO 3 Recarbonation by bubbling CO after softening Recrabonation is used to stabilize limetreated water, thus reducing its scaleforming potential. Carbon dioxide is used for the recarbonation process. It converts lime to calcium carbonate. Further recarbonation will convert carbonate to bicarbonate. CO CO CO + Ca( OH ) 3 + Mg( OH ) + CaCO CaCO + H O Ca( HCO ) 3 MgCO + H 3 + H O 3 O King Saud University 45 King Saud University 46 Stabilization Process of making water less corrosive and less depositing Increasing Ca hardness, alkalinity or ph: Increase scaling and decrease corrosive tendency Increasing temperature: Increase scaling and corrosive tendency TDS can affect scaling and corrosivity Stabilization Caustic soda is commonly used Unstable water: red water, lead and copper corrosion problems Orthophosphates, silicates used to prevent lead & copper corrosion, sequester Ca & carbonate Final ph should be selected by looking at scaling indices: LSI: Langlier Saturation index CCPP: Calcium Carbonate Precipitation Potential Ryznar index Lead solubility King Saud University 47 King Saud University 48

13 Bargraph: Hypothetical chemical composition Bargraph: Hypothetical chemical composition King Saud University 49 King Saud University 50 Bargraph: Hypothetical chemical composition Bargraph: Hypothetical chemical composition King Saud University 51 King Saud University 5

14 Bargraph: Hypothetical chemical composition Bargraph: Hypothetical chemical composition King Saud University 53 King Saud University 54 King Saud University 55