Hardness and Water Softening Dr. Akepati S. Reddy Associate Professor School of Energy and Environment Thapar University, Patiala
Hardness • Hardness: multivalent metal ions (di or multivalent cations) that form precipitates with soaps • Based on the hardness waters are classified as – Soft water (0-70 mg/L), – Moderately hard water (70-150 mg/L), – Hard water (>150 mg/L) – water with hardness >300 is often known as very hard water
• Cations causing hardness: Ca2+, Mg2+, Sr2+, Fe2+, Mn2+, Al • Hardness is associated with: HCO3-, SO42-, Cl-, NO3-, SiO32- , PO43• Hardness categories – Calcium Hardness (Ca2+H), Magnesium Hardness (Mg2+) and Total Hardness (TH) based on the responsible cations • Mg2+ H = T H - Ca2+ H – Carbonate Hardness (CH) and Non-carbonate Hardness (NCH) based on the associated anions • CH is also known as temporary hardness (bicarbonates) and it can be removed by boiling and by lime (CaO or Ca(OH)2) • NCH (also known as permanent hardness) is associated with SO42and Cl-- can be removed by soda ash &lime –unaffected by boiling
Hardness • If Alkalinity < TH, then CH = Alkalinity – If Alkalinity ≥ TH, then CH = TH – NCH = TH – Total Alkalinity – Sodium Alkalinity = Total Alkalinity – Total Hardness (TH)
• Method used for hardness measurement: EDTA (Ethylene-diaminetetra-acetic acid) method – Eriochrome Black T is indicator – forms complex with free EDTA and change colour from red to blue – Boiling off and filtering removes temporary hardness and permanent hardness can be used – Calcium hardness is also measured by EDTA method but after precipitating the Mg at 13 pH and using different indicator (calcon) – Hardness is expressed in mg/L as CaCO3 and at times in meq/L
• Concentrations in mg/L can be converted into meq/L by dividing with equivalent weight of the species – – – –
X mg/L of Ca2+ = X/20 meq/L of Ca2+ (20 is equi. wgt. of Ca2+) X mg/L of Mg2+ = X/12.2 meq/L of Mg2+ (12.2 is equi. wgt. of Mg2+) X mg/L of TH as CaCO3 = X/50 meq/L of TH (50 is equi. wgt. of CaCO3) X mg/L of CO2 = X/22 meq of CO2 (22 is equi. wgt. of CO2)
At >8.3 pH carbon dioxide does not exisit At >11.3 even biocarbonate disappears
Measurement of Calcium hardness • Samples pH is raised to ~13 for precipitating magnesium as hydroxide and not allowing its reaction with EDTA - in the titration with EDTA, calcon (Hydroxy naphthol) is used as an indicator • Preliminary titration to find approximate volume of EDTA needed – To the sample, add a few drops of polyvinyl alcohol and 1-2 ml of 50% NaOH, and heat to 60-70o – Add 2 shots of hydroxy naphthol powder indicator and titrate rapidly with EDTA to clear blue colour
• Final titration to find accurate volume of EDTA needed – To the sample, add a few drops of polyvinyl alcohol, 2 shots of hydroxy naphthol powder indicator and accurately measured ~95% of the EDTA volume used in the preliminary titration – Heat to 60-70o, add 1-2 ml of 50% NaOH, mix and titrate with EDTA to a clear blue end point • Note: – Addition of 95% of the EDTA (prior to precipitating magnesium as Mg(OH)2) reduces the calcium ion level to a very low value – Precipitated Mg(OH)2 obscures the end point, and the ppt. formed adsorbs and coprecipitates calcium ions – Polyvinyl alcohol and heating to 70o reduces adsorption of calcium ions – Eriochrome Black T does not give satisfactory color change at high pH hydroxy naphthol functions at a high pH but in the same manner
Why to soften water and by what methods? • Hard water forms scales in water heating systems like boilers • Forms foam/leather with soap and increase the soap consumption • Water softening processes – Ion-exchange process – Lime-soda ash process • Advantages (and disadvantages) of the lime – soda ash process – The lime added to water in the process is totally removed. – TDS (total dissolved solids) of the water is significantly reduced – Lime also precipitates the soluble Fe and Mn – Excess lime treatment can provide disinfection (through raising the water pH!) and aid in the coagulation process – Removal of non-carbonate hardness requires soda ash and when soda ash is applied, Na+ remains in the finished water • Schemes of water softening by lime-soda ash process – Excess lime treatment – Selective calcium removal – Split treatment
Indices (describing behavior of CaCO3 in water) Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) Langelier Saturation Index (LSI) • A calculated number developed by Wilfred Langelier (1936) • It predicts the CaCO3 stability of water (whether the carbonate will precipitate, dissolve, or remain in equilibrium) and stated as LSI = pH - pHs • pHs (the pH at which water is saturated in CaCO3) is calculated by pHs = (9.3 + A + B) - (C + D) A = (Log10[TDS] - 1)/10 B = -13.12 x Log10(oC + 273) + 34.55 – (2.09 at 25°C) C = Log10[Ca2+ as CaCO3] - 0.4 - - (2.5(Ca2+) D = Log10[alkalinity as CaCO3]
• LSI is temp. sensitive (increasing temp. increases the LSI value)
Indices (describing behavior of CaCO3 in water) • Water with an LSI between -0.5 and +0.5 will not display any mineral dissolving or scale forming properties – LSI > 0 indicates super-saturation of water with CaCO3 and tendency of precipitation of CaCO3 scale layer - LSI < 0 indicates under-saturated and dissolution of solid CaCO3
Ryznar Stability Index (RSI) • Developed from empirical observations of corrosion rates and film formation in steel mains • Defines as RSI = 2 pHs – pH (measured) • RSI between 6.5 and 7.0 indicates that the water is at saturation equilibrium with calcium carbonate – RSI > 8 indicates water is under saturated and tends to dissolve any existing solid CaCO3 – RSI < 6.5 indicates super-saturation of water with CaCO3 and tendency to form CaCO3 scale
Example: find LSI and RSI for the water with pH = 7.5; TDS = 320 mg/L; Calcium = 150 mg/L (or ppm) as CaCO3; Alkalinity = 34 mg/L (or ppm) as CaCO3
Lime-Soda ash process: Chemistry • A chemical precipitation method - uses lime (CaO/Ca(OH)2) and soda ash (Na2CO3)
– Lime removes carbonate hardness (CH), and Soda ash removes non-carbonate hardness (NCH) from water – Hardness is removed as Calcium Carbonate (CaCO3) and Magnesium Hydroxide (Mg(OH)2) precipitates
• The lime added first reacts with CO2 to form carbonate precipitate and then with carbonate hardness – – – –
Mg2+ CH hardness reacts with lime to form MgCO3 & CaCO3 ppt. MgCO3 in turn reacts with lime added to form Mg(OH)2 ppt. One mole of Ca2+ CH requires one mole of lime One mole of Mg2+ CH requires two moles of lime
• Lime added converts Mg2+ NCH into Ca2+ NCH and forms CaCO3 ppt.
– Soda ash added converts Ca2+ NCH into CaCO3 precipitate – Removal of 1 mole of Ca2+ NCH requires 1 mole of soda ash – Removal of 1 mole of Mg2+ NCH requires 1 mole each of lime
Lime-Soda ash process: Chemistry • CaCO3 and Mg(OH)2 are slightly soluble in water
– 0.6 meq/L (30 mg/L as CaCO3) of CaCO3 and 0.2 meq/L (10 mg/L as CaCO3) of Mg(OH)2 do not be removed through coagulationflocculation-settling/filtration – Theoretical solubility of CaCO3 is 17 mg/L and of Mg(OH)2 is 9 mg/L – Practical minimum TH for the softened water is taken as 50-80 mg/L
• Minimum practical limit of softening is 40 mg/L – Goal set for the softening is 75-120 mg/L as CaCO3 – Limited completion of the chemical reactions, inadequate mixing and not complete removal of the ppt. affect the limits the softening
• Residual hardness in the form of CaCO3 and Mg(OH)2 can result in easy scaling – recarbonation involving conversion of the hardness into Ca2+ and Mg2+ bicarbonate is practiced – In the 1st stage of recarbonation the OH- is neutralized to <10.5 (?) – Ca(OH)2 added in excess and residual MG(OH)2 are converted into Ca2+ and Mg2+ carbonate – In the 2nd stage of recarbonation, carbonate is converted into bicarbonate through brining down pH to ~8.5 – Both Ca2+ and Mg2+ are converted into Ca2+ and Mg2+ bicarbonates
Lime-Soda ash process: Chemistry • Water with high magnesium hardness requires excess lime treatment (to increase of pH to >11.0 for softening) and two stage recarbonation – Softened water has both calcium and magnesium hydroxides and 1st stage recarbonation converts these hydroxides into carbonates – The 2nd stage recarbonation reduces the pH to 8.4-8.6 and converts the carbonate into bicarbonate – CO2 is totally neutralized by 8.3 pH, bicarbonate is totally converted to carbonate by 11.3 pH, and above 11.3 pH all the alkalinity will be carbonate and hydroxide
• Split treatment is preferred when magnesium hardness is high – the softening process is bypassed by part of the water and added with the softened water – Overall lime requirement and carbon dioxide requirement for recarbonation can be reduced here
• When the magnesium hardness is low, addition of excess lime is not needed and a single stage recarbonation is sufficient • Softened water is finally filtered after the recarbonation for removing if any suspended particles left in water
Lime Addition
Lime and soda ash addition
Recarbonation MgOH 2 CO2 MgCO3 H 2O
MgCO3 CO2 H 2O MgHCO3 2
CaCO3 CO2 H 2O CaHCO3 2
Bar diagram showing ion composition of water • Used in – dose calculations of lime, soda ash and carbon dioxide for softening – presenting the ionic composition of water before, after and after recarbonation steps of softening
• Has two rows, a top row showing cations (CO2, Ca2+, Mg2+, Na+, and K+) concentrations and a bottom row showing anion (OH-, CO32-, HCO3-, SO42-, Cl-, NO3-) concentrations in meq/L – – – –
X mg/L of Ca2+ = X/20 meq/L of Ca2+ (20 is eq. weight of Ca2+) X mg/L of Mg2+ = X/12.2 meq/L of Mg2+ (12.2 is eq. weight of Mg2+) X mg/L of TH as CaCO3 = X/50 meq/L of TH (50 is eq. weight of CaCO3) X mg/L of CO2 = X/22 meq of CO2 (22 is eq. weight of CO2)
• Ion balance is used as the basis for deciding the acceptability of results presented in the bar diagram
cations anions Ion balance cations anions 100
– Ion balance should be <5% for results acceptability
Hypothetical combination of positive and negative ions in the water sample
A water softening case
A water softening case (Chemical requirements calculation)
A water softening case (ion composition after addition of softening chemicals
A water softening case (ion composition after addition of softening chemicals
Excess lime treatment Treatment is done in 2 stages (each stage includes a recarbonation step), and the final effluent is filtered In the 1st stage excess lime (more than stoichiometrically required) upto 1.25 meq/L is added to raise the pH to 11.0 and removing the carbonate harness, and then recarbonation to reduce pH to 10.3 for converting the excess OH- into CO32Second stage involves dosing of soda ash for removing the noncarbonate hardness and recarbonation to convert OH- and CO32- into bicarbonate
Selective calcium hardness removal system Water having Mg2+ hardness <40 mg/L as CaCO3 is subjected to this scheme of water softening Mg2+ hardness is not removed here
Both carbonate and non-carbonate hardness of Ca2+ are removed through dosing both lime and soda ash Excess lime is not added The process is carried out in a single stage
Split treatment Part of the raw water is bypassed the first stage softening and the rest is softened by excess lime (for CH removal) Fraction bypassed is decided on the requirement of satisfying 40 mg Mg+2/L as CaCO3 in the finished water
Excess lime, added in the 1st, is neutralized by the bypass flow, and the 1st stage recarbonation is eliminated (lowers CO2 needs) Since all the raw water is not passing through the 1st stage of soften, lime requirements of the softening process are lower Sludge recycling is believed to further reduce the chemical requirements
