Cvg 2132 Assignment Satisfaction

Unformatted text preview: CE 3230 Chemical Equilibrium Homework 1. Can you oxidize sulfide with nitrate via the following reaction at pH 8 if the concentration of all the reactants and products (except H+) is 10-4 M? What other factor would need to be considered? 2 H + + NO -3 HS - H 2O SO4 NH 4 K 1085.3 Yes, the rate of reaction (i.e., the time required) would also need to be considered 2. If the initial concentration of Mg2+ is 30 mg/L, how much Mg2+ can theoretically be removed by the precipitation of Mg(OH)2(s) via the following reaction if the pH is raised to 11 during water softening (removal of hardness, Mg2+ and Ca2+)? Mg(OH)2(s) Mg2+ + 2 OH- Kso = 10-10.7 98% 3. What is the concentration of H 2 CO* in pure water in equilibrium with an atmosphere 3 containing PCO2 = 10-3.5 atm and PCO2 = 10-2.0 atm? Which solution would have the lower pH? 10-5 and 10-3.5 M 4. At a wastewater treatment plant, FeCl3(s) is added to remove excess phosphate from the effluent. Assume that the reactions that occur are FeCl3 (s) Fe3+ + 3ClFe3+ + PO3 FePO 4 (s) 4 The equilibrium constant for the second reaction is 1026.4. What would the concentration of Fe3+ need to be to maintain the phosphate concentration below the limit of 1 mg/L PO4-P? [Fe3+] ≥ 1.2 x 10-22 M 5. What is the alkalinity (mg/L as CaCO3) of water in equilibrium with CaCO3(s) at pH 8 and 400 ppm Ca2+? 5 mg/L as CaCO3 6. Acetic acid is a weak acid (Ka = 10-4.6) CH3COOH ↔ H CH3COO‐ What is the ratio of acetic acid (CH3COOH) to acetate (CH3COO-) at pH 5.2? ¼ 7. If log KSO, calcite (CaCO3(s)) = -8.61 at 50 ºC, would calcite be over- or undersaturated in a hot water heater at 50 ºC if the water was in equilibrium with calcite at 25 ºC (log KSO = -8.30) when it entered the house? Supersaturated by a factor of 2 8. Given the equilibrium constants for the following reactions: H 2CO* 3 H+ HCO3 HCO3 H+ 2 CO3 H 2O H+ OH K 106.3 K 10 10.3 K 1014 Calculate the concentrations of all species appearing in the alkalinity equation in a solution of pH 7.1 with 50 mg/L alkalinity as CaCO3 (Hint: assume initially that the alkalinity is due primarily to bicarbonate, HCO3 , solve for the concentration of the remaining species, and then check the assumption). [HCO -3 ] = 10-3 M, [CO 2- ] = 10-6.2 M, [OH - ] = 10-6.9 M 3 ...
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Unformatted text preview: WATER SOFTENING 1 Objec2ve of Water So9ening Remove POLYVALENT CATIONS from solu2on Equivalent Weight (EW) An equivalent weight is the molecular weight of a species divided by n, where n is: •  Number of replaceable hydrogen atoms needed to replace the ca2on – for compounds •  Valence of the elements in ques2on – for precipita2on reac2ons •  Change in valence or number of electrons transferred oxida2on/reduc2on (redox) rxns •  Number of electrons transferred – redox rxns •  Number of protons transferred in acid/base rxns The following equa2on can be used to determine the equivalent weight: –  EW = Equivalent Weight (in units of meq or eq) –  MW = molecular weight –  n = # equivalents of a species (charge, valence, number of H+ ion, or number of OH-­‐ ions) Note: 1 eq = 1000 meq 1 eq/m3 = 1 meq/L The first step to determining the equivalent weight is to determine the number of equivalents of a species. For CaCO3 CaCO3 Ca2+ CO32-­‐ For Fe2(SO4)3 Fe2(SO4)3 2Fe3+ 3SO42-­‐ For FeSO4: FeSO4 Fe2+ SO42-­‐ The calcium has a 2+ charge and the carbonate has a 2-­‐ charge. Therefore, the number of hydrogens that can be replaced are 2 so the equivalents is 2. The iron has a 3+ charge and the sulfate has a 2-­‐ charge. The charge on the iron ion is mul2plied by the number of irons needed to fill the valence of the sulfate, which is 2. Therefore, the number of hydrogens that can be subs2tuted are 2 x 3 = 6; therefore, so the equivalents equals 6. The iron has a 2+ charge and the sulfate has a 2-­‐ charge. The number of hydrogens that can be subs2tuted are 2 and the equivalents equals 2. Now that we can find the equivalents of a species, to determine the equivalent weight we determine the molecular weight of the species and divide it by its equivalents. For CaCO3: The molecular weight is Ca2+: 40.08 g/mol 100.08 g/mol CO32-­‐: 60.00 g/mol The equivalent weight is: For FeSO4: The molecular weight is Fe2+: 55.85 g/mol 151.92 g/mol SO42-­‐: 96.07 g/mol The equivalent weight is: For Fe2(SO4)3: The molecular weight is 2 Fe3+: 55.85 g/mol x 2 = 111.7 g/mol 399.91 g/mol 2-­‐ 3 SO4 : 96.07 g/mol x 3 = 288.21 g/mol The equivalent weight is: Normality (N) Equivalent weight of a solute per volume of solu2on: !"##!!"!!"#$%&!(!) ! =! = !"/!! ! (!", )(!"#$%&!!"!!"#$%&"', !) !" Related to molarity by: ! = !!! = !"/!! 6 Concentra2on in mg/L as CaCO3 Concentra2on of a species in terms of CaCO3 !" !" !"!!"!# ! !!!!"!#! = ! !"!!"#$%#&!×! ! !"!!"#$%#& ! 7 Example: Calculate the concentra2on of 15 mg/ L of H2SO4 as CaCO3 and in meq/L 8 Example: Calculate the concentra2on of 150 mg/L of Na2CO3 as CaCO3 and in meq/L 9 What is Hard Water? •  “Hard” water is caused by a large concentra2on of polyvalent metallic ca2ons, usually Fe2+ Mn2+ –  Ca2+ and Mg2+ are the most dominant mul2valent ca2ons in most natural waters –  Fe2+ and Mn2+ cause staining problems at low concentra2ons but typically contribute lihle to hardness effects •  Adverse effects of hard water –  Reduces the effec2veness of detergents and soaps –  Produces a scale on appliances and fixtures •  Efficiency and cost considera2ons: –  May produce scale in hot water heaters and in distribu2on piping –  Chemical precipita2on of CaCO3(s) 10 What Causes Hard Water? •  Contact with bedrock •  Carbonic acid dissolves Ca2+ and Mg2+ minerals CaCO3(s) + H2CO3 ↔ Ca2+ + 2HCO3-­‐ MgCO3(s) + H2CO3 ↔ Mg2+ + 2HCO3-­‐ dissolution of limestone •  Minerals containing other metals can also dissolve •  Water hardness is a func2on of the contact between source waters and geologic forma2ons •  Surface waters tend to be much so9er than groundwater, so9ening applica2on mostly to groundwater sources Figure 6-­‐10: Natural process by which water is made hard 11 Hardness •  Hardness is defined as the sum of all the soluble concentra2ons of polyvalent ca2ons (Total of all polyvalent ca2ons) •  Comes from the dissolu2on of minerals from geologic forma2ons that contain calcium and magnesium •  Indicator of poten2al precipita2on of mul2valent ca2ons – calcium and magnesium •  Expressed in equivalent concentra2on of calcium carbonate Classifica;on Range as CaCO3 So9 0 -­‐ <50 mg/L Moderately Hard 50 to <100 mg/L Hard 100 to <150 mg/L Very Hard >150 mg/L 12 Types of Hardness •  Two general types of hardness of interest: –  Carbonate – HCO3-­‐ and CO32-­‐ –  Non carbonate – associated with other ions such as Cl-­‐ and SO42-­‐ •  Balance between the two is important in water so9ening and scale forma2on –  HCO3-­‐ dissociates at higher temperatures due to CaCO3 precipita2on –  Plugs pipes, decreases the heat transfer coefficients and changes the fric2onal resistance to flow in pipes •  HOW DO WE DETERMINE THE WATER HARDNESS? 13 Ionic Composi2on Diagram Cons;tuent Conc. Conc. Conc. (mg/L) (meq/L) (mg/L CaCO3) 75 3.75 188 Ca2+ 2+ 40 3.28 164 Mg 2+ Mn 3.0 0.109 5.45 Na+ 10 0.435 21.7 7.57 379 Total MUST HAVE A CHARGE BALANCE Ca2ons 3.75 meq/L 7.03 188 mg/L as CaCO3 352 Ca2+ Mg2+ Cons;tuent Conc. Conc. Conc. (mg/L) (meq/L) (mg/L CaCO3) HCO3-­‐ SO42-­‐ F-­‐ Cl-­‐ Total 300 95 19 35.5 4.92 1.98 0.258 0.423 7.58 7.14 357 246 99 12.9 21.1 379 7.57 379 Na+ Mn2+ Anions HCO3-­‐ SO42-­‐ 246 mg/L 4.92 meq/L Cl-­‐ F-­‐ 345 6.90 7.16 358 7.58 379 14 Hardness Diagram MUST HAVE A CHARGE BALANCE 2+ 3.75 meq/L 188 mg/L Ca 7.03 352 Mg2+ HCO3-­‐ 246 mg/L 4.92 meq/L Carbonate Hardness (CH) Noncarbonate Hardness (NCH) Mg-­‐CH Ca-­‐CH Mg-­‐NCH Total Hardness (TH) 15 Example: Total Hardness Determine the total hardness (as mg/L as CaCO3) for a raw water source containing 36 mg/L of Ca2+ and 14 mg/L of Mg2+. 16 Example: Carbonate and Non-­‐Carbonate Hardness Determine the TH, CH, and NCH (as mg/L as CaCO3) for a raw water source containing 36 mg/L of Ca2+, 14 mg/L of Mg2+ and 30 mg/L of HCO3-­‐. 17 So9ening •  Relies on the rela2ve insolubili2es of calcium carbonate and magnesium hydroxide •  Choice of process and chemicals depends on raw water quality and cost •  FUNCTION OF pH •  Most commonly used chemicals –  Lime – Calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) –  Caus2c soda – NaOH or sodium hydroxide •  Chemical reac2ons depend on the type of hardness •  The minimum hardness that can be achieved depends on the solubili2es of the calcium carbonate and magnesium hydroxide 18 Chemical So9ening •  Most commonly used chemicals –  Lime – Calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) •  Quick Lime –  Granular –  Greater than 90% CaO with magnesium oxide as the primary impurity –  Must be hydrated or slaked to Ca(OH)2 and used in slurry feed of 5% Ca(OH)2 •  Hydrated Lime –  Powder –  Contains about 70% Ca(OH)2 –  Fluidized in a tank containing a turbine mixer •  •  –  Soda Ash •  Grayish-­‐white powder •  Approximately 98% Na2CO3 •  Can be added simultaneously with the lime or sequen2ally following the lime addi2on –  Caus2c soda – NaOH or sodium hydroxide Chemical reac2ons depend on the type of hardness Tradi2onal method 19 Lime So9ening Carbonate Hardness •  The chemical reac2ons for lime So9ening are •  Range of lime dosage needed for removal of MgCO3 as Mg(OH)2 is 30 to 70 mg/L as CaCO3 •  This is in excess of the stoichiometric dosage •  Excess calcium will precipitate out with carbonate 20 Lime-­‐Soda So9ening Carbonate and Non-­‐carbonate Hardness •  If there is a lack of carbonate alkalinity to react with lime have to add an external source •  Soda Ash – Na2CO3 •  The amount of soda ash required depends on the amount of non-­‐ carbonate hardness to be removed 21 Caus2c Soda So9ening: Carbonate and Non-­‐ Carbonate Hardness •  Caus2c Soda – NaOH may be used when there is an insufficient carbonate hardness present in the raw water to react with lime •  Purchased as a 50% solu2on •  Use will depend on economics, ease of handling and magnesium content •  Results in the replacement of divalent hardness ions with sodium 22 Recarbona2on – pH Adjustment •  Used to lower the pH •  Addi2on of CO2 is the most common and economical method to precipitate out excess calcium and reduce the pH •  When excess noncarbonate alkalinity is present: 2!"! + !"! ⇌ !"!!! + !! !! ! !"!!! + !"! + !! ! ⇌ 2!"#!! ! When CO32-­‐ is formed – reacts to precipitate calcium present above the satura2on of Calcium carbonate Addi2onal CO2 will lower the pH •  Used for water that requires excess calcium and magnesium removal – addi2on of excess lime (final pH above 11.0 requires pH adjustment) 23 Treatment Equipment and Parameters Lime So9ening •  Equipment and contact 2mes: –  Reactors/clarifiers – 20 minutes –  Sludge blanket clarifiers – 1 to 2 hours •  20 to 30 minutes of contact 2me is required for Ca2+ precipita2on •  pH range 9.0 to 12.0 •  Recycling the sludge will increase the kine2cs of CaCO3 forma2on Recarbona2on •  Equipment and contact 2mes: –  CO2 diffusion – 3 minutes –  Recarbona2on tanks – 20 minutes –  Basin with baffled channels and walls – 3.7 m or 12 9 deep 24 Types of So9ening Process Configura2ons Several process trains used in prac2ce Selec2on depends on raw-­‐water quality and treated-­‐water quality objec2ves Single-­‐Stage Lime: •  Used for waters with high Ca, low Mg, low carbonate hardness •  Carbonic acid concentra2on + calcium carbonate hardness •  Lime addi2on for so9ening and carbon dioxide for pH adjustment a9er so9ening Excess Lime: •  Used for water with high Ca, high Mg, may be one or two stages •  Lime addi2on for so9ening •  Carbon dioxide for pH adjustment a9er so9ening Single-­‐Stage Lime-­‐Soda Ash: •  Used for water with high Ca, low Mg, low carbonate and noncarbonate hardness •  Lime and Soda Ash addi2on for so9ening •  Carbon dioxide for pH adjustment a9er so9ening Excess Lime-­‐Soda Ash (may be one or two stage) •  Used for high Ca, high Mg, carbonate and noncarbonate hardness •  Lime and Soda Ash addi2on for so9ening •  Carbon dioxide for pH adjustment a9er so9ening 25 Single-­‐Stage So9ening •  •  •  •  •  Some2mes called underso9ening Used for water that does not require removal of magnesium Uses flash mixers to prevent precipita2on from forming on the blades Recarbona2on is required to lower pH Final hardness is between 70 to 100 mg/L as CaCO3 hhp://www.engineeringexcelspreadsheets.com/ 2012/05/lime-­‐soda-­‐water-­‐so9ening-­‐spreadsheets/ 26 Two-­‐Stage So9ening •  Excess lime is added with a flash mixer •  CO2 recarbona2on is used to reduce the pH •  Soda ash is added to precipitate the excess lime added for magnesium removal •  Addi2on of a second stage precipita2on step is followed by sedimenta2on and then recarbona2on is used to lower pH hhp://www.engineeringexcelspreadsheets.com/2012/05/lime-­‐soda-­‐water-­‐ so9ening-­‐spreadsheets/ 27 Split-­‐Treatment So9ening •  Process water split to different streams •  Water treated using different methods to varying degrees of so9ness then blended to obtain effluent water quality objec2ves •  Include –  Parallel so9ening and coagula2on –  Parallel lime so9ening and ion exchange or reverse osmosis –  Split treatment with excess lime hhp://www.engineeringexcelspreadsheets.com/2012/05/lime-­‐soda-­‐water-­‐ so9ening-­‐spreadsheets/ 28 Split Treatment With Excess Lime •  Used when Magnesium must be reduced and raw water contains very lihle noncarbonate hardness •  Streams –  Excess lime for both calcium and magnesium hardness removal –  Other part is blended with so9ened water prior to sedimenta2on •  Alkalinity of the bypassed water is used to neutralize the excess caus2c alkalinity required to reduce the magnesium in the treated water •  Recarbona2on is usually not needed •  Best for groundwaters 29 Parallel So9ening and Coagula2on •  Part of the water is treated with excess lime – remove calcium and magnesium •  Remaining coagulated to remove turbidity and color •  Water blended to achieve water quality goal •  Some2mes a third stream is added to neutralize the excess caus2c alkalinity in the so9ened water stream 30 Parallel So9ening and Ion Exchange or Reverse Osmosis •  Used to so9en water with high noncarbonate and dissolved solids concentra2ons •  Lime so9ening stream •  Ion exchange or reverse osmosis stream –  Ion exchange is useful for noncarbonate hardness if soda ash is not economical –  Reverse osmosis is good for demineraliza2on if the raw water requires addi2ons reduc2on in dissolved solids 31 Reac2ons in Lime-­‐Soda So9ening 32 Process Limita2ons •  The reac2ons are considered ideal but are really equilibrium reac2ons •  Must take into account that at minimum the water hardness will be –  40 mg/L CaCO3 (0.8 meq/L) –  30 mg/L CaCO3 of Ca2+ and 10 mg/L CaCO3 of Mg2+ •  Maximum allowable Mg2+: Less than 40 mg/L as CaCO3 (Mg scales on components, very temperature sensi4ve) •  Typical desired hardness: 75 – 120 mg/L as CaCO3 (1.5 – 2.4 meq/L) •  Economics: 140 – 150 mg/L of total hardness 33 Water So9ening Slow Mix G values for slow mix So9ening floc is heavier than coagula2on floc, requiring a greater G value to stay in suspension 34 35 ***Algorithm for Solving Problems*** Figure 6-­‐14: Flow Diagram for solving so9ening problems 1.  Calculate H2CO3* demand in terms of lime requirement (use alkalinity, pH and Ka values) 2.  Draw bar diagram 3.  Determine excess lime required (look at Mg2+ concentra2on) 4.  Set up a table of concentra2ons 5.  Complete table 36 ***Hardness Diagram*** MUST HAVE A CHARGE BALANCE 2+ 3.75 meq/L 188 mg/L Ca 7.03 352 Mg2+ HCO3-­‐ 246 mg/L 4.92 meq/L Carbonate Hardness (CH) Noncarbonate Hardness (NCH) Mg-­‐CH Ca-­‐CH Mg-­‐NCH Total Hardness (TH) 37 *** TABLE FOR CHEMICAL ADDITION *** Determine the concentra2ons of each of the components using the bar diagram. Using stoichiometry from the reac2ons determine the amount of lime or soda ash that must be added and the amount of sludge that has to be removed. Component Conc.** (meq/L) Lime** (meq/L) Soda Ash** CaCO3(s)** Mg(OH)2(s)** (meq/L) (meq/L) (meq/L) H2CO3* And/or CO2 Ca Mg CH NCH EXCESS TOTALS **Concentra2on can be in terms of meq/L or mg/L as CaCO3 38 ***So\ening Reac;ons*** 1. Carbonic Acid Neutraliza2on (base buffering) !! !"! + !"(!")! ⇌ !"!#! (!) + 2!! !! This is not a so9ening reac2on – All it does is raise the pH 2. Ca-­‐CH removal !"!! + 2!"#!! + !"(!")! ⇌ 2!"!#! (!) + !! !! 3. Mg-­‐CH removal Lime !"!! + 2!"#!! + 2!"(!")! → !"(!")! ! + 2!"!#! (!) + 2!! !! Lime 4. Ca-­‐NCH removal !"!! + !"! !"! → !"!#! ! + 2!"! ! So9ening RXN – Take out 2 Ca and 1 Mg Add Soda Ash Soda Ash 5. Mg-­‐NCH removal !"!! + !"! !"! + !"(!")! → !"(!")! Carbonate is used to remove the lime Soda Ash ! + !"!#! (!) + 2!"! ! Lime 6. Recarbona2on 2!"! + !"! ⇌ !"#!! ! 7. Addi2on of calcium carbonate – See RULE OF THUMB Must add lime and soda ash 39 ***Rule of Thumb – Excess Lime Addi;on*** Process Limita2ons •  At minimum the water hardness will be –  40 mg/L CaCO3 (0.8 meq/L) –  30 mg/L CaCO3 of Ca2+ and 10 mg/L CaCO3 of Mg2+ –  Typical desired hardness: 75 – 120 mg/L as CaCO3 (1.5 – 2.4 meq/L) •  RULE OF THUMB FOR EXCESS LIME ADDITION –  If Mg2+ to be removed is ≤ 20 mg/L CaCO3 (0.4 meq/L), then add 20 mg/L (0.4 meq/L) CaCO3 of lime –  If Mg2+ to be removed is between 20 to 40 mg/L CaCO3, then add lime equal to Mg2+ to be removed (in mg/L CaCO3) –  If Mg2+ to be removed is ≥ 40 mg/L CaCO3 (0.8 meq/L), then add 40 mg/L CaCO3(0.8 meq/L) of lime equal Want a total of 10 mg/L CaCO3 of Mg2+ to be removed. Any addi2onal leave in. 40 So9ening Example 1 From the water analysis presented below, determine the amount of lime and soda (in mg/L as CaCO3) necessary to soften the water to 80.00 mg/L hardness as CaCO3. Species Concentra;on (mg/L) Ca2+ Mg2+ Na+ CO2 HCO3-­‐ SO42-­‐ Cl-­‐ 95.20 13.44 25.76 19.36 241.46 53.77 67.81 41 So9ening Example 2 From the water analysis presented below, determine the amount of lime and soda (in mg/L as CaCO3) necessary to soften the water to 90.00 mg/L hardness as CaCO3. Species Concentra;on (mg/L) Ca2+ Mg2+ Na+ CO2 HCO3-­‐ SO42-­‐ Cl-­‐ 149.2 65.8 17.4 29.3 185.0 29.8 17.6 42 So9ening Example 3 From the water analysis presented below, determine the amount of lime and soda (in mg/L as CaCO3) necessary to soften the water to 120.00 mg/L hardness as CaCO3. Species Concentra;on (mg/L) Ca2+ Mg2+ Na+ CO2 HCO3-­‐ SO42-­‐ Cl-­‐ 179.2 75.8 17.4 29.3 195.0 49.8 17.6 43 Manual Control Panel in Water Treatment Plant 44 Lime slaker and feed system. 45 Upflow solids contact tank. Effluent weir on right. 46 Sehling tank with radial weirs 47 Rapid sand filter with exposed under drain block. Wash water troughs run from le9 into gullet on right. 48 Rapid sand filter with water level just below backwash troughs. Note iron stain at high water line. 49 Rapid sand filter during filtra2on. Reflec2on shows water level above gullet and backwash troughs. 50 Rotameter used to measure flowrate of gaseous chlorine into water. 51 Hydrofluorosilicic acid drum and pump metering system for feeding fluoride into water. 52 So9ening sludge lagoon. Winter view shows ice (light blue) where supernatant water is standing. 53 Overview of water treatment plant showing sludge lagoons and building housing treatment facili2es. 54 ***Algorithm for Solving Problems*** Figure 6-­‐14: Flow Diagram for solving so9ening problems 1.  Calculate H2CO3* demand in terms of lime requirement (use alkalinity, pH and Ka values) 2.  Draw bar diagram 3.  Determine excess lime required (look at Mg2+ concentra2on) 4.  Set up a table of concentra2ons 5.  Complete table 55 ***Hardness Diagram*** MUST HAVE A CHARGE BALANCE 2+ 3.75 meq/L 188 mg/L Ca 7.03 352 Mg2+ HCO3-­‐ 246 mg/L 4.92 meq/L Carbonate Hardness (CH) Noncarbonate Hardness (NCH) Mg-­‐CH Ca-­‐CH Mg-­‐NCH Total Hardness (TH) 56 *** TABLE FOR CHEMICAL ADDITION *** Determine the concentra2ons of each of the components using the bar diagram. Using stoichiometry from the reac2ons determine the amount of lime or soda ash that must be added and the amount of sludge that has to be removed. Component Conc.** (meq/L) Lime** (meq/L) Soda Ash** CaCO3(s)** Mg(OH)2(s)** (meq/L) (meq/L) (meq/L) H2CO3* And/or CO2 Ca Mg CH NCH EXCESS TOTALS **Concentra2on can be in terms of meq/L or mg/L as CaCO3 57 ***So\ening Reac;ons*** 1. Carbonic Acid Neutraliza2on (base buffering) !! !"! + !"(!")! ⇌ !"!#! (!) + 2!! !! This is not a so9ening reac2on – All it does is raise the pH 2. Ca-­‐CH removal !"!! + 2!"#!! + !"(!")! ⇌ 2!"!#! (!) + !! !! 3. Mg-­‐CH removal Lime !"!! + 2!"#!! + 2!"(!")! → !"(!")! ! + 2!"!#! (!) + 2!! !! Lime 4. Ca-­‐NCH removal !"!! + !"! !"! → !"!#! ! + 2!"! ! So9ening RXN – Take out 2 Ca and 1 Mg Add Soda Ash Soda Ash 5. Mg-­‐NCH removal !"!! + !"! !"! + !"(!")! → !"(!")! Carbonate is used to remove the lime Soda Ash ! + !"!#! (!) + 2!"! ! Lime 6. Recarbona2on 2!"! + !"! ⇌ !"#!! ! 7. Addi2on of calcium carbonate – See RULE OF THUMB Must add lime and soda ash 58 ***Rule of Thumb – Excess Lime Addi;on*** Process Limita2ons •  At minimum the water hardness will be –  40 mg/L CaCO3 (0.8 meq/L) –  30 mg/L CaCO3 of Ca2+ and 10 mg/L CaCO3 of Mg2+ –  Typical desired hardness: 75 – 120 mg/L as CaCO3 (1.5 – 2.4 meq/L) •  RULE OF THUMB FOR EXCESS LIME ADDITION –  If Mg2+ to be removed is ≤ 20 mg/L CaCO3 (0.4 meq/L), then add 20 mg/L (0.4 meq/L) CaCO3 of lime –  If Mg2+ to be removed is between 20 to 40 mg/L CaCO3, then add lime equal to Mg2+ to be removed (in mg/L CaCO3) –  If Mg2+ to be removed is ≥ 40 mg/L CaCO3 (0.8 meq/L), then add 40 mg/L CaCO3(0.8 meq/L) of lime equal Want a total of 10 mg/L CaCO3 of Mg2+ to be removed. Any addi2onal leave in. 59 ...
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