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WTP Operator Training: Water Chemistry

WTP Operator Training: Water Chemistry

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WTP Operator Training: Water Chemistry

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  1. WTP Operator Training:Water Chemistry Missouri Water and Wastewater Conference Dr. John T. O’Connor, PE Tom O’Connor, PE

  2. Elements of Water Chemistry • Molecular Properties of Water • Acids and Bases - pH, pOH, Alkalinity • Solubility Equilibria - Precipitation • Oxidation-Reduction Reactions

  3. The Atom

  4. Water Molecule - - + +

  5. Hydrogen Bonding - H18O9

  6. Hydrophobia

  7. Ionization of H2O

  8. What are Acids & Bases? An Acid is a Proton (H+) Donor A Base is a Proton (H+) Acceptor H2O + H2O = H3O+ +OH- [H+] = [OH-], moles/litre = 10-7 M pH + pOH = pKw constant 7 + 7 = 14 8 + 6 = 14

  9. Chemical Shorthand pH = - log [H+] pOH = - log [OH-] pK = - log [K] e.g., [10 -7 M] = 7

  10. HCl + H2O  H3O+ + Cl-

  11. Acids and Bases Acid pK Base HCl -3 strongCl- H2SO4 -3 SO42- HNO3 -1 NO3 - H2CO3 6.3 HCO3 - H2S 7.1 HS - NH4 + 9.3 NH3 HCO3- 10.3 weak CO32-

  12. Carbonate Equilibria H2CO3HCO3-  CO32-

  13. Alkalinity Acid Neutralizing Capacity of Water (Ability to accept protons) = HCO3- + 2CO3- + OH- - H+ Titration End-Point: H2CO3 (pH≈4.5)

  14. Hydrogen Sulfide H2S  HS- S2-

  15. Ammonium Ion Ammonia NH4+ NH3

  16. Precipitation & Solution Precipitation of Iron, Manganese Formation of Coagulant Floc Corrosion of Copper, Lead Calcium Carbonate Stability

  17. Coagulation withFerric Hydroxide

  18. Inorganic Coagulants • Aluminum Sulfate ( Filter Alum) Al2(SO4)3 · 14H2OAl(OH)3 • Ferric Sulfate Fe2(SO4)3 · 9H2O Fe(OH)3

  19. Metal Solubility • OH- : Zn(OH) 2Cu(OH) 2Cd(OH) 2 • CO32- :ZnCO3 CuCO3 CdCO3 • S2- : ZnS CuS CdS

  20. Metal Hydroxide Solubility vs pH

  21. Metals Removal in WWTP At.Wt. Influent Effluent g/mole µg/l µg/l Zinc 65.4 5730 690 Copper 63.5 2760 297 Cadmium 112.4 94 15 Lead 207.2 100 0 Nickel 58.7 37 13 Iron 55.8 1700 100

  22. Oxidation-Reduction Oxidation is the loss of an electron Fe2+(ferrous ion)- e- Fe3+ (ferric ion) Reduction is the gain of an electron Clo (chlorine) + e- Cl- (chloride ion) Rapid to glacially slow reaction rates

  23. Oxidation-Reduction

  24. Oxidizing Agents Oxygen O°2 - 4e- O2- (H2O) Chlorine Cl°2 - 2e- 2Cl- Potassium Permanganate KMnO4 - 3e-MnO2 Hydrogen Peroxide H2O2 - 2e- O2 + H2O

  25. Reducing Agents Hydrogen Sulfide H2S + 2e- S0 Nitrite Ion NO2- + 2e- NO3- Ferrous Ion Fe2+ + e- Fe3+ Carbon C° + 4e- CO2

  26. Chlorination Cl2 + H2S  Cl-+S0 Cl2 + NO2-Cl-+NO3- Cl2 + 2Fe2+ 2Cl-+2Fe3+ Rapid Reaction - Immediate Chlorine Demand

  27. Ozonation O3 + H2S O2+S° O3 + NO2-O2 +NO3- O3 + 2Fe2+ O2+2Fe3+ Rapid Reduction: O°  O2- - 2e-

  28. The Human Elements

  29. Groundwater Constituents All water is the same. It is only the stuff in it that is different. • INORGANIC • ORGANIC • GAS

  30. Inorganic Constituents Major Ions: Ca2+, Mg2+, Na+; HCO3-, SO42-, Cl- Nonionic: SiO2 Minor Ions: NH4+, K+; F-, PO4 3- Trace Constituents: Al, As, Ba, B, Br, Cd, Cs, Cr, Co, Cu, I, Fe, Pb, Mn, Hg, Mo, Ni, Ra, Ru, Se, Ag, Sr, Sn, Ti, U, Zn

  31. Electroneutrality Conditions Missouri River Dec. 2000 Kansas City Well Water

  32. Organic Constituents Total Organic Carbon (TOC) = Purgeable organic carbon (e.g., methane) + Non-Purgeable organic carbon (NPOC) Dissolved (natural color) (DOC) + Particulate (bacteria, algae)

  33. TOC in Missouri Waters • Source WaterTOC (avg) • Lakes 4.8 • Rivers 3.6 • Wells < 100 ft. 1.2 • Wells > 100 ft. 0.2

  34. Dissolved Gas Composition Abundance in Ground Waters Methane CH4 Nitrogen N2 Carbon dioxide CO2 Hydrogen sulfide H2S Radon Rn

  35. Methane • up to 77 mg CH4/l in Illinois Groundwaters • early methane wells used for home heating, cooking • wellhouse explosions • flaming the tap

  36. Groundwater Characteristics Midwestern United States Physical & Inorganic Water Qualities - Constant Cations: Hardness (calcium, magnesium), sodium, potassium, ammonium ions Anions: Alkalinity (bicarbonate), chloride, sulfate fluoride, arsenite ions; little or no nitrate Temperature: 12 ± 2 °Celsius; density, viscosity constant

  37. Microbially-Mediated Reducing Conditions in Midwestern U.S. Ground Waters GASES DO absent (converted to carbon dioxide) H2S: 0 to 1 mg/l (sulfate converted to sulfides) Methane: 0 to ~ 70 mg CH4/lMETALS Iron, manganese, lead, copper, cadmium, zinc concentrations limited by carbonate, sulfide Arsenic (AsO2-) - commonly 0 to 50 µg /l NITROGEN Ammonium Ion: 0 to > 5 mg N/l Nitrate Ion: low (converted to nitrogen gas, N2) Nitrite Ion: low

  38. Historical TOC DataMissouri Finished Drinking Water Supplies

  39. Missouri Finished Drinking Water Supplies

  40. Metals in Municipal WatersMissouri Finished Drinking Water Supplies

  41. Hardness + Sodium Missouri Finished Drinking Water Supplies

  42. Missouri Finished Drinking Water Supplies

  43. Hardness - MO Surface Waters