Water Treatment Part 3 Groundwater Treatment - PowerPoint PPT Presentation

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Water Treatment Part 3 Groundwater Treatment
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Water Treatment Part 3 Groundwater Treatment

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  1. Water TreatmentPart 3Groundwater Treatment Dr. Abdel Fattah Hasan

  2. Groundwater (GW) are usually: • Cool and uncontaminated • Has uniform quality • Usually used directly for municipal use (just chlorine is added to avoid post contamination) Sometimes GW is polluted or contaminated with: • Hardness • Fertilizers • WW • Pesticides • Radionuclides • Toxic metals, such as Arsenic

  3. GW Treatment Options

  4. Hardness Removal - Precipitation softening • Hardness of water is caused by divalent cations, such as Ca & Mg ions • Max. hardness for public supply: 300 -500 mg/l as CaCO3 • Moderate hardness for public supply: 60 -120 mg/l as CaCO3 • Precipitation softening uses lime CaO and soda ash Na2CO3 to remove Ca and Mg • Lime slurries are usually has the form Ca(OH)2 • Lime treatment has the incidental benefits of bacterial actions, removal of iron and aid in clarification of turbid surface water • Carbon dioxide can be applied after lime treatment to lower pH by converting the excess hydroxide ion and carbonate ion to bicarbonate ion

  5. Converting Ca and Mg into mg/l CaCO3 • Ca Hardness as mg/l CaCO3 = Ca (meq/l) X 50 • Mg Hardness as mg/l CaCO3 = Mg (meq/l) X 50

  6. Chemical reactions in precipitation softening 1- Lime added to water reacts first with any available CO2 • CO2 + Ca(OH)2 = CaCO3 + H2O • Ca(HCO3)2 + Ca(OH)2 = 2 CaCO3 + 2H2O Mg(HCO3)2 + Ca(OH)2 = CaCO3 + MgCO3+ 2H2O MgCO3 + Ca(OH)2 = CaCO3 + Mg(OH)2 • Mg(HCO3)2 + 2Ca(OH)2 = 2 CaCO3 + Mg(OH)2 + 2H2O • MgSO4 + Ca(OH)2 = CaSO4 + Mg(OH)2 • CaSO4 + Na2CO3 = CaCO3+Na2SO4 1 eq to one eq 2- Then Lime reacts with any calcium bicarbonate present in water 3- Then Lime reacts with magnesium bicarbonate 2eq of lime to one eq Mg(HCO3)2 4- Non-carbonate Ca (sulfate or chloride) require addition of soda ash and non-carbonate Mg (sulfate or chloride) require both lime and soda ash 1 eq to one eq 1 eq to one eq Note: Ca ion can be effectively removed by lime addition (pH = 10.3), but Mg ion demand higher pH, so lime should be added in excess of about (35 mg/l; 1.25 meq/l)

  7. Re-carbonation • Used to stabilize excess lime of treated water by adding CO2: • Ca(OH)2 +CO2 = CaCO3 + H2O • CaCO3 +CO2 +H2O = Ca(HCO3)2

  8. Excess lime softening • Calcium can effectively reduced by lime addition, but magnesium removal need excess lime • Lime and Soda ash dosages to be estimated by chemical equations PLUS excess lime for Mg removal • Practical limits (remains after estimation of theoretical dosages from chemical equations) for hardness removal are: - CaCO3: 30 mg/l as CaCO3 (= 0.6 meq/l Ca++) • Mg(OH)2 :10 mg/l as CaCO3 (= 0.2 meq/l Mg++) Note: Sodium (Na) concentration is usually increased by the amount added in the Soda ash

  9. Excess Lime Softening

  10. Selective Calcium Carbonate Removal • Used to soften water with low Mg hardness (less than 40 mg/l as CaCO3) • Enough lime is added to remove Ca without Excess. • Soda ash mayor may not be required, depending on the contents of non-carbonate hardness. • Recarbonation is usually performed to reduce scale formation.

  11. Selective Calcium Carbonate Process

  12. Split-Treatment Softening By dividing the raw water into two portions for softening in a two stage system QP MgR QR MgR QE MgE • Split treatment can result in chemical savings • Recarbonation my not be required • Split around 1st stage is determined by the level of Mg desired in treated water • Mg in treated water = (QP X MgR + QE X 10)/QR

  13. Example: Water defined by the following analysis is to be softened by excess lime treatment. Assume that the practical limit of hardness removal for CaCO3 is 30 mg/l and of Mg(OH)2 is 10 mg/l as CaCO3 CO2= 8.8 mg/l Ca++ = 40mg/l Mg++ =14.7mg/l Na+ = 13.7mg/l Alk (HCO3-) =135 mg/l as CaCO3 SO4= 29mg/l Cl- = 17.8mg/l (a) Sketch a meg/l bar graph and list the hypothetical combination of chemical compounds in solution (b) Calculate the softening chemicals required, expressing lime dosage as CaO and soda ash as Na2CO (d)Draw a bar graph for softened water before and after recarbonation. Assume that half the alkalinity in the softened water is in the bicarbonate form.

  14. Calcium hardness = 2X 50 = 100 mg/l as CaCO3 Magnesium hardness = 1.2X 50= 60.5 mg/l as CaCO3 Required lime dosage = 4.31 X28 +35 = 156 Dosage of Soda ash = 0.51* 53= 27 mg/l Na2CO3

  15. 3.21 0.0 2.00 3.81 (A) 0.0 2.70 3.30 3.81 0.0 0.6 0.8 1.91 Ca++ OH- (B) 1.25 of excess lime 0.0 0.2 0.8 1.40 1.91 0.0 0.6 0.8 1.91 (C) 0.0 0.4 0.8 1.40 1.91

  16. Iron and manganese removal • Fe++ and Mn++ soluble in groundwater exposed to air these reduced to insoluble Fe+++ and Mn++++ • Rate of oxidation depend on pH, alkalinity, organic content and present of oxidizing agents • Filtration – sedimentation and filtration • Fe++ ( ferrous) + oxygen Fe Ox ( ferric oxidizes) • Manganese can not oxidized as easily as iron need to increase pH • ِaeration –chemical oxidation – sedimentation- filtration • Fe++ + Mn++ + oxygen FeOx + MnO2 ( ferric oxidizes) Free chlorine residual

  17. Iron and manganese removal • Fe (HCO3)2 + KMn O4 Fe (OH) 3 + Mn O2 • Mn(HCO3)2 + KMnO4 MnO2 Potassium permanganate Potassium permanganate

  18. Iron and manganese removal • Manganese zeolite process • Figure 7-20

  19. Figure 7-20 Aeration optional Well water Anthracite medium Dry KMnO4 ................................................................................................................................................ Manganese treated greensand Dissolving tank and solution feeder Under drain Pressure filter Finished water

  20. Water Stabilization • Ferrous metal when placed in contact with water results in an electric current caused by the reaction between the metal surfaces and existing chemicals in water • Fe Fe++ + 2electron • 2 elec + H2O + ½ O2 OH- • 2Fe++ + 5H2O + ½ O2 Fe(OH)3 + 4 H+ • To Protect ductile iron pipe against internal corrosion is by lining with thin layer of cement mortar placed during manufacturing

  21. Ion- exchange softening and nitrate removal • Ions of a particular species in solution are replaced by ions of a different species attached to an insoluble resin

  22. Ion Exchanger

  23. Cation exchange softening Ca ++ Mg ++ CaR MgR + Na+ In Process of Removal +Na2R CaR MgR Ca ++ Mg ++ excess Regeneration + NaCl Na2R + NaCl

  24. Anion exchange for Nitrate Removal Nitrate removal So =4 NO-3 RSO4 RNO3 RCl + + Cl - Regeneration with NaCl Disadvantages : high operating costs and problem of brine disposal

  25. Removal of dissolved salts • Distillation : (desalination of sea water) • heating sea water (35000 mg/l mostly NaCl) to boiling point and converting it into steam to form water vapor that is condensed yielding salt free water

  26. Removal of dissolved salts • Reserves osmosis • Forced passage of the natural osmotic pressure to accomplish separation of water and ions

  27. Semi permeable Membrane P> P0 Po .......................................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................ ...................................................................................................................................................................................... Saline water Fresh water Osmosis Reverse osmosis Osmosis equilibrium (b) (c) (a)

  28. Reverse osmosis system • Pretreatment unit • Pump to provide high pressure • Post-treatment • Brine disposal

  29. Reverse osmosis models Alkali Saline water Permeate (product water) Scale inhibitor Waste brine Pump Granular-media filter Acid 10-30% of saline feed Cartridge filter

  30. Source of wastes in water treatment • Residue from chemical coagulation • Precipitation from softening • Filter back wash • Settled solid from pre-sedimentation Total Sludge Solids produced in WT (lb/mil gal) = 8.34 x [0.44 x alum dosage (mg/l)+ 0.74 x Turbidity (NTU)]

  31. Example 7-16 • A surface water treatment plant coagulant a raw water having a turbidity of 9 units by applying an alum dosage of 30 mg/l. • Estimate the total sludge solids production in pounds per million gallons of water processed. • Compute the volume of sludge from the settling basin and filter backwash water using 1% solid concentration in the sludge and 500 mg/l of solids in the waste water. Assume 30% of total solids are removed in the filter.

  32. Applying Eq. 7-39 Total sludge solids = 8.34 (0.44 X 30 + 0.74 X 9)= 166 lb/ mil gal Solids in sludge = 0.70 X 166 = 116 lb/ mil gal Solids in backwash water = 0.30 X 166 = 50 lb/ mil gal Volume = Sludge solids (lb) Solids fraction X 8.34 (lb/ gal) 116 Sludge volume = = 1390 gal/mil gal 1.0 100 X 8.34 Wash- water volume = 50 500 1,000,000 X 8.34 = 12,000 gal/mil gal

  33. PRECIPITATE PRODUCED COMPONENT IN WATER

  34. Dewatering and waste disposal of wastes from water treatment plants • Lagoons • Drying beds • Gravity thickening • Centrifugation • Pressure filter

  35. Gravity thickening Sludge in flow Inlet baffle Supernatant overflow Weir Pickets Scraper blades Sludge discharge

  36. Filtration pressure

  37. Centrifugation

  38. Example • Case 1: Groundwater source with small infrequent possible contamination used for domestic use • Solution : chlorination or ozonation or filtration

  39. Example 2 • Surface water: floating matter, high suspended matter, high turbidity, considerable, biological contamination, clay • Solution: screening, sedimentation, coagulation, flocculation, filtration, chlorination