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Environmental Engineering 441

Philadelphia University Faculty of Engineering Department of Civil Engineering First Semester, 2013/2014. Environmental Engineering 441. Lecture 5: Water Treatment (2) Coagulation and flocculation. Objectives of Water Treatment.

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Environmental Engineering 441

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  1. Philadelphia University Faculty of Engineering Department of Civil Engineering First Semester, 2013/2014 Environmental Engineering 441 Lecture 5: Water Treatment (2) Coagulation and flocculation

  2. Objectives of Water Treatment “ To remove harmful microorganisms or chemicals, thereby preventing the spread of disease in order to supply clean and safe water for public demand”

  3. Water Source Potable water/Palatable water Water source could be • Ground water (well) most likely better quality than surface water) • Deep • Shallow • Surface Water • Lakes • River • Reservoir • Raw water quality varies with the source, and if the source is surface water, the quality vary seasonally, particularly with flooding, upland and lowland, etc

  4. Water Treatment System Treatment system can be classified as • Simple disinfection ; i.e ground water (simply chlorination if only has bacteria) • Filter plant • Rapid mixing, flocculation, sedimentation, filtration and disinfestation to remove color, turbidity, taste & odor, organic matter and bacteria) • Softening plant; use the same unit operation as filter system but use different chemical. Softening plant in primarily used to remove hardness

  5. Selection of Treatment Processes Selection of the set of Treatment Processes will be based on : • Sources of water intake • Characteristic of water Detailed raw water quality analysis for a minimum of one year, or longer, at periods of high, low and medium flows. The parameters to be looked at all those listed in the legislative standards • Cost • Suitability of the processes for removing particulate impurities. Example: • Turbidity; suitable process might be coagulation , sedimentation or filtration • For pathogen, pre or post chlorination, UV, ozone, or chloramines

  6. Coagulation • Colloids are suspended particle very small to settle in a reasonable time (sedimentation) and too small to trap to be trapped in the pores of a filter (Filtration); • They possess a negative charges that repels other colloidal particles before they colloid with one another . • In order to destabilize the colloids, we must neutralize the charge by addition of an ions of the opposite charge for the colloids

  7. Coagulation • Positive ions is added to water to reduce the surface charge to the point where the colloids are not repelled from each other. • Coagulants tend to be positively charged.  Due to their positive charge, they are attracted to the negative particles in the water • The combination of positive and negative charge results in a neutral , or lack, of charge

  8. Water treatment Coagulants • Particles in water are negative; coagulants usually positively charged. The two most common coagulants are: • Alum- aluminum sulfate 2. Ferric chloride or ferrous sulfate Two important factors in coagulation addition are pH and dose; which must be determined from lab test

  9. Coagulant Alum Alum- (aluminum sulfate): Alum is attracted to the positive charges created by aluminum hydroxides. 1. Trivalent Al+3 charge attracts neg – particles 2. Forms flocs of aluminum hydroxide (AlOH3). 3. Impacted by mixing, alkalinity, turbidity and temp. 4. Ideal pH range 5-8

  10. Alum- (aluminum sulfate)- Al2(SO4)3·14H2O + 6 HCO3- 2(Al(OH3).3H2O(S) + 6CO2+8H2O + 3SO4-2 2Al+3+ negatively charged colloids makes neutral surface charge WHY IS ALKALINITY SO IMPORTANT?? No bicarbonate (low alkalinity, low pH sulfuric acid!): Al2(SO4)3·14H2O 2Al(OH3).3H2O(S) +3H2SO4+2H2O • Optimum pH: 5.5 to 6.5 • Operating pH: 5 to 8

  11. Jar Test • One of the most common method to evaluate coagulation efficiency • To determine the optimal pH and the coagulant dose • Measure : • pH • Turbidity- SS removal • Sludge volume

  12. Water Treatment -Iron Iron salt- (Iron chloride or sulfate)- FeCl3+ 3 HCO3- +3H2O Fe(OH3). 3H2O(S) + 3CO2 +3Cl- No bicarbonate (low alkalinity, low pH hydrochloric acid!): FeCl3+ 6 H2O Fe(OH3). 3H2O(S) + 3HCl 2. Work over a larger pH range (4-9) than alum

  13. Water Treatment- Coagulant aid • Activated silica (sodium silicate)- helps improve coagulation, decreases volume of coagulant necessary. Typically is sodium silicate. • Produce stable solution, that has negative surface charges • Resulting larger, denser flocs and settle faster • For treating highly colored, low turbidity water • Require proper equipment and close operational control

  14. Water Treatment- Coagulant aid Bentonite (clay)- helps improve coagulation, decreases volume of coagulant necessary. 1. high in color, low turbidity, low mineral content water 2. 10-50 mg/L dosage 3. Heavier denser floc that settles faster

  15. Water Treatment- Coagulant aid Polymers: Water-soluble organic polymers Long chained carbon, high MWt, many active sites used as both primary coagulants and coagulant aids. Act as "bridges" between the already formed particles : • Anionic—ionize in solution to form negative sites along the polymer molecule. • Cationic—ionize to form positive sites. • Positive and negative charges- polyamorphotype • Non-ionic—very slight ionization. effectiveness: particles type, turbidity present, and the turbulence (mixing) available during coagulation.

  16. Water Treatment- Coagulant aid • pH adjuster • Sulfuric Acid for lowering the pH • Lime Ca(OH)2 or Soda Ash Na2CO3 for raising the pH

  17. Softening Objective: to remove hardness Hardness: Sum of All polyvalent cations express as CaCO3 • Carbonate hardness (soft) (CH) • Non-Carbonate hardness (hard) If pH< 8.3; that means all HCO3 is the dominant form of alkalinity NCH= Total Hardness (TH) – Alkalinity (HCO3-) Softening can be accomplished by either lime soda or Ion exchange

  18. Softening- lime Soda • The objective is to precipitate the Ca as CaCO3 at pH 10.3 and Mg as Mg(OH)2 at pH 11 Ca2+ + CO32- CaCO3(s) Mg2+ + 2OH- Mg(OH)2 (s) • Mg is more expensive to remove than Ca • More expensive to remove NCH than CH, because we need to add another chemical to provide CO32- • Can not produce a water completely free of hardness because of the solubility of CaCO3 and Ma(OH)2 in water; physical limitation in mixing and contact, and the lack of sufficient time for the reaction

  19. Ion exchange Softening Reversible interchange of ion between a soild and liquid phase in which no permanent change in the structure of the solids The water containing the hardness (such as Ca or Mg) is passed through the ion exchange material. The hardness in the water exchange with an ion (Na) from the ion- exchange materials. Ca(HCO3)2 + 2NaR CaR2 + 2NaHCO3 • Alkalinity does not changing • Removal of 100% hardness from water until the exchange capacity is reached “breakthrough” • Regeneration of the material by using NaCl; CaR2 + 2NaCl 2NaR + CaCl2

  20. Ion exchange Softening Ion exchange material: 1- Zeolite: 2- Synthetic resin The synthetically resin has a much higher exchange capacity and require less amount of salt for regeneration

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