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Coagulation

Coagulation. CE 547. Overview. Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, or organic compounds. Colloidal Particles Difficult to settle Pass through small pores of conventional filters

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Coagulation

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  1. Coagulation CE 547

  2. Overview Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, or organic compounds. Colloidal Particles • Difficult to settle • Pass through small pores of conventional filters How to remove colloidal particles? • By aggregation (making them bigger sizes) Why aggregation is difficult? • Small size of particles • Physical and electrical forces How to aggregate? • Use of chemical agents

  3. Chemical Coagulation It is the process of destroying the stabilizing forces and causing aggregation Aggregation occurs in two steps: 1. reduction or elimination of the inter-particle forces responsible for stability (by addition of chemicals) 2. collision due to • molecular motion • mechanical mixing (using rapid or flash mixing for very short time, less than 1 minute)

  4. After destabilization, gentle mixing is provided to increase the rate of particle collision without breaking the aggregates or flocs (this process is called flocculation)

  5. Stability of Particulates Colloidal particulates remain in suspension for very long periods due to their stability (is it possible to give such particles sufficient time to settle?)

  6. Particulate Characteristics 1. Size • colloidal materials • upper limit of ≈ 1 m • lower limit of ≈ 5 nm • suspended solids • larger than ≈ 0.5 m Particles larger than 5 nm are in suspension

  7. 2. Nature of solid-water interface • Hydrophobic • Well defined interface • Low affinity for water molecules • Thermodynamically unstable and will aggregate over time (irreversible) • Generally inorganic • Hydrophilic (Fig 12.1) • Lack of clear interface • Generally organic matter such as proteins • Can aggregate (but reversible)

  8. Mechanisms of Stability The main mechanism of particle stability is the electrical repulsion Presence of adsorbed water molecules (this will provide barrier to successful collision)

  9. How electrical charges exist at particle surfaces? There are three principle ways: 1. Crystal Imperfections Silicon atoms in crystals can be replaced by atoms with lower valence (such as Al) giving excess negative charge to the crystal material. 2. Preferential Adsorption of Specific Ions When particles are dispersed in water, soluble polyelectrolytes of natural origin may adsorb on particle surfaces (for example, fulvi acid, -vely charged, can be adsorbed on CaCO3, +vely charged)

  10. 3. Specific Chemical Reactions of Inorganic Groups on Particulate Surfaces many particulate surfaces contain inorganic groups such as hydroxyl or carboxyl functional groups which dissociate in water producing a surface electrical charge (that depends on the pH of the solution)

  11. - - - - - - - - - - - - - - + + + + + Origin of the Double Layer When particles are dispersed in water, ions with opposite charge to the particle surface accumulate closer to the particle to produce electro-neutrality.

  12. - + Diffusion Electro-static Attraction This accumulation is opposed by the tendency of ions to diffuse in the direction of decreasing concentration The result is a diffuse cloud of ions surrounding the particle, which is known as the electrical double layer (Fig 12.3)

  13. As ionic strength (TDS) increases, this will compress the diffuse layer.

  14. Mechanisms of Destabilization Removal of colloidal and suspended particulates depends on reduction in particulate stability. Destabilization can be achieved through: • compression of the electrical double layer • electrostatic attraction • interparticle bridging • enmeshment or sweep floc

  15. Double Layer Compression Increasing the ionic strength will compress the double layer, causing a decrease in its thickness. This will result in decreasing the Zeta Potential The amount of dissolved ions that produce rapid coagulation is defined as the Critical Coagulation Concentration (CCC), which depends on: • type of particulate • type of dissolved ions

  16. for hydrophobic particles, CCC is inversely proportional to the sixth power of the charge on the ion. So, for mono-, di-, and trivalent ions, the CCC values are in the ratio: As an Example: 3000 mg/l NaCl is equivalent to 44 mg/l CaCl2

  17. Electrostatic Attraction This occurs when surfaces are oppositely charged, which is promoted by the adsorption of specific ions on the surface of the particle. Particles in natural waters exhibit both +ve and –ve charges based on the pH of the water. Zero Point of Charge (ZPC): is the pH corresponding to a surface charge of zero. • Above ZPC, surface charge is –ve (anionic) • Below ZPC, surface charge is +ve

  18. Reduction of surface charge can be achieved by: • pH alteration • addition of specifically adsorbed ions

  19. Interparticle Bridging Long-chain polymers carrying –ve charges can form bridges between particle, thus destabilizing the suspension. This mechanism was shown to be the major mechanism controlling the aggregation of bacterial and alga suspensions.

  20. Enmeshment (Sweep Floc) This mechanism is predominant in water treatment where pH values are between 6 and 8 and Al or Fe salts are used at concentrations exceeding saturation with respect to amorphous metal hydroxide solid that is formed. In this mechanism, finely divided particles are entrapped in the amorphous precipitate formed.

  21. Chemistry of Coagulation Functions of Coagulants • destabilization • strengthening of flocs to reduce floc breakup Selection of Coagulants • low cost • availability • stability during storage • ease of handling • must form highly insoluble compounds to minimize the concentration of soluble residuals

  22. Selection of Type and Dose of Coagulants Depends on: • characteristics of the coagulant • characteristics of the particles • characteristics of the water Jar test is used to investigate: • what coagulant to be used • optimum pH • optimum dose

  23. In selection of coagulants, cost and quantity and dewaterability of produced solids (sludge) should be taken into consideration. Full-scale testing is necessary to determine: • optimum dose • optimum coagulants combination (usually inorganic coagulant + polyelectrolyte) Inorganic Coagulants • Aluminum salts • Ferric salts • Usually in forms of sulfates or chlorides and available in solid and liquid forms • Aluminum or Ferric Ions React With OH-, SO42-, or PO43- To Form Soluble and Insoluble Products.

  24. One mole of trivalent ion produces one mole of the metal hydroxide. So, 1 mg of aluminum sulfate (alum) [Al2(SO4)3 . 14H2O] produces about 0.26 mg of insoluble Al(OH)3 and consumes about 0.5 mg of alkalinity expressed as CaCO3. • Similarly, 1 mg of ferric sulfate [Fe2(SO4)3] produces about 0.5 mg of Fe(OH)3 as precipitate and consumes about 0.75 mg alkalinity (as CaCO3) • Actual amount of precipitate and acidity (H+) formed depend on system pH and concentration of reactive liquids.

  25. The rate of reaction of coagulants with water depends on: • pH (plays a dominant role) • ionic species in water • temperature • type and concentration of particles • concentration of coagulant • mixing condition at the point of coagulant addition

  26. Aluminum At pH values less than 6: • +vely charged Al species remain in solution long enough to interact with particles and destabilize them by charge neutralization • turbidity-causing particles are destabilized by adsorption

  27. Ferric Similar to Aluminum, but at pH below 4. Above pH 6 for Aluminum and pH 4 for ferric, formation of amorphous precipitates occurs reapidly causing entrapment of particles “sweep floc”. This sweep floc mechanism requires greater quantity of coagulant than charge neutralization which will result in producing more sludge.

  28. Organic Coagulants Organic polymers are used as coagulants and are termed as “ployelectrlytes”. They are used as: • primary coagulants • coagulants or filter aids • sludge conditioners The use of polymers is restricted in water treatment due to: • high cost • uncertainties regarding chemical impurities

  29. Polymers natural (sodium alginate and chitosan; very high cost) synthetic (predominant in water treatment) Functions of Polymers • destabilization of particles • form larger and more shear-resistant flocs

  30. Chitosan

  31. Destabilization • charge neutralization (-vely charged particles can be destabilized by cationic polymers) • polymer bridging (-vely charged particles can be destabilized by anionic polymers under appropriate conditions. The mechanism will result in: • increase floc size • increase floc strength

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