Ch3_2 coagulation and flocculation

Ch3_2 coagulation and flocculation - Chapter 3-2...

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Chapter 3-2 Coagulation/Flocculation Why do we need coagulation and flocculation Why do we need coagulation and flocculation processes rather than just clarification only? processes rather than just clarification only? Settling velocities of various size particles* Particle diameter (mm) Size typical of Settling velocity (m/s) 10 Pebble 0.73 1 Coarse sand 0.23 0.1 Fine sand 1.0 × 10 -2 0.01 Silt 1.0 × 10 -4 0.0001 Large colloid 1.0 × 10 -8 0.000001 Small colloid 1.0 × 10 -13 * Spheres with specific gravity of 2.65 in water at 20 o C (Table 4-1 in Text) If we can agglomerate particles into groups, the effective size and settling velocities of particles will increase to promote settling.

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In nature, particles are generally negatively charged. Colloidal Stability Theory: Repulsion force: Electrostatic potential Attraction force: van der Waals force particle particle + + + + + + + + - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + Double layer particle + + + + + + + + - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + +

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Van der Waals force 0 Distance between colloids Net force (Zeta - van der Waals) Zeta potential (or Stern potential) Energy barrier Distance from particle Number of ions Counter ions Co-ions Bulk solution Double Layer How thick? 300 nm 300 nm (Repulsive force) (Attractive force) Montgomery, 1985)
Coagulation Theory Coagulation: Electrochemical process to destablize colloids by 1) Ionic layer compression: Compressing the double layer by high ionic concentration so that van der Waals force is predominant thereby only net attractive force present with no energy barrier (That’s why seawater is so clear). 2) Adsorption and charge neutralization: Neutralizing the surface charge by adsorbing aqueous metallic cations --> disappearance of the electrostatic potentials. 3) Sweep coagulation: Entrapping colloids into a floc of aluminum hydroxide with high density and settling by gravity. Al(OH) x

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Ionic layer compression Distance from particle Number of ions Counter ions (+) Co-ions (-) Bulk solution Double Layer w/ low ionic strength + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Double layer w/ high ionic strength
Van der Waals force 0 Distance between colloids Net force (Zeta - van der Waals) Zeta potential (or Stern potential) Energy barrier (Repulsive force) (Attractive force) Ionic layer compression

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Adsorption and charge neutralization Van der Waals force 0 Distance between colloids Net force (Zeta - van der Waals) Zeta potential (or Stern potential) Energy barrier (Repulsive force) (Attractive force)
Coagulation Theory Inter-particle Bridging: Linear or branched polymers with high surface reactivity may attached to

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Ch3_2 coagulation and flocculation - Chapter 3-2...

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