Water Treatment: Coagulation and Flocculation

Coagulation and Flocculation

• The main goal of coagulation/flocculation process is to reduce turbidity in water
• Turbidity is a measure of the amount of suspended solids in water sample that causes cloudiness
• Removing turbidity can remove unwanted taste and odor compounds
• Lower turbidity allows for higher rates of disinfection

What is a Coagulant?

• A chemical that is added to water to cause small particles to come together to form large particles that can settle due to gravity
• Typical coagulants: Alum (Aluminum sulfate) and Ferric sulfate (Fe2(SO4)3)

Coagulation Process

• A chemical process in which a coagulant is added and suspended in water by rapid mixing to aggregate dispersed non-settleable solids by destabilization
• Keywords:
  • Coagulant: a floc-forming chemical reagent
  • Nonsettleable solids: solids that cannot settle down by virtue of gravity, having negligible settling velocities

Theory of Coagulation

• Solid colloids stay in suspension due to repulsive forces, making the system stable
• Colloids can be destabilized by charge neutralization using positively charged ions (Na+, Mg2+, Al3+, Fe3+, etc.) to neutralize the colloidal negative charges
• With destabilization, colloids aggregate in size and start to settle

Points to Consider

• Low particulate concentration: sludge recycling
• Large coagulant dosage: restabilization of colloids with positive charge

Coagulation of Colloids by Organic Polymers

• Coagulation by bridging: the ionizable groups on the polymer surface react with the groups or the active sites on the surfaces of the colloids
• Several colloids may bind to a single polymer

Coagulants

• Aluminum sulfate (Al2(SO4)3.14 H2O)
  • 1 mole of alum consumes 6 moles of bicarbonate (HCO3-)
  • If alkalinity is not enough, pH will reduce greatly, which can be overcome by adding Ca(OH)2 or NaHCO3 (soda ash)
• Ferrous sulfate (FeSO4.7H2O)
  • Forms dense, quick-settling floc
  • pH must be raised to 9.5
• Ferric sulfate (Fe2(SO4)3)
  • pH should be in the range from 4 to 12
• Ferric chloride (FeCl3)
  • pH should be in the range from 4 to 12

Coagulation and Flocculation Process

• Coagulation: rapid mixing to aggregate dispersed non-settleable solids
• Flocculation: gentle mixing to form large flocs
• Agitation devices:
  • Mechanical agitators
  • Pneumatic agitators
  • Baffle basins

Jar Test

• A laboratory technique used to determine:
  • The proper coagulant and coagulant aid
  • The chemical dosage required
• Variables:
  • Speed of rotation
  • Time of settling
  • Dosage
  • Size of flocs
  • Turbidity removal
  • Color removal
  • pH

Velocity Gradient

• Measures the degree of mixing
• Velocity gradient (G): relative velocity of the two fluid particles divided by distance
• Higher G value, intenser mixing

Mixing by Impellers and Pneumatic Mixing

• Equations for power input and velocity gradient for mechanical and pneumatic agitation