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Civil Engineering QUESTION #2129
Question 1
In water treatment, coagulation and flocculation are distinct processes where:
  • Coagulation uses slow mixing to form large settleable flocs; flocculation uses rapid mixing to destabilize colloids
  • Coagulation (rapid mixing) destabilizes colloidal particles by neutralizing their surface charge (zeta potential) using alum or ferric salts; flocculation (slow, gentle mixing) then aggregates the destabilized particles into larger, settleable floc through physical contact and van der Waals forces✔️
  • Both processes are identical and occur simultaneously in the same mixing unit
  • Coagulation removes dissolved solids; flocculation removes suspended solids
Correct Answer Explanation
Coagulation: rapid mixing (30–60 sec, G = 300–1000 s⁻¹) with coagulant (alum: Al₂(SO₄)₃, produces Al(OH)₃) neutralizes the negative surface charge of colloids, reducing electrostatic repulsion (double layer compression). Flocculation: slow mixing (10–30 min, G = 10–75 s⁻¹) promotes gentle particle collisions, building floc size through orthokinetic flocculation — floc must be large/dense enough to settle in clarifiers.